rfc9262.original   rfc9262.txt 
Network Working Group T.T.E. Eckert, Ed. Internet Engineering Task Force (IETF) T. Eckert, Ed.
Internet-Draft Futurewei Request for Comments: 9262 Futurewei
Intended status: Standards Track M.M. Menth Category: Standards Track M. Menth
Expires: 27 October 2022 University of Tuebingen ISSN: 2070-1721 University of Tuebingen
G.C. Cauchie G. Cauchie
KOEVOO KOEVOO
April 2022 October 2022
Tree Engineering for Bit Index Explicit Replication (BIER-TE) Tree Engineering for Bit Index Explicit Replication (BIER-TE)
draft-ietf-bier-te-arch-13
Abstract Abstract
This memo describes per-packet stateless strict and loose path This memo describes per-packet stateless strict and loose path
steered replication and forwarding for "Bit Index Explicit steered replication and forwarding for "Bit Index Explicit
Replication" (BIER, RFC8279) packets. It is called BIER Tree Replication" (BIER) packets (RFC 8279); it is called "Tree
Engineering (BIER-TE) and is intended to be used as the path steering Engineering for Bit Index Explicit Replication" (BIER-TE) and is
mechanism for Traffic Engineering with BIER. intended to be used as the path steering mechanism for Traffic
Engineering with BIER.
BIER-TE introduces a new semantic for "bit positions" (BP). They BIER-TE introduces a new semantic for "bit positions" (BPs). These
indicate adjacencies of the network topology, as opposed to (non-TE) BPs indicate adjacencies of the network topology, as opposed to (non-
BIER in which BPs indicate "Bit-Forwarding Egress Routers" (BFER). A TE) BIER in which BPs indicate "Bit-Forwarding Egress Routers"
BIER-TE packets BitString therefore indicates the edges of the (loop- (BFERs). A BIER-TE "packets BitString" therefore indicates the edges
free) tree that the packet is forwarded across by BIER-TE. BIER-TE of the (loop-free) tree across which the packets are forwarded by
can leverage BIER forwarding engines with little changes. Co- BIER-TE. BIER-TE can leverage BIER forwarding engines with little
existence of BIER and BIER-TE forwarding in the same domain is changes. Co-existence of BIER and BIER-TE forwarding in the same
possible, for example by using separate BIER "sub-domains" (SDs). domain is possible -- for example, by using separate BIER
Except for the optional routed adjacencies, BIER-TE does not require "subdomains" (SDs). Except for the optional routed adjacencies,
a BIER routing underlay, and can therefore operate without depending BIER-TE does not require a BIER routing underlay and can therefore
on an "Interior Gateway Routing protocol" (IGP). operate without depending on a routing protocol such as the "Interior
Gateway Protocol" (IGP).
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
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Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on 3 October 2022. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9262.
Copyright Notice Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Overview
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 2. Introduction
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Requirements Language
2.1. Basic Examples . . . . . . . . . . . . . . . . . . . . . 5 2.2. Basic Examples
2.2. BIER-TE Topology and adjacencies . . . . . . . . . . . . 8 2.3. BIER-TE Topology and Adjacencies
2.3. Relationship to BIER . . . . . . . . . . . . . . . . . . 9 2.4. Relationship to BIER
2.4. Accelerated/Hardware forwarding comparison . . . . . . . 11 2.5. Accelerated Hardware Forwarding Comparison
3. Components . . . . . . . . . . . . . . . . . . . . . . . . . 11 3. Components
3.1. The Multicast Flow Overlay . . . . . . . . . . . . . . . 12 3.1. The Multicast Flow Overlay
3.2. The BIER-TE Control Plane . . . . . . . . . . . . . . . . 12 3.2. The BIER-TE Control Plane
3.2.1. The BIER-TE Controller . . . . . . . . . . . . . . . 14 3.2.1. The BIER-TE Controller
3.2.1.1. BIER-TE Topology discovery and creation . . . . . 14 3.2.1.1. BIER-TE Topology Discovery and Creation
3.2.1.2. Engineered Trees via BitStrings . . . . . . . . . 15 3.2.1.2. Engineered Trees via BitStrings
3.2.1.3. Changes in the network topology . . . . . . . . . 16 3.2.1.3. Changes in the Network Topology
3.2.1.4. Link/Node Failures and Recovery . . . . . . . . . 16 3.2.1.4. Link/Node Failures and Recovery
3.3. The BIER-TE Forwarding Plane . . . . . . . . . . . . . . 16 3.3. The BIER-TE Forwarding Plane
3.4. The Routing Underlay . . . . . . . . . . . . . . . . . . 17 3.4. The Routing Underlay
3.5. Traffic Engineering Considerations . . . . . . . . . . . 17 3.5. Traffic Engineering Considerations
4. BIER-TE Forwarding . . . . . . . . . . . . . . . . . . . . . 18 4. BIER-TE Forwarding
4.1. The BIER-TE Bit Index Forwarding Table (BIFT) . . . . . . 18 4.1. The BIER-TE Bit Index Forwarding Table (BIFT)
4.2. Adjacency Types . . . . . . . . . . . . . . . . . . . . . 20 4.2. Adjacency Types
4.2.1. Forward Connected . . . . . . . . . . . . . . . . . . 21 4.2.1. Forward Connected
4.2.2. Forward Routed . . . . . . . . . . . . . . . . . . . 21 4.2.2. Forward Routed
4.2.3. ECMP . . . . . . . . . . . . . . . . . . . . . . . . 21 4.2.3. ECMP
4.2.4. Local Decap(sulation) . . . . . . . . . . . . . . . . 22 4.2.4. Local Decap(sulation)
4.3. Encapsulation / Co-existence with BIER . . . . . . . . . 22 4.3. Encapsulation / Co-existence with BIER
4.4. BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . 23 4.4. BIER-TE Forwarding Pseudocode
4.5. BFR Requirements for BIER-TE forwarding . . . . . . . . . 26 4.5. BFR Requirements for BIER-TE Forwarding
5. BIER-TE Controller Operational Considerations . . . . . . . . 27 5. BIER-TE Controller Operational Considerations
5.1. Bit Position Assignments . . . . . . . . . . . . . . . . 27 5.1. Bit Position Assignments
5.1.1. P2P Links . . . . . . . . . . . . . . . . . . . . . . 27 5.1.1. P2P Links
5.1.2. BFER . . . . . . . . . . . . . . . . . . . . . . . . 27 5.1.2. BFERs
5.1.3. Leaf BFERs . . . . . . . . . . . . . . . . . . . . . 27 5.1.3. Leaf BFERs
5.1.4. LANs . . . . . . . . . . . . . . . . . . . . . . . . 29 5.1.4. LANs
5.1.5. Hub and Spoke . . . . . . . . . . . . . . . . . . . . 30 5.1.5. Hub and Spoke
5.1.6. Rings . . . . . . . . . . . . . . . . . . . . . . . . 30 5.1.6. Rings
5.1.7. Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . 31 5.1.7. Equal-Cost Multipath (ECMP)
5.1.8. Forward Routed adjacencies . . . . . . . . . . . . . 34 5.1.8. Forward Routed Adjacencies
5.1.8.1. Reducing bit positions . . . . . . . . . . . . . 34 5.1.8.1. Reducing Bit Positions
5.1.8.2. Supporting nodes without BIER-TE . . . . . . . . 35 5.1.8.2. Supporting Nodes without BIER-TE
5.1.9. Reuse of bit positions (without DNC) . . . . . . . . 35 5.1.9. Reuse of Bit Positions (without DNC)
5.1.10. Summary of BP optimizations . . . . . . . . . . . . . 36 5.1.10. Summary of BP Optimizations
5.2. Avoiding duplicates and loops . . . . . . . . . . . . . . 37 5.2. Avoiding Duplicates and Loops
5.2.1. Loops . . . . . . . . . . . . . . . . . . . . . . . . 38 5.2.1. Loops
5.2.2. Duplicates . . . . . . . . . . . . . . . . . . . . . 38 5.2.2. Duplicates
5.3. Managing SI, sub-domains and BFR-ids . . . . . . . . . . 39 5.3. Managing SIs, Subdomains, and BFR-ids
5.3.1. Why SI and sub-domains . . . . . . . . . . . . . . . 39 5.3.1. Why SIs and Subdomains?
5.3.2. Assigning bits for the BIER-TE topology . . . . . . . 40 5.3.2. Assigning Bits for the BIER-TE Topology
5.3.3. Assigning BFR-id with BIER-TE . . . . . . . . . . . . 41 5.3.3. Assigning BFR-ids with BIER-TE
5.3.4. Mapping from BFR to BitStrings with BIER-TE . . . . . 42 5.3.4. Mapping from BFRs to BitStrings with BIER-TE
5.3.5. Assigning BFR-ids for BIER-TE . . . . . . . . . . . . 43 5.3.5. Assigning BFR-ids for BIER-TE
5.3.6. Example bit allocations . . . . . . . . . . . . . . . 43 5.3.6. Example Bit Allocations
5.3.6.1. With BIER . . . . . . . . . . . . . . . . . . . . 43 5.3.6.1. With BIER
5.3.6.2. With BIER-TE . . . . . . . . . . . . . . . . . . 44 5.3.6.2. With BIER-TE
5.3.7. Summary . . . . . . . . . . . . . . . . . . . . . . . 45 5.3.7. Summary
6. Security Considerations . . . . . . . . . . . . . . . . . . . 46 6. Security Considerations
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47 7. IANA Considerations
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 47 8. References
9. Change log [RFC Editor: Please remove] . . . . . . . . . . . 48 8.1. Normative References
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 61 8.2. Informative References
10.1. Normative References . . . . . . . . . . . . . . . . . . 61 Appendix A. BIER-TE and Segment Routing (SR)
10.2. Informative References . . . . . . . . . . . . . . . . . 61 Acknowledgements
Appendix A. BIER-TE and Segment Routing (SR) . . . . . . . . . . 64 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 65
1. Overview 1. Overview
BIER-TE is based on the (non-TE) BIER architecture, terminology and "Tree Engineering for Bit Index Explicit Replication" (BIER-TE) is
packet formats as described in [RFC8279] and [RFC8296]. This based on the (non-TE) BIER architecture, terminology, and packet
document describes BIER-TE in the expectation that the reader is formats as described in [RFC8279] and [RFC8296]. This document
familiar with these two documents. describes BIER-TE, with the expectation that the reader is familiar
with these two documents.
BIER-TE introduces a new semantic for "bit positions" (BP). They
indicate adjacencies of the network topology, as opposed to (non-TE)
BIER in which BPs indicate "Bit-Forwarding Egress Routers" (BFER). A
BIER-TE packets BitString therefore indicates the edges of the (loop-
free) tree that the packet is forwarded across by BIER-TE. With
BIER-TE, the "Bit Index Forwarding Table" (BIFT) of each "Bit
Forwarding Router" (BFR) is only populated with BP that are adjacent
to the BFR in the BIER-TE Topology. Other BPs are empty in the BIFT.
The BFR replicate and forwards BIER packets to adjacent BPs that are BIER-TE introduces a new semantic for "bit positions" (BPs). These
set in the packet. BPs are normally also cleared upon forwarding to BPs indicate adjacencies of the network topology, as opposed to (non-
avoid duplicates and loops. TE) BIER in which BPs indicate "Bit-Forwarding Egress Routers"
(BFERs). A BIER-TE "packets BitString" therefore indicates the edges
of the (loop-free) tree across which the packets are forwarded by
BIER-TE. With BIER-TE, the "Bit Index Forwarding Table" (BIFT) of
each "Bit-Forwarding Router" (BFR) is only populated with BPs that
are adjacent to the BFR in the BIER-TE topology. Other BPs are empty
in the BIFT. The BFR replicates and forwards BIER packets to
adjacent BPs that are set in the packets. BPs are normally also
cleared upon forwarding to avoid duplicates and loops.
BIER-TE can leverage BIER forwarding engines with little or no BIER-TE can leverage BIER forwarding engines with little or no
changes. It can also co-exist with BIER forwarding in the same changes. It can also co-exist with BIER forwarding in the same
domain, for example by using separate BIER sub-domains. Except for domain -- for example, by using separate BIER subdomains. Except for
the optional routed adjacencies, BIER-TE does not require a BIER the optional routed adjacencies, BIER-TE does not require a BIER
routing underlay, and can therefore operate without depending on an routing underlay and can therefore operate without depending on a
"Interior Gateway Routing protocol" (IGP). routing protocol such as the "Interior Gateway Protocol" (IGP).
This document is structured as follows: This document is structured as follows:
* Section 2 introduces BIER-TE with two forwarding examples, * Section 2 introduces BIER-TE with two forwarding examples,
followed by an introduction of the new concepts of the BIER-TE followed by an introduction to the new concepts of the BIER-TE
(overlay) topology and finally a summary of the relationship (overlay) topology, and finally a summary of the relationship
between BIER and BIER-TE and a discussion of accelerated hardware between BIER and BIER-TE and a discussion of accelerated hardware
forwarding. forwarding.
* Section 3 describes the components of the BIER-TE architecture, * Section 3 describes the components of the BIER-TE architecture:
Flow overlay, BIER-TE layer with the BIER-TE control plane the multicast flow overlay, the BIER-TE layer with the BIER-TE
(including the BIER-TE controller) and BIER-TE forwarding plane, control plane (including the BIER-TE controller), the BIER-TE
and the routing underlay. forwarding plane, and the routing underlay.
* Section 4 specifies the behavior of the BIER-TE forwarding plane * Section 4 specifies the behavior of the BIER-TE forwarding plane
with the different type of adjacencies and possible variations of with the different types of adjacencies and possible variations of
BIER-TE forwarding pseudocode, and finally the mandatory and BIER-TE forwarding pseudocode, and finally the mandatory and
optional requirements. optional requirements.
* Section 5 describes operational considerations for the BIER-TE * Section 5 describes operational considerations for the BIER-TE
controller, foremost how the BIER-TE controller can optimize the controller, primarily how the BIER-TE controller can optimize the
use of BP by using specific type of BIER-TE adjacencies for use of BPs by using specific types of BIER-TE adjacencies for
different type of topological situations, but also how to assign different types of topological situations. It also describes how
bits to avoid loops and duplicates (which in BIER-TE does not come to assign bits to avoid loops and duplicates (which, in BIER-TE,
for free), and finally how "Set Identifier" (SI), "sub-domain" does not come "for free"). Finally, it discusses how "Set
(SD) and BFR-ids can be managed by a BIER-TE controller, examples Identifiers" (SIs), "subdomains" (SDs), and BFR-ids can be managed
and summary. by a BIER-TE controller; examples and a summary are provided.
* Appendix A concludes the technology specific sections of the * Appendix A concludes this document; details regarding the
document by further relating BIER-TE to Segment Routing (SR). relationship between BIER-TE and "Segment Routing" (SR) are
discussed.
Note that related work, [I-D.ietf-roll-ccast] uses Bloom filters Note that related work [CONSTRAINED-CAST] uses Bloom filters
[Bloom70] to represent leaves or edges of the intended delivery tree. [Bloom70] to represent leaves or edges of the intended delivery tree.
Bloom filters in general can support larger trees/topologies with Bloom filters in general can support larger trees/topologies with
fewer addressing bits than explicit BitStrings, but they introduce fewer addressing bits than explicit BitStrings, but they introduce
the heuristic risk of false positives and cannot clear bits in the the heuristic risk of false positives and cannot clear bits in the
BitString during forwarding to avoid loops. For these reasons, BIER- BitStrings during forwarding to avoid loops. For these reasons,
TE uses explicit BitStrings like BIER. The explicit BitStrings of BIER-TE, like BIER, uses explicit BitStrings. Explicit BitStrings as
BIER-TE can also be seen as a special type of Bloom filter, and this used by BIER-TE can also be seen as a special type of Bloom filter,
is how related work [ICC] describes it. and this is how other related work [ICC] describes it.
1.1. Requirements Language 2. Introduction
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Introduction 2.2. Basic Examples
2.1. Basic Examples
BIER-TE forwarding is best introduced with simple examples. These BIER-TE forwarding is best introduced with simple examples. These
examples use formal terms defined later in the document (Figure 4), examples use formal terms defined later in this document (Figure 4 in
including forward_connected(), forward_routed() and local_decap(). Section 4.1), including forward_connected(), forward_routed(), and
local_decap().
Consider the simple network in the BIER-TE overview example shown in
Figure 1, with six BFRs. p1...p15 are the bit positions used. All
BFRs can act as a "Bit-Forwarding Ingress Router" (BFIR); BFR1, BFR3,
BFR4, and BFR6 can also be BFERs. "Forward_connected()" is the name
used for adjacencies that represent subnet adjacencies of the
network. "Local_decap()" is the name used for the adjacency that
decapsulates BIER-TE packets and passes their payload to higher-layer
processing.
BIER-TE Topology: BIER-TE Topology:
Diagram: Diagram:
p5 p6 p5 p6
--- BFR3 --- --- BFR3 ---
p3/ p13 \p7 p15 p3/ p13 \p7 p15
BFR1 ---- BFR2 BFR5 ----- BFR6 BFR1 ---- BFR2 BFR5 ----- BFR6
p1 p2 p4\ p14 /p10 p11 p12 p1 p2 p4\ p14 /p10 p11 p12
--- BFR4 --- --- BFR4 ---
p8 p9 p8 p9
(simplified) BIER-TE Bit Index Forwarding Tables (BIFT): (simplified) BIER-TE Bit Index Forwarding Tables (BIFTs):
BFR1: p1 -> local_decap() BFR1: p1 -> local_decap()
p2 -> forward_connected() to BFR2 p2 -> forward_connected() to BFR2
BFR2: p1 -> forward_connected() to BFR1 BFR2: p1 -> forward_connected() to BFR1
p5 -> forward_connected() to BFR3 p5 -> forward_connected() to BFR3
p8 -> forward_connected() to BFR4 p8 -> forward_connected() to BFR4
BFR3: p3 -> forward_connected() to BFR2 BFR3: p3 -> forward_connected() to BFR2
p7 -> forward_connected() to BFR5 p7 -> forward_connected() to BFR5
skipping to change at page 6, line 41 skipping to change at line 259
p10 -> forward_connected() to BFR5 p10 -> forward_connected() to BFR5
p14 -> local_decap() p14 -> local_decap()
BFR5: p6 -> forward_connected() to BFR3 BFR5: p6 -> forward_connected() to BFR3
p9 -> forward_connected() to BFR4 p9 -> forward_connected() to BFR4
p12 -> forward_connected() to BFR6 p12 -> forward_connected() to BFR6
BFR6: p11 -> forward_connected() to BFR5 BFR6: p11 -> forward_connected() to BFR5
p15 -> local_decap() p15 -> local_decap()
Figure 1: BIER-TE basic example Figure 1: BIER-TE Basic Example
Consider the simple network in the above BIER-TE overview example
picture with 6 BFRs. p1...p15 are the bit positions used. All BFRs
can act as an ingress BFR (BFIR), BFR1, BFR3, BFR4 and BFR6 can also
be BFERs. Forward_connected() is the name for adjacencies that are
representing subnet adjacencies of the network. Local_decap() is the
name of the adjacency to decapsulate BIER-TE packets and pass their
payload to higher layer processing.
Assume a packet from BFR1 should be sent via BFR4 to BFR6. This Assume that a packet from BFR1 should be sent via BFR4 to BFR6. This
requires a BitString (p2,p8,p10,p12,p15). When this packet is requires a BitString (p2,p8,p10,p12,p15). When this packet is
examined by BIER-TE on BFR1, the only bit position from the BitString examined by BIER-TE on BFR1, the only bit position from the BitString
that is also set in the BIFT is p2. This will cause BFR1 to send the that is also set in the BIFT is p2. This will cause BFR1 to send the
only copy of the packet to BFR2. Similarly, BFR2 will forward to only copy of the packet to BFR2. Similarly, BFR2 will forward to
BFR4 because of p8, BFR4 to BFR5 because of p10 and BFR5 to BFR6 BFR4 because of p8, BFR4 to BFR5 because of p10, and BFR5 to BFR6
because of p12. p15 finally makes BFR6 receive and decapsulate the because of p12. p15 finally makes BFR6 receive and decapsulate the
packet. packet.
To send a copy to BFR6 via BFR4 and also a copy to BFR3, the To send a copy to BFR6 via BFR4 and also a copy to BFR3, the
BitString needs to be (p2,p5,p8,p10,p12,p13,p15). When this packet BitString needs to be (p2,p5,p8,p10,p12,p13,p15). When this packet
is examined by BFR2, p5 causes one copy to be sent to BFR3 and p8 one is examined by BFR2, p5 causes one copy to be sent to BFR3 and p8 one
copy to BFR4. When BFR3 receives the packet, p13 will cause it to copy to BFR4. When BFR3 receives the packet, p13 will cause it to
receive and decapsulate the packet. receive and decapsulate the packet.
If instead the BitString was (p2,p6,p8,p10,p12,p13,p15), the packet If instead the BitString was (p2,p6,p8,p10,p12,p13,p15), the packet
would be copied by BFR5 towards BFR3 because of p6 instead of being would be copied by BFR5 towards BFR3 because of p6 instead of being
copied by BFR2 to BFR3 because of p5 in the prior case. This is copied by BFR2 to BFR3 because of p5 in the prior case. This
showing the ability of the shown BIER-TE Topology to make the traffic demonstrates the ability of the BIER-TE topology, as shown in
pass across any possible path and be replicated where desired. Figure 1, to make the traffic pass across any possible path and be
replicated where desired.
BIER-TE has various options to minimize BP assignments, many of which BIER-TE has various options for minimizing BP assignments, many of
are based on out-of-band knowledge about the required multicast which are based on out-of-band knowledge about the required multicast
traffic paths and bandwidth consumption in the network, such as from traffic paths and bandwidth consumption in the network, e.g., from
pre-deployment planning. predeployment planning.
Figure 2 shows a modified example, in which Rtr2 and Rtr5 are assumed Figure 2 shows a modified example, in which Rtr2 and Rtr5 are assumed
not to support BIER-TE, so traffic has to be unicast encapsulated not to support BIER-TE, so traffic has to be unicast encapsulated
across them. To emphasize non-L2, but routed/tunneled forwarding of across them. To explicitly distinguish routed/tunneled forwarding of
BIER-TE packets, these adjacencies are called "forward_routed". BIER-TE packets from Layer 2 forwarding (forward_connected()), these
Otherwise, there is no difference in their processing over the adjacencies are called "forward_routed()" adjacencies. Otherwise,
aforementioned forward_connected() adjacencies. there is no difference in their processing over the aforementioned
forward_connected() adjacencies.
In addition, bits are saved in the following example by assuming that In addition, bits are saved in the following example by assuming that
BFR1 only needs to be BFIR but not BFER or transit BFR. BFR1 only needs to be a BFIR -- not a BFER or a transit BFR.
BIER-TE Topology: BIER-TE Topology:
Diagram: Diagram:
p1 p3 p7 p1 p3 p7
....> BFR3 <.... p5 ....> BFR3 <.... p5
........ ........> ........ ........>
BFR1 (Rtr2) (Rtr5) BFR6 BFR1 (Rtr2) (Rtr5) BFR6
........ ........> p9 ........ ........> p9
....> BFR4 <.... p6 ....> BFR4 <.... p6
p2 p4 p8 p2 p4 p8
(simplified) BIER-TE Bit Index Forwarding Tables (BIFT): (simplified) BIER-TE Bit Index Forwarding Tables (BIFTs):
BFR1: p1 -> forward_routed() to BFR3 BFR1: p1 -> forward_routed() to BFR3
p2 -> forward_routed() to BFR4 p2 -> forward_routed() to BFR4
BFR3: p3 -> local_decap() BFR3: p3 -> local_decap()
p5 -> forward_routed() to BFR6 p5 -> forward_routed() to BFR6
BFR4: p4 -> local_decap() BFR4: p4 -> local_decap()
p6 -> forward_routed() to BFR6 p6 -> forward_routed() to BFR6
BFR6: p7 -> forward_routed() to BFR3 BFR6: p7 -> forward_routed() to BFR3
p8 -> forward_routed() to BFR4 p8 -> forward_routed() to BFR4
p9 -> local_decap() p9 -> local_decap()
Figure 2: BIER-TE basic overlay example Figure 2: BIER-TE Basic Overlay Example
To send a BIER-TE packet from BFR1 via BFR3 to be received by BFR6, To send a BIER-TE packet from BFR1 via BFR3 to be received by BFR6,
the BitString is (p1,p5,p9). From BFR1 via BFR4 to be received by the BitString is (p1,p5,p9). A packet from BFR1 via BFR4 to be
BFR6, the BitString is (p2,p6,p9). A packet from BFR1 to be received received by BFR6 uses the BitString (p2,p6,p9). A packet from BFR1
by BFR3,BFR4 and from BFR3 to be received by BFR6 uses to be received by BFR3,BFR4 and from BFR3 to be received by BFR6 uses
(p1,p2,p3,p4,p5,p9). A packet from BFR1 to be received by BFR3,BFR4 (p1,p2,p3,p4,p5,p9). A packet from BFR1 to be received by BFR3,BFR4
and from BFR4 to be received by BFR6 uses (p1,p2,p3,p4,p6,p9). A and from BFR4 to be received by BFR6 uses (p1,p2,p3,p4,p6,p9). A
packet from BFR1 to be received by BFR4, and from BFR4 to be received packet from BFR1 to be received by BFR4, then from BFR4 to be
by BFR6 and from there to be received by BFR3 uses received by BFR6, and finally from BFR6 to be received by BFR3, uses
(p2,p3,p4,p6,p7,p9). A packet from BFR1 to be received by BFR3, and (p2,p3,p4,p6,p7,p9). A packet from BFR1 to be received by BFR3, then
from BFR3 to be received by BFR6 there to be received by BFR4 uses from BFR3 to be received by BFR6, and finally from BFR6 to be
(p1,p3,p4,p5,p8,p9). received by BFR4, uses (p1,p3,p4,p5,p8,p9).
2.2. BIER-TE Topology and adjacencies 2.3. BIER-TE Topology and Adjacencies
The key new component in BIER-TE compared to (non-TE) BIER is the The key new component in BIER-TE compared to (non-TE) BIER is the
BIER-TE topology as introduced through the two examples in BIER-TE topology as introduced through the two examples in
Section 2.1. It is used to control where replication can or should Section 2.2. It is used to control where replication can or should
happen and how to minimize the required number of BP for adjacencies. happen and how to minimize the required number of BPs for
adjacencies.
The BIER-TE Topology consists of the BIFTs of all the BFR and can The BIER-TE topology consists of the BIFTs of all the BFRs and can
also be expressed as a directed graph where the edges are the also be expressed as a directed graph where the edges are the
adjacencies between the BFRs labelled with the BP used for the adjacencies between the BFRs labeled with the BP used for the
adjacency. Adjacencies are naturally unidirectional. BP can be adjacency. Adjacencies are naturally unidirectional. A BP can be
reused across multiple adjacencies as long as this does not lead to reused across multiple adjacencies as long as this does not lead to
undesired duplicates or loops as explained in Section 5.2. undesired duplicates or loops, as explained in Section 5.2.
If the BIER-TE topology represents (a subset of) the underlying If the BIER-TE topology represents (a subset of) the underlying
(layer 2) topology of the network as shown in the first example, this (Layer 2) topology of the network as shown in the first example, this
may be called a "native" BIER-TE topology. A topology consisting may be called an "underlay" BIER-TE topology. A topology consisting
only of "forward_routed" adjacencies as shown in the second example only of "forward_routed()" adjacencies as shown in the second example
may be called an "overlay" BIER-TE topology. A BIER-TE topology with may be called an "overlay" BIER-TE topology. A BIER-TE topology with
both forward_connected() and forward_routed() adjacencies may be both forward_connected() and forward_routed() adjacencies may be
called a "hybrid" BIER-TE topology. called a "hybrid" BIER-TE topology.
2.3. Relationship to BIER 2.4. Relationship to BIER
BIER-TE is designed so that its forwarding plane is a simple BIER-TE is designed so that its forwarding plane is a simple
extension to the (non-TE) BIER forwarding plane, hence allowing for extension to the (non-TE) BIER forwarding plane, hence allowing it to
it to be added to BIER deployments where it can be beneficial. be added to BIER deployments where it can be beneficial.
BIER-TE is also intended as an option to expand the BIER architecture BIER-TE is also intended as an option to expand the BIER architecture
into deployments where (non-TE) BIER may not be the best fit, such as into deployments where (non-TE) BIER may not be the best fit, such as
statically provisioned networks with needs for path steering but statically provisioned networks that need path steering but do not
without desire for distributed routing protocols. want distributed routing protocols.
1. BIER-TE inherits the following aspects from BIER unchanged: 1. BIER-TE inherits the following aspects from BIER unchanged:
1. The fundamental purpose of per-packet signaled replication 1.a The fundamental purpose of per-packet signaled replication
and delivery via a BitString. and delivery via a BitString.
2. The overall architecture consisting of three layers, flow 1.b The overall architecture, which consists of three layers:
overlay, BIER(-TE) layer and routing underlay. the flow overlay, the BIER(-TE) layer, and the routing
underlay.
3. The supported encapsulations [RFC8296]. 1.c The supported encapsulations [RFC8296].
4. The semantic of all [RFC8296] header elements used by the 1.d The semantics of all BIER header elements [RFC8296] used by
BIER-TE forwarding plane other than the semantic of the BP in the BIER-TE forwarding plane, other than the semantic of the
the BitString. BP in the BitString.
5. The BIER forwarding plane, except for how bits have to be 1.e The BIER forwarding plane, except for how bits have to be
cleared during replication. cleared during replication.
2. BIER-TE has the following key changes with respect to BIER: 2. BIER-TE has the following key changes with respect to BIER:
1. In BIER, bits in the BitString of a BIER packet header 2.a In BIER, bits in the BitString of a BIER packet header
indicate a BFER and bits in the BIFT indicate the BIER indicate a BFER, and bits in the BIFT indicate the BIER
control plane calculated next-hop toward that BFER. In BIER- control plane's calculated next hop towards that BFER. In
TE, a bit in the BitString of a BIER packet header indicates BIER-TE, a bit in the BitString of a BIER packet header
an adjacency in the BIER-TE topology, and only the BFR that indicates an adjacency in the BIER-TE topology, and only the
is the upstream of that adjacency has its BP populated with BFR that is the upstream of that adjacency has its BP
the adjacency in its BIFT. populated with the adjacency in its BIFT.
2. In BIER, the implied reference options for the core part of 2.b In BIER, the implied reference options for the core part of
the BIER layer control plane are the BIER extensions for the BIER layer control plane are the BIER extensions for
distributed routing protocols. This includes ISIS/OSPF distributed routing protocols. These include IS-IS and OSPF
extensions for BIER, [RFC8401] and [RFC8444]. extensions for BIER, as specified in [RFC8401] and
[RFC8444], respectively.
3. The reference option for the core part of the BIER-TE control 2.c The reference option for the core part of the BIER-TE
plane is the BIER-TE controller. Nevertheless, both the BIER control plane is the BIER-TE controller. Nevertheless, both
and BIER-TE BIFTs forwarding plane state could equally be the BIER and BIER-TE BIFTs' forwarding plane state could
populated by any mechanism. equally be populated by any mechanism.
4. Assuming the reference options for the control plane, BIER-TE 2.d Assuming the reference options for the control plane, BIER-
replaces in-network autonomous path calculation by explicit TE replaces in-network autonomous path calculations with
paths calculated by the BIER-TE controller. explicit paths calculated by the BIER-TE controller.
3. The following elements/functions described in the BIER 3. The following elements/functions described in the BIER
architecture are not required by the BIER-TE architecture: architecture are not required by the BIER-TE architecture:
1. "Bit Index Routing Tables" (BIRTs) are not required on BFRs 3.a "Bit Index Routing Tables" (BIRTs) are not required on BFRs
for BIER-TE when using a BIER-TE controller because the for BIER-TE when using a BIER-TE controller, because the
controller can directly populate the BIFTs. In BIER, BIRTs controller can directly populate the BIFTs. In BIER, BIRTs
are populated by the distributed routing protocol support for are populated by the distributed routing protocol support
BIER, allowing BFRs to populate their BIFTs locally from for BIER, allowing BFRs to populate their BIFTs locally from
their BIRTs. Other BIER-TE control plane or management plane their BIRTs. Other BIER-TE control plane or management
options may introduce requirements for BIRTs for BIER-TE plane options may introduce requirements for BIRTs for BIER-
BFRs. TE BFRs.
2. The BIER-TE layer forwarding plane does not require BFRs to 3.b The BIER-TE layer forwarding plane does not require BFRs to
have a unique BP and therefore also no unique BFR-id. See have a unique BP; see Section 5.1.3. Therefore, BFRs may
Section 5.1.3. not have a unique BFR-id; see Section 5.3.3.
3. Identification of BFRs by the BIER-TE control plane is 3.c Identification of BFRs by the BIER-TE control plane is
outside the scope of this specification. Whereas the BIER outside the scope of this specification. Whereas the BIER
control plane uses BFR-ids in its BFR to BFR signaling, a control plane uses BFR-ids in its BFR-to-BFR signaling, a
BIER-TE controller may choose any form of identification BIER-TE controller may choose any form of identification
deemed appropriate. deemed appropriate.
4. BIER-TE forwarding does not require the BFIR-id field of the 3.d BIER-TE forwarding does not require the BFIR-id field of the
BIER packet header. BIER packet header.
4. Co-existence of BIER and BIER-TE in the same network requires the 4. Co-existence of BIER and BIER-TE in the same network requires the
following: following:
1. The BIER/BIER-TE packet header needs to allow addressing both 4.a The BIER/BIER-TE packet header needs to allow the addressing
BIER and BIER-TE BIFTs. Depending on the encapsulation of both BIER and BIER-TE BIFTs. Depending on the
option, the same SD may or may not be reusable across BIER encapsulation option, the same SD may or may not be reusable
and BIER-TE. See Section 4.3. In either case, a packet is across BIER and BIER-TE. See Section 4.3. In either case,
always only forwarded end-to-end via BIER or via BIER-TE a packet is always forwarded only end to end via BIER or via
(ships in the nights forwarding). BIER-TE ("ships in the night" forwarding).
2. BIER-TE deployments will have to assign BFR-ids to BFRs and 4.b BIER-TE deployments will have to assign BFR-ids to BFRs and
insert them into the BFIR-id field of BIER packet headers as insert them into the BFIR-id field of BIER packet headers,
BIER does, whenever the deployment uses (unchanged) as does BIER, whenever the deployment uses (unchanged)
components developed for BIER that use BFR-id, such as components developed for BIER that use BFR-ids, such as
multicast flow overlays or BIER layer control plane elements. multicast flow overlays or BIER layer control plane
See also Section 5.3.3. elements. See also Section 5.3.3.
2.4. Accelerated/Hardware forwarding comparison 2.5. Accelerated Hardware Forwarding Comparison
BIER-TE forwarding rules, especially the BitString parsing are BIER-TE forwarding rules, especially BitString parsing, are designed
designed to be as close as possible to those of BIER in the to be as close as possible to those of BIER, with the expectation
expectation that this eases the programming of BIER-TE forwarding that this eases the programming of BIER-TE forwarding code and/or
code and/or BIER-TE forwarding hardware on platforms supporting BIER. BIER-TE forwarding hardware on platforms supporting BIER. The
The pseudocode in Section 4.4 shows how existing (non-TE) BIER/BIFT pseudocode in Section 4.4 shows how existing (non-TE) BIER/BIFT
forwarding can be modified to support the required BIER-TE forwarding forwarding can be modified to support the required BIER-TE forwarding
functionality (Section 4.5), by using BIER BIFT's "Forwarding Bit functionality (Section 4.5), by using the BIER BIFT's "Forwarding Bit
Mask" (F-BM): Only the clearing of bits to avoid duplicate packets to Mask" (F-BM): only the clearing of bits to avoid sending duplicate
a BFR's neighbor is skipped in BIER-TE forwarding because it is not packets to a BFR's neighbor is skipped in BIER-TE forwarding, because
necessary and could not be done when using BIER F-BM. it is not necessary and could not be done when using a BIER F-BM.
Whether to use BIER or BIER-TE forwarding is simply a choice of the Whether to use BIER or BIER-TE forwarding is simply a choice of the
mode of the BIFT indicated by the packet (BIER or BIER-TE BIFT). mode of the BIFT indicated by the packet (BIER or BIER-TE BIFT).
This is determined by the BFR configuration for the encapsulation, This is determined by the BFR configuration for the encapsulation;
see Section 4.3. see Section 4.3.
3. Components 3. Components
BIER-TE can be thought of being constituted from the same three BIER-TE can be thought of as being composed of the same three layers
layers as BIER: The "multicast flow overlay", the "BIER layer" and as BIER: the "multicast flow overlay", the "BIER layer", and the
the "routing underlay". The following picture also shows how the "routing underlay". Figure 3 also shows how the BIER layer is
"BIER layer" is constituted from the "BIER-TE forwarding plane" and composed of the "BIER-TE forwarding plane" and the "BIER-TE control
the "BIER-TE control plane" represent by the "BIER-TE Controller". plane" as represented by the "BIER-TE controller".
<------BGP/PIM-----> <------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->| |<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay overlay
BIER-TE [BIER-TE Controller] <=> [BIER-TE Topology] BIER-TE [BIER-TE Controller] <=> [BIER-TE Topology]
control ^ ^ ^ control ^ ^ ^
plane / | \ BIER-TE control protocol plane / | \ BIER-TE control protocol
| | | e.g. YANG/NETCONF/RESTCONF | | | (e.g., YANG/NETCONF/RESTCONF
| | | PCEP/... | | | PCEP/...)
v v v v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr
|<----------------->| |<----------------->|
BIER-TE forwarding plane BIER-TE forwarding plane
|<- BIER-TE domain->| |<- BIER-TE domain->|
|<--------------------->| |<--------------------->|
Routing underlay Routing underlay
Figure 3: BIER-TE architecture Figure 3: BIER-TE Architecture
3.1. The Multicast Flow Overlay 3.1. The Multicast Flow Overlay
The Multicast Flow Overlay has the same role as described for BIER in The multicast flow overlay has the same role as that described for
[RFC8279], Section 4.3. See also Section 3.2.1.2. BIER in [RFC8279], Section 4.3. See also Section 3.2.1.2.
When a BIER-TE controller is used, then the signaling for the When a BIER-TE controller is used, it might also be preferable that
Multicast Flow Overlay may also be preferred to operate through a multicast flow overlay signaling be performed through a central point
central point of control. For BGP based overlay flow services such of control. For BGP-based overlay flow services such as "Multicast
as "Multicast VPN Using BIER" ([RFC8556]) this can be achieved by VPN Using Bit Index Explicit Replication (BIER)" [RFC8556], this can
making the BIER-TE controller operate as a BGP Route Reflector be achieved by making the BIER-TE controller operate as a BGP Route
([RFC4456]) and combining it with signaling through BGP or a Reflector [RFC4456] and combining it with signaling through BGP or a
different protocol for the BIER-TE controller calculated BitStrings. different protocol for the BIER-TE controller's calculated
See Section 3.2.1.2 and Section 5.3.4. BitStrings. See Sections 3.2.1.2 and 5.3.4.
3.2. The BIER-TE Control Plane 3.2. The BIER-TE Control Plane
In the (non-TE) BIER architecture [RFC8279], the BIER control plane In the (non-TE) BIER architecture [RFC8279], the BIER layer is
is not explicitly separated from the BIER forwarding plane, but summarized in Section 4.2 of [RFC8279]. This summary includes both
instead their functions are summarized together in Section 4.2. the functions of the BIER-layer control plane and forwarding plane,
Example standardized options for the BIER control plane include ISIS/ without using those terms. Example standardized options for the BIER
OSPF extensions for BIER, [RFC8401] and [RFC8444]. control plane include IS-IS and OSPF extensions for BIER, as
specified in [RFC8401] and [RFC8444], respectively.
For BIER-TE, the control plane includes at minimum the following For BIER-TE, the control plane includes, at a minimum, the following
functionality. functionality.
1. BIER-TE topology control: During initial provisioning of the 1. BIER-TE topology control: During initial provisioning of the
network and/or during modifications of its topology and/or network and/or during modifications of its topology and/or
services, the protocols and/or procedures to establish BIER-TE services, the protocols and/or procedures to establish BIER-TE
BIFTs: BIFTs:
1. Determine the desired BIER-TE topology for a BIER-TE sub- 1.a Determine the desired BIER-TE topology for BIER-TE
domains: the native and/or overlay adjacencies that are subdomains: the adjacencies that are assigned to BPs.
assigned to BPs. Topology discovery is discussed in Topology discovery is discussed in Section 3.2.1.1, and the
Section 3.2.1.1 and the various aspects of the BIER-TE various aspects of the BIER-TE controller's determinations
controllers determinations about the topology are discussed regarding the topology are discussed throughout Section 5.
throughout Section 5
2. Determine the per-BFR BIFT from the BIER-TE topology. This is 1.b Determine the per-BFR BIFT from the BIER-TE topology. This
achieved by simply extracting the adjacencies of the BFR from is achieved by simply extracting the adjacencies of the BFR
the BIER-TE topology and populating the BFRs BIFT with them. from the BIER-TE topology and populating the BFR's BIFT with
them.
3. Optionally assign BFR-ids to BFIRs for later insertion into 1.c Optionally assign BFR-ids to BFIRs for later insertion into
BIER headers on BFIRs as BFIR-id. Alternatively, BFIR-id in BIER headers on BFIRs as BFIR-ids. Alternatively, BFIR-ids
BIER packet headers may be managed solely by the flow overlay in BIER packet headers may be managed solely by the flow
layer and/or be unused. This is discussed in Section 5.3.3. overlay layer and/or be unused. This is discussed in
Section 5.3.3.
4. Install/update the BIFTs into the BFRs and optionally BFR-ids 1.d Install/update the BIFTs into the BFRs and, optionally, BFR-
into BFIRs. This is discussed in Section 3.2.1.1. ids into BFIRs. This is discussed in Section 3.2.1.1.
2. BIER-TE tree control: During operations of the network, 2. BIER-TE tree control: During network operations, protocols and/or
protocols and/or procedures to support creation/change/removal of procedures to support creation/change/removal of overlay flows on
overlay flows on BFIRs: BFIRs:
1. Process the BIER-TE requirements for the multicast overlay 2.a Process the BIER-TE requirements for the multicast overlay
flow: BFIR and BFERs of the flow as well as policies for the flow: BFIRs and BFERs of the flow as well as policies for
path selection of the flow. This is discussed in Section 3.5. the path selection of the flow. This is discussed in
Section 3.5.
2. Determine the BitStrings and optionally Entropy. This is 2.b Determine the BitStrings and, optionally, entropy.
discussed in Section 3.2.1.2, Section 3.5 and Section 5.3.4. BitStrings are discussed in Sections 3.2.1.2, 3.5, and
5.3.4. Entropy is discussed in Section 4.2.3.
3. Install state on the BFIR to impose the desired BIER packet 2.c Install state on the BFIR to impose the desired BIER packet
header(s) for packets of the overlay flow. Different aspects header(s) for packets of the overlay flow. Different
of this and the next point are discussed throughout aspects of this point, as well as the next point, are
Section 3.2.1 and in Section 4.3, but the main responsibility discussed throughout Section 3.2.1 and in Section 4.3. The
of these two points is with the Multicast Flow Overlay main component responsible for these two points is the
(Section 3.1), which is architecturally inherited from BIER. multicast flow overlay (Section 3.1), which is
architecturally inherited from BIER.
4. Install the necessary state on the BFERs to decapsulate the 2.d Install the necessary state on the BFERs to decapsulate the
BIER packet header and properly dispatch its payload. BIER packet header and properly dispatch its payload.
3.2.1. The BIER-TE Controller 3.2.1. The BIER-TE Controller
[RFC-Editor: the following text has three references to anchors This architecture describes the BIER-TE control plane, as shown in
topology-control, topology-control-1 and tree-control. Figure 3, as consisting of:
Unfortunately, XMLv2 does not offer any tagging that reasonable
references are generated (i had this problem already in RFCs last
year. Please make sure there are useful-to-read cross-references in
the RFC in these three places after you convert to XMLv3.]
This architecture describes the BIER-TE control plane as shown in
Figure 3 to consist of:
* A BIER-TE controller. * A BIER-TE controller.
* BFR data-models and protocols to communicate between controller * BFR data models and protocols to communicate between the
and BFRs in support of BIER-TE topology control (Section 3.2), controller and BFRs in support of BIER-TE topology control (see
such as YANG/NETCONF/RESTCONF ([RFC7950]/[RFC6241]/[RFC8040]). the list under "BIER-TE topology control"), such as YANG/NETCONF/
RESTCONF [RFC7950] [RFC6241] [RFC8040].
* BFR data-models and protocols to communicate between controller * BFR data models and protocols to communicate between the
and BFIR in support of BIER-TE tree control (Section 3.2), such as controller and BFIRs in support of BIER-TE tree control (see
BIER-TE extensions for [RFC5440]. Section 3.2, point 2.), such as BIER-TE extensions for [RFC5440].
The single, centralized BIER-TE controller is used in this document The single, centralized BIER-TE controller is used in this document
as reference option for the BIER-TE control plane but other options as the reference option for the BIER-TE control plane, but other
are equally feasible. The BIER-TE control plane could equally be options are equally feasible. The BIER-TE control plane could
implemented without automated configuration/protocols, by an operator equally be implemented without automated configuration/protocols, by
via CLI on the BFRs. In that case, operator configured local policy an operator via a CLI on the BFRs. In that case, operator-configured
on the BFIR would have to determine how to set the appropriate BIER local policy on the BFIR would have to determine how to set the
header fields. The BIER-TE control plane could also be decentralized appropriate BIER header fields. The BIER-TE control plane could also
and/or distributed, but this document does not consider any be decentralized and/or distributed, but this document does not
additional protocols and/or procedures which would then be necessary consider any additional protocols and/or procedures that would then
to coordinate its (distributed/decentralized) entities to achieve the be necessary to coordinate its (distributed/decentralized) entities
above described functionality. to achieve the above-described functionality.
3.2.1.1. BIER-TE Topology discovery and creation 3.2.1.1. BIER-TE Topology Discovery and Creation
The first item of BIER-TE topology control (Section 3.2, Paragraph 3, The first item listed for BIER-TE topology control (Section 3.2,
Item 2.2.1) includes network topology discovery and BIER-TE topology point 1.a.) includes network topology discovery and BIER-TE topology
creation. The latter describes the process by which a Controller creation. The latter describes the process by which a controller
determines which routers are to be configured as BFRs and the determines which routers are to be configured as BFRs and the
adjacencies between them. adjacencies between them.
In statically managed networks, such as in industrial environments, In statically managed networks, e.g., industrial environments, both
both discovery and creation can be a manual/offline process. discovery and creation can be a manual/offline process.
In other networks, topology discovery may rely on protocols including In other networks, topology discovery may rely on such protocols as
extending a "Link-State-Protocol" based IGP into the BIER-TE those that include extending an IGP based on a link-state protocol
controller itself, [RFC7752] (BGP-LS) or [RFC8345] (YANG topology) as into the BIER-TE controller itself, e.g., BGP-LS [RFC7752] or YANG
well as BIER-TE specific methods, for example via topology [RFC8345], as well as methods specific to BIER-TE -- for
[I-D.ietf-bier-te-yang]. These options are non-exhaustive. example, via [BIER-TE-YANG]. These options are non-exhaustive.
Dynamic creation of the BIER-TE topology can be as easy as mapping Dynamic creation of the BIER-TE topology can be as easy as mapping
the network topology 1:1 to the BIER-TE topology by assigning a BP the network topology 1:1 to the BIER-TE topology by assigning a BP
for every network subnet adjacency. In larger networks, it likely for every network subnet adjacency. In larger networks, it likely
involves more complex policy and optimization decisions including how involves more complex policy and optimization decisions, including
to minimize the number of BPs required and how to assign BPs across how to minimize the number of BPs required and how to assign BPs
different BitStrings to minimize the number of duplicate packets across different BitStrings to minimize the number of duplicate
across links when delivering an overlay flow to BFER using different packets across links when delivering an overlay flow to BFERs using
SIs/BitStrings. These topics are discussed in Section 5. different SIs:BitStrings. These topics are discussed in Section 5.
When the BIER-TE topology is determined, the BIER-TE Controller then When the BIER-TE topology has been determined, the BIER-TE controller
pushes the BitPositions/adjacencies to the BIFT of the BFRs. On each pushes the BPs/adjacencies to the BIFT of the BFRs. On each BFR,
BFR only those SI:BitPositions are populated that are adjacencies to only those SIs:BPs that are adjacencies to other BFRs in the BIER-TE
other BFRs in the BIER-TE topology. topology are populated.
Communications between the BIER-TE Controller and BFRs for both BIER- Communications between the BIER-TE controller and BFRs for both BIER-
TE topology control and BIER-TE tree control is ideally via TE topology control and BIER-TE tree control are ideally via
standardized protocols and data-models such as NETCONF/RESTCONF/YANG/ standardized protocols and data models such as NETCONF/RESTCONF/YANG/
PCEP. Vendor-specific CLI on the BFRs is also an option (as in many PCEP. A vendor-specific CLI on the BFRs is also an option (as in
other SDN solutions lacking definition of standardized data models). many other "Software-Defined Network" (SDN) solutions lacking
definitions of standardized data models).
3.2.1.2. Engineered Trees via BitStrings 3.2.1.2. Engineered Trees via BitStrings
In BIER, the same set of BFER in a single sub-domain is always In BIER, the same set of BFERs in a single subdomain is always
encoded as the same BitString. In BIER-TE, the BitString used to encoded as the same BitString. In BIER-TE, the BitString used to
reach the same set of BFER in the same sub-domain can be different reach the same set of BFERs in the same subdomain can be different
for different overlay flows because the BitString encodes the paths for different overlay flows because the BitString encodes the paths
towards the BFER, so the BitStrings from different BFIR to the same towards the BFERs, so the BitStrings from different BFIRs to the same
set of BFER will often be different. Likewise, the BitString from set of BFERs will often be different. Likewise, the BitString from
the same BFIR to the same set of BFER can be different for different the same BFIR to the same set of BFERs can be different for different
overlay flows for policy reasons such as shortest path trees, Steiner overlay flows if different policies should be applied to those
trees (minimum cost trees), diverse path trees for redundancy and so overlay flows, such as shortest path trees, Steiner trees (minimum
on. cost trees), diverse path trees for redundancy, and so on.
See also [I-D.ietf-bier-multicast-http-response] for an application See also [BIER-MCAST-OVERLAY] for an application leveraging BIER-TE
leveraging BIER-TE engineered trees. engineered trees.
3.2.1.3. Changes in the network topology 3.2.1.3. Changes in the Network Topology
If the network topology changes (not failure based) so that If the network topology changes (not failure based) so that
adjacencies that are assigned to bit positions are no longer needed, adjacencies that are assigned to bit positions are no longer needed,
the BIER-TE Controller can re-use those bit positions for new the BIER-TE controller can reuse those bit positions for new
adjacencies. First, these bit positions need to be removed from any adjacencies. First, these bit positions need to be removed from any
BFIR flow state and BFR BIFT state, then they can be repopulated, BFIR flow state and BFR BIFT state. Then, they can be repopulated,
first into BIFT and then into the BFIR. first into the BIFT and then into the BFIR.
3.2.1.4. Link/Node Failures and Recovery 3.2.1.4. Link/Node Failures and Recovery
When link or nodes fail or recover in the topology, BIER-TE could When links or nodes fail or recover in the topology, BIER-TE could
quickly respond with FRR procedures such as [I-D.eckert-bier-te-frr], quickly respond with "Fast Reroute" (FRR) procedures such as those
the details of which are out of scope for this document. It can also described in [BIER-TE-PROTECTION], the details of which are out of
more slowly react by recalculating the BitStrings of affected scope for this document. It can also more slowly react by
multicast flows. This reaction is slower than the FRR procedure recalculating the BitStrings of affected multicast flows. This
because the BIER-TE Controller needs to receive link/node up/down reaction is slower than the FRR procedure because the BIER-TE
indications, recalculate the desired BitStrings and push them down controller needs to receive link/node up/down indications,
into the BFIRs. With FRR, this is all performed locally on a BFR recalculate the desired BitStrings, and push them down into the
receiving the adjacency up/down notification. BFIRs. With FRR, this is all performed locally on a BFR receiving
the adjacency up/down notification.
3.3. The BIER-TE Forwarding Plane 3.3. The BIER-TE Forwarding Plane
[RFC-editor Q: "is constituted from" / "consists of" / "composed The BIER-TE forwarding plane consists of the following components:
from..." ???]
The BIER-TE Forwarding Plane is constituted from the following
components:
1. On a BFIR, imposition of the BIER header for packets from overlay 1. On a BFIR, imposition of the BIER header for packets from overlay
flows. This is driven by a combination of state established by flows. This is driven by state established by the BIER-TE
the BIER-TE control plane and/or the multicast flow overlay as control plane, the multicast flow overlay as explained in
explained in Section 3.1. Section 3.1, or a combination of both.
2. On BFRs (including BFIR and BFER), forwarding/replication of BIER 2. On BFRs (including BFIRs and BFERs), forwarding/replication of
packets according to their SD, SI, "BitStringLength" (BSL), BIER packets according to their SD, SI, "BitStringLength" (BSL),
BitString and optionally Entropy fields as explained in BitString, and, optionally, entropy fields as explained in
Section 4. Processing of other BIER header fields such as DSCP Section 4. Processing of other BIER header fields, such as the
is outside the scope of this document. "Differentiated Services Code Point" (DSCP) field, is outside the
scope of this document.
3. On BFERs, removal of the BIER header and dispatching of the 3. On BFERs, removal of the BIER header and dispatching of the
payload according to state created by the BIER-TE control plane payload according to state created by the BIER-TE control plane
and/or overlay layer. and/or overlay layer.
When the BIER-TE Forwarding Plane receives a packet, it simply looks When the BIER-TE forwarding plane receives a packet, it simply looks
up the bit positions that are set in the BitString of the packet in up the bit positions that are set in the BitString of the packet in
the BIFT that was populated by the BIER-TE Controller. For every BP the BIFT that was populated by the BIER-TE controller. For every BP
that is set in the BitString, and that has one or more adjacencies in that is set in the BitString and has one or more adjacencies in the
the BIFT, a copy is made according to the type of adjacencies for BIFT, a copy is made according to the types of adjacencies for that
that BP in the BIFT. Before sending any copy, the BFR clears all BPs BP in the BIFT. Before sending any copies, the BFR clears all BPs in
in the BitString of the packet for which the BFR has one or more the BitString of the packet for which the BFR has one or more
adjacencies in the BIFT. Clearing these bits inhibits packets from adjacencies in the BIFT. Clearing these bits prevents packets from
looping when the BitStrings erroneously includes a forwarding loop. looping when a BitString erroneously includes a forwarding loop.
When a forward_connected() adjacency has the "DoNotClear" (DNC) flag When a forward_connected() adjacency has the "DoNotClear" (DNC) flag
set, then this BP is re-set for the packet copied to that adjacency. set, this BP is reset for the packet copied to that adjacency. See
See Section 4.2.1. Section 4.2.1.
3.4. The Routing Underlay 3.4. The Routing Underlay
For forward_connected() adjacencies, BIER-TE is sending BIER packets For forward_connected() adjacencies, BIER-TE sends BIER packets to
to directly connected BIER-TE neighbors as L2 (unicasted) BIER directly connected BIER-TE neighbors as L2 (unicast) BIER packets
packets without requiring a routing underlay. For forward_routed() without requiring a routing underlay. For forward_routed()
adjacencies, BIER-TE forwarding encapsulates a copy of the BIER adjacencies, BIER-TE forwarding encapsulates a copy of the BIER
packet so that it can be delivered by the forwarding plane of the packet so that it can be delivered by the forwarding plane of the
routing underlay to the routable destination address indicated in the routing underlay to the routable destination address indicated in the
adjacency. See Section 4.2.2 for the adjacency definition. adjacency. See Section 4.2.2 for details on forward_routed()
adjacencies.
BIER relies on the routing underlay to calculate paths towards BFERs BIER relies on the routing underlay to calculate paths towards BFERs
and derive next-hop BFR adjacencies for those paths. This commonly and derive next-hop BFR adjacencies for those paths. These two steps
relies on BIER specific extensions to the routing protocols of the commonly rely on BIER-specific extensions to the routing protocols of
routing underlay but may also be established by a controller. In the routing underlay but may also be established by a controller. In
BIER-TE, the next-hops of a packet are determined by the BitString BIER-TE, the next hops for a packet are determined by the BitString
through the BIER-TE Controller established adjacencies on the BFR for through the BIER-TE controller-established adjacencies on the BFR for
the BPs of the BitString. There is thus no need for BFR specific the BPs of the BitString. There is thus no need for BFR-specific
routing underlay extensions to forward BIER packets with BIER-TE routing underlay extensions to forward BIER packets with BIER-TE
semantics. semantics.
Encapsulation parameters can be provisioned by the BIER-TE controller Encapsulation parameters can be provisioned by the BIER-TE controller
into the forward_connected() or forward_routed() adjacencies directly into the forward_connected() or forward_routed() adjacencies directly
without relying on a routing underlay. without relying on a routing underlay.
If the BFR intends to support FRR for BIER-TE, then the BIER-TE If the BFR intends to support FRR for BIER-TE, then the BIER-TE
forwarding plane needs to receive fast adjacency up/down forwarding plane needs to receive fast adjacency up/down
notifications: Link up/down or neighbor up/down, e.g. from BFD. notifications: link up/down or neighbor up/down, e.g., from
Providing these notifications is considered to be part of the routing "Bidirectional Forwarding Detection" (BFD). Providing these
underlay in this document. notifications is considered to be part of the routing underlay in
this document.
3.5. Traffic Engineering Considerations 3.5. Traffic Engineering Considerations
Traffic Engineering ([I-D.ietf-teas-rfc3272bis]) provides performance Traffic Engineering [TE-OVERVIEW] provides performance optimization
optimization of operational IP networks while utilizing network of operational IP networks while utilizing network resources
resources economically and reliably. The key elements needed to economically and reliably. The key elements needed to effect Traffic
effect TE are policy, path steering and resource management. These Engineering are policy, path steering, and resource management.
elements require support at the control/controller level and within These elements require support at the control/controller level and
the forwarding plane. within the forwarding plane.
Policy decisions are made within the BIER-TE control plane, i.e., Policy decisions are made within the BIER-TE control plane, i.e.,
within BIER-TE Controllers. Controllers use policy when composing within BIER-TE controllers. Controllers use policy when composing
BitStrings and BFR BIFT state. The mapping of user/IP traffic to BitStrings and BFR BIFT state. The mapping of user/IP traffic to
specific BitStrings/BIER-TE flows is made based on policy. The specific BitStrings / BIER-TE flows is made based on policy. The
specific details of BIER-TE policies and how a controller uses them specific details of BIER-TE policies and how a controller uses them
are out of scope of this document. are out of scope for this document.
Path steering is supported via the definition of a BitString. Path steering is supported via the definition of a BitString.
BitStrings used in BIER-TE are composed based on policy and resource BitStrings used in BIER-TE are composed based on policy and resource
management considerations. For example, when composing BIER-TE management considerations. For example, when composing BIER-TE
BitStrings, a Controller must take into account the resources BitStrings, a controller must take into account the resources
available at each BFR and for each BP when it is providing available at each BFR and for each BP when it is providing
congestion-loss-free services such as Rate Controlled Service congestion-loss-free services such as Rate-Controlled Service
Disciplines [RCSD94]. Resource availability could be provided for Disciplines [RCSD94]. Resource availability could be provided, for
example via routing protocol information, but may also be obtained example, via routing protocol information but may also be obtained
via a BIER-TE control protocol such as NETCONF or any other protocol via a BIER-TE control protocol such as NETCONF or any other protocol
commonly used by a Controller to understand the resources of the commonly used by a controller to understand the resources of the
network it operates on. The resource usage of the BIER-TE traffic network on which it operates. The resource usage of the BIER-TE
admitted by the BIER-TE controller can be solely tracked on the BIER- traffic admitted by the BIER-TE controller can be solely tracked on
TE Controller based on local accounting as long as no the BIER-TE controller based on local accounting as long as no
forward_routed() adjacencies are used (see Section 4.2.1 for the forward_routed() adjacencies are used (see Section 4.2.2 for the
definition of forward_routed() adjacencies). When forward_routed() definition of forward_routed() adjacencies). When forward_routed()
adjacencies are used, the paths selected by the underlying routing adjacencies are used, the paths selected by the underlying routing
protocol need to be tracked as well. protocol need to be tracked as well.
Resource management has implications on the forwarding plane beyond Resource management has implications for the forwarding plane beyond
the BIER-TE defined steering of packets. This includes allocation of the BIER-TE-defined steering of packets; this includes allocation of
buffers to guarantee the worst case requirements of admitted RCSD buffers to guarantee the worst-case requirements for admitted RCSD
traffic and potentially policing and/or rate-shaping mechanisms, traffic and potentially policing and/or rate-shaping mechanisms,
typically done via various forms of queuing. This level of resource typically done via various forms of queuing. This level of resource
control, while optional, is important in networks that wish to control, while optional, is important in networks that wish to
support congestion management policies to control or regulate the support congestion management policies to control or regulate the
offered traffic to deliver different levels of service and alleviate offered traffic to deliver different levels of service and alleviate
congestion problems, or those networks that wish to control latencies congestion problems, or those networks that wish to control latencies
experienced by specific traffic flows. experienced by specific traffic flows.
4. BIER-TE Forwarding 4. BIER-TE Forwarding
4.1. The BIER-TE Bit Index Forwarding Table (BIFT) 4.1. The BIER-TE Bit Index Forwarding Table (BIFT)
The BIER-TE BIFT is the equivalent to the BIER BIFT for (non-TE) The BIER-TE BIFT is equivalent to the (non-TE) BIER BIFT. It exists
BIER. It exists on every BFR running BIER-TE. For every BIER sub- on every BFR running BIER-TE. For every BIER "subdomain" (SD) in use
domain (SD) in use for BIER-TE, it is a table as shown shown in for BIER-TE, the BIFT is constructed per the example shown in
Figure 4. That example BIFT assumes a BSL of 8 bit positions (BPs) Figure 4. The BIFT in the figure assumes a BSL of 8 "bit positions"
in the packets BitString. As in [RFC8279] this BSL is purely used (BPs) in the packets BitString. As in [RFC8279], this BSL is purely
for the example and not a BIER/BIER-TE supported BSL (minimum BSL is used as an example and is not a BSL supported by BIER/BIER-TE
64). (minimum BSL is 64).
A BIER-TE BIFT compares to a BIER BIFT as shown in [RFC8279] as A BIER-TE BIFT is compared to a BIER BIFT as shown in [RFC8279] as
follows. follows.
In both BIER and BIER-TE, BIFT rows/entries are indexed in their In both BIER and BIER-TE, BIFT rows/entries are indexed in their
respective BIER pseudocode ([RFC8279] Section 6.5) and BIER-TE respective BIER pseudocode ([RFC8279], Section 6.5) and BIER-TE
pseudocode (Section 4.4) by the BIFT-index derived from the packets pseudocode (Section 4.4) by the BIFT-index derived from the packet's
SI, BSL and the one bit position of the packets BitString (BP) SI, BSL, and the one bit position of the packets BitString (BP)
addressing the BIFT row: BIFT-index = SI * BSL + BP - 1. BP within a addressing the BIFT row: BIFT-index = SI * BSL + BP - 1. BPs within
BitString are numbered from 1 to BSL, hence the - 1 offset when a BitString are numbered from 1 to BSL -- hence, the - 1 offset when
converting to a BIFT-index. This document also uses the notion SI:BP converting to a BIFT-index. This document also uses the notion
to indicate BIFT rows, [RFC8279] uses the equivalent notion "SI:BP" to indicate BIFT rows. [RFC8279] uses the equivalent notion
SI:BitString, where the BitString is filled with only the BP for the "SI:BitString", where the BitString is filled with only the BPs for
BIFT row. the BIFT row.
In BIER, each BIFT-index addresses one BFER by its BFR-id = BIFT- In BIER, each BIFT-index addresses one BFER by its BFR-id = BIFT-
index + 1 and is populated on each BFR with the next-hop "BFR index + 1 and is populated on each BFR with the next-hop "BFR
Neighbor" (BFR-NBR) towards that BFER. Neighbor" (BFR-NBR) towards that BFER.
In BIER-TE, each BIFT-index and therefore SI:BP indicates one or more In BIER-TE, each BIFT-index and, therefore, SI:BP indicates one or,
adjacencies between BFRs in the topology and is only populated with in the case of reuse of SI:BP, more than one adjacency between BFRs
those adjacencies forwarding entries on the BFR that is the upstream in the topology. The SI:BP is populated with the adjacency on the
for these adjacencies. The BIFT entry are empty on all other BFRs. upstream BFR of the adjacency. The BIFT entries are empty on all
other BFRs.
In BIER, each BIFT row also requires a "Forwarding Bit Mask" (F-BM) In BIER, each BIFT row also requires a "Forwarding Bit Mask" (F-BM)
entry for BIER forwarding rules. In BIER-TE forwarding, F-BM is not entry for BIER forwarding rules. In BIER-TE forwarding, an F-BM is
required, but can be used when implementing BIER-TE on forwarding not required but can be used when implementing BIER-TE on forwarding
hardware derived from BIER forwarding, that must use F-BM. This is hardware, derived from BIER forwarding, that must use an F-BM. This
discussed in the first BIER-TE forwarding pseudocode in Section 4.4. is discussed in the first variation of BIER-TE forwarding pseudocode
shown in Section 4.4.
------------------------------------------------------------------ -------------------------------------------------------------------
| BIFT-index | | Adjacencies: | | BIFT-index | | Adjacencies: |
| (SI:BP) |(FBM)| <empty> or one or more per entry | | (SI:BP) |(F-BM)| <empty> or one or more per entry |
================================================================== ===================================================================
| BIFT indices for Packets with SI=0 | | BIFT indices for Packets with SI=0 |
------------------------------------------------------------------ -------------------------------------------------------------------
| 0 (0:1) | ... | forward_connected(interface,neighbor{,DNC}) | | 0 (0:1) | ... | forward_connected(interface,neighbor{,DNC}) |
------------------------------------------------------------------ -------------------------------------------------------------------
| 1 (0:2) | ... | forward_connected(interface,neighbor{,DNC}) | | 1 (0:2) | ... | forward_connected(interface,neighbor{,DNC}) |
| | ... | forward_connected(interface,neighbor{,DNC}) | | | ... | forward_connected(interface,neighbor{,DNC}) |
------------------------------------------------------------------ -------------------------------------------------------------------
| ... | ... | ... | | ... | ... | ... |
------------------------------------------------------------------ -------------------------------------------------------------------
| 4 (0:5) | ... | local_decap({VRF}) | | 4 (0:5) | ... | local_decap({VRF}) |
------------------------------------------------------------------ -------------------------------------------------------------------
| 5 (0:6) | ... | forward_routed({VRF,}l3-neighbor) | | 5 (0:6) | ... | forward_routed({VRF,}l3-neighbor) |
------------------------------------------------------------------ -------------------------------------------------------------------
| 6 (0:7) | ... | <empty> | | 6 (0:7) | ... | <empty> |
------------------------------------------------------------------ -------------------------------------------------------------------
| 7 (0:8) | ... | ECMP((adjacency1,...adjacencyN){,seed}) | | 7 (0:8) | ... | ECMP((adjacency1,...adjacencyN){,seed}) |
----------------------------------------------------------------- -------------------------------------------------------------------
| BIFT indices for BitString/Packet with SI=1 | | BIFT indices for BitString/Packet with SI=1 |
------------------------------------------------------------------ -------------------------------------------------------------------
| 9 (1:1) | | ... | | 9 (1:1) | | ... |
| ... |... | ... | | ... | ... | ... |
------------------------------------------------------------------ -------------------------------------------------------------------
BIER-TE Bit Index Forwarding Table (BIFT)
Figure 4: BIER-TE BIFT with different adjacencies Figure 4: BIER-TE Bit Index Forwarding Table (BIFT) with
Different Adjacencies
The BIFT is configured for the BIER-TE data plane of a BFR by the The BIFT is configured for the BIER-TE data plane of a BFR by the
BIER-TE Controller through an appropriate protocol and data-model. BIER-TE controller through an appropriate protocol and data model.
The BIFT is then used to forward packets, according to the rules The BIFT is then used to forward packets, according to the procedures
specified in the BIER-TE Forwarding Procedures. for the BIER-TE forwarding plane as specified in Section 3.3.
Note that a BIFT index (SI:BP) may be populated in the BIFT of more Note that a BIFT-index (SI:BP) may be populated in the BIFT of more
than one BFR to save BPs. See Section 5.1.6 for an example of how a than one BFR to save BPs. See Section 5.1.6 for an example of how a
BIER-TE controller could assign BPs to (logical) adjacencies shared BIER-TE controller could assign BPs to (logical) adjacencies shared
across multiple BFRs, Section 5.1.3 for an example of assigning the across multiple BFRs, Section 5.1.3 for an example of assigning the
same BP to different adjacencies, and Section 5.1.9 for general same BP to different adjacencies, and Section 5.1.9 for general
guidelines regarding re-use of BPs across different adjacencies. guidelines regarding the reuse of BPs across different adjacencies.
{VRF} indicates the Virtual Routing and Forwarding context into which {VRF} indicates the Virtual Routing and Forwarding context into which
the BIER payload is to be delivered. This is optional and depends on the BIER payload is to be delivered. This is optional and depends on
the multicast flow overlay. the multicast flow overlay.
4.2. Adjacency Types 4.2. Adjacency Types
4.2.1. Forward Connected 4.2.1. Forward Connected
A "forward_connected()" adjacency is towards a directly connected BFR A "forward_connected()" adjacency is an adjacency towards a directly
neighbor using an interface address of that BFR on the connecting connected BFR-NBR using an interface address of that BFR on the
interface. A forward_connected() adjacency does not route packets connecting interface. A forward_connected() adjacency does not route
but only L2 forwards them to the neighbor. packets; only L2 forwards them to the neighbor.
Packets sent to an adjacency with "DoNotClear" (DNC) set in the BIFT Packets sent to an adjacency with "DoNotClear" (DNC) set in the BIFT
MUST NOT have the bit position for that adjacency cleared when the MUST NOT have the bit position for that adjacency cleared when the
BFR creates a copy for it. The bit position will still be cleared BFR creates a copy for it. The bit position will still be cleared
for copies of the packet made towards other adjacencies. This can be for copies of a packet made towards other adjacencies. This can be
used for example in ring topologies as explained in Section 5.1.6. used, for example, in ring topologies as explained in Section 5.1.6.
For protection against loops from misconfiguration (see For protection against loops caused by misconfiguration (see
Section 5.2.1), DNC is only permissible for forward_connected() Section 5.2.1), DNC is only permissible for forward_connected()
adjacencies. No need or benefit of DNC for other type of adjacencies adjacencies. No need or benefit of DNC for other types of
was identified and their risk was not analyzed. adjacencies was identified, and associated risks were not analyzed.
4.2.2. Forward Routed 4.2.2. Forward Routed
A "forward_routed()" adjacency is an adjacency towards a BFR that A "forward_routed()" adjacency is an adjacency towards a BFR that
uses a (tunneling) encapsulation which will cause the packet to be uses a (tunneling) encapsulation that will cause a packet to be
forwarded by the routing underlay toward the adjacent BFR. This can forwarded by the routing underlay towards the adjacent BFR indicated
leverage any feasible encapsulation, such as MPLS or tunneling over via the l3-neighbor parameter of the forward_routed() adjacency.
IP/IPv6, as long as the BIER-TE packet can be identified as a This can leverage any feasible encapsulation, such as MPLS or
payload. This identification can either rely on the BIER/BIER-TE co- tunneling over IP/IPv6, as long as the BIER-TE packet can be
existence mechanisms described in Section 4.3, or by explicit support identified as a payload. This identification can rely on either the
for a BIER-TE payload type in the tunneling encapsulation. BIER/BIER-TE co-existence mechanisms described in Section 4.3 or
explicit support for a BIER-TE payload type in the tunneling
encapsulation.
forward_routed() adjacencies are necessary to pass BIER-TE traffic Forward_routed() adjacencies are necessary to pass BIER-TE traffic
across non BIER-TE capable routers or to minimize the number of across routers that are not BIER-TE capable or to minimize the number
required BP by tunneling over (BIER-TE capable) routers on which of required BPs by tunneling over (BIER-TE-capable) routers on which
neither replication nor path-steering is desired, or simply to neither replication nor path steering is desired, or simply to
leverage path redundancy and FRR of the routing underlay towards the leverage the routing underlay's path redundancy and FRR towards the
next BFR. They may also be useful to a multi-subnet adjacent BFR to next BFR. They may also be useful to a multi-subnet adjacent BFR for
leverage the routing underlay ECMP independent of BIER-TE ECMP leveraging the routing underlay ECMP independently of BIER-TE ECMP
(Section 4.2.3). (Section 4.2.3).
4.2.3. ECMP 4.2.3. ECMP
(non-TE) BIER ECMP is tied to the BIER BIFT processing semantic and (Non-TE) BIER ECMP is tied to the BIER BIFT processing semantic and
is therefore not directly usable with BIER-TE. is therefore not directly usable with BIER-TE.
A BIER-TE "Equal Cost Multipath" (ECMP()) adjacency as shown in A BIER-TE "Equal-Cost Multipath" (ECMP()) adjacency as shown in
Figure 4 for BIFT-index 7 has a list of two or more non-ECMP Figure 4 for BIFT-index 7 has a list of two or more non-ECMP()
adjacencies as parameters and an optional seed parameter. When a adjacencies as parameters and an optional seed parameter. When a
BIER-TE packet is copied onto such an ECMP() adjacency, an BIER-TE packet is copied onto such an ECMP() adjacency, an
implementation specific so-called hash function will select one out implementation-specific so-called hash function will select one out
of the list's adjacencies to which the packet is forwarded. If the of the list's adjacencies to which the packet is forwarded. If the
packet's encapsulation contains an entropy field, the entropy field packet's encapsulation contains an entropy field, the entropy field
SHOULD be respected; two packets with the same value of the entropy SHOULD be respected; two packets with the same value of the entropy
field SHOULD be sent on the same adjacency. The seed parameter field SHOULD be sent on the same adjacency. The seed parameter
allows to design hash functions that are easy to implement at high permits the design of hash functions that are easy to implement at
speed without running into polarization issues across multiple high speed without running into polarization issues across multiple
consecutive ECMP hops. See Section 5.1.7 for more explanations. consecutive ECMP hops. See Section 5.1.7 for details.
4.2.4. Local Decap(sulation) 4.2.4. Local Decap(sulation)
A "local_decap()" adjacency passes a copy of the payload of the BIER- A "local_decap()" adjacency passes a copy of the payload of the BIER-
TE packet to the protocol ("NextProto") within the BFR (IPv4/IPv6, TE packet to the protocol ("NextProto") within the BFR (IP/IPv6,
Ethernet,...) responsible for that payload according to the packet Ethernet,...) responsible for that payload according to the packet
header fields. A local_decap() adjacency turns the BFR into a BFER header fields. A local_decap() adjacency turns the BFR into a BFER
for matching packets. Local_decap() adjacencies require the BFER to for matching packets. Local_decap() adjacencies require the BFER to
support routing or switching for NextProto to determine how to support routing or switching for NextProto to determine how to
further process the packet. further process the packets.
4.3. Encapsulation / Co-existence with BIER 4.3. Encapsulation / Co-existence with BIER
Specifications for BIER-TE encapsulation are outside the scope of Specifications for BIER-TE encapsulation are outside the scope of
this document. This section gives explanations and guidelines. this document. This section gives explanations and guidelines.
Like [RFC8279], handling of "Maximum Transmission Unit" (MTU) The handling of "Maximum Transmission Unit" (MTU) limitations is
limitations is outside the scope of this document and instead part of outside the scope of this document and is not discussed in [RFC8279]
the BIER-TE packet encapsulation and/or flow overlay. See for either. Instead, this process is part of the BIER-TE packet
example [RFC8296], Section 3. It applies equally to BIER-TE as it encapsulation and/or flow overlay; for example, see [RFC8296],
does to BIER. Section 3. It applies equally to BIER-TE and BIER.
Because a BFR needs to interpret the BitString of a BIER-TE packet Because a BFR needs to interpret the BitString of a BIER-TE packet
differently from a (non-TE) BIER packet, it is necessary to differently from a (non-TE) BIER packet, it is necessary to
distinguish BIER from BIER-TE packets. In the BIER encapsulation distinguish BIER packets from BIER-TE packets. In BIER encapsulation
[RFC8296], the BIFT-id field of the packet indicates the BIFT of the [RFC8296], the BIFT-id field of the packet indicates the BIFT of the
packet. BIER and BIER-TE can therefore be run simultaneously, when packet. BIER and BIER-TE can therefore be run simultaneously, when
the BIFT-id address space is shared across BIER BIFT and BIER-TE the BIFT-id address space is shared across BIER BIFTs and BIER-TE
BIFT. Partitioning the BIFT-id address space is subject to BIER-TE/ BIFTs. Partitioning the BIFT-id address space is subject to BIER-TE/
BIER control plane procedures. BIER control plane procedures.
When [RFC8296] is used for BIER with MPLS, BIFT-id address ranges can When [RFC8296] is used for BIER with MPLS, BIFT-id address ranges can
be dynamically allocated from MPLS label space only for the set of be dynamically allocated from MPLS label space only for the set of
actually used SD:BSL BIFT. This allows to also allocate non- actually used SD:BSL BIFTs. This also permits the allocation of non-
overlapping label ranges for BIFT-id that are to be used with BIER-TE overlapping label ranges for BIFT-ids that are to be used with BIER-
BIFTs. TE BIFTs.
With MPLS, it is also possible to reuse the same SD space for both With MPLS, it is also possible to reuse the same SD space for both
BIER-TE and BIER, so that the same SD has both a BIER BIFT with a BIER-TE and BIER, so that the same SD has both a BIER BIFT with a
corresponding range of BIFT-ids and disjoint BIER-TE BIFTs with a corresponding range of BIFT-ids and disjoint BIER-TE BIFTs with a
non-overlapping range of BIFT-ids. non-overlapping range of BIFT-ids.
When a fixed mapping from BSL, SD and SI to BIFT-id is used which Assume that a fixed mapping from BSL, SD, and SI to a BIFT-id is
does not explicitly partition the BIFT-id space between BIER and used, which does not explicitly partition the BIFT-id space between
BIER-TE, such as proposed for non-MPLS forwarding with [RFC8296] BIER and BIER-TE -- for example, as proposed for non-MPLS forwarding
encapsulation in [I-D.ietf-bier-non-mpls-bift-encoding] revision 04, with BIER encapsulation [RFC8296] in [NON-MPLS-BIER-ENCODING],
section 5, then it is necessary to allocate disjoint SDs to BIER and Section 5. In this case, it is necessary to allocate disjoint SDs to
BIER-TE BIFTs so that both can be addressed by the BIFT-ids. The BIER and BIER-TE BIFTs so that both can be addressed by the BIFT-ids.
encoding proposed in section 6. of the same document does not The encoding proposed in Section 6 of [NON-MPLS-BIER-ENCODING] does
statically encode BSL or SD into the BIFT-id, but allows for a not statically encode the BSL or SD into the BIFT-id, but the
mapping, and hence could provide for the same freedom as when MPLS is encoding permits a mapping and hence could provide the same freedom
being used (same or different SD for BIER/BIER-TE). as when MPLS is being used (the same SD, or different SDs for BIER/
BIER-TE).
forward_routed() requires an encapsulation that permits to direct Forward_routed() requires an encapsulation that permits directing
unicast encapsulated BIER-TE packets to a specific interface address unicast encapsulated BIER-TE packets to a specific interface address
on a target BFR. With MPLS encapsulation, this can simply be done on a target BFR. With MPLS encapsulation, this can simply be done
via a label stack with that addresses label as the top label - via a label stack with that address's label as the top label,
followed by the label assigned to the (BSL,SD,SI) BitString. With followed by the label assigned to the (BSL,SD,SI) BitString. With
non-MPLS encapsulation, some form of IP encapsulation would be non-MPLS encapsulation, some form of IP encapsulation would be
required (for example IP/GRE). required (for example, IP/GRE).
The encapsulation used for forward_routed() adjacencies can equally The encapsulation used for forward_routed() adjacencies can equally
support existing advanced adjacency information such as "loose source support existing advanced adjacency information such as "loose source
routes" via e.g. MPLS label stacks or appropriate header extensions routes" via, for example, MPLS label stacks or appropriate header
(e.g. for IPv6). extensions (e.g., for IPv6).
4.4. BIER-TE Forwarding Pseudocode 4.4. BIER-TE Forwarding Pseudocode
The following pseudocode, Figure 5, for BIER-TE forwarding is based The pseudocode for BIER-TE forwarding, as shown in Figure 5, is based
on the (non-TE) BIER forwarding pseudocode of [RFC8279], section 6.5 on the (non-TE) BIER forwarding pseudocode provided in [RFC8279],
with one modification. Section 6.5, with one modification.
void ForwardBitMaskPacket_withTE (Packet) void ForwardBitMaskPacket_withTE (Packet)
{ {
SI=GetPacketSI(Packet); SI=GetPacketSI(Packet);
Offset=SI*BitStringLength; Offset=SI*BitStringLength;
for (Index = GetFirstBitPosition(Packet->BitString); Index ; for (Index = GetFirstBitPosition(Packet->BitString); Index ;
Index = GetNextBitPosition(Packet->BitString, Index)) { Index = GetNextBitPosition(Packet->BitString, Index)) {
F-BM = BIFT[Index+Offset]->F-BM; F-BM = BIFT[Index+Offset]->F-BM;
if (!F-BM) continue; [3] if (!F-BM) continue; [3]
BFR-NBR = BIFT[Index+Offset]->BFR-NBR; BFR-NBR = BIFT[Index+Offset]->BFR-NBR;
PacketCopy = Copy(Packet); PacketCopy = Copy(Packet);
PacketCopy->BitString &= F-BM; [2] PacketCopy->BitString &= F-BM; [2]
PacketSend(PacketCopy, BFR-NBR); PacketSend(PacketCopy, BFR-NBR);
// The following must not be done for BIER-TE: // The following must not be done for BIER-TE:
// Packet->BitString &= ~F-BM; [1] // Packet->BitString &= ~F-BM; [1]
} }
} }
Figure 5: BIER-TE Forwarding Pseudocode for required functions,
based on BIER Pseudocode
In step [2], the F-BM is used to clear bit(s) in PacketCopy. This Figure 5: BIER-TE Forwarding Pseudocode for Required Functions,
step exists in both BIER and BIER-TE, but the F-BMs need to be Based on BIER Pseudocode
populated differently for BIER-TE than for BIER for the desired
clearing. In step [2], the F-BM is used to clear one or more bits in
PacketCopy. This step exists in both BIER and BIER-TE, but the F-BMs
need to be populated differently for BIER-TE than for BIER for the
desired clearing.
In BIER, multiple bits of a BitString can have the same BFR-NBR. In BIER, multiple bits of a BitString can have the same BFR-NBR.
When a received packets BitString has more than one of those bits When a received packets BitString has more than one of those bits
set, the BIER replication logic has to avoid that more than one set, BIER's replication logic has to prevent more than one PacketCopy
PacketCopy is sent to that BFR-NBR ([1]). Likewise, the PacketCopy from being sent to that BFR-NBR ([1]). Likewise, the PacketCopy sent
sent to a BFR-NBR must clear all bits in its BitString that are not to a BFR-NBR must clear all bits in its BitString that are not routed
routed across BFR-NBR. This protects against BIER replication on any across a BFR-NBR. This prevents BIER's replication logic from
possible further BFR to create duplicates ([2]). creating duplicates on any possible further BFRs ([2]).
To solve both [1] and [2] for BIER, the F-BM of each bit index needs To solve both [1] and [2] for BIER, the F-BM of each bit index needs
to have all bits set that this BFR wants to route across BFR-NBR. [2] to have all bits set that this BFR wants to route across a BFR-
clears all other bits in PacketCopy->BitString, and [1] clears those NBR. [2] clears all other bits in PacketCopy->BitString, and [1]
bits from Packet->BitString after the first PacketCopy. clears those bits from Packet->BitString after the first PacketCopy.
In BIER-TE, a BFR-NBR in this pseudocode is an adjacency, In BIER-TE, a BFR-NBR in this pseudocode is an adjacency --
forward_connected(), forward_routed() or local_decap(). There is no forward_connected(), forward_routed(), or local_decap(). There is no
need for [2] to suppress duplicates in the way BIER does because in need for [2] to suppress duplicates in the same way that BIER does,
general, different BP would never have the same adjacency. If a because in general, different BPs would never have the same
BIER-TE controller actually finds some optimization in which this adjacency. If a BIER-TE controller actually finds some optimization
would be desirable, then the controller is also responsible to ensure in which this would be desirable, then the controller is also
that only one of those bits is set in any Packet->BitString, unless responsible for ensuring that only one of those bits is set in any
the controller explicitly wants for duplicates to be created. Packet->BitString, unless the controller explicitly wants duplicates
to be created.
The following points describe how the forwarding bit mask (F-BM) for The following points describe how the F-BM for each BP is configured
each BP is configured in the BIFT and how this impacts the BitString in the BIFT and how this impacts the BitString of the packet being
of the packet being processed with that BIFT: processed with that BIFT:
1. The F-BMs of all BIFT BPs without an adjacency have all their 1. The F-BMs of all BIFT BPs without an adjacency have all their
bits clear. This will cause [3] to skip further processing of bits clear. This will cause [3] to skip further processing of
such a BP. such a BP.
2. All BIFT BPs with an adjacency (with DNC flag clear) have an F-BM 2. All BIFT BPs with an adjacency (with the DNC flag clear) have an
that has only those BPs set for which this BFR does not have an F-BM that has only those BPs set for which this BFR does not have
adjacency. This causes [2] to clear all bits from an adjacency. This causes [2] to clear all bits from
PacketCopy->BitString for which this BFR does have an adjacency. PacketCopy->BitString for which this BFR does have an adjacency.
3. [1] is not performed for BIER-TE. All bit clearing required by 3. [1] is not performed for BIER-TE. All bit clearing required by
BIER-TE is performed by [2]. BIER-TE is performed by [2].
This Forwarding Pseudocode can support the required BIER-TE This forwarding pseudocode can support the required BIER-TE
forwarding functions (see Section 4.5), forward_connected(), forwarding functions (see Section 4.5) -- forward_connected(),
forward_routed() and local_decap(), but not the recommended functions forward_routed(), and local_decap() -- but cannot support the
DNC flag and multiple adjacencies per bit nor the optional function, recommended functions (DNC flag and multiple adjacencies per bit) or
ECMP() adjacencies. The DNC flag cannot be supported when using only the optional function (i.e., ECMP() adjacencies). The DNC flag
[1] to mask bits. cannot be supported when using only [1] to mask bits.
The modified and expanded Forwarding Pseudocode in Figure 6 specifies The modified and expanded forwarding pseudocode in Figure 6 specifies
how to support all BIER-TE forwarding functions (required, how to support all BIER-TE forwarding functions (required,
recommended and optional): recommended, and optional):
* This pseudocode eliminates per-bit F-BM, therefore reducing the 1. This pseudocode eliminates per-bit F-BMs, therefore reducing the
size of BIFT state by BSL^2*SI and eliminating the need for per- size of BIFT state by SI*BSL^2 and eliminating the need for per-
packet-copy BitString masking operations except for adjacencies packet-copy BitString masking operations, except for adjacencies
with the DNC flag set: with the DNC flag set:
- AdjacentBits[SI] are bit positions with a non-empty list of 1.a AdjacentBits[SI] are bit positions with a non-empty list of
adjacencies in this BFR BIFT. This can be computed whenever adjacencies in this BFR BIFT. This can be computed whenever
the BIER-TE Controller updates (add/removes) adjacencies in the the BIER-TE controller updates (adds/removes) adjacencies in
BIFT. the BIFT.
- The BFR needs to create packet copies for these adjacent bits 1.b The BFR needs to create packet copies for these adjacent
when they are set in the packets BitString. This set of bits bits when they are set in the packets BitString. This set
is calculated in PktAdjacentBits. of bits is calculated in PktAdjacentBits.
- All bit positions to which the BFR creates copies have to be 1.c All bit positions for which the BFR creates copies have to
cleared in packet copies to avoid loops. This is done by be cleared in packet copies to avoid loops. This is done by
masking the BitString of the packet with ~AdjacentBits[SI]. masking the BitString of the packet with ~AdjacentBits[SI].
When an adjacency has DNC set, this bit position is set again When an adjacency has DNC set, this bit position is set
only for the packet copy towards that bit position. again only for the packet copy towards that bit position.
* BIFT entries may contain more than one adjacency in support of 2. BIFT entries may contain more than one adjacency in support of
specific configurations such as Section 5.1.5. The code therefore specific configurations, such as a hub and multiple spokes
includes a loop over these adjacencies. (Section 5.1.5). The code therefore includes a loop over these
adjacencies.
* The ECMP() adjacency is shown. Its parameters are a seed and a 3. The ECMP() adjacency is also shown in the figure. Its parameters
ListOfAdjacencies from which one is picked. are a seed and "ListOfAdjacencies", from which one is picked.
* The forward_connected(), forward_routed(), local_decap() 4. The forward_connected(), forward_routed(), and local_decap()
adjacencies are shown with their parameters. adjacencies are shown with their parameters.
void ForwardBitMaskPacket_withTE (Packet) void ForwardBitMaskPacket_withTE (Packet)
{ {
SI = GetPacketSI(Packet); SI = GetPacketSI(Packet);
Offset = SI * BitStringLength; Offset = SI * BitStringLength;
// Determine adjacent bits in the Packets BitString // Determine adjacent bits in the packets BitString
PktAdjacentBits = Packet->BitString & AdjacentBits[SI]; PktAdjacentBits = Packet->BitString & AdjacentBits[SI];
// Clear adjacent bits in Packet header to avoid loops // Clear adjacent bits in the packet header to avoid loops
Packet->BitString &= ~AdjacentBits[SI]; Packet->BitString &= ~AdjacentBits[SI];
// Loop over PktAdjacentBits to create packet copies // Loop over PktAdjacentBits to create packet copies
for (Index = GetFirstBitPosition(PktAdjacentBits); Index ; for (Index = GetFirstBitPosition(PktAdjacentBits); Index ;
Index = GetNextBitPosition(PktAdjacentBits, Index)) { Index = GetNextBitPosition(PktAdjacentBits, Index)) {
for adjacency in BIFT[Index+Offset]->Adjacencies { for adjacency in BIFT[Index+Offset]->Adjacencies {
if(adjacency.type == ECMP(ListOfAdjacencies,seed) ) { if(adjacency.type == ECMP(ListOfAdjacencies,seed) ) {
I = ECMP_hash(sizeof(ListOfAdjacencies), I = ECMP_hash(sizeof(ListOfAdjacencies),
Packet->Entropy,seed); Packet->Entropy,seed);
adjacency = ListOfAdjacencies[I]; adjacency = ListOfAdjacencies[I];
skipping to change at page 26, line 42 skipping to change at line 1160
SendToL3(PacketCopy,{VRF,}l3-neighbor); SendToL3(PacketCopy,{VRF,}l3-neighbor);
case local_decap({VRF},neighbor): case local_decap({VRF},neighbor):
DecapBierHeader(PacketCopy); DecapBierHeader(PacketCopy);
PassTo(PacketCopy,{VRF,}Packet->NextProto); PassTo(PacketCopy,{VRF,}Packet->NextProto);
} }
} }
} }
} }
Figure 6: Complete BIER-TE Forwarding Pseudocode for required, Figure 6: Complete BIER-TE Forwarding Pseudocode for Required,
recommended and optional functions Recommended, and Optional Functions
4.5. BFR Requirements for BIER-TE forwarding 4.5. BFR Requirements for BIER-TE Forwarding
BFR that support BIER-TE and BIER MUST support configuration that BFRs that support BIER-TE and BIER MUST support a configuration that
enables BIER-TE instead of (non-TE) BIER forwarding rules for all enables BIER-TE instead of (non-TE) BIER forwarding rules for all
BIFT of one or more BIER sub-domains. Every BP in a BIER-TE BIFT BIFTs of one or more BIER subdomains. Every BP in a BIER-TE BIFT
MUST support to have zero or one adjacency. BIER-TE forwarding MUST MUST support having zero or one adjacency. BIER-TE forwarding MUST
support the adjacency types forward_connected() with the DNC flag not support the adjacency types forward_connected() with the DNC flag not
set, forward_routed() and local_decap(). As explained in set, forward_routed(), and local_decap(). As explained in
Section 4.4, these required BIER-TE forwarding functions can be Section 4.4, these required BIER-TE forwarding functions can be
implemented via the same Forwarding Pseudocode as BIER forwarding implemented via the same forwarding pseudocode as that used for BIER
except for one modification (skipping one masking with F-BM). forwarding, except for one modification (skipping one masking with an
F-BM).
BIER-TE forwarding SHOULD support forward_connected() adjacencies BIER-TE forwarding SHOULD support forward_connected() adjacencies
with a set DNC flag, as this is highly useful to save bits in rings with the DNC flag set, as this is very useful for saving bits in
(see Section 5.1.6). rings (see Section 5.1.6).
BIER-TE forwarding SHOULD support more than one adjacency on a bit. BIER-TE forwarding SHOULD support more than one adjacency on a bit.
This allows to save bits in hub and spoke scenarios (see This allows bits to be saved in hub-and-spoke scenarios (see
Section 5.1.5). Section 5.1.5).
BIER-TE forwarding MAY support ECMP() adjacencies to save bits in BIER-TE forwarding MAY support ECMP() adjacencies to save bits in
ECMP scenarios, see Section 5.1.7 for an example. This is an ECMP scenarios; see Section 5.1.7 for an example. This is an
optional requirement, because for ECMP deployments using BIER-TE one optional requirement, because for ECMP deployments using BIER-TE one
can also leverage ECMP of the routing underlay via forwarded_routed can also leverage the routing underlay ECMP via forward_routed()
adjacencies and/or might prefer to have more explicit control of the adjacencies and/or might prefer to have more explicit control of the
path chosen via explicit BP/adjacencies for each ECMP path path chosen via explicit BPs/adjacencies for each ECMP path
alternative. alternative.
5. BIER-TE Controller Operational Considerations 5. BIER-TE Controller Operational Considerations
5.1. Bit Position Assignments 5.1. Bit Position Assignments
This section describes how the BIER-TE Controller can use the This section describes how the BIER-TE controller can use the
different BIER-TE adjacency types to define the bit positions of a different BIER-TE adjacency types to define the bit positions of a
BIER-TE domain. BIER-TE domain.
Because the size of the BitString limits the size of the BIER-TE Because the size of the BitString limits the size of the BIER-TE
domain, many of the options described exist to support larger domain, many of the options described here exist to support larger
topologies with fewer bit positions. topologies with fewer bit positions.
5.1.1. P2P Links 5.1.1. P2P Links
On a P2P link that connects two BFRs, the same bit position can be On a "point-to-point" (P2P) link that connects two BFRs, the same bit
used on both BFRs for the adjacency to the neighboring BFR. A P2P position can be used on both BFRs for the adjacency to the
link requires therefore only one bit position. neighboring BFR. A P2P link therefore requires only one bit
position.
5.1.2. BFER 5.1.2. BFERs
Every non-Leaf BFER is given a unique bit position with a Every non-leaf BFER is given a unique bit position with a
local_decap() adjacency. local_decap() adjacency.
5.1.3. Leaf BFERs 5.1.3. Leaf BFERs
A leaf BFER is one where incoming BIER-TE packets never need to be
forwarded to another BFR but are only sent to the BFER to exit the
BIER-TE domain. For example, in networks where "Provider Edge" (PE)
routers are spokes connected to Provider (P) routers, those PEs are
leaf BFERs, unless there is a U-turn between two PEs.
Consider how redundant disjoint traffic can reach BFER1/BFER2 as
shown in Figure 7: when BFER1/BFER2 are non-leaf BFERs as shown on
the right-hand side, one traffic copy would be forwarded to BFER1
from BFR1, but the other one could only reach BFER1 via BFER2, which
makes BFER2 a non-leaf BFER. Likewise, BFER1 is a non-leaf BFER when
forwarding traffic to BFER2. Note that the BFERs on the left-hand
side of the figure are only guaranteed to be leaf BFERs by correctly
applying a routing configuration that prohibits transit traffic from
passing through the BFERs, which is commonly applied in these
topologies.
BFR1(P) BFR2(P) BFR1(P) BFR2(P) BFR1(P) BFR2(P) BFR1(P) BFR2(P)
| \ / | | | | \ / | | |
| X | | | | X | | |
| / \ | | | | / \ | | |
BFER1(PE) BFER2(PE) BFER1(PE)----BFER2(PE) BFER1(PE) BFER2(PE) BFER1(PE)----BFER2(PE)
^ U-turn link ^ U-turn link
Leaf BFER / Non-Leaf BFER / Leaf BFER / Non-leaf BFER /
PE-router PE-router PE router PE router
Figure 7: Leaf vs. non-Leaf BFER Example
A leaf BFER is one where incoming BIER-TE packets never need to be
forwarded to another BFR but are only sent to the BFER to exit the
BIER-TE domain. For example, in networks where Provider Edge (PE)
router are spokes connected to Provider (P) routers, those PEs are
Leaf BFERs unless there is a U-turn between two PEs.
Consider how redundant disjoint traffic can reach BFER1/BFER2 in Figure 7: Leaf vs. Non-Leaf BFER Example
Figure 7: When BFER1/BFER2 are Non-Leaf BFER as shown on the right-
hand side, one traffic copy would be forwarded to BFER1 from BFR1,
but the other one could only reach BFER1 via BFER2, which makes BFER2
a non-Leaf BFER. Likewise, BFER1 is a non-Leaf BFER when forwarding
traffic to BFER2. Note that the BFERs in the left-hand picture are
only guaranteed to be leaf-BFER by fitting routing configuration that
prohibits transit traffic to pass through the BFERs, which is
commonly applied in these topologies.
In most situations, leaf-BFER that are to be addressed via the same In most situations, leaf BFERs that are to be addressed via the same
BitString can share a single bit position for their local_decap() BitString can share a single bit position for their local_decap()
adjacency in that BitString and therefore save bit positions. On a adjacency in that BitString and therefore save bit positions. On a
non-leaf BFER, a received BIER-TE packet may only need to transit the non-leaf BFER, a received BIER-TE packet may only need to transit the
BFER or it may need to also be decapsulated. Whether or not to BFER, or it may also need to be decapsulated. Whether or not to
decapsulate the packet therefore needs to be indicated by a unique decapsulate the packet therefore needs to be indicated by a unique
bit position populated only on the BIFT of this BFER with a bit position populated only on the BIFT of this BFER with a
local_decap() adjacency. On a leaf-BFER, packets never need to pass local_decap() adjacency. On a leaf BFER, packets never need to pass
through; any packet received is therefore usually intended to be through; any packet received is therefore usually intended to be
decapsulated. This can be expressed by a single, shared bit position decapsulated. This can be expressed by a single, shared bit position
that is populated with a local_decap() adjacency on all leaf-BFER that is populated with a local_decap() adjacency on all leaf BFERs
addressed by the BitString. addressed by the BitString.
The possible exception from this leaf-BFER bit position optimization The possible exceptions to this leaf BFER bit position optimization
can be cases where the bit position on the prior BIER-TE BFR (which scenario can be cases where the bit position on the prior BIER-TE BFR
created the packet copy for the leaf-BFER in question) is populated (which created the packet copy for the leaf BFER in question) is
with multiple adjacencies as an optimization, such as in populated with multiple adjacencies as an optimization -- for
Section 5.1.4 or Section 5.1.5. With either of these two example, as described in Sections 5.1.4 and 5.1.5. With either of
optimizations, the sender of the packet could only control explicitly these two optimizations, the sender of the packet could only control
whether the packet was to be decapsulated on the leaf-BFER in explicitly whether the packet was to be decapsulated on the leaf BFER
question, if the leaf-BFER has a unique bit position for its in question, if the leaf BFER has a unique bit position for its
local_decap() adjacency. local_decap() adjacency.
However, if the bit position is shared across leaf-BFER, and packets However, if the bit position is shared across a leaf BFER and packets
are therefore decapsulated potentially unnecessarily, this may still are therefore decapsulated -- potentially unnecessarily -- this may
be appropriate if the decapsulated payload of the BIER-TE packet still be appropriate if the decapsulated payload of the BIER-TE
indicates whether or not the packet needs to be further processed/ packet indicates whether or not the packets need to be further
received. This is typically true for example if the payload is IP processed/received. This is typically true, for example, if the
multicast because IP multicast on a BFER would know the membership payload is IP multicast, because IP multicast on a BFER would know
state of the IP multicast payload and be able to discard it if the the membership state of the IP multicast payload and be able to
packet was delivered unnecessarily by the BIER-TE layer. If the discard it if the packets were delivered unnecessarily by the BIER-TE
payload has no such membership indication, and the BFIR wants to have layer. If the payload has no such membership indication and the BFIR
explicit control about which BFER are to receive and decapsulate a wants to have explicit control regarding which BFERs are to receive
packet, then these two optimizations can not be used together with and decapsulate a packet, then these two optimizations cannot be used
shared bit positions optimization for leaf-BFER. together with shared bit position optimization for a leaf BFER.
5.1.4. LANs 5.1.4. LANs
In a LAN, the adjacency to each neighboring BFR is given a unique bit In a LAN, the adjacency to each neighboring BFR is given a unique bit
position. The adjacency of this bit position is a position. The adjacency of this bit position is a
forward_connected() adjacency towards the BFR and this bit position forward_connected() adjacency towards the BFR, and this bit position
is populated into the BIFT of all the other BFRs on that LAN. is populated into the BIFT of all the other BFRs on that LAN.
BFR1 BFR1
|p1 |p1
LAN1-+-+---+-----+ LAN1-+-+---+-----+
p3| p4| p2| p3| p4| p2|
BFR3 BFR4 BFR7 BFR3 BFR4 BFR7
Figure 8: LAN Example Figure 8: LAN Example
If Bandwidth on the LAN is not an issue and most BIER-TE traffic If bandwidth on the LAN is not an issue and most BIER-TE traffic
should be copied to all neighbors on a LAN, then bit positions can be should be copied to all neighbors on a LAN, then bit positions can be
saved by assigning just a single bit position to the LAN and saved by assigning just a single bit position to the LAN and
populating the bit position of the BIFTs of each BFRs on the LAN with populating the bit position of the BIFTs of each BFR on the LAN with
a list of forward_connected() adjacencies to all other neighbors on a list of forward_connected() adjacencies to all other neighbors on
the LAN. the LAN.
This optimization does not work in the case of BFRs redundantly This optimization does not work in the case of BFRs redundantly
connected to more than one LAN with this optimization because these connected to more than one LAN with this optimization. These BFRs
BFRs would receive duplicates and forward those duplicates into the would receive duplicates and forward those duplicates into the other
opposite LANs. Adjacencies of such BFRs into their LAN still need a LANs. Such BFRs require separate bit positions for each LAN they
separate bit position. connect to.
5.1.5. Hub and Spoke 5.1.5. Hub and Spoke
In a setup with a hub and multiple spokes connected via separate p2p In a setup with a hub and multiple spokes connected via separate P2P
links to the hub, all p2p adjacencies from the hub to the spokes links to the hub, all P2P adjacencies from the hub to the spokes'
links can share the same bit position. The bit position on the hub's links can share the same bit position. The bit position on the hub's
BIFT is set up with a list of forward_connected() adjacencies, one BIFT is set up with a list of forward_connected() adjacencies, one
for each Spoke. for each spoke.
This option is similar to the bit position optimization in LANs: This option is similar to the bit position optimization in LANs:
Redundantly connected spokes need their own bit positions, unless redundantly connected spokes need their own bit positions, unless
they are themselves Leaf-BFER. they are themselves leaf BFERs.
This type of optimized BP could be used for example when all traffic This type of optimized BP could be used, for example, when all
is "broadcast" traffic (very dense receiver set) such as live-TV or traffic is "broadcast" traffic (very dense receiver sets), such as
many-to-many telemetry including situation-awareness (SA). This BP live TV or many-to-many telemetry, including situational awareness.
optimization can then be used to explicitly steer different traffic This BP optimization can then be used to explicitly steer different
flows across different ECMP paths in Data-Center or broadband- traffic flows across different ECMP paths in data-center or
aggregation networks with minimal use of BPs. broadband-aggregation networks with minimal use of BPs.
5.1.6. Rings 5.1.6. Rings
In L3 rings, instead of assigning a single bit position for every p2p In L3 rings, instead of assigning a single bit position for every P2P
link in the ring, it is possible to save bit positions by setting the link in the ring, it is possible to save bit positions by setting the
"DoNotClear" (DNC) flag on forward_connected() adjacencies. "DoNotClear" (DNC) flag on forward_connected() adjacencies.
For the rings shown in Figure 9, a single bit position will suffice For the ring shown in Figure 9, a single bit position will suffice to
to forward traffic entering the ring at BFRa or BFRb all the way up forward traffic entering the ring at BFRa or BFRb all the way up to
to BFR1: BFR1, as follows.
On BFRa, BFRb, BFR30,... BFR3, the bit position is populated with a On BFRa, BFRb, BFR30,... BFR3, the bit position is populated with a
forward_connected() adjacency pointing to the clockwise neighbor on forward_connected() adjacency pointing to the clockwise neighbor on
the ring and with DNC set. On BFR2, the adjacency also points to the the ring and with DNC set. On BFR2, the adjacency also points to the
clockwise neighbor BFR1, but without DNC set. clockwise neighbor BFR1, but without DNC set.
Handling DNC this way ensures that copies forwarded from any BFR in Handling DNC this way ensures that copies forwarded from any BFRs in
the ring to a BFR outside the ring will not have the ring bit the ring to a BFR outside the ring will not have the ring bit
position set, therefore minimizing the chance to create loops. position set, therefore minimizing the risk of creating loops.
v v v v
| | | |
L1 | L2 | L3 L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\ /-------- BFRa ---- BFRb --------------------\
| | | |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/ \- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | | | | L4 | |
p33| p15| p33| p15|
BFRd BFRc BFRd BFRc
skipping to change at page 31, line 4 skipping to change at line 1359
v v v v
| | | |
L1 | L2 | L3 L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\ /-------- BFRa ---- BFRb --------------------\
| | | |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/ \- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | | | | L4 | |
p33| p15| p33| p15|
BFRd BFRc BFRd BFRc
Figure 9: Ring Example Figure 9: Ring Example
Note that this example only permits for packets intended to make it Note that this example only permits packets intended to make it all
all the way around the ring to enter it at BFRa and BFRb, and that the way around the ring to enter it at BFRa and BFRb. Note also that
packets will always travel clockwise. If packets should be allowed packets will always travel clockwise. If packets should be allowed
to enter the ring at any ring BFR, then one would have to use two to enter the ring at any of the ring's BFRs, then one would have to
ring bit positions. One for each direction: clockwise and use two ring bit positions, one for each direction: clockwise and
counterclockwise. counterclockwise.
Both would be set up to stop rotating on the same link, e.g. L1. Both would be set up to stop rotating on the same link, e.g., L1.
When the ingress ring BFR creates the clockwise copy, it will clear When the ring's BFIR creates the clockwise copy, it will clear the
the counterclockwise bit position because the DNC bit only applies to counterclockwise bit position because the DNC bit only applies to the
the bit for which the replication is done. Likewise for the bit for which the replication is done (likewise for the clockwise bit
clockwise bit position for the counterclockwise copy. As a result, position for the counterclockwise copy). As a result, the ring's
the ring ingress BFR will send a copy in both directions, serving BFIR will send a copy in both directions, serving BFRs on either side
BFRs on either side of the ring up to L1. of the ring up to L1.
5.1.7. Equal Cost MultiPath (ECMP)
[RFC-Editor: A reviewer (Lars Eggert) noted that the infinite "to 5.1.7. Equal-Cost Multipath (ECMP)
use" in the following sentence is not correct. The same was also
noted for several other similar instances. The following URL seems
to indicate though that this is a per-case decision, which seems
undefined: https://writingcenter.gmu.edu/guides/choosing-between-
infinitive-and-gerund-to-do-or-doing. What exactly should be done
about this ?].
An ECMP() adjacency allows to use just one BP to deliver packets to An ECMP() adjacency allows the use of just one BP to deliver packets
one of N adjacencies instead of one BP for each adjacency. In the to one of N adjacencies instead of one BP for each adjacency. In the
common example case Figure 10, a link-bundle of three links L1,L2,L3 common example case shown in Figure 10, a link bundle of three links
connects BFR1 and BFR2, and only one BP is used instead of three BP L1,L2,L3 connects BFR1 and BFR2, and only one BP is used instead of
to deliver packets from BFR1 to BFR2. three BPs to deliver packets from BFR1 to BFR2.
--L1----- --L1-----
BFR1 --L2----- BFR2 BFR1 --L2----- BFR2
--L3----- --L3-----
BIFT entry in BFR1: BIFT entry in BFR1:
------------------------------------------------------------------ ------------------------------------------------------------------
| Index | Adjacencies | | Index | Adjacencies |
================================================================== ==================================================================
| 0:6 | ECMP({forward_connected(L1, BFR2), | | 0:6 | ECMP({forward_connected(L1, BFR2), |
skipping to change at page 32, line 31 skipping to change at line 1411
================================================================== ==================================================================
| 0:6 | ECMP({forward_connected(L1, BFR1), | | 0:6 | ECMP({forward_connected(L1, BFR1), |
| | forward_connected(L2, BFR1), | | | forward_connected(L2, BFR1), |
| | forward_connected(L3, BFR1)}, seed) | | | forward_connected(L3, BFR1)}, seed) |
------------------------------------------------------------------ ------------------------------------------------------------------
Figure 10: ECMP Example Figure 10: ECMP Example
This document does not standardize any ECMP algorithm because it is This document does not standardize any ECMP algorithm because it is
sufficient for implementations to document their freely chosen ECMP sufficient for implementations to document their freely chosen ECMP
algorithm. Figure 11 shows an example ECMP algorithm, and would algorithm. Figure 11 shows an example ECMP algorithm and would
double as its documentation: A BIER-TE controller could determine double as its documentation: a BIER-TE controller could determine
which adjacency is chosen based on the seed and adjacencies which adjacency is chosen based on the seed and adjacencies
parameters and the packet entropy. parameters and on packet entropy.
forward(packet, ECMP(adj(0), adj(1),... adj(N-1), seed)): forward(packet, ECMP(adj(0), adj(1),... adj(N-1), seed)):
i = (packet(bier-header-entropy) XOR seed) % N i = (packet(bier-header-entropy) XOR seed) % N
forward packet to adj(i) forward packet to adj(i)
Figure 11: ECMP algorithm Example Figure 11: ECMP Algorithm Example
In the following example, all traffic from BFR1 towards BFR10 is In the example shown in Figure 12, all traffic from BFR1 towards
intended to be ECMP load split equally across the topology. This BFR10 is intended to be ECMP load-split equally across the topology.
example is not meant as a likely setup, but to illustrate that ECMP This example is not meant as a likely setup; rather, it illustrates
can be used to share BPs not only across link bundles, but also that ECMP can be used to share BPs not only across link bundles but
across alternative paths across different transit BFR, and it also across alternative paths across different transit BFRs, and it
explains the use of the seed parameter. explains the use of the seed parameter.
BFR1 (BFIR) BFR1 (BFIR)
/L11 \L12 /L11 \L12
/ \ / \
BFR2 BFR3 BFR2 BFR3
/L21 \L22 /L31 \L32 /L21 \L22 /L31 \L32
/ \ / \ / \ / \
BFR4 BFR5 BFR6 BFR7 BFR4 BFR5 BFR6 BFR7
\ / \ / \ / \ /
skipping to change at page 34, line 5 skipping to change at line 1478
| 0:8 | forward_connected(Lxx, BFR9) |xx differs on BFR6/BFR7| | 0:8 | forward_connected(Lxx, BFR9) |xx differs on BFR6/BFR7|
------------------------------------------------------------------ ------------------------------------------------------------------
BIFT entry in BFR8, BFR9: BIFT entry in BFR8, BFR9:
------------------------------------------------------------------ ------------------------------------------------------------------
| 0:9 | forward_connected(Lxx, BFR10) |xx differs on BFR8/BFR9| | 0:9 | forward_connected(Lxx, BFR10) |xx differs on BFR8/BFR9|
------------------------------------------------------------------ ------------------------------------------------------------------
Figure 12: Polarization Example Figure 12: Polarization Example
Note that for the following discussion of ECMP, only the BIFT ECMP Note that for the following discussion of ECMP, only the BIFT ECMP()
adjacencies on BFR1, BFR2, BFR3 are relevant. The re-use of BP adjacencies on BFR1, BFR2, and BFR3 are relevant. The reuse of BPs
across BFR in this example is further explained in Section 5.1.9 across BFRs in this example is further explained in Section 5.1.9
below. below.
With the setup of ECMP in the topology above, traffic would not be With the ECMP setup shown in the topology above, traffic would not be
equally load-split. Instead, links L22 and L31 would see no traffic equally load-split. Instead, links L22 and L31 would see no traffic
at all: BFR2 will only see traffic from BFR1 for which the ECMP hash at all: BFR2 will only see traffic from BFR1, for which the ECMP hash
in BFR1 selected the first adjacency in the list of 2 adjacencies in BFR1 selected the first adjacency in the list of two adjacencies
given as parameters to the ECMP. It is link L11-to-BFR2. BFR2 given as parameters to the ECMP: link L11-to-BFR2. BFR2 again
performs again ECMP with two adjacencies on that subset of traffic performs ECMP with two adjacencies on that subset of traffic using
using the same seed1, and will therefore again select the first of the same seed1 and will therefore again select the first of its two
its two adjacencies: L21-to-BFR4. And therefore L22 and BFR5 sees no adjacencies: L21-to-BFR4. Therefore, L22 and BFR5 see no traffic
traffic. Likewise for L31 and BFR6. (likewise for L31 and BFR6).
This issue in BFR2/BFR3 is called polarization. It results from the This issue in BFR2/BFR3 is called "polarization". It results from
re-use of the same hash function across multiple consecutive hops in the reuse of the same hash function across multiple consecutive hops
topologies like these. To resolve this issue, the ECMP() adjacency in topologies like these. To resolve this issue, the ECMP()
on BFR1 can be set up with a different seed2 than the ECMP() adjacency on BFR1 can be set up with a different seed2 than the
adjacencies on BFR2/BFR3. BFR2/BFR3 can use the same hash because ECMP() adjacencies on BFR2/BFR3. BFR2/BFR3 can use the same hash
packets will not sequentially pass across both of them. Therefore, because packets will not sequentially pass across both of them.
they can also use the same BP 0:7. Therefore, they can also use the same BP (i.e., 0:7).
Note that ECMP solutions outside of BIER often hide the seed by auto- Note that ECMP solutions outside of BIER often hide the seed by auto-
selecting it from local entropy such as unique local or next-hop selecting it from local entropy such as unique local or next-hop
identifiers. Allowing the BIER-TE Controller to explicitly set the identifiers. Allowing the BIER-TE controller to explicitly set the
seed gives the ability for it to control same/different path seed gives the BIER-TE controller the ability to control the
selection across multiple consecutive ECMP hops. selection of the same path or different paths across multiple
consecutive ECMP hops.
5.1.8. Forward Routed adjacencies 5.1.8. Forward Routed Adjacencies
5.1.8.1. Reducing bit positions 5.1.8.1. Reducing Bit Positions
Forward_routed() adjacencies can reduce the number of bit positions Forward_routed() adjacencies can reduce the number of bit positions
required when the path steering requirement is not hop-by-hop required when the path steering requirement is not hop-by-hop
explicit path selection, but loose-hop selection. Forward_routed() explicit path selection but rather is loose-hop selection.
adjacencies can also allow to operate BIER-TE across intermediate hop Forward_routed() adjacencies can also permit BIER-TE operation across
routers that do not support BIER-TE. intermediate-hop routers that do not support BIER-TE.
Assume that the requirement in Figure 13 is to explicitly steer
traffic flows that have arrived at BFR1 or BFR4 via a path in the
routing underlay "Network Area 1" to one of the following next three
segments: (1) BFR2 via link L1, (2) BFR2 via link L2, or (3) via BFR3
and then not caring whether the packet is forwarded via L3 or L4.
............... ...............
...BFR1--... ...--L1-- BFR2... ...BFR1--... ...--L1-- BFR2...
... .Routers. ...--L2--/ ... .Routers. ...--L2--/
...BFR4--... ...--L3-- BFR3... ...BFR4--... ...--L3-- BFR3...
... ...--L4--/ | ... ...--L4--/ |
............... | ............... |
LO LO
Network Area 1 Network Area 1
Figure 13: Forward Routed Adjacencies Example Figure 13: Forward Routed Adjacencies Example
Assume the requirement in Figure 13 is to explicitly steer traffic To enable this, both BFR1 and BFR4 are set up with a forward_routed()
flows that have arrived at BFR1 or BFR4 via a path in the routing
underlay "Network Area 1" to one of the following three next
segments: (1) BFR2 via link L1, (2) BFR2 via link L2, or (3) via BFR3
and then nor caring whether the packet is forwarded via L3 or L4.
To enable this, both BFR1 and BFR4 are set up with a forward_routed
adjacency bit position towards an address of BFR2 on link L1, another adjacency bit position towards an address of BFR2 on link L1, another
forward_routed() bit position towards an address of BFR2 on link L2 forward_routed() bit position towards an address of BFR2 on link L2,
and a third forward_routed() bit position towards a node address LO and a third forward_routed() bit position towards a node address LO
of BFR3. of BFR3.
5.1.8.2. Supporting nodes without BIER-TE 5.1.8.2. Supporting Nodes without BIER-TE
Forward_routed() adjacencies also enable incremental deployment of Forward_routed() adjacencies also enable incremental deployment of
BIER-TE. Only the nodes through which BIER-TE traffic needs to be BIER-TE. Only the nodes through which BIER-TE traffic needs to be
steered - with or without replication - need to support BIER-TE. steered -- with or without replication -- need to support BIER-TE.
Where they are not directly connected to each other, forward_routed Where they are not directly connected to each other, forward_routed()
adjacencies are used to pass over non BIER-TE enabled nodes. adjacencies are used to pass over nodes that are not BIER-TE enabled.
5.1.9. Reuse of bit positions (without DNC) 5.1.9. Reuse of Bit Positions (without DNC)
Bit positions can be re-used across multiple BFRs to minimize the BPs can be reused across multiple BFRs to minimize the number of BPs
number of BP needed. This happens when adjacencies on multiple BFRs needed. This happens when adjacencies on multiple BFRs use the DNC
use the DNC flag as described above, but it can also be done for non- flag as described above, but it can also be done for non-DNC
DNC adjacencies. This section only discusses this non-DNC case. adjacencies. This section only discusses this non-DNC case.
Because BP are cleared when passing a BFR with an adjacency for that Because a given BP is cleared when passing a BFR with an adjacency
BP, reuse of BP across multiple BFRs does not introduce any problems for that BP, reusing BPs across multiple BFRs does not introduce any
with duplicates or loops that do not also exist when every adjacency problems with duplicates or loops that do not also exist when every
has a unique BP. Instead, the challenge when reusing BP is whether adjacency has a unique BP. Instead, the challenge when reusing BPs
it allows to still achieve the desired Tree Engineering goals. is whether the desired Tree Engineering goals can still be achieved.
BP cannot be reused across two BFRs that would need to be passed A BP cannot be reused across two BFRs that would need to be passed
sequentially for some path: The first BFR will clear the BP, so those sequentially for some path: the first BFR will clear the BP, so those
paths cannot be built. BP can be set across BFR that would (A) only paths cannot be built. A BP can be set across BFRs that would only
occur across different paths or (B) across different branches of the occur across (A) different paths or (B) different branches of the
same tree. same tree.
An example of (A) was given in Figure 12, where BP 0:7, BP 0:8 and BP An example of (A) was given in Figure 12, where BP 0:7, BP 0:8, and
0:9 are each reused across multiple BFRs because a single packet/path BP 0:9 are each reused across multiple BFRs because a single packet/
would never be able to reach more than one BFR sharing the same BP. path would never be able to reach more than one BFR sharing the same
BP.
Assume the example was changed: BFR1 has no ECMP() adjacency for BP Assume that the example was changed: BFR1 has no ECMP() adjacency for
0:6, but instead BP 0:5 with forward_connected() to BFR2 and BP 0:6 BP 0:6 but instead has BP 0:5 with forward_connected() to BFR2 and BP
with forward_connected() to BFR3. Packets with both BP 0:5 and BP 0:6 with forward_connected() to BFR3. Packets with both BP 0:5 and
0:6 would now be able to reach both BFR2 and BFR3 and the still BP 0:6 would now be able to reach both BFR2 and BFR3, and the still-
existing re-use of BP 0:7 between BFR2 and BFR3 is a case of (B) existing reuse of BP 0:7 between BFR2 and BFR3 is a case of (B) where
where reuse of BP is perfect because it does not limit the set of reusing a BP is perfect because it does not limit the set of useful
useful path choices: path choices, as in the following example.
If instead of reusing BP 0:7, BFR3 used a separate BP 0:10 for its If instead of reusing BP 0:7 BFR3 used a separate BP 0:10 for its
ECMP() adjacency, no useful additional path steering options would be ECMP() adjacency, no useful additional path steering options would be
enabled. If duplicates at BFR10 where undesirable, this would be enabled. If duplicates at BFR10 were undesirable, this would be done
done by not setting BP 0:5 and BP 0:6 for the same packet. If the by not setting BP 0:5 and BP 0:6 for the same packet. If the
duplicates where desirable (e.g.: resilient transmission), the duplicates were desirable (e.g., resilient transmission), the
additional BP 0:10 would also not render additional value. additional BP 0:10 would also not render additional value.
Reuse may also save BPs in larger topologies. Consider the topology
shown in Figure 14.
area1 area1
BFR1a BFR1b BFR1a BFR1b
/ \ / \
.................................... ....................................
. Core . . Core .
.................................... ....................................
| / \ / \ | | / \ / \ |
BFR2a BFR2b BFR3a BFR3b BFR6a BFR6b BFR2a BFR2b BFR3a BFR3b BFR6a BFR6b
/-------\ /---------\ /--------\ /-------\ /---------\ /--------\
| area2 | | area3 | ... | area6 | | area2 | | area3 | ... | area6 |
| ring | | ring | | ring | | ring | | ring | | ring |
\-------/ \---------/ \--------/ \-------/ \---------/ \--------/
more BFR more BFR more BFR more BFRs more BFRs more BFRs
Figure 14: Reuse of BP Figure 14: Reuse of BPs
Reuse may also save BPs in larger topologies. Consider the topology A BFIR/sender (e.g., video headend) is attached to area 1, and the
shown in Figure 14. A BFIR/sender (e.g.: video headend) is attached five areas 2...6 contain receivers/BFERs. Assume that each area has
to area 1, and area 2...6 contain receivers/BFER. Assume each area a distribution ring, each with two BPs to indicate the direction (as
had a distribution ring, each with two BPs to indicate the direction explained before). These two BPs could be reused across the five
(as explained before). These two BPs could be reused across the 5 areas. Packets would be replicated through other BPs from the core
areas. Packets would be replicated through other BPs for the Core to to the desired subset of areas, and once a packet copy reaches the
the desired subset of areas, and once a packet copy reaches the ring ring of the area, the two ring BPs come into play. This reuse is a
of the area, the two ring BPs come into play. This reuse is a case case of (B), but it limits the topology choices: packets can only
of (B), but it limits the topology choices: Packets can only flow flow around the same direction in the rings of all areas. This may
around the same direction in the rings of all areas. This may or may or may not be acceptable based on the desired path steering options:
not be acceptable based on the desired path steering options: If if resilient transmission is the path engineering goal, then it is
resilient transmission is the path engineering goal, then it is likely a good optimization; however, if the bandwidth of each ring
likely a good optimization, if the bandwidth of each ring was to be were to be optimized separately, it would not be a good limitation.
optimized separately, it would not be a good limitation.
5.1.10. Summary of BP optimizations 5.1.10. Summary of BP Optimizations
This section reviewed a range of techniques by which a BIER-TE In this section, we reviewed a range of techniques by which a BIER-TE
Controller can create a BIER-TE topology in a way that minimizes the controller can create a BIER-TE topology in a way that minimizes the
number of necessary BPs. number of necessary BPs.
Without any optimization, a BIER-TE Controller would attempt to map Without any optimization, a BIER-TE controller would attempt to map
the network subnet topology 1:1 into the BIER-TE topology and every the network subnet topology 1:1 into the BIER-TE topology, every
subnet adjacent neighbor requires a forward_connected() BP and every adjacent neighbor in the subnet would require a forward_connected()
BFER requires a local_decap() BP. BP, and every BFER would require a local_decap() BP.
The optimizations described are then as follows: The optimizations described in this document are then as follows:
* P2P links require only one BP (Section 5.1.1). 1. P2P links require only one BP (Section 5.1.1).
* All leaf-BFER can share a single local_decap() BP (Section 5.1.3). 2. All leaf BFERs can share a single local_decap() BP
(Section 5.1.3).
* A LAN with N BFR needs at most N BP (one for each BFR). It only 3. A LAN with N BFRs needs at most N BPs (one for each BFR). It
needs one BP for all those BFR that are not redundantly connected only needs one BP for all those BFRs that are not redundantly
to multiple LANs (Section 5.1.4). connected to multiple LANs (Section 5.1.4).
* A hub with p2p connections to multiple non-leaf-BFER spokes can 4. A hub with P2P connections to multiple non-leaf BFER spokes can
share one BP to all spokes if traffic can be flooded to all share one BP with all of the spokes if traffic can be flooded to
spokes, e.g.: because of no bandwidth concerns or dense receiver all of those spokes, e.g., because of no bandwidth concerns or
sets (Section 5.1.5). dense receiver sets (Section 5.1.5).
* Rings of BFR can be built with just two BP (one for each 5. Rings of BFRs can be built with just two BPs (one for each
direction) except for BFR with multiple ring connections - similar direction), except for BFRs with multiple ring connections --
to LANs (Section 5.1.6). similar to LANs (Section 5.1.6).
* ECMP() adjacencies to N neighbors can replace N BP with 1 BP. 6. ECMP() adjacencies to N neighbors can replace N BPs with one BP.
Multihop ECMP can avoid polarization through different seeds of Multihop ECMP can avoid polarization through different seeds of
the ECMP algorithm (Section 5.1.7). the ECMP algorithm (Section 5.1.7).
* Forward_routed() adjacencies allow to "tunnel" across non-BIER-TE 7. Forward_routed() adjacencies permit "tunneling" across routers
capable routers and across BIER-TE capable routers where no that are either BIER-TE capable or not BIER-TE capable where no
traffic-steering or replications are required (Section 5.1.8). traffic steering or replications are required (Section 5.1.8).
* BP can generally be reused across a set of nodes where it can be 8. A BP can generally be reused across a set of nodes where it can
guaranteed that no path will ever need to traverse more than one be guaranteed that no path will ever need to traverse more than
node of the set. Depending on scenario, this may limit the one node of the set. Depending on the scenario, this may limit
feasible path steering options (Section 5.1.9). the feasible path steering options (Section 5.1.9).
Note that the described list of optimizations is not exhaustive. Note that this list of optimizations is not exhaustive. Further
Especially when the set of required path steering choices is limited optimizations of BPs are possible, especially when both the set of
and the set of possible subsets of BFERs that should be able to required path steering choices and the possible subsets of BFERs that
receive traffic is limited, further optimizations of BP are possible. should be able to receive traffic are limited. The hub-and-spoke
The hub and spoke optimization is a simple example of such traffic optimization is a simple example of such traffic-pattern-dependent
pattern dependent optimizations. optimizations.
5.2. Avoiding Duplicates and Loops
5.2. Avoiding duplicates and loops
5.2.1. Loops 5.2.1. Loops
Whenever BIER-TE creates a copy of a packet, the BitString of that Whenever BIER-TE creates a copy of a packet, the BitString of that
copy will have all bit positions cleared that are associated with copy will have all bit positions cleared that are associated with
adjacencies on the BFR. This inhibits looping of packets. The only adjacencies on the BFR. This prevents packets from looping. The
exception are adjacencies with DNC set. only exceptions are adjacencies with DNC set.
With DNC set, looping can happen. Consider in Figure 15 that link L4
from BFR3 is (inadvertently) plugged into the L1 interface of BFRa
(instead of BFR2). This creates a loop where the ring's clockwise
bit position is never cleared for copies of the packets traveling
clockwise around the ring.
v v v v
| | | |
L1 | L2 | L3 L1 | L2 | L3
/-------- BFRa ---- BFRb ---------------------\ /-------- BFRa ---- BFRb ---------------------\
| . | | . |
| ...... Wrong link wiring | | ...... Wrong link wiring |
| . | | . |
\- BFR1 - BFR2 BFR3 - ... - BFR29 - BFR30 -/ \- BFR1 - BFR2 BFR3 - ... - BFR29 - BFR30 -/
| | L4 | | | | L4 | |
p33| p15| p33| p15|
BFRd BFRc BFRd BFRc
Figure 15: Miswired Ring Example Figure 15: Miswired Ring Example
With DNC set, looping can happen. Consider in Figure 15 that link L4
from BFR3 is (inadvertently) plugged into the L1 interface of BFRa
(instead of BFR2). This creates a loop where the rings clockwise bit
position is never cleared for copies of the packets traveling
clockwise around the ring.
To inhibit looping in the face of such physical misconfiguration, To inhibit looping in the face of such physical misconfiguration,
only forward_connected() adjacencies are permitted to have DNC set, only forward_connected() adjacencies are permitted to have DNC set,
and the link layer port unique unicast destination address of the and the link layer port unique unicast destination address of the
adjacency (e.g. MAC address) protects against closing the loop. adjacency (e.g., "Media Access Control" (MAC) address) protects
Link layers without port unique link layer addresses should not be against closing the loop. Link layers without port unique link layer
used with the DNC flag set. addresses should not be used with the DNC flag set.
5.2.2. Duplicates 5.2.2. Duplicates
Duplicates happen when the graph expressed by a BitString is not a
tree but is redundantly connecting BFRs with each other. In
Figure 16, a BitString of p2,p3,p4,p5 would result in duplicate
packets arriving on BFER4. The BIER-TE controller must therefore
ensure that only BitStrings that are trees are created.
BFIR1 BFIR1
/ \ / \
/ p2 \ p3 / p2 \ p3
BFR2 BFR3 BFR2 BFR3
\ p4 / p5 \ p4 / p5
\ / \ /
BFER4 BFER4
Figure 16: Duplicates Example Figure 16: Duplicates Example
Duplicates happen when the graph expressed by a BitString is not a When links are incorrectly physically reconnected before the BIER-TE
tree but redundantly connecting BFRs with each other. In Figure 16, controller updates BitStrings in BFIRs, duplicates can happen. Like
a BitString of p2,p3,p4,p5 would result in duplicate packets to
arrive on BFER4. The BIER-TE Controller must therefore ensure to
only create BitStrings that are trees.
When links are incorrectly physically re-connected before the BIER-TE
Controller updates BitStrings in BFIRs, duplicates can happen. Like
loops, these can be inhibited by link layer addressing in loops, these can be inhibited by link layer addressing in
forward_connected() adjacencies. forward_connected() adjacencies.
If interface or loopback addresses used in forward_routed() If interface or loopback addresses used in forward_routed()
adjacencies are moved from one BFR to another, duplicates can equally adjacencies are moved from one BFR to another, duplicates are equally
happen. Such re-addressing operations must be coordinated with the likely to happen. Such readdressing operations must be coordinated
BIER-TE Controller. with the BIER-TE controller.
5.3. Managing SI, sub-domains and BFR-ids 5.3. Managing SIs, Subdomains, and BFR-ids
When the number of bits required to represent the necessary hops in When the number of bits required to represent the necessary hops in
the topology and BFER exceeds the supported BitStringLength (BSL), the topology and BFERs exceeds the supported "BitStringLength" (BSL),
multiple SIs and/or sub-domains must be used. This section discusses multiple SIs and/or subdomains must be used. This section discusses
how. how this is done.
BIER-TE forwarding does not require the concept of BFR-id, but BIER-TE forwarding does not require the concept of BFR-ids, but
routing underlay, flow overlay and BIER headers may. This section routing underlay, flow overlay, and BIER headers may. This section
also discusses how BFR-ids can be assigned to BFIR/BFER for BIER-TE. also discusses how BFR-ids can be assigned to BFIRs/BFERs for BIER-
TE.
5.3.1. Why SI and sub-domains 5.3.1. Why SIs and Subdomains?
For (non-TE) BIER and BIER-TE forwarding, the most important result For (non-TE) BIER and BIER-TE forwarding, the most important result
of using multiple SI and/or sub-domains is the same: Packets that of using multiple SIs and/or subdomains is the same: multicast flow
need to be sent to BFERs in different SIs or sub-domains require overlay packets that need to be sent to BFERs in different SIs or
different BIER packets: each one with a BitString for a different subdomains require multiple BIER packets, each one with a BitString
(SI,sub-domain) combination. Each such BitString uses one BSL sized for a different (SI,subdomain) combination. Each such BitString uses
SI block in the BIFT of the sub-domain. We call this a BIFT:SI one BSL-sized SI block in the BIFT of the subdomain. We call this a
(block). BIFT:SI (block).
For BIER and BIER-TE forwarding themselves there is also no SIs and subdomains have different purposes in the BIER architecture
difference whether different SIs and/or sub-domains are chosen, but and also the BIER-TE architecture. This impacts how operators manage
SI and sub-domain have different purposes in the BIER architecture them and especially how flow overlays will likely use them.
shared by BIER-TE. This impacts how operators are managing them and
how especially flow overlays will likely use them.
By default, every possible BFIR/BFER in a BIER network would likely By default, every possible BFIR/BFER in a BIER network would likely
be given a BFR-id in sub-domain 0 (unless there are > 64k BFIR/BFER). be given a BFR-id in subdomain 0 (unless there are > 64k BFIRs/
BFERs).
If there are different flow services (or service instances) requiring If there are different flow services (or service instances) requiring
replication to different subsets of BFERs, then it will likely not be replication to different subsets of BFERs, then it will likely not be
possible to achieve the best replication efficiency for all of these possible to achieve the best replication efficiency for all of these
service instances via sub-domain 0. Ideal replication efficiency for service instances via subdomain 0. Ideal replication efficiency for
N BFER exists in a sub-domain if they are split over not more than N BFERs exists in a subdomain if they are split over no more than
ceiling(N/BitStringLength) SI. ceiling(N/BitStringLength) SIs.
If service instances justify additional BIER:SI state in the network, If service instances justify additional BIER:SI state in the network,
additional sub-domains will be used: BFIR/BFER are assigned BFR-id in additional subdomains will be used: BFIRs/BFERs are assigned BFR-ids
those sub-domains and each service instance is configured to use the in those subdomains, and each service instance is configured to use
most appropriate sub-domain. This results in improved replication the most appropriate subdomain. This results in improved replication
efficiency for different services. efficiency for different services.
Even if creation of sub-domains and assignment of BFR-id to BFIR/BFER Even if creation of subdomains and assignment of BFR-ids to BFIRs/
in those sub-domains is automated, it is not expected that individual BFERs in those subdomains is automated, it is not expected that
service instances can deal with BFER in different sub-domains. A individual service instances can deal with BFERs in different
service instance may only support configuration of a single sub- subdomains. A service instance may only support configuration of a
domain it should rely on. single subdomain it should rely on.
To be able to easily reuse (and modify as little as possible) To be able to easily reuse (and modify as little as possible)
existing BIER procedures including flow-overlay and routing underlay, existing BIER procedures (including flow overlay and routing
when BIER-TE forwarding is added, we therefore reuse SI and sub- underlay), when BIER-TE forwarding is added, we therefore reuse SIs
domain logically in the same way as they are used in BIER: All and subdomains logically in the same way as they are used in BIER:
necessary BFIR/BFER for a service use a single BIER-TE BIFT and are all necessary BFIRs/BFERs for a service use a single BIER-TE BIFT and
split across as many SIs as necessary (see Section 5.3.2). Different are split across as many SIs as necessary (see Section 5.3.2).
services may use different sub-domains that primarily exist to Different services may use different subdomains that primarily exist
provide more efficient replication (and for BIER-TE desirable path to provide more efficient replication (and, for BIER-TE, desirable
steering) for different subsets of BFIR/BFER. path steering) for different subsets of BFIRs/BFERs.
5.3.2. Assigning bits for the BIER-TE topology 5.3.2. Assigning Bits for the BIER-TE Topology
In BIER, BitStrings only need to carry bits for BFERs, which leads to In BIER, BitStrings only need to carry bits for BFERs; this leads to
the model that BFR-ids map 1:1 to each bit in a BitString. the model where BFR-ids map 1:1 to each bit in a BitString.
In BIER-TE, BitStrings need to carry bits to indicate not only the In BIER-TE, BitStrings need to carry bits to indicate not only the
receiving BFER but also the intermediate hops/links across which the receiving BFER but also the intermediate hops/links across which the
packet must be sent. The maximum number of BFER that can be packet must be sent. The maximum number of BFERs that can be
supported in a single BitString or BIFT:SI depends on the number of supported in a single BitString or BIFT:SI depends on the number of
bits necessary to represent the desired topology between them. bits necessary to represent the desired topology between them.
"Desired" topology because it depends on the physical topology, and "Desired" topology means that it depends on the physical topology and
on the desire of the operator to allow for explicit path steering the operator's desire to
across every single hop (which requires more bits), or reducing the
number of required bits by exploiting optimizations such as unicast 1. permit explicit path steering across every single hop (which
(forward_routed()), ECMP() or flood (DNC) over "uninteresting" sub- requires more bits), or
parts of the topology - e.g. parts where different trees do not need
to take different paths due to path steering reasons. 2. reduce the number of required bits by exploiting optimizations
such as unicast (forward_routed()), ECMP(), or flood (DNC) over
"uninteresting" sub-parts of the topology, e.g., parts where, for
path steering reasons, different trees do not need to take
different paths.
The total number of bits to describe the topology vs. the number of The total number of bits to describe the topology vs. the number of
BFERs in a BIFT:SI can range widely based on the size of the topology BFERs in a BIFT:SI can range widely based on the size of the topology
and the amount of alternative paths in it. In a BIER-TE topology and the amount of alternative paths in it. In a BIER-TE topology
crafted by a BIER-TE expert, the higher the percentage of non-BFER crafted by a BIER-TE expert, the higher the percentage of non-BFER
bits, the higher the likelihood, that those topology bits are not bits, the higher the likelihood that those topology bits are not just
just BIER-TE overhead without additional benefit, but instead that BIER-TE overhead without additional benefit but instead will allow
they will allow to express desirable path steering alternatives. the expression of desirable path steering alternatives.
5.3.3. Assigning BFR-id with BIER-TE 5.3.3. Assigning BFR-ids with BIER-TE
BIER-TE forwarding does not use the BFR-id, nor does it require for BIER-TE forwarding does not use BFR-ids, nor does it require that the
the BFIR-id field of the BIER header to be set to a particular value. BFIR-id field of the BIER header be set to a particular value.
However, other parts of a BIER-TE deployment may need a BFR-id, However, other parts of a BIER-TE deployment may need a BFR-id --
specifically multicast flow overlay signaling and multicast flow specifically, multicast flow overlay signaling and multicast flow
overlay packet disposition, and in that case BFRs need to also have overlay packet disposition; in that case, BFRs need to also have BFR-
BFR-ids for BIER-TE SDs. ids for BIER-TE SDs.
For example, for BIER overlay signaling, BFIRs need to have a BFR-id, For example, for BIER overlay signaling, BFIRs need to have a BFR-id,
because this BFIR BFR-id is carried in the BFIR-id field of the BIER because this BFIR BFR-id is carried in the BFIR-id field of the BIER
header to indicate to the overlay signaling on the receiving BFER header to indicate to the overlay signaling on the receiving BFER
which BFIR originated the packet. which BFIR originated the packet.
In BIER, BFR-id = SI * BSL + BP, such that the SI and BP of a BFER In BIER, BFR-id = SI * BSL + BP, such that the SI and BP of a BFER
can be calculated from the BFR-id and vice versa. This also means can be calculated from the BFR-id and vice versa. This also means
that every BFR with a BFR-id has a reserved BP in an SI, even if that that every BFR with a BFR-id has a reserved BP in an SI, even if that
is not necessary for BIER forwarding, because the BFR may never be a is not necessary for BIER forwarding, because the BFR may never be a
BFER but only a BFIR. BFER (i.e., will only be a BFIR).
In BIER-TE, for a non-leaf BFER, there is usually a single BP for In BIER-TE, for a non-leaf BFER, there is usually a single BP for
that BFER with a local_decap() adjacency on the BFER. The BFR-id for that BFER with a local_decap() adjacency on the BFER. The BFR-id for
such a BFER can therefore be determined using the same procedure as such a BFER can therefore be determined using the same procedure as
in (non-TE) BIER: BFR-id = SI * BSL + BP. that used for (non-TE) BIER: BFR-id = SI * BSL + BP.
As explained in Section 5.1.3, leaf BFERs do not need such a unique As explained in Section 5.1.3, leaf BFERs do not need such a unique
local_decap() adjacency. Likewise, BFIRs that are not also BFERs may local_decap() adjacency. Likewise, BFIRs that are not also BFERs may
not have a unique local_decap() adjacency either. For all those not have a unique local_decap() adjacency either. For all those
BFIRs and (leaf) BFERs, the controller needs to determine unique BFR- BFIRs and (leaf) BFERs, the controller needs to determine unique BFR-
ids that do not collide with the BFR-ids derived from the non-leaf ids that do not collide with the BFR-ids derived from the non-leaf
BFER local_decap() BPs. BFER local_decap() BPs.
While this document defines no requirements on how to allocate such While this document defines no requirements on how to allocate such
BFR-id, a simple option is to derive it from the (SI,BP) of an BFR-ids, a simple option is to derive it from the (SI,BP) of an
adjacency that is unique to the BFR in question. For a BFIR this can adjacency that is unique to the BFR in question. For a BFIR, this
be the first adjacency only populated on this BFIR, for a leaf-BFER, can be the first adjacency that is only populated on this BFIR; for a
this could be the first BP with an adjacency towards that BFER. leaf BFER, this could be the first BP with an adjacency towards that
BFER.
5.3.4. Mapping from BFR to BitStrings with BIER-TE 5.3.4. Mapping from BFRs to BitStrings with BIER-TE
In BIER, applications of the flow overlay on a BFIR can calculate the In BIER, applications of the flow overlay on a BFIR can calculate the
(SI,BP) of a BFER from the BFR-id of the BFER and can therefore (SI,BP) of a BFER from the BFR-id of the BFER and can therefore
easily determine the BitStrings for a BIER packet to a set of BFERs easily determine the BitStrings for a BIER packet to a set of BFERs
with known BFR-ids. with known BFR-ids.
In BIER-TE this mapping needs to be equally supported for flow In BIER-TE, this mapping needs to be equally supported for flow
overlays. This section outlines two core options, based on what type overlays. This section outlines two core options, based on what type
of Tree Engineering the BIER-TE controller needs to performs for a of Tree Engineering the BIER-TE controller needs to perform for a
particular application. particular application.
"Independent branches": For a given flow overlay instance, the "Independent branches": For a given flow overlay instance, the
branches from a BFIR to every BFER are calculated by the BIER-TE branches from a BFIR to every BFER are calculated by the BIER-TE
controller to be independent of the branches to any other BFER. controller to be independent of the branches to any other BFER.
Shortest path trees are the most common examples of trees with Shortest path trees are the most common examples of trees with
independent branches. independent branches.
"Interdependent branches": When a BFER is added or deleted from a "Interdependent branches": When a BFER is added to or deleted from a
particular distribution tree, the BIER-TE controller has to particular distribution tree, the BIER-TE controller has to
recalculate the branches to other BFER, because they may need to recalculate the branches to other BFERs, because they may need to
change. Steiner trees are examples of interdependent branch trees. change. Steiner trees are examples of interdependent branch
trees.
If "independent branches" are used, the BIER-TE Controller can signal If "independent branches" are used, the BIER-TE controller can signal
to the BFIR flow overlay for every BFER an SI:BitString that to the BFIR flow overlay for every BFER an SI:BitString that
represents the branch to that BFER. The flow overlay on the BIFR can represents the branch to that BFER. The flow overlay on the BFIR can
then independently of the controller calculate the SI:BitString for then, independently of the controller, calculate the SI:BitString for
all desired BFERs by OR'ing their BitStrings. This allows for flow all desired BFERs by ORing their BitStrings. This allows flow
overlay applications to operate independently of the controller overlay applications to operate independently of the controller
whenever it needs to determine which subset of BFERs need to receive whenever they need to determine which subset of BFERs needs to
a particular packet. receive a particular packet.
If "interdependent branches" are required, the application would need If "interdependent branches" are required, an application would need
to inquire the SI:BitString for a given set of BFER whenever the set to query the SI:BitString for a given set of BFERs whenever the set
changes. changes.
Note that in either case (unlike in BIER), the bits may need to Note that in either case (unlike the scenario for BIER), the bits may
change upon link/node failure/recovery, network expansion and network need to change upon link/node failure/recovery, network expansion, or
resource consumption by other traffic as part of traffic engineering network resource consumption by other traffic as part of achieving
goals (e.g.: re-optimization of lower priority traffic flows). Traffic Engineering goals (e.g., reoptimization of lower-priority
Interactions between such BFIR applications and the BIER-TE traffic flows). Interactions between such BFIR applications and the
Controller do therefore need to support dynamic updates to the BIER-TE controller do therefore need to support dynamic updates to
SI:BitStrings. the SIs:BitStrings.
Communications between the BFIR flow overlay and the BIER-TE Communications between the BFIR flow overlay and the BIER-TE
controller requires some way to identify the BFER. If BFR-ids are controller require some way to identify the BFERs. If BFR-ids are
used in the deployment, as outlined in Section 5.3.3, then those are used in the deployment, as outlined in Section 5.3.3, then those are
the natural BFR identifier. If BFR-ids are not used, then any other the "natural" BFR-ids. If BFR-ids are not used, then any other
unique identifier, such as the BFR-prefix of the BFR ([RFC8279]) unique identifier, such as a BFR's BFR-prefix [RFC8279], could be
could be used. used.
5.3.5. Assigning BFR-ids for BIER-TE 5.3.5. Assigning BFR-ids for BIER-TE
It is not currently determined if a single sub-domain could or should It is not currently determined if a single subdomain could or should
be allowed to forward both (non-TE) BIER and BIER-TE packets. If be allowed to forward both (non-TE) BIER and BIER-TE packets. If
this should be supported, there are two options: this should be supported, there are two options:
A. BIER and BIER-TE have different BFR-id in the same sub-domain. A. BIER and BIER-TE have different BFR-ids in the same subdomain.
This allows higher replication efficiency for BIER because their BFR- This allows higher replication efficiency for BIER because the
id can be assigned sequentially, while the BitStrings for BIER-TE BIER BFR-ids can be assigned sequentially, while the BitStrings
will have also the additional bits for the topology. There is no for BIER-TE will also have to assign the additional bits for the
relationship between a BFR BIER BFR-id and its BIER-TE BFR-id. topology adjacencies. There is no relationship between a BFR
BIER BFR-id and its BIER-TE BFR-id.
B. BIER and BIER-TE share the same BFR-id. The BFR-ids are assigned B. BIER and BIER-TE share the same BFR-id. The BFR-ids are assigned
as explained above for BIER-TE and simply reused for BIER. The as explained above for BIER-TE and simply reused for BIER. The
replication efficiency for BIER will be as low as that for BIER-TE in replication efficiency for BIER will be as low as that for BIER-
this approach. TE in this approach.
5.3.6. Example bit allocations 5.3.6. Example Bit Allocations
5.3.6.1. With BIER 5.3.6.1. With BIER
Consider a network setup with a BSL of 256 for a network topology as Consider a network setup with a BSL of 256 for a network topology as
shown in Figure 17. The network has 6 areas, each with 170 BFERs, shown in Figure 17. The network has six areas, each with 170 BFERs,
connecting via a core with 4 (core) BFRs. To address all BFERs with connecting via a core with four (core) BFRs. To address all BFERs
BIER, 4 SIs are required. To send a BIER packet to all BFER in the with BIER, four SIs are required. To send a BIER packet to all BFERs
network, 4 copies need to be sent by the BFIR. On the BFIR it does in the network, four copies need to be sent by the BFIR. On the
not make a difference how the BFR-ids are allocated to BFER in the BFIR, it does not matter how the BFR-ids are allocated to BFERs in
network, but for efficiency further down in the network it does make the network, but it does matter for efficiency further down in the
a difference. network.
area1 area2 area3 area1 area2 area3
BFR1a BFR1b BFR2a BFR2b BFR3a BFR3b BFR1a BFR1b BFR2a BFR2b BFR3a BFR3b
| \ / \ / | | \ / \ / |
................................ ................................
. Core . . Core .
................................ ................................
| / \ / \ | | / \ / \ |
BFR4a BFR4b BFR5a BFR5b BFR6a BFR6b BFR4a BFR4b BFR5a BFR5b BFR6a BFR6b
area4 area5 area6 area4 area5 area6
Figure 17: Scaling BIER-TE bits by reuse Figure 17: Scaling BIER-TE Bits by Reuse
With random allocation of BFR-id to BFER, each receiving area would With random allocation of BFR-ids to BFERs, each receiving area would
(most likely) have to receive all 4 copies of the BIER packet because (most likely) have to receive all four copies of the BIER packet
there would be BFR-id for each of the 4 SIs in each of the areas. because there would be BFR-ids for each of the four SIs in each of
Only further towards each BFER would this duplication subside - when the areas. Only further towards each BFER would this duplication
each of the 4 trees runs out of branches. subside -- when each of the four trees runs out of branches.
If BFR-ids are allocated intelligently, then all the BFER in an area If BFR-ids are allocated intelligently, then all the BFERs in an area
would be given BFR-id with as few as possible different SIs. Each would be given BFR-ids with as few different SIs as possible. Each
area would only have to forward one or two packets instead of 4. area would only have to forward one or two packets instead of four.
Given how networks can grow over time, replication efficiency in an Given how networks can grow over time, replication efficiency in an
area will then also go down over time when BFR-ids are only allocated area will then also go down over time when BFR-ids are only allocated
sequentially, network wide. An area that initially only has BFR-id sequentially, network wide. An area that initially only has BFR-ids
in one SI might end up with many SIs over a longer period of growth. in one SI might end up with many SIs over a longer period of growth.
Allocating SIs to areas with initially sufficiently many spare bits Allocating SIs to areas that initially have sufficiently many spare
for growths can help to alleviate this issue. Or renumber BFERs bits for growth can help alleviate this issue. Alternatively, BFERs
after network expansion. In this example one may consider to use 6 can be renumbered after network expansion. In this example, one may
SIs and assign one to each area. consider using six SIs and assigning one to each area.
This example shows that intelligent BFR-id allocation within at least This example shows that intelligent BFR-id allocation within at least
sub-domain 0 can even be helpful or even necessary in BIER. subdomain 0 can be helpful or even necessary in BIER.
5.3.6.2. With BIER-TE 5.3.6.2. With BIER-TE
In BIER-TE one needs to determine a subset of the physical topology In BIER-TE, one needs to determine a subset of the physical topology
and attached BFERs so that the "desired" representation of this and attached BFERs so that the "desired" representation of this
topology and the BFER fit into a single BitString. This process topology and the BFERs fit into a single BitString. This process
needs to be repeated until the whole topology is covered. needs to be repeated until the whole topology is covered.
Once bits/SIs are assigned to topology and BFERs, BFR-id is just a Once bits/SIs are assigned to the topology and BFERs, BFR-ids are
derived set of identifiers from the operator/BIER-TE Controller as just a derived set of identifiers from the operator / BIER-TE
explained above. controller as explained above.
Every time that different sub-topologies have overlap, bits need to Whenever different subtopologies have overlap, bits need to be
be repeated across the BitStrings, increasing the overall amount of repeated across the BitStrings, increasing the overall amount of bits
bits required across all BitString/SIs. In the worst case, one required across all BitStrings/SIs. In the worst case, one assigns
assigns random subsets of BFERs to different SIs. This will result random subsets of BFERs to different SIs. This will result in an
in an outcome much worse than in (non-TE) BIER: It maximizes the outcome much worse than in (non-TE) BIER: it maximizes the amount of
amount of unnecessary topology overlap across SI and therefore unnecessary topology overlap across SIs and therefore reduces the
reduces the number of BFER that can be reached across each individual number of BFERs that can be reached across each individual SI.
SI. Intelligent BFER to SI assignment and selecting specific Intelligent BFER-to-SI assignment and selecting specific "desired"
"desired" subtopologies can minimize this problem. subtopologies can minimize this problem.
To set up BIER-TE efficiently for the topology of Figure 17, the To set up BIER-TE efficiently for the topology shown in Figure 17,
following bit allocation method can be used. This method can easily the following bit allocation method can be used. This method can
be expanded to other, similarly structured larger topologies. easily be expanded to other, similarly structured larger topologies.
Each area is allocated one or more SIs depending on the number of Each area is allocated one or more SIs, depending on the number of
future expected BFERs and number of bits required for the topology in future expected BFERs and the number of bits required for the
the area. In this example, 6 SIs, one per area. topology in the area. In this example, six SIs are used, one per
area.
In addition, we use 4 bits in each SI: bia, bib, bea, beb: (b)it In addition, we use four bits in each SI:
(i)ngress (a), (b)it (i)ngress (b), (b)it (e)gress (a), (b)it
(e)gress (b). These bits will be used to pass BIER packets from any
BFIR via any combination of ingress area a/b BFR and egress area a/b
BFR into a specific target area. These bits are then set up with the
right forward_routed() adjacencies on the BFIR and area edge BFR:
On all BFIRs in an area j|j=1...6, bia in each BIFT:SI is populated bia: (b)it (i)ngress (a)
with the same forward_routed(BFRja), and bib with
forward_routed(BFRjb). On all area edge BFR, bea in bib: (b)it (i)ngress (b)
bea: (b)it (e)gress (a)
beb: (b)it (e)gress (b)
These bits will be used to pass BIER packets from any BFIR via any
combination of ingress area a/b BFRs and egress area a/b BFRs into a
specific target area. These bits are then set up with the right
forward_routed() adjacencies on the BFIRs and area edge BFRs as
follows.
On all BFIRs in an area, j|j=1...6, bia in each BIFT:SI is populated
with the same forward_routed(BFRja) and bib with
forward_routed(BFRjb). On all area edge BFRs, bea in
BIFT:SI=k|k=1...6 is populated with forward_routed(BFRka) and beb in BIFT:SI=k|k=1...6 is populated with forward_routed(BFRka) and beb in
BIFT:SI=k with forward_routed(BFRkb). BIFT:SI=k with forward_routed(BFRkb).
For BIER-TE forwarding of a packet to a subset of BFERs across all For BIER-TE forwarding of a packet to a subset of BFERs across all
areas, a BFIR would create at most 6 copies, with SI=1...SI=6, In areas, a BFIR would create at most six copies, with SI=1...SI=6. In
each packet, the bits indicate bits for topology and BFER in that each packet, the BitString includes bits for one area and the BFERs
topology plus the four bits to indicate whether to pass this packet in that area, plus the four bits to indicate whether to pass this
via the ingress area a or b border BFR and the egress area a or b packet via the ingress area a or b border BFR and the egress area a
border BFR, therefore allowing path steering for those two "unicast" or b border BFR, therefore allowing path steering for those two
legs: 1) BFIR to ingress area edge and 2) core to egress area edge. "unicast" legs: 1) BFIR to ingress area edge and 2) core to egress
Replication only happens inside the egress areas. For BFER in the area edge. Replication only happens inside the egress areas. For
same area as in the BFIR, these four bits are not used. BFERs that are in the same area as the BFIR, these four bits are not
used.
5.3.7. Summary 5.3.7. Summary
BIER-TE can, like BIER, support multiple SIs within a sub-domain. BIER-TE can, like BIER, support multiple SIs within a subdomain.
This allows to apply the mapping BFR-id = SI * BSL + BP. This allows This allows application of the mapping BFR-id = SI * BSL + BP. This
to re-use the BIER architecture concept of BFR-id and therefore also permits the reuse of the BIER architecture concept of BFR-ids
minimize BIER-TE specific functions in possible BIER layer control and, therefore, minimization of BIER-TE-specific functions in
plane mechanisms with BIER-TE, including flow overlay methods and possible BIER layer control plane mechanisms with BIER-TE, including
BIER header fields. flow overlay methods and BIER header fields.
The number of BFIR/BFER possible in a sub-domain is smaller than in The number of BFIRs/BFERs possible in a subdomain is smaller than in
BIER because BIER-TE uses additional bits for topology. BIER because BIER-TE uses additional bits for the topology.
Sub-domains (SDs) in BIER-TE can be used like in BIER to create more Subdomains in BIER-TE can be used as they are in BIER to create more
efficient replication to known subsets of BFERs. efficient replication to known subsets of BFERs.
Assigning bits for BFERs intelligently into the right SI is more Assigning bits for BFERs intelligently into the right SI is more
important in BIER-TE than in BIER because of replication efficiency important in BIER-TE than in BIER because of replication efficiency
and overall amount of bits required. and the overall amount of bits required.
6. Security Considerations 6. Security Considerations
If [RFC8296] is used, BIER-TE shares its security considerations. If "Encapsulation for Bit Index Explicit Replication (BIER) in MPLS
and Non-MPLS Networks" [RFC8296] is used, its security considerations
also apply to BIER-TE.
BIER-TE shares the security considerations of BIER, [RFC8279], with The security considerations of "Multicast Using Bit Index Explicit
the following overriding or additional considerations. Replication (BIER)" [RFC8279] also apply to BIER-TE, with the
following overriding or additional considerations.
BIER-TE forwarding explicitly supports unicast "tunneling" of BIER BIER-TE forwarding explicitly supports unicast "tunneling" of BIER
packets via forward_routed() adjacencies. The BIER domain security packets via forward_routed() adjacencies. The BIER domain security
model is based on a subset of interfaces on a BFR that connect to model is based on a subset of interfaces on a BFR that connect to
other BFRs of the same BIER domain. For BIER-TE, this security model other BFRs of the same BIER domain. For BIER-TE, this security model
equally applies to such unicast "tunneled" BIER packets. This does equally applies to such unicast "tunneled" BIER packets. This not
not only include the need to filter received unicast "tunneled" BIER only includes the need to filter received unicast "tunneled" BIER
packets to prohibit injection of such "tunneled" BIER packets from packets to prohibit the injection of such "tunneled" BIER packets
outside the BIER domain, but also prohibiting forward_routed() from outside the BIER domain but also the need to prohibit
adjacencies to leak BIER packets from the BIER domain. It SHOULD be forward_routed() adjacencies from leaking BIER packets from the BIER
possible to configure interfaces to be part of a BIER domain solely domain. It SHOULD be possible to configure interfaces to be part of
for sending and receiving of unicast "tunneled" BIER packets even if a BIER domain solely for sending and receiving unicast "tunneled"
the interface can not send/receive BIER encapsulated packets. BIER packets even if the interface cannot send/receive BIER
encapsulated packets.
In BIER, the standardized methods for the routing underlays are IGPs In BIER, the standardized methods for the routing underlays are IGPs
with extensions to distribute BFR-ids and BFR-prefixes. [RFC8401] with extensions to distribute BFR-ids and BFR-prefixes. [RFC8401]
specifies the extensions for IS-IS and [RFC8444] specifies the specifies the extensions for IS-IS, and [RFC8444] specifies the
extensions for OSPF. Attacking the protocols for the BIER routing extensions for OSPF. Attacking the protocols for the BIER routing
underlay or (non-TE) BIER layer control plane, or impairment of any underlay or (non-TE) BIER layer control plane, or the impairment of
BFR in a domain may lead to successful attacks against the results of any BFRs in a domain, may lead to successful attacks against the
the routing protocol, enabling DoS attacks against paths or the information that BIER-TE learns from the routing protocol (routes,
addressing (BFR-id, BFR-prefixes) used by BIER. next hops, BFR-ids, ...), enabling DoS attacks against paths or the
addressing (BFR-ids, BFR-prefixes) used by BIER.
The reference model for the BIER-TE layer control plane is a BIER-TE The reference model for the BIER-TE layer control plane is a BIER-TE
controller. When such a controller is used, impairment of an controller. When such a controller is used, the impairment of an
individual BFR in a domain causes no impairment of the BIER-TE individual BFR in a domain causes no impairment of the BIER-TE
control plane on other BFRs. If a routing protocol is used to control plane on other BFRs. If a routing protocol is used to
support forward_routed() adjacencies, then this is still an attack support forward_routed() adjacencies, then this is still an attack
vector as in BIER, but only for BIER-TE forward_routed() adjacencies, vector as in BIER, but only for BIER-TE forward_routed() adjacencies
and not other adjacencies. and not other adjacencies.
Whereas IGP routing protocols are most often not well secured through Whereas IGP routing protocols are most often not well secured through
cryptographic authentication and confidentiality, communications cryptographic authentication and confidentiality, communications
between controllers and routers such as those to be considered for between controllers and routers such as those to be considered for
the BIER-TE controller/control-plane can be and are much more the BIER-TE controller / control plane can be, and are, much more
commonly secured with those security properties, for example by using commonly secured with those security properties -- for example, by
Secure SHell (SSH), [RFC4253] for NETCONF ([RFC6242]), or via using "Secure Shell" (SSH) [RFC4253] for NETCONF [RFC6242]; or via
Transport Layer Security (TLS), such as [RFC8253] for PCEP, "Transport Layer Security" (TLS), such as [RFC8253] for PCEP
[RFC5440], or [RFC7589] for NETCONF. BIER-TE controllers SHOULD use [RFC5440] or [RFC7589] for NETCONF. BIER-TE controllers SHOULD use
security equal to or better than these mechanisms. security equal to or better than these mechanisms.
When any of these security mechanisms/protocols are used for When any of these security mechanisms/protocols are used for
communications between a BIER-TE controller and BFRs, their security communications between a BIER-TE controller and BFRs, their security
considerations apply to BIER-TE. In addition, the security considerations apply to BIER-TE. In addition, the security
considerations of PCE, [RFC4655] apply. considerations of "A Path Computation Element (PCE)-Based
Architecture" [RFC4655] apply.
The most important attack vector in BIER-TE is misconfiguration, The most important attack vector in BIER-TE is misconfiguration,
either on the BFR themselves or via the BIER-TE controller. either on the BFRs themselves or via the BIER-TE controller.
Forwarding entries with DNC could be set up to create persistent Forwarding entries with DNC could be set up to create persistent
loops, in which packets only expire because of TTL. To minimize the loops, in which packets only expire because of TTL. To minimize the
impact of such attacks (or more likely unintentional misconfiguration impact of such attacks (or, more likely, unintentional
by operators and/or bad BIER-TE controller software), the BIER-TE misconfiguration by operators and/or bad BIER-TE controller
forwarding rules are defined to be as strict in clearing bits as software), the BIER-TE forwarding rules are defined to be as strict
possible. The clearing of all bits with an adjacency on a BFR in clearing bits as possible. The clearing of all bits with an
prohibits that a looping packet creates additional packet adjacency on a BFR prohibits a looping packet from creating
amplification through the misconfigured loop on the packet's second additional packet amplification through the misconfigured loop on the
or further times around the loop, because all relevant adjacency bits packet's second time or subsequent times around the loop, because all
would have been cleared on the first round through the loop. In relevant adjacency bits would have been cleared on the first round
result, BIER-TE has the same degree of looping packets as possible through the loop. As a result, looping packets can occur in BIER-TE
with unintentional or malicious loops in the routing underlay with to the same degree as is possible with unintentional or malicious
BIER or even with unicast traffic. loops in the routing underlay with BIER, or even with unicast
traffic.
Deployments where BIER-TE would likely be beneficial may include Deployments where BIER-TE would likely be beneficial may include
operational models where actual configuration changes from the operational models where actual configuration changes from the
controller are only required during non-production phases of the controller are only required during non-production phases of the
network's life-cycle, such as in embedded networks or in network's life cycle, e.g., in embedded networks or in manufacturing
manufacturing networks during e.g. plant reworking/repairs. In these networks during such activities as plant reworking or repairs. In
type of deployments, configuration changes could be locked out when these types of deployments, configuration changes could be locked out
the network is in production state and could only be (re-)enabled when the network is in production state and could only be
through reverting the network/installation into non-production state. (re-)enabled through reverting the network/installation to non-
Such security designs would not only allow to provide additional production state. Such security designs would not only allow a
layers of protection against configuration attacks, but would deployment to provide additional layers of protection against
foremost protect the active production process from such configuration attacks but would, first and foremost, protect the
configuration attacks. active production process from such configuration attacks.
7. IANA Considerations 7. IANA Considerations
This document requests no action by IANA. This document has no IANA actions.
8. Acknowledgements
The authors would like to thank Greg Shepherd, Ijsbrand Wijnands,
Neale Ranns, Dirk Trossen, Sandy Zheng, Lou Berger, Jeffrey Zhang,
Carsten Borman and Wolfgang Braun for their reviews and suggestions.
Special thanks to Xuesong Geng for shepherding the document and for
IESG review/suggestions by Alvaro Retana (responsible AD/RTG),
Benjamin Kaduk (SEC), Tommy Pauly (TSV), Zaheduzzaman Sarker (TSV),
Eric Vyncke (INT), Martin Vigoureux (RTG), Robert Wilton (OPS), Eric
Kline (INT), Lars Eggert (GEN), Roman Danyliv (SEC), Ines Robles
(RTGDIR), Robert Sparks (Gen-ART), Yingzhen Qu (RTGdir), Martin Duke
(TSV).
9. Change log [RFC Editor: Please remove]
draft-ietf-bier-te-arch:
13:
Changed Gregs author association/email.
Fixed Nits in -12 from Ben Kaduk.
Fixed Alvaro's concerns: (1) Removed references to SR in Abstract/
Overview (2) removed section 4.5.
12:
AD review Alvaro Retana.
Various textual/editorial nits including adding () to all
instances of forwarding adjacency name instances.
3.1 Added new paragraph outlining possible use of BGP as RR in
BIER-TE controller as core of multicast flow overlay component of
BIER-TE.
3.2 added xref's to relevant sections to the listed control plane
points.
4.1 rewrote paragraphs of 4.1 leading up to Figure 4. to eliminate
any confusion in how the BIFT work and how it compares to the
notions in rfc8279, as well as better linking it to the
Pseudocode.
Moved SR section into appendix.
TSV review Martin Duke.
Text/editorial nits.
4.4 improved text describing handling of F-BM.
RTGdir review Yingzhen Qu.
Various text/editorial nits.
Added notion that BitStrings represent loop free tree for packet
to abstract and intro.
Various text nit and editorial improvements.
Fixed some BFR-id field -> BFIR-id field mistakes.
Capitalized NETCONF/RESTCONF/YANG, added RFC references.
Improved Figure 16 with explicitly two links into BFR3 and
explanatory text.
Gen-ART review Robert Sparks.
Various textual nits, editorial improvements.
3.2 Introduced terms "BIER-TE topology control" and "BIER-TE tree
control" for the two functional components of the control plane.
3.2.1 - 3.2 change introduces the open RFC-editor issue of
appropriate xrfs (to be resolved by RFC-editor).
3.3 Rewrote last paragraph to better describe loop prevention
through clearing of bits in BitString.
4.1 Fixed up text/formula describing mapping between bfr-id, SI:BP
and SI,BSL and BP. Fix offset bug.
5.3.6.2 Improved description paragraph explaining overlap of
topology for different SI.
5.3.7 Improved first summary paragraph.
7. Rephrased applicability statement of control plane protocol
security considerations to BIER-TE security.
RTGDIR review Ines Robles.
Fixed up adjacencies in Example 2 and explanation text to be
explicit about which BFR not only passes, but also receives the
packet.
7. (security considerations). Added paragraph about
forward_routed() and prohibiting BIER packet leaking in/out of
domain.
IESG review Roman Danyliv (SEC).
Several textual/sentence nits/editorials.
IESG review Lars Eggert (GEN).
Various good editorial word fixed.
Pointer to non-false-positive bloom filter work that looks like it
happened after our IETF discussions documented in this doc, so
will not add it to doc, but here is URL for folks interested:
https://ieeexplore.ieee.org/document/8486415.
Did not change "native" to a different word for inclusivity
because of my worry there is no estavblished single replacement
word, making reading/searching/understanding more difficult.
IESG review Martin Vigeureux (RTG).
Added back reference to RFC8402. Textual fixes.
IESG review Eric Kline (INT).
2.1 Fixed typo in BFR* explanations.
4.3 Added explanatio about MTU handling.
IESG review Eric Vyncke (INT).
Fixed up initial text to introduce various abbreviations.
2.4 refined wording to "with the _intent_ to easily build common
forwarding planes...".
4.2.3 refined text about entropy in ECMP - now taken text from
rfc8279.
IESG review Zaheduzzaman Sarker (TSV).
5.1.7 Refined text explaining documentation of ECMP algorithm.
5.3.6.2. fixed range of areas/SI over which to build the example
large network BPs - removed explanation of the large network shown
to be only used for sources in area 1 (IPTV), because it was a
stale explanation.
IESG review Ben Kaduk (round 2):
4.4 Advanced pseudocode still had one wrong "~". Root cause seems
to have been day 0 problem in pseudocode written for -01, "~" was
inserted in the wrong one of two code lines. Also enhanced
textual description and comments in pseudocode, changed variable
name AdjacentBits to PktAdjacentBits to avoid confusion with
AdjacentBits[SI].
5.1.3 Rewrote last two paragraphs explaining the sharing of bit
positions for lead-BFER hopefully better. Also detailled how it
interacts with other optimizations and the type of payload BIER-TE
packets may carry.
4.4 (from Carsten Borman) changed spacing in pseudocode to be
consistent. Fixed {VRF}, clarified pseudocode object syntax,
typos.
11: IESG review Ben Kaduk, summary:
One discuss for bug in pseudocode. turned out to be one cahrcter
typo.
Added (non-TE) prefix in places where BIER by itsels had to be
better disambiguated.
enhanced text for hub-and-spoke to indicate we're only talking
about hub to spoke traffic.
long list ot language fixes/improvement (nits). Thanks a lot!.
add suggestion to SHOULD use known confidentiality protocols
between controller and BFR.
10: AD review Alvaro Retana, summary:
Note: rfcdiff shows more changes than actually exist because text
moved around.
Summary:
1. restructuring: merged all controller sections under common
controller ops main section, moved unfitting stuff out to
other parts of doc. Split Intro section into Overview and
Intro. Shortened Abstract, moved text into Overview, added
sections overview.
2. enhanced/rewrote: 2.3 Comparison with -> Relationship to BIER-
TE
3. enhanced/rewrote: 3.2 BIER-TE controller -> BIER-TE control
plane, 3.2.1 BIER-TE controller, for consistency with rfc8279
4. additional subsections for Alvaros asks
5. added to: 3.3 BIER-TE forwarding plane (consistency with
rfc8279)
6. Enhanced description of 4.3/encap considerations to better
explain how BIER/BIER-TE can run together.
Notation: Markers (a),(b),... at end of points are references from
the review discussion with Alvaro to the changes made.
Details:.
Throughout text: changed term spelling to rfc8279 - bit positions,
sub-domain, ... (i).
Reset changed to clear, also DNR changed to DNC (Do Not Clear)
(q).
Abstract: Shortened. Removed name explanation note (Tree
Engineering), (a).
1. Introduction -> Overview: Moved important explanation
paragraph from abstract to Introduction. Fixed text, (a).
Added bullet point list explanation of structure of document (e).
Renamed to Overview because that is now more factually correct.
1.1. Fixed bug in example adding bit p15.(l).
2. (New - Introduction): Moved section 1.1 - 1.3 (examples,
comparison with BIER-TE) from Introduction into new "Overview"
section. Primarily so that "requirements language" section (at
end of Introduction) is not in middle of document after all the
Introduction.
2.1 Removed discussion of encap, moved to 4.2.2 (m).
2.2 enhanced paragraph suggesting native/overlay topology types,
also sugest type hybrid (n).
2.3 Overhauled comparison text BIER/BIER-TE, structured into
common, different, not-required-by-te, integration-bier-bier-te.
Changed title to "Relationship" to allow including last point.
(f).
2.4 moved Hardware forwarding comparison section into section 2 to
allow coalescing of sections into section 5 about the controller
operations (hardware forwarding was in the middle of it, wrong
place). Shortened/improved third paragraph by pointing to BIFT as
deciding element for selection between BIER/BIER-TE. Removed
notion of experimentation (this now targets standard) (g).
3. (Components): Aligned component name and descriptions better
with RFC8279. Now describe exactly same three layers. BIER layer
constituted from BIER-TE control plane and BIER-TE forwarding
plane. BIER-TE controller is now simply component of BIER-TE
control plane. (b).
3.1. shortened/improved paragraph explaining use of SI:BP instead
of also bfr-id as index into BIFT, rewrote paragraph talking about
reuse of BPs(o).
3.2. rewrote explanation of BIER-TE control plane in the style of
RFC8729 Section 4.2 (BIER layer) with numbered points. Note that
RFC8729 mixes control and forwarding plane bullet points (this doc
does not). Merged text from old sections 2.2.1 and 2.2.3 into
list. (b).
3.2.1. Expanded/improved explanation of BIER-TE Controller (b).
3.2.1.1. Added subsection for topology discovery and creation
(d).
3.2.1.2. Added subsection for engineered BitStrings as key novel
aspect not found in BIER. (X).
3.3. Added numbered list for components of BIER-TE forwarding
plane (completing the comparable text from RFC8729 Section 4.2).
3.4 Alvaro does not mind additional example, fixed bugs.
3.5 Removed notion about using IGP BIER extensions for BIER-TE,
such as BIFT address ranges. After -10 making use of BIFT
clearer, it now looks to authors as if use of IGP extensions would
not be beneficial, as long as we do need to use the BIER-TE
controller, e.g. unlike in BIER, a BFR could not learn from the
IGP information what traffic to send towards a particular BIFT-ID,
but instead that is the core of what the controller needs to
provide.
4.2.2 Improved text to explain requirement to identify BIER-TE in
the tunnel encap and compress description of use-cases (m).
4.2.3 enhanced ECMP text (p).
4.3. rewrote most of Encapsulation Considerations to better
explain to Alvaros question re sharing or not sharing SD via BIER/
BIER-TE. Added reference to I-D.ietf-bier-non-mpls-bift-encoding
as a very helpful example. (f).
4.3 Renamed title to "...Co-Existence with BIER" as this is what
it is about and to help finding it from abstract/intro ("co-
exist") (j).
4.4. Moved BIER-TE Forwarding Pseudocode here to coalesce text
logically. Changed text to better compare with BIER pseudo
forwarding code. Numerical list of how F-BM works for BIER-TE.
Removed efficiency comparison with BIER (too difficult to provide
sufficient justification, derails from focus of section) (j).
4.6. (Requirements) Restructured: Removed notion of "basic" BIER-
TE forwarding, simply referring to it now as "mandatory" BIER-TE
forwarding. Cleaned up text to have requirements for different
adjacencies in different paragraphs. (c).
5. Created new main section "BIER-TE Controller operational
considerations", coalesced old sections 4., 5., 7. into this new
main section. No text changes. (k).
5.1.9 Added new separate picture instead of referring to a picture
later in text, adjusted text (r).
5.3.2 Changed title to not include word "comparison" to avoid this
being accounted against Alvaros concern about scattering
comparison (IMHO text already has little comparison, so title was
misleading) (h).
co-authors internal review:
4.4 Added xref to Figure 5.
5.2.1 Duplicated ring picture, added visuals for described
miswiring (s).
5.2.2 replace "topology" with graph (wrong word).
5.3.3 rewrote explanation of how to map BFR-id to SI:BP and assign
them, clarified BFR-id is option. Retitled to better explain
scope of section.
5.3.4 Removed considerations in 5.3.4 for sharing BFR-id across
BIER/BIER-TE (t), changed title to explain how BFIR/BIER-TE
controller interactions need some form of identifying BFR but this
does not have to be BFR-id.
7. Added new security considerations (u).
09: Incorporated fixes for feedback from Shepherd (Xuesong Geng).
Added references for Bloom Filters and Rate Controlled Service
Disciplines.
1.1 Fixed numbering of example 1 topology explanation. Improved
language on second example (less abbreviating to avoid confusion
about meaning).
1.2 Improved explanation of BIER-TE topology, fixed terminology of
graphs (BIER-TE topology is a directed graph where the edges are
the adjacencies).
2.4 Fixed and amended routing underlay explanations: detailed why
no need for BFER routing underlay routing protocol extensions, but
potential to re-use BIER routing underlay routing protocol
extensions for non-BFER related extensions.
3.1 Added explanation for VRF and its use in adjacencies.
08: Incorporated (with hopefully acceptable fixes) for Lou
suggested section 2.5, TE considerations.
Fixes are primarily to the point to a) emphasize that BIER-TE does
not depend on the routing underlay unless forward_routed()
adjacencies are used, and b) that the allocation and tracking of
resources does not explicitly have to be tied to BPs, because they
are just steering labels. Instead, it would ideally come from
per-hop resource management that can be maintained only via local
accounting in the controller.
07: Further reworking text for Lou.
Renamed BIER-PE to BIER-TE standing for "Tree Engineering" after
votes from BIER WG.
Removed section 1.1 (introduced by version 06) because not
considered necessary in this doc by Lou (for framework doc).
Added [RFC editor pls. remove] Section to explain name change to
future reviewers.
06: Concern by Lou Berger re. BIER-TE as full traffic engineering
solution.
Changed title "Traffic Engineering" to "Path Engineering"
Added intro section of relationship BIER-PE to traffic
engineering.
Changed "traffic engineering" term in text" to "path engineering",
where appropriate
Other:
Shortened "BIER-TE Controller Host" to "BIER-TE Controller".
Fixed up all instances of controller to do this.
05: Review Jeffrey Zhang.
Part 2:
4.3 added note about leaf-BFER being also a propery of routing
setup.
4.7 Added missing details from example to avoid confusion with
routed adjacencies, also compressed explanatory text and better
justification why seed is explicitly configured by controller.
4.9 added section discussing generic reuse of BP methods.
4.10 added section summarizing BP optimizations of section 4.
6. Rewrote/compressed explanation of comparison BIER/BIER-TE
forwarding difference. Explained benefit of BIER-TE per-BP
forwarding being independent of forwarding for other BPs.
Part 1:
Explicitly ue forwarded_connected adjcency in ECMP adjcency
examples to avoid confusion.
4.3 Add picture as example for leav vs. non-leaf BFR in topology.
Improved description.
4.5 Exampe for traffic that can be broadcast -> for single BP in
hub&spoke.
4.8.1 Simplified example picture for routed adjacency, explanatory
text.
Review from Dirk Trossen:
Fixed up explanation of ICC paper vs. bloom filter.
04: spell check run.
Addded remaining fixes for Sandys (Zhang Zheng) review:
4.7 Enhance ECMP explanations:
example ECMP algorithm, highlight that doc does not standardize
ECMP algorithm.
Review from Dirk Trossen:
1. Added mentioning of prior work for traffic engineered paths
with bloom filters.
2. Changed title from layers to components and added "BIER-TE
control plane" to "BIER-TE Controller" to make it clearer, what it
does.
2.2.3. Added reference to I-D.ietf-bier-multicast-http-response
as an example solution.
2.3. clarified sentence about resetting BPs before sending copies
(also forgot to mention DNR here).
3.4. Added text saying this section will be removed unless IESG
review finds enough redeeming value in this example given how -03
introduced section 1.1 with basic examples.
7.2. Removed explicit numbers 20%/80% for number of topology bits
in BIER-TE, replaced with more vague (high/low) description,
because we do not have good reference material Added text saying
this section will be removed unless IESG review finds enough
redeeming value in this example given how -03 introduced section
1.1 with basic examples.
many typos fixed. Thanks a lot.
03: Last call textual changes by authors to improve readability:
removed Wolfgang Braun as co-authors (as requested).
Improved abstract to be more explanatory. Removed mentioning of
FRR (not concluded on so far).
Added new text into Introduction section because the text was too
difficult to jump into (too many forward pointers). This
primarily consists of examples and the early introduction of the
BIER-TE Topology concept enabled by these examples.
Amended comparison to SR.
Changed syntax from [VRF] to {VRF} to indicate its optional and to
make idnits happy.
Split references into normative / informative, added references.
02: Refresh after IETF104 discussion: changed intended status back
to standard. Reasoning:
Tighter review of standards document == ensures arch will be
better prepared for possible adoption by other WGs (e.g. DetNet)
or std. bodies.
Requirement against the degree of existing implementations is self
defined by the WG. BIER WG seems to think it is not necessary to
apply multiple interoperating implementations against an
architecture level document at this time to make it qualify to go
to standards track. Also, the levels of support introduced in -01
rev. should allow all BIER forwarding engines to also be able to
support the base level BIER-TE forwarding.
01: Added note comparing BIER and SR to also hopefully clarify
BIER-TE vs. BIER comparison re. SR.
- added requirements section mandating only most basic BIER-TE
forwarding features as MUST.
- reworked comparison with BIER forwarding section to only
summarize and point to pseudocode section.
- reworked pseudocode section to have one pseudocode that mirrors
the BIER forwarding pseudocode to make comparison easier and a
second pseudocode that shows the complete set of BIER-TE
forwarding options and simplification/optimization possible vs.
BIER forwarding. Removed MyBitsOfInterest (was pure
optimization).
- Added captions to pictures.
- Part of review feedback from Sandy (Zhang Zheng) integrated.
00: Changed target state to experimental (WG conclusion), updated
references, mod auth association.
- Source now on https://www.github.com/toerless/bier-te-arch
- Please open issues on the github for change/improvement requests
to the document - in addition to posting them on the list
(bier@ietf.). Thanks!.
draft-eckert-bier-te-arch:
06: Added overview of forwarding differences between BIER, BIER-
TE.
05: Author affiliation change only.
04: Added comparison to Live-Live and BFIR to FRR section
(Eckert).
04: Removed FRR content into the new FRR draft [I-D.eckert-bier-
te-frr] (Braun).
- Linked FRR information to new draft in Overview/Introduction
- Removed BTAFT/FRR from "Changes in the network topology"
- Linked new draft in "Link/Node Failures and Recovery"
- Removed FRR from "The BIER-TE Forwarding Layer"
- Moved FRR section to new draft
- Moved FRR parts of Pseudocode into new draft
- Left only non FRR parts
- removed FrrUpDown(..) and //FRR operations in
ForwardBierTePacket(..)
- New draft contains FrrUpDown(..) and ForwardBierTePacket(Packet)
from bier-arch-03
- Moved "BIER-TE and existing FRR to new draft
- Moved "BIER-TE and Segment Routing" section one level up
- Thus, removed "Further considerations" that only contained this
section
- Added Changes for version 04
03: Updated the FRR section. Added examples for FRR key concepts.
Added BIER-in-BIER tunneling as option for tunnels in backup
paths. BIFT structure is expanded and contains an additional
match field to support full node protection with BIER-TE FRR.
03: Updated FRR section. Explanation how BIER-in-BIER
encapsulation provides P2MP protection for node failures even
though the routing underlay does not provide P2MP.
02: Changed the definition of BIFT to be more inline with BIER.
In revs. up to -01, the idea was that a BIFT has only entries for
a single BitString, and every SI and sub-domain would be a
separate BIFT. In BIER, each BIFT covers all SI. This is now
also how we define it in BIER-TE.
02: Added Section 5.3 to explain the use of SI, sub-domains and
BFR-id in BIER-TE and to give an example how to efficiently assign
bits for a large topology requiring multiple SI.
02: Added further detailed for rings - how to support input from
all ring nodes.
01: Fixed BFIR -> BFER for section 4.3.
01: Added explanation of SI, difference to BIER ECMP,
consideration for Segment Routing, unicast FRR, considerations for
encapsulation, explanations of BIER-TE Controller and CLI.
00: Initial version.
10. References 8. References
10.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
skipping to change at page 61, line 36 skipping to change at line 2181
Explicit Replication (BIER)", RFC 8279, Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017, DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>. <https://www.rfc-editor.org/info/rfc8279>.
[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., [RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non- for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>. 2018, <https://www.rfc-editor.org/info/rfc8296>.
10.2. Informative References 8.2. Informative References
[Bloom70] Bloom, B. H., "Space/time trade-offs in hash coding with
allowable errors", Comm. ACM 13(7):422-6, July 1970,
<https://dl.acm.org/doi/10.1145/362686.362692>.
[I-D.eckert-bier-te-frr]
Eckert, T., Cauchie, G., Braun, W., and M. Menth,
"Protection Methods for BIER-TE", Work in Progress,
Internet-Draft, draft-eckert-bier-te-frr-03, 5 March 2018,
<https://www.ietf.org/archive/id/draft-eckert-bier-te-frr-
03.txt>.
[I-D.ietf-bier-multicast-http-response] [BIER-MCAST-OVERLAY]
Trossen, D., Rahman, A., Wang, C., and T. Eckert, Trossen, D., Rahman, A., Wang, C., and T. Eckert,
"Applicability of BIER Multicast Overlay for Adaptive "Applicability of BIER Multicast Overlay for Adaptive
Streaming Services", Work in Progress, Internet-Draft, Streaming Services", Work in Progress, Internet-Draft,
draft-ietf-bier-multicast-http-response-06, 10 July 2021, draft-ietf-bier-multicast-http-response-06, 10 July 2021,
<https://www.ietf.org/archive/id/draft-ietf-bier- <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
multicast-http-response-06.txt>. multicast-http-response-06>.
[I-D.ietf-bier-non-mpls-bift-encoding] [BIER-TE-PROTECTION]
Wijnands, I., Mishra, M., Xu, X., and H. Bidgoli, "An Eckert, T., Cauchie, G., Braun, W., and M. Menth,
Optional Encoding of the BIFT-id Field in the non-MPLS "Protection Methods for BIER-TE", Work in Progress,
BIER Encapsulation", Work in Progress, Internet-Draft, Internet-Draft, draft-eckert-bier-te-frr-03, 5 March 2018,
draft-ietf-bier-non-mpls-bift-encoding-04, 30 May 2021, <https://datatracker.ietf.org/doc/html/draft-eckert-bier-
<https://www.ietf.org/archive/id/draft-ietf-bier-non-mpls- te-frr-03>.
bift-encoding-04.txt>.
[I-D.ietf-bier-te-yang] [BIER-TE-YANG]
Zhang, Z., Wang, C., Chen, R., Hu, F., Sivakumar, M., and Zhang, Z., Wang, C., Chen, R., Hu, F., Sivakumar, M., and
H. Chen, "A YANG data model for Tree Engineering for Bit H. Chen, "A YANG data model for Tree Engineering for Bit
Index Explicit Replication (BIER-TE)", Work in Progress, Index Explicit Replication (BIER-TE)", Work in Progress,
Internet-Draft, draft-ietf-bier-te-yang-04, 7 November Internet-Draft, draft-ietf-bier-te-yang-05, 1 May 2022,
2021, <https://www.ietf.org/archive/id/draft-ietf-bier-te- <https://datatracker.ietf.org/doc/html/draft-ietf-bier-te-
yang-04.txt>. yang-05>.
[I-D.ietf-roll-ccast] [Bloom70] Bloom, B. H., "Space/time trade-offs in hash coding with
allowable errors", Comm. ACM 13(7):422-6,
DOI 10.1145/362686.362692, July 1970,
<https://dl.acm.org/doi/10.1145/362686.362692>.
[CONSTRAINED-CAST]
Bergmann, O., Bormann, C., Gerdes, S., and H. Chen, Bergmann, O., Bormann, C., Gerdes, S., and H. Chen,
"Constrained-Cast: Source-Routed Multicast for RPL", Work "Constrained-Cast: Source-Routed Multicast for RPL", Work
in Progress, Internet-Draft, draft-ietf-roll-ccast-01, 30 in Progress, Internet-Draft, draft-ietf-roll-ccast-01, 30
October 2017, <https://www.ietf.org/archive/id/draft-ietf- October 2017, <https://datatracker.ietf.org/doc/html/
roll-ccast-01.txt>. draft-ietf-roll-ccast-01>.
[I-D.ietf-teas-rfc3272bis]
Farrel, A., "Overview and Principles of Internet Traffic
Engineering", Work in Progress, Internet-Draft, draft-
ietf-teas-rfc3272bis-16, 24 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-teas-
rfc3272bis-16.txt>.
[ICC] Reed, M. J., Al-Naday, M., Thomos, N., Trossen, D., [ICC] Reed, M. J., Al-Naday, M., Thomos, N., Trossen, D.,
Petropoulos, G., and S. Spirou, "Stateless multicast Petropoulos, G., and S. Spirou, "Stateless multicast
switching in software defined networks", IEEE switching in software defined networks", IEEE
International Conference on Communications (ICC), Kuala International Conference on Communications (ICC), Kuala
Lumpur, Malaysia, 2016, May 2016, Lumpur, Malaysia, DOI 10.1109/ICC.2016.7511036, May 2016,
<https://ieeexplore.ieee.org/document/7511036>. <https://ieeexplore.ieee.org/document/7511036>.
[RCSD94] Zhang, H. and D. Domenico, "Rate-Controlled Service [NON-MPLS-BIER-ENCODING]
Disciplines", Journal of High-Speed Networks, 1994, May Wijnands, IJ., Mishra, M., Xu, X., and H. Bidgoli, "An
1994, <https://dl.acm.org/doi/10.5555/2692227.2692232>. Optional Encoding of the BIFT-id Field in the non-MPLS
BIER Encapsulation", Work in Progress, Internet-Draft,
draft-ietf-bier-non-mpls-bift-encoding-04, 30 May 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
non-mpls-bift-encoding-04>.
[RCSD94] Zhang, H. and D. Ferrari, "Rate-Controlled Service
Disciplines", Journal of High Speed Networks, Volume 3,
Issue 4, pp. 389-412, DOI 10.3233/JHS-1994-3405, October
1994, <https://content.iospress.com/articles/journal-of-
high-speed-networks/jhs3-4-05>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>. January 2006, <https://www.rfc-editor.org/info/rfc4253>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006, (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>. <https://www.rfc-editor.org/info/rfc4456>.
skipping to change at page 64, line 37 skipping to change at line 2324
Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2 Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2
Extensions for Bit Index Explicit Replication (BIER)", Extensions for Bit Index Explicit Replication (BIER)",
RFC 8444, DOI 10.17487/RFC8444, November 2018, RFC 8444, DOI 10.17487/RFC8444, November 2018,
<https://www.rfc-editor.org/info/rfc8444>. <https://www.rfc-editor.org/info/rfc8444>.
[RFC8556] Rosen, E., Ed., Sivakumar, M., Przygienda, T., Aldrin, S., [RFC8556] Rosen, E., Ed., Sivakumar, M., Przygienda, T., Aldrin, S.,
and A. Dolganow, "Multicast VPN Using Bit Index Explicit and A. Dolganow, "Multicast VPN Using Bit Index Explicit
Replication (BIER)", RFC 8556, DOI 10.17487/RFC8556, April Replication (BIER)", RFC 8556, DOI 10.17487/RFC8556, April
2019, <https://www.rfc-editor.org/info/rfc8556>. 2019, <https://www.rfc-editor.org/info/rfc8556>.
[TE-OVERVIEW]
Farrel, A., Ed., "Overview and Principles of Internet
Traffic Engineering", Work in Progress, Internet-Draft,
draft-ietf-teas-rfc3272bis-21, 11 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
rfc3272bis-21>.
Appendix A. BIER-TE and Segment Routing (SR) Appendix A. BIER-TE and Segment Routing (SR)
SR ([RFC8402]) aims to enable lightweight path steering via loose SR [RFC8402] aims to enable lightweight path steering via loose
source routing. Compared to its more heavy-weight predecessor RSVP- source routing. For example, compared to its more heavyweight
TE, SR does for example not require per-path signaling to each of predecessor, RSVP-TE, SR does not require per-path signaling to each
these hops. of these hops.
BIER-TE supports the same design philosophy for multicast. Like in BIER-TE supports the same design philosophy for multicast. Like SR,
SR, it relies on source-routing - via the definition of a BitString. BIER-TE
Like SR, it only requires to consider the "hops" on which either
replication has to happen, or across which the traffic should be
steered (even without replication). Any other hops can be skipped
via the use of routed adjacencies.
BIER-TE bit position (BP) can be understood as the BIER-TE equivalent * relies on source routing (via a BitString), and
of "forwarding segments" in SR, but they have a different scope than
SR forwarding segments. Whereas forwarding segments in SR are global * only requires consideration of the "hops" either (1) on which
or local, BPs in BIER-TE have a scope that is the group of BFR(s) replication has to happen or (2) across which the traffic should
that have adjacencies for this BP in their BIFT. This can be called be steered (even without replication).
"adjacency" scoped forwarding segments.
Any other hops can be skipped via the use of routed adjacencies.
BIER-TE "bit positions" (BPs) can be understood as the BIER-TE
equivalent of "forwarding segments" in SR, but they have a different
scope than do forwarding segments in SR. Whereas forwarding segments
in SR are global or local, BPs in BIER-TE have a scope that is
comprised of one or more BFRs that have adjacencies for the BPs in
their BIFTs. These segments can be called "adjacency-scoped"
forwarding segments.
Adjacency scope could be global, but then every BFR would need an Adjacency scope could be global, but then every BFR would need an
adjacency for this BP, for example a forward_routed() adjacency with adjacency for a given BP -- for example, a forward_routed() adjacency
encapsulation to the global SR SID of the destination. Such a BP with encapsulation to the global SR "Segment Identifier" (SID) of the
would always result in ingress replication though (as in [RFC7988]). destination. Such a BP would always result in ingress replication,
The first BFR encountering this BP would directly replicate to it. though (as in [RFC7988]). The first BFR encountering this BP would
Only by using non-global adjacency scope for BPs can traffic be directly replicate traffic on it. Only by using non-global adjacency
steered and replicated on non-ingress BFR. scope for BPs can traffic be steered and replicated on a non-BFIR.
SR can naturally be combined with BIER-TE and help to optimize it. SR can naturally be combined with BIER-TE and can help optimize it.
For example, instead of defining bit positions for non-replicating For example, instead of defining bit positions for non-replicating
hops, it is equally possible to use segment routing encapsulations hops, it is equally possible to use SR encapsulations (e.g., SR-MPLS
(e.g. SR-MPLS label stacks) for the encapsulation of label stacks) for the encapsulation of "forward_routed()"
"forward_routed" adjacencies. adjacencies.
Note that (non-TE) BIER itself can also be seen to be similar to SR. Note that (non-TE) BIER itself can also be seen as being similar to
BIER BPs act as global destination Node-SIDs and the BIER BitString SR. BIER BPs act as global destination Node-SIDs, and the BIER
is simply a highly optimized mechanism to indicate multiple such SIDs BitString is simply a highly optimized mechanism to indicate multiple
and let the network take care of effectively replicating the packet such SIDs and let the network take care of effectively replicating
hop-by-hop to each destination Node-SID. What BIER does not allow is the packet hop by hop to each destination Node-SID. BIER does not
to indicate intermediate hops, or in terms of SR the ability to allow the indication of intermediate hops or, in terms of SR, the
indicate a sequence of SID to reach the destination. This is what ability to indicate a sequence of SIDs to reach the destination. On
BIER-TE and its adjacency scoped BP enables. the other hand, BIER-TE and its adjacency-scoped BPs provide these
capabilities.
Acknowledgements
The authors would like to thank Greg Shepherd, IJsbrand Wijnands,
Neale Ranns, Dirk Trossen, Sandy Zheng, Lou Berger, Jeffrey Zhang,
Carsten Bormann, and Wolfgang Braun for their reviews and
suggestions.
Special thanks to Xuesong Geng for shepherding this document.
Special thanks also for IESG review/suggestions by Alvaro Retana
(responsible AD/RTG), Benjamin Kaduk (SEC), Tommy Pauly (TSV),
Zaheduzzaman Sarker (TSV), Éric Vyncke (INT), Martin Vigoureux (RTG),
Robert Wilton (OPS), Erik Kline (INT), Lars Eggert (GEN), Roman
Danyliw (SEC), Ines Robles (RTGDIR), Robert Sparks (Gen-ART),
Yingzhen Qu (RTGDIR), and Martin Duke (TSV).
Authors' Addresses Authors' Addresses
Toerless Eckert (editor) Toerless Eckert (editor)
Futurewei Technologies Inc. Futurewei Technologies Inc.
2330 Central Expy 2330 Central Expy
Santa Clara, 95050 Santa Clara, CA 95050
United States of America United States of America
Email: tte+ietf@cs.fau.de Email: tte@cs.fau.de
Michael Menth Michael Menth
University of Tuebingen University of Tuebingen
Germany
Email: menth@uni-tuebingen.de Email: menth@uni-tuebingen.de
Gregory Cauchie Gregory Cauchie
KOEVOO KOEVOO
France
Email: gregory@koevoo.tech Email: gregory@koevoo.tech
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