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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft-ietf-bier-te-arch-13" category="std"> number="9262" submissionType="IETF" category="std" consensus="true" obsoletes="" updates="" xml:lang="en" tocInclude="true" tocDepth="4" symRefs="true" sortRefs="true" version="3">

  <!-- xml2rfc v2v3 conversion 3.12.2 -->
  <front>
    <title abbrev="BIER-TE ARCH">Tree Engineering for Bit Index Explicit Replication (BIER-TE)</title>
    <seriesInfo name="RFC" value="9262"/>
    <author role="editor" fullname="Toerless Eckert" initials="T.T.E." initials="T." surname="Eckert">
      <organization abbrev="Futurewei">Futurewei Technologies Inc.</organization>
      <address>
        <postal>
          <street>2330 Central Expy</street>
          <city>Santa Clara</city>
          <code>95050</code>
                      <country>USA</country>
          <region>CA</region>
          <country>United States of America</country>
        </postal>
                    <email>tte+ietf@cs.fau.de</email>
        <email>tte@cs.fau.de</email>
      </address>
    </author>
    <author fullname="Michael Menth" initials="M.M." initials="M." surname="Menth">
      <organization>University of Tuebingen</organization>
      <address>
      <postal>
       <country>Germany</country>
      </postal>
        <email>menth@uni-tuebingen.de</email>
      </address>
    </author>
    <author fullname="Gregory Cauchie" initials="G.C." initials="G." surname="Cauchie">
      <organization>KOEVOO</organization>
      <address>
      <postal>
       <country>France</country>
      </postal>
        <email>gregory@koevoo.tech</email>
      </address>
    </author>

    <date month="April" month="October" year="2022"/>
    <area>rtg</area>
    <workgroup>bier</workgroup>

<keyword>BIER</keyword>
<keyword>BIER-TE</keyword>
<keyword>controller</keyword>
<keyword>ECMP</keyword>
<keyword>forwarding</keyword>
<keyword>traffic-engineering</keyword>
<keyword>multicast</keyword>
<keyword>pseudocode</keyword>
<keyword>routing</keyword>
<keyword>traffic-steering</keyword>
<keyword>tree-steering</keyword>

    <abstract>
      <t> This memo describes per-packet stateless strict and loose path
steered replication and forwarding for "Bit Index Explicit Replication" (BIER, RFC8279) packets.  It (BIER)  packets (RFC 8279); it is called BIER Tree "Tree Engineering for Bit Index Explicit Replication" (BIER-TE) and is intended to be used as the path steering mechanism for Traffic Engineering
with BIER.</t>
      <t>BIER-TE introduces a new semantic for "bit positions" (BP). They (BPs). These BPs indicate adjacencies
of the network topology, as opposed to (non-TE) BIER in which BPs indicate
 "Bit-Forwarding Egress Routers" (BFER). (BFERs).  A BIER-TE packets BitString "packets BitString" therefore indicates the
edges of the (loop-free) tree that across which the packet is packets are forwarded across by BIER-TE.
BIER-TE can leverage BIER forwarding engines with little changes.
Co-existence of BIER and BIER-TE forwarding in the same domain is possible, possible -- for example example, by using
separate BIER "sub-domains" "subdomains" (SDs). Except for the optional routed adjacencies, BIER-TE does not
require a BIER routing underlay, underlay and can therefore operate without depending
on an a routing protocol such as the "Interior Gateway Routing protocol" Protocol" (IGP).</t>
    </abstract>
  </front>
  <middle>
    <section anchor="overview" title="Overview">

<t> BIER-TE numbered="true" toc="default">
      <name>Overview</name>
      <t>"Tree Engineering for Bit Index Explicit Replication" (BIER-TE) is based on the (non-TE) BIER architecture, terminology terminology, and packet formats as described
in <xref target="RFC8279"/> target="RFC8279" format="default"/> and <xref target="RFC8296"/>. target="RFC8296" format="default"/>.
This document describes BIER-TE in BIER-TE, with the expectation that the reader is familiar
with these two documents.</t>
      <t>BIER-TE introduces a new semantic for "bit positions" (BP). They (BPs). These BPs indicate adjacencies
of the network topology, as opposed to (non-TE) BIER in which BPs indicate
 "Bit-Forwarding Egress Routers" (BFER). (BFERs).  A BIER-TE packets BitString "packets BitString" therefore indicates the
edges of the (loop-free) tree that across which the packet is packets are forwarded across by BIER-TE.
With BIER-TE, the "Bit Index Forwarding Table" (BIFT) of each "Bit Forwarding "Bit-Forwarding Router" (BFR)
is only populated with BP BPs that are adjacent to the BFR
in the BIER-TE Topology. topology. Other BPs are empty in the BIFT. The BFR replicate replicates
and forwards BIER packets to adjacent BPs that are set in the packet. packets.
BPs are normally also cleared upon forwarding to avoid duplicates and loops.
</t>
      <t>BIER-TE can leverage BIER forwarding engines with little or no changes.
It can also co-exist with BIER forwarding in the same domain, domain -- for example example, by using
separate BIER sub-domains. subdomains. Except for the optional routed adjacencies, BIER-TE does not
require a BIER routing underlay, underlay and can therefore operate without depending
on an a routing protocol such as the "Interior Gateway Routing protocol" Protocol" (IGP).</t>
      <t>This document is structured as follows:
<list style="symbols">
    <t><xref target="introduction"/>
</t>
      <ul spacing="normal">
        <li>
          <xref target="introduction" format="default"/> introduces BIER-TE with two
    forwarding examples, followed by an introduction of to the new concepts of the BIER-TE
    (overlay) topology topology, and finally a summary of the relationship between BIER and BIER-TE and a discussion of accelerated hardware forwarding.</t>
    <t><xref target="components"/> forwarding.</li>
        <li>
          <xref target="components" format="default"/> describes the components of the BIER-TE architecture,
     Flow architecture: the multicast
     flow overlay, the BIER-TE layer with the BIER-TE control plane (including the BIER-TE controller) and controller), the BIER-TE forwarding plane, and the routing underlay.</t>
    <t><xref target="forwarding"/> underlay.</li>
        <li>
          <xref target="forwarding" format="default"/> specifies the behavior of the BIER-TE forwarding plane with the different type types of adjacencies and possible variations of BIER-TE forwarding pseudocode, and finally the mandatory and optional requirements.</t>
    <t><xref target="controller-ops"/> requirements.</li>
        <li>
          <xref target="controller-ops" format="default"/> describes operational considerations for the BIER-TE controller, foremost primarily how the BIER-TE controller can optimize the use of BP BPs by using specific type types of BIER-TE adjacencies for different type types of topological situations, but situations.  It also describes how to assign bits to avoid loops and duplicates (which (which, in BIER-TE BIER-TE, does not come for free), and finally "for free").  Finally, it discusses how "Set Identifier" (SI), "sub-domain" (SD) Identifiers" (SIs), "subdomains" (SDs), and BFR-ids can be managed by a BIER-TE controller, controller; examples and summary.</t>
    <t><xref target="SR"/> a summary are provided.</li>
        <li>
          <xref target="SR" format="default"/> concludes this document; details regarding the technology specific sections of the document by further relating relationship between BIER-TE to Segment Routing (SR).</t>
</list></t> and "Segment Routing" (SR) are discussed.</li>
      </ul>
      <t>Note that related work, work <xref target="I-D.ietf-roll-ccast"/> target="I-D.ietf-roll-ccast" format="default"/>
uses Bloom filters <xref target="Bloom70"/> target="Bloom70" format="default"/> to represent leaves or edges of the intended delivery tree.  Bloom filters
in general can support larger trees/topologies with fewer addressing bits than explicit BitStrings,
but they introduce the heuristic risk of false positives and cannot clear bits in
the BitString BitStrings during forwarding to avoid loops. For these reasons, BIER-TE
uses explicit BitStrings BIER-TE, like BIER. The BIER,
uses explicit BitStrings. Explicit BitStrings of as used by BIER-TE can also
be seen as a special type of Bloom filter, and this is how other related work <xref target="ICC"/> target="ICC" format="default"/>
describes it.</t>

<!--  Removed for now by review with Lou Berger

  <section anchor="te" title="BIER-TE and Traffic Engineering (BIER-TE)">

<t>BIER-TE is not a standalone, complete traffic engineering signaling solution such as RSVP with RSVP-TE
extensions (<xref target="RFC2205"/>, <xref target="RFC3209"/>). Instead it is a (non-TE) BIER derived architecture
and forwarding plane that allows to signal "source-routed" paths and replication points without
per-path, per-replication-point state on the transit nodes. This document introduces the name
"Tree Engineering" for BitStrings using this semantic. BIER-TE is therefore more similar to Segment Routing
(SR, (<xref target="RFC8402"/>)) than RSVP-TE. Note that SR does not provide stateless replication point
and receiver set signaling in its packet header.  See <xref target="SR"/> for a more detailed discussion of
BIER-TE and SR.</t>

<t>BIER-TE can be used alone in use cases not requiring bandwidth or buffer resource reservations,
such as high resilient services through dual transmission with path diversity or optimization
of network capacity utilization through calculated paths/trees ("load balancing across non-ECMP paths").
Due to its stateless BIER approach, BIER-TE does not create per-flow/per-tree state on intermedia nodes.</t>

<t>BIER-TE can also be combined with bandwidth and buffer management functions to support
traffic engineering such as per-flow guaranteed bandwidth and guaranteed latency across BIER-TE
steered paths / trees. Combinations of BIER or BIER-TE with such per-tree/per-flow resource
 guarantees are called BIER-TE. The following paragraphs summarize options and considerations.</t>

<t>In <xref target="components"/> below, the BIER-TE architecture specifies the BIER-TE Controller
as an entity calculating both the BIER-TE topology and desired paths/trees for overlay flows
based on the desired policies. A Path Computation Engine (PCE, see <xref target="RFC4655"/>)
that can calculate the BitString for BIER-TE is an instance of such a BIER-TE Controller.
If the PCE can also perform resource management such as per-flow bandwidth reservations and
 optional latency guarantees, then it becomes a PCE for BIER-TE with traffic engineering.</t>

<t>To support bandwidth guarantees in the forwarding plane, the ingres BIER-TE node
(BFIR) may need to have a per-flow policer if ingressed traffic is not trusted to stay within
its admitted traffic envelope. This is a well understood policy function that can be deployed
without changes to BIER-TE.</t>

<t>If latency guarantees as required as for example by Guaranteed Services (<xref target="RFC2212"/>),
then additional per-hop latency control in the forwarding plane can be required. This can also
be added to BIER-TE deployments without changes to BIER-TE. Per-hop stateless solutions for this
such as in <xref target="I-D.qiang-detnet-large-scale-detnet"/> would allow to maintain
the per-hop stateless design goal of BIER-TE and expand it into BIER-TE. Per-hop stateful solutions like
per-flow, per-hop shaping may also be beneficial given how BIER-TE eliminates the need for
per-flow, per-hop multicast replication and steering state.</t>

<t>Mechanisms how to combine BIER-TE or BIER with other mechanisms to build BIER-TE are outside
the scope of this document.  See <xref target="I-D.eckert-teas-bier-te-framework"/>.</t>
    </section>

-->
    <section anchor="introduction" numbered="true" toc="default">
      <name>Introduction</name>
    <section anchor="boilerplate" title="Requirements Language">

<t>
 The numbered="true" toc="default">
        <name>Requirements Language</name>
        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",  "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
        "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",
        "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
        "<bcp14>SHOULD NOT</bcp14>",
        "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
        "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document
        are to be interpreted as described in BCP 14 BCP&nbsp;14
        <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only
        when, they appear in all capitals, as shown here.
</t>

    </section> here.</t>
      </section>
      <section anchor="introduction" title="Introduction">

  <section anchor="examples" title="Basic Examples"> numbered="true" toc="default">
        <name>Basic Examples</name>
        <t>BIER-TE forwarding is best introduced with simple examples. These examples
use formal terms defined later in the this document (<xref target="adjacencies"/>), target="adjacencies" format="default"/> in <xref target="btft"/>),
including forward_connected(), forward_routed() forward_routed(), and local_decap().
</t>
        <t>Consider the simple network in the BIER-TE overview example shown in
<xref target="basic-example"/>, with six BFRs. &nbsp;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.
</t>
        <figure anchor="basic-example" title="BIER-TE basic example"> anchor="basic-example">
          <name>BIER-TE Basic Example</name>
          <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
BIER-TE Topology:

   Diagram:

                    p5    p6
                  --- BFR3 ---
               p3/    p13     \p7          p15
   BFR1 ---- BFR2              BFR5 ----- BFR6
      p1   p2  p4\    p14     /p10 p11   p12
                  --- BFR4 ---
                    p8    p9

   (simplified) BIER-TE Bit Index Forwarding Tables (BIFT): (BIFTs):

   BFR1:   p1  -> local_decap()
           p2  -> forward_connected() to BFR2

   BFR2:   p1  -> forward_connected() to BFR1
           p5  -> forward_connected() to BFR3
           p8  -> forward_connected() to BFR4

   BFR3:   p3  -> forward_connected() to BFR2
           p7  -> forward_connected() to BFR5
           p13 -> local_decap()

   BFR4:   p4  -> forward_connected() to BFR2
           p10 -> forward_connected() to BFR5
           p14 -> local_decap()

   BFR5:   p6  -> forward_connected() to BFR3
           p9  -> forward_connected() to BFR4
           p12 -> forward_connected() to BFR6

   BFR6:   p11 -> forward_connected() to BFR5
           p15 -> local_decap()

]]></artwork></figure>

<t>
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.
</t>
]]></artwork>
        </figure>
        <t>
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 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 only copy of the packet
to BFR2. Similarly, BFR2 will forward to BFR4 because of p8, BFR4 to BFR5
because of p10 p10, and BFR5 to BFR6 because of p12. p15 &nbsp;p15 finally makes BFR6 receive
and decapsulate the packet.
</t>
        <t>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 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 receive and decapsulate
the packet.
</t>
        <t>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 copied by
BFR2 to BFR3 because of p5 in the prior case. This is showing demonstrates the ability of the shown
BIER-TE Topology topology, as shown in <xref target="basic-example"/>, to make the traffic pass across any possible path and be
replicated where desired.
</t>
        <t>BIER-TE has various options to minimize for minimizing BP assignments,
many of which are based on out-of-band knowledge about the required multicast traffic
paths and bandwidth consumption in the network, such as e.g., from pre-deployment predeployment planning.</t>
        <t><xref target="basic-overlay"/> target="basic-overlay" format="default"/> shows a modified example, in which Rtr2 and Rtr5 are
assumed not to support BIER-TE, so traffic has to be unicast encapsulated across
them.  To emphasize non-L2, but explicitly distinguish routed/tunneled forwarding of BIER-TE packets, packets
from Layer 2 forwarding (forward_connected()), these adjacencies are called "forward_routed". "forward_routed()" adjacencies. Otherwise, there is no difference
in their processing over the aforementioned forward_connected() adjacencies.</t>
        <t>In addition, bits are saved in the following example by assuming that BFR1 only
needs to be a BFIR but -- not a BFER or a transit BFR.</t>
        <figure anchor="basic-overlay" title="BIER-TE basic overlay example"> anchor="basic-overlay">
          <name>BIER-TE Basic Overlay Example</name>
          <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
BIER-TE Topology:

   Diagram:

                   p1  p3  p7
                ....> BFR3 <....       p5
        ........                ........>
   BFR1       (Rtr2)          (Rtr5)      BFR6
        ........                ........> p9
                ....> BFR4 <....       p6
                   p2  p4  p8

   (simplified) BIER-TE Bit Index Forwarding Tables (BIFT): (BIFTs):

   BFR1:   p1  -> forward_routed() to BFR3
           p2  -> forward_routed() to BFR4

   BFR3:   p3  -> local_decap()
           p5  -> forward_routed() to BFR6

   BFR4:   p4  -> local_decap()
           p6  -> forward_routed() to BFR6

   BFR6:   p7  -> forward_routed() to BFR3
           p8  -> forward_routed() to BFR4
           p9  -> local_decap()

]]></artwork></figure>
]]></artwork>
        </figure>
        <t>To send a BIER-TE packet from BFR1 via BFR3 to be received by BFR6,
the BitString is (p1,p5,p9). From A packet from BFR1 via BFR4 to be received by BFR6, BFR6 uses the BitString is (p2,p6,p9). A packet from BFR1 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 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 then from BFR4 to be
received by BFR6 BFR6, and finally from there BFR6 to be received by BFR3 BFR3, uses
(p2,p3,p4,p6,p7,p9).  A packet from BFR1 to be received by BFR3, and
then from BFR3 to be received by BFR6, and finally from BFR6
 there to be
received by BFR4 BFR4, uses (p1,p3,p4,p5,p8,p9).</t> (p1,p3,p4,p5,p8,p9).
</t>
      </section>
      <section anchor="topology" title="BIER-TE numbered="true" toc="default">
        <name>BIER-TE Topology and adjacencies"> Adjacencies</name>
        <t>The key new component in BIER-TE compared to (non-TE) BIER is the BIER-TE topology
as introduced through the two examples in <xref target="examples"/>. target="examples" format="default"/>.
It is used to control where replication can or should happen and how to
minimize the required number of BP BPs for adjacencies.
</t>
        <t>
The BIER-TE Topology topology consists of the BIFTs of all the BFR BFRs and
can also be expressed as a directed graph where the edges are the adjacencies
between the BFRs labelled labeled with the BP used for the adjacency. Adjacencies are
naturally unidirectional.  A BP can be reused across multiple adjacencies as long as this does not
lead to undesired duplicates or loops loops, as explained in <xref target="avoiding"/>. target="avoiding" format="default"/>.
</t>
        <t>If the BIER-TE topology represents (a subset of) the underlying (layer (Layer 2)
topology of the network as shown in the first example, this may be called a "native" an "underlay"
BIER-TE topology. A topology consisting only of "forward_routed" "forward_routed()" adjacencies as
shown in the second example may be called an "overlay" BIER-TE topology.
A BIER-TE topology with both forward_connected() and forward_routed() adjacencies
may be called a "hybrid" BIER-TE topology.</t>
      </section>
    <!-- topology -->

<section anchor="comparison" title="Relationship numbered="true" toc="default">
        <name>Relationship to BIER"> BIER</name>
        <t>BIER-TE is designed so that its forwarding plane is a simple extension to the (non-TE) BIER forwarding plane, hence allowing for it to be added to BIER deployments where it can be beneficial.</t>
        <t>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 statically provisioned networks with needs for that need path steering but without desire for do not want distributed routing protocols.</t>

<t><list style="numbers">
        <ol spacing="normal" type="1"><li>
            <t>BIER-TE inherits the following aspects from BIER unchanged:
<list style="numbers">
    <t>The
</t>
            <ol spacing="normal" type="%p%c"><li>The fundamental purpose of per-packet signaled replication and delivery via a BitString.</t>
    <t>The BitString.</li>
              <li>The overall architecture consisting architecture, which consists of three layers, layers: the flow overlay, the BIER(-TE) layer layer, and the routing underlay.</t>
    <t>The underlay.</li>
              <li>The supported encapsulations <xref target="RFC8296"/>.</t>
    <t>The semantic target="RFC8296" format="default"/>.</li>
              <li>The semantics of all <xref target="RFC8296"/> BIER header elements <xref target="RFC8296" format="default"/> used by the BIER-TE forwarding plane plane, other than the semantic of the BP in the BitString.</t>
    <t>The BitString.</li>
              <li>The BIER forwarding plane, except for how bits have to be cleared during replication.</t>
</list></t> replication.</li>
            </ol>
          </li>
          <li>
            <t>BIER-TE has the following key changes with respect to BIER:
<list style="numbers">
     <t>In
</t>
            <ol spacing="normal" type="%p%c"><li>In BIER, bits in the BitString of a BIER packet header indicate a BFER BFER,
        and bits in the BIFT indicate the BIER control plane plane's calculated next-hop
        toward next hop
        towards that BFER. In BIER-TE, a bit in the BitString of a BIER packet
        header indicates an adjacency in the BIER-TE topology, and only the
        BFR that is the upstream of that adjacency has its BP populated with
        the adjacency in its BIFT.</t>

    <t>In BIFT.</li>
              <li>In BIER, the implied reference options for the core part of the
 BIER layer
       control plane are the BIER extensions for distributed routing protocols.
       This includes ISIS/OSPF
       These include IS-IS and OSPF extensions for BIER, as specified in <xref target="RFC8401"/> target="RFC8401" format="default"/>
       and <xref target="RFC8444"/>.</t>

    <t>The target="RFC8444" format="default"/>, respectively.</li>
              <li>The reference option for the core part of the BIER-TE control plane is
       the BIER-TE controller. Nevertheless, both the BIER and BIER-TE BIFTs BIFTs' forwarding
       plane state could equally be populated by any mechanism.</t>
    <t>Assuming mechanism.</li>
              <li>Assuming the reference options for the control plane, BIER-TE replaces in-network autonomous path calculation by calculations with explicit paths calculated by the BIER-TE controller.</t>
</list></t> controller.</li>
            </ol>
          </li>
          <li>
            <t>The following elements/functions described in the BIER architecture are not required by the BIER-TE architecture:
<list style="numbers">
    <t>"Bit
</t>
            <ol spacing="normal" type="%p%c"><li>"Bit Index Routing Tables" (BIRTs) are not required on BFRs for BIER-TE when using a BIER-TE controller controller, because the controller can directly populate the BIFTs. In BIER, BIRTs are populated by the distributed routing protocol support for BIER, allowing BFRs to populate their BIFTs locally from their BIRTs. Other BIER-TE control plane  or management plane options may introduce requirements for BIRTs for BIER-TE BFRs.</t>
    <t>The BFRs.</li>
              <li>The BIER-TE layer forwarding plane does not require BFRs to have a unique BP and therefore also no BP; see <xref target="leaf-bfer" format="default"/>. Therefore, BFRs may not have a unique BFR-id.  See BFR-id; see <xref target="leaf-bfer"/>.</t>
    <t>Identification target="bfr-id"/>.</li>
              <li>Identification of BFRs by the BIER-TE control plane is outside the scope of this specification. Whereas the BIER control plane uses BFR-ids in its BFR to BFR BFR-to-BFR signaling, a BIER-TE controller may choose any form of identification deemed appropriate.</t>
    <t>BIER-TE appropriate.</li>
              <li>BIER-TE forwarding does not require the BFIR-id field of the BIER packet header.</t>
</list></t> header.</li>
            </ol>
          </li>
          <li>
            <t>Co-existence of BIER and BIER-TE in the same network requires the following:
<list style="numbers">
<t>The
</t>
            <ol spacing="normal" type="%p%c"><li>The BIER/BIER-TE packet header needs to allow the addressing of both BIER and BIER-TE BIFTs. Depending on the encapsulation option, the same SD may or may not be reusable across BIER and BIER-TE.  See <xref target="encapsulation"/>. target="encapsulation" format="default"/>.
In either case, a packet is always only forwarded end-to-end only end to end via BIER or via BIER-TE (ships ("ships in the nights forwarding).</t>

<t>BIER-TE night" forwarding).</li>
              <li>BIER-TE deployments will have to assign BFR-ids to BFRs and insert them into the BFIR-id field of BIER packet headers headers, as BIER does, does BIER, whenever the deployment uses (unchanged) components developed for BIER that use BFR-id, BFR-ids, such as multicast flow overlays or BIER layer control plane elements. See also <xref target="bfr-id"/>.</t>

</list></t>

</list></t> target="bfr-id" format="default"/>.</li>
            </ol>
          </li>
        </ol>
      </section>
      <section anchor="fwd-comparison" title="Accelerated/Hardware forwarding comparison"> numbered="true" toc="default">
        <name>Accelerated Hardware Forwarding Comparison</name>
        <t>BIER-TE forwarding rules, especially the BitString parsing parsing, are designed to be as close
as possible to those of BIER in BIER, with the expectation that this eases the programming of BIER-TE forwarding
code and/or BIER-TE forwarding hardware on platforms supporting BIER.
The pseudocode in <xref target="pseudocode"/> target="pseudocode" format="default"/> shows how existing
(non-TE) BIER/BIFT forwarding can be modified to support the required BIER-TE forwarding
functionality (<xref target="requirements"/>), target="requirements" format="default"/>), by using the BIER BIFT's "Forwarding Bit Mask" (F-BM):
Only
only the clearing of bits to avoid sending duplicate
packets to a BFR's neighbor is skipped in BIER-TE forwarding forwarding, because it is not necessary
and could not be done when using a BIER F-BM.</t>
        <t>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). This is determined
by the BFR configuration for the encapsulation, encapsulation; see <xref target="encapsulation"/>.</t> target="encapsulation" format="default"/>.</t>
      </section>
  <!-- fwd-comparison -->

</section>
<!-- overview -->

  <section anchor="components" title="Components"> numbered="true" toc="default">
      <name>Components</name>
      <t>BIER-TE can be thought of as being constituted from composed of the same three
layers as BIER: The the "multicast flow overlay", the "BIER layer" layer", and
the "routing underlay".  The following picture  <xref target="architecture"/> also shows how the "BIER layer" BIER layer
is constituted from composed of the "BIER-TE forwarding plane" and the "BIER-TE control plane"
represent as
represented by the "BIER-TE Controller".</t> controller".
</t>
      <figure anchor="architecture" title="BIER-TE architecture"> anchor="architecture">
        <name>BIER-TE Architecture</name>
        <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
                <------BGP/PIM----->
   |<-IGMP/PIM->  multicast flow   <-PIM/IGMP->|
                     overlay

       BIER-TE  [BIER-TE Controller] <=> [BIER-TE Topology]
       control     ^      ^     ^
       plane      /       |      \   BIER-TE control protocol
                 |        |       |  e.g.  (e.g., YANG/NETCONF/RESTCONF
                 |        |       |       PCEP/...       PCEP/...)
                 v        v       v
 Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr

                |<----------------->|
              BIER-TE forwarding plane

                |<- BIER-TE domain->|

              |<--------------------->|
                  Routing underlay
]]></artwork></figure>
]]></artwork>
      </figure>
      <section anchor="flow-overlay" title="The numbered="true" toc="default">
        <name>The Multicast Flow Overlay"> Overlay</name>
        <t>The Multicast Flow Overlay multicast flow overlay has the same role as that described for BIER
in <xref target="RFC8279"/>, Section 4.3. target="RFC8279" sectionFormat="comma" section="4.3"/>. See also <xref target="engineered-bitstrings"/>.</t> target="engineered-bitstrings" format="default"/>.</t>
        <t>When a BIER-TE controller is used, then the signaling for the Multicast Flow Overlay may it might also be preferred to operate preferable that
multicast flow overlay signaling be performed through a central point of control. For BGP based BGP-based
overlay flow services such as "Multicast VPN Using BIER" (<xref target="RFC8556"/>) "<xref target="RFC8556" format="title"/>" <xref target="RFC8556" format="default"/>, this
can be achieved by making the BIER-TE controller operate as a BGP Route
Reflector (<xref target="RFC4456"/>) <xref target="RFC4456" format="default"/> and combining it with signaling through BGP
or a different protocol for the BIER-TE controller controller's calculated BitStrings.
See <xref target="engineered-bitstrings"/> Sections&nbsp;<xref target="engineered-bitstrings" format="counter"/> and <xref target="bitstring-mappings"/>.</t> target="bitstring-mappings" format="counter"/>.</t>
      </section>
    <!-- flow-overlay -->

    <section anchor="control-plane" title="The numbered="true" toc="default">
        <name>The BIER-TE Control Plane"> Plane</name>
        <t>In the (non-TE) BIER architecture <xref target="RFC8279"/>, target="RFC8279" format="default"/>, the BIER control
plane layer is not explicitly separated from summarized in <xref target="RFC8279" sectionFormat="of" section="4.2"/>. This summary includes both the BIER functions
of the BIER-layer control plane and forwarding plane,
but instead their functions are summarized together in Section 4.2. without using those terms.
Example standardized options for the BIER control plane include
ISIS/OSPF
IS-IS and OSPF extensions for BIER, as specified in <xref target="RFC8401"/> target="RFC8401" format="default"/> and <xref target="RFC8444"/>.</t> target="RFC8444" format="default"/>, respectively.</t>
        <t>For BIER-TE, the control plane includes includes, at minimum a minimum, the following functionality.</t>

<t><list style="hanging">
    <t anchor="topology-control" hangText="1. BIER-TE

        <ol spacing="normal" type="1"><li>

          <t>BIER-TE topology control:">During control: During initial provisioning of the network and/or during modifications of its topology and/or services, the protocols and/or procedures to establish BIER-TE BIFTs:
    <list style="numbers">
        <t BIFTs:</t>
            <ol spacing="normal" type="%p%c">
              <li anchor="topology-control-1">Determine the desired BIER-TE topology for a BIER-TE sub-domains: subdomains: the native and/or overlay adjacencies that are assigned to BPs. Topology discovery is discussed in <xref target="topology-discovery"/> target="topology-discovery" format="default"/>, and the various aspects of the BIER-TE controllers controller's determinations about regarding the topology are discussed throughout <xref target="controller-ops"/></t>
        <t>Determine target="controller-ops" format="default"/>.</li>
              <li>Determine the per-BFR BIFT from the BIER-TE topology. This is achieved by simply extracting the adjacencies of the BFR from the BIER-TE topology and populating the BFRs BFR's BIFT with them.</t>
        <t>Optionally them.</li>
              <li>Optionally assign BFR-ids to BFIRs for later insertion into BIER headers on BFIRs as BFIR-id. BFIR-ids. Alternatively, BFIR-id BFIR-ids in BIER packet headers may be managed solely by the flow overlay layer and/or be unused. This is discussed in <xref target="bfr-id"/>.</t>
        <t>Install/update target="bfr-id" format="default"/>.</li>
              <li>Install/update the BIFTs into the BFRs and optionally and, optionally, BFR-ids into BFIRs. This is discussed in <xref target="topology-discovery"/>.</t>
    </list></t> target="topology-discovery" format="default"/>.</li>
            </ol>
        </li>
        <li>
          <t anchor="tree-control" hangText="2. anchor="tree-control">
            BIER-TE tree control:">During operations of the network, control:
            During network operations, protocols and/or procedures to support creation/change/removal of overlay flows on BFIRs:
    <list style="numbers">
        <t>Process BFIRs:</t>
            <ol spacing="normal" type="%p%c"><li>Process the BIER-TE requirements for the multicast overlay flow: BFIR BFIRs and BFERs of the flow as well as policies for the path selection of the flow. This is discussed in <xref target="te-considerations"/>.</t>
        <t>Determine target="te-considerations" format="default"/>.</li>
              <li>Determine the BitStrings and optionally Entropy. This is and, optionally, entropy.
BitStrings are discussed in Sections&nbsp;<xref target="engineered-bitstrings" format="counter"/>, <xref target="engineered-bitstrings"/>, <xref target="te-considerations"/> target="te-considerations" format="counter"/>, and <xref target="bitstring-mappings"/>.</t>
        <t>Install target="bitstring-mappings" format="counter"/>.  Entropy is discussed in <xref target="forward-ecmp"/>.</li>
              <li>Install state on the BFIR to impose the desired BIER packet header(s) for packets of the overlay flow. Different aspects of this and point, as well as the next point point, are discussed throughout <xref target="bier-te-controller"/> target="bier-te-controller" format="default"/> and in <xref target="encapsulation"/>, but the target="encapsulation" format="default"/>. The main responsibility of component responsible for these two points is with the Multicast Flow Overlay multicast flow overlay (<xref target="flow-overlay"/>), target="flow-overlay" format="default"/>), which is architecturally inherited from BIER.</t>
        <t>Install BIER.</li>
              <li>Install the necessary state on the BFERs to decapsulate the BIER packet header and properly dispatch its payload.</t>
    </list></t>
</list></t> payload.</li>
            </ol>
          </li>
        </ol>
        <section anchor="bier-te-controller" title="The numbered="true" toc="default">
          <name>The BIER-TE Controller">

<t>[RFC-Editor: the following text has three references to anchors topology-control, topology-control-1 and tree-control. 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.]</t> Controller</name>
          <t>This architecture describes the
BIER-TE control plane plane, as shown in <xref target="architecture"/> to consist target="architecture" format="default"/>, as consisting of:
    <list style="symbols">
        <t>A
          </t>
          <ul spacing="normal">
            <li>A BIER-TE controller.</t>
        <t>BFR data-models controller.</li>
            <li>BFR data models and protocols to communicate between the controller and BFRs
           in support of <xref target="topology-control">BIER-TE target="topology-control-1" format="none">BIER-TE topology control</xref>, control</xref> (see the list under "BIER-TE topology control"),
           such as YANG/NETCONF/RESTCONF (<xref target="RFC7950"/>/<xref target="RFC6241"/>/<xref target="RFC8040"/>).</t>
        <t>BFR data-models <xref target="RFC7950" format="default"/> <xref target="RFC6241" format="default"/> <xref target="RFC8040" format="default"/>.</li>
            <li>BFR data models and protocols to communicate between the controller and BFIR BFIRs in support of
           <xref target="tree-control">BIER-TE target="tree-control" format="none">BIER-TE tree control</xref>, control</xref> (see <xref target="control-plane"/>, point 2.), such as BIER-TE extensions
           for <xref target="RFC5440"/>.</t>
    </list>
</t> target="RFC5440" format="default"/>.</li>
          </ul>
          <t>The single, centralized BIER-TE controller is used in this document as the reference option for the BIER-TE control plane plane, but other options are equally feasible.

The BIER-TE control plane could equally be implemented without automated configuration/protocols,
by an operator via a CLI on the BFRs.

 In that case, operator configured operator-configured local policy on the BFIR would have to
determine how to set the appropriate BIER header fields.  The BIER-TE control plane could also be decentralized
and/or distributed, but this document does not consider any additional protocols and/or procedures
which
that would then be necessary to coordinate its (distributed/decentralized) entities to achieve the above described above-described functionality.</t>
          <section anchor="topology-discovery" title="BIER-TE numbered="true" toc="default">
            <name>BIER-TE Topology discovery Discovery and creation">

<t><xref target="topology-control-1">The Creation</name>
            <t>
<xref target="topology-control-1" format="none">The first item of listed for BIER-TE topology control</xref> (<xref target="control-plane"/>, point 1.a.)
includes network topology discovery and BIER-TE topology creation. The latter describes
the process by which a Controller controller determines which routers are to be configured as BFRs and the
adjacencies between them.</t>
            <t>In statically managed networks, such as in e.g., industrial environments, both discovery and creation can be a manual/offline process.</t>
            <t>In other networks, topology discovery may rely on such protocols including as those that include extending a "Link-State-Protocol" based an IGP based on a link-state protocol into the BIER-TE controller itself, e.g., BGP-LS <xref target="RFC7752"/> (BGP-LS) target="RFC7752" format="default"/> or YANG topology <xref target="RFC8345"/> (YANG topology) target="RFC8345" format="default"/>, as well as BIER-TE methods specific methods, to BIER-TE -- for example example, via <xref target="I-D.ietf-bier-te-yang"/>. target="BIER-TE-YANG" format="default"/>. These options are non-exhaustive.</t>
            <t>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 for every network subnet adjacency. In larger networks, it likely involves more complex policy and optimization decisions decisions, including how to minimize the number of BPs required and how to assign BPs across different BitStrings to minimize the number of duplicate packets across links when delivering an overlay flow to BFER BFERs using different SIs/BitStrings. SIs:BitStrings. These topics are discussed in <xref target="controller-ops"/>.</t> target="controller-ops" format="default"/>.</t>
            <t>When the BIER-TE topology is has been determined, the BIER-TE Controller then controller  pushes
the BitPositions/adjacencies BPs/adjacencies to the BIFT of the BFRs. On each BFR BFR, only those SI:BitPositions
are populated SIs:BPs
that are adjacencies to other BFRs in the BIER-TE topology.</t> topology are populated.</t>
            <t>Communications between the BIER-TE Controller controller and BFRs for both BIER-TE topology
control and BIER-TE tree control is are ideally via standardized protocols and data-models data models such
as NETCONF/RESTCONF/YANG/PCEP.  Vendor-specific  A vendor-specific CLI on the BFRs is also an option (as in many other SDN "Software-Defined Network" (SDN)
solutions lacking definition definitions of standardized data models).</t>
          </section>
          <section anchor="engineered-bitstrings" title="Engineered numbered="true" toc="default">
            <name>Engineered Trees via BitStrings"> BitStrings</name>
            <t>In BIER, the same set of BFER BFERs in a single sub-domain subdomain is always encoded as the same BitString.
In BIER-TE, the BitString used to reach the same set of BFER BFERs in the same sub-domain subdomain can be
different for different overlay flows because the BitString encodes the paths towards the BFER, BFERs,
so the BitStrings from different BFIR BFIRs to the same set of BFER BFERs will often be different. Likewise, the BitString from
the same BFIR to the same set of BFER BFERs can be different for different
overlay flows for policy reasons if different policies should be applied to those overlay
flows, such as shortest path trees, Steiner
trees (minimum cost trees), diverse path trees for redundancy redundancy, and so on.</t> on.
</t>
            <t>See also <xref target="I-D.ietf-bier-multicast-http-response"/> target="BIER-MCAST-OVERLAY" format="default"/> for an application
leveraging BIER-TE engineered trees.</t>
          </section>
          <section anchor="changes-in-topo" title="Changes numbered="true" toc="default">
            <name>Changes in the network topology"> Network Topology</name>
            <t>If the network topology changes (not failure based) so that adjacencies
that are assigned to bit positions are no longer needed, the BIER-TE Controller controller can
re-use
reuse those bit positions for new adjacencies.  First, these bit positions
need to be removed from any BFIR flow state and BFR BIFT state, then state. Then, they
can be repopulated, first into the BIFT and then into the BFIR.</t>
          </section>
      <!-- changes-in-topo -->

      <section anchor="failures" title="Link/Node numbered="true" toc="default">
            <name>Link/Node Failures and Recovery"> Recovery</name>
            <t>When link links or nodes fail or recover in the topology, BIER-TE could quickly
respond with FRR "Fast Reroute" (FRR) procedures such as those described in <xref target="I-D.eckert-bier-te-frr"/>, target="BIER-TE-PROTECTION" format="default"/>, the details of which are out of scope for this document. It can also more slowly react by
recalculating the BitStrings of affected multicast flows. This reaction is
slower than the FRR procedure because the BIER-TE Controller controller needs to receive
link/node up/down indications, recalculate the desired BitStrings BitStrings, and push
them down into the BFIRs. With FRR, this is all performed locally on a BFR
receiving the adjacency up/down notification.</t>
          </section>
      <!-- failures -->

    </section>
    <!-- control-plane -->
</section>
    <!-- XXX -->

    <section anchor="forwarding-plane" title="The numbered="true" toc="default">
        <name>The BIER-TE Forwarding Plane">

<t>[RFC-editor Q: "is constituted from" / "consists of" / "composed from..." ???]</t> Plane</name>
        <t>The BIER-TE Forwarding Plane is constituted from forwarding plane consists of the following components:
    <list style="numbers">
        <t>On
        </t>
        <ol spacing="normal" type="1"><li>On a BFIR, imposition of the BIER header for packets from overlay flows. This is driven by a combination of state established by the BIER-TE control plane and/or plane, the multicast flow overlay as explained in <xref target="flow-overlay"/>.</t>
        <t>On target="flow-overlay" format="default"/>, or a combination of both.</li>
          <li>On BFRs (including BFIR BFIRs and BFER), BFERs), forwarding/replication of BIER packets according to their SD, SI, "BitStringLength" (BSL), BitString and optionally Entropy BitString, and, optionally, entropy fields as explained in <xref target="forwarding"/>. target="forwarding" format="default"/>. Processing of other BIER header fields fields, such as DSCP the "Differentiated Services Code Point" (DSCP) field, is outside the scope of this document.</t>
        <t>On document.</li>
          <li>On BFERs, removal of the BIER header and dispatching of the payload according to state created by the BIER-TE control plane and/or overlay layer.</t>
    </list>
</t> layer.</li>
        </ol>
        <t>When the BIER-TE Forwarding Plane forwarding plane receives a packet, it simply looks
up the bit positions that are set in the BitString of the packet in the
BIFT that was populated by the BIER-TE Controller. controller.
For every BP that is set in the BitString, BitString and that has one or
more adjacencies in the BIFT, a copy is made according to the type types
of adjacencies for that BP in the BIFT. Before sending any copy, copies, the
BFR clears all BPs in the BitString of the packet for which the
BFR has one or more adjacencies in the BIFT.  Clearing these bits inhibits prevents
packets from looping when the BitStrings a BitString erroneously includes a forwarding loop.
When a forward_connected() adjacency has the "DoNotClear" (DNC) flag
set, then this BP is re-set reset for the packet copied to that adjacency.
See <xref target="forward-connected"/>.</t> target="forward-connected" format="default"/>.</t>
      </section>
    <!-- forwarding-plane -->

    <section anchor="routing-underlay" title="The numbered="true" toc="default">
        <name>The Routing Underlay"> Underlay</name>
        <t>For forward_connected() adjacencies, BIER-TE is sending sends BIER packets to directly connected
BIER-TE neighbors as L2 (unicasted) (unicast) BIER packets without requiring a
routing underlay. For forward_routed() adjacencies, BIER-TE forwarding encapsulates
a copy of the BIER packet so that it can be delivered by the forwarding plane
of the routing underlay to the routable destination address indicated in the adjacency.
See <xref target="forward-routed"/> target="forward-routed" format="default"/> for the adjacency definition.</t> details on forward_routed() adjacencies.</t>
        <t>BIER relies on the routing underlay to calculate paths towards BFERs and derive next-hop BFR adjacencies for those paths. This These two steps commonly relies rely on BIER specific BIER-specific extensions to the routing protocols of the routing underlay but may also be established
by a controller. In BIER-TE, the next-hops of next hops for a packet are determined by the BitString
through the BIER-TE Controller established controller-established adjacencies on the BFR for the BPs of the BitString.
There is thus no need for BFR specific BFR-specific routing underlay extensions to forward BIER packets with
BIER-TE semantics.</t>
        <t>Encapsulation parameters can be provisioned by the BIER-TE controller into
the forward_connected() or forward_routed() adjacencies directly without relying on a routing underlay.
</t>
        <t>If the BFR intends to support FRR for BIER-TE, then the BIER-TE
forwarding plane needs to receive fast adjacency up/down notifications:
Link
link up/down or neighbor up/down, e.g. e.g., from BFD. "Bidirectional Forwarding Detection" (BFD). Providing these notifications
is considered to be part of the routing underlay in this document.</t>
      </section>
    <!-- routing-underlay -->

    <section anchor="te-considerations" title="Traffic numbered="true" toc="default">
        <name>Traffic Engineering Considerations"> Considerations</name>
        <t>Traffic Engineering (<xref target="I-D.ietf-teas-rfc3272bis"/>) <xref target="TE-OVERVIEW" format="default"/>
provides performance optimization of operational IP networks while utilizing
 network resources economically and
reliably.  The key elements needed to effect TE Traffic Engineering are policy, path steering steering,
and resource management. These elements require support at the
control/controller level and within the forwarding plane.</t>
        <t>Policy decisions are made within the BIER-TE control plane, i.e., within
BIER-TE Controllers. controllers.  Controllers use policy when composing BitStrings
and BFR BIFT state. The mapping of user/IP traffic to specific
BitStrings/BIER-TE
BitStrings / BIER-TE flows is made based on policy. The specific details of
BIER-TE policies and how a controller uses them are out of scope of for this
document.</t>
        <t>Path steering is supported via the definition of a BitString.  BitStrings
used in BIER-TE are composed based on policy and resource management
considerations.  For example, when composing BIER-TE BitStrings, a Controller controller must take
into account the resources available at each BFR and for each BP
when it is providing congestion-loss-free services such as
Rate Controlled
Rate-Controlled Service Disciplines <xref target="RCSD94"/>. target="RCSD94" format="default"/>.  Resource availability
could be provided provided, for example example, via routing protocol information, information but
may also be obtained via a BIER-TE control protocol such as NETCONF or
any other protocol commonly used by a Controller controller to understand the resources
of the network on which it operates on. operates.  The
resource usage of the BIER-TE traffic admitted by the BIER-TE controller
can be solely tracked on the BIER-TE Controller controller based on local accounting
as long as no forward_routed() adjacencies are used (see <xref target="forward-connected"/> target="forward-routed" format="default"/> for the definition
of forward_routed() adjacencies).  When forward_routed() adjacencies are used,
the paths selected by the underlying routing protocol need to be tracked as well.</t>
        <t>Resource management has implications on for the forwarding plane beyond
the BIER-TE defined BIER-TE-defined steering of packets.  This packets; this includes allocation of
buffers to guarantee the worst case worst-case requirements of for admitted RCSD traffic
and potentially policing and/or rate-shaping mechanisms, typically done
via various forms of queuing.  This level of resource control,
while optional, is important in networks that wish to
support congestion management policies to control or regulate the offered
traffic to deliver different levels of service and alleviate congestion
problems, or those networks that wish to control latencies experienced by
specific traffic flows.</t>
      </section>
    <!-- te-considerations -->

  </section>
  <!-- components -->

  <section anchor="forwarding" title="BIER-TE Forwarding"> numbered="true" toc="default">
      <name>BIER-TE Forwarding</name>
      <section anchor="btft" title="The numbered="true" toc="default">
        <name>The BIER-TE Bit Index Forwarding Table (BIFT)"> (BIFT)</name>
        <t>The BIER-TE BIFT is the equivalent to the BIER BIFT for (non-TE) BIER. BIER BIFT. It
exists on every BFR running BIER-TE. For every BIER sub-domain "subdomain" (SD) in use for BIER-TE,
it
the BIFT is a table as shown constructed per the example shown in <xref target="adjacencies"/>. That example target="adjacencies" format="default"/>. The
BIFT in the figure assumes a BSL of 8 bit positions "bit positions" (BPs) in the packets BitString.
As in <xref target="RFC8279"/> target="RFC8279" format="default"/>, this BSL is purely used for the as an example and is not a BIER/BIER-TE
supported BSL supported by BIER/BIER-TE
(minimum BSL is 64).</t>
        <t>A BIER-TE BIFT compares is compared to a BIER BIFT as shown in <xref target="RFC8279"/> target="RFC8279" format="default"/> as
follows.</t>
        <t>In both BIER and BIER-TE, BIFT rows/entries are indexed in their respective BIER pseudocode
(<xref target="RFC8279"/> Section 6.5) target="RFC8279" sectionFormat="comma" section="6.5"/>) and BIER-TE pseudocode (<xref target="pseudocode"/>) target="pseudocode" format="default"/>)
by the BIFT-index derived from the packets packet's SI, BSL BSL, and the one bit position of the
packets BitString (BP) addressing the BIFT row: BIFT-index = SI * BSL + BP - 1.
BP
BPs within a BitString are numbered from 1 to BSL, hence BSL -- hence, the - 1 offset when converting
to a BIFT-index. This document also uses the notion SI:BP "SI:BP" to indicate BIFT rows, rows.
<xref target="RFC8279"/> target="RFC8279" format="default"/> uses the equivalent notion SI:BitString, "SI:BitString", where the BitString is
filled with only the BP BPs for the BIFT row.</t>
        <t>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 Neighbor" (BFR-NBR) towards that BFER.</t>
        <t>In BIER-TE, each BIFT-index and therefore and, therefore, SI:BP indicates one or or, in the case of reuse of SI:BP, more adjacencies than one adjacency between BFRs in the topology and topology. The SI:BP
is only populated with those adjacencies  forwarding
entries the adjacency on the upstream BFR that is of the upstream for these adjacencies. adjacency.  The BIFT entry entries are empty on all other BFRs.</t>
        <t>In BIER, each BIFT row also requires a "Forwarding Bit Mask" (F-BM) entry
for BIER forwarding rules. In BIER-TE forwarding, an F-BM is not required, required but can be used
when implementing BIER-TE on forwarding hardware hardware, derived from BIER forwarding, that
must use an F-BM. This is discussed in the first variation of BIER-TE forwarding pseudocode shown in
 <xref target="pseudocode"/>.</t> target="pseudocode" format="default"/>.</t>

<figure anchor="adjacencies" title="BIER-TE BIFT anchor="adjacencies">
<name>BIER-TE Bit Index Forwarding Table (BIFT) with different adjacencies"> Different Adjacencies</name>
<artwork align="left"><![CDATA[
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 | BIFT-index |      | Adjacencies:                                |
 | (SI:BP)    |(FBM)|    |(F-BM)| <empty> or one or more per entry            |
  ==================================================================
 ===================================================================
 |               BIFT indices for Packets with SI=0                |
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 | 0 (0:1)    | ...  | forward_connected(interface,neighbor{,DNC}) |
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 | 1 (0:2)    | ...  | forward_connected(interface,neighbor{,DNC}) |
 |            | ...  | forward_connected(interface,neighbor{,DNC}) |
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 |  ...       | ...  | ...                                         |
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 | 4 (0:5)    | ...  | local_decap({VRF})                          |
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 | 5 (0:6)    | ...  | forward_routed({VRF,}l3-neighbor)           |
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 | 6 (0:7)    | ...  | <empty>                                     |
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 | 7 (0:8)    | ...  | ECMP((adjacency1,...adjacencyN){,seed})     |
  -----------------------------------------------------------------
 -------------------------------------------------------------------
 |           BIFT indices for BitString/Packet with SI=1           |
  ------------------------------------------------------------------
 -------------------------------------------------------------------
 | 9 (1:1)    |      | ...                                         |
 |  ...       |...       | ...  |
  ------------------------------------------------------------------
               BIER-TE Bit Index Forwarding Table (BIFT) ...                                         |
 -------------------------------------------------------------------
]]></artwork></figure>
        <t>The BIFT is configured for the BIER-TE data plane of a BFR by the BIER-TE
Controller
controller through an appropriate protocol and data-model. data model. The BIFT is then
used to forward packets, according to the rules
specified in procedures for the BIER-TE Forwarding Procedures.</t> forwarding plane as specified in <xref target="forwarding-plane"/>.</t>
        <t>Note that a BIFT index BIFT-index (SI:BP) may be populated in the BIFT of more
than one BFR to save BPs. See <xref target="rings"/> target="rings" format="default"/> for an example of how a BIER-TE controller
could assign BPs to (logical) adjacencies shared across multiple BFRs,
<xref target="leaf-bfer"/> target="leaf-bfer" format="default"/> for an example of assigning the same BP to different
adjacencies, and <xref target="reuse"/> target="reuse" format="default"/> for general guidelines regarding re-use the reuse of BPs across different adjacencies.</t>
        <t>{VRF} indicates the Virtual Routing and Forwarding context into which
the BIER payload is to be delivered. This is optional and depends
on the multicast flow overlay.</t>
      </section>
    <!-- btft -->

    <section anchor="atypes" title="Adjacency Types"> numbered="true" toc="default">
        <name>Adjacency Types</name>
        <section anchor="forward-connected" title="Forward Connected"> numbered="true" toc="default">
          <name>Forward Connected</name>
          <t>A "forward_connected()" adjacency is an adjacency towards a directly connected
BFR neighbor
BFR-NBR using an interface address of that BFR on the connecting
interface. A forward_connected() adjacency does not route packets
but packets;
only L2 forwards them to the neighbor.</t>
          <t>Packets sent to an adjacency with "DoNotClear" (DNC) set in the
BIFT MUST NOT <bcp14>MUST NOT</bcp14> have the bit position for that adjacency cleared when the
BFR creates a copy for it. The bit position will still be cleared for
copies of the a packet made towards other adjacencies. This can be
used
used, for example example, in ring topologies as explained in <xref target="rings"/>.</t> target="rings" format="default"/>.</t>
          <t>For protection against loops from caused by misconfiguration (see <xref target="loops"/>), target="loops" format="default"/>),
DNC is only permissible for forward_connected() adjacencies. No need or benefit
of DNC for other type types of adjacencies was identified identified, and their risk was associated risks were not analyzed.</t>
        </section>
      <!-- forward-connected -->

      <section anchor="forward-routed" title="Forward Routed"> numbered="true" toc="default">
          <name>Forward Routed</name>
          <t>A "forward_routed()" adjacency is an adjacency towards a BFR that
uses a (tunneling) encapsulation which that will cause the a packet to be
forwarded by the routing underlay toward towards the adjacent BFR. BFR indicated via the l3-neighbor parameter of the forward_routed() adjacency.  This can
leverage any feasible encapsulation, such as MPLS or tunneling over IP/IPv6,
as long as the BIER-TE packet can be identified as a payload. This identification
can either rely on either the BIER/BIER-TE co-existence mechanisms described in
<xref target="encapsulation"/>, target="encapsulation" format="default"/> or by explicit support for a BIER-TE payload type
in the tunneling encapsulation.</t>

<t>forward_routed()
          <t>Forward_routed() adjacencies are necessary to pass BIER-TE traffic across
non
routers that are not BIER-TE capable routers or to minimize the number of required BP BPs by
tunneling over (BIER-TE capable) (BIER-TE-capable) routers on which neither replication nor
path-steering
path steering is desired, or simply to leverage the routing underlay's path redundancy and FRR of the
routing underlay towards the next BFR. They may also be useful to a
multi-subnet adjacent BFR to leverage for leveraging the routing underlay ECMP
independent
independently of BIER-TE ECMP (<xref target="forward-ecmp"/>).</t> target="forward-ecmp" format="default"/>).</t>
        </section>
      <!-- forward-routed -->

      <section anchor="forward-ecmp" title="ECMP">

<t>(non-TE) numbered="true" toc="default">
          <name>ECMP</name>
          <t>(Non-TE) BIER ECMP is tied to the BIER BIFT processing semantic and is therefore
not directly usable with BIER-TE.</t>
          <t>A BIER-TE "Equal Cost "Equal-Cost Multipath" (ECMP()) adjacency as shown in <xref target="adjacencies"/> target="adjacencies" format="default"/>
for BIFT-index 7 has a list of two or more non-ECMP non-ECMP() adjacencies as parameters and an optional
seed parameter. When a BIER-TE packet is copied
onto such an ECMP() adjacency, an implementation specific implementation-specific so-called hash function
will select one out of the list's adjacencies to which the packet is forwarded.
If the packet's encapsulation contains an entropy field, the entropy field SHOULD <bcp14>SHOULD</bcp14>
be respected; two packets with the same value of the entropy field SHOULD <bcp14>SHOULD</bcp14> be sent on
the same adjacency.  The seed parameter allows to permits the design of
hash functions that are easy to implement at high speed without running into
polarization issues across multiple consecutive ECMP hops. See <xref target="ecmp"/> target="ecmp" format="default"/>
for more explanations.</t> details.</t>
        </section>
      <!-- forward-ecmp -->

      <section anchor="forward-local" title="Local Decap(sulation)"> numbered="true" toc="default">
          <name>Local Decap(sulation)</name>
          <t>A "local_decap()" adjacency passes a copy of the payload of
the BIER-TE packet to the protocol ("NextProto") within the BFR (IPv4/IPv6, (IP/IPv6, Ethernet,...) responsible for
that payload according to the packet header fields.
A local_decap() adjacency turns the BFR into a BFER for matching
packets. Local_decap() adjacencies require the BFER to support
routing or switching for NextProto to determine how to further
process the packet.</t> packets.</t>
        </section>
      <!-- forward-local -->

    </section>
    <!-- atypes -->

    <section anchor="encapsulation" title="Encapsulation numbered="true" toc="default">
        <name>Encapsulation / Co-existence with BIER"> BIER</name>
        <t>Specifications for BIER-TE encapsulation are outside the scope of this document.
This section gives explanations and guidelines.</t>

<t>Like <xref target="RFC8279"/>,
        <t>The handling of "Maximum Transmission Unit" (MTU) limitations is
outside the scope of this document and instead is not discussed in
<xref target="RFC8279" format="default"/> either.  Instead, this process is part of the BIER-TE packet encapsulation and/or flow overlay. See overlay; for example example, see
<xref target="RFC8296"/>, Section 3. target="RFC8296" sectionFormat="comma" section="3"/>.
It applies equally to BIER-TE as it does to and BIER.</t>
        <t>Because a BFR needs to interpret the BitString of a BIER-TE packet differently
from a (non-TE) BIER packet, it is necessary to distinguish BIER packets from BIER-TE packets.
In the BIER encapsulation <xref target="RFC8296"/>, target="RFC8296" format="default"/>,
the BIFT-id field of the packet indicates the BIFT of the packet. BIER and BIER-TE can
therefore be run simultaneously, when the BIFT-id address space is shared across
BIER BIFT BIFTs and BIER-TE BIFT. BIFTs. Partitioning the BIFT-id address space is subject
to BIER-TE/BIER control plane procedures.</t>
        <t>When <xref target="RFC8296"/> target="RFC8296" format="default"/> is used for BIER with MPLS, BIFT-id address ranges
can be dynamically allocated from MPLS label space only for the set of actually
used SD:BSL BIFT. BIFTs.  This allows to also allocate permits the allocation of non-overlapping label ranges for BIFT-id BIFT-ids
that are to be used with BIER-TE BIFTs.</t>
        <t>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 corresponding range of BIFT-ids and disjoint BIER-TE BIFTs with a non-overlapping range of BIFT-ids.</t>

<t>When
        <t>Assume that a fixed mapping from BSL, SD SD, and SI to a BIFT-id is used used,
which does not explicitly partition the BIFT-id space between BIER
and BIER-TE,
such BIER-TE -- for example, as proposed for non-MPLS forwarding with <xref target="RFC8296"/>
BIER encapsulation <xref target="RFC8296" format="default"/>
in <xref target="I-D.ietf-bier-non-mpls-bift-encoding"/>
revision 04, section 5, then target="NON-MPLS-BIER-ENCODING" sectionFormat="comma" section="5"/>.
In this case, it is necessary to allocate disjoint SDs to BIER and BIER-TE BIFTs
so that both can be addressed by the BIFT-ids.  The encoding
proposed in section 6. of the same document <xref target="NON-MPLS-BIER-ENCODING" sectionFormat="of" section="6"/> does not statically encode the BSL or SD into the BIFT-id, but allows for the encoding
permits a mapping, mapping and hence could provide for the same freedom as when
MPLS is being used (same (the same SD, or different SD SDs for BIER/BIER-TE).</t>

<t>forward_routed() BIER/BIER-TE).
</t>
        <t>Forward_routed() requires an encapsulation that permits to direct directing unicast encapsulated BIER-TE packets to a specific interface address on a target BFR. With MPLS encapsulation, this can
simply be done via a label stack with that addresses address's label as the top label - label, followed
by the label assigned to the (BSL,SD,SI) BitString.
With non-MPLS encapsulation, some form of IP encapsulation would be required (for example example, IP/GRE).
</t>
        <t>The encapsulation used for forward_routed() adjacencies can equally support
existing advanced adjacency information such as "loose source routes" via e.g. via, for example, MPLS
label stacks or appropriate header extensions (e.g. (e.g., for IPv6).</t>
      </section>
    <!-- encapsulation -->

  <section anchor="pseudocode" title="BIER-TE numbered="true" toc="default">
        <name>BIER-TE Forwarding Pseudocode"> Pseudocode</name>
        <t>
The following pseudocode, <xref target="simple-pseudocode-picture"/>, pseudocode for BIER-TE forwarding forwarding, as shown in <xref target="simple-pseudocode-picture" format="default"/>, is based
on the (non-TE) BIER forwarding pseudocode of provided in <xref target="RFC8279"/>, section 6.5 target="RFC8279" sectionFormat="comma" section="6.5"/>, with one modification.</t>
        <figure anchor="simple-pseudocode-picture" title="BIER-TE anchor="simple-pseudocode-picture">
          <name>BIER-TE Forwarding Pseudocode for required functions, based Required Functions, Based on BIER Pseudocode">
<artwork align="left"><![CDATA[ Pseudocode</name>
<sourcecode name="" type="pseudocode"><![CDATA[
   void ForwardBitMaskPacket_withTE (Packet)
   {
       SI=GetPacketSI(Packet);
       Offset=SI*BitStringLength;
       for (Index = GetFirstBitPosition(Packet->BitString); Index ;
            Index = GetNextBitPosition(Packet->BitString, Index)) {
           F-BM = BIFT[Index+Offset]->F-BM;
           if (!F-BM) continue;                            [3]
           BFR-NBR = BIFT[Index+Offset]->BFR-NBR;
           PacketCopy = Copy(Packet);
           PacketCopy->BitString &= F-BM;                  [2]
           PacketSend(PacketCopy, BFR-NBR);
           // The following must not be done for BIER-TE:
           // Packet->BitString &= ~F-BM;                  [1]
       }
   }
]]></artwork></figure>
]]></sourcecode>
        </figure>
        <t>In step [2], the F-BM is used to clear bit(s) 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.</t>
        <t>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 set,
the BIER
BIER's replication logic has to avoid that prevent more than one PacketCopy is from being
sent to that BFR-NBR ([1]). Likewise, the PacketCopy sent to a BFR-NBR
must clear all bits in its BitString that are not routed across a BFR-NBR.
This protects against BIER prevents BIER's replication logic from creating duplicates on any possible further
BFR to create duplicates BFRs ([2]).</t>
        <t>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 a BFR-NBR. [2] &nbsp;[2] clears
all other bits in PacketCopy->BitString, PacketCopy-&gt;BitString, and [1] clears those bits from
Packet->BitString
Packet-&gt;BitString after the first PacketCopy.</t>
        <t>In BIER-TE, a BFR-NBR in this pseudocode is an adjacency, adjacency -- forward_connected(), forward_routed() forward_routed(),
or local_decap(). There is no need for [2] to suppress duplicates in the same way
that BIER does does, because in general, different BP BPs would never have the same
adjacency. If a BIER-TE controller actually finds some optimization in
which this would be desirable, then the controller is also responsible to
ensure for
ensuring that only one of those bits is set in any Packet->BitString, Packet-&gt;BitString, unless
 the controller explicitly wants for duplicates to be created.</t>
        <t>The following points describe how the forwarding bit mask (F-BM) F-BM for each BP is configured in the BIFT and how this impacts the BitString of the packet being processed with that BIFT:
<list style="numbers">
  <t>The
</t>
        <ol spacing="normal" type="1"><li>The F-BMs of all BIFT BPs without an adjacency have all their bits clear.
  This will cause [3] to skip further processing of such a BP.</t>
  <t>All BP.</li>
          <li>All BIFT BPs with an adjacency (with the DNC flag clear) have an F-BM
  that has only those BPs set for which this BFR does not have an adjacency.
  This causes [2] to clear all bits from PacketCopy->BitString PacketCopy-&gt;BitString for which this
  BFR does have an adjacency.</t>
  <t>[1] adjacency.</li>
          <li>[1] is not performed for BIER-TE. All bit clearing required by BIER-TE
  is performed by [2].</t>
</list></t> [2].</li>
        </ol>
        <t>This Forwarding Pseudocode forwarding pseudocode can support the required BIER-TE forwarding
functions (see <xref target="requirements"/>), target="requirements" format="default"/>) -- forward_connected(),
forward_routed()
forward_routed(), and local_decap(), local_decap() -- but not cannot support the recommended functions DNC (DNC flag and multiple adjacencies per bit nor bit) or the optional function, function (i.e., ECMP() adjacencies. adjacencies).
The DNC flag cannot be supported when using only [1] to mask bits.</t>
        <t>The modified and expanded Forwarding Pseudocode forwarding pseudocode in <xref target="pseudocode-picture"/> target="pseudocode-picture" format="default"/> specifies how to
support all BIER-TE forwarding functions (required, recommended recommended, and optional):
<list style="symbols">
</t>
        <ol spacing="normal">
          <li>
            <t>This pseudocode eliminates per-bit F-BM, F-BMs, therefore reducing the size of BIFT state by BSL^2*SI SI*BSL<sup>2</sup> and eliminating the need for per-packet-copy BitString masking operations operations, except for adjacencies with the DNC flag set:
    <list style="symbols">
        <t>AdjacentBits[SI]
            </t>
            <ol spacing="normal" type="%p%c">
              <li>AdjacentBits[SI] are bit positions with a non-empty list of adjacencies in this BFR BIFT. This can be computed whenever the BIER-TE Controller controller updates (add/removes) (adds/removes) adjacencies in the BIFT.</t>
        <t>The BIFT.</li>
              <li>The BFR needs to create packet copies for these adjacent bits when they are set in the packets BitString. This set of bits is calculated in PktAdjacentBits.</t>
        <t>All PktAdjacentBits.</li>
              <li>All bit positions to for which the BFR creates copies have to be cleared in packet copies to avoid loops. This is done by masking the BitString of the packet with ~AdjacentBits[SI]. When an adjacency has DNC set, this bit position is set again only for the packet copy towards that bit position.</t>
    </list></t>
    <t>BIFT position.</li>
            </ol>
          </li>
          <li>BIFT entries may contain more than one adjacency in support of specific configurations configurations, such as <xref target="hubnspoke"/>. a hub and multiple spokes (<xref target="hubnspoke" format="default"/>). The code therefore includes a loop over these adjacencies.</t>
    <t>The adjacencies.</li>
          <li>The ECMP() adjacency is shown. also shown in the figure. Its parameters are a seed and a ListOfAdjacencies "ListOfAdjacencies", from which one is picked.</t>
    <t>The picked.</li>
          <li>The forward_connected(), forward_routed(), and local_decap() adjacencies are shown with their parameters.</t>
</list></t> parameters.</li>
        </ol>
        <figure anchor="pseudocode-picture" title="Complete anchor="pseudocode-picture">
          <name>Complete BIER-TE Forwarding Pseudocode for required, recommended Required, Recommended, and optional functions">
<artwork align="left"><![CDATA[ Optional Functions</name>
<sourcecode name="" type="pseudocode"><![CDATA[
 void ForwardBitMaskPacket_withTE (Packet)
 {
     SI = GetPacketSI(Packet);
     Offset = SI * BitStringLength;
     // Determine adjacent bits in the Packets packets BitString
     PktAdjacentBits = Packet->BitString & AdjacentBits[SI];

     // Clear adjacent bits in Packet the packet header to avoid loops
     Packet->BitString &= ~AdjacentBits[SI];

     // Loop over PktAdjacentBits to create packet copies
     for (Index = GetFirstBitPosition(PktAdjacentBits); Index ;
          Index = GetNextBitPosition(PktAdjacentBits, Index)) {
         for adjacency in BIFT[Index+Offset]->Adjacencies {
             if(adjacency.type == ECMP(ListOfAdjacencies,seed) ) {
                 I = ECMP_hash(sizeof(ListOfAdjacencies),
                               Packet->Entropy,seed);
                 adjacency = ListOfAdjacencies[I];
             }
             PacketCopy = Copy(Packet);
             switch(adjacency.type) {
                 case forward_connected(interface,neighbor,DNC):
                     if(DNC)
                         PacketCopy->BitString |= 1<<(Index-1);
                     SendToL2Unicast(PacketCopy,interface,neighbor);

                 case forward_routed({VRF,}l3-neighbor):
                     SendToL3(PacketCopy,{VRF,}l3-neighbor);

                 case local_decap({VRF},neighbor):
                     DecapBierHeader(PacketCopy);
                     PassTo(PacketCopy,{VRF,}Packet->NextProto);
             }
         }
     }
 }
]]></artwork></figure>
]]></sourcecode>
        </figure>
      </section>
  <!-- pseudocode -->

<section anchor="requirements" title="BFR numbered="true" toc="default">
        <name>BFR Requirements for BIER-TE forwarding">

<t>BFR Forwarding</name>
        <t>BFRs that support BIER-TE and BIER MUST <bcp14>MUST</bcp14> support a configuration that enables
BIER-TE instead of (non-TE) BIER forwarding rules for all BIFT BIFTs of one or more
BIER sub-domains. subdomains. Every BP in a BIER-TE BIFT MUST <bcp14>MUST</bcp14> support to have having
zero or one adjacency. BIER-TE forwarding MUST <bcp14>MUST</bcp14> support the adjacency types forward_connected() with the DNC flag not set, forward_routed() forward_routed(), and local_decap().
As explained in <xref target="pseudocode"/>, target="pseudocode" format="default"/>, these required BIER-TE forwarding functions
can be implemented via the same Forwarding Pseudocode forwarding pseudocode as that used for BIER forwarding forwarding, except for
one modification (skipping one masking with an F-BM).</t>
        <t>BIER-TE forwarding SHOULD <bcp14>SHOULD</bcp14> support forward_connected() adjacencies with a set the DNC flag, flag set,
as this is highly very useful to save for saving bits in rings (see <xref target="rings"/>).</t> target="rings" format="default"/>).</t>
        <t>BIER-TE forwarding SHOULD <bcp14>SHOULD</bcp14> support more than one adjacency on a bit.
This allows to save bits to be saved in hub and spoke hub-and-spoke scenarios (see <xref target="hubnspoke"/>).</t> target="hubnspoke" format="default"/>).</t>
        <t>BIER-TE forwarding MAY <bcp14>MAY</bcp14> support ECMP() adjacencies to save bits in ECMP
scenarios,
scenarios; see <xref target="ecmp"/> target="ecmp" format="default"/> for an example.
This is an optional requirement, because for ECMP deployments using BIER-TE
one can also leverage ECMP of the routing underlay ECMP via forwarded_routed forward_routed()
adjacencies and/or might prefer to have more explicit control of the path
chosen via explicit BP/adjacencies BPs/adjacencies for each ECMP path alternative.</t>
      </section>
    </section>
  <!-- forwarding -->

<section anchor="controller-ops" title="BIER-TE numbered="true" toc="default">
      <name>BIER-TE Controller Operational Considerations"> Considerations</name>
      <section anchor="bitpositions" title="Bit numbered="true" toc="default">
        <name>Bit Position Assignments"> Assignments</name>
        <t>This section describes how the BIER-TE Controller controller can use the
different BIER-TE adjacency types to define the bit positions of a BIER-TE domain.</t>
        <t>Because the size of the BitString limits the size of the
BIER-TE domain, many of the options described here exist to support larger
topologies with fewer bit positions.</t>
        <section anchor="p2p-links" title="P2P Links"> numbered="true" toc="default">
          <name>P2P Links</name>
          <t>On a P2P "point-to-point" (P2P) link that connects two BFRs, the same bit position can be used on
both BFRs for the adjacency to the neighboring BFR. A P2P link requires therefore requires
only one bit position.</t>
        </section>
    <!-- p2p-links -->

    <section anchor="bfer" title="BFER"> numbered="true" toc="default">
          <name>BFERs</name>
          <t>Every non-Leaf non-leaf BFER is given a unique bit position with a local_decap() adjacency.</t>
        </section>
    <!-- bfer -->

    <section anchor="leaf-bfer" title="Leaf BFERs">

<figure anchor="leaf-bfer-picture" title="Leaf vs. non-Leaf BFER Example">
<artwork align="left"><![CDATA[
        BFR1(P) BFR2(P)             BFR1(P)  BFR2(P)
          |  \ /  |                    |       |
          |   X   |                    |       |
          |  / \  |                    |       |
     BFER1(PE)  BFER2(PE)        BFER1(PE)----BFER2(PE)

                                           ^ U-turn link

         Leaf BFER /               Non-Leaf BFER /
          PE-router                  PE-router
]]></artwork></figure> numbered="true" toc="default">
          <name>Leaf BFERs</name>
          <t>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 "Provider Edge" (PE) router routers
are spokes connected to Provider (P) routers, those PEs are Leaf BFERs leaf BFERs, unless
there is a U-turn between two PEs.</t>
          <t>Consider how redundant disjoint
traffic can reach BFER1/BFER2 as shown in <xref target="leaf-bfer-picture"/>: When target="leaf-bfer-picture" format="default"/>: when BFER1/BFER2
are Non-Leaf BFER 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 non-leaf
BFER. Likewise, BFER1 is a non-Leaf non-leaf BFER when forwarding traffic to BFER2.
Note that the BFERs in on the left-hand picture side of the figure are only guaranteed to
be leaf-BFER leaf BFERs by fitting correctly applying a routing configuration that prohibits transit
traffic to pass from passing through the BFERs, which is commonly applied in these
topologies.</t>
          <figure anchor="leaf-bfer-picture">
            <name>Leaf vs. Non-Leaf BFER Example</name>
            <artwork align="left" name="" type="" alt=""><![CDATA[
        BFR1(P) BFR2(P)             BFR1(P)  BFR2(P)
          |  \ /  |                    |       |
          |   X   |                    |       |
          |  / \  |                    |       |
     BFER1(PE)  BFER2(PE)        BFER1(PE)----BFER2(PE)

                                           ^ U-turn link

         Leaf BFER /               Non-leaf BFER /
          PE router                  PE router
]]></artwork>
          </figure>
          <t>In most situations, leaf-BFER leaf BFERs that are to be addressed via the same BitString can share a single bit position for their local_decap() 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 BFER BFER, or it may also need to also be decapsulated. Whether or not to decapsulate the packet therefore needs to be indicated by a unique bit position populated only on the BIFT of this BFER with a local_decap() adjacency. On a leaf-BFER, leaf BFER, packets never need to pass through; any packet received is therefore usually intended to be decapsulated. This can be expressed by a single, shared bit position that is populated with a local_decap() adjacency on all leaf-BFER leaf BFERs addressed by the BitString.</t>
          <t>The possible exception from exceptions to this leaf-BFER leaf BFER bit position optimization scenario can be cases where the bit position on the prior BIER-TE BFR (which created the packet copy for the leaf-BFER leaf BFER in question) is populated with multiple adjacencies as an optimization, such optimization -- for example, as described in Sections&nbsp;<xref target="lans" format="counter"/> and <xref target="lans"/> or <xref target="hubnspoke"/>. target="hubnspoke" format="counter"/>. With either of these two optimizations, the sender of the packet could only control explicitly whether the packet was to be decapsulated on the leaf-BFER leaf BFER in question, if the leaf-BFER leaf BFER has a unique bit position for its local_decap() adjacency.</t>
          <t>However, if the bit position is shared across leaf-BFER, a leaf BFER and packets are therefore decapsulated -- potentially unnecessarily, unnecessarily -- this may still be appropriate if the decapsulated payload of the BIER-TE packet indicates whether or not the packet needs packets need to be further processed/received. This is typically true true, for example example, if the payload is IP multicast multicast, because IP multicast on a BFER would know the membership state of the IP multicast payload and be able to discard it if the packet was packets were delivered unnecessarily by the BIER-TE layer. If the payload has no such membership indication, indication and the BFIR wants to have explicit control about regarding which BFER BFERs are to receive and decapsulate a packet, then these two optimizations can not cannot be used together with shared bit positions position optimization for leaf-BFER.</t> a leaf BFER.</t>
        </section>
    <!-- leaf-bfer -->

    <section anchor="lans" title="LANs"> numbered="true" toc="default">
          <name>LANs</name>
          <t>In a LAN, the adjacency to each neighboring BFR
is given a unique bit position. The adjacency of this bit position
is a forward_connected() adjacency towards the BFR BFR, and this bit position
is populated into the BIFT of all the other BFRs on that LAN.</t>
          <figure anchor="lan-picture" title="LAN Example"> anchor="lan-picture">
            <name>LAN Example</name>
            <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
                                 BFR1
                                  |p1
                           LAN1-+-+---+-----+
                              p3|   p4|   p2|
                              BFR3  BFR4  BFR7
]]></artwork></figure>
]]></artwork>
          </figure>
          <t>If Bandwidth 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 saved by assigning just a single bit position to the LAN
and populating the bit position of the BIFTs of each BFRs BFR on
the LAN with a list of forward_connected() adjacencies to all other
neighbors on the LAN.</t>
          <t>This optimization does not work in the case of BFRs redundantly
connected to more than one LAN with this optimization because
these optimization. These
BFRs would receive duplicates and forward those duplicates into the opposite
other LANs. Adjacencies of such Such BFRs into their LAN still
need a require separate bit position.</t> positions for each LAN they
connect to.</t>
        </section>
    <!-- lans -->

    <section anchor="hubnspoke" title="Hub numbered="true" toc="default">
          <name>Hub and Spoke"> Spoke</name>
          <t>In a setup with a hub and multiple spokes connected via separate
p2p
P2P links to the hub, all p2p P2P adjacencies from the hub to the spokes spokes' 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 for each Spoke.</t> spoke.</t>
          <t>This option is similar to the bit position optimization in
LANs: Redundantly redundantly connected spokes need their own bit positions,
unless they are themselves Leaf-BFER.</t> leaf BFERs.</t>
          <t>This type of optimized BP could be used used, for example example, when all
traffic is "broadcast" traffic (very dense receiver set) sets),
such as live-TV live TV or many-to-many telemetry telemetry, including situation-awareness (SA). situational awareness.
This BP optimization can then be used to explicitly steer different traffic
flows across different ECMP paths in Data-Center data-center or broadband-aggregation
networks with minimal use of BPs.</t>
        </section>
    <!-- hubnspoke -->

    <section anchor="rings" title="Rings"> numbered="true" toc="default">
          <name>Rings</name>
          <t>In L3 rings, instead of assigning a single bit position for
every p2p P2P link in the ring, it is possible to save bit positions by
setting the "DoNotClear" (DNC) flag on forward_connected() adjacencies.</t>
          <t>For the rings ring shown in <xref target="ring-picture"/>, target="ring-picture" format="default"/>, a single bit position
will suffice to forward traffic entering the ring at BFRa or BFRb
all the way up to BFR1:</t> BFR1, as follows.</t>
          <t>On BFRa, BFRb, BFR30,... BFR3, the bit position is populated with
a forward_connected() adjacency pointing to the clockwise neighbor
on the ring and with DNC set. On BFR2, the adjacency also points
to the clockwise neighbor BFR1, but without DNC set.</t>
          <t>Handling DNC this way ensures that copies forwarded from any BFR BFRs in
the ring to a BFR outside the ring will not have the ring bit position set,
therefore minimizing the chance to create risk of creating loops.</t>
          <figure anchor="ring-picture" title="Ring Example"> anchor="ring-picture">
            <name>Ring Example</name>
            <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
               v        v
               |        |
        L1     |   L2   |   L3
    /-------- BFRa ---- BFRb --------------------\
    |                                            |
    \- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
        |      |    L4               |      |
     p33|                         p15|
        BFRd                       BFRc
]]></artwork></figure>
]]></artwork>
          </figure>
          <t>Note that this example only permits for packets intended to make it all
the way around the ring to enter it at
BFRa and BFRb, and BFRb.  Note also that packets will always travel clockwise. If
packets should be allowed to enter the ring at any ring BFR, of the ring's BFRs, then one
would have to use two ring bit positions. One positions, one for each direction:
clockwise and counterclockwise.</t>
          <t>Both would be set up to stop rotating on the same link, e.g. e.g., L1. When the
ingress ring BFR
ring's BFIR creates the clockwise copy, it will clear the counterclockwise
bit position because the DNC bit only applies to the bit for which the
replication is done. Likewise done (likewise for the clockwise
bit position for the counterclockwise copy. copy). As a result, the ring ingress
BFR ring's
BFIR will send a copy in both directions, serving BFRs on either side of the
ring up to L1.</t>
        </section>
    <!-- rings -->

    <section anchor="ecmp" title="Equal Cost MultiPath (ECMP)">

<t>[RFC-Editor: A reviewer (Lars Eggert) noted that the infinite "to 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 ?].</t> numbered="true" toc="default">
          <name>Equal-Cost Multipath (ECMP)</name>
          <t>An ECMP() adjacency allows to the use of just one BP to deliver packets
to one of N adjacencies instead of one BP for each adjacency.
In the common example case shown in <xref target="ecmp-picture"/>, target="ecmp-picture" format="default"/>,
a link-bundle link bundle of three links L1,L2,L3 connects BFR1 and BFR2, and
only one BP is used instead of three BP BPs to deliver packets from
BFR1 to BFR2.</t>
          <figure anchor="ecmp-picture" title="ECMP Example"> anchor="ecmp-picture">
            <name>ECMP Example</name>
            <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
             --L1-----
        BFR1 --L2----- BFR2
             --L3-----

  BIFT entry in BFR1:
  ------------------------------------------------------------------
  | Index |  Adjacencies                                           |
  ==================================================================
  | 0:6   |  ECMP({forward_connected(L1, BFR2),                    |
  |       |        forward_connected(L2, BFR2),                    |
  |       |        forward_connected(L3, BFR2)}, seed)             |
  ------------------------------------------------------------------

  BIFT entry in BFR2:
  ------------------------------------------------------------------
  | Index |  Adjacencies                                           |
  ==================================================================
  | 0:6   |  ECMP({forward_connected(L1, BFR1),                    |
  |       |        forward_connected(L2, BFR1),                    |
  |       |        forward_connected(L3, BFR1)}, seed)             |
  ------------------------------------------------------------------
]]></artwork></figure>
]]></artwork>
          </figure>
          <t>This document does not standardize any ECMP algorithm because it
is sufficient for implementations to document their freely chosen
ECMP algorithm.
<xref target="ecmp-algo-picture"/> target="ecmp-algo-picture" format="default"/> shows an example ECMP algorithm, algorithm
and would double as its documentation: A a BIER-TE controller could
determine which adjacency is chosen based on the seed and adjacencies parameters
and the on packet entropy.</t>
          <figure anchor="ecmp-algo-picture" title="ECMP algorithm Example"> anchor="ecmp-algo-picture">
            <name>ECMP Algorithm Example</name>
            <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
   forward(packet, ECMP(adj(0), adj(1),... adj(N-1), seed)):
      i = (packet(bier-header-entropy) XOR seed) % N
      forward packet to adj(i)

]]></artwork></figure>
]]></artwork>
          </figure>
          <t>In the following example, example shown in <xref target="polarization-picture"/>, all traffic from BFR1 towards BFR10 is
intended to be ECMP load split load-split equally across the topology. This
example is not meant as a likely setup, but to illustrate setup; rather, it illustrates that ECMP can
be used to share BPs not only across link bundles, bundles but also across
alternative paths across different transit BFR, BFRs, and it explains
the use of the seed parameter.</t>
          <figure anchor="polarization-picture" title="Polarization Example"> anchor="polarization-picture">
            <name>Polarization Example</name>
            <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
                 BFR1         (BFIR)
               /L11  \L12
              /       \
          BFR2         BFR3
         /L21 \L22    /L31 \L32
        /      \     /      \
       BFR4  BFR5   BFR6  BFR7
        \      /     \      /
         \    /       \    /
          BFR8         BFR9
              \       /
               \     /
                BFR10         (BFER)

  BIFT entry in BFR1:
  ------------------------------------------------------------------
  | 0:6   |  ECMP({forward_connected(L11, BFR2),                   |
  |       |        forward_connected(L12, BFR3)}, seed1)           |
  ------------------------------------------------------------------

  BIFT entry in BFR2:
  ------------------------------------------------------------------
  | 0:7   |  ECMP({forward_connected(L21, BFR4),                   |
  |       |        forward_connected(L22, BFR5)}, seed1)           |
  ------------------------------------------------------------------

  BIFT entry in BFR3:
  ------------------------------------------------------------------
  | 0:7   |  ECMP({forward_connected(L31, BFR6),                   |
  |       |        forward_connected(L32, BFR7)}, seed1)           |
  ------------------------------------------------------------------

  BIFT entry in BFR4, BFR5:
  ------------------------------------------------------------------
  | 0:8   |  forward_connected(Lxx, BFR8)  |xx differs on BFR4/BFR5|
  ------------------------------------------------------------------

  BIFT entry in BFR6, BFR7:
  ------------------------------------------------------------------
  | 0:8   |  forward_connected(Lxx, BFR9)  |xx differs on BFR6/BFR7|
  ------------------------------------------------------------------

  BIFT entry in BFR8, BFR9:
  ------------------------------------------------------------------
  | 0:9   |  forward_connected(Lxx, BFR10) |xx differs on BFR8/BFR9|
  ------------------------------------------------------------------

]]></artwork></figure>
]]></artwork>
          </figure>
          <t>Note that for the following discussion of ECMP, only the BIFT ECMP ECMP()
adjacencies on BFR1, BFR2, and BFR3 are relevant. The re-use reuse of BP BPs across
BFR
BFRs in this example is further explained in <xref target="reuse"/> target="reuse" format="default"/>
below.</t>
          <t> With the setup of ECMP setup shown in the topology above, traffic would not be
equally load-split. Instead, links L22 and L31 would see no traffic
at all: BFR2 will only see traffic from BFR1 BFR1, for which the ECMP
hash in BFR1 selected the first adjacency in the list of 2 two adjacencies
given as parameters to the ECMP. It is ECMP: link L11-to-BFR2. BFR2 performs again performs
ECMP with two adjacencies on that subset of traffic using the same
seed1,
seed1 and will therefore again select the first of its two adjacencies:
L21-to-BFR4. And therefore Therefore, L22 and BFR5 sees see no traffic. Likewise traffic (likewise for
L31 and BFR6.</t> BFR6).</t>
          <t>This issue in BFR2/BFR3 is called polarization. "polarization". It results from the
re-use
reuse of the same hash function across multiple consecutive hops in
topologies like these.  To resolve this issue, the ECMP() adjacency on BFR1
can be set up with a different seed2 than the ECMP() adjacencies on BFR2/BFR3.
BFR2/BFR3 can use the same hash because packets will not sequentially
pass across both of them. Therefore, they can also use the same BP 0:7.</t> (i.e., 0:7).</t>
          <t>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 identifiers. Allowing the BIER-TE Controller controller to explicitly set the seed gives
the BIER-TE controller the ability for it to control same/different path the selection of the same path or different paths across multiple
consecutive ECMP hops.</t>
        </section>
    <!-- ecmp -->
    <section anchor="routed" title="Forward numbered="true" toc="default">
          <name>Forward Routed adjacencies"> Adjacencies</name>
          <section anchor="reducing" title="Reducing bit positions"> numbered="true" toc="default">
            <name>Reducing Bit Positions</name>
            <t>Forward_routed() adjacencies can reduce the number of bit positions
required when the path steering requirement is not hop-by-hop
explicit path selection, selection but rather is loose-hop selection. Forward_routed() adjacencies
can also allow to operate permit BIER-TE operation across intermediate hop intermediate-hop routers
that do not support BIER-TE.</t>

<figure anchor="routed-picture" title="Forward Routed Adjacencies Example">
<artwork align="left"><![CDATA[
                   ...............
         ...BFR1--...           ...--L1-- BFR2...
                  ... .Routers. ...--L2--/
         ...BFR4--...           ...--L3-- BFR3...
                  ...           ...--L4--/ |
                   ...............         |
                                          LO
                    Network Area 1
]]></artwork></figure>
            <t>Assume that the requirement in <xref target="routed-picture"/> target="routed-picture" format="default"/> 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 three next three
segments: (1) BFR2 via link L1, (2) BFR2 via link L2, or (3) via BFR3 and then
nor
not caring whether the packet is forwarded via L3 or L4.</t>

            <figure anchor="routed-picture">
              <name>Forward Routed Adjacencies Example</name>
              <artwork align="left" name="" type="" alt=""><![CDATA[
                   ...............
         ...BFR1--...           ...--L1-- BFR2...
                  ... .Routers. ...--L2--/
         ...BFR4--...           ...--L3-- BFR3...
                  ...           ...--L4--/ |
                   ...............         |
                                          LO
                    Network Area 1
]]></artwork>
            </figure>

            <t>To enable this, both BFR1 and BFR4 are set up with a forward_routed forward_routed()
adjacency bit position towards an address of BFR2 on link L1, another
forward_routed() bit position towards an address of BFR2 on link L2 L2, and a third
forward_routed() bit position towards a node address LO of BFR3.</t>
          </section>
      <!-- reducing -->

      <section anchor="without" title="Supporting nodes numbered="true" toc="default">
            <name>Supporting Nodes without BIER-TE"> BIER-TE</name>
            <t>Forward_routed() adjacencies also enable incremental deployment of BIER-TE.
Only the nodes through which BIER-TE traffic needs to be steered - --
with or without replication - -- need to support BIER-TE. Where
they are not directly connected to each other, forward_routed forward_routed()
adjacencies are used to pass over non nodes that are not BIER-TE enabled nodes.</t> enabled.</t>
          </section>
      <!-- without -->

    </section>
    <!-- routed -->

      <section anchor="reuse" title="Reuse numbered="true" toc="default">
          <name>Reuse of bit positions Bit Positions (without DNC)">

<t>Bit positions DNC)</name>
          <t>BPs can be re-used reused across multiple BFRs to minimize the number
of BP BPs needed. This happens when adjacencies on multiple BFRs use the DNC
flag as described above, but it can also be done for non-DNC adjacencies.
This section only discusses this non-DNC case.</t>
          <t>Because a given BP are is cleared when passing a BFR with an adjacency for that
BP, reuse of BP reusing BPs across multiple BFRs does not introduce any problems
with duplicates or loops that do not also exist when every adjacency has
a unique BP. Instead, the challenge when reusing BP BPs is whether it
allows to still achieve the desired
Tree Engineering goals.</t>

<t>BP goals can still be achieved.</t>
          <t>A BP cannot be reused across two BFRs that would need to be passed
sequentially for some path: The the first BFR will clear the BP, so those
paths cannot be built. A BP can be set across BFR BFRs that would (A) only occur across
(A) different paths or (B) across different branches of the same tree.</t>
          <t>An example of (A) was given in <xref target="polarization-picture"/>, target="polarization-picture" format="default"/>,
where BP 0:7, BP 0:8 0:8, and BP 0:9 are each reused across multiple BFRs because
a single packet/path would never be able to reach more than one BFR
sharing the same BP.</t>
          <t>Assume that the example was changed: BFR1 has no ECMP() adjacency for BP 0:6, 0:6
but instead has BP 0:5 with forward_connected() to BFR2 and BP 0:6 with
forward_connected() to BFR3.  Packets with both BP 0:5 and BP 0:6 would
now be able to reach both BFR2 and BFR3 BFR3, and the still existing re-use still-existing reuse
of BP 0:7 between BFR2 and BFR3 is a case of (B) where reuse of reusing a BP
is perfect because it does not limit the set of useful path choices:</t> choices, as in the following example.</t>
          <t>If instead of reusing BP 0:7, 0:7 BFR3 used a separate BP 0:10 for its
ECMP() adjacency, no useful additional path steering options would be enabled.
If duplicates at BFR10 where were undesirable, this would be done by not
setting BP 0:5 and BP 0:6 for the same packet. If the duplicates where were
desirable (e.g.: (e.g., resilient transmission), the additional BP 0:10
would also not render additional value.</t>
          <t>Reuse may also save BPs in larger topologies.  Consider the topology
shown in <xref target="scaling-picture2" format="default"/>.</t>

          <figure anchor="scaling-picture2" title="Reuse anchor="scaling-picture2">
            <name>Reuse of BP"> BPs</name>
            <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
                       area1
                   BFR1a BFR1b
                     /    \
        ....................................
        .                Core              .
        ....................................
        |    /       \    /           \  |
      BFR2a BFR2b  BFR3a BFR3b      BFR6a BFR6b
       /-------\   /---------\      /--------\
       | area2 |   |  area3  | ...  | area6  |
       | ring  |   |  ring   |      | ring   |
       \-------/   \---------/      \--------/
       more BFR BFRs    more BFR BFRs        more BFR
]]></artwork></figure>

<t>Reuse may also save BPs in larger topologies.  Consider the topology
shown in <xref target="scaling-picture2"/>. A BFRs
]]></artwork>
          </figure>
<t>A BFIR/sender (e.g.: (e.g., video headend) is attached to area 1,
and area the five areas 2...6 contain receivers/BFER. receivers/BFERs. Assume that each area had has a distribution
ring, each with two BPs to indicate the direction (as explained before).
These two BPs could be reused across the 5 five areas.  Packets would be replicated
through other BPs for from the Core core to the desired subset of areas, and once a packet copy
reaches the ring of the area, the two ring BPs come into play. This reuse is
a case of (B), but it limits the topology choices: Packets packets
can only flow around the same direction in the rings of all areas. This may or may not
be acceptable based on the desired path steering options: If if resilient
transmission is the path engineering goal, then it is likely a good
optimization,
optimization; however, if the bandwidth of each ring was were to be optimized separately,
it would not be a good limitation.</t>
        </section>
        <section anchor="bits-summary" title="Summary numbered="true" toc="default">
          <name>Summary of BP optimizations">

<t>This section Optimizations</name>
          <t>In this section, we reviewed a range of techniques by which a BIER-TE Controller controller can create
a BIER-TE topology in a way that minimizes the number of necessary BPs.</t>
          <t>Without any optimization, a BIER-TE Controller controller would attempt to map the network
subnet topology 1:1 into the BIER-TE topology and topology, every subnet adjacent
neighbor requires in the subnet would require a forward_connected() BP BP, and every BFER requires would require a local_decap() BP.</t>
          <t>The optimizations described in this document are then as follows:<list style="symbols">
  <t>P2P follows:</t>
          <ol spacing="normal">
            <li>P2P links require only one BP (<xref target="p2p-links"/>).</t>
  <t>All leaf-BFER target="p2p-links" format="default"/>).</li>
            <li>All leaf BFERs can share a single local_decap() BP (<xref target="leaf-bfer"/>).</t>
  <t>A target="leaf-bfer" format="default"/>).</li>
            <li>A LAN with N BFR BFRs needs at most N BP BPs (one for each BFR). It only needs one BP for all those BFR BFRs that are not redundantly connected to multiple LANs (<xref target="lans"/>).</t>
  <t>A target="lans" format="default"/>).</li>
            <li>A hub with p2p P2P connections to multiple non-leaf-BFER non-leaf BFER spokes can share one BP to with all of the spokes if traffic can be flooded to all of those spokes, e.g.: e.g., because of no bandwidth concerns or dense receiver sets (<xref target="hubnspoke"/>).</t>
  <t>Rings target="hubnspoke" format="default"/>).</li>
            <li>Rings of BFR BFRs can be built with just two BP BPs (one for each direction) direction), except for BFR BFRs with multiple ring connections - -- similar to LANs (<xref target="rings"/>).</t>
  <t>ECMP() target="rings" format="default"/>).</li>
            <li>ECMP() adjacencies to N neighbors can replace N BP BPs with 1 one BP. Multihop ECMP can avoid polarization through different seeds of the ECMP algorithm (<xref target="ecmp"/>).</t>
  <t>Forward_routed() target="ecmp" format="default"/>).</li>
            <li>Forward_routed() adjacencies allow to "tunnel" permit "tunneling" across non-BIER-TE capable routers and across that are either BIER-TE capable or not BIER-TE capable routers where no traffic-steering traffic steering or replications are required (<xref target="routed"/>).</t>
  <t>BP target="routed" format="default"/>).</li>
            <li>A BP can generally be reused across a set of nodes where it can be guaranteed that no path will
ever need to traverse more than one node of the set. Depending on the scenario, this may limit the feasible path steering options (<xref target="reuse"/>).</t>

</list></t> target="reuse" format="default"/>).</li>
          </ol>
          <t>Note that the described this list of optimizations is not exhaustive. Especially Further optimizations of BPs are possible, especially when both the set of required path steering choices is limited and the set of possible subsets of BFERs that should be able to receive traffic is limited, further optimizations of BP are possible. limited. The hub and spoke hub-and-spoke optimization is a simple example of such traffic pattern dependent traffic-pattern-dependent optimizations.</t>
        </section>
      </section>
  <!-- bitpositions -->

  <section anchor="avoiding" title="Avoiding duplicates numbered="true" toc="default">
        <name>Avoiding Duplicates and loops"> Loops</name>
        <section anchor="loops" title="Loops"> numbered="true" toc="default">
          <name>Loops</name>
          <t>Whenever BIER-TE creates a copy of a packet, the BitString of
that copy will have all bit positions cleared that are associated
with adjacencies on the BFR. This inhibits looping of packets. prevents packets from looping.
The only exception exceptions are adjacencies with DNC set.</t>

          <t>With DNC set, looping can happen.  Consider in <xref target="ring-picture2" format="default"/>
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.</t>
          <figure anchor="ring-picture2" title="Miswired anchor="ring-picture2">
            <name>Miswired Ring Example"> Example</name>
            <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
               v        v
               |        |
        L1     |   L2   |   L3
    /-------- BFRa ---- BFRb ---------------------\
    |        .                                    |
    |         ......  Wrong link wiring           |
    |               .                             |
    \- BFR1 - BFR2   BFR3 - ... - BFR29 - BFR30 -/
        |      |    L4               |      |
     p33|                         p15|
        BFRd                       BFRc
]]></artwork></figure>

<t>With DNC set, looping can happen.  Consider in <xref target="ring-picture2"/>
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.</t>
]]></artwork>
          </figure>
          <t>To inhibit looping in the face of such physical misconfiguration,
only forward_connected() adjacencies are permitted to have DNC set,
and the link layer port unique unicast destination address of the adjacency (e.g. MAC (e.g., "Media Access Control" (MAC) address)
protects against closing the loop. Link layers without port unique
link layer addresses should not be used with the DNC flag set.</t>
        </section>
      <!-- loops -->

      <section anchor="duplicates" title="Duplicates">

<figure anchor="duplicates-picture" title="Duplicates Example">
<artwork align="left"><![CDATA[
                 BFIR1
                /    \
               / p2   \ p3
              BFR2   BFR3
               \ p4   / p5
                \    /
                 BFER4
]]></artwork></figure> numbered="true" toc="default">
          <name>Duplicates</name>

          <t>Duplicates happen when the graph expressed by a BitString is not a
tree but is redundantly connecting BFRs with each other. In <xref target="duplicates-picture"/>, target="duplicates-picture" format="default"/>,
a BitString of p2,p3,p4,p5 would result in duplicate packets to arrive arriving on BFER4.
The BIER-TE Controller controller must therefore ensure to that only create BitStrings that are trees.</t> trees are created.</t>
          <figure anchor="duplicates-picture">
            <name>Duplicates Example</name>
            <artwork align="left" name="" type="" alt=""><![CDATA[
                 BFIR1
                /    \
               / p2   \ p3
              BFR2   BFR3
               \ p4   / p5
                \    /
                 BFER4
]]></artwork>
          </figure>
          <t>When links are incorrectly physically re-connected reconnected before the
BIER-TE Controller controller updates BitStrings in BFIRs, duplicates can happen.
Like loops, these can be inhibited by link layer addressing
in forward_connected() adjacencies.</t>
          <t>If interface or loopback addresses used in forward_routed() adjacencies
are moved from one BFR to another, duplicates can are equally likely to happen.
Such re-addressing readdressing operations must be coordinated with the BIER-TE Controller.</t> controller.</t>
        </section>
    <!-- duplicates -->

  </section>
  <!-- avoiding -->

  <section anchor="mgmt-stuff" title="Managing SI, sub-domains numbered="true" toc="default">
        <name>Managing SIs, Subdomains, and BFR-ids"> BFR-ids</name>
        <t>When the number of bits required to represent the necessary hops
in the topology and BFER BFERs exceeds the supported BitStringLength "BitStringLength" (BSL),
multiple SIs and/or sub-domains subdomains must be used. This section discusses how.</t> how this is done.</t>
        <t>BIER-TE forwarding does not require the concept of BFR-id, BFR-ids, but routing
underlay, flow overlay overlay, and BIER headers may. This section also discusses
how BFR-ids can be assigned to BFIR/BFER BFIRs/BFERs for BIER-TE.</t>
        <section anchor="why" title="Why SI numbered="true" toc="default">
          <name>Why SIs and sub-domains"> Subdomains?</name>
          <t>For (non-TE) BIER and BIER-TE forwarding, the most important result of using multiple
SI
SIs and/or sub-domains subdomains is the same: Packets multicast flow overlay packets that need to be sent to BFERs in
different SIs or sub-domains subdomains require different multiple BIER packets: packets, each one with a
BitString for a different (SI,sub-domain) (SI,subdomain) combination. Each such BitString uses
one BSL sized BSL-sized SI block in the BIFT of the sub-domain. subdomain. We call this
a BIFT:SI (block).</t>

<t>For BIER and BIER-TE forwarding themselves there is also no difference whether
different SIs and/or sub-domains are chosen, but SI
          <t>SIs and sub-domain subdomains have different purposes in the BIER architecture shared by BIER-TE.
and also the BIER-TE architecture.  This impacts how operators are managing manage them and how
especially how flow overlays will likely use them.</t>
          <t>By default, every possible BFIR/BFER in a BIER network would likely be given
a BFR-id in sub-domain subdomain 0 (unless there are > &gt; 64k BFIR/BFER). BFIRs/BFERs). </t>
          <t>If there are different flow services (or service instances) requiring replication
to different subsets of BFERs, then it will likely not be possible to achieve
the best replication efficiency for all of these service instances via sub-domain subdomain 0.

Ideal replication efficiency for N BFER BFERs exists in a sub-domain subdomain if they are
split over not no more than ceiling(N/BitStringLength) SI.</t> SIs.</t>
          <t>If service instances justify additional BIER:SI state in the network, additional
sub-domains
subdomains will be used: BFIR/BFER BFIRs/BFERs are assigned BFR-id BFR-ids in those sub-domains subdomains,
and each service instance is configured to use the most appropriate sub-domain. subdomain.
This results in improved replication efficiency for different services.</t>
          <t>Even if creation of sub-domains subdomains and assignment of BFR-id BFR-ids to BFIR/BFER BFIRs/BFERs in those
sub-domains
subdomains is automated, it is not expected that individual
service instances can deal with BFER BFERs in different sub-domains. subdomains. A service
instance may only support configuration of a single sub-domain subdomain it should rely on.</t>
          <t>To be able to easily reuse (and modify as little as possible) existing
BIER procedures including flow-overlay (including flow overlay and routing underlay, underlay), when BIER-TE
forwarding is added, we therefore reuse SI SIs and sub-domain subdomains logically in the
same way as they are used in BIER: All all necessary BFIR/BFER BFIRs/BFERs for a service use
a single BIER-TE BIFT and are split across as many SIs as necessary (see <xref target="bit-requirements"/>). target="bit-requirements" format="default"/>).
Different services may use different sub-domains subdomains that primarily exist to
provide more efficient replication (and (and, for BIER-TE BIER-TE, desirable path steering)
for different subsets of BFIR/BFER.</t> BFIRs/BFERs.</t>
        </section>
    <!-- why -->

    <section anchor="bit-requirements" title="Assigning bits numbered="true" toc="default">
          <name>Assigning Bits for the BIER-TE topology"> Topology</name>
          <t>In BIER, BitStrings only need to carry bits for BFERs, which BFERs; this leads to the
model that where BFR-ids map 1:1 to each bit in a BitString.</t>
          <t>In BIER-TE, BitStrings need to carry bits to indicate not only the receiving
BFER but also the intermediate hops/links across which the packet must be sent.
The maximum number of BFER BFERs that can be supported in a single BitString or BIFT:SI
depends on the number of bits necessary to represent the desired topology between
them.</t>
          <t>"Desired" topology because means that it depends on the physical topology, topology and
on
the operator's desire of the operator to allow for to</t>
<ol spacing="normal">
<li>permit explicit path steering across
every single hop (which requires more bits), or reducing or</li>
<li>reduce the number of required
bits by exploiting optimizations such as unicast (forward_routed()), ECMP() ECMP(), or flood
(DNC) over "uninteresting" sub-parts of the topology - e.g. topology, e.g., parts where where, for path steering reasons, different trees do not need to take different paths due to path steering reasons.</t> paths.</li>
</ol>
          <t>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 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 bits, the higher the likelihood, likelihood that those topology
bits are not just BIER-TE overhead without additional benefit, benefit but instead that they
will allow to express the expression of desirable path steering alternatives.</t>
        </section>
        <section anchor="bfr-id" title="Assigning BFR-id numbered="true" toc="default">
          <name>Assigning BFR-ids with BIER-TE"> BIER-TE</name>
          <t>BIER-TE forwarding does not use the BFR-id, BFR-ids, nor does it require for that
the BFIR-id field of the BIER header to be set to a particular value.
However, other parts of a BIER-TE deployment may need a BFR-id, specifically BFR-id -- specifically,
multicast flow overlay signaling and multicast flow overlay packet disposition,
and disposition;
in that case case, BFRs need to also have BFR-ids for BIER-TE SDs.</t>
          <t>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 header to indicate
to the overlay signaling on the receiving BFER which BFIR originated the packet.</t>
          <t>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
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 BFER but (i.e., will only be a BFIR.</t> BFIR).</t>
          <t>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 such a BFER can therefore
be determined using the same procedure as in that used for (non-TE) BIER: BFR-id = SI * BSL + BP.</t>
          <t>As explained in <xref target="leaf-bfer"/>, target="leaf-bfer" format="default"/>, leaf BFERs do not need such
a unique local_decap() adjacency. Likewise, BFIRs that are not also BFERs
may not have a unique local_decap() adjacency either. For all those 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 BFER local_decap() BPs.</t>
          <t>While this document defines no requirements on how to allocate such BFR-id, 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 BFIR, this can be the first adjacency that is
only populated on this BFIR, BFIR; for a leaf-BFER, leaf BFER, this could be the first BP
with an adjacency towards that BFER.</t>
        </section>
        <section anchor="bitstring-mappings" title="Mapping numbered="true" toc="default">
          <name>Mapping from BFR BFRs to BitStrings with BIER-TE"> BIER-TE</name>
          <t>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 easily determine the BitStrings
for a BIER packet to a set of BFERs with known BFR-ids.</t>
          <t>In BIER-TE BIER-TE, this mapping needs to be equally supported for flow overlays.
This section outlines two core options, based on what type of Tree Engineering
the BIER-TE controller needs to performs perform for a particular application.</t>

<t>"Independent branches": For
    <dl spacing="normal">
          <dt>"Independent branches":</dt><dd>For a given flow overlay instance, the branches
from a BFIR to every BFER are calculated by the BIER-TE controller to be
independent of the branches to any other BFER.  Shortest path trees are the most common
examples of trees with independent branches.</t>

<t>"Interdependent branches": When branches.</dd>
          <dt>"Interdependent branches":</dt><dd>When a BFER is added to or deleted from a particular
distribution tree, the BIER-TE controller has to recalculate the branches to other BFER, BFERs,
because they may need to change.  Steiner trees are examples of interdependent branch trees.</t> trees.</dd>
    </dl>
          <t>If "independent branches" are used, the BIER-TE Controller controller
can signal to the BFIR flow overlay for every BFER an SI:BitString that
represents the branch to that BFER.  The flow overlay on the BIFR BFIR can then then, independently
of the controller controller, calculate the SI:BitString for all desired BFERs by OR'ing ORing their BitStrings.
This allows for flow overlay applications to operate independently of the controller
whenever it needs they need to determine which subset of BFERs need needs to receive a particular packet.</t>
          <t>If "interdependent branches" are required, the an application would need to inquire query
the SI:BitString for a given set of BFER BFERs whenever the set changes.</t>
          <t>Note that in either case (unlike in the scenario for BIER), the bits may need to
change upon link/node failure/recovery, network expansion and expansion, or network resource consumption
by other traffic as part of traffic engineering achieving Traffic Engineering goals (e.g.: re-optimization (e.g., reoptimization of lower
priority
lower-priority traffic flows). Interactions between such BFIR applications and the BIER-TE Controller controller
do therefore need to support dynamic updates to the SI:BitStrings.</t> SIs:BitStrings.</t>
          <t>Communications between the BFIR flow overlay and the BIER-TE controller
requires
require some way to identify the BFER. BFERs. If BFR-ids are used in the deployment, as
outlined in <xref target="bfr-id"/>, target="bfr-id" format="default"/>, then those are the natural BFR identifier. "natural" BFR-ids. If
BFR-ids are not used, then any other unique identifier, such as the a BFR's BFR-prefix
of the BFR (<xref target="RFC8279"/>)
<xref target="RFC8279" format="default"/>, could be used.</t>
        </section>
    <!-- bfr-id -->

    <section anchor="assigning" title="Assigning numbered="true" toc="default">
          <name>Assigning BFR-ids for BIER-TE"> BIER-TE</name>
          <t>It is not currently determined if a single sub-domain subdomain could or should be
allowed to forward both (non-TE) BIER and BIER-TE packets. If this should be
supported, there are two options:</t>

<t>A. BIER
          <ol spacing="normal" type="A">
          <li>BIER and BIER-TE have different BFR-id BFR-ids in the same sub-domain. subdomain. This allows higher replication efficiency for BIER because their BFR-id the BIER BFR-ids can be assigned
sequentially, while the BitStrings for BIER-TE will have also have to assign the additional
bits for the topology. topology adjacencies. There is no relationship between a BFR BIER BFR-id and its
BIER-TE BFR-id.</t>

<t>B. BIER BFR-id.</li>
          <li>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 replication efficiency for BIER will
be as low as that for BIER-TE in this approach.</t> approach.</li>
	  </ol>
        </section>
    <!-- assigning -->

    <section anchor="allocation-example" title="Example bit allocations"> numbered="true" toc="default">
          <name>Example Bit Allocations</name>
          <section anchor="with-bier" title="With BIER"> numbered="true" toc="default">
            <name>With BIER</name>
            <t>Consider a network setup with a BSL of 256 for a network
topology as shown in <xref target="scaling-picture"/>. target="scaling-picture" format="default"/>. The network has 6 six areas, each with
170 BFERs, connecting via a core with 4 four (core) BFRs. To address all BFERs with BIER,
4
four SIs are required. To send a BIER
packet to all BFER BFERs in the network, 4 four copies need to be sent by the BFIR. On the
BFIR
BFIR, it does not make a difference matter how the BFR-ids are allocated to BFER BFERs
in the network, but it does matter for efficiency further down in the network it does
make a difference.</t> network.</t>
            <figure anchor="scaling-picture" title="Scaling anchor="scaling-picture">
              <name>Scaling BIER-TE bits Bits by reuse"> Reuse</name>
              <artwork align="left"><![CDATA[ align="left" name="" type="" alt=""><![CDATA[
             area1           area2        area3
            BFR1a BFR1b  BFR2a BFR2b   BFR3a BFR3b
              |  \         /    \        /  |
              ................................
              .                Core          .
              ................................
              |    /       \    /        \  |
            BFR4a BFR4b  BFR5a BFR5b   BFR6a BFR6b
             area4          area5        area6
]]></artwork></figure>
]]></artwork>
            </figure>
            <t>With random allocation of BFR-id BFR-ids to BFER, BFERs, each receiving area would (most likely)
have to receive all 4 four copies of the BIER packet because there would be
BFR-id
BFR-ids for each of the 4 four SIs in each of the areas. Only further towards each
BFER would this duplication subside - -- when each of the 4 four trees runs out of
branches.</t>
            <t>If BFR-ids are allocated intelligently, then all the BFER BFERs in an area
would be given BFR-id BFR-ids with as few as possible different SIs. SIs as possible.
Each area would only have to forward one or two packets instead of 4.</t> four.</t>
            <t>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 sequentially, network wide.
An area that initially only has BFR-id BFR-ids in one SI
might end up with many SIs over a longer period of growth. Allocating SIs
to areas with that initially have sufficiently many spare bits for growths growth can help
to
alleviate this issue. Or renumber Alternatively, BFERs can be renumbered after network expansion. In
this example example, one may consider to use 6 using six SIs and assign assigning one to each area.</t>
            <t>This example shows that intelligent BFR-id allocation within at least
sub-domain
subdomain 0 can even be helpful or even necessary in BIER.</t>
          </section>
      <!-- with-bier -->

      <section anchor="with-bier-te" title="With BIER-TE"> numbered="true" toc="default">
            <name>With BIER-TE</name>
            <t>In BIER-TE BIER-TE, one needs to determine a subset of the physical topology
and attached BFERs so that the "desired" representation of this topology
and the BFER BFERs fit into a single BitString. This process needs to be
repeated until the whole topology is covered.</t>
            <t>Once bits/SIs are assigned to the topology and BFERs, BFR-id is BFR-ids are just a derived
set of identifiers from the operator/BIER-TE Controller operator / BIER-TE controller as explained above.</t>

<t>Every time that
            <t>Whenever different sub-topologies subtopologies have overlap, bits need to
be repeated across the BitStrings, increasing the overall amount of bits
required across all BitString/SIs. BitStrings/SIs. In the worst case, one assigns random subsets of BFERs
to different SIs. This will result in an outcome much worse than in (non-TE) BIER: It it
maximizes the amount of unnecessary topology overlap across SI SIs and therefore
reduces the number of BFER BFERs that can be reached across each individual SI.
Intelligent BFER to SI BFER-to-SI assignment and selecting specific "desired" subtopologies can
minimize this problem.</t>
            <t>To set up BIER-TE efficiently for the topology of shown in <xref target="scaling-picture"/>, target="scaling-picture" format="default"/>, the following bit
allocation method can be used. This method can easily be expanded to
other, similarly structured larger topologies.</t>
            <t>Each area is allocated one or more SIs SIs, depending on the number of future
expected BFERs and the number of bits required for the topology in the area.
In this example, 6 SIs, six SIs are used, one per area.</t>
            <t>In addition, we use 4 four bits in each SI: bia, bib, bea, beb: (b)it SI:</t>
            <dl spacing="normal">
             <dt>bia:</dt><dd>(b)it (i)ngress (a),
(b)it (a)</dd>
             <dt>bib:</dt><dd>(b)it (i)ngress (b), (b)it (b)</dd>
             <dt>bea:</dt><dd>(b)it (e)gress (a), (b)it (a)</dd>
             <dt>beb:</dt><dd>(b)it (e)gress (b). These (b)</dd>
	    </dl>
<t>These bits will be used to pass BIER
packets from any BFIR via any combination of ingress area a/b BFR BFRs and egress area
 a/b BFR BFRs into a specific target area. These bits are then set up with the right
 forward_routed() adjacencies on the BFIR BFIRs and area edge BFR:</t> BFRs as follows.</t>
            <t>On all BFIRs in an area area, j|j=1...6, bia in each BIFT:SI is populated with the same
forward_routed(BFRja),
forward_routed(BFRja) and bib with forward_routed(BFRjb). On all area
edge BFR, BFRs, bea in BIFT:SI=k|k=1...6 is populated with forward_routed(BFRka) and
beb in BIFT:SI=k with forward_routed(BFRkb).</t>
            <t>For BIER-TE forwarding of a packet to a subset of BFERs across all areas,
a BFIR would create at most 6 six copies, with SI=1...SI=6, SI=1...SI=6. In each packet,
the bits indicate BitString includes bits for topology one area and BFER the BFERs in that topology area, plus the four bits
to indicate whether to pass this packet via the ingress area a or b border BFR
and the egress area a or b border BFR, therefore allowing path steering
for those two "unicast" legs: 1) BFIR to ingress area edge and 2) core to egress
area edge. Replication only happens inside the egress areas. For BFER BFERs that are in the
same area as in the BFIR, these four bits are not used.</t>
          </section>
      <!-- with-bier-te -->

    </section>
    <!-- example -->

    <section anchor="summary" title="Summary"> numbered="true" toc="default">
          <name>Summary</name>
          <t>BIER-TE can, like BIER, support multiple SIs within a sub-domain. subdomain. This allows
to apply
application of the mapping BFR-id = SI * BSL + BP. This allows to re-use also permits the reuse of
the BIER architecture concept of BFR-id and therefore minimize BIER-TE specific BFR-ids and, therefore, minimization of BIER-TE-specific functions in possible
BIER layer control plane mechanisms with BIER-TE, including flow overlay methods and BIER header fields.</t>
          <t>The number of BFIR/BFER BFIRs/BFERs possible in a sub-domain subdomain is smaller than in BIER
because BIER-TE uses additional bits for the topology.</t>

<t>Sub-domains (SDs)
          <t>Subdomains in BIER-TE can be used like as they are in BIER to create more efficient
replication to known subsets of BFERs.</t>
          <t>Assigning bits for BFERs intelligently into the right SI is more important in
BIER-TE than in BIER because of replication efficiency and the overall amount of
bits required.</t>
        </section>
    <!-- example -->

  </section>
  <!-- mgmt-stuff -->
</section>
  <!-- controller-ops -->

  <section anchor="security" title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>If "<xref target="RFC8296" format="title"/>" <xref target="RFC8296"/> target="RFC8296" format="default"/> is used, BIER-TE shares its security considerations.</t>

<t>BIER-TE shares the considerations also apply to BIER-TE.</t>
      <t>The security considerations of BIER, "<xref target="RFC8279" format="title"/>" <xref target="RFC8279"/>, target="RFC8279" format="default"/> also apply to BIER-TE, with
the following overriding or additional considerations.</t>
      <t>BIER-TE forwarding explicitly supports unicast "tunneling" of BIER packets via forward_routed()
adjacencies. The BIER domain security 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 equally applies
to such unicast "tunneled" BIER packets. This does not only include includes the need to filter
received unicast "tunneled" BIER packets to prohibit the injection of such "tunneled" BIER
packets from outside the BIER domain, domain but also prohibiting the need to prohibit forward_routed() adjacencies
to leak
from leaking BIER packets from the BIER domain. It SHOULD <bcp14>SHOULD</bcp14> be possible to configure
interfaces to be part of a BIER domain solely for sending and receiving of unicast
"tunneled" BIER packets even if the interface can not cannot send/receive BIER encapsulated packets.</t>
      <t>In BIER, the standardized methods for the routing underlays are IGPs
with extensions to distribute BFR-ids and BFR-prefixes.
<xref target="RFC8401"/> target="RFC8401" format="default"/> specifies the extensions for IS-IS IS-IS, and <xref target="RFC8444"/> target="RFC8444" format="default"/> specifies the extensions for OSPF.
Attacking the protocols for the BIER routing underlay or (non-TE) BIER layer control
plane, or the impairment of any BFR BFRs in a domain domain, may lead to successful attacks
against the results of information that BIER-TE learns from the routing protocol, protocol (routes, next hops, BFR-ids, ...), enabling DoS attacks against
paths or the addressing (BFR-id, (BFR-ids, BFR-prefixes) used by BIER.</t>
      <t>The reference model for the BIER-TE layer control plane is a BIER-TE controller.
When such a controller is used, the impairment of an individual BFR in a domain causes
no impairment of the BIER-TE control plane on other BFRs. If a routing
protocol is used to support forward_routed() adjacencies, then this is still an
attack vector as in BIER, but only for BIER-TE forward_routed() adjacencies, adjacencies and
not other adjacencies.</t>
      <t>Whereas IGP routing protocols are most often not well secured through
cryptographic authentication and confidentiality, communications between controllers and routers such as those
to be considered for the BIER-TE controller/control-plane controller / control plane can be be, and are are, much more commonly
secured with those security properties, properties -- for example example, by using Secure SHell (SSH), "Secure Shell" (SSH) <xref target="RFC4253"/> target="RFC4253" format="default"/> for NETCONF (<xref target="RFC6242"/>), <xref target="RFC6242" format="default"/>; or via Transport "Transport Layer Security Security" (TLS), such as <xref target="RFC8253"/> target="RFC8253" format="default"/> for PCEP, PCEP <xref target="RFC5440"/>, target="RFC5440" format="default"/> or <xref target="RFC7589"/> target="RFC7589" format="default"/> for NETCONF. BIER-TE controllers SHOULD <bcp14>SHOULD</bcp14> use security equal to or better than these mechanisms.</t>
      <t>When any of these security mechanisms/protocols are used for communications
between a BIER-TE controller and BFRs, their security considerations apply to BIER-TE.
In addition, the security considerations of PCE, "<xref target="RFC4655" format="title"/>" <xref target="RFC4655"/> target="RFC4655" format="default"/> apply.</t>
      <t>The most important attack vector in BIER-TE is misconfiguration,
either on the BFR BFRs themselves or via the BIER-TE controller.
Forwarding entries with DNC could be set up to create persistent loops, in which
packets only expire because of TTL. To minimize the impact of such attacks
(or
(or, more likely likely, unintentional misconfiguration by operators and/or bad BIER-TE controller software),
the BIER-TE forwarding rules are defined to be as strict in clearing
bits as possible. The clearing of all bits with an adjacency on
a BFR prohibits that a looping packet creates from creating additional packet amplification
through the misconfigured loop on the packet's second time or further subsequent times around the
loop, because all relevant adjacency bits would have been cleared on the first round
through the loop.  In As a result, looping packets can occur in BIER-TE has to the same degree of looping packets
as is possible with unintentional or malicious loops in the routing underlay with BIER BIER, or even with unicast traffic.</t>
      <t>Deployments where BIER-TE would likely be beneficial
may include operational models where actual configuration changes
from the controller are only required during non-production phases of
the network's life-cycle, such as life cycle, e.g., in embedded networks or in manufacturing
networks during e.g. such activities as plant reworking/repairs. reworking or repairs. In these
type
types of deployments, configuration changes could be locked out when the
network is in production state and could only be (re-)enabled through
reverting the network/installation into to non-production state. Such
security designs would not only allow a deployment to provide additional layers
of protection against configuration attacks, attacks but would foremost would, first and foremost,
protect the active production process from such configuration attacks.</t>
    </section>
  <!-- security -->

  <section anchor="iana" title="IANA Considerations"> numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>This document requests has no action by IANA. </t> IANA actions.</t>
    </section>
  <!-- iana -->

  <section anchor="ack" title="Acknowledgements">

  <t>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.</t>
  <t> 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).</t>
  </section>
  <!-- ack -->

  <section anchor="changes" title="Change log [RFC Editor: Please remove]">
  <t>draft-ietf-bier-te-arch:
  <list>
      <t>13:</t>
      <t>Changed Gregs author association/email.</t>
      <t>Fixed Nits in -12 from Ben Kaduk.</t>
      <t>Fixed Alvaro's concerns: (1) Removed references to SR in Abstract/Overview (2) removed section 4.5.</t>

      <t>12:</t>
      <t>AD review Alvaro Retana.</t>
      <t>Various textual/editorial nits including adding () to all instances of forwarding adjacency name instances.</t>
      <t>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.</t>
      <t>3.2 added xref's to relevant sections to the listed control plane points.</t>
      <t>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.</t>
      <t>Moved SR section into appendix.</t>

      <t>TSV review Martin Duke.</t>
      <t>Text/editorial nits.</t>
      <t>4.4 improved text describing handling of F-BM.</t>
      <t>RTGdir review Yingzhen Qu.</t>
      <t>Various text/editorial nits.</t>
      <t>Added notion that BitStrings represent loop free tree for packet to abstract and intro.</t>
      <t>Various text nit and editorial improvements.</t>
      <t>Fixed some BFR-id field -> BFIR-id field mistakes.</t>
      <t>Capitalized NETCONF/RESTCONF/YANG, added RFC references.</t>
      <t>Improved Figure 16 with explicitly two links into BFR3 and explanatory text.</t>

      <t>Gen-ART review Robert Sparks.</t>
      <t>Various textual nits, editorial improvements.</t>
      <t>3.2 Introduced terms "BIER-TE topology control" and "BIER-TE tree control" for the two functional components of the control plane. </t>
      <t>3.2.1 - 3.2 change introduces the open RFC-editor issue of appropriate xrfs (to be resolved by RFC-editor).</t>
      <t>3.3 Rewrote last paragraph to better describe loop prevention through clearing of bits in BitString.</t>
      <t>4.1 Fixed up text/formula describing mapping between bfr-id, SI:BP and SI,BSL and BP. Fix offset bug.</t>
      <t>5.3.6.2 Improved description paragraph explaining overlap of topology for different SI.</t>
      <t>5.3.7 Improved first summary paragraph.</t>
      <t>7. Rephrased applicability statement of control plane protocol security considerations to BIER-TE security.</t>

      <t>RTGDIR review Ines Robles.</t>
      <t>Fixed up adjacencies in Example 2 and explanation text to be explicit about which BFR not
         only passes, but also receives the packet.</t>
      <t>7. (security considerations). Added paragraph about forward_routed() and prohibiting BIER packet leaking in/out of domain.</t>
      <t>IESG review Roman Danyliv (SEC).</t>
      <t>Several textual/sentence nits/editorials.</t>

      <t>IESG review Lars Eggert (GEN).</t>
      <t>Various good editorial word fixed.</t>
      <t>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.</t>
      <t>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.</t>
      <t>IESG review Martin Vigeureux (RTG).</t>
      <t>Added back reference to RFC8402. Textual fixes.</t>
      <t>IESG review Eric Kline (INT).</t>
      <t>2.1 Fixed typo in BFR* explanations.</t>
      <t>4.3 Added explanatio about MTU handling.</t>
      <t>IESG review Eric Vyncke (INT).</t>
      <t>Fixed up initial text to introduce various abbreviations.</t>
      <t>2.4 refined wording to "with the _intent_ to easily build common forwarding planes...".</t>
      <t>4.2.3 refined text about entropy in ECMP - now taken text from rfc8279.</t>

      <t>IESG review Zaheduzzaman Sarker (TSV).</t>
      <t>5.1.7 Refined text explaining documentation of ECMP algorithm.</t>
      <t>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.</t>

      <t>IESG review Ben Kaduk (round 2):</t>
      <t>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].</t>
      <t>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.</t>
      <t>4.4 (from Carsten Borman) changed spacing in pseudocode to be consistent. Fixed {VRF}, clarified pseudocode object syntax, typos.</t>

      <t>11:  IESG review Ben Kaduk, summary:</t>
      <t>One discuss for bug in pseudocode. turned out to be one cahrcter typo.</t>
      <t>Added (non-TE) prefix in places where BIER by itsels had to be better disambiguated.</t>
      <t>enhanced text for hub-and-spoke to indicate we're only talking about hub to spoke traffic.</t>
      <t> long list ot language fixes/improvement (nits). Thanks a lot!.</t>
      <t>add suggestion to SHOULD use known confidentiality protocols between controller and BFR.</t>
      <t>10:  AD review Alvaro Retana, summary:</t>
<t>Note: rfcdiff shows more changes than actually exist because text moved around.</t>
<t>Summary:<list style="numbers">
 <t>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.</t>
 <t>enhanced/rewrote: 2.3 Comparison with -> Relationship to BIER-TE</t>
 <t>enhanced/rewrote: 3.2 BIER-TE controller -> BIER-TE control plane, 3.2.1 BIER-TE controller, for consistency with rfc8279</t>
 <t>additional subsections for Alvaros asks</t>
 <t>added to: 3.3 BIER-TE forwarding plane (consistency with rfc8279)</t>
 <t>Enhanced description of 4.3/encap considerations to better explain how BIER/BIER-TE can run together.</t>
</list></t>

<t>Notation: Markers (a),(b),... at end of points are references from the review discussion with Alvaro to the changes made.</t>

<t>Details:.</t>
 <t>Throughout text: changed term spelling to rfc8279 - bit positions, sub-domain, ... (i).</t>
 <t>Reset changed to clear, also DNR changed to DNC (Do Not Clear) (q).</t>
 <t>Abstract: Shortened. Removed name explanation note (Tree Engineering), (a).</t>
 <t>1. Introduction -> Overview: Moved important explanation paragraph from abstract to Introduction. Fixed text,  (a).</t>
 <t>   Added bullet point list explanation of structure of document (e).</t>
 <t>   Renamed
</middle>
  <back>

<displayreference target="I-D.ietf-roll-ccast" to="CONSTRAINED-CAST"/>

    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8279.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8296.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4253.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4456.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4655.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5440.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6241.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6242.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7589.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7752.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7950.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7988.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8040.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8253.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8345.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8401.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8402.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8444.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8556.xml"/>

<!-- draft-ietf-teas-rfc3272bis (I-D Exists)
     (have to Overview do "long way" because that is now more factually correct.</t>
 <t>1.1. Fixed bug in example adding bit p15.(l).</t>
 <t>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) Adrian Farrel is not in middle of document after all the Introduction.</t>
 <t>2.1 Removed discussion of encap, moved to 4.2.2 (m).</t>
 <t>2.2 enhanced paragraph suggesting native/overlay topology types, also sugest type hybrid (n).</t>
 <t>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).</t>
 <t>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).</t>
 <t>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).</t>
 <t>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).</t>
 <t>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 editor) -->
<reference anchor="TE-OVERVIEW">
   <front>
      <title>Overview and 2.2.3 into list. (b).</t>
 <t>3.2.1. Expanded/improved explanation Principles of BIER-TE Controller (b).</t>
 <t>3.2.1.1. Added subsection for topology discovery and creation (d).</t>
 <t>3.2.1.2. Added subsection for engineered BitStrings as key novel aspect not found in BIER. (X).</t>
 <t>3.3. Added numbered list for components of BIER-TE forwarding plane (completing the comparable text from RFC8729 Section 4.2).</t>
 <t>3.4 Alvaro does not mind additional example, fixed bugs.</t>
 <t>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 Internet Traffic Engineering</title>
      <author fullname="Adrian Farrel" surname="Farrel" initials="A" role="editor">
      </author>
      <date month="September" day="11" year="2022" />
   </front>
   <seriesInfo name="Internet-Draft" value="draft-ietf-teas-rfc3272bis-21" />
</reference>

<!-- draft-ietf-bier-multicast-http-response (Expired)
     (have 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.</t>
 <t>4.2.2 Improved text to explain requirement to identify BIER-TE in the tunnel encap and compress description of use-cases (m).</t>
 <t>4.2.3 enhanced ECMP text (p).</t>
 <t>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).</t>
 <t>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).</t>
 <t>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).</t>
 <t>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).</t>
 <t>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).</t>
 <t>5.1.9 Added new separate picture instead of referring to a picture later in text, adjusted text (r).</t>
 <t>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).</t>

 <t>co-authors internal review:</t>
 <t>4.4 Added xref to Figure 5.</t>
 <t>5.2.1 Duplicated ring picture, added visuals for described miswiring (s).</t>
 <t>5.2.2 replace "topology" with graph (wrong word).</t>
 <t>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.</t>
 <t>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.</t>
 <t>7. Added new security considerations (u).</t>
 <t></t>

      <t>09: Incorporated fixes for feedback from Shepherd (Xuesong Geng).</t>
      <t>    Added references for Bloom Filters and Rate Controlled Service Disciplines.</t>
      <t>    1.1 Fixed numbering of example 1 topology explanation. Improved language on second example (less abbreviating "long way" to avoid confusion about meaning).</t>
      <t>    1.2 Improved explanation of BIER-TE topology, fixed terminology fix Toerless Eckert's initial) -->
<reference anchor="BIER-MCAST-OVERLAY">
   <front>
      <title>Applicability of graphs (BIER-TE topology is a directed graph where the edges are the adjacencies).</t>
      <t>    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.</t>
      <t>    3.1 Added explanation for VRF and its use in adjacencies.</t>
      <t>08: Incorporated (with hopefully acceptable fixes) for Lou suggested section 2.5, TE considerations.</t>
      <t>    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.</t>
      <t>07: Further reworking text for Lou.</t>
      <t>    Renamed BIER-PE to BIER-TE standing Multicast Overlay for "Tree Engineering" after votes from BIER WG.</t>
      <t>    Removed section 1.1 (introduced by version 06) because not considered necessary in this doc by Lou (for framework doc).</t>
      <t>    Added [RFC editor pls. remove] Section to explain name change to future reviewers.</t>
      <t>06: Concern by Lou Berger re. BIER-TE as full traffic engineering solution.</t>
      <t>    Changed title "Traffic Engineering" to "Path Engineering"</t>
      <t>    Added intro section of relationship BIER-PE to traffic engineering.</t>
      <t>    Changed "traffic engineering" term in text" to "path engineering", where appropriate</t>
      <t>    Other:</t>
      <t>    Shortened "BIER-TE Controller Host" to "BIER-TE Controller". Fixed up all instances of controller Adaptive Streaming Services</title>
      <author initials="D." surname="Trossen" fullname="Dirk Trossen">
         <organization>Huawei Technologies Duesseldorf GmbH</organization>
      </author>
      <author initials="A." surname="Rahman" fullname="Akbar Rahman">
         <organization>InterDigital Communications, LLC</organization>
      </author>
      <author initials="C." surname="Wang" fullname="Chonggang Wang">
         <organization>InterDigital Communications, LLC</organization>
      </author>
      <author initials="T." surname="Eckert" fullname="Toerless Eckert">
         <organization>Futurewei Technologies Inc.</organization>
      </author>
      <date month="July" day="10" year="2021" />
   </front>
   <seriesInfo name="Internet-Draft" value="draft-ietf-bier-multicast-http-response-06" />
</reference>

<!-- draft-eckert-bier-te-frr (Expired)
     (have to do this.</t>
      <t>05: Review Jeffrey Zhang.</t>
      <t>    Part 2:</t>
      <t>    4.3 added note about leaf-BFER being also a propery of routing setup.</t>
      <t>    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.</t>
      <t>    4.9 added section discussing generic reuse of BP methods.</t>
      <t>    4.10 added section summarizing BP optimizations of section 4.</t>
      <t>    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.</t>
      <t>    Part 1:</t>
      <t>    Explicitly ue forwarded_connected adjcency in ECMP adjcency examples "long way" to avoid confusion.</t>
      <t>    4.3 Add picture as example for leav vs. non-leaf BFR in topology. Improved description.</t>
      <t>    4.5 Exampe for traffic that can be broadcast -> for single BP in hub&amp;spoke.</t>
      <t>    4.8.1 Simplified example picture fix Toerless Eckert's initial) -->
<reference anchor="BIER-TE-PROTECTION">
   <front>
      <title>Protection Methods for routed adjacency, explanatory text.</t>
      <t>    Review from Dirk Trossen:</t>
      <t>    Fixed up explanation BIER-TE</title>
      <author initials="T." surname="Eckert" fullname="Toerless Eckert">
         <organization>Huawei USA - Futurewei Technologies Inc.</organization>
      </author>
      <author initials="G." surname="Cauchie" fullname="Gregory Cauchie">
         <organization>Bouygues Telecom</organization>
      </author>
      <author initials="W." surname="Braun" fullname="Wolfgang Braun">
         <organization>University of ICC paper vs. bloom filter.</t>
      <t>04: spell check run.</t>
      <t>    Addded remaining fixes for Sandys (Zhang Zheng) review:</t>
      <t>    4.7 Enhance ECMP explanations:</t>
      <t>    example ECMP algorithm, highlight that doc does not standardize ECMP algorithm.</t>
      <t>    Review from Dirk Trossen:</t>
      <t>    1. Added mentioning Tuebingen</organization>
      </author>
      <author initials="M." surname="Menth" fullname="Michael Menth">
         <organization>University of prior work for traffic engineered paths with bloom filters.</t>
      <t>    2. Changed title from layers to components and added "BIER-TE control plane" to "BIER-TE Controller" to make it clearer, what it does.</t>
      <t>    2.2.3. Added reference to I-D.ietf-bier-multicast-http-response as an example solution.</t>
      <t>    2.3. clarified sentence about resetting BPs before sending copies (also forgot Tuebingen</organization>
      </author>
      <date month="March" day="5" year="2018" />
   </front>
   <seriesInfo name="Internet-Draft" value="draft-eckert-bier-te-frr-03" />
</reference>

<!-- draft-ietf-roll-ccast (Expired) -->
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.ietf-roll-ccast.xml"/>

<!-- draft-ietf-bier-te-yang (I-D Exists)
     (have to mention DNR here).</t>
      <t>    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.</t>
      <t>    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.</t>
      <t>    many typos fixed. Thanks a lot.</t>
      <t>03: Last call textual changes by authors to improve readability:</t>
      <t>    removed Wolfgang Braun as co-authors (as requested).</t>
      <t>    Improved abstract "long way" to be more explanatory. Removed mentioning of FRR (not concluded on so far).</t>
      <t>    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 get correct capping of the BIER-TE Topology concept enabled by these examples.</t>
      <t>    Amended comparison to SR.</t>
      <t>    Changed syntax from [VRF] to {VRF} to indicate its optional "Hu" and to make idnits happy.</t>
      <t>    Split references into normative / informative, added references.</t>
      <t>02: Refresh after IETF104 discussion: changed intended status back to standard. Reasoning:</t>
      <t>    Tighter review of standards document == ensures arch will be better prepared fix
     erroneous "chenhuanan" for possible adoption by other WGs (e.g. DetNet) or std. bodies.</t>
      <t>    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.</t>
      <t>01: Added note comparing BIER and SR to also hopefully clarify BIER-TE vs. BIER comparison re. SR.</t>
      <t> - added requirements section mandating only most basic BIER-TE forwarding features as MUST.</t>
      <t> - reworked comparison with BIER forwarding section to only summarize and point to pseudocode section.</t>
      <t> - 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).</t>
      <t> - Added captions to pictures.</t>
      <t> - Part of review feedback from Sandy (Zhang Zheng) integrated.</t>
      <t>00: Changed target state to experimental (WG conclusion), updated references, mod auth association.</t>
      <t> - Source now on https://www.github.com/toerless/bier-te-arch</t>
      <t> - Please open issues on the github H. Chen -->
<reference anchor="BIER-TE-YANG">
   <front>
      <title>A YANG data model for change/improvement requests to the document - in addition to posting them on the list (bier@ietf.). Thanks!.</t>
    </list>
  </t>
  <t>draft-eckert-bier-te-arch:
    <list>
      <t>06: Added overview of forwarding differences between BIER, BIER-TE.</t>
      <t>05: Author affiliation change only.</t>
      <t>04: Added comparison to Live-Live and BFIR to FRR section (Eckert).</t>
      <t>04: Removed FRR content into the new FRR draft [I-D.eckert-bier-te-frr] (Braun).</t>
      <t> - Linked FRR information to new draft in Overview/Introduction</t>
      <t> - Removed BTAFT/FRR from "Changes in the network topology"</t>
      <t> - Linked new draft in "Link/Node Failures and Recovery"</t>
      <t> - Removed FRR from "The BIER-TE Forwarding Layer"</t>
      <t> - Moved FRR section Tree Engineering for Bit Index Explicit Replication (BIER-TE)</title>
      <author initials="Z." surname="Zhang" fullname="Zheng Zhang">
         <organization>ZTE Corporation</organization>
      </author>
      <author initials="C." surname="Wang" fullname="Cui(Linda) Wang">
         <organization>Individual</organization>
      </author>
      <author initials="R." surname="Chen" fullname="Ran Chen">
         <organization>ZTE Corporation</organization>
      </author>
      <author initials="F." surname="Hu" fullname="fangwei hu">
         <organization>Individual</organization>
      </author>
      <author initials="M." surname="Sivakumar" fullname="Mahesh Sivakumar">
         <organization>Juniper networks</organization>
      </author>
      <author initials="H." surname="Chen" fullname="Huanan Chen">
         <organization>China Telecom</organization>
      </author>
      <date month="May" day="1" year="2022" />
   </front>
   <seriesInfo name="Internet-Draft" value="draft-ietf-bier-te-yang-05" />
</reference>

<!-- draft-ietf-bier-non-mpls-bift-encoding (Expired)
     (have to new draft</t>
      <t> - Moved FRR parts of Pseudocode into new draft</t>
      <t> - Left only non FRR parts</t>
      <t> - removed FrrUpDown(..) and //FRR operations in ForwardBierTePacket(..)</t>
      <t> - New draft contains FrrUpDown(..) and ForwardBierTePacket(Packet) from bier-arch-03</t>
      <t> - Moved "BIER-TE and existing FRR do "long way" to new draft</t>
      <t> - Moved "BIER-TE and Segment Routing" section one level up</t>
      <t> - Thus, removed "Further considerations" that only contained this section</t>
      <t> - Added Changes for version 04</t>
      <t></t>
      <t>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 get "IJ. Wijnands" and contains an additional match field to support full node protection with BIER-TE FRR.</t>
      <t>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.</t>
      <t>02: Changed the definition "M. Mishra") -->
<reference anchor="NON-MPLS-BIER-ENCODING">
   <front>
      <title>An Optional Encoding 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 BIFT-id Field in BIER-TE.</t>
      <t>02: Added <xref target="mgmt-stuff"/> 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.</t>
      <t>02: Added further detailed for rings - how to support input from all ring nodes.</t>
      <t>01: Fixed BFIR -> BFER for section 4.3.</t>
      <t>01: Added explanation of SI, difference to non-MPLS BIER ECMP, consideration for Segment Routing, unicast FRR, considerations for encapsulation, explanations of BIER-TE Controller and CLI.</t>
      <t>00: Initial version.</t>
    </list>
  </t>
  </section>
  <!-- changes -->

</middle>

<back>

<references title="Normative References">
      &RFC2119;
      &RFC8279;
      &RFC8174;
      &RFC8296;
</references>

<references title="Informative References">
      &RFC4253;
      &RFC4456;
      &RFC4655;
      &RFC5440;
      &RFC6241;
      &RFC6242;
      &RFC7589;
      &RFC7752;
      &RFC7950;
      &RFC7988;
      &RFC8040;
      &RFC8253;
      &RFC8345;
      &RFC8401;
      &RFC8402;
      &RFC8444;
      &RFC8556;
<!--
      &RFC2205;
      &RFC2212;
      &RFC3209;
      <?rfc include="reference.I-D.eckert-teas-bier-te-framework"?>
      <?rfc include="reference.I-D.qiang-detnet-large-scale-detnet"?>
-->

      <?rfc include="reference.I-D.ietf-teas-rfc3272bis"?>
      <?rfc include="reference.I-D.ietf-bier-multicast-http-response"?>
      <?rfc include="reference.I-D.eckert-bier-te-frr"?>
      <?rfc include="reference.I-D.ietf-roll-ccast"?>
      <?rfc include="reference.I-D.ietf-bier-te-yang"?>
      <?rfc include="reference.I-D.ietf-bier-non-mpls-bift-encoding"?> Encapsulation</title>
     <author fullname="IJsbrand Wijnands" surname="Wijnands" initials="IJ"></author>
     <author fullname="Mankamana Mishra" surname="Mishra" initials="M"></author>
     <author fullname="Xiaohu Xu" surname="Xu" initials="X"></author>
     <author fullname="Hooman Bidgoli" surname="Bidgoli" initials="H"></author>
    <date month="May" day="30" year="2021" />
  </front>
 <seriesInfo name="Internet-Draft" value="draft-ietf-bier-non-mpls-bift-encoding-04" />
</reference>

        <reference anchor="ICC" target="https://ieeexplore.ieee.org/document/7511036">
          <front>
            <title>
            Stateless multicast switching in software defined networks
            </title>
            <author initials="M. J." surname="Reed"/>
            <author initials="M." surname="Al-Naday"/>
            <author initials="N." surname="Thomos"/>
            <author initials="D." surname="Trossen"/>
            <author initials="G." surname="Petropoulos"/>
            <author initials="S." surname="Spirou"/>
            <date month="May" year="2016"/>
          </front>
        <seriesInfo name="" value="IEEE
          <refcontent>IEEE International Conference on Communications (ICC), Kuala Lumpur, Malaysia, 2016"/> Malaysia</refcontent>
        <seriesInfo name="DOI" value="10.1109/ICC.2016.7511036"/>
        </reference>

        <reference anchor="RCSD94" target="https://dl.acm.org/doi/10.5555/2692227.2692232"> target="https://content.iospress.com/articles/journal-of-high-speed-networks/jhs3-4-05">
          <front>
            <title>
            Rate-Controlled Service Disciplines
            </title>
            <author initials="H." surname="Zhang"/>
            <author initials="D." surname="Domenico"/> surname="Ferrari"/>
            <date month="May" month="October" year="1994"/>
          </front>
        <seriesInfo name="" value="Journal
          <refcontent>Journal of High-Speed High Speed Networks, 1994"/> Volume 3, Issue 4, pp. 389-412</refcontent>
        <seriesInfo name="DOI" value="10.3233/JHS-1994-3405"/>
        </reference>

        <reference anchor="Bloom70"> anchor="Bloom70" target="https://dl.acm.org/doi/10.1145/362686.362692">
          <front>
            <title>Space/time trade-offs in hash coding with allowable errors</title>
            <author initials="B. H." surname="Bloom" fullname="Burton H. Bloom">
              <organization/>
            </author>
            <date month="July" year="1970"/>
          </front>
        <seriesInfo name="Comm.
          <refcontent>Comm. ACM " value="13(7):422-6"/>
        <format type="PDF" target="https://dl.acm.org/doi/10.1145/362686.362692"/> 13(7):422-6</refcontent>
          <seriesInfo name="DOI" value="10.1145/362686.362692"/>
        </reference>

      <!-- TODO change reference below as soon as its available from tool chain-->
      <!-- <?rfc include="reference.I-D.eckert-bier-te-frr"?> -->

      <!---->
</references>
    </references>
    <section anchor="SR" title="BIER-TE numbered="true" toc="default">
      <name>BIER-TE and Segment Routing (SR)"> (SR)</name>
      <t>SR (<xref target="RFC8402"/>) <xref target="RFC8402" format="default"/> aims to enable lightweight path steering
via loose source routing. Compared For example, compared to its more heavy-weight predecessor heavyweight predecessor, RSVP-TE, SR does for example not require per-path signaling to each of these hops.</t>
      <t>BIER-TE supports the same design philosophy for multicast.
Like in SR, it relies BIER-TE</t>
<ul spacing="normal">
<li>relies on source-routing -
via the definition of source routing (via a BitString. Like SR, it only BitString), and</li>
<li>only requires to consider consideration of
the "hops" either (1) on which either replication has to happen, happen or (2) across which the
traffic should be steered (even without replication).  Any replication).</li>
</ul>
<t>Any other hops can
be skipped via the use of routed adjacencies.</t>
      <t>BIER-TE bit position (BP) "bit positions" (BPs) can be understood as the BIER-TE equivalent of
"forwarding segments" in SR, but they have a different scope than SR do forwarding
segments.
segments in SR. Whereas forwarding segments in SR are global or local, BPs in BIER-TE
have a scope that is the group comprised of BFR(s) one or more BFRs that have adjacencies for this BP the BPs in
their BIFT. This BIFTs. These segments can be called "adjacency" scoped "adjacency-scoped" forwarding segments.</t>
      <t>Adjacency scope could be global, but then every BFR would need an adjacency
for this BP, a given BP -- for example example, a forward_routed() adjacency with encapsulation to
the global SR SID "Segment Identifier" (SID) of the destination. Such a BP would always result in ingress
replication
replication, though (as in <xref target="RFC7988"/>). target="RFC7988" format="default"/>). The first BFR encountering this BP would directly
replicate to traffic on it. Only by using non-global adjacency scope for BPs can
traffic be steered and replicated on non-ingress BFR.</t> a non-BFIR.</t>
      <t>SR can naturally be combined with BIER-TE and can help to optimize it. For example,
instead of defining bit positions for non-replicating hops, it is equally
possible to use segment routing SR encapsulations (e.g. (e.g., SR-MPLS label stacks)
for the encapsulation of "forward_routed" "forward_routed()" adjacencies.</t>
      <t>Note that (non-TE) BIER itself can also be seen to be as being similar to SR. BIER BPs act
as global destination Node-SIDs Node-SIDs, and the BIER BitString is simply a highly optimized
mechanism to indicate multiple such SIDs and let the network take care of effectively
replicating the packet hop-by-hop hop by hop to each destination Node-SID.  What  BIER does not allow is to
indicate the
indication of intermediate hops, or hops or, in terms of SR SR, the ability to indicate a sequence of SID SIDs
to reach the destination. This is what On the other hand, BIER-TE and its adjacency scoped BP enables.</t> adjacency-scoped BPs provide these capabilities.</t>
    </section>
  <section anchor="ack" numbered="false" toc="default">
      <name>Acknowledgements</name>
      <t>The authors would like to thank <contact fullname="Greg Shepherd"/>, <contact fullname="IJsbrand Wijnands"/>, <contact fullname="Neale Ranns"/>,
<contact fullname="Dirk Trossen"/>, <contact fullname="Sandy Zheng"/>, <contact fullname="Lou Berger"/>, <contact fullname="Jeffrey Zhang"/>, <contact fullname="Carsten Bormann"/>, and <contact fullname="Wolfgang Braun"/> for their reviews and suggestions.</t>
      <t> Special thanks to <contact fullname="Xuesong Geng"/> for shepherding this document.  Special thanks also for IESG review/suggestions by <contact fullname="Alvaro Retana"/> (responsible AD/RTG), <contact fullname="Benjamin Kaduk"/> (SEC), <contact fullname="Tommy Pauly"/> (TSV), <contact fullname="Zaheduzzaman Sarker"/> (TSV), <contact fullname="Éric Vyncke"/> (INT), <contact fullname="Martin Vigoureux"/> (RTG), <contact fullname="Robert Wilton"/> (OPS), <contact fullname="Erik Kline"/> (INT), <contact fullname="Lars Eggert"/> (GEN), <contact fullname="Roman Danyliw"/> (SEC), <contact fullname="Ines Robles"/> (RTGDIR), <contact fullname="Robert Sparks"/> (Gen-ART), <contact fullname="Yingzhen Qu"/> (RTGDIR), and <contact fullname="Martin Duke"/> (TSV).</t>
    </section>
  <!-- SR -->
</back>
</rfc>