DetNet
Internet Engineering Task Force (IETF) B. Varga, Ed.
Internet-Draft
Request for Comments: 9550 J. Farkas
Intended status:
Category: Informational Ericsson
Expires: 18 July 2024
ISSN: 2070-1721 S. Kehrer
T. Heer
Hirschmann Automation and Control GmbH
15 January
March 2024
Deterministic Networking (DetNet): Packet Ordering Function
draft-ietf-detnet-pof-11
Abstract
Replication
The replication and Elimination elimination functions of DetNet Architecture the Deterministic
Networking (DetNet) architecture can result in out-of-order packets,
which is not acceptable for some time-sensitive applications. The
Packet Ordering Function (POF)
algorithm algorithms described herein enables to restore in this document
enable restoration of the correct packet order when the replication
and elimination functions are used in DetNet networks. The POF only
provides ordering within the latency bound of a DetNet flow, and flow; it does
not provide any additional reliability.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Terms Used in This Document . . . . . . . . . . . . . . . 4
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements on for POF Implementations . . . . . . . . . . . . . 4
4. POF Algorithms . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Prerequisites and Assumptions . . . . . . . . . . . . . . 5
4.2. POF building blocks . . . . . . . . . . . . . . . . . . . 5 Building Blocks
4.3. The Basic POF Algorithm . . . . . . . . . . . . . . . . . 6
4.4. The Advanced POF Algorithm . . . . . . . . . . . . . . . 8
4.5. Further enhancements Enhancements of the POF algorithms . . . . . . . . . 9 Algorithms
4.6. Selecting and using Using the POF algorithm . . . . . . . . . . 10 Algorithms
5. Control and Management Plane Parameters for POF . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1.
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2.
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The DetNet Working Group has defined packet replication
[RFC8655] defines the Packet Replication Function (PRF) and
packet elimination Packet
Elimination Function (PEF) functions in DetNet for achieving extremely low
packet loss. The PRF and PEF are described in [RFC8655] and provide service protection for DetNet
flows. This service protection method relies on copies of the same
packet sent over multiple maximally disjoint paths and uses
sequencing information to eliminate duplicates. A possible
implementation of the PRF and PEF functions is described in
[IEEE8021CB] [IEEE8021CB], and
the related YANG model is defined in [IEEEP8021CBcv].
In general, use of per packet per-packet replication and elimination functions
can result in out-of-order delivery of packets, which is not
acceptable for some deterministic applications. Correcting packet
order is not a trivial task, therefore task; therefore, details of a Packet Ordering
Function (POF) are specified herein. The IETF DetNet WG has defined in this document. [RFC8655] defines the
external observable result of a POF function, i.e., (i.e., that packets are reordered,
reordered) but without does not specify any implementation details.
So far in packet networks, out-of-order delivery situations were have been
handled at higher OSI layers at the end-points/hosts endpoints/hosts (e.g., in the TCP
stack when packets are sent to the application layer) and not within
a network in nodes acting at the Layer-2 Layer 2 or Layer-3 Layer 3 OSI layers.
Figure 1 shows a DetNet flow on which packet replication, elimination Packet Replication,
Elimination, and ordering Ordering Functions (PREOF) functions are applied during
forwarding from source to destination.
+------------+
+-----------E1----+ | |
+----+ | | +---R3---+ | +----+
|src |------R1 +---+ | E3----O1---+ dst|
+----+ | | E2-------+ +----+
+-------R2 |
+-----------------+
R: replication point (PRF)
E: elimination point (PEF)
O: ordering function (POF)
Figure 1: PREOF scenario Scenario in a DetNet network Network
In general, the use of PREOF functions require requires sequencing information to be
included in the packets of a DetNet compound flow. This can be done
by adding a sequence number as part of DetNet encapsulation
[RFC8655]. Sequencing information is typically added once, at or
close to the source.
Important
It is important to note that different applications can react
differently to out-of-order delivery. A single out-of-order packet (E.g.,
(e.g., packet
order: order #1, #3, #2, #4, #5) is interpreted by some
application as a single error, but some other applications treat it as 3
three errors in-
a-row situation. in a row. For example, in industrial scenarios 3 scenarios, three
errors in-
a-row in a row is a usual typical error threshold and can cause the
application to stop (e.g., to go to a fail-safe state).
The POF ensures in-order delivery for packets being within the latency
bound of the (DetNet) DetNet flow. The POF does not correct errors in the
packet flow e.g., (e.g., duplicate packets, packets or packets that are too late packets. late).
2. Terminology
2.1. Terms Used in This Document
This document uses the terminology established in the DetNet
architecture [RFC8655], and [RFC8655]; the reader is assumed to be familiar with
that document and its terminology.
2.2. Abbreviations
The following abbreviations are used in this document:
DetNet Deterministic Networking. Networking
PEF Packet Elimination Function. Function
POF Packet Ordering Function. Function
PREOF Packet Replication, Elimination Elimination, and Ordering Functions. Functions
PRF Packet Replication Function. Function
3. Requirements on for POF Implementations
The requirements on a for POF function implementations are:
* to To solve the out-of-order delivery problem of the Replication replication and
Elimination
elimination functions of DetNet networks.
* to To consider the delay bound requirement of a DetNet Flow. flow.
* to To be simple and to require in network nodes only a minimum set of
states/configuration parameters states,
configuration parameters, and resources per DetNet Flow. flow in network
nodes.
* to To add only minimal or no delay to the forwarding process of packets.
* To not to require synchronization between PREOF nodes.
Some aspects are explicitly out-of-scope out of scope for a POF function: POF:
* to To eliminate the delay variation caused by the packet ordering.
Dealing with delay variation is a DetNet forwarding sub-layer
target
target, and it can be achieved achieved, for example example, by placing a de-jitter de-
jitter buffer or flow regulator (e.g., shaping) function after the POF
functionality.
POF.
4. POF Algorithms
4.1. Prerequisites and Assumptions
The POF Algorithm algorithms discussed in this document makes make some assumptions
and tradeoffs trade-offs regarding the characteristics of the sequence of
received packets. In particular, the algorithm assumes algorithms assume that a Packet
Elimination Function (PEF) PEF is
performed on the incoming packets before they are handed to the POF function. POF.
Hence, the sequence of incoming packets can be out of order out-of-order or
incomplete but cannot contain duplicate packets. However, the PREOF functions
run independently without any state exchange required between the PEF
and the POF or the PRF and the POF. Error cases in which duplicate
packets are presented to the POF can lead to out of order out-of-order delivery of
duplicate packets as well as and to increased delays.
The algorithm algorithms further requires require that the delay difference between two
replicated packets that arrive at the PEF before the POF is bounded
and known. Error cases that violate this condition (e.g., a packet
that arrives later than this bound) will result in out-of order out-of-order
packets.
The algorithm algorithms also makes make some tradeoffs. trade-offs. For simplicity, it is
designed in a way that allows to allow for some out of order out-of-order packets directly after
initialization. If this is not acceptable, Section 4.5 provides an
alternative initialization scheme that prevents out-of-
order out-of-order packets
in the initialization phase.
4.2. POF building blocks Building Blocks
The method described herein in this document provides a POF for DetNet
networks. The configuration parameters of the POF can be derived during
when engineering the DetNet flow through the network.
The POF method is provided via:
1. via the following:
Delay calculator: calculates Calculates buffering time for out-of-order
packets. Buffering time considers (i) the delay difference of
paths used for forwarding the replicated packets and (ii) the
bounded delay requirement of the given DetNet flow.
2.
Conditional delay buffer: Used for buffering the out-of-order
packets of a DetNet flow for a given time.
Note: the The conditional delay buffer of the POF increases the
burstiness of the traffic as it only adds delay only for some of the
packets.
Figure 2 shows the building blocks of a possible POF implementation.
+------------+ +--------------+
| Delay calc | | Conditional |
+--| for packet >--->>---| Delay Buffer >--+
| +------------+ +--------------+ |
| |
+------^--------+ |
->>--| POF selector >---------------------------------+-->>----
| (Flow ident.) |
+---------------+
->>- packet flow
Figure 2: POF Building Blocks
4.3. The Basic POF Algorithm
The basic POF algorithm delays all out-of-order packets until all
previous packet arrives packets arrive or a given time (POFMaxDelay) ("POFMaxDelay") elapses. The
basic POF algorithm works as follows:
* The sequence number of the last forwarded packet (POFLastSent) ("POFLastSent")
is stored for each DetNet Flow. flow.
* The sequence number (seq_num) of a received packet is compared to
that of the last forwarded one (POFLastSent). ("POFLastSent").
* If (seq_num <= POFLastSent + 1)
- Then the packet is forwarded without buffering buffering, and
"POFLastSent" is updated (POFLastSent = seq_num).
- Else Else, the received packet is buffered.
* A buffered packet is forwarded from the buffer when its seq_num
becomes equal to "POFLastSent +1," + 1" OR a predefined time
("POFMaxDelay") elapses.
* When a packet is forwarded from the buffer buffer, "POFLastSent" is
updated with its seq_num (POFLastSent = seq_num).
Note: the
Notes:
* The difference of between sequence number numbers in consecutive packets is
bounded due to the history window of the Elimination elimination function
before the POF. Therefore Therefore, "<=" can be evaluated despite of the
circular sequence number space. A possible implementation of the
PEF function and the impact of the history window is are described in
[IEEE8021CB].
Note2:
* The basic POF algorithm can be extended to cope with multiple
failure scenarios (i.e., simultaneous packet loss and out-of-order packets),
packets) when the expiration of the timer for a packet with
sequence number N
trigger triggers the release of some number of packets with a
sequence number smaller than N.
The state used by the basic POF algorithm (i.e., "POFLastSent") needs
initialization and maintenance. This works as follows:
* The next received packet is forwarded and the POFLastSent "POFLastSent"
updated when the POF function was is reset OR no packet was is received for a
predefined time ("POFTakeAnyTime").
* The reset of the POF erases all packets from the time-based buffer
used by the POF.
The basic POF algorithm has two parameters to engineer:
* "POFMaxDelay", which cannot be smaller than the delay difference
of the paths used by the flow.
* "POFTakeAnyTime", which is calculated based on several factors,
for example example, the RECOVERY_TIMEOUT related settings of the
Elimination elimination function(s) relating
to RECOVERY_TIMEOUT before the POF, the flow characteristics
(e.g., inter packet inter-packet time), and the delay difference of the paths
used by the flow.
Design of these parameters is out-of-scope in out of scope for this document.
Note: multiple Multiple network failures can impact the POF function (e.g., complete
outage of all redundant paths).
The basic POF algorithm increases the delay of packets with maximum
"POFMaxDelay" time. Packets being in order In-order packets are not delayed. This basic
POF method can be applied in all network scenarios where the
remaining delay budget of a flow at the POF point is larger than
"POFMaxDelay" time.
Figure 3 shows the delay budget relations situation at the POF point.
Path delay
difference
/-------------/
<- path with min delay -> /-- remaining delay budget --/
|-----------------------|-------------|----------------------------|
0 t1 t2 T
<-------- path with max delay -------->
/-------------------- delay budget at POF point -------------------/
Figure 3: Delay Budget Relations Situation at the POF Point
4.4. The Advanced POF Algorithm
In network scenario scenarios where the remaining delay budget of a flow at
the POF point is smaller than "POFMaxDelay" time time, the basic method
needs extensions.
The issue is that packets on the longest path cannot be buffered in
order to keep the delay budget of the flow. It must be noted that
such a packet (i.e., forwarded over the longest path) needs no
buffering as it is the "last chance" last chance to deliver a packet with a given
sequence number. This is because all replicas are already arrived via a
shorter path(s).
The advanced POF algorithm needs two requires extensions of the basic POF
algorithm:
* to identify the received packet's path at the POF location and
* to make the value of "POFMaxDelay" for buffered packets path
dependent ("POFMaxDelay_i", where "i" notes the path the packet
has used).
By identifying
The advanced POF algorithm identifies the path of a given packet, the POF algorithm can use packet and
uses this information to select what the predefined time "POFMaxDelay_i" ("POFMaxDelay_i")
to apply for the buffered packet. So, in the advanced POF algorithm algorithm,
"POFMaxDelay" is an array, array that contains the predefined and path path-
specific buffering time for each redundant path of a flow. Values in
the "POFMaxDelay" array are engineered to fulfill the delay budget
requirement.
Design of these parameters is out-of-scope in out of scope for this document.
Note: for For the "Advanced advanced POF Algorithm" algorithm, the path dependent path-dependent delays might
result in multiple packets triggering the "maximum delay reached" at
exactly the same time. The transmission order of these packets then
should be done in their seq_num order.
The method for identification of identifying the packet's path at the POF location
depends on the network scenario. It can be implemented via various
techniques, for example example, using ingress interface information,
encoding the path in the packet itself (e.g., replication functions
can
set a different FlowID per member flow at their egress what can be and such a
FlowID is used as to identify the path of a PathID), packet at the POF), or
in other
means. Method Methods for identification of identifying the packet's path is are out of scope in
for this document.
Note: in case of When using the advanced POF algorithm algorithm, it might be advantageous
to combine PEF and POF locations in the DetNet network, as it this can
simplify the method used for identification of identifying the packet's path at the POF
location.
4.5. Further enhancements Enhancements of the POF algorithms Algorithms
POF algorithms can be further enhanced by distinguishing the case of
initialization from normal operation at the price of more states and
more sophisticated implementation. Such enhancements could could, for
example
example, react better after some failure scenarios (e.g., complete
outage of all paths of a DetNet flow) and can be dependent on the PEF
implementation.
The challenge for POF initialization is that for example after a
reset it is not known whether
the first received packet is in-order or out-of-order. out-of-order (for example,
after a reset). The original initialization (see before) Section 4.3)
considers the first packet as in-order, so out-of-order packet(s)
during "POFMaxTime"/"POFMaxTime_path_i" time - -- after the first
packet was is received - -- cannot be corrected. Motivation The motivation behind
such an initialization is simplicity of POF implementation simplicity. implementation.
A possible enhancement of POF initialization works as follows:
* After a reset reset, all received packets are buffered with their
predefined timer ("POFMaxTime"/"POFMaxTime_path_i").
* No basic/advanced basic or advanced POF rules are applied until the first timer
expires.
* When the first timer expires expires, the packet with lowest seq_num in
the buffer is selected, selected and forwarded, and "POFLastSent" is set
with its seq_num.
* The basic/advanced basic or advanced POF rules are applied for the packet(s) in
the buffer and the subsequently received packets.
4.6. Selecting and using Using the POF algorithm Algorithms
The selection of the POF algorithm depends on the network scenario
and the remaining delay budget of a flow. Using the POF algorithms
and calculating
its their parameters require proper design. Knowing the
path delay difference is essential for the POF algorithms described
here. Failure scenarios breaking the design assumptions can impact
the result of the POF (e.g., packet received out of the expected
worst-case delay window - -- calculated based on the path delay
difference - -- can result in unwanted out-of-order delivery).
In DetNet scenarios scenarios, there is always an Elimination elimination function before
the POF (therefore (therefore, duplicates are not considered by the POF).
Implementing them together in the same node allows the POF to
consider PEF events/states during the re-ordering. reordering. For example, under
normal
circumstances circumstances, the difference of between sequence number numbers in
consecutive packets is bounded due to the history window of the PEF.
However, in some scenarios (e.g., reset of sequence number) number), the
difference can be much larger than the size of the history window size. window.
5. Control and Management Plane Parameters for POF
POF algorithms needs setting of require the following parameters: parameters to be set:
* Basic POF
- "POFMaxDelay"
- "POFTakeAnyTime"
* Advanced POF
- "POFMaxDelay_i" for each possible path i
- "POFTakeAnyTime"
- Network Configuration(s) related to network path identification related configuration(s)
Note, that in
Note: In a proper design design, "POFTakeAnyTime" is always larger than
"POFMaxDelay".
6. Security Considerations
PREOF related
PREOF-related security considerations (including POF) are described
in section Section 3.3 of [RFC9055]. There are no additional POF related POF-related
security considerations originating from this document.
7. IANA Considerations
This document makes has no IANA requests.
9. actions.
8. References
9.1.
8.1. Normative References
[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
[RFC9055] Grossman, E., Ed., Mizrahi, T., and A. Hacker,
"Deterministic Networking (DetNet) Security
Considerations", RFC 9055, DOI 10.17487/RFC9055, June
2021, <https://www.rfc-editor.org/info/rfc9055>.
9.2.
8.2. Informative References
[IEEE8021CB]
IEEE, "IEEE Standard for Local and metropolitan area
networks -- Frame Replication and Elimination for
Reliability", IEEE Std 802.1CB-2017,
DOI 10.1109/IEEESTD.2017.8091139, October 2017,
<https://standards.ieee.org/standard/802_1CB-2017.html>.
[IEEEP8021CBcv]
Kehrer, S., "FRER
IEEE, "IEEE Standard for Local and metropolitan area
networks -- Frame Replication and Elimination for
Reliability - Amendment 1: Information Model, YANG Data Model
Model, and Management Information Base Module", IEEE P802.1CBcv
/D1.2 P802.1CBcv, March 2021,
<https://www.ieee802.org/1/files/private/cv-drafts/d1/802-
1CBcv-d1-2.pdf>.
8. Std
802.1CBcv-2001, DOI 10.1109/IEEESTD.2022.9715061, February
2022, <https://standards.ieee.org/ieee/802.1CBcv/7285/>.
Acknowledgements
Authors extend their appreciation to Gyorgy Miklos, Mohammadpour
Ehsan, Ehsan
Mohammadpour, Ludovic Thomas, Greg Mirsky, Jeong-dong Ryoo, Shirley Yangfan, Fan Yang,
Toerless Eckert, Norman Finn Finn, and Ethan Grossman for their insightful
comments and productive discussion that helped to improve the
document.
Authors' Addresses
Balazs Varga (editor)
Ericsson
Budapest
Magyar Tudosok krt. 11.
1117
Hungary
Email: balazs.a.varga@ericsson.com
Janos Farkas
Ericsson
Budapest
Magyar Tudosok krt. 11.
1117
Hungary
Email: janos.farkas@ericsson.com
Stephan Kehrer
Hirschmann Automation and Control GmbH
Stuttgarter Strasse 45-51.
72654 Neckartenzlingen
Germany
Email: Stephan.Kehrer@belden.com
Tobias Heer
Hirschmann Automation and Control GmbH
Stuttgarter Strasse 45-51.
72654 Neckartenzlingen
Germany
Email: Tobias.Heer@belden.com