Internet Engineering Task Force (IETF) B. Varga, Ed.

Internet Engineering Task Force (IETF) B. Varga, Ed.
Request for Comments: 9550 J. Farkas
Category: Informational Ericsson
ISSN: 2070-1721 S. Kehrer
T. Heer
Belden
March 2024

Deterministic Networking (DetNet): Packet Ordering Function

Abstract

The replication and elimination functions of 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) algorithms described 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; it does
not provide any additional reliability.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9550.

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Table of Contents

1. Introduction
2. Terminology
2.1. Terms Used in This Document
2.2. Abbreviations
3. Requirements for POF Implementations
4. POF Algorithms
4.1. Prerequisites and Assumptions
4.2. POF Building Blocks
4.3. The Basic POF Algorithm
4.4. The Advanced POF Algorithm
4.5. Further Enhancements of the POF Algorithms
4.6. Selecting and Using the POF Algorithms
5. Control and Management Plane Parameters for POF
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

[RFC8655] defines the Packet Replication Function (PRF) and Packet
Elimination Function (PEF) in DetNet for achieving extremely low
packet loss. The PRF and PEF 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 is described in [IEEE8021CB], and
the related YANG model is defined in [IEEEP8021CBcv].

In general, use of 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, details of a Packet Ordering
Function (POF) are specified in this document. [RFC8655] defines the
external observable result of a POF (i.e., that packets are
reordered) but does not specify any implementation details.

So far in packet networks, out-of-order delivery situations have been
handled at higher OSI layers at the 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 or Layer 3 OSI layers.

Figure 1 shows a DetNet flow on which Packet Replication,
Elimination, and Ordering Functions (PREOF) 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 in a DetNet Network

In general, the use of PREOF 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.

It is important to note that different applications can react
differently to out-of-order delivery. A single out-of-order packet
(e.g., packet order #1, #3, #2, #4, #5) is interpreted by some
application as a single error, but other applications treat it as
three errors in a row. For example, in industrial scenarios, three
errors in a row is a typical error threshold and can cause the
application to stop (e.g., go to a fail-safe state).

The POF ensures in-order delivery for packets within the latency
bound of the DetNet flow. The POF does not correct errors in the
packet flow (e.g., duplicate packets or packets that are too late).

2. Terminology

2.1. Terms Used in This Document

This document uses the terminology established in the DetNet
architecture [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

PEF Packet Elimination Function

POF Packet Ordering Function

PREOF Packet Replication, Elimination, and Ordering Functions

PRF Packet Replication Function

3. Requirements for POF Implementations

The requirements for POF implementations are:

* To solve the out-of-order delivery problem of the replication and
elimination functions of DetNet networks.

* To consider the delay bound requirement of a DetNet flow.

* To be simple and to require only a minimum set of states,
configuration parameters, and resources per DetNet flow in network
nodes.

* To add minimal or no delay to the forwarding process of packets.

* To not require synchronization between PREOF nodes.

Some aspects are explicitly out of scope for a POF:

* To eliminate the delay variation caused by the packet ordering.
Dealing with delay variation is a DetNet forwarding sub-layer
target, and it can be achieved, for example, by placing a de-
jitter buffer or flow regulator (e.g., shaping) function after the
POF.

4. POF Algorithms

4.1. Prerequisites and Assumptions

The POF algorithms discussed in this document make some assumptions
and trade-offs regarding the characteristics of the sequence of
received packets. In particular, the algorithms assume that a PEF is
performed on the incoming packets before they are handed to the POF.
Hence, the sequence of incoming packets can be out-of-order or
incomplete but cannot contain duplicate packets. However, the PREOF
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 delivery of
duplicate packets and to increased delays.

The algorithms further 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
packets.

The algorithms also make some trade-offs. For simplicity, it is
designed to allow for some 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 packets
in the initialization phase.

4.2. POF Building Blocks

The method described in this document provides a POF for DetNet
networks. The configuration parameters of the POF can be derived
when engineering the DetNet flow through the network.

The POF method is provided via the following:

Delay calculator: 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.

Conditional delay buffer: Used for buffering the out-of-order
packets of a DetNet flow for a given time.

Note: The conditional delay buffer of the POF increases the
burstiness of the traffic as it only adds delay 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 packets arrive or a given time ("POFMaxDelay") elapses. The
basic POF algorithm works as follows:

* The sequence number of the last forwarded packet ("POFLastSent")
is stored for each DetNet flow.

* The sequence number (seq_num) of a received packet is compared to
that of the last forwarded one ("POFLastSent").

* If (seq_num <= POFLastSent + 1)

- Then the packet is forwarded without buffering, and
"POFLastSent" is updated (POFLastSent = seq_num).

- Else, the received packet is buffered.

* A buffered packet is forwarded from the buffer when its seq_num
becomes equal to "POFLastSent + 1" OR a predefined time
("POFMaxDelay") elapses.

* When a packet is forwarded from the buffer, "POFLastSent" is
updated with its seq_num (POFLastSent = seq_num).

Notes:

* The difference between sequence numbers in consecutive packets is
bounded due to the history window of the elimination function
before the POF. Therefore, "<=" can be evaluated despite the
circular sequence number space. A possible implementation of the
PEF and the impact of the history window are described in
[IEEE8021CB].

* The basic POF algorithm can be extended to cope with multiple
failure scenarios (i.e., simultaneous packet loss and out-of-order
packets) when the expiration of the timer for a packet with
sequence number N triggers the release of some 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"
updated when the POF is reset OR no packet 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, the settings of the elimination function(s) relating
to RECOVERY_TIMEOUT before the POF, the flow characteristics
(e.g., inter-packet time), and the delay difference of the paths
used by the flow.

Design of these parameters is out of scope for this document.

Note: Multiple network failures can impact the POF (e.g., complete
outage of all redundant paths).

The basic POF algorithm increases the delay of packets with maximum
"POFMaxDelay" time. 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 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 Situation at the POF Point

4.4. The Advanced POF Algorithm

In network scenarios where the remaining delay budget of a flow at
the POF point is smaller than "POFMaxDelay" 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 to deliver a packet with a given
sequence number. This is because all replicas already arrived via a
shorter path(s).

The advanced POF algorithm 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).

The advanced POF algorithm identifies the path of a given packet and
uses this information to select the predefined time ("POFMaxDelay_i")
to apply for the buffered packet. So, in the advanced POF algorithm,
"POFMaxDelay" is an array that contains the predefined and 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 for this document.

Note: For the advanced POF algorithm, the path-dependent delays might
result in multiple packets triggering the "maximum delay reached" at
exactly the same time. The transmission order of these packets
should be done in their seq_num order.

The method for identifying the packet's path at the POF location
depends on the network scenario. It can be implemented via various
techniques, for example, using ingress interface information,
encoding the path in the packet itself (e.g., replication functions
set a different FlowID per member flow at their egress and such a
FlowID is used to identify the path of a packet at the POF), or other
means. Methods for identifying the packet's path are out of scope
for this document.

Note: When using the advanced POF algorithm, it might be advantageous
to combine PEF and POF locations in the DetNet network, as this can
simplify the method used for identifying the packet's path at the POF
location.

4.5. Further Enhancements of the POF 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, for
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 it is not known whether
the first received packet is in-order or out-of-order (for example,
after a reset). The original initialization (see 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 is received -- cannot be corrected. The motivation behind
such an initialization is simplicity of POF implementation.

A possible enhancement of POF initialization works as follows:

* After a reset, all received packets are buffered with their
predefined timer ("POFMaxTime"/"POFMaxTime_path_i").

* No basic or advanced POF rules are applied until the first timer
expires.

* When the first timer expires, the packet with lowest seq_num in
the buffer is selected and forwarded, and "POFLastSent" is set
with its seq_num.

* The basic or advanced POF rules are applied for the packet(s) in
the buffer and the subsequently received packets.

4.6. Selecting and Using the POF 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 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, there is always an elimination function before
the POF (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 reordering. For example, under
normal circumstances, the difference between sequence numbers in
consecutive packets is bounded due to the history window of the PEF.
However, in some scenarios (e.g., reset of sequence number), the
difference can be much larger than the size of the history window.

5. Control and Management Plane Parameters for POF

POF algorithms require the following parameters to be set:

* Basic POF

- "POFMaxDelay"

- "POFTakeAnyTime"

* Advanced POF

- "POFMaxDelay_i" for each possible path i

- "POFTakeAnyTime"

- Configuration(s) related to network path identification

Note: In a proper design, "POFTakeAnyTime" is always larger than
"POFMaxDelay".

6. Security Considerations

PREOF-related security considerations (including POF) are described
in Section 3.3 of [RFC9055]. There are no additional POF-related
security considerations originating from this document.

7. IANA Considerations

This document has no IANA actions.

8. References

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>.

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]
IEEE, "IEEE Standard for Local and metropolitan area
networks -- Frame Replication and Elimination for
Reliability - Amendment 1: Information Model, YANG Data
Model, and Management Information Base Module", IEEE 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, Ehsan
Mohammadpour, Ludovic Thomas, Greg Mirsky, Jeong-dong Ryoo, Fan Yang,
Toerless Eckert, Norman 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: [email protected]

Janos Farkas
Ericsson
Budapest
Magyar Tudosok krt. 11.
1117
Hungary
Email: [email protected]

Stephan Kehrer
Belden Electronics GmbH
Stuttgarter Strasse 45-51.
72654 Neckartenzlingen
Germany
Email: [email protected]

Tobias Heer
Belden Electronics GmbH
Stuttgarter Strasse 45-51.
72654 Neckartenzlingen
Germany
Email: [email protected]