Deterministic Networking (DetNet): Packet Ordering Function
draft-varga-detnet-pof-01
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| Authors | Balazs Varga , János Farkas , Stephan Kehrer , Tobias Heer | ||
| Last updated | 2021-05-21 (Latest revision 2021-04-19) | ||
| Replaced by | draft-ietf-detnet-pof | ||
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draft-varga-detnet-pof-01
DetNet B. Varga, Ed.
Internet-Draft J. Farkas
Intended status: Informational Ericsson
Expires: November 22, 2021 S. Kehrer
T. Heer
Hirschmann Automation and Control GmbH
May 21, 2021
Deterministic Networking (DetNet): Packet Ordering Function
draft-varga-detnet-pof-01
Abstract
Replication and Elimination functions of DetNet [RFC8655] may result
in out-of-order packets, which may not be acceptable for some time-
sensitive applications. The Packet Ordering Function (POF) algorithm
described herein enables to restore the correct packet order when
replication and elimination functions are used in DetNet networks.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 22, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Terms Used in This Document . . . . . . . . . . . . . . . 3
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3
2.3. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. Requirements on POF Implementations . . . . . . . . . . . . . 4
4. POF Algorithms . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Prerequisites and Assumptions . . . . . . . . . . . . . . 4
4.2. POF building blocks . . . . . . . . . . . . . . . . . . . 5
4.3. The Basic POF Algorithm . . . . . . . . . . . . . . . . . 6
4.4. The Advanced POF Algorithm . . . . . . . . . . . . . . . 7
4.5. Further enhancements of POF algorithms . . . . . . . . . 8
4.6. Selecting and using the POF algorithm . . . . . . . . . . 9
5. Control and Management Plane Parameters for POF . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The DetNet Working Group has defined packet replication (PRF) and
packet elimination (PEF) functions for achieving extremely low packet
loss. 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 PRF and PEF functions is described in
[IEEE8021CB] and the related YANG model is defined in
[IEEEP8021CBcv].
In general, use of per packet replication and elimination functions
may result in out-of-order delivery of packets, which may not be
acceptable for some deterministic applications. Correcting packet
order is not a trivial task, therefore details of a Packet Ordering
Function (POF) are specified herein. The IETF DetNet WG has defined
in [RFC8655] the external observable result of a POF function, i.e.,
that packets are reordered, but without any implementation details.
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So far in packet networks, out-of-order delivery situations were
handled at higher OSI layers at the end-points/hosts (e.g., in the
TCP stack when ackets are sent to 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 PREOF functions are applied
during forwarding from source to destination.
+------------+
+---------------E----+ | |
+----+ | | +----R---+ | +----+
|src |-------R +---+ | E-----O----+ dst|
+----+ | | E--------+ +----+
+-----------R |
+-----------------+
R: replication point (PRF)
E: elimination point (PEF)
O: ordering function (POF)
Figure 1: PREOF scenario in a DetNet network
Important to note, that application may react differently on out-of-
order delivery. A single out-of-order packet (E.g., packet order:
#1, #3, #2, #4, #5) may be interpreted by some applications as a
single error, but some other applications may treat it as a 3 errors
in-a-row situation. 3 errors in-a-row is a usual error threshold and
may cause the application to stop (e.g., to tranistion to a fail safe
state).
2. Terminology
2.1. Terms Used in This Document
This document uses the terminology established in the DetNet
architecture [RFC8655], and 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.
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POF Packet Ordering Function.
PREOF Packet Replication, Elimination and Ordering Functions.
PRF Packet Replication Function.
2.3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Requirements on POF Implementations
The requirements on a POF function are:
o to solve the out-of-order delivery problem of the Replication and
Elimination functions of DetNet networks.
o to consider the delay bound requirement of a DetNet Flow.
o to be simple and to require in network nodes only a minimum set of
states/configuration parameters and resources per DetNet Flow.
o to add only minimal or no delay to the forwarding process of
packets.
o not to require synchronization between PREOF nodes.
4. POF Algorithms
4.1. Prerequisites and Assumptions
The POF Algorithm discussed in this document makes some assumptions
and tradeoffs regarding the characteristics of the sequence of
received packets. In particular, the algorithm assumes that a Packet
Elimination Function (PEF) is performed on the incoming packets
before they are handed to the POF function. Hence, the sequence of
incoming packets can be 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 the POF is presented
duplicate packets may lead to out of order delivery of duplicate
packets as well as to increased delays.
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The algorithm further requires that the delay difference between two
replicated packets that arrive at the PRF 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 algorithm also makes some tradeoffs. For simplicity, it is
designed in a way that allows 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 herein provides POF for DetNet networks. The
configuration parameters of POF can be derived during engineering the
DetNet flow through the network.
The POF method is provided via:
1. Conditional buffer: for buffering the out-of-order packets of a
DetNet flow for a given time.
2. Delay calculator: 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.
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
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4.3. The Basic POF Algorithm
The basic POF algorithm delays all out-of-order packets until all
previous packet arrives or a given time (POFMaxDelay) elapses. The
basic POF algorithm works as follows:
o The sequence number of the last forwarded packet (POFLastSent) is
stored for each DetNet Flow.
o The sequence number (seq_num) of a received packet is compared to
that of the last forwarded one (POFLastSent).
o If (seq_num <= POFLastSent + 1)
* Then the packet is forwarded and "POFLastSent" is updated
(POFLastSent = seq_num).
* Else the received packet is buffered.
o A buffered packet is forwarded from the buffer when its seq_num
becomes equal to "POFLastSent +1," OR a predefined time
("POFMaxDelay") elapses.
o When a packet is forwarded from the buffer "POFLastSent" is
updated with its seq_num (POFLastSent = seq_num).
Note: the difference of sequence number in consecutive packets is
bounded due to the history window of the Elimination function before
the POF. Therefore "<=" can be evaluated despite of the circular
sequence number space.
The state used by the basic POF algorithm (i.e., "POFLastSent") needs
initialization and maintenance. This works as follows:
o The next received packet must be forwarded and the POFLastSent
updated when the POF function was reset OR no packet was received
for a predefined time ("POFTakeAnyTime").
o The reset of POF erases all frames/packets from the time-based
buffer used by POF.
The basic POF algorithm has two parameters to engineer:
o "POFMaxDelay", which cannot be smaller than the delay difference
of the paths used by the flow.
o "POFTakeAnyTime", which is calculated based on several factors,
for example the RECOVERY_TIMEOUT related settings of the
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Elimination function(s) before the POF, the flow characteristics
(e.g., inter frame/packet time), and the delay difference of the
paths used by the flow.
Design of these parameters is out-of-scope in this document.
Note: multiple network failures may 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 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 at the POF point.
Path delay
difference
/-------------/
<--- fast path delay ---> /--- remaining delay budget ---/
|-----------------------|-------------|------------------------------|
0 t1 t2 T
<----------- slow path delay --------->
/-------------------- delay budget at POF point ---------------------/
Figure 3: Delay Budget Relations at the POF Point
4.4. The Advanced POF Algorithm
In network scenario 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 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 must be arrived via
shorter path(s).
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The advanced POF algorithm needs two extensions of the basic POF
algorithm:
o to identify the received packet's path at the POF location and
o 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 path of a given frame, the POF algorithm can use
this information to select what predefined time "POFMaxDelay_i" to
apply for the buffered frame/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.
The method for identification of 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
can set different FlowID per egress what can be used as a PathID), or
in other means. Method for identification of the packet's path is
out of scope in this document.
Note: in case of using the advanced POF algorithm it might be
advantageous to combine PEF and POF locations in the DetNet network,
as it can simplify the method used for identification of the packet's
path at the POF location.
4.5. Further enhancements of 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 may 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. The original initialization (see before) considers
the first packet as in-order, so out-of-order packet(s) during
"POFMaxTime"/"POFMaxTime_path_i" time - after the first packet was
received - may not be corrected. Motivation behind such an
initialization is POF implementation simplicity.
A possible enhancement of POF initialization works as follows:
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o After a reset all received packets are buffered with their
predefined timer ("POFMaxTime"/"POFMaxTime_path_i").
o No basic/advanced POF rules are applied until the first timer
expires.
o When the first timer expires the packet with lowest seq_num in
buffer is selected, forwarded, and "POFLastSent" is set with its
seq_num.
o The basic/advanced POF rules are applied for the packet(s) in the
buffer and the subsequently received packets.
4.6. Selecting and using the POF algorithm
The selection of the POF algorithm depends on the network scenario
and the remaining delay budget of a flow. Using POF and calculating
its parameters require proper design. Knowing the path delay
difference is essential for the POF algorithms described here.
Failure scenarios breaking the design assumptions may impact the
result of POF (e.g., packet received out of the expected worst-case
delay window - calculated based on the path delay difference - may
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 POF to consider
PEF events/states during the re-ordering. For example, under normal
circumstances the difference of sequence number in consecutive
packets is bounded due to the history window of PEF. However, in
some scenarios (e.g., reset of sequence number) the difference can be
much larger than the history window size.
5. Control and Management Plane Parameters for POF
POF algorithms needs setting of the following parameters:
o Basic POF
* "POFMaxDelay"
* "POFTakeAnyTime"
o Advanced POF
* "POFMaxDelay_i"
* "POFTakeAnyTime"
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* Network path identification related configuration(s)
Note, that in a proper design "POFTakeAnyTime" must be always larger
than "POFMaxDelay".
6. Security Considerations
There are no POF related security considerations for DetNet.
7. IANA Considerations
This document makes no IANA requests.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
8.2. Informative References
[IEEE8021CB]
IEEE, "IEEE Standard for Local and metropolitan area
networks -- Frame Replication and Elimination for
Reliability", DOI 10.1109/IEEESTD.2017.8091139, October
2017,
<https://standards.ieee.org/standard/802_1CB-2017.html>.
[IEEEP8021CBcv]
Kehrer, S., "FRER YANG Data 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>.
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Authors' Addresses
Balazs Varga (editor)
Ericsson
Magyar Tudosok krt. 11.
Budapest 1117
Hungary
Email: balazs.a.varga@ericsson.com
Janos Farkas
Ericsson
Magyar Tudosok krt. 11.
Budapest 1117
Hungary
Email: janos.farkas@ericsson.com
Stephan Kehrer
Hirschmann Automation and Control GmbH
Stuttgarter Strasse 45-51.
Neckartenzlingen 72654
Germany
Email: Stephan.Kehrer@belden.com
Tobias Heer
Hirschmann Automation and Control GmbH
Stuttgarter Strasse 45-51.
Neckartenzlingen 72654
Germany
Email: Tobias.Heer@belden.com
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