Coding and congestion control in transport
draft-irtf-nwcrg-coding-and-congestion-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9265.
|
|
|---|---|---|---|
| Authors | Nicolas Kuhn , Emmanuel Lochin , François Michel , Michael Welzl | ||
| Last updated | 2020-03-09 (Latest revision 2020-03-05) | ||
| Replaces | draft-kuhn-coding-congestion-transport | ||
| RFC stream | Internet Research Task Force (IRTF) | ||
| Formats | |||
| Reviews | |||
| IETF conflict review | conflict-review-irtf-nwcrg-coding-and-congestion, conflict-review-irtf-nwcrg-coding-and-congestion, conflict-review-irtf-nwcrg-coding-and-congestion, conflict-review-irtf-nwcrg-coding-and-congestion, conflict-review-irtf-nwcrg-coding-and-congestion, conflict-review-irtf-nwcrg-coding-and-congestion | ||
| Additional resources | Mailing list discussion | ||
| Stream | IRTF state | (None) | |
| Consensus boilerplate | Unknown | ||
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| IESG | IESG state | Became RFC 9265 (Informational) | |
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draft-irtf-nwcrg-coding-and-congestion-02
NWCRG N. Kuhn
Internet-Draft CNES
Intended status: Informational E. Lochin
Expires: September 6, 2020 ISAE-SUPAERO
F. Michel
UCLouvain
M. Welzl
University of Oslo
March 5, 2020
Coding and congestion control in transport
draft-irtf-nwcrg-coding-and-congestion-02
Abstract
FEC coding is a reliability mechanism that is distinct and separate
from the loss detection of congestion controls. Using FEC coding can
be a useful way to deal with transfer tail losses or with networks
having non-congestion losses. However, FEC coding mechanisms should
not hide congestion signals. This memo offers a discussion of how
FEC coding and congestion control can coexist. Another objective is
to encourage the research community to also consider congestion
control aspects when proposing and comparing FEC coding solutions in
communication systems.
This document is the product of the Coding for Efficient Network
Communications Research Group (NWCRG). The scope of the document is
end-to-end communications: FEC coding for tunnels is out-of-the scope
of the document.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 6, 2020.
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Copyright Notice
Copyright (c) 2020 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
(https://trustee.ietf.org/license-info) in effect on the date of
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. Separate channels, separate entities . . . . . . . . . . . . 3
3. FEC and CC layering . . . . . . . . . . . . . . . . . . . . . 5
3.1. FEC above the transport . . . . . . . . . . . . . . . . . 5
3.2. FEC within the transport . . . . . . . . . . . . . . . . 6
3.3. FEC below the transport . . . . . . . . . . . . . . . . . 7
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Informative References . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
There are cases where deploying FEC coding improves the performance
of a transmission. As an example, it may take time for the sender to
detect transfer tail losses (losses that occur at the end of a
transfer, where e.g. TCP obtains no more ACKs to quickly repair the
loss via retransmission). This would improve the experience of
applications using short flows. Another example are networks where
non-congestion losses are persistent and prevent a sender from
exploiting the link capacity.
Coding is a reliability mechanism that is distinct and separate from
the loss detection of congestion controls. [RFC5681] defines TCP as
a loss-based congestion control; because FEC coding repairs such
losses, blindly applying it may easily lead to an implementation that
also hides a congestion signal to the sender. It is important to
ensure that such information hiding does not occur.
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FEC coding and congestion control can be seen as two separate
channels. In practice, implementations may mix the signals that are
exchanged on these channels. This memo offers a discussion of how
FEC coding and congestion control can coexist. Another objective is
to encourage the research community to also consider congestion
control aspects when proposing and comparing FEC coding solutions in
communication systems. This being said, this document does not aim
at proposing guidelines for characterizing FEC coding solutions:
comparing FEC Schemes without considering congestion control can be
relevant if the goal is to compare those schemes only.
The proposed document considers FEC coding at the transport or
application layer for an end-to-end unicast data transfer. The
typical application scenario that is considered in the current
version of the document is a client browsing the web. This memo may
be extended to cases with multiple paths.
This document represents the collaborative work and consensus of the
Coding for Efficient Network Communications Research Group (NWCRG);
it is not an IETF product and is not a standard. The document
follows the terminology proposed in the taxonomy document [RFC8406].
2. Separate channels, separate entities
Figure 1 presents the notations that will be used in this document
and introduces the Congestion Control (CC) and Forward Erasure
Correction (FEC) channels. The Congestion Control channel carries
source packets from a sender to a receiver, and packets signaling
information about the network (number of packets received vs. lost,
ECN marks, etc.) from the receiver to the sender. The Forward
Erasure Correction channel carries repair packets (from the sender to
the receiver) and potential information signaling which packets have
been repaired (from the receiver to the sender). It is worth
pointing out that there are cases where these channels are not
separated.
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Sender Receiver
+------+ +------+
| | ----- source packets ---->| |
| CC | | CC |
| | <--- network information ---| |
+------+ +------+
+------+ +------+
| | ----- repair packets ---->| |
| FEC | | FEC |
| | <--- info: repaired packets --| |
+------+ +------+
Figure 1: Notations and separate channels
Inside a host, the CC and FEC entities can be regarded as
conceptually separate:
| ^ | ^
| source | coding |packets | sending
| packets | rate |requirements | rate (or
v | v | window)
+-------------------+source and/or +--------------------+
| FEC |=> repair | CC |=> packets
+-------------------+ packets +--------------------+
^ ^
| signaling about | network
| losses and/or | information
| repaired packets
Figure 2: Separate entities (sender-side)
Figure 2 provides more details than Figure 1 by focusing on the
sender-side. Some elements are introduced:
o 'network information' (input control plane for the transport
including CC): refers not only to the network information that is
explicitly signaled from the receiver, but all the information a
congestion control obtains from a network (e.g. TCP can estimate
the latency and the available capacity at the bottleneck).
o 'requirements' (input control plane for the transport including
CC): refers to application requirements such as upper/lower rate
bounds, periods of quiescence, or a priority.
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o 'sending rate (or window)' (output control plane for the transport
including CC): refers to the rate at which a congestion control
decides to transmit packets, based on 'network information'.
o 'signaling about losses and/or repaired packets' (input control
plane for the FEC): refers to the information a FEC sender can
obtain from a FEC receiver about the performance of the FEC
solution as seen from the receiver.
o 'coding rate' (output control plane for the FEC): refers to the
coding rate that is used by the FEC solution.
o 'source and/or repair packets' (data plane for both the FEC and
the CC): refers to the packets that are transmitted. There can
only be source packets (if the coding rate is 0), repair packets
(if the solution decides not to send the original source packets)
or a mix of both.
The inputs to FEC (packets to work upon, and signaling from the
receiver about losses and/or repaired packets) are distinct from the
inputs to CC. The latter calculates a sending rate or window from
network information, and it takes the packet to send as input,
sometimes along with application requirements such as upper/lower
rate bounds, periods of quiescence, or a priority.
It is not clear that the ACK signals feeding into a congestion
control algorithm are useful to FEC in their raw form, and vice versa
- information about repaired blocks may be quite irrelevant to a CC
algorithm.
3. FEC and CC layering
This section discusses how FEC and CC can relate in different cases
(FEC above the transport, FEC within the transport, FEC below the
transport).
3.1. FEC above the transport
Figure 3 illustrates the FEC above the transport case.
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| source
| packets
v
+------------------------------------+
| FEC |
+------------------------------------+
^ | source ^
| signaling about | and/or | sending rate
| losses and/or | repair | (or window)
| repaired packets v packets |
+-----------------+
| Transport | source and/or
| (including CC) | repair packets
+-----------------+ ===>
^
| network
| information
Figure 3: FEC above the transport (sender-side)
Figure 3 presents an architecture where FEC is on top of the
transport. The repair packets are sent within what the congestion
window allows.
The advantage of this approach is that the FEC does not contribute to
adding congestion in the network.
While CC is in principle independent of other transport functions
such as reliability, we note that CC is often embedded in reliable
transfer protocols (e.g. TCP). This approach requires that the
transport protocol does not implement a fully reliable data transfer
service (e.g., based on lost packet retransmission). UDP is an
example of a protocol for which this approach is relevant.
3.2. FEC within the transport
Figure 4 illustrates the FEC within the transport case.
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|
| source packets,
| requirements
v
+---------------------+
| Transport | source and/or
| +-----+ +-----+ | repair packets
| | FEC | | CC | | ===>
| +-----+ +-----+ |
| |
+---------------------+
^ ^
| |
signaling about network
losses and/or information
repaired packets
Figure 4: FEC in the transport (sender-side)
Figure 4 presents an architecture where FEC is within the transport.
The repair packets are sent within what the congestion window allows,
such as in [CTCP]. Examples of the solution would be sending repair
packets when there is no more data to transmit or preferably send
repair packets instead of the following packets in the send buffer.
The advantage of this approach is that it can enable conjoint
optimization between the CC and the FEC. Moreover, the transmission
of repair packets does not add congestion in potentially congested
networks but helps repair lost packets (such as tail losses).
The drawback of this approach is that it may not result in much gains
as opposed to classical retransmission mechanisms and it costs
bandwidth that could have been exploited to transmit source packets.
The coding ratio needs to be carefully designed.
3.3. FEC below the transport
Figure 5 illustrates the FEC below the transport case.
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| source packets | sending rate
| requirements | (or window)
v v
+------------------------------------+
| Transport (including CC) |
+------------------------------------+
^ |
| network | source packets
| information |
v
+-----------------+ source and/or
| FEC | repair packets
| | ===>
+-----------------+
^
| signaling about
| losses and/or
| repaired packets
Figure 5: FEC below the transport (sender-side)
Figure 5 presents an architecture where FEC is below the transport.
The repair packets are sent on top of what is allowed by the
congestion control. Examples of the solution could be adding a given
percentage of the congestion window as supplementary packets or
sending a given amount of repair packets at a given rate. The
redundancy flow can be decorrelated from the congestion control that
manages source packets: a secondary congestion control could be
introduced underneath the FEC layer. The separate congestion control
mechanisms could be made to work together while adhering to
priorities, as in coupled congestion control for RTP media
[I-D.ietf-rmcat-coupled-cc]. Another possibility would be to exploit
a lower than best-effort congestion control [RFC6297] for repair
packets.
The advantage of this approach is that it can result in performance
gains when there are persistent transmission losses along the path.
The drawback of this approach is that it can induce congestion in
already congested networks. The coding ratio needs to be carefully
designed.
4. Acknowledgements
Many thanks to Spencer Dawkins, Dave Oran, Carsten Bormann, Vincent
Roca and Marie-Jose Montpetit for their useful comments that helped
improve the document.
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5. IANA Considerations
This memo includes no request to IANA.
6. Security Considerations
FEC and CC schemes can contribute to DoS attacks. This is not
specific to this document.
In case of FEC below the transport, the aggregate rate of source and
repair packets may exceed the rate at which a congestion control
mechanism allows an application to send. This could result in an
application obtaining more than its fair share of the network
capacity.
7. Informative References
[CTCP] Kim (et al.), M., "Network Coded TCP (CTCP)",
arXiv 1212.2291v3, 2013.
[I-D.ietf-rmcat-coupled-cc]
Islam, S., Welzl, M., and S. Gjessing, "Coupled congestion
control for RTP media", draft-ietf-rmcat-coupled-cc-09
(work in progress), August 2019.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC6297] Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort
Transport Protocols", RFC 6297, DOI 10.17487/RFC6297, June
2011, <https://www.rfc-editor.org/info/rfc6297>.
[RFC8406] Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek,
F., Ghanem, S., Lochin, E., Masucci, A., Montpetit, M-J.,
Pedersen, M., Peralta, G., Roca, V., Ed., Saxena, P., and
S. Sivakumar, "Taxonomy of Coding Techniques for Efficient
Network Communications", RFC 8406, DOI 10.17487/RFC8406,
June 2018, <https://www.rfc-editor.org/info/rfc8406>.
Authors' Addresses
Nicolas Kuhn
CNES
Email: nicolas.kuhn@cnes.fr
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Emmanuel Lochin
ISAE-SUPAERO
Email: emmanuel.lochin@isae-supaero.fr
Francois Michel
UCLouvain
Email: francois.michel@uclouvain.be
Michael Welzl
University of Oslo
Email: michawe@ifi.uio.no
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