Internet Research Task Force (IRTF) N. Kuhn

Internet Research Task Force (IRTF) N. Kuhn
Request for Comments: 9265 CNES
Category: Informational E. Lochin
ISSN: 2070-1721 ENAC
F. Michel
UCLouvain
M. Welzl
University of Oslo
July 2022

Forward Erasure Correction (FEC) Coding and Congestion Control in
Transport

Abstract

Forward Erasure Correction (FEC) is a reliability mechanism that is
distinct and separate from the retransmission logic in reliable
transfer protocols such as TCP. FEC coding can help deal with losses
at the end of transfers 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 document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Research Task Force
(IRTF). The IRTF publishes the results of Internet-related research
and development activities. These results might not be suitable for
deployment. This RFC represents the consensus of the Network Coding
for Efficient Network Communications Research Group of the Internet
Research Task Force (IRTF). Documents approved for publication by
the IRSG are not 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/rfc9265.

Copyright Notice

Copyright (c) 2022 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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to this document.

Table of Contents

1. Introduction
2. Context
2.1. Fairness, Quantifying and Limiting Harm, and Policy
Concerns
2.2. Separate Channels, Separate Entities
2.3. Relation between Transport Layer and Application
Requirements
2.4. Scope of the Document Concerning Transport Multipath and
Multistream Applications
2.5. Types of Coding
3. FEC above the Transport
3.1. Fairness and Impact on Non-coded Flows
3.2. Congestion Control and Recovered Symbols
3.3. Interactions between Congestion Control and Coding Rates
3.4. On Useless Repair Symbols
3.5. On Partial Ordering at FEC Level
3.6. On Partial Reliability at FEC Level
3.7. On Multipath Transport and FEC Mechanism
4. FEC within the Transport
4.1. Fairness and Impact on Non-coded Flows
4.2. Interactions between Congestion Control and Coding Rates
4.3. On Useless Repair Symbols
4.4. On Partial Ordering at FEC and/or Transport Level
4.5. On Partial Reliability at FEC Level
4.6. On Transport Multipath and Subpath FEC Coding Rate
5. FEC below the Transport
5.1. Fairness and Impact on Non-coded Flows
5.2. Congestion Control and Recovered Symbols
5.3. Interactions between Congestion Control and Coding Rates
5.4. On Useless Repair Symbols
5.5. On Partial Ordering at FEC Level with In-Order Delivery
Transport
5.6. On Partial Reliability at FEC Level
5.7. FEC Not Aware of Transport Multipath
6. Research Recommendations and Questions
6.1. Activities Related to Congestion Control and Coding
6.2. Open Research Questions
6.2.1. Parameter Derivation
6.2.2. New Signaling Methods and Fairness
6.3. Recommendations and Advice for Evaluating Coding Mechanisms
7. IANA Considerations
8. Security Considerations
9. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

There are cases where deploying FEC coding improves the performance
of a transmission. As an example, it may take time for a sender to
detect transfer tail losses (losses that occur at the end of a
transfer where, e.g., TCP obtains no more ACKs that would enable it
to quickly repair the loss via retransmission). Allowing the
receiver to recover such losses instead of having to rely on a
retransmission could improve the experience of applications using
short flows. Another example is a network where non-congestion
losses are persistent and prevent a sender from exploiting the link
capacity.

Coding and the loss detection of congestion controls are two distinct
and separate reliability mechanisms. Since FEC coding repairs
losses, blindly applying FEC may easily lead to an implementation
that also hides a congestion signal from the sender. It is important
to ensure that such hiding of information does not occur, because
loss may be the only congestion signal available to the sender (e.g.,
TCP [RFC5681]).

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 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 does not aim to propose
guidelines for characterizing FEC coding solutions.

We consider three architectures for end-to-end unicast data transfer:

* with FEC coding in the application (above the transport)
(Section 3),

* within the transport (Section 4), or

* directly below the transport (Section 5).

A typical scenario for the considerations in this document is a
client browsing the Web or watching a live video.

This document represents the collaborative work and consensus of the
Coding for Efficient Network Communications Research Group (NWCRG);
it is not an IETF product nor a standard. The document follows the
terminology proposed in the taxonomy document [RFC8406].

2. Context

2.1. Fairness, Quantifying and Limiting Harm, and Policy Concerns

Traffic from or to different end users may share various types of
bottlenecks. When such a shared bottleneck does not implement some
form of flow protection, the share of the available capacity between
single flows can help assess when one flow starves the other.

As one example, for residential accesses, the data rate can be
guaranteed for the customer premises equipment but not necessarily
for the end user. The quality of service that guarantees fairness
between the different clients can be seen as a policy concern
[FLOW-RATE-FAIRNESS].

While past efforts have focused on achieving fairness, quantifying
and limiting harm caused by new algorithms (or algorithms with
coding) is more practical [BEYONDJAIN]. This document considers
fairness as the impact of the addition of coded flows on non-coded
flows when they share the same bottleneck. It is assumed that the
non-coded flows respond to congestion signals from the network. This
document does not contribute to the definition of fairness at a wider
scale.

2.2. Separate Channels, Separate Entities

Figures 1 and 2 present the notations that will be used in this
document and introduce the Forward Erasure Correction (FEC) and
Congestion Control (CC) channels. The FEC channel carries repair
symbols (from the sender to the receiver) and information from the
receiver to the sender (e.g., signaling which symbols have been
recovered, loss rate prior and/or after decoding, etc.). The CC
channel carries network packets from a sender to a receiver and
packets signaling information about the network (number of packets
received vs. lost, Explicit Congestion Notification (ECN) marks
[RFC3168], etc.) from the receiver to the sender. The network
packets that are sent by the CC channel may be composed of source
packets and/or repair symbols.

SENDER RECEIVER

+------+ +------+
| | ----- network packets ---->| |
| CC | | CC |
| | <--- network information ---| |
+------+ +------+

Figure 1: Congestion Control (CC) Channel

SENDER RECEIVER

+------+ +------+
| | source and/or | |
| | ----- repair symbols ---->| |
| FEC | | FEC |
| | signaling | |
| | <--- recovered symbols ----| |
+------+ +------+

Figure 2: Forward Erasure Correction (FEC) Channel

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 +-----------------+
| FEC |and/or | CC |
| |repair | |network
| |symbols | |packets
+---------------+==> +-----------------+==>
^ ^
| signaling | network
| recovered symbols | information

Figure 3: Separate Entities (Sender-Side)

| |
| source and/or | network
| repair symbols | packets
v v
+---------------+ +-----------------+
| FEC |signaling | CC |
| |recovered | |network
| |symbols | |information
+---------------+==> +-----------------+==>

Figure 4: Separate Entities (Receiver-Side)

Figures 3 and 4 provide more details than Figures 1 and 2. Some
elements are introduced:

'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.

'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.

'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'.

'signaling recovered symbols' (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 by the
receiver.

'coding rate' (output control plane for the FEC):
refers to the coding rate that is used by the FEC solution (i.e.,
proportion of transmitted symbols that carry useful data).

'network packets' (output data plane for the CC):
refers to the data that is transmitted by a CC sender to a CC
receiver. The network packets may contain source and/or repair
symbols.

'source and/or repair symbols' (data plane for the FEC):
refers to the data that is transmitted by a FEC sender to a FEC
receiver. The sender can decide to send source symbols only
(meaning that the coding rate is 0), repair symbols only (if the
solution decides not to send the original source symbols), or a
mix of both.

The inputs to FEC (incoming data packets without repair symbols and
signaling from the receiver about losses and/or recovered symbols)
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 recovered blocks may be quite irrelevant to a CC
algorithm.

2.3. Relation between Transport Layer and Application Requirements

The choice of the adequate transport layer may be related to
application requirements and the services offered by a transport
protocol [RFC8095]:

The transport layer may implement a retransmission mechanism to
guarantee the reliability of a data transfer (e.g., TCP).
Depending on how the FEC and CC functions are scheduled (FEC above
CC (Section 3), FEC in CC (Section 4), and FEC below CC
(Section 5)), the impact of reliable transport on the FEC
reliability mechanisms is different.

The transport layer may provide an unreliable transport service
(e.g., UDP or the Datagram Congestion Control Protocol (DCCP)
[RFC4340]) or a partially reliable transport service (e.g., the
Stream Control Transmission Protocol (SCTP) with the partial
reliability extension [RFC3758] or QUIC with the unreliable datagram
extension [RFC9221]). Depending on the amount of redundancy and
network conditions, there could be cases where it becomes impossible
to carry traffic. This is further discussed in Section 3 where a
"FEC above CC" case is assessed and in Sections 4 and 5 where "FEC in
CC" and "FEC below CC" are assessed, respectively.

2.4. Scope of the Document Concerning Transport Multipath and
Multistream Applications

The application layer can be composed of several streams above FEC
and transport-layer instances. The transport layer can exploit a
multipath mechanism. The different streams could exploit different
paths between the sender and the receiver. Moreover, a single-stream
application could also exploit a multipath transport mechanism. This
section describes what is in the scope of this document with regard
to multistream applications and multipath transport protocols.

The different combinations between multistream applications and
multipath transport are the following: (1) one application-layer
stream as input packets above a combination of FEC and multipath
(Mpath) transport layers (Figure 5) and (2) multiple application-
layer streams as input packets above a combination of FEC and
multipath (Mpath) or single path (Spath) transport layers (Figure 6).
This document further details cases I (in Section 3.7), II (in
Section 4.6), and III (in Section 5.7) as illustrated in Figure 5.
Cases IV, V, and VI of Figure 6 are related to how multiple streams
are managed by a single transport or FEC layer; this does not
directly concern the interaction between FEC and the transport and is
out of the scope of this document.

CASE I CASE II CASE III
+---------------+ +---------------+ +---------------+
| Stream 1 | | Stream 2 | | Stream 3 |
+---------------+ +---------------+ +---------------+

+---------------+ +---------------+ +---------------+
| FEC | | FEC | |Mpath Transport|
+---------------+ | in | +---------------+
|Mpath Transport|
+---------------+ | | +-----+ +-----+
|Mpath Transport| | | |Flow1|...|FlowM|
+---------------+ +---------------+ +-----+ +-----+

+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
|Flow1|...|FlowM| |Flow1|...|FlowM| | FEC |...| FEC |
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+

Figure 5: Transport Multipath and Single-Stream Applications - in
the Scope of the Document

CASE IV CASE V CASE VI
+-------+ +-------+ +-------+ +-------+ +-------+ +-------+
|Stream1|...|StreamM| |Stream1|...|StreamM| |Stream1|...|StreamM|
+-------+ +-------+ +-------+ +-------+ +-------+ +-------+

+-------------------+ +-------------------+ +-------------------+
| | | FEC | | Mpath Transport |
| FEC | +-------------------+ +-------------------+
| above/in/below |
| Spath Transport | +-------------------+ +-------------------+
| | | Mpath Transport | | FEC |
+-------------------+ +-------------------+ +-------------------+

+-------------------+ +-----+ +-----+ +-----+ +-----+
| Flow | |Flow1| ... |FlowM| |Flow1| ... |FlowM|
+-------------------+ +-----+ +-----+ +-----+ +-----+

Figure 6: Transport Single Path, Transport Multipath, and Multistream
Applications - out of the Scope of the Document

2.5. Types of Coding

[RFC8406] summarizes recommended terminology for Network Coding
concepts and constructs. In particular, the document identifies the
following coding types (among many others):

Block Coding: Coding technique where the input Flow must first be
segmented into a sequence of blocks; FEC encoding and decoding are
performed independently on a per-block basis.

Sliding Window Coding: General class of coding techniques that rely
on a sliding encoding window.

The decoding scheme may not be able to decode all the symbols. The
chance of decoding the erased packets depends on the size of the
encoding window, the coding rate, and the distribution of erasure in
the transmission channel. The FEC channel may let the client
transmit information related to the need of supplementary symbols to
adapt the level of reliability. Partial and full reliability could
be envisioned.

Full reliability: The receiver may hold symbols until the decoding
of source symbols is possible. In particular, if the codec does
not enable a subset of the system to be inverted, the receiver
would have to wait for a certain minimum amount of repair packets
before it can recover all the source symbols.

Partial reliability: The receiver cannot deliver source symbols that
could not have been decoded to the upper layer. For a fixed size
of encoding window (for Sliding Window Coding) or of blocks (for
Block Coding) containing the source symbols, increasing the amount
of repair symbols would increase the chances of recovering the
erased symbols. However, this would have an impact on memory
requirements, the cost of encoding and decoding processes, and the
network overhead.

3. FEC above the Transport

| source ^ source
| packets | packets
v |
+-------------+ +-------------+
|FEC | signaling|FEC |
| | recovered| |
| | symbols| |
| | <==| |
+-------------+ +-------------+
| source ^ ^ source
| and/or | sending | and/or
| repair | rate | repair
| symbols | (or window) | symbols
v | |
+-------------+ +-------------+
|Transport | network|Transport |
|(incl. CC) | information| |
| |network <==| |
| |packets | |
+-------------+==> +-------------+

SENDER RECEIVER

Figure 7: FEC above the Transport

Figure 7 presents an architecture where FEC operates on top of the
transport.

The advantage of this approach is that the FEC overhead does not
contribute to congestion in the network when congestion control is
implemented at the transport layer, because the repair symbols are
sent following the congestion window or rate determined by the CC
mechanism. This can result in an improved quality of experience for
latency-sensitive applications such as Voice over IP (VoIP) or any
not-fully reliable services.

This approach requires that the transport protocol does not implement
a fully reliable in-order data transfer service (e.g., like TCP).
QUIC with the unreliable datagram extension [RFC9221] is an example
of a protocol for which this is relevant. In cases where the
partially reliable transport is blocked and a fallback to a reliable
transport is proposed, there is a risk for bad interactions between
reliability at the transport level and coding schemes. For reliable
transfers, coding usage does not guarantee better performance;
instead, it would mainly reduce goodput.

3.1. Fairness and Impact on Non-coded Flows

The addition of coding within the flow does not influence the
interaction between coded and non-coded flows. This interaction
would mainly depend on the congestion controls associated with each
flow.

3.2. Congestion Control and Recovered Symbols

The congestion control mechanism receives network packets and may not
be able to differentiate repair symbols from actual source ones.
This differentiation requires a transport protocol to provide more
than the services described in [RFC8095], such as specifically
indicating what information has been repaired. The relevance of
adding coding at the application layer is related to the needs of the
application. For real-time applications using an unreliable or
partially reliable transport, this approach may reduce the number of
losses perceived by the application.

3.3. Interactions between Congestion Control and Coding Rates

The coding rate applied at the application layer mainly depends on
the available rate or congestion window given by the congestion
control underneath. The coding rate could be adapted to avoid adding
overhead when the minimum required data rate of the application is
not provided by the congestion control underneath. When the
congestion control allows sending faster than the application needs,
adding coding can reduce packet losses and improve the quality of
experience (provided that an unreliable or partially reliable
transport is used).

3.4. On Useless Repair Symbols

The only case where adding useless repair symbols does not obviously
result in reduced goodput is when the application rate is limited
(e.g., VoIP traffic). In this case, useless repair symbols would
only impact the amount of data generated in the network. Extra data
in the network can, however, increase the likelihood of increasing
delay and/or packet loss, which could provoke a congestion control
reaction that would degrade goodput.

3.5. On Partial Ordering at FEC Level

Irrespective of the transport protocol, a FEC mechanism does not
require implementing a reordering mechanism if the application does
not need it. However, if the application needs in-order delivery of
packets, a reordering mechanism at the receiver is required.

3.6. On Partial Reliability at FEC Level

The application may require partial reliability. In this case, the
coding rate of a FEC mechanism could be adapted based on inputs from
the application and the trade-off between latency and packet loss.
Partial reliability impacts the type of FEC and type of codec that
can be used, such as discussed in Section 2.5.

3.7. On Multipath Transport and FEC Mechanism

Whether the transport protocol exploits multiple paths or not does
not have an impact on the FEC mechanism.

4. FEC within the Transport

| source ^ source
| packets | packets
v |
+------------+ +------------+
| Transport | | Transport |
| | | |
| +---+ +--+ | signaling| +---+ +--+ |
| |FEC| |CC| | recovered| |FEC| |CC| |
| +---+ +--+ | symbols| +---+ +--+ |
| | <==| |
| |network network| |
| |packets information| |
+------------+ ==> <==+------------+

SENDER RECEIVER

Figure 8: FEC in the Transport

Figure 8 presents an architecture where FEC operates within the
transport. The repair symbols are sent within what the congestion
window or calculated rate allows, such as in [CTCP].

The advantage of this approach is that it allows a joint optimization
of CC and FEC. Moreover, the transmission of repair symbols does not
add congestion in potentially congested networks but helps repair
lost packets (such as tail losses). This joint optimization is the
key to prevent flows to consume the whole available capacity. The
amount of repair traffic injected should not lead to congestion. As
denoted in [FEC-CONGESTION-CONTROL], an increase of the repair ratio
should be done conjointly with a decrease of the source sending rate.

The drawback of this approach is that it may require specific
signaling and transport services that may not be described in
[RFC8095]. Therefore, development and maintenance may require
specific efforts at both the transport and the coding levels, and the
design of the solution may end up being complex to suit different
deployment needs.

For reliable transfers, including redundancy reduces goodput for long
transfers, but the amount of repair symbols can be adapted, e.g.,
depending on the congestion window size. There is a trade-off
between 1) the capacity that could have been exploited by application
data instead of transmitting source packets and 2) the benefits
derived from transmitting repair symbols (e.g., unlocking the receive
buffer if it is limiting). The coding ratio needs to be carefully
designed. For small files, sending repair symbols when there is no
more data to transmit could help to reduce the transfer time.
Sending repair symbols can avoid the silence period between the
transmission of the last packet in the send buffer and 1) firing a
retransmission of lost packets or 2) the transmission of new packets.

Examples of the solution could be to add a given percentage of the
congestion window or rate as supplementary symbols or to send a fixed
amount of repair symbols at a fixed rate. The redundancy flow can be
decorrelated from the congestion control that manages source packets;
a separate congestion control entity could be introduced to manage
the amount of recovered symbols to transmit on the FEC channel. The
separate congestion control instances could be made to work together
while adhering to priorities, as in coupled congestion control for
RTP media [RFC8699] in case all traffic can be assumed to take the
same path, or otherwise with a multipath congestion window coupling
mechanism as in Multipath TCP [RFC6356]. Another possibility would
be to exploit a lower-than-best-effort congestion control [RFC6297]
for repair symbols.

4.1. Fairness and Impact on Non-coded Flows

Specific interaction between congestion controls and coding schemes
can be proposed (see Sections 4.2 and 4.3). If no specific
interaction is introduced, the coding scheme may hide congestion
losses from the congestion controller, and the description of
Section 5 may apply.

4.2. Interactions between Congestion Control and Coding Rates

The receiver can differentiate between source packets and repair
symbols. The receiver may indicate both the number of source packets
received and the repair symbols that were actually useful in the
recovery process of packets. The congestion control at the sender
can then exploit this information to tune congestion control
behavior.

There is an important flexibility in the trade-off, inherent to the
use of coding, between (1) reducing goodput when useless repair
symbols are transmitted and (2) helping to recover from losses
earlier than with retransmissions. The receiver may indicate to the
sender the number of packets that have been received or recovered.
The sender may use this information to tune the coding ratio. For
example, coupling an increased transmission rate with an increasing
or decreasing coding rate could be envisioned. A server may use a
decreasing coding rate as a probe of the channel capacity and adapt
the congestion control transmission rate.

4.3. On Useless Repair Symbols

The sender may exploit the information given by the receiver to
reduce the number of useless repair symbols and improve goodput.

4.4. On Partial Ordering at FEC and/or Transport Level

The application may require in-order delivery of packets. In this
case, both FEC and transport-layer mechanisms should guarantee that
packets are delivered in order. If partial ordering is requested by
the application, both the FEC and transport could relax the
constraints related to in-order delivery; partial ordering impacts
both the congestion control and the type of FEC and type of codec
that can be used.

4.5. On Partial Reliability at FEC Level

The application may require partial reliability. The reliability
offered by FEC may be sufficient with no retransmission required.
This depends on application needs and the trade-off between latency
and loss. Partial reliability impacts the type of FEC and type of
codec that can be used, such as discussed in Section 2.5.

4.6. On Transport Multipath and Subpath FEC Coding Rate

The sender may adapt the coding rate of each of the single subpaths
whether the congestion control is coupled or not. There is an
important flexibility on how the coding rate is tuned depending on
the characteristics of each subpath.

5. FEC below the Transport

| source ^ source
| packets | packets
v |
+--------------+ +--------------+
|Transport | network|Transport |
|(including CC)| information| |
| | <==| |
+--------------+ +--------------+
| network packets ^ network packets
v |
+--------------+ +--------------+
| FEC |source | FEC |
| |and/or signaling| |
| |repair recovered| |
| |symbols symbols| |
| |==> <==| |
+--------------+ +--------------+

SENDER RECEIVER

Figure 9: FEC below the Transport

Figure 9 presents an architecture where FEC is applied end to end
below the transport layer but above the link layer. Note that it is
common to apply FEC at the link layer on one or more of the links
that make up the end-to-end path. The application of FEC at the link
layer contributes to the total capacity that a link exposes to upper
layers, but it may not be visible to either the end-to-end sender or
the receiver, if the end-to-end sender and receiver are separated by
more than one link; this is therefore out of scope for this document.
This includes the use of FEC on top of a link layer in scenarios
where the link is known by configuration. In the scenario considered
here, the repair symbols are not visible to the end-to-end congestion
controller and may be sent on top of what is allowed by the
congestion control.

Including redundancy adds traffic without reducing goodput but incurs
potential fairness issues. The effective bit rate is higher than the
CC's computed fair share due to the transmission of repair symbols,
and losses are hidden from the transport. This may cause a problem
for loss-based congestion detection, but it is not a problem for
delay-based congestion detection.

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.

Examples of the solution could be to add a given percentage of the
congestion window or rate as supplementary symbols or to send a fixed
amount of repair symbols at a fixed rate. The redundancy flow can be
decorrelated from the congestion control that manages source packets;
a separate congestion control entity could be introduced to manage
the amount of recovered symbols to transmit on the FEC channel. The
separate congestion control instances could be made to work together
while adhering to priorities, as in coupled congestion control for
RTP media [RFC8699] in case all traffic can be assumed to take the
same path, or otherwise with a multipath congestion window coupling
mechanism as in Multipath TCP [RFC6356]. Another possibility would
be to exploit a lower-than-best-effort congestion control [RFC6297]
for repair symbols.

5.1. Fairness and Impact on Non-coded Flows

The coding scheme may hide congestion losses from the congestion
controller. There are cases where this can drastically reduce the
goodput of non-coded flows. Depending on the congestion control, it
may be possible to signal to the congestion control mechanism that
there was congestion (loss) even when a packet has been recovered,
e.g., using ECN, to reduce the impact on the non-coded flows (see
Section 5.2 and [TENTET]).

5.2. Congestion Control and Recovered Symbols

The congestion control may not be aware of the existence of a coding
scheme underneath it. The congestion control may behave as if no
coding scheme had been introduced. The only way for a coding channel
to indicate that symbols have been lost but recovered is to exploit
existing signaling that is understood by the congestion control
mechanism. An example would be to indicate to a TCP sender that a
packet has been received, yet congestion has occurred, by using ECN
signaling [TENTET].

5.3. Interactions between Congestion Control and Coding Rates

The coding rate can be tuned depending on the number of recovered
symbols and the rate at which the sender transmits data. If the
coding scheme is not aware of the congestion control implementation,
it is hard for the coding scheme to apply the relevant coding rate.

5.4. On Useless Repair Symbols

Useless repair symbols only impact the load on the network without
actual gain for the coded flow. Using feedback signaling, FEC
mechanisms can measure the ratio between the number of symbols that
were actually used and the number of symbols that were useless, and
adjust the coding rate.

5.5. On Partial Ordering at FEC Level with In-Order Delivery Transport

The transport above the FEC channel may support out-of-order delivery
of packets; reordering mechanisms at the receiver may not be
necessary. In cases where the transport requires in-order delivery,
the FEC channel may need to implement a reordering mechanism.
Otherwise, spurious retransmissions may occur at the transport level.

5.6. On Partial Reliability at FEC Level

The transport or application layer above the FEC channel may require
partial reliability only. FEC may provide an unnecessary service
unless it is aware of the reliability requirements. Partial
reliability impacts the type of FEC and codec that can be used, such
as discussed in Section 2.5.

5.7. FEC Not Aware of Transport Multipath

The transport may exploit multiple paths without the FEC channel
being aware of it. If FEC is aware that multiple paths are in use,
FEC can be applied to all subflows as an aggregate, or to each of the
subflows individually. If FEC is not aware that multiple paths are
in use, FEC can only be applied to each subflow individually. When
FEC is applied to all the flows as an aggregate, the varying
characteristics of the individual paths may lead to a risk for the
coding rate to be inadequate for the characteristics of the
individual paths.

6. Research Recommendations and Questions

This section provides a short state-of-the art overview of activities
related to congestion control and coding. The objective is to
identify open research questions and contribute to advice when
evaluating coding mechanisms.

6.1. Activities Related to Congestion Control and Coding

We map activities related to congestion control and coding with the
organization presented in this document:

For the FEC above transport case: [RFC8680]

For the FEC within transport case: [CODING-FOR-QUIC], [QUIC-FEC],
and [RFC5109]

For the FEC below transport case: [NCTCP] and [TETRYS]

6.2. Open Research Questions

There is a general trade-off, inherent to the use of coding, between
(1) reducing goodput when useless repair symbols are transmitted and
(2) helping to recover from transmission and congestion losses.

6.2.1. Parameter Derivation

There is a trade-off related to the amount of redundancy to add as a
function of the transport-layer protocol and application
requirements.

[RFC8095] describes the mechanisms provided by existing IETF
protocols such as TCP, SCTP, or RTP. [RFC8406] describes the variety
of coding techniques. The number of combinations makes the
determination of an optimum parameters derivation very complex. This
depends on application requirements and deployment context.

Appendix C of [RFC8681] describes how to tune the parameters for a
target use case. However, this discussion does not integrate
congestion-controlled end points.

Research question 1: "Is there a way to dynamically adjust the codec
characteristics depending on the transmission channel, the
transport protocol, and application requirements?"

Research question 2: "Should we apply specific per-stream FEC
mechanisms when multiple streams with different reliability needs
are carried out?"

6.2.2. New Signaling Methods and Fairness

Recovering lost symbols may hide congestion losses from the
congestion control. Disambiguating ACKed packets from rebuilt
packets would help the sender adapt its sending rate accordingly.
There are opportunities for introducing interaction between
congestion control and coding schemes to improve the quality of
experience while guaranteeing fairness with other flows.

Some existing solutions already propose to disambiguate ACKed packets
from rebuilt packets [QUIC-FEC]. New signaling methods and FEC-
recovery-aware congestion controls could be proposed. This would
allow the design of adaptive coding rates.

Research question 3: "Should we quantify the harm that a coded flow
would induce on a non-coded flow? How can this be reduced while
still benefiting from advantages brought by FEC?"

Research question 4: "If transport and FEC senders are collocated
and close to the client, and FEC is applied only on the last mile,
e.g., to ignore losses on a noisy wireless link, would this raise
fairness issues?"

Research question 5: "Should we propose a generic API to allow
dynamic interactions between a transport protocol and a coding
scheme? This should consider existing APIs between application
and transport layers."

6.3. Recommendations and Advice for Evaluating Coding Mechanisms

Research Recommendation 1: "From a congestion control point of view,
a recovered packet must be considered as a lost packet. This does
not apply to the usage of FEC on a path that is known to be
lossy."

Research Recommendation 2: "New research contributions should be
mapped following the organization of this document (above, below,
and in the congestion control) and should consider congestion
control aspects when proposing and comparing FEC coding solutions
in communication systems."

Research Recommendation 3: "When a research work aims at improving
throughput by hiding the packet loss signal from congestion
control (e.g., because the path between the sender and receiver is
known to consist of a noisy wireless link), the authors should 1)
discuss the advantages of using the proposed FEC solution compared
to replacing the congestion control by one that ignores a portion
of the encountered losses and 2) critically discuss the impact of
hiding packet loss from the congestion control mechanism."

7. IANA Considerations

This document has no IANA actions.

8. Security Considerations

FEC and CC schemes can contribute to DoS attacks. Moreover, the
transmission of signaling messages from the client to the server
should be protected and reliable; otherwise, an attacker may
compromise FEC rate adaptation. Indeed, an attacker could either
modify the values indicated by the client or drop signaling messages.

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.

9. Informative References

[BEYONDJAIN]
Ware, R., Mukerjee, M. K., Seshan, S., and J. Sherry,
"Beyond Jain's Fairness Index: Setting the Bar For The
Deployment of Congestion Control Algorithms", HotNets '19:
Proceedings of the 18th ACM Workshop on Hot Topics in
Networks, DOI 10.1145/3365609.3365855, November 2019,
<https://doi.org/10.1145/3365609.3365855>.

[CODING-FOR-QUIC]
Swett, I., Montpetit, M., Roca, V., and F. Michel, "Coding
for QUIC", Work in Progress, Internet-Draft, draft-swett-
nwcrg-coding-for-quic-04, 9 March 2020,
<https://datatracker.ietf.org/doc/html/draft-swett-nwcrg-
coding-for-quic-04>.

[CTCP] Kim, M., Cloud, J., ParandehGheibi, A., Urbina, L., Fouli,
K., Leith, D., and M. Medard, "Network Coded TCP (CTCP)",
arXiv: 1212.2291v3, DOI 10.48550/arXiv.1212.2291, April
2013, <https://doi.org/10.48550/arXiv.1212.2291>.

[FEC-CONGESTION-CONTROL]
Singh, V., Nagy, M., Ott, J., and L. Eggert, "Congestion
Control Using FEC for Conversational Media", Work in
Progress, Internet-Draft, draft-singh-rmcat-adaptive-fec-
03, 20 March 2016, <https://datatracker.ietf.org/doc/html/
draft-singh-rmcat-adaptive-fec-03>.

[FLOW-RATE-FAIRNESS]
Briscoe, B., "Flow Rate Fairness: Dismantling a Religion",
Work in Progress, Internet-Draft, draft-briscoe-tsvarea-
fair-02, 11 July 2007,
<https://datatracker.ietf.org/doc/html/draft-briscoe-
tsvarea-fair-02>.

[NCTCP] Sundararajan, J., Shah, D., Médard, M., Jakubczak, S.,
Mitzenmacher, M., and J. Barros, "Network Coding Meets
TCP: Theory and Implementation", Proceedings of the IEEE
(Volume: 99, Issue: 3), DOI 10.1109/JPROC.2010.2093850,
March 2011, <https://doi.org/10.1109/JPROC.2010.2093850>.

[QUIC-FEC] Michel, F., De Coninck, Q., and O. Bonaventure, "QUIC-FEC:
Bringing the benefits of Forward Erasure Correction to
QUIC", DOI 10.23919/IFIPNetworking.2019.8816838, May 2019,
<https://doi.org/10.23919/IFIPNetworking.2019.8816838>.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.

[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758,
DOI 10.17487/RFC3758, May 2004,
<https://www.rfc-editor.org/info/rfc3758>.

[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.

[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <https://www.rfc-editor.org/info/rfc5109>.

[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>.

[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, DOI 10.17487/RFC6356, October 2011,
<https://www.rfc-editor.org/info/rfc6356>.

[RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>.

[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>.

[RFC8680] Roca, V. and A. Begen, "Forward Error Correction (FEC)
Framework Extension to Sliding Window Codes", RFC 8680,
DOI 10.17487/RFC8680, January 2020,
<https://www.rfc-editor.org/info/rfc8680>.

[RFC8681] Roca, V. and B. Teibi, "Sliding Window Random Linear Code
(RLC) Forward Erasure Correction (FEC) Schemes for
FECFRAME", RFC 8681, DOI 10.17487/RFC8681, January 2020,
<https://www.rfc-editor.org/info/rfc8681>.

[RFC8699] Islam, S., Welzl, M., and S. Gjessing, "Coupled Congestion
Control for RTP Media", RFC 8699, DOI 10.17487/RFC8699,
January 2020, <https://www.rfc-editor.org/info/rfc8699>.

[RFC9221] Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
Datagram Extension to QUIC", RFC 9221,
DOI 10.17487/RFC9221, March 2022,
<https://www.rfc-editor.org/info/rfc9221>.

[TENTET] Lochin, E., "On the joint use of TCP and Network Coding",
NWCRG Session, IETF 100, November 2017,
<https://datatracker.ietf.org/meeting/100/materials/
slides-100-nwcrg-07-lochin-on-the-joint-use-of-tcp-and-
network-coding-00>.

[TETRYS] Detchart, J., Lochin, E., Lacan, J., and V. Roca, "Tetrys,
an On-the-Fly Network Coding protocol", Work in Progress,
Internet-Draft, draft-detchart-nwcrg-tetrys-08, 17 October
2021, <https://datatracker.ietf.org/doc/html/draft-
detchart-nwcrg-tetrys-08>.

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.

Authors' Addresses

Nicolas Kuhn
CNES
Email: [email protected]

Emmanuel Lochin
ENAC
Email: [email protected]

François Michel
UCLouvain
Email: [email protected]

Michael Welzl
University of Oslo
Email: [email protected]