NWCRG                                                            N. Kuhn
Internet-Draft                                                      CNES
Intended status: Informational                                 E. Lochin
Expires: September 29, 2021                                         ENAC
                                                               F. Michel
                                                               UCLouvain
                                                                M. Welzl
                                                      University of Oslo
                                                          March 28, 2021


               Coding and congestion control in transport
               draft-irtf-nwcrg-coding-and-congestion-07

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 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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 29, 2021.



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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
   (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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Context . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Separate channels, separate entities  . . . . . . . . . .   4
     2.2.  Relation between transport layer and application
           requirements  . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Transport multipath . . . . . . . . . . . . . . . . . . .   7
     2.4.  Types of coding . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  Fairness, a policy concern  . . . . . . . . . . . . . . .   8
   3.  FEC above the transport . . . . . . . . . . . . . . . . . . .   9
     3.1.  Fairness and impact on non-coded flows  . . . . . . . . .  10
     3.2.  Congestion control and recovered symbols  . . . . . . . .  10
     3.3.  Interactions between congestion control and coding rates   10
     3.4.  On useless repair symbols . . . . . . . . . . . . . . . .  10
     3.5.  On partial ordering . . . . . . . . . . . . . . . . . . .  10
     3.6.  On partial reliability  . . . . . . . . . . . . . . . . .  11
     3.7.  On multipath transport  . . . . . . . . . . . . . . . . .  11
   4.  FEC within the transport  . . . . . . . . . . . . . . . . . .  11
     4.1.  Fairness and impact on non-coded flows  . . . . . . . . .  12
     4.2.  Congestion control and recovered symbols  . . . . . . . .  12
     4.3.  Interactions between congestion control and coding rates   12
     4.4.  On useless repair symbols . . . . . . . . . . . . . . . .  12
     4.5.  On partial ordering . . . . . . . . . . . . . . . . . . .  13
     4.6.  On partial reliability  . . . . . . . . . . . . . . . . .  13
     4.7.  On transport multipath  . . . . . . . . . . . . . . . . .  13
   5.  FEC below the transport . . . . . . . . . . . . . . . . . . .  13
     5.1.  Fairness and impact on non-coded flows  . . . . . . . . .  15
     5.2.  Congestion control and recovered symbols  . . . . . . . .  15
     5.3.  Interactions between congestion control and coding rates   15
     5.4.  On useless repair symbols . . . . . . . . . . . . . . . .  15
     5.5.  On partial ordering . . . . . . . . . . . . . . . . . . .  16
     5.6.  On partial reliability  . . . . . . . . . . . . . . . . .  16



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     5.7.  On transport multipath  . . . . . . . . . . . . . . . . .  16
   6.  Research considerations . . . . . . . . . . . . . . . . . . .  16
     6.1.  Activities related to congestion control and coding . . .  16
     6.2.  Open research questions . . . . . . . . . . . . . . . . .  17
       6.2.1.  Parameter derivation  . . . . . . . . . . . . . . . .  17
       6.2.2.  New signaling methods and fairness  . . . . . . . . .  17
     6.3.  Advice for evaluating coding mechanisms . . . . . . . . .  18
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   10. Informative References  . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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 is a reliability mechanism that is distinct and separate from
   the loss detection of congestion controls.  [RFC5681] defines the
   loss-based congestion control of TCP; since FEC coding repairs such
   losses, blindly applying it may easily lead to an implementation that
   also hides a congestion signal from the sender.  It is important to
   ensure that such information hiding does not occur.

   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 also to consider congestion control
   aspects when proposing and comparing FEC coding solutions in
   communication systems.  This document does not aim at proposing
   guidelines for characterizing FEC coding solutions.

   We consider an end-to-end unicast data transfer with FEC coding in
   the application (above the transport), within the transport or
   directly below the transport.  A typical scenario for the
   considerations in this document is a client browsing the web or
   watching a live video.




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   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.  Context

2.1.  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,
   Explicit Congestion Notification (ECN) marks, etc.) from the receiver
   to the sender.  The Forward Erasure Correction channel carries repair
   symbols (from the sender to the receiver) and information from the
   receiver to the sender (e.g. signaling which packets have been
   repaired, loss rate prior and/or after decoding, etc.).

    SENDER                                RECEIVER

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

   +------+                               +------+
   |      | -----    repair symbols  ---->|      |
   | FEC  |                               | FEC  |
   |      | <--- info: repaired symbols --|      |
   +------+                               +------+

                 Figure 1: Notations and separate channels

   Inside a host, the CC and FEC entities can be regarded as
   conceptually separate:













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     |            ^             |             ^
     | source     | coding      |packets      | sending
     | packets    | rate        |requirements | rate (or
     v            |             v             | window)
   +---------------+source     +-----------------+
   |    FEC        |and/or     |    CC           |
   |               |repair     |                 |source
   |               |symbols    |                 |packets
   +---------------+==>        +-----------------+==>
     ^                                       ^
     | signaling about                       | network
     | losses and/or                         | information
     | repaired symbols

                 Figure 2: Separate entities (sender-side)

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

                Figure 3: Separate entities (receiver-side)

   Figure 2 and Figure 3 provide more details than Figure 1.  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.

   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 repaired symbols' (input control plane for the FEC):
      refers to the information a FEC sender can obtain from a FEC




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      receiver about the performance of the FEC solution as seen by the
      receiver.

   o  '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).

   o  'source and/or repair symbols' (data plane for both the FEC and
      the CC): refers to the data that is transmitted.  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 packets) or a mix of both.

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

2.2.  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]:

   o  The transport layer may provide an unreliable transport service
      (e.g.  UDP or DCCP [RFC4340]) or a partially reliable transport
      service (e.g.  SCTP with the partial reliability extension
      [RFC3758] or QUIC with the unreliable datagram extension
      [I-D.ietf-quic-datagram]).  Depending on the amount of redundancy
      and network conditions, there could be cases where it becomes
      impossible to carry traffic.

   o  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, FEC in CC, FEC below CC), the impact of reliable transport on
      the FEC reliability mechanisms is different.








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2.3.  Transport multipath

   The application layer may be composed of several streams above FEC
   and transport layers instances.  The different streams could exploit
   different paths between the sender and the receiver or not.  The
   document considers one application layer stream as input packets
   above a combination of FEC and transport layers.  In Figure 4, each
   of stream 1 and stream 2 are considered in the scope of the document
   but a combination of stream 1 and stream 2 is not.  The case of
   multiple application level streams above multiple FEC and transport
   layers instances is out of the scope of the document and not further
   described.

     +-----------------------------------------------+
     |                   Application                 |
     |                                               |
     |  +-------------------+ +-------------------+  |
     |  |      Stream 1     | |      Stream 2     |  |
     |  +-------------------+ +-------------------+  |
     |                                               |
     |  +-------------------+ +-------------------+  |
     |  |        FEC        | |Multipath Transport|  |
     |  +-------------------+ +-------------------+  |
     |                                               |
     |  +-------------------+ +-----+       +-----+  |
     |  |Multipath Transport| |Flow1|  ...  |FlowM|  |
     |  +-------------------+ +-----+       +-----+  |
     |                                               |
     |  +-----+       +-----+ +-----+       +-----+  |
     |  |Flow1|  ...  |FlowM| | FEC |  ...  | FEC |  |
     |  +-----+       +-----+ +-----+       +-----+  |
     +-----------------------------------------------+

                Figure 4: Scope of the transport multipath

2.4.  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):

   o  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.

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




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   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
   tencoding 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.

   o  Full reliability: The receiver may hold symbols until the decoding
      of source packets is possible.  In particular, if the codec does
      not enable a subset of the linear 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 packets.

   o  Partial reliability: The receiver cannot deliver source packets
      that could not have been decoded to the upper layer.  If the size
      of the encoding window (for Sliding Window Coding) and the size of
      the blocks (for Block Coding) are large, the chances of recovering
      the erased symbols would increase.  However, this would impact on
      memory requirements and the cost of encoding and decoding
      processes.

2.5.  Fairness, a policy concern

   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
   [I-D.briscoe-tsvarea-fair].

   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.








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3.  FEC above the transport

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

        SENDER                                 RECEIVER

                     Figure 5: FEC above the transport

   Figure 5 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, the repair symbols are sent
   following the congestion window or rate determined by the CC
   mechanism.  This can result in improved quality of experience for
   latency sensitive applications such as VoIP or any not-fully reliable
   services.

   This approach requires that the transport protocol does not implement
   a fully reliable data transfer service (e.g., based on lost packet
   retransmission).  QUIC with unreliable datagram extension
   [I-D.ietf-quic-datagram] is an example of a protocol for which this
   is relevant.  In cases where QUIC traffic is blocked and a fall-back
   to TCP is proposed, there is a risk for bad interactions between
   TCP's full reliability and coding schemes.  For reliable transfers,
   coding usage does not guarantee better performance; instead, it would
   mainly reduce goodput.




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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 may not be able to differentiate
   repair symbols from actual source packets.  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 discussion depends on application needs.  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

   Irrespective of the transport protocol, a FEC mechanism does not
   require to implement 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.






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3.6.  On partial reliability

   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 2.4.

3.7.  On multipath transport

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

4.  FEC within the transport

    | source  | sending                    ^ source
    | packets | rate                       | packets
    v         v                            |
   +------------+                      +------------+
   | Transport  |                      | Transport  |
   |            |                      |            |
   | +---+ +--+ |             signaling| +---+ +--+ |
   | |FEC| |CC| |              repaired| |FEC| |CC| |
   | +---+ +--+ |source         symbols| +---+ +--+ |
   |            |and/or             <==|            |
   |            |repair         network|            |
   |            |packets    information|            |
   +------------+ ==>               <==+------------+

       SENDER                              RECEIVER

                      Figure 6: FEC in the transport

   Figure 6 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).

   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



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   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.

4.1.  Fairness and impact on non-coded flows

   The addition of coding within the transport may impact the congestion
   control mechanism and hide congestion losses.  Specific interaction
   between congestion controls and coding schemes can be proposed (see
   Section 4.2, Section 4.3 and Section 4.4).  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.  Congestion control and recovered symbols

   The receiver can differentiate between source packets and repair
   symbols.  The receiver may indicate both the number of source packets
   received and repair symbols that were actually useful in the recovery
   process of packets.

4.3.  Interactions between congestion control and coding rates

   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.

4.4.  On useless repair symbols

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








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4.5.  On partial ordering

   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: reordering mechanisms at
   the receiver may not be necessary.

4.6.  On partial reliability

   The application may require partial reliability.  In this case, the
   transport and FEC mechanisms could be conjointly designed.  As one
   example, 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 2.4.

4.7.  On transport multipath

   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
























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

        SENDER                                 RECEIVER

                     Figure 7: FEC below the transport

   Figure 7 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, where it contributes to the
   total capacity that a link exposes to upper layers.  This application
   of FEC is out of scope of this document.  In the scenario considered
   here, the repair symbols are 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 bitrate 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



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   packets: a separate congestion control entity could be introduced to
   manage the amount of repaired packets 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 actually used and useless
   symbols, and adjust the coding rate.







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5.5.  On partial ordering

   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

   The transport or application layer above the FEC channel may require
   partial reliability only.  In this case, FEC may provide an
   unnecessary service if it is not aware of the reliability
   requirements.  Partial reliability impacts the type of FEC and type
   of codec that can be used, such as discussed in Section 2.4.

5.7.  On transport multipath

   The transport may exploit multiple paths without the FEC channel
   being aware of it.  This depends on whether FEC is applied to all
   subflows or each of the subflows individually.  When FEC is applied
   to all the flows, there is a risk for the coding rate to be
   inadequate for the characteristics of the individual paths.

6.  Research considerations

   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:

   o  For the FEC above transport case: [RFC8680].

   o  For the FEC within transport case:
      [I-D.swett-nwcrg-coding-for-quic], [QUIC-FEC], [RFC5109].

   o  For the FEC below transport case: [NCTCP],
      [I-D.detchart-nwcrg-tetrys].








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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 important level of combinations makes the
   determination of an optimum parameters derivation very complex.  This
   depends on application requirements and deployment context.

   Appendix&nbsp;C of [RFC8681] describes how to tune the parameters for
   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.

   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 ?"





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   Research question 4 : "If transport and FEC senders are not
   collocated, if the FEC is applied only on the last mile, 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.  Advice for evaluating coding mechanisms

   Our advice is that new research contributions should be mapped
   following the organization of this document.  Otherwise, this may
   lead to wrong assumptions on the validity of the proposal and wrong
   ideas about the relevance of coding for a given use case.

   The discussion provided in this document aims to encourage the
   research community to also consider congestion control aspects when
   proposing and comparing FEC coding solutions in communication
   systems.  As one example, this draft proposes discussions on the
   impact of the proposed FEC solution on congestion control, especially
   loss-based congestion control mechanisms.  When a research work aims
   at improving throughput by hiding the packet loss signal from
   congestion control, 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, 2)
   critically discuss the impact of hiding packet loss from the
   congestion control mechanism.

7.  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.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  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




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   application obtaining more than its fair share of the network
   capacity.

10.  Informative References

   [BEYONDJAIN]
              Ware (et al.), R., "Beyond Jain's Fairness Index: Setting
              the Bar For The Deployment of Congestion Control
              Algorithms", HotNets '19 10.1145/3365609.3365855, 2019.

   [CTCP]     Kim (et al.), M., "Network Coded TCP (CTCP)",
              arXiv 1212.2291v3, 2013.

   [I-D.briscoe-tsvarea-fair]
              Briscoe, B., "Flow Rate Fairness: Dismantling a Religion",
              draft-briscoe-tsvarea-fair-02 (work in progress), July
              2007.

   [I-D.detchart-nwcrg-tetrys]
              Detchart, J., Lochin, E., Lacan, J., and V. Roca, "Tetrys,
              an On-the-Fly Network Coding protocol", draft-detchart-
              nwcrg-tetrys-06 (work in progress), December 2020.

   [I-D.ietf-quic-datagram]
              Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", draft-ietf-quic-datagram-01
              (work in progress), August 2020.

   [I-D.swett-nwcrg-coding-for-quic]
              Swett, I., Montpetit, M., Roca, V., and F. Michel, "Coding
              for QUIC", draft-swett-nwcrg-coding-for-quic-04 (work in
              progress), March 2020.

   [NCTCP]    Sundararajan (et al.), J., "Network Coding Meets TCP:
              Theory and Implementation", IEEE
              INFOCOM 10.1109/JPROC.2010.2093850, 2009.

   [QUIC-FEC]
              Michel (et al.), F., "QUIC-FEC: Bringing the benefits of
              Forward Erasure Correction to QUIC", IFIP
              Networking 10.23919/IFIPNetworking.2019.8816838, 2019.

   [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>.




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   [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>.



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   [TENTET]   Lochin, E., "On the joint use of TCP and Network Coding",
              NWCRG session IETF 100, 2017.

Authors' Addresses

   Nicolas Kuhn
   CNES

   Email: nicolas.kuhn@cnes.fr


   Emmanuel Lochin
   ENAC

   Email: emmanuel.lochin@enac.fr


   Francois Michel
   UCLouvain

   Email: francois.michel@uclouvain.be


   Michael Welzl
   University of Oslo

   Email: michawe@ifi.uio.no
























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