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Versions: (draft-kuhn-coding-congestion-transport) 00 01 02 03 04 05 06 07

NWCRG                                                            N. Kuhn
Internet-Draft                                                      CNES
Intended status: Informational                                 E. Lochin
Expires: August 19, 2021                                            ENAC
                                                               F. Michel
                                                               UCLouvain
                                                                M. Welzl
                                                      University of Oslo
                                                       February 15, 2021


               Coding and congestion control in transport
               draft-irtf-nwcrg-coding-and-congestion-05

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.  Using 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
   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 August 19, 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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.  Fairness, a policy concern  . . . . . . . . . . . . . . .   7
   3.  FEC above the transport . . . . . . . . . . . . . . . . . . .   7
     3.1.  Fairness and impact on non-coded flows  . . . . . . . . .   9
     3.2.  Congestion control and recovered symbols  . . . . . . . .   9
     3.3.  Interactions between congestion control and coding rates    9
     3.4.  On the useless repair symbols . . . . . . . . . . . . . .   9
     3.5.  On partial ordering . . . . . . . . . . . . . . . . . . .   9
     3.6.  On partial reliability  . . . . . . . . . . . . . . . . .   9
     3.7.  On multipath transport  . . . . . . . . . . . . . . . . .  10
   4.  FEC within the transport  . . . . . . . . . . . . . . . . . .  10
     4.1.  Fairness and impact on non-coded flows  . . . . . . . . .  11
     4.2.  Congestion control and recovered symbols  . . . . . . . .  11
     4.3.  Interactions between congestion control and coding rates   11
     4.4.  On the useless repair symbols . . . . . . . . . . . . . .  11
     4.5.  On partial ordering . . . . . . . . . . . . . . . . . . .  11
     4.6.  On partial reliability  . . . . . . . . . . . . . . . . .  12
     4.7.  On transport multipath  . . . . . . . . . . . . . . . . .  12
   5.  FEC below the transport . . . . . . . . . . . . . . . . . . .  12
     5.1.  Fairness and impact on non-coded flows  . . . . . . . . .  13
     5.2.  Congestion control and recovered symbols  . . . . . . . .  13
     5.3.  Interactions between congestion control and coding rates   14
     5.4.  On the useless repair symbols . . . . . . . . . . . . . .  14
     5.5.  On partial ordering . . . . . . . . . . . . . . . . . . .  14
     5.6.  On partial reliability  . . . . . . . . . . . . . . . . .  14
     5.7.  On transport multipath  . . . . . . . . . . . . . . . . .  14
   6.  Open research questions . . . . . . . . . . . . . . . . . . .  14



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     6.1.  Activities related to congestion control and coding . . .  15
     6.2.  Open research questions . . . . . . . . . . . . . . . . .  15
     6.3.  Advice for evaluating coding mechanisms . . . . . . . . .  15
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   10. Informative References  . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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 repair the loss via
   retransmission quickly).  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 TCP as
   a loss-based congestion control; 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 at
   the application (above the transport), within the transport or
   directly below the transport.  The typical application scenario
   considered in the current version of the 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 and is not a standard.  The document
   follows the terminology proposed in the taxonomy document [RFC8406].




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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,
   ECN marks, etc.) from the receiver to the sender.  The Forward
   Erasure Correction channel carries repair symbols (from the sender to
   the receiver) and potential information signaling which packets have
   been repaired (from the receiver to the sender).  There are cases
   where these channels are not separated.

    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.

   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  In the case of an unreliable data transfer, the transport layer
      may provide a non-reliable transport service (e.g.  UDP or DCCP
      [RFC4340] or a partially reliable transport protocol such as SCTP
      with partial reliability [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  In the case of a reliable data transfer, the transport layer may
      implement a retransmission mechanism to guarantee the reliability
      of the file 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.

   The document considers one application layer stream as input packets.
   The application layer may exploit several paths above both the FEC
   and transport layers; this case is not further described in the
   document.



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2.3.  Fairness, a policy concern

   The contract between the client and the operator may guarantee a
   minimum data-rate (e.g. mobile networks).  However, for residential
   accesses, the data-rate can be guaranteed for the customer premises
   equipment, but not necessarily for the client.  The quality of
   service that guarantees fairness between the different clients can be
   seen as a policy concern [I-D.briscoe-tsvarea-fair].

   While flow level fairness does not embody the actual application
   level fairness, the share of available capacity between single flows
   can help assess when one flow starves the other.  While past efforts
   have focused on achieving fairness, quantifying and limiting harm
   caused by new algorithm (or algorithms with coding) is more practical
   [BEYONDJAIN].  Clients may share a bottleneck that may not be ruled
   by a quality of service mechanism, e.g. in case of:

   o  a mobile network client running several applications;

   o  two clients on a residential access.

   This document considers fairness as an index to quantify the impact
   of the addition of coded flows on non-coded flows when they share the
   same bottleneck.  This document does not aim at contributing to the
   definition of fairness at a wider scale.  This document assumes that
   the non-coded flows respond to congestion signals from the network.

3.  FEC above the transport























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    | 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 4: FEC above the transport

   Figure 4 present 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.  This approach 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
   approach is relevant.  In cases where QUIC traffic is blocked and a
   fallback to TCP mechanism is proposed, there is a risk for bad
   interactions between the TCP reliability mechanisms and coding
   schemes.  For reliable transfers, coding usage does not guarantee
   better performance and would mainly reduce goodput for large file
   transfers.  Moreover, the recovered symbols may not be known to the
   transport.





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3.1.  Fairness and impact on non-coded flows

   The addition of coding within the flow does not impact on the
   interaction between coded and non-coded flows.  This interaction
   would mainly depend on the congestion controls embedded in each host.

3.2.  Congestion control and recovered symbols

   The congestion control 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, this approach may reduce the number of
   retransmission.  The usage of a non-reliable transport is more
   adequate in this case.

3.3.  Interactions between congestion control and coding rates

   The coding rate applied at the application layer mainly depends on
   the available capacity given by the congestion control underneath.
   Adapting the coding rate to the minimum required data rate of the
   application may reduce packet losses and improve the quality of
   experience.

3.4.  On the 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 needs a limited amount of goodput
   (e.g., VoIP traffic).  In this case, the 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

   Whether the transport protocol includes a reordering mechanism or
   not, the FEC mechanism does not require to implement a reordering
   mechanism if the application does not require it.  However, if the
   application requires in-order delivery of packets, a reordering
   mechanism at the client is required.

3.6.  On partial reliability

   The application may require partial reliability only.  In this case,
   the coding rates of the FEC mechanisms could be adapted accordingly
   based on inputs of the application and the trade-off between latency
   and packet loss.



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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 5: FEC in the transport

   Figure 5 presents an architecture where FEC operates within the
   transport.  The repair symbols are sent within what the congestion
   window allows, such as in [CTCP].

   The advantage of this approach allows a joint optimization of the CC
   and the 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
   large file transfers but the amount of repair symbols can be adapted,
   e.g. depending on the congestion window size.  There is a trade-off
   between the cost of capacity used to transmit source packets and the
   benefits brought out by transmitting repair symbols (e.g. unlocking
   the receive buffer if this 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.  In general, sending repair symbols could avoid a silent period
   between the transmission of the last packet in the send buffer and 1)
   firing the retransmission of lost packets, or 2) the transmission of
   new packets.





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4.1.  Fairness and impact on non-coded flows

   The addition of coding within the flow 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

   If the FEC and CC channels are decoupled, the endpoint may exploit
   different protocols for each channel.  The channels may be coupled
   and one single protocol may be exploited.  In both cases, the
   receiver can differentiate 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 sooner from
   transmission and congestion losses.  The receiver may indicate to the
   sender the number of packets that have been received or recovered.
   The sender may exploit this information to tune the coding ratio.  As
   one example of flexibility of this case, coupling an increased
   transmission rate with an increasing or decreasing coding rate could
   be envisioned.  A server may use an increasing coding rate as a probe
   of the channel capacity and adapt the congestion control transmission
   rate.

4.4.  On the 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.

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 release the
   constraints related to in-order delivery of packets : reordering
   mechanisms at the receiver may not be necessary.



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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 and no
   retransmission required.  This depends on application requirements
   and the trade-off between latency and loss.

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

    | 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 6: FEC below the transport

   Figure 6 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, in which 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.





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   Including redundancy adds traffic without reducing goodput but leads
   to potential fairness issues.  The effective bitrate is indeed higher
   than the CC's computed fair share due to the sending of repair
   symbols and the 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 adding a given percentage of the
   congestion window as supplementary symbols or sending a given amount
   of repair symbols at a given 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 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 know what is going on in the network
   underneath and whether a coding scheme is introduced or not.  The
   congestion control may behave as if no coding scheme is introduced.
   The only way for a coding channel to indicate that symbols have been
   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 recovered (i.e., congestion has
   occurred), by using ECN signaling [TENTET].



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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.  The coding
   scheme is not aware of the congestion control implementation, making
   it hard for the coding scheme to apply the relevant coding rate.

5.4.  On the useless repair symbols

   The useless repair symbols only impact the load of the network
   without actual gain for the coded flow.  That being said, using
   feedback signaling, FEC mechanisms can measure the actually used
   symbols and tune the amount of useless repair symbols.

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 there may be spurious retransmissions 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.

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.  Open research 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.







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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].

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.

   For the FEC above transport case, there is a trade-off related to the
   amount of redundancy to add, as a function of the transport layer
   protocol and application requirements.

   For the FEC within transport case, recovering lost symbols may hide
   congestion losses to the congestion control.  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.

   For the FEC below transport case, there are opportunities for
   introducing interaction between congestion control and coding schemes
   to improve the quality of experience while guaranteeing fairness with
   other flows.  New signaling methods and FEC-recovery-aware congestion
   controls could be proposed.  An open question also resides in the
   relevance of FEC when there are multiple streams that exploit the FEC
   channel.

6.3.  Advice for evaluating coding mechanisms

   The contribution to research questions 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 at encouraging 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



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   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
   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.







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   [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>.

   [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>.





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   [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>.

   [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>.

   [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



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   Michael Welzl
   University of Oslo

   Email: michawe@ifi.uio.no















































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