TSVWG                                                            V. Roca
Internet-Draft                                                     INRIA
Updates: 6363 (if approved)                                     A. Begen
Intended status: Standards Track                         Networked Media
Expires: March 23, July 8, 2019                                    January 4, 2019                               September 19, 2018

  Forward Error Correction (FEC) Framework Extension to Sliding Window
                                 Codes
                    draft-ietf-tsvwg-fecframe-ext-06
                    draft-ietf-tsvwg-fecframe-ext-07

Abstract

   RFC 6363 describes a framework for using Forward Error Correction
   (FEC) codes to provide protection against packet loss.  The framework
   supports applying FEC to arbitrary packet flows over unreliable
   transport and is primarily intended for real-time, or streaming,
   media.  However FECFRAME as per RFC 6363 is restricted to block FEC
   codes.  The present  This document updates FECFRAME RFC 6363 to support FEC Codes based on
   a sliding encoding window, in addition to Block FEC Codes, in a backward compatible
   backward-compatible way.  During multicast/broadcast real-time
   content delivery, the use of sliding window codes significantly
   improves robustness in harsh environments, with less repair traffic
   and lower FEC-related added latency.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 23, July 8, 2019.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions and Abbreviations . . . . . . . . . . . . . . . .   4
   3.  Summary of Architecture Overview  . . . . . . . . . . . . . .   7
   4.  Procedural Overview . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Sender Operation with Sliding Window FEC Codes  . . . . .  10
     4.3.  Receiver Operation with Sliding Window FEC Codes  . . . .  13
   5.  Protocol Specification  . . . . . . . . . . . . . . . . . . .  15
     5.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  FEC Framework Configuration Information . . . . . . . . .  16
     5.3.  FEC Scheme Requirements . . . . . . . . . . . . . . . . .  16
   6.  Feedback  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Transport Protocols . . . . . . . . . . . . . . . . . . . . .  17
   8.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  17
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .  17
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   11. Operations and Management Considerations  . . . . . . . . . .  18
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     14.2.  Informative References . . . . . . . . . . . . . . . . .  18  19
   Appendix A.  About Sliding Encoding Window Management (non
                Normative) . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Many applications need to transport a continuous stream of packetized
   data from a source (sender) to one or more destinations (receivers)
   over networks that do not provide guaranteed packet delivery.  In
   particular packets may be lost, which is strictly the focus of this
   document: we assume that transmitted packets are either lost (e.g.,
   because of a congested router, of a poor signal-to-noise ratio in a
   wireless network, or because the number of bit errors exceeds the
   correction capabilities of the physical-layer error correcting code)
   or received by the transport protocol without any corruption (i.e.,
   the bit-errors, if any, have been fixed by the physical-layer error
   correcting code and therefore are hidden to the upper layers).

   For these use-cases, Forward Error Correction (FEC) applied within
   the transport or application layer, layer is an efficient technique to
   improve packet transmission robustness in presence of packet losses
   (or "erasures"), without going through packet retransmissions that
   create a delay often incompatible with real-time constraints.  The
   FEC Building Block defined in [RFC5052] provides a framework for the
   definition of Content Delivery Protocols (CDPs) that make use of
   separately defined
   separately-defined FEC schemes.  Any CDP defined according to the
   requirements of the FEC Building Block can then easily be used with
   any FEC Scheme that is also defined according to the requirements of
   the FEC Building Block.

   Then FECFRAME [RFC6363] provides a framework to define Content
   Delivery Protocols (CDPs) that provide FEC protection for arbitrary
   packet flows over an unreliable datagram service transports transport such as
   UDP.  It is primarily intended for real-time or streaming media
   applications, using broadcast, multicast, or on-demand delivery.

   However [RFC6363] only considers block FEC schemes defined in
   accordance with the FEC Building Block [RFC5052] (e.g., [RFC6681],
   [RFC6816] or [RFC6865]).  These codes require the input flow(s) to be
   segmented into a sequence of blocks.  Then FEC encoding (at a sender
   or an encoding middlebox) and decoding (at a receiver or a decoding
   middlebox) are both performed on a per-block basis.  For instance if
   the current block encompasses the 100's to 119's source symbols
   (i.e., block of size 20 symbols) of an input flow, encoding (and
   decoding) will be performed on this block independantly of other
   blocks.  This approach has major impacts on FEC encoding and decoding
   delays.  The data packets of continuous media flow(s) may be passed
   to the transport layer immediately, without delay.  But the block
   creation time, that depends on the number of source symbols in this
   block, impacts both the FEC encoding delay (since encoding requires
   that all source symbols be known), and mechanically the packet loss
   recovery delay at a receiver (since no repair symbol for the current
   block can be generated and therefore received before that time).
   Therefore a good value for the block size is necessarily a balance
   between the maximum FEC decoding latency at the receivers (which must
   be in line with the most stringent real-time requirement of the
   protected flow(s), hence an incentive to reduce the block size), and
   the desired robustness against long loss bursts (which increases with
   the block size, hence an incentive to increase this size).

   This document updates [RFC6363] in order to also support FEC codes
   based on a sliding encoding window (A.K.A. convolutional codes)

   [RFC8406].  This encoding window, either of fixed or variable size,
   slides over the set of source symbols.  FEC encoding is launched
   whenever needed, from the set of source symbols present in the
   sliding encoding window at that time.  This approach significantly
   reduces FEC-related latency, since repair symbols can be generated
   and passed to the transport layer on-the-fly, at any time, and can be
   regularly received by receivers to quickly recover packet losses.
   Using sliding window FEC codes is therefore highly beneficial to
   real-time flows, one of the primary targets of FECFRAME.  [RLC-ID]
   provides an example of such FEC Scheme for FECFRAME, built upon the
   simple sliding window Random Linear Codes (RLC).

   This document is fully backward compatible with [RFC6363].  Indeed:

   o  this extension FECFRAME update does not prevent nor compromise in any way
      the support of block FEC codes.  Both types of codes can nicely co-
      exist,
      co-exist, just like different block FEC schemes can co-exist;

   o  each sliding window FEC Scheme is associated to a specific FEC
      Encoding ID subject to IANA registration, just like block FEC
      Schemes;

   o  any receiver, for instance a legacy receiver that only supports
      block FEC schemes, can easily identify the FEC Scheme used in a
      FECFRAME session.  This is made possible with  Indeed, the FEC Encoding ID that identifies the
      FEC Scheme used and which is carried in the FEC Framework Configuration
      Information (see section 5.5 of [RFC6363]).  For instance, when
      the Session Description Protocol (SDP) is used to carry the FEC
      Framework Configuration Information, the FEC Encoding ID can be
      communicated in the "encoding-id=" parameter of a "fec-repair-flow" "fec-repair-
      flow" attribute [RFC6364].  This mechanism is the basic approach
      for a FECFRAME receiver to determine whether or not it supports
      the FEC Scheme used in a given FECFRAME session;

   This document leverages on [RFC6363] and re-uses its structure.  It
   proposes new sections specific to sliding window FEC codes whenever
   required.  The only exception is Section 3 that provides a quick
   summary of FECFRAME in order to facilitate the understanding of this
   document to readers not familiar with the concepts and terminology.

2.  Definitions and Abbreviations

   The following list of definitions and abbreviations is copied from
   [RFC6363], adding only the Block/sliding window FEC Code and
   Encoding/Decoding Window definitions (tagged with "ADDED"):

   Application Data Unit (ADU):  The unit of source data provided as
       payload to the transport layer.  For instance, it can be a
       payload containing the result of the RTP packetization of a
       compressed video frame.

   ADU Flow:  A sequence of ADUs associated with a transport-layer flow
       identifier (such as the standard 5-tuple {source IP address,
       source port, destination IP address, destination port, transport
       protocol}).

   AL-FEC:  Application-layer Forward Error Correction.

   Application Protocol:  Control protocol used to establish and control
       the source flow being protected, e.g., the Real-Time Streaming
       Protocol (RTSP).

   Content Delivery Protocol (CDP):  A complete application protocol
       specification that, through the use of the framework defined in
       this document, is able to make use of FEC schemes to provide FEC
       capabilities.

   FEC Code:  An algorithm for encoding data such that the encoded data
       flow is resilient to data loss.  Note that, in general, FEC codes
       may also be used to make a data flow resilient to corruption, but
       that is not considered in this document.

   Block FEC Code: (ADDED)  An FEC Code that operates on blocks, i.e.,
       for which the input flow MUST be segmented into a sequence of
       blocks, FEC encoding and decoding being performed independently
       on a per-block basis.

   Sliding Window FEC Code: (ADDED)  An FEC Code that can generate
       repair symbols on-the-fly, at any time, from the set of source
       symbols present in the sliding encoding window at that time.
       These codes are also known as convolutional codes.

   FEC Framework:  A protocol framework for the definition of Content
       Delivery Protocols using FEC, such as the framework defined in
       this document.

   FEC Framework Configuration Information:  Information that controls
       the operation of the FEC Framework.

   FEC Payload ID:  Information that identifies the contents and
       provides positional information of a packet with respect to the
       FEC Scheme.

   FEC Repair Packet:  At a sender (respectively, at a receiver), a
       payload submitted to (respectively, received from) the transport
       protocol containing one or more repair symbols along with a
       Repair FEC Payload ID and possibly an RTP header.

   FEC Scheme:  A specification that defines the additional protocol
       aspects required to use a particular FEC code with the FEC
       Framework.

   FEC Source Packet:  At a sender (respectively, at a receiver), a
       payload submitted to (respectively, received from) the transport
       protocol containing an ADU along with an optional Explicit Source
       FEC Payload ID.

   Repair Flow:  The packet flow carrying FEC data.

   Repair FEC Payload ID:  A FEC Payload ID specifically for use with
       repair packets.

   Source Flow:  The packet flow to which FEC protection is to be
       applied.  A source flow consists of ADUs.

   Source FEC Payload ID:  A FEC Payload ID specifically for use with
       source packets.

   Source Protocol:  A protocol used for the source flow being
       protected, e.g., RTP.

   Transport Protocol:  The protocol used for the transport of the
       source and repair flows, using an unreliable datagram service
       such as UDP.

   Encoding Window: (ADDED)  Set of Source Symbols available at the
       sender/coding node that are used to generate a repair symbol,
       with a Sliding Window FEC Code.

   Decoding Window: (ADDED)  Set of received or decoded source and
       repair symbols available at a receiver that are used to decode
       erased source symbols, with a Sliding Window FEC Code.

   Code Rate:  The ratio between the number of source symbols and the
       number of encoding symbols.  By definition, the code rate is such
       that 0 < code rate <= 1.  A code rate close to 1 indicates that a
       small number of repair symbols have been produced during the
       encoding process.

   Encoding Symbol:  Unit of data generated by the encoding process.
       With systematic codes, source symbols are part of the encoding
       symbols.

   Packet Erasure Channel:  A communication path where packets are
       either lost (e.g., in our case, by a congested router, or because
       the number of transmission errors exceeds the correction
       capabilities of the physical-layer code) or received.  When a
       packet is received, it is assumed that this packet is not
       corrupted (i.e., in our case, the bit-errors, if any, are fixed
       by the physical-layer code and therefore hidden to the upper
       layers).

   Repair Symbol:  Encoding symbol that is not a source symbol.

   Source Block:  Group of ADUs that are to be FEC protected as a single
       block.  This notion is restricted to Block FEC Codes.

   Source Symbol:  Unit of data used during the encoding process.

   Systematic Code:  FEC code in which the source symbols are part of
       the encoding symbols.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Summary of Architecture Overview

   The architecture of [RFC6363], Section 3, equally applies to this
   FECFRAME extension and is not repeated here.  However we provide
   hereafter a quick summary to facilitate the understanding of this
   document to readers not familiar with the concepts and terminology.

   +----------------------+
   |     Application      |
   +----------------------+
              |
              | (1) Application Data Units (ADUs)
              |
              v
   +----------------------+                           +----------------+
   |    FEC Framework     |                           |                |
   |                      |-------------------------->|   FEC Scheme   |
   |(2) Construct source  |(3) Source Block           |                |
   |    blocks            |                           |(4) FEC Encoding|
   |(6) Construct FEC     |<--------------------------|                |
   |    Source and Repair |                           |                |
   |    Packets           |(5) Explicit Source FEC    |                |
   +----------------------+    Payload IDs            +----------------+
              |                Repair FEC Payload IDs
              |                Repair symbols
              |
              |(7) FEC Source and Repair Packets
              v
   +----------------------+
   |  Transport Protocol  |
   +----------------------+

               Figure 1: FECFRAME architecture at a sender.

   The FECFRAME architecture is illustrated in Figure 1 from the
   sender's point of view, in case of a block FEC Scheme.  It shows an
   application generating an ADU flow (other flows, from other
   applications, may co-exist).  These ADUs, of variable size, must be
   somehow mapped to source symbols of fixed size. size (this fixed size is a
   requirement of all FEC Schemes that comes from the way mathematical
   operations are applied to symbols content).  This is the goal of an ADU to symbols
   ADU-to-symbols mapping process that is FEC Scheme FEC-Scheme specific (see
   below).  Once the source block is built, taking into account both the
   FEC Scheme constraints (e.g., in terms of maximum source block size)
   and the application's flow constraints (e.g., in terms of real-time
   constraints), the associated source symbols are handed to the FEC
   Scheme in order to produce an appropriate number of repair symbols.
   FEC Source Packets (containing ADUs) and FEC Repair Packets
   (containing one or more repair symbols each) are then generated and
   sent using an appropriate transport protocol (more precisely
   [RFC6363], Section 7, requires a transport protocol providing an
   unreliable datagram service, such as UDP).  In practice FEC Source
   Packets may be passed to the transport layer as soon as available,
   without having to wait for FEC encoding to take place.  In that case
   a copy of the associated source symbols needs to be kept within
   FECFRAME for future FEC encoding purposes.

   At a receiver (not shown), FECFRAME processing operates in a similar
   way, taking as input the incoming FEC Source and Repair Packets
   received.  In case of FEC Source Packet losses, the FEC decoding of
   the associated block may recover all (in case of successful decoding)
   or a subset potentially empty (otherwise) of the missing source
   symbols.  After source symbol to ADU source-symbol-to-ADU mapping, when lost ADUs are
   recovered, they are then assigned to their respective flow (see
   below).  ADUs are returned to the application(s), either in their
   initial transmission order (in that case ADUs received after an
   erased one will be delayed until FEC decoding has taken place) or not
   (in that case each ADU is returned as soon as it is received or
   recovered), depending on the application requirements.

   FECFRAME features two subtle mechanisms:

   o  ADUs to source symbols  ADUs-to-source-symbols mapping: in order to manage variable size
      ADUs, FECFRAME and FEC Schemes can use small, fixed size symbols
      and create a mapping between ADUs and symbols.  To each ADU this
      mechanism prepends a length field (plus a flow identifier, see
      below) and pads the result to a multiple of the symbol size.  A
      small ADU may be mapped to a single source symbol while a large
      one may be mapped to multiple symbols.  The mapping details are
      FEC Scheme dependant
      FEC-Scheme-dependent and must be defined in the associated
      document;

   o  Assignment of decoded ADUs to flows in multi-flow configurations:
      when multiple flows are multiplexed over the same FECFRAME
      instance, a problem is to assign a decoded ADU to the right flow
      (UDP port numbers and IP addresses traditionally used to map
      incoming ADUs to flows are not recovered during FEC decoding).  To
      make it possible, at the FECFRAME sending instance, each ADU is
      prepended with a flow identifier (1 byte) during the ADU to source
      symbols ADU-to-
      source-symbols mapping (see above).  The flow identifiers are also
      shared between all FECFRAME instances as part of the FEC Framework
      Configuration Information.  This (flow identifier + length +
      application payload + padding), called ADUI, is then FEC
      protected.  Therefore a decoded ADUI contains enough information
      to assign the ADU to the right flow.

   A few aspects are not covered by FECFRAME, namely:

   o  [RFC6363] section 8 does not detail any congestion control
      mechanism, but only provides high level normative requirements;

   o  the possibility of having feedbacks from receiver(s) is considered
      out of scope, although such a mechanism may exist within the
      application (e.g., through RCTP RTCP control messages);

   o  flow adaptation at a FECFRAME sender (e.g., how to set the FEC
      code rate based on transmission conditions) is not detailed, but
      it needs to comply with the congestion control normative
      requirements (see above).

4.  Procedural Overview

4.1.  General

   The general considerations of [RFC6363], Section 4.1, that are
   specific to block FEC codes are not repeated here.

   With a Sliding Window FEC Code, the FEC Source Packet MUST contain
   information to identify the position occupied by the ADU within the
   source flow, in terms specific to the FEC Scheme.  This information
   is known as the Source FEC Payload ID, and the FEC Scheme is
   responsible for defining and interpreting it.

   With a Sliding Window FEC Code, the FEC Repair Packets MUST contain
   information that identifies the relationship between the contained
   repair payloads and the original source symbols used during encoding.
   This information is known as the Repair FEC Payload ID, and the FEC
   Scheme is responsible for defining and interpreting it.

   The Sender Operation ([RFC6363], Section 4.2.) and Receiver Operation
   ([RFC6363], Section 4.3) are both specific to block FEC codes and
   therefore omitted below.  The following two sections detail similar
   operations for Sliding Window FEC codes.

4.2.  Sender Operation with Sliding Window FEC Codes

   With a Sliding Window FEC Scheme, the following operations,
   illustrated in Figure 2 for the generic case (non-RTP repair flows),
   and in Figure 3 for the case of RTP repair flows, describe a possible
   way to generate compliant source and repair flows:

   1.   A new ADU is provided by the application.

   2.   The FEC Framework communicates this ADU to the FEC Scheme.

   3.   The sliding encoding window is updated by the FEC Scheme.  The
        ADU to source symbols
        ADU-to-source-symbols mapping as well as the encoding window
        management details are both the responsibility of the FEC Scheme
        and MUST be detailed there.  Appendix A provides non normative non-normative
        hints about what FEC Scheme designers need to consider;

   4.   The Source FEC Payload ID information of the source packet is
        determined by the FEC Scheme.  If required by the FEC Scheme,
        the Source FEC Payload ID is encoded into the Explicit Source
        FEC Payload ID field and returned to the FEC Framework.

   5.   The FEC Framework constructs the FEC Source Packet according to
        [RFC6363] Figure 6, using the Explicit Source FEC Payload ID
        provided by the FEC Scheme if applicable.

   6.   The FEC Source Packet is sent using normal transport-layer
        procedures.  This packet is sent using the same ADU flow
        identification information as would have been used for the
        original source packet if the FEC Framework were not present
        (e.g., the source and destination addresses and UDP port numbers
        on the IP datagram carrying the source packet will be the same
        whether or not the FEC Framework is applied).

   7.   When the FEC Framework needs to send one or several FEC Repair
        Packets (e.g., according to the target Code Rate), it asks the
        FEC Scheme to create one or several repair packet payloads from
        the current sliding encoding window along with their Repair FEC
        Payload ID.

   8.   The Repair FEC Payload IDs and repair packet payloads are
        provided back by the FEC Scheme to the FEC Framework.

   9.   The FEC Framework constructs FEC Repair Packets according to
        [RFC6363] Figure 7, using the FEC Payload IDs and repair packet
        payloads provided by the FEC Scheme.

   10.  The FEC Repair Packets are sent using normal transport-layer
        procedures.  The port(s) and multicast group(s) to be used for
        FEC Repair Packets are defined in the FEC Framework
        Configuration Information.

   +----------------------+
   |     Application      |
   +----------------------+
              |
              | (1) New Application Data Unit (ADU)
              v
   +---------------------+                           +----------------+
   |    FEC Framework    |                           |   FEC Scheme   |
   |                     |-------------------------->|                |
   |                     | (2) New ADU               |(3) Update of   |
   |                     |                           |    encoding    |
   |                     |<--------------------------|    window      |
   |(5) Construct FEC    | (4) Explicit Source       |                |
   |    Source Packet    |     FEC Payload ID(s)     |(7) FEC         |
   |                     |<--------------------------|    encoding    |
   |(9) Construct FEC    | (8) Repair FEC Payload ID |                |
   |    Repair Packet(s) |     + Repair symbol(s)    +----------------+
   +---------------------+
              |
              | (6)  FEC Source Packet
              | (10) FEC Repair Packets
              v
   +----------------------+
   |  Transport Protocol  |
   +----------------------+

         Figure 2: Sender Operation with Sliding Window FEC Codes

   +----------------------+
   |     Application      |
   +----------------------+
              |
              | (1) New Application Data Unit (ADU)
              v
   +---------------------+                           +----------------+
   |    FEC Framework    |                           |   FEC Scheme   |
   |                     |-------------------------->|                |
   |                     | (2) New ADU               |(3) Update of   |
   |                     |                           |    encoding    |
   |                     |<--------------------------|    window      |
   |(5) Construct FEC    | (4) Explicit Source       |                |
   |    Source Packet    |     FEC Payload ID(s)     |(7) FEC         |
   |                     |<--------------------------|    encoding    |
   |(9) Construct FEC    | (8) Repair FEC Payload ID |                |
   |    Repair Packet(s) |     + Repair symbol(s)    +----------------+
   +---------------------+
       |             |
       |(6) Source   |(10) Repair payloads
       |    packets  |
       |      + -- -- -- -- -+
       |      |     RTP      |
       |      +-- -- -- -- --+
       v             v
   +----------------------+
   |  Transport Protocol  |
   +----------------------+

     Figure 3: Sender Operation with Sliding Window FEC Codes and RTP
                               Repair Flows

4.3.  Receiver Operation with Sliding Window FEC Codes

   With a Sliding Window FEC Scheme, the following operations,
   illustrated in Figure 4 for the generic case (non-RTP repair flows),
   and in Figure 5 for the case of RTP repair flows.  The only
   differences with respect to block FEC codes lie in steps (4) and (5).
   Therefore this section does not repeat the other steps of [RFC6363],
   Section 4.3, "Receiver Operation".  The new steps (4) and (5) are:

   4.  The FEC Scheme uses the received FEC Payload IDs (and derived FEC
       Source Payload IDs when the Explicit Source FEC Payload ID field
       is not used) to insert source and repair packets into the
       decoding window in the right way.  If at least one source packet
       is missing and at least one repair packet has been received and
       the rank of the associated linear system permits it, received, then
       FEC decoding can be performed in order is attempted to recover missing source payloads.
       The FEC Scheme determines whether source packets have been lost
       and whether enough repair packets have been received to decode
       any or all of the missing source payloads.

   5.  The FEC Scheme returns the received and decoded ADUs to the FEC
       Framework, along with indications of any ADUs that were missing
       and could not be decoded.

   +----------------------+
   |     Application      |
   +----------------------+
              ^
              |(6) ADUs
              |
   +----------------------+                           +----------------+
   |    FEC Framework     |                           |   FEC Scheme   |
   |                      |<--------------------------|                |
   |(2)Extract FEC Payload|(5) ADUs                   |(4) FEC Decoding
   |   IDs and pass IDs & |-------------------------->|                |
   |   payloads to FEC    |(3) Explicit Source FEC    +----------------+
   |   scheme             |            Payload IDs
   +----------------------+    Repair FEC Payload IDs
              ^                Source payloads
              |                Repair payloads
              |(1) FEC Source
              |    and Repair Packets
   +----------------------+
   |  Transport Protocol  |
   +----------------------+

        Figure 4: Receiver Operation with Sliding Window FEC Codes

   +----------------------+
   |     Application      |
   +----------------------+
              ^
              |(6) ADUs
              |
   +----------------------+                           +----------------+
   |    FEC Framework     |                           |   FEC Scheme   |
   |                      |<--------------------------|                |
   |(2)Extract FEC Payload|(5) ADUs                   |(4) FEC Decoding|
   |   IDs and pass IDs & |-------------------------->|                |
   |   payloads to FEC    |(3) Explicit Source FEC    +----------------+
   |   scheme             |            Payload IDs
   +----------------------+    Repair FEC Payload IDs
       ^             ^         Source payloads
       |             |         Repair payloads
       |Source pkts  |Repair payloads
       |             |
   +-- |- -- -- -- -- -- -+
   |RTP| | RTP Processing |
   |   | +-- -- -- --|-- -+
   | +-- -- -- -- -- |--+ |
   | | RTP Demux        | |
   +-- -- -- -- -- -- -- -+
              ^
              |(1) FEC Source and Repair Packets
              |
   +----------------------+
   |  Transport Protocol  |
   +----------------------+

    Figure 5: Receiver Operation with Sliding Window FEC Codes and RTP
                               Repair Flows

5.  Protocol Specification

5.1.  General

   This section discusses the protocol elements for the FEC Framework
   specific to Sliding Window FEC schemes.  The global formats of source
   data packets (i.e., [RFC6363], Figure 6) and repair data packets
   (i.e., [RFC6363], Figures 7 and 8) remain the same with Sliding
   Window FEC codes.  They are not repeated here.

5.2.  FEC Framework Configuration Information

   The FEC Framework Configuration Information considerations of
   [RFC6363], Section 5.5, equally applies to this FECFRAME extension
   and is not repeated here.

5.3.  FEC Scheme Requirements

   The FEC Scheme requirements of [RFC6363], Section 5.6, mostly apply
   to this FECFRAME extension and are not repeated here.  An exception
   though is the "full specification of the FEC code", item (4), that is
   specific to block FEC codes.  The following item (4-bis) applies in
   case of Sliding Window FEC schemes:

   4-bis.  A full specification of the Sliding Window FEC code

       This specification MUST precisely define the valid FEC-Scheme-
       Specific Information values, the valid FEC Payload ID values, and
       the valid packet payload sizes (where packet payload refers to
       the space within a packet dedicated to carrying encoding
       symbols).

       Furthermore, given valid values of the FEC-Scheme-Specific
       Information, a valid Repair FEC Payload ID value, a valid packet
       payload size, and a valid encoding window (i.e., a set of source
       symbols), the specification MUST uniquely define the values of
       the encoding symbol (or symbols) to be included in the repair
       packet payload with the given Repair FEC Payload ID value.

   Additionally, the FEC Scheme associated to a Sliding Window FEC Code:

   o  MUST define the relationships between ADUs and the associated
      source symbols (mapping);

   o  MUST define the management of the encoding window that slides over
      the set of ADUs.  Appendix A provides non normative hints about
      what FEC Scheme designers need to consider;

   o  MUST define the management of the decoding window, consisting of window.  This usually
      consists in managing a system of linear equations (in case of a
      linear FEC code);

6.  Feedback

   The discussion of [RFC6363], Section 6, equally applies to this
   FECFRAME extension and is not repeated here.

7.  Transport Protocols

   The discussion of [RFC6363], Section 7, equally applies to this
   FECFRAME extension and is not repeated here.

8.  Congestion Control

   The discussion of [RFC6363], Section 8, equally applies to this
   FECFRAME extension and is not repeated here.

9.  Implementation Status

   Editor's notes: RFC Editor, please remove this section motivated by
   RFC 7942 before publishing the RFC.  Thanks!

   An implementation of FECFRAME extended to Sliding Window codes
   exists:

   o  Organisation: Inria

   o  Description: This is an implementation of FECFRAME extended to
      Sliding Window codes and supporting the RLC FEC Scheme [RLC-ID].
      It is based on: (1) a proprietary implementation of FECFRAME, made
      by Inria and Expway for which interoperability tests have been
      conducted; and (2) a proprietary implementation of RLC Sliding
      Window FEC Codes.

   o  Maturity: the basic FECFRAME maturity is "production", the
      FECFRAME extension maturity is "under progress".

   o  Coverage: the software implements a subset of [RFC6363], as
      specialized by the 3GPP eMBMS standard [MBMSTS].  This software
      also covers the additional features of FECFRAME extended to
      Sliding Window codes, in particular the RLC FEC Scheme.

   o  Lincensing: proprietary.

   o  Implementation experience: maximum.

   o  Information update date: March 2018.

   o  Contact: vincent.roca@inria.fr

10.  Security Considerations

   This FECFRAME extension does not add any new security consideration.
   All the considerations of [RFC6363], Section 9, apply to this
   document as well.  However, for the sake of completeness, the
   following goal can be added to the list provided in Section 9.1
   "Problem Statement" of [RFC6363]:

   o  Attacks can try to corrupt source flows in order to modify the
      receiver application's behavior (as opposed to just denying
      service).

11.  Operations and Management Considerations

   This FECFRAME extension does not add any new Operations and
   Management Consideration.  All the considerations of [RFC6363],
   Section 10, apply to this document as well.

12.  IANA Considerations

   No IANA actions are required for this document.

   A FEC Scheme for use with this FEC Framework is identified via its
   FEC Encoding ID.  It is subject to IANA registration in the "FEC
   Framework (FECFRAME) FEC Encoding IDs" registry.  All the rules of
   [RFC6363], Section 11, apply and are not repeated here.

13.  Acknowledgments

   The authors would like to thank Christer Holmberg, David Black, Gorry
   Fairhurst, and Emmanuel Lochin Lochin, Spencer Dawkins, Ben Campbell,
   Benjamin Kaduk, Eric Rescorla, and Adam Roach for their valuable
   feedbacks on this document.  This document being an extension to
   [RFC6363], the authors would also like to thank Mark Watson as the
   main author this of that RFC.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6363]  Watson, M., Begen, A., and V. Roca, "Forward Error
              Correction (FEC) Framework", RFC 6363,
              DOI 10.17487/RFC6363, October 2011,
              <https://www.rfc-editor.org/info/rfc6363>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

14.2.  Informative References

   [MBMSTS]   3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
              Protocols and codecs", 3GPP TS 26.346, March 2009,
              <http://ftp.3gpp.org/specs/html-info/26346.htm>.

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error
              Correction (FEC) Building Block", RFC 5052,
              DOI 10.17487/RFC5052, August 2007,
              <https://www.rfc-editor.org/info/rfc5052>.

   [RFC6364]  Begen, A., "Session Description Protocol Elements for the
              Forward Error Correction (FEC) Framework", RFC 6364,
              DOI 10.17487/RFC6364, October 2011,
              <https://www.rfc-editor.org/info/rfc6364>.

   [RFC6681]  Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward
              Error Correction (FEC) Schemes for FECFRAME", RFC 6681,
              DOI 10.17487/RFC6681, August 2012,
              <https://www.rfc-editor.org/info/rfc6681>.

   [RFC6816]  Roca, V., Cunche, M., and J. Lacan, "Simple Low-Density
              Parity Check (LDPC) Staircase Forward Error Correction
              (FEC) Scheme for FECFRAME", RFC 6816,
              DOI 10.17487/RFC6816, December 2012,
              <https://www.rfc-editor.org/info/rfc6816>.

   [RFC6865]  Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K.
              Matsuzono, "Simple Reed-Solomon Forward Error Correction
              (FEC) Scheme for FECFRAME", RFC 6865,
              DOI 10.17487/RFC6865, February 2013,
              <https://www.rfc-editor.org/info/rfc6865>.

   [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>.

   [RLC-ID]   Roca, V. and B. Teibi, "Sliding Window Random Linear Code
              (RLC) Forward Erasure Correction (FEC) Scheme for
              FECFRAME", Work in Progress, Transport Area Working Group
              (TSVWG) draft-ietf-tsvwg-rlc-fec-scheme (Work in
              Progress), September 2018, <https://tools.ietf.org/html/
              draft-ietf-tsvwg-rlc-fec-scheme>.

Appendix A.  About Sliding Encoding Window Management (non Normative)

   The FEC Framework does not specify the management of the sliding
   encoding window which is the responsibility of the FEC Scheme.  This
   annex only provides a few non normative hints.

   Source symbols are added to the sliding encoding window each time a
   new ADU is available at the sender, after the ADU to source symbol ADU-to-source-symbol
   mapping specific to the FEC Scheme.

   Source symbols are removed from the sliding encoding window, for
   instance:

   o  after a certain delay, when an "old" ADU of a real-time flow times
      out.  The source symbol retention delay in the sliding encoding
      window should therefore be initialized according to the real-time
      features of incoming flow(s) when applicable;

   o  once the sliding encoding window has reached its maximum size
      (there is usually an upper limit to the sliding encoding window
      size).  In that case the oldest symbol is removed each time a new
      source symbol is added.

   Several considerations can impact the management of this sliding
   encoding window:

   o  at the source flows level: real-time constraints can limit the
      total time source symbols can remain in the encoding window;

   o  at the FEC code level: theoretical or practical limitations (e.g.,
      because of computational complexity) can limit the number of
      source symbols in the encoding window;

   o  at the FEC Scheme level: signaling and window management are
      intrinsically related.  For instance, an encoding window composed
      of a non sequential set of source symbols requires an appropriate
      signaling to inform a receiver of the composition of the encoding
      window, and the associated transmission overhead can limit the
      maximum encoding window size.  On the opposite, an encoding window
      always composed of a sequential set of source symbols simplifies
      signaling: providing the identity of the first source symbol plus
      their number is sufficient, which creates a fixed and relatively
      small transmission overhead.

Authors' Addresses

   Vincent Roca
   INRIA
   Univ. Grenoble Alpes
   France

   EMail: vincent.roca@inria.fr

   Ali Begen
   Networked Media
   Konya
   Turkey

   EMail: ali.begen@networked.media