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Versions: (draft-roca-tsvwg-fecframev2) 00

TSVWG                                                            V. Roca
Internet-Draft                                                     INRIA
Intended status: Standards Track                                A. Begen
Expires: January 18, 2018                                Networked Media
                                                           July 17, 2017


  Forward Error Correction (FEC) Framework Extension to Sliding Window
                                 Codes
                    draft-ietf-tsvwg-fecframe-ext-00

Abstract

   RFC 6363 describes a framework for using Forward Error Correction
   (FEC) codes with applications in public and private IP networks 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 document extends FECFRAME to support FEC Codes based on a
   sliding encoding window, in addition to Block FEC Codes, in a
   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

   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 http://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 January 18, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions and Abbreviations . . . . . . . . . . . . . . . .   4
   3.  Architecture Overview . . . . . . . . . . . . . . . . . . . .   7
   4.  Procedural Overview . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Sender Operation with Sliding Window FEC Codes  . . . . .  10
     4.3.  Receiver Operation with Sliding Window FEC Codes  . . . .  12
   5.  Protocol Specification  . . . . . . . . . . . . . . . . . . .  14
     5.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.2.  FEC Framework Configuration Information . . . . . . . . .  15
     5.3.  FEC Scheme Requirements . . . . . . . . . . . . . . . . .  15
   6.  Feedback  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  Transport Protocols . . . . . . . . . . . . . . . . . . . . .  16
   8.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  16
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .  16
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. Operations and Management Considerations  . . . . . . . . . .  17
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     14.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  About Sliding Encoding Window Management (non
                Normative) . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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 received
   without any corruption or totally lost (e.g., because of a congested
   router, of a poor signal-to-noise ratio in a wireless network, or




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   because the number of bit errors exceeds the correction capabilities
   of a low-layer error correcting code).

   For these use-cases, Forward Error Correction (FEC) applied within
   the transport or application 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 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 unreliable transports 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.  This approach
   has major impacts on FEC encoding and decoding delays.  The data
   packets of continuous media flow(s) can be sent immediately, without
   delay.  But the block creation time, that depends on the number k of
   source symbols in this block, impacts the FEC encoding delay since
   encoding requires that all source symbols be known.  This block
   creation time also impacts the decoding delay a receiver will
   experience in case of erasures, since no repair symbol for the
   current block can be received before.  Therefore a good value for the
   block size is necessarily a balance between the maximum 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 extends [RFC6363] in order to also support FEC codes
   based on a sliding encoding window (A.K.A. convolutional codes).
   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



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   latency, since repair symbols can be generated and sent 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] that it
   extends but does not replace.  Indeed:

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

   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 thanks to the associated SDP file and its FEC
      Encoding ID information (i.e., the "encoding-id=" parameter of a
      "fec-repair-flow" attribute, [RFC6364]).  This mechanism is not
      specific to this extension but 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 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:

   Application Data Unit (ADU):  The unit of source data provided as
       payload to the transport layer.

   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.





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   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:  An FEC Code that operates in a block manner, 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 (or Convolutional) FEC Code:  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.

   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 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.




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   Protection Amount:  The relative increase in data sent due to the use
       of FEC.

   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, e.g., UDP and the Datagram Congestion
       Control Protocol (DCCP).

   Encoding Window:  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:  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., by a congested router, or because the number
       of transmission errors exceeds the correction capabilities of the
       physical-layer codes) or received.  When a packet is received, it
       is assumed that this packet is not corrupted.

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





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   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].

3.  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 Layer    |
   |     (e.g., UDP)      |
   +----------------------+

               Figure 1: FECFRAME architecture at a sender.





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   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.  This is the goal of
   an ADU to symbols mapping process that is 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 UDP (more precisely [RFC6363], Section 7, requires a
   transport protocol providing an unreliable datagram service, like UDP
   or DCCP).  In practice FEC Source Packets can be sent as soon as
   available, without having to wait for FEC encoding to take place.  In
   that case a copy of the associated source symbols need 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 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 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 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



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      (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 mapping to
      source symbols (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 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.



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4.2.  Sender Operation with Sliding Window FEC Codes

   With a Sliding Window FEC scheme, the following operations,
   illustrated in Figure 2 for the case of UDP 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 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 some hints on
        the way it might be performed.

   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
        (for example, in the UDP case, the UDP source and destination
        addresses and ports 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.




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   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 Layer    |
   |     (e.g., UDP)      |
   +----------------------+

          Figure 2: Sender Operation with Convolutional FEC Codes


















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   +----------------------+
   |     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 Layer    |
   |     (e.g., UDP)      |
   +----------------------+

             Figure 3: Sender Operation with 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 case of UDP 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, then FEC
       decoding can be performed in order to recover missing source



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       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 Layer    |
   |     (e.g., UDP)      |
   +----------------------+

        Figure 4: Receiver Operation with Sliding Window FEC Codes



















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   +----------------------+
   |     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 Layer    |
   |     (e.g., UDP)      |
   +----------------------+

            Figure 5: Receiver Operation with 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.








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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) applies instead:

   4.  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 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 a non normative example;

   o  MUST define the management of the decoding window, consisting of 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.








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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 2017.

   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.



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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

   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

   TBD

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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6363>.

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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6364>.





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   [RFC6681]  Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward
              Error Correction (FEC) Schemes for FECFRAME", RFC 6681,
              DOI 10.17487/RFC6681, August 2012,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6865>.

   [RLC-ID]   Roca, V., "Sliding Window Random Linear Code (RLC) Forward
              Erasure Correction (FEC) Scheme for FECFRAME", Work
              in Progress, Transport Area Working Group (TSVWG) draft-
              roca-tsvwg-rlc-fec-scheme (Work in Progress), June 2017,
              <https://tools.ietf.org/html/draft-roca-tsvwg-rlc-fec-
              scheme>.




























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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 provides a few hints with respect to the management of this
   encoding window.

   Source symbols are added to the sliding encoding window each time a
   new ADU arrives, where the following information is provided for this
   ADU by the FEC Framework: a description of the source flow with which
   the ADU is associated, the ADU itself, and the length of the ADU.
   This information is sufficient for the FEC scheme to map the ADU to
   the corresponding source symbols.

   Source symbols and the corresponding ADUs 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).

   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 aspects exist that can impact the sliding encoding window
   management:

   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: there may be theoretical or practical
      limitations (e.g., because of computational complexity) that 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.  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.






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Authors' Addresses

   Vincent Roca
   INRIA
   Grenoble
   France

   EMail: vincent.roca@inria.fr


   Ali Begen
   Networked Media
   Konya
   Turkey

   EMail: ali.begen@networked.media



































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