FEC Framework                                                   U. Kozat
Internet-Draft                                           DoCoMo USA Labs
Intended status:  Standards Track                               A. Begen
Expires:  April 30,  September 8, 2009                                Cisco Systems
                                                        October 27, 2008
                                                           March 7, 2009

 Pseudo Content Delivery Protocol (CDP) for Protecting Multiple Source
                         Flows in FEC Framework

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   This document provides a pseudo Content Delivery Protocol (CDP) to
   protect multiple source flows with one or more repair flows based on
   the FEC Framework document and the Session Description Protocol (SDP)
   elements defined for the framework.  The purpose of the document is
   not to provide a full-pledged protocol, but to show how the defined
   framework and SDP elements can be combined together to design a CDP.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  3
   3.  Definitions/Abbreviations  . . . . . . . . . . . . . . . . . .  3
   4.  Construction of a Repair Flow from Multiple Source Flows . . .  4
     4.1.  Example: Two Source Flows Protected by a Single Repair
           Flow . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Reconstruction of Source Flows from Repair Flow(s) . . . . . . 10
     5.1.  Example: Multiple Source Flows Protected by a Single
           Repair Flow  . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  Normative References . . . . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

1.  Introduction

   The Forward Error Correction (FEC) Framework (described in
   [I-D.ietf-fecframe-framework]) and SDP Elements for FEC Framework
   (described in [I-D.ietf-fecframe-sdp-elements]) together define
   mechanisms sufficient enough to build an actual Content Delivery
   Protocol (CDP). (CDP) with FEC protection.  Methods to convey FEC Framework
   Configuration Information (described in
   [I-D.ietf-fecframe-config-signaling]) on the other hand provides the
   signaling protocols that may be used as part of CDP to communicate
   FEC Scheme-Specific Information from FEC sender to a single as well
   as multiple FEC receivers.  This document aims at providing a
   guideline on how the mechanisms defined in each document become useful
   [I-D.ietf-fecframe-framework] and [I-D.ietf-fecframe-sdp-elements]
   can be sufficiently used to design a CDP over a non-
   trivial non-trivial scenario,
   namely protection of multiple source flows with one or more repair

   In particular, we provide clarifications and descriptions on how:

   o  source and repair flows may be uniquely identified,

   o  source blocks may be generated from one or more source flows,

   o  repair flows may be paired with the source flows,

   o  the receiver explicitly and implicitly identifies individual

   o  source blocks are regenerated at the receiver and the missing
      source symbols in a source block are recovered.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Definitions/Abbreviations

   This document uses the following definitions.  For further
   definitions that apply to FEC Framework in general, see

   CDP:  Content Delivery Protocol.

   FEC:  Forward Error Correction.

   Source Flow:  The packet flow or flows to which FEC protection is to
      be applied.

   Repair Flow:  The packet flow or flows carrying FEC data.

   Transport Protocol:  The protocol used for transport of the source
      data flow being protected.

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

   Source Block:  The group of source data packets which are to be FEC
      protected as a single block.

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

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

4.  Construction of a Repair Flow from Multiple Source Flows

   At the sender side, CDP constructs the source blocks (SB) by
   multiplexing transport payloads from multiple flows (See Figure 1 and
   Figure 2).  According to the FEC Framework, each source block is FEC
   protected separately.  Each source block is given to the specific FEC
   encoder used within the CDP as input and as the outputs Explicit
   Source FEC Payload ID, Repair FEC Payload ID, and Repair Payloads
   corresponding to that source block are generated.  Note that Explicit
   Source FEC payload ID is optional and if CDP has implicit means of
   constructing the source block at the sender/receiver (e.g., by using
   any existing sequence numbers in the payload), the Explicit Source
   FEC payload ID might not be output.

    s_1 --------> |            |
     .   Source   | Source     |     +--------+ +--------+ +--------+
     .   Flows    | Block      |=> ..|SB_(j+1)| |  SB_j  | |SB_(j-1)| ..
    s_n --------> | Generation |     +--------+ +--------+ +--------+
            Figure 1: Source Block generation for an FEC scheme

   Figure 2 shows the structure of a source block.  A CDP MUST clearly
   specify which payload corresponds to which source flow and the length
   of each payload.

         <------------------ Source Block (SB) ------------------->

         +-------...-----+-------...-----+-      -+-------...-----+
         |   Payload_1   |   Payload_2   |  . . . |   Payload_n   |
         +-------...-----+-------...-----+-      -+-------...-----+
         \______  _______|______  _______|        |______  _______|
                \/              \/                       \/
            FID_1,Len_1     FID_2,Len_2              FID_n,Len_n

                   Figure 2: Structure of a Source Block

   Flow ID (FID) value provides a unique short-hand identifier for the
   source flows.  FID is specified and associated with the possibly
   wildcarded tuple of {Source IP Address, Destination IP Address,
   Source Transport Port, Destination Transport Port, Transport
   Protocol} in the SDP file.  When wildcarded, certain fields in the
   tuple are not needed for distinguishing the source flows.  The tuple
   is carried in the IP and transport headers of the source packets.
   Since FID is utilized by the CDP and FEC scheme to distinguish
   between the source packets, the tuple MUST have a one-to-one mapping
   to a valid FID.  This point will be clearer in the specific example
   given later in this section.  The length of FID must be a priori
   fixed and known to both the receiver and sender.  Alternatively, it
   might be specified in the FEC-Scheme-Specific Information field in
   the SDP element [I-D.ietf-fecframe-sdp-elements].

   The payload length (Len) information is needed to figure out how many
   bits, bytes, or symbols (depending on the FEC scheme) from a
   particular source flow are included in the source block.  If the
   payload is not an integer multiple of the specified symbol length,
   the remaining portion is padded with zeros (See Figure 3 and
   Figure 4).

         +--------+ +--------+ +--------+        |      | -------> r_1
      .. |SB_(j+1)| |  SB_j  | |SB_(j-1)| .. --> | FEC  |  Repair   .
         +--------+ +--------+ +--------+        |Scheme|  Flows    .
                                                 |      | -------> r_k
             Figure 3: Repair flow generation by an FEC scheme

        <------------------ Source Block (SB) ------------------->
        |          |          |          |              |          |
        +-------...-----+-------...-----+-      -+-------...-----+ |
        |   Payload_1   |   Payload_2   |  . . . |   Payload_n   |0|
        +-------...-----+-------...-----+-      -+-------...-----+ |
        |          |          |          |              |          |
        | Symbol_1 | Symbol_2 | Symbol_3 |      . . .   | Symbol_m |
        |<-------->|<-------->|<-------->|              |<-------->|

        Symbol_1,..,Symbol_m => | FEC  | => Symbol_u,..,Symbol_1
                                | Enc. |

                 Figure 4: Repair flow payload generation

   FEC schemes typically expect a source block of certain size, say m
   symbols.  Therefore, the FEC encoder divides each source block into m
   symbols (with some padding if the source block is shorter than the
   expected m symbols) and generates u repair symbols which are
   functions of the m symbols in the original source block.  The repair
   symbols are grouped by the FEC scheme into repair payloads with each
   repair payload assigned a Repair FEC Payload ID in order to associate
   each repair payload with a particular source block at the receiver.
   If the payloads in a given source block have sequence numbers that
   can uniquely specify their location in the source block, an Explicit
   Source FEC Payload ID may not be generated for these payloads.
   Otherwise, Explicit Source FEC Payload IDs are generated for each
   payload and indicate the order the payloads appear in the source

   Note that FID and length information are not actually transmitted
   with the source payloads since both information can be gathered by
   other means as it will be clear in the next sections.

4.1.  Example: Two Source Flows Protected by a Single Repair Flow

   In this section, we present an example of source flow and repair flow
   generation by the CDP.  We have two source flows with flow IDs of 0
   and 1 to be protected by a single repair flow (See Figure 5).  The
   first source flow is multicast to and the second source
   flow is multicast to  Both flows use the port number
   30000.  The SDP description below states that the source flow defined
   by the tuple {*,,*,30000} is identified with FID=0 and the
   source flow defined by the tuple {*,,*,30000} is identified
   with FID=1.  The SDP description also states that the repair flow is
   to be received at the multicast address of and at port

                 SOURCE FLOWS             | INSTANCE #1
                 0: Source Flow |_________| 2: Repair Flow
                 1: Source Flow |

          Figure 5: Example: Two source flows and one repair flow

        o=ali 1122334455 1122334466 IN IP4 fec.example.com
        s=FEC Framework Examples
        t=0 0
        a=group:FEC S1 S2 R1
        m=video 30000 RTP/AVP 100
        c=IN IP4
        a=rtpmap:100 MP2T/90000
        a=fec-source-flow: id=0
        m=video 30000 RTP/AVP 101
        c=IN IP4
        a=rtpmap:101 MP2T/90000
        a=fec-source-flow: id=1
        m=application 30000 udp/fec
        c=IN IP4
        a=fec-repair-flow: encoding-id=0; ss-fssi=5hu=
        a=repair-window: 200

   Figure 7 6 shows the first and the second source blocks (SB_1 and SB_2)
   generated from these two source flows.  In this example, SB_1 is of
   length 10000 bytes.  Suppose that the FEC scheme uses a symbol length
   of 512 bytes.  Then SB_1 can be divided into 20 symbols after padding
   the source block for 240 bytes.  Assume that the FEC scheme is
   rate-2/3 erasure code, hence, it generates 10 repair symbols from 20
   original symbols for SB_1.  On the other hand, SB_2 is 7000-byte long
   and can be divided into 14 symbols after padding 168 bytes.  Using
   the same encoder, suppose that 7 repair symbols are generated for

                  <-------- Source Block 1 -------->
                  | $1 $2 $3 $4| #1 #2 #3 #4 #5 #6 | 0..00
                  \__________________  __________________/
                        @1 @2 @3 @4 @5 @6 @7 @8 @9 @10

                  <---- Source Block 2 ---->
                  | $5 $6 $7 $8 $9 | #7 #8 |0..00
                  \______________  _____________/
                    @11 @12 @13 @14 @15 @16 @17

                  $: 1000-byte payload from source flow 1
                  #: 1000-byte payload from source flow 2
                  @: Repair symbol

               Figure 7: 6: Source block with two source flows

   The information on the unit of payload length, FEC scheme, symbol
   size, and coding rates can be specified in the FEC Scheme Specific Scheme-Specific
   Information (FSSI) field of the SDP element.  If the values of the
   payload lengths from each source flow and the order of appearance of
   source flows in every source block are fixed during the session,
   these values may be also provided in the FSSI field.  To carry FSSI
   information to the FEC receivers, one may utilize the signaling
   methods described in [I-D.ietf-fecframe-config-signaling].  In our
   example, we will consider the case where the ordering is fixed and
   known both at the sender and the receiver, but the payload lengths
   will be variable from one source block to another.  We assume that
   the payload of a source flow with an FID smaller than another flow's
   FID precedes other payloads in a source block.

   The FEC scheme gets the source blocks as input and generates the
   parity blocks for each source block to protect the whole source
   block.  In the example, the repair payloads for SB_1 consist of 512-
   byte symbols, denoted by @1 to @10.  Similarly @11 to @17 constitute
   the repair payloads for SB_2.  The FEC scheme outputs the repair
   payloads along with the Repair FEC Payload IDs.  In our example,
   Repair FEC Payload ID provides information on the source block
   sequence number and the order the repair symbols are generated.  For
   instance @3 is the third FEC repair symbol for SB_1 and the three
   tuple {@3, SB_1,3} can uniquely deliver this information.  In our
   example, the FEC scheme also provides Explicit Source FEC Payload IDs
   that carry information to indicate which source symbols correspond to
   which source block sequence number and its relative position in the
   source block.  For instance the two tuple {SB_2,2} can be attached to
   $6 as the Explicit Source FEC Payload ID to indicate that $6 is
   protected together with packets belonging to SB_2, and $6 is the
   second payload in SB_2.

   The source packets are generated from the source symbols by
   concatenating consecutive symbols in one packet.  There SHOULD NOT be
   any fragmentation of a source symbol, e.g., symbols #7 and #8 can be
   concatenated in one transport payload of 2000-bytes (The
   implementation SHOULD make sure that the size of the resulting source
   packet - payload plus the overhead - is not larger than the path
   MTU), but one portion of symbol #7 SHOULD NOT be put in one source
   packet and the remaining portion in another source packet.  The
   simplest implementation is to place each source symbol in a different
   source packet as shown in Figure 8. 7.

                   |       IP header {}        |
                   |      Transport header {30000}      |
                   |   Original Transport Payload {$6}  |
                   |   Source FEC Payload ID  {SB_2,2}  |

                   Figure 8: 7: Example of a source packet

   The repair packets are generated from the repair symbols belonging to
   the same source block by grouping consecutive symbols in one packet.
   There should not be any fragmentation of a repair symbol, e.g.,
   symbols @4, @5, and @6 can be concatenated in one transport payload
   of 1536-bytes, but @6 SHOULD NOT be divided into smaller sub-symbols
   and spread over multiple repair packets.  The Repair FEC Payload ID
   MUST carry sufficient information for the decoding process and in our
   example indicating source block sequence number, length of each
   source payload, and the order that the first parity block in a repair
   packet is generated are sufficient.  The exact header format of
   Repair FEC Payload ID may be specified in the FSSI field of the SDP
   element.  In Figure 9 8 for instance, the repair symbols @4, @5, and @6
   are concatenated together.  The Payload ID {SB_1,4,4,6} states that
   the repair symbols protect SB_1, the first repair symbol in the
   payload is generated as the 4th symbol and the source block consists
   of two source flows carrying 4 and 6 packets from each.

                   |       IP header {}        |
                   |      Transport header {30000}      |
                   | Repair FEC Payload ID {SB_1,4,4,6} |
                   |      Repair Symbols {@4,@5,@6}     |

                   Figure 9: 8: Example of a repair packet

5.  Reconstruction of Source Flows from Repair Flow(s)

5.1.  Example: Multiple Source Flows Protected by a Single Repair Flow

   At the receiver, source flows 1 and 2 are received at
   {,30000} and {,30000}, while the repair flow is
   received at {,30000}.  The CDP can map these tuples to the
   flow IDs using the SDP elements.  Accordingly, the payloads received
   at {,30000} and {,30000} are mapped to flow IDs 0
   and 1, respectively.

   The CDP passes the flow IDs and received payloads along with the
   Explicit Source FEC Payload ID to the FEC scheme defined in the SDP
   description.  The CDP also passes the received repair packet payloads
   and Repair FEC Payload ID to the FEC scheme.  The FEC scheme can
   construct the original source block with missing packets by using the
   information given in the FEC Payload IDs.  The FEC Repair Payload ID
   provides the information that SB_1 has packets from two flows with 4
   packets from the first one and 6 packets from the second one.  Flow
   IDs state that the packets from source flow 0 precedes the packets
   from source flow 1.  Explicit Source FEC Payload IDs on the other
   hand provide the information about which source payload appears in
   what order.  Therefore, the FEC scheme can depict an source block
   with exact locations of the missing packets.  Figure 10 9 depicts the
   case for SB_1.  Since the original source block with missing packets
   can be constructed at the decoder and the FEC scheme knows the coding
   rate (e.g., it might be carried in the FSSI field in the SDP
   description), a proper decoding operation can start as soon as the
   repair symbols are provided to the FEC scheme.

            <-------- Source Block 1 -------->
            | $1 $2 X  X | #1 X  #3 #4 #5 #6 |

            O: Symbols received from the source flow 1 for SB_1
            #: Symbols received from the source flow 2 for SB_1
            X: Lost source symbols

                    Figure 10: 9: Source block regeneration

   When the FEC scheme can recover any missing block while more repair
   symbols are arriving, it provides the recovered blocks along with the
   source flow IDs of the recovered blocks as outputs to the CDP.  The
   receiver knows how long to wait to repair the remaining missing
   packets (e.g., specified by the 'repair-window' attribute in the SDP
   description).  After the associated timer expires, the CDP hands over
   whatever could be recovered from the source flow to the application
   layer and continues with processing the next source block.

6.  Security Considerations


   For the general security considerations related to the FEC Framework,
   refer to [I-D.ietf-fecframe-framework].  There are no additional
   security considerations that apply to this document.

7.  IANA Considerations


   There are no IANA related issues considered in this document.

8.  Acknowledgments


   The authors would like to thank the FEC Framework Design Team for
   their inputs, suggestions and contributions.

9.  Normative References

              Watson, M., "Forward Error Correction (FEC) Framework",
              draft-ietf-fecframe-framework-03 (work in progress),
              October 2008.

              Begen, A., "SDP Elements for FEC Framework",
              draft-ietf-fecframe-sdp-elements-02 (work in progress),
              November 2008.

              Asati, R., "Methods to convey FEC Framework Configuration
              Information", draft-ietf-fecframe-config-signaling-01
              (work in progress), November 2008.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

Authors' Addresses

   Ulas C. Kozat
   DoCoMo USA Labs
   3240 Hillview Avenue
   Palo Alto, CA  94304-1201

   Phone:  +1 650 496 4739
   Email:  kozat@docomolabs-usa.com

   Ali Begen
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134

   Email:  abegen@cisco.com

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