FEC Framework                                                   A. Begen
Internet-Draft                                             Cisco Systems
Intended status:  Informational                           T. Stockhammer
Expires:  July 31, 2009                                 Digital Fountain
                                                        January 27,  February 12, 2010                               Nomor Research
                                                         August 11, 2009

              DVB Application-Layer Hybrid FEC Protection

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   This document describes the Application-layer Forward Error
   Correction (FEC) protocol that was developed by the Digital Video
   Broadcasting (DVB) consortium for the protection of media streams
   over IP networks.  The DVB AL-FEC protocol uses two layers for FEC
   protection.  The first (base) layer is based on the 1-D interleaved
   parity code.  The second (enhancement) layer is based on the Raptor
   code.  By offering a layered approach, the DVB AL-FEC offers a good
   protection against both bursty and random packet losses at a cost of
   decent complexity.  The 1-D interleaved parity code and Raptor code
   have already been specified in separate documents and the current
   document normatively references these specifications.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  5
   3.  DVB AL-FEC Specification . . . . . . . . . . . . . . . . . . .  5
     3.1.  Base-Layer FEC . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Enhancement-Layer FEC  . . . . . . . . . . . . . . . . . .  7
     3.3.  Hybrid Decoding Procedures . . . . . . . . . . . . . . . .  8
   4.  Session Description Protocol (SDP) Signaling . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

1.  Introduction

   In 2007, the Digital Video Broadcasting (DVB) consortium published a
   technical specification [ETSI-TS-102-034v1.3.1] through European
   Telecommunications Standards Institute (ETSI).  This specification
   covers several areas related to the transmission of MPEG2 transport
   stream-based services over IP networks.

   The Annex E of [ETSI-TS-102-034v1.3.1] defines an optional protocol
   for Application-layer Forward Error Correction (AL-FEC) to protect
   the streaming media for DVB-IP services carried over RTP [RFC3550]
   transport.  In 2008, DVB updated the specification in a new revision
   that has been published as a DVB Bluebook [DVB-A086r7] and serves as
   draft ETSI TS-102-034v1.4.1 until the final ETSI publication
   (expected in early 2009).  Among others, some updates and
   modifications to the AL-FEC protocol have been made.

   The DVB AL-FEC protocol uses two layers for protection:  a base layer
   that is produced by the 1-D interleaved parity code, and an
   enhancement layer that is produced by the Raptor code.  Whenever a
   receiver supports the DVB AL-FEC protocol, the decoding support for
   the base-layer FEC is mandatory while the decoding support for the
   enhancement-layer FEC is optional.  Both the interleaved parity code
   and the Raptor code are systematic FEC codes, meaning that source
   packets are not modified in any way during the FEC encoding process.

   The normative DVB AL-FEC protocol considers protection of single-
   sequence source RTP flows only.  The source can be any type of media
   such as audio, video, text or application.  However, in the AL-FEC
   protocol, the source stream can only be an MPEG-2 transport stream.
   The FEC data at each layer are generated based on some configuration
   information, which also determines the exact associations and
   relationships between the source and repair packets.  This document
   shows how this configuration may be communicated out-of-band in the
   Session Description Protocol (SDP) [RFC4566].

   In DVB AL-FEC, the source packets are carried in the source RTP
   stream and the generated FEC repair packets at each layer are carried
   in separate streams.  At the receiver side, if all of the source
   packets are successfully received, there is no need for FEC recovery
   and the repair packets may be discarded.  However, if there are
   missing source packets, the repair packets can be used to recover the
   missing information.

   The block diagram of the encoder side for the systematic DVB AL-FEC
   protection is sketched in Figure 1.  Here, the source packets are fed
   into the parity encoder to produce the parity repair packets.  The
   source packets may also be fed to the Raptor encoder to produce the
   Raptor repair packets.  Source packets as well as the repair packets
   are then sent to the receiver(s) over an IP network.

   +--+  +--+  +--+  +--+ --> |  Systematic  | -> +--+  +--+  +--+  +--+
   +--+  +--+  +--+  +--+     |FEC Protection|    +--+  +--+  +--+  +--+
                              |    Parity    | -> +==+  +==+  +==+
                              |    Encoder   |    +==+  +==+  +==+
                              |    Raptor    | -> +~~+  +~~+
                              |    Encoder   |    +~~+  +~~+

   Source Packet: +--+

   Base-layer Repair Packet: +==+

   Enhancement-layer Repair Packet: +~~+

            Figure 1: Block diagram for the DVB AL-FEC encoder

   The block diagram of the decoder side for the systematic DVB AL-FEC
   protection is sketched in Figure 2.  This is a Minimum Performance
   Decoder since the receiver only supports decoding the base-layer
   repair packets.  If there is a loss among the source packets, the
   parity decoder attempts to recover the missing source packets by
   using the base-layer repair packets.

   +--+   X     X    +--+ --> |  Systematic  | -> +--+  +--+  +--+  +--+
   +--+              +--+     |FEC Protection|    +--+  +--+  +--+  +--+
         +==+  +==+  +==+ --> |    Parity    |
         +==+  +==+  +==+     |    Decoder   |

   Lost Packet: X

      Figure 2: Block diagram for the DVB AL-FEC minimum performance

   On the other hand, if the receiver supports decoding both the base-
   layer and enhancement-layer repair packets, a combined (hybrid)
   decoding approach is employed to improve the recovery rate of the
   lost packets.  In this case, the decoder is called an Enhanced
   Decoder.  Section 3.3 outlines the procedures for hybrid decoding.

   +--+   X     X     X   --> |  Systematic  | -> +--+  +--+  +--+  +--+
   +--+                       |FEC Protection|    +--+  +--+  +--+  +--+
         +==+  +==+  +==+ --> |    Parity    |
         +==+  +==+  +==+     |    Decoder   |
               +~~+  +~~+ --> |    Raptor    |
               +~~+  +~~+     |    Decoder   |

   Lost Packet: X

        Figure 3: Block diagram for the DVB AL-FEC enhanced decoder

2.  Requirements Notation

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

3.  DVB AL-FEC Specification

   The DVB AL-FEC protocol comprises two layers of FEC protection:  1-D
   interleaved parity FEC for the base layer and Raptor FEC for the
   enhancement layer.  The performance of these FEC codes has been
   examined in detail in [DVB-A115].

3.1.  Base-Layer FEC

   The 1-D interleaved parity FEC uses the exclusive OR (XOR) operation
   to generate the repair symbols.  In a group of D x L source packets,
   the XOR operation is applied to the group of the source packets whose
   sequence numbers are L apart from each other to generate L repair
   packets.  Due to interleaving, this FEC is effective against bursty
   packet losses up to burst sizes of length L.

   The DVB AL-FEC protocol requires the D x L block of the source
   packets protected by the 1-D interleaved FEC code to be wholly
   contained within a single source block of the Raptor code, if both
   FEC layers are used.

   Originally, the DVB AL-FEC protocol had adopted the 1-D interleaved
   FEC payload format from [SMPTE2022-1] in [ETSI-TS-102-034v1.3.1].
   However, some incompatibilities with RTP [RFC3550] have been
   discovered in this specification.  These issues have all been
   addressed in [I-D.ietf-fecframe-interleaved-fec-scheme] (For details,
   refer to Section 1 of [I-D.ietf-fecframe-interleaved-fec-scheme]).
   Some of the changes required by
   [I-D.ietf-fecframe-interleaved-fec-scheme] are, however, not backward
   compatible with the existing implementations that were based on

   In a recent liaison from IETF AVT WG to DVB IPI, it has been
   recommended that DVB IPI defines a new RTP profile for the AL-FEC
   protocol since in the new profile, several of the issues could easily
   be addressed without jeopardizing the compliance to RTP [RFC3550].

   At the writing of this document, it was not clear whether or not a
   new RTP profile would be defined for the AL-FEC protocol.  DVB
   attempted to address some of the issues in the updated specification
   [DVB-A086r7], however, there are still outstanding issues.  Note that
   [DVB-A086r7] does not obsolete [ETSI-TS-102-034v1.3.1] but DVB will
   exclusively use [DVB-A086r7] for any future revisions of the DVB IPTV

   The following is a list of the exceptions that MUST be considered by
   an implementation adopting [I-D.ietf-fecframe-interleaved-fec-scheme]
   to be in compliant with the AL-FEC protocol as specified in

   o  SSRC
      In the DVB AL-FEC protocol, the SSRC fields of the FEC packets
      MUST be set to 0.

      This requirement conflicts with RTP [RFC3550].  Unless signaled
      otherwise, RTP uses random SSRC values with collision detection.
      An explicit SSRC signaling mechanism is currently defined in
      [RFC5576].  It is RECOMMENDED that the DVB AL-FEC protocol uses
      this mechanism for explicit SSRC signaling.

   o  CSRC
      The DVB AL-FEC protocol does not support the protection of the
      CSRC entries in the source packets.  Thus, the source stream MUST
      NOT have any CSRC entries in its packets and the CC fields of the
      source RTP packets MUST be zero.

      Note that if there are no RTP mixers used in a system running the
      DVB AL-FEC protocol, the CC field of the source RTP packets will
      be 0 and this is no longer an issue.  Thus, if defined, the new
      RTP profile for the AL-FEC protocol SHOULD forbid the use of any
      RTP mixers.

   o  Timestamp
      In the DVB AL-FEC protocol, the timestamp fields of the FEC
      packets SHALL be ignored by the receivers.

   o  Payload Type
      In the DVB AL-FEC protocol, the PT fields of the FEC packets MUST
      be set to 96.

      A static payload type assignment for the base-layer FEC packets is
      outside the scope of [I-D.ietf-fecframe-interleaved-fec-scheme].
      If defined, the new RTP profile for the AL-FEC protocol MAY assign
      96 as the payload type for the base-layer FEC packets.

   In implementations that are based on
   [I-D.ietf-fecframe-interleaved-fec-scheme] and are willing to be in
   compliant with the AL-FEC protocol as specified in
   [ETSI-TS-102-034v1.3.1], all these exceptions MUST be considered as
   well, however, in this case, the sender does not have to select a
   random initial sequence number for the FEC stream as suggested by

   Note that neither [ETSI-TS-102-034v1.3.1] nor [DVB-A086r7] implements
   the 1-D interleaved parity code as specified in
   [I-D.ietf-fecframe-interleaved-fec-scheme].  Thus, the payload format
   registered in [I-D.ietf-fecframe-interleaved-fec-scheme] MUST NOT be
   used by the implementations that are compliant with the
   [ETSI-TS-102-034v1.3.1] or [DVB-A086r7] specification.

3.2.  Enhancement-Layer FEC

   The Raptor code is a fountain code where as many encoding symbols as
   needed can be generated by the encoder on-the-fly from source data.
   Due to the fountain property of the Raptor code, multiple enhancement
   layers may also be specified, if needed.

   The details of the Raptor code are provided in
   [I-D.ietf-fecframe-raptor].  The RTP payload format for Raptor FEC is
   specified in [I-D.watson-fecframe-rtp-raptor].

   It is important to note that the DVB AL-FEC protocol in the latest
   specification [DVB-A086r7] allows only RTP-over-UDP encapsulation for
   the enhancement-layer FEC stream.  The initial specification
   [ETSI-TS-102-034v1.3.1] exclusively permits UDP-only encapsulation
   for the enhancement-layer FEC stream.

   When SDP is used for signaling, the transport protocol identifier
   permits to distinguish whether an RTP-over-UDP or UDP-only
   encapsulation is used.  In case of any other signaling framework, the
   differentiation of the protocol for the enhancement-layer stream is
   achieved either explicitly through a protocol identifier or
   implicitly by the version number of the DVB IPTV Handbook.  If none
   of the above signaling is provided, the receiver shall concur from
   the packet size of the repair packets if RTP-over-UDP or UDP-only
   encapsulation is used.

3.3.  Hybrid Decoding Procedures

   The receivers that support receiving and decoding both the base and
   enhancement-layer FEC perform hybrid decoding to improve the repair
   performance.  The following steps may be followed to perform hybrid

   1.  Base-layer (Parity) Decoding:  In this step, the repair packets
       that are encoded by the parity encoder are processed as usual to
       repair as many missing source packets as possible.

   2.  Enhancement-layer (Raptor) Decoding:  If there are still missing
       source packets after the first step, the repair packets that are
       Raptor encoded are processed with the source packets already
       received and the source packets that are recovered in the first

   3.  Hybrid Decoding:  If there are still missing source packets after
       the second step, the unprocessed base-layer (parity) repair
       packets are converted to a form in which they can be added to the
       Raptor decoding process.  With this additional information,
       Raptor decoding may potentially recover any remaining missing
       source packet.

   The procedure that should be followed to benefit from the base-layer
   repair packets in the Raptor decoding process is explained in detail
   in Section E.5.2 of [ETSI-TS-102-034v1.3.1] and [DVB-A086r7].

4.  Session Description Protocol (SDP) Signaling

   This section provides an SDP [RFC4566] example for [DVB-A086r7].  The
   example uses the FEC grouping semantics [RFC4756].

   In the example, we have one source video stream (mid:S1), one FEC
   repair stream (mid:R1) that is produced by the 1-D interleaved parity
   FEC code as well as another FEC repair stream (mid:R2) that is
   produced by the Raptor FEC code.  We form one FEC group with the
   "a=group:FEC S1 R1 R2" line.  The source and repair streams are sent
   to the same port on different multicast groups.  The source, base-
   layer FEC and enhancement-layer FEC streams are all encapsulated in

   Due to the exceptions described in Section 3.1, a [DVB-A086r7]-
   compliant implementation MUST NOT use the RTP payload format defined
   in [I-D.ietf-fecframe-interleaved-fec-scheme].  Instead, it may use
   the payload format that has been registered by DVB IPI for

        o=ali 1122334455 1122334466 IN IP4 fec.example.com
        s=DVB AL-FEC Example
        t=0 0
        a=group:FEC S1 R1 R2
        m=video 30000 RTP/AVP 100
        c=IN IP4
        a=rtpmap:100 MP2T/90000
        m=application 30000 RTP/AVP 96
        c=IN IP4
        a=rtpmap:96 vnd.dvb.iptv.alfec-base/90000
        m=application 30000 RTP/AVP 111
        c=IN IP4
        a=rtpmap:111 vnd.dvb.iptv.alfec-enhancement/90000

   Note that in the example above, the payload type has been chosen as
   96 for the base-layer FEC stream and there is no "a=fmtp:" line to
   specify the format parameters.  Due to the lack of the format
   parameters, it is not possible to learn the FEC parameters from the
   SDP description.  This severely limits the ability of using multiple
   FEC streams that are generated with different settings.

5.  Security Considerations

   There are no security considerations in this document.

6.  IANA Considerations

   There are no IANA considerations in this document.

7.  Acknowledgments

   This document is based on [ETSI-TS-102-034v1.3.1] and [DVB-A086r7].
   Thus, the authors would like to thank the editors of
   [ETSI-TS-102-034v1.3.1] and [DVB-A086r7].

8.  References

8.1.  Normative References

              ETSI TS 102 034 V1.3.1, "Transport of MPEG 2 TS Based DVB
              Services over IP Based Networks", October 2007.

              DVB Document A086 Rev. 7 (Draft ETSI TS 102 034 V1.4.1),
              "Transport of MPEG 2 TS Based DVB Services over IP Based
              Networks", September 2008.

              Begen, A., "RTP Payload Format for 1-D Interleaved Parity
              FEC", draft-ietf-fecframe-interleaved-fec-scheme-01 draft-ietf-fecframe-interleaved-fec-scheme-05 (work
              in progress), October 2008. May 2009.

              Watson, M., "Raptor FEC Schemes for FECFRAME",
              draft-ietf-fecframe-raptor-01 (work in progress),
              October 2008.
              July 2009.

              Watson, M., "RTP Payload Format for Raptor FEC",
              draft-watson-fecframe-rtp-raptor-00 (work in progress),
              October 2008.

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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.


   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", draft-ietf-mmusic-sdp-source-attributes-02 (work
              in progress), October 2008. RFC 5576, June 2009.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4756]  Li, A., "Forward Error Correction Grouping Semantics in
              Session Description Protocol", RFC 4756, November 2006.

8.2.  Informative References

              Available at: http://www.dvb.org/technology/standards/
              a115.tm3783.AL-FEC_Evaluation.pdf, "DVB Application Layer
              FEC Evaluations (DVB Document A115)", May 2007.

              SMPTE 2022-1-2007, "Forward Error Correction for Real-Time
              Video/Audio Transport over IP Networks", 2007.

Authors' Addresses

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

   Email:  abegen@cisco.com

   Thomas Stockhammer
   Digital Fountain
   39141 Civic Center Drive
   Suite 300
   Fremont, CA  94538
   Nomor Research
   Brecherspitzstrasse 8
   Munich,   81541

   Email:  stockhammer@digitalfountain.com  stockhammer@nomor.de