FECFRAME Working Group                                      Rajiv Asati
Internet Draft                                            Cisco Systems
Intended status: Standards Track                           July 4, 2008
Expires: January May 2009

   Signaling Protocol                                      November 1, 2008

         Methods to convey FEC Framework Configuration Information

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   FEC Framework document [FECARCH] defines the FEC Framework
   Configuration Information necessary for the FEC framework operation.
   This document describes one how to use existing signaling protocol protocols to
   determine and dynamically communicate the Configuration information
   between sender(s) and receiver(s).

Conventions used in this document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.

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

Table of Contents

   1. Introduction...................................................2
   2. Terminology/Abbreviations......................................3
   3. FEC Framework Configuration Information........................4
      3.1. Encoding Format...........................................5
   4. Signaling Protocol.............................................6
      4.1. Signaling Protocol for Multicasting.......................7
         4.1.1. Sender Procedure.....................................9
         4.1.2. Receiver Procedure..................................10
      4.2. Signaling Protocol for Unicasting........................11
         4.2.1. SIP.................................................12
         4.2.2. RSTP................................................12
         4.2.3. DSM-CC..............................................13
   5. Security Considerations.......................................13
   6. IANA Considerations...........................................13
   7. Conclusions...................................................14 Conclusions...................................................13
   8. Acknowledgments...............................................14
   9. References....................................................15
      9.1. Normative References.....................................15
      9.2. Informative References...................................15
   Author's Addresses...............................................16
   Intellectual Property Statement..................................16
   Disclaimer of Validity...........................................16

1. Introduction

   FEC Framework document [FECARCH] defines the FEC Framework
   Configuration Information that governs the overall FEC framework
   operation common to any FEC scheme. This information MUST be
   available at both sender and reciever(s). This document describes one
   signalling protocol how
   to use various signaling protocols to determine and communicate the
   Configuration information between sender and receiver(s). The
   configuration information may be encoded in any compatible format
   such as SDP [RFC4566], XML etc. The A signaling protocol is intended to be generic
   and could be
   utilized by any FEC scheme and/or any Content Delivery Protocol

   This document doesn't describe any FEC scheme specifics information
   (for example, how are source blocks are constructed) or any sender or
   receiver side operation for a particular FEC scheme (for example,
   whether the receiver makes use of one or more repair flows that are
   received) etc. Such FEC scheme specifics should be covered in
   separate document(s). This document doesn't mandate a particular
   encoding format for the configuration information either.

   <What is CDP>

   The FEC Framework document [FECARCH] defines a

   Content Delivery Protocol (CDP) is defined as a complete (suite of)
   specification which, through the use of FEC Framework, is able to
   make use of a particular FEC
   scheme scheme(s) to provide FEC capabilities. In other
   words, CDP is specific
   to a FEC scheme, but makes use of common building blocks (including signaling
   protocol) as defined in the FEC Framework document [FECARCH].

   This document is structured such that Section 2 describes the terms
   used in this document, section 3 describes the FEC Framework
   configuration information, section 4 describes the signalling how to use signaling
   protocol for the multicast, multicast application, section 5 describes the signalling
   signaling protocol for the unicast, unicast application, and section 6
   describes security consideration.

   Copyright (C) The IETF Trust (2008).  This version of this MIB module
   is part of RFC XXXX; see the RFC itself for full legal notices.

   Copyright (C) The IETF Trust (2008).  The initial version of this MIB
   module was published in RFC XXXX; for full legal notices see the RFC
   itself.  Supplementary information may be available at:

2. Terminology/Abbreviations

   This document makes use of the terms/abbreviations defined in the FEC
   Framework document [FECARCH]. Additionally, it defines

   o  Media Sender   -  Node performing the Media encoding and producing
      the original media flow to the 'FEC Sender'

   o  Media Receiver -  Node performing the Media decoding;

   o  FEC Sender     -  Node performing the FEC encoding on the original
      stream to produce the FEC stream
   o  FEC Receiver   -  Node performing the FEC decoding, as needed, and
      providing the original media flow to the Media receiver.

   o  Sender         -  Same as FEC Sender

   o  Receiver       -  Same as FEC Receiver

   o  (Media) Stream -  A single media instance i.e. an audio stream or
      a video stream.

   This documents document deliberately refers to the 'FEC Sender' and 'FEC
   Receiver' as the 'Sender' and 'Receiver' respectively.

3. FEC Framework Configuration Information

   The FEC Framework [FECARCH] defines a minimum set of information that
   MUST be communicated between the sender and receiver(s) for a proper
   operation of an FEC scheme.  This information is referred to as "FEC
   Framework Configuration Information". This is the information that
   the FEC Framework needs in order to apply FEC protection to the
   transport flows.

   A single instance of the FEC Framework provides FEC protection for
   all packets of a specified set of source packet flows, by means of
   one or more packet flows consisting of repair packets. As per the FEC
   Framework document [FECARCH], the FEC Framework Configuation
   Information includes, for each instance of the FEC Framework:

   1. Identification of Source Flow(s)

   2. Identification of the repair flow(s)

   3. Identification of FEC Scheme

   4. Length of Source FEC payload ID

   5. FEC Scheme Specific Information (FSSI)
   FSSI basically provides an opaque container to encode FEC scheme
   specific configuration information such as buffer size, decoding
   wait-time etc. Please refer to the FEC Framework document [FECARCH]
   for more details.

   The usage of signaling protocol protocols described in this document requires
   that the application layer responsible for the FEC Framework instance
   i.e. FEC scheme provide the value for each of the configuration
   information parameter (listed above) encoded as per the chosen
   encoding format. Failure to receive the complete information, the
   signaling protocol module must return an error for the OAM purposes
   and optionally convey to the application layer. Please refer to the
   figure 1 of the FEC Framework document [FECARCH] for further

   This document does not make any assumption that the 'FEC sender and
   receiver' functionality and the 'Media Source/Receiver' functionality
   are implemented on the single device, though it is likely to may be the case.

3.1. Encoding Format

   The FEC Framework configuration information (listed above in section
   3) may be encoded in any format such as SDP, XML etc. as chosen or
   prefered by a particular FEC Framework instance i.e. FEC Scheme. The
   selection of such encoding format or syntax is independent of the
   signaling protocol and beyond the scope of this document.

   Whatever encoding format is selected for a particular FEC framework
   instance, it must be known by the signaling protocol. This is to
   provide a mean (e.g. a field such as Payload Type) in the signaling
   protocol message(s) to convey the chosen encoding format for the
   configuration information so that the Payload i.e. configuration
   information can be correctly parsed as per the semantics of the
   chosen encoding format. format at the receiver. Please note that the the encoding
   format is not a negotiated parameter, but rather a property of a
   particular FEC Framework instance i.e. FEC scheme and/or its

   Additionally, the encoding format for each FEC Framework
   configuration parameter must be defined in terms of a sequence of
   octets that can be embedded within the payload of the signaling
   protocol message(s).  The length of the encoding format MUST either
   be fixed, or derived from by examining the encoded octets themselves.  For
   example, the initial octets may include some kind of length

   Each instance of the FEC Framework muse use a single encoding format
   to describe e.g. encode all of the configuration information
   associated with that instance. The signaling protocol may not
   validate the encoded information, though it may validate the syntax
   or length of the encoded information.

   The reader may refer to the SDP elements document [FECSDP], which
   describes the usage of 'SDP' encoding format as an example encoding
   format for FEC framework configuration information.

4. Signaling Protocol Usage

   FEC Framework [FECARCH] requires certain FEC Framework Configuration
   Information to be available to both sender and receiver(s). This
   configuration information is almost always formulated at the sender
   (or on behalf of a sender), the sender),the receiver(s) somehow must get this
   configuration information. While one may envision a static method to
   populate the configuration information at both sender and
   receiver(s), it would require the knowledge of every receiver in
   advance and that is something not always feasible. Hence, there is a
   desire to define and describe dynamic method using methods i.e. signaling protocol to convey
   the configuration information from between sender to and one or more

   It is important to note that there may be either only one receiver
   needing the FEC Framework configuration information to FEC protect a
   "unicasted multimedia stream" (such as Video On Demand stream), or
   one or more receivers needing the FEC Framework configuration
   information to FEC protect a "multicasted multimedia stream" (such as
   Broadcast TV or IPTV). While the unicasted stream requires the
   identification of the receiver (which typically initiates the
   communication) at the sender, the multicasted stream
   doesn't necessarily require the identification of the receiver at the

   Such diversity necessitates describing using at least two types of
   signaling protocols - one to deliver the configuration information to
   many receivers via multicasting (described in section 4.1), and the
   other to deliver the configuration information to one and only one
   receiver via unicasting (described in section 4.2).

   Figure 1 below illustrates a sample topology showing the FEC sender
   and FEC receiver (that may or may not be the Media Sender and Media
   Receiver respectively) such that FEC_Sender1 is serving
   FEC_Reciver11,12,13 via the multicast signaling protocol, whereas the
   FEC_Sender2 is serving only FEC_Reciever2 via the unicast signaling

   FEC_Sender2---------|      |--------FEC_Receiver2
                       |      |
   FEC_Sender1-----IP/MPLS network

                Figure 1 Topology using Sender and Receiver

   The rest of the section continues to use the terms 'Sender' and
   'Receiver' to refer to the 'FEC Sender' and 'FEC Receiver'

4.1. Signaling Protocol for Multicasting

   A one-to-many signaling protocol is desired in order to effectively
   deliver the FEC Framework configuration from one sender to many
   receivers. The

   This specification describes using  Session Announcement Protocol
   (SAP) version 2 [RFC2974] is used as the signaling protocol to multicast the
   configuration information. information from one sender to many receivers. The
   apparent advantage is that the server doesn't need to maintain any
   state for any receiver using SAP.

   At the high level, a sender, acting as the SAP announcer, signals the
   FEC Framework Configuration Information for each FEC Framework
   instance available at the sender, using the SAP message(s). The
   configuration information, encoded in a suitable format as per the
   section 3.1, is carried in the Payload of the SAP message(s). A
   receiver, acting as the SAP listener, listens on a well known UDP
   port and at least one well known multicast group IP address. This
   enables the receiver to receives receive the SAP message(s) and obtains the
   FEC Framework Configuration Information for each FEC Framework

   Using the configuration information, the receiver becomes aware of
   available FEC protection options, and may subscribe to one or more
   multicast trees to receive the FEC streams using out-of-band
   multicasting techniques such as PIM [RFC4601]. This, however, is
   outside the specification scope of this document.

   SAP message is carried over UDP over IP. The destination UDP port
   must be 9875 and source UDP port may be any available number. The SAP
   message(s) SHOULD contain an authentication header and MAY be
   subjected to the cryptography. One cryptography method suggested by
   this specification is the usage of Group Cryptography as specified in
   GDOI [RFC3547].

   Figure 2 below illustrates the SAP packet format (it is reprinted
   from the RFC2974) -

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      | V=1 |A|R|T|E|C|   auth len    |         msg id hash           |
      |                                                               |
      :                originating source (32 or 128 bits)            :
      :                                                               :
      |                    optional authentication data               |
      :                              ....                             :
      |                      optional payload type                    |
      +                                         +-+- - - - - - - - - -+
      |                                         |0|                   |
      + - - - - - - - - - - - - - - - - - - - - +-+                   |
      |                                                               |
      :                            payload                            :
      |                                                               |

                        Figure 2 SAP Message format

   While the RFC2974 includes explanation for each field, the most
   interesting it is worth
   discussing the 'Payload' and 'Payload Type' field. This field is required, by this
   specification, must be
   used to carry the the FEC Framework configuration information.
   Subsequently, the optional 'Payload Type' field, which is a MIME
   content type specifier, must describe the encoding format used to
   encode the Payload. For example, the 'Payload Type' field may be
   application/sdp if the FEC framework configuration information was is
   encoded in SDP format and placed as carried in the SAP payload. Similarly, it
   would be application/xml if the FEC framework configuration
   information was encoded in XML format.

4.1.1. Sender Procedure

   The sender signals the FEC framework configuration for each FEC
   framework instance in a periodic SAP announcement message. The SAP
   announcement message is sent to a well known multicast IP address and
   port. The announcement is multicasted with the same scope as the
   session it is announcing. being announced.

   The SAP module at the sender obtains the FEC Framework configuration
   information per Instance from the 'FEC Framework' module and places
   that in the SAP payload accordingly. A single SAP (announcement)
   message may carry the FEC Framework Configuration Information for
   each FEC Framework Instance. This is a the preferred method, though the
   other method may be to aggregate more than one SAP (announcement)
   messages in a single UDP datagram as long as the resulting UDP
   datagram length is less than the IP MTU of the outgoing interface.

   The IP packet carrying the SAP message must be sent with to destination IP
   address of either (if IPv4 administrative scope 239. is selected) or (if IPv4
   global scope is selected) or
   FF0X:0:0:0:0:0:2:7FFE (if IPv6 is selected, where X is the 4-bit
   scope value) with UDP destination port 9875. The default IP TTL value
   should be 255, though the implementation should allow to set it to
   any other value. The IP DSCP field may be set to any value that
   indicates a desired QoS treatment in the IP network.

   The IP packet carrying the SAP message must be sent with source IP
   address that is reachable by the receiver. The sender may assign the
   same IP address in the "originating source" field of the SAP message,
   as the one used in the source IP address of the IP packet.

   Furthermore, the FEC Framework Configuration Information must NOT
   include any of the reserved multicast group IP addresses for the FEC
   streams (i.e. source or repair flows), though it may use the same IP
   address as the 'originating source' address to identify the FEC
   streams (i.e. source or repair flows). Please refer to IANA
   assignments for multicast addresses.

   The sender must periodically send the 'SAP announcement' message.
   This is required so that the receiver doesn't purge the cached
   entry(s) from the database and doesn't trigger the deletion of FEC
   Framework configuration information. While the time interval between
   repetitions of an announcement can be calculated as per the very
   sophisticated but complex formula explained in RFC2974, the preferred
   and simpler mean is to let the user specify the time interval from
   the range of 1-200 seconds with suggested default being 60 seconds.
   The time interval must be chosen to ensure that SAP announcement
   message is sent out before the corresponding multicast routing entry
   (S,G) or (*,G) on the router doesn't time out. It is worth noting
   that the default time-out period for the multicast routing entry is
   210 seconds, per the PIM specification [RFC4601], but though it depends may
   depend on the implementation. The SAP implementation of signaling protocol should provide
   the flexibility to the operator to choose the complex method over the
   simpler method of determining the SAP announcement time interval.
   Additionally, the 'time interval' should be signaled within the FEC
   Framework configuration Information.

   The sender may choose to delete the announced FEC framework
   configuration information by sending a 'SAP deletion' message. This
   may be used if the sender no longer desires to send any FEC streams.
   If the sender needs to modify the announced FEC Framework
   configuration Information for one or more FEC instances, then the
   sender must send a new announcement message with a different 'Message
   Identifier Hash' value as per the rules described in section 5 of
   RFC2974. Such announcement message should be sent immediately
   (without having to wait for the time-interval) to ensure that the
   modifications are received by the receiver as soon as possible. The
   sender must send the SAP deletion message to delete the previous SAP
   announcement message (i.e. with the previous 'Message Identifier
   Hash' value).

4.1.2. Receiver Procedure

   The receiver must listen on UDP port 9875 for packets arriving with
   IP destination address of either (if IPv4
   administrative scope is selected) or (if IPv4 global
   scope is selected) or FF0X:0:0:0:0:0:2:7FFE (if IPv6 is selected,
   where X is the 4-bit scope value).

   The receiver, upon receiving a SAP announcement message, creates an
   entry, if it doesn't already exists, in a local database and passes
   the FEC Framework configuration information from the SAP Payload
   field to the 'FEC Framework' module. When the same annoucement
   (please see section 5 of RFC2974) is received the next time, the
   timer of the corresponding entry should be reset to the three times
   the time-interval value that is signaled by the sender or one hour,
   whichever is greater.

   Editor's Note: Unfortunately, SAP doesn't allow the time-interval to
   be signaled in the SAP header. Hence, we need this to the time-interval should be
   specified in the FEC Framework Configuration Information (allowed by SAP). Information. For
   example, the usage of "r=" (repeat time) field in SDP.

   The receiver, upon receiving a SAP delete message, must delete the
   matching SAP entry in its database. This should result in the
   receiver no longer using the relevant FEC framework configuration
   information for every instance, and should no longer subscribe to any
   related FEC streams.

4.2. Signaling Protocol for Unicasting


   This document describes leveraging any signaling protocol that is
   already used by the unicast application, for unicasting enables two nodes, which wish
   to communicate one-to-one across an IP network, to exchange exchanging the FEC
   Framework configuration Information. This exchange may be
   unidirectional or bidirectional depending on the application desiring
   the FEC protection for its communication. Information between two nodes.

   For example, a multimedia (VoD) client may send a request via
   unicasting for a particular content to the multimedia (VoD) server,
   which may offer various options such as encodings, bitrates,
   transport etc. for the content. The client selects the suitable
   options and answers to the server, paving the way for the content to
   be unicasted on the chosen transport from server to the client. This
   offer/answer signaling, described in [RFC3264], is commonly utilized
   by many application protocols such as SIP, RTSP etc.

   The fact that two nodes desiring unicast communication almost always
   rely on an application to first exchange the application related
   parameters via the signaling protocol, it is logical to enhance such
   signaling protocol(s) to (a) convey the desire for the FEC protection
   and (b) subsequently also exchange FEC parameters i.e. FEC Framework
   Configuration information. This enables the node acting as the
   offerer to offer 'FEC Framework Configuration Information' for each
   of available FEC instances, and the node acting as the answerer
   conveying the chosen FEC Framework instance(s) to the offerer. The
   usage of FEC framework instance i.e. FEC scheme is beyond the scope
   of this document. Please refer to explained the FEC
   Framework document [FECARCH].

   While enhancing the application's anapplication's signaling protocol to exchange FEC
   parameters is one method (briefly explained above), another method
   would be to have a unicast based generic protocol that could be used
   by two nodes independent of the application's signaling protocol. The
   latter method is under investigation and may be covered in future. by a separate

   The remainder of this section provides example signaling protocols
   and explains how they can be used to exchange FEC Framework
   Configuration information.

4.2.1. SIP

   SIP [RFC3261] is an application-level signaling protocol to create,
   modify, and terminate multimedia sessions with one or more
   participants. SIP also enables the participants to discover one
   another and to agree on a characterization of a multimedia session
   they would like to share. SIP runs on either TCP or UDP or SCTP
   transport, and uses SDP as the encoding format to describe multmedia
   session attributes.

   SIP already uses offer/answer model with SDP, described in [RFC3264],
   to exchange the information between two nodes to establish unicast
   sessions between them. This specification document extends the usage of this model
   for exchanging the FEC Framework Configuration Information, explained
   in section 3, between two SIP speaking nodes. 3. Any SDP specific enhancements to accommodate the FEC
   Framework are covered in the SDP Elements specification [FECSDP].

4.2.2. RSTP

   RTSP [RFC2326] is an application-level signaling protocol for control
   over the delivery of data with real-time properties. RTSP provides an
   extensible framework to enable controlled, on-demand delivery of
   real-time data, such as audio and video. RTSP runs on either TCP or
   UDP transports.

   RTSP already provides an ability to extend the existing method with
   new parameters. This specification suggests requesting for the FEC
   protection options by including "FEC Protection Required" in the
   "Require:" header of SETUP (method) request message. The node
   receiving such request either responds with "200 OK" message that
   includes offers i.e. available FEC options (e.g. FEC Framework
   Configuration Information for each Instnace) or "551 Option not
   supported" message. A sample of related message exchange is shown
   below -
   Node1->Node2:  SETUP < . . .> ... > RTSP/1.0
                  CSeq: 1
                  Transport: <omitted for simplicity>
                  Require: FEC Protections Required

   Node2->Node1:  RTSP/1.0 200 OK
                  Cseq: 1
                  Transport: <omitted for simplicity>


   Node2->Node1:  RTSP/1.0 551 Option Not supported
                  CSeq: 1
                  Transport: <omitted for simplicity>

   The requesting node (Node1) may then send either the SETUP message
   without using the Require: header, if the remote node didn't support
   the "FEC protection", or a new SETUP message to request the selected
   FEC protection streams.

4.2.3. DSM-CC

   DSM-CC is a predominant suite of protocols including the signaling
   protocol used for the video control plane in Cable/MSO networks that
   have offered video services for decades. Unfortunately, DSM-CC is
   actually standardised in
   MPEG-2 ISO/IEC 13818-6 (part 6 of the MPEG-2 standard), not within
   the IETF yet, hence, DSM-CC related enhancements aren't covered in
   this document. The same is applicable to Session Setup protocol (SSP)
   and Lightweight Stream Control Protocol (LSCP) that are derived from
   DSM-CC, as well.

5. Security Considerations

   There are is no additional security consideration other than what's
   already covered in RFC2974 for SAP, RFC2326 for RTSP, RFC3261 for SIP

6. IANA Considerations


7. Conclusions


8. Acknowledgments

   Thanks to Colin Perkins for pointing out the issue with the time-
   interval for the SAP messages. Additionally, many thanks to Mark Watson,
   Ali Begen and Ulas Kozat for helping with solidifying this proposal.

   This document was prepared using 2-Word-v2.0.template.dot.

9. References

9.1. Normative References

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

   [FECARCH] Watson, M., "Forward Error Correction (FEC) Framework",
             draft-ietf-fecframe-framework-03 (work in progress),,
             November 2007. progress),
             October 2008.

   [FECSDP]  Begen, A., "SDP Elements for FEC Framework", draft-begen-
             fecframe-sdp-elements-00 draft-ietf-
             fecframe-sdp-elements-01 (work in progress), November 11
             2007. July 14 2008.

9.2. Informative References

   [RFC2974] Handley, M., Perkins, C. and E. Whelan, "Session
             Announcement Protocol", RFC 2974, October 2000.

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

   [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
             with Session Description Protocol (SDP)", RFC 3264, June

   [RFC2326] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time
             Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC3261] Handley, M., Schulzrinne, H., Schooler, E. and J.
             Rosenberg, "SIP: Session Initiation Protocol", RFC 3261,
             June 2002.

   [RFC4601] Fenner, etc., "Protocol Independent Multicast - Sparse Mode
             (PIM-SM) : Protocol Specification", RFC 4601, August 2006.

   [RFC3547] Baugher, etc., "The Group Domain of Interpretation", RFC
             3547, July 2003.

Author's Addresses

   Rajiv Asati
   Cisco Systems,
   7025-6 Kit Creek Rd, RTP, NC, 27709-4987
   Email: rajiva@cisco.com

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