FECFRAME Working Group                                      Rajiv Asati
Internet Draft                                            Cisco Systems
Intended status: Informational
Expires: July 2012

                                                      February 21, 25, 2012

         Methods to convey FEC Framework Configuration Information


   FEC Framework document [RFC6363] defines the FEC Framework
   Configuration Information necessary for the FEC framework operation.
   This document describes using existing how to use signaling protocols such as
   Session Announcement Protocol (SAP), Session Initiation Protocol
   (SIP), Real Time Stream Protocol (RTSP) etc. to determine for determining and
   dynamically communicate
   communicating the Configuration information between sender(s) and

   This document doesn't define any new signaling protocol.

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Table of Contents

   1. Introduction...................................................3
   2. Specification Language.........................................3 Language.........................................4
   3. Terminology/Abbreviations......................................4
   4. FEC Framework Configuration Information........................4 Information........................5
      4.1. Encoding Format...........................................6
   5. Signaling Protocol Usage.......................................6 Usage.......................................7
      5.1. Signaling Protocol for Multicasting.......................7 Multicasting.......................8
         5.1.1. Sender Procedure.....................................9
         5.1.2. Receiver Procedure..................................12
      5.2. Signaling Protocol for Unicasting........................13
         5.2.1. SIP.................................................13
         5.2.2. RTSP................................................14
   6. Security Considerations.......................................15
   7. IANA Considerations...........................................15
   8. Acknowledgments...............................................15
   9. References....................................................16
      9.1. Normative References.....................................16
      9.2. Informative References...................................16
   Author's Addresses...............................................17

1. Introduction

   FEC Framework document [RFC6363] 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 how various signaling protocols such as
   Session Announcement Protocol (SAP)[RFC2974], Session Initiation
   Protocol (SIP)[RFC3261], Real Time Stream Protocol (RTSP)[RFC2326]
   etc. could be used by the FEC scheme (and/or Content Delivery
   Protocol (CDP))to 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., though
   this document references to SDP encoding usage quite extensively.

     Note that this document doesn't define any new signaling protocol;
     rather it just provides examples of how existing protocols should
     be used. Also, the list of signaling protocols for unicast is not
     intended to be a complete list.

   This document doesn't describe any FEC scheme specific information
   (FSSI) (for example, how 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). 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.

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

2. Specification Language

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

3. Terminology/Abbreviations

   This document makes use of the terms/abbreviations defined in the
   FEC Framework document [RFC6363] and defines the following
   additional terms:

   o  Media Sender   -  Node providing original media flow(s) to the
      'FEC Sender'

   o  Media Receiver -  Node performing the Media decoding;

   o  FEC Sender     -  Node performing the FEC encoding on the
      original media flow(s) to produce the FEC repair flow(s)

   o  FEC Receiver      -  Node performing the FEC decoding, as needed,
      and providing the original media flow(s) to the Media receiver.

   o  Sender         -  Same as FEC Sender

   o  Receiver       -  Same as FEC Receiver

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

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

4. FEC Framework Configuration Information

   The FEC Framework [RFC6363] defines a minimum set of information
   that is 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 [RFC6363] section 6.5, the FEC Framework
   Configuration Information includes the following for each FEC
   Framework instance:

   1. Identification of the repair flow(s)

   2. Identification of Source Flow(s)

   3. Identification of FEC Scheme

   4. Length of Explicit 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 [RFC6363]
   for more details.

   The usage of signaling protocols described in this document requires
   that the application layer responsible for the FEC Framework
   instance provide the value for each of the configuration information
   parameter (listed above) encoded as per the chosen encoding format.
   In case of failure to receive the complete information, the
   signaling protocol module must return an error for the Operation,
   Administration and Maintenance (OAM) purposes and optionally convey
   this error to the application layer. Please refer to the figure 1 of
   the FEC Framework document [RFC6363] for further illustration.

   This document does not make any assumption that the 'FEC sender' and
   'Media Sender' functionalities are implemented on the same device,
   though that may be the case. Similarly, this document does not make
   any assumption that 'FEC receiver' and 'Media Receiver'
   functionalities are implemented on the same device, though that may
   be the case. There may also be more than one Media Sender.

4.1. Encoding Format

   The FEC Framework Configuration Information (listed above in section
   4) may be encoded in any format such as SDP, XML etc. as chosen or
   prefered by a particular FEC Framework instance. 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 to the signaling protocol. This is to
   provide a means (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 at the receiver. Please note that the
   encoding format is not a negotiated parameter, but rather a property
   of a particular FEC Framework instance and/or its implementation.

   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 by examining the encoded octets themselves.
   For example, the initial octets may include some kind of length

   Independent of the encoding formats supported by an FEC scheme, each
   instance of the FEC Framework must use a single encoding format to
   describe all of the configuration information associated with that
   instance. The signaling protocol specified in this document should
   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 [RFC6364], which
   describes the usage of 'SDP' encoding format as an example encoding
   format for FEC Framework Configuration Information.

5. Signaling Protocol Usage

   FEC Framework [RFC6363] 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), and somehow made available at the
   receiver(s). While one may envision a static method to populate the
   configuration information at both sender and receiver(s), it would
   not be optimal since it would (a) require the knowledge of every
   receiver in advance, (b) require the time and means to configure
   each receiver and sender, and (c) increase the misconfiguration
   possibility. Hence, there is a benefit in using a dynamic method
   i.e., signaling protocol to convey the configuration information
   between sender and one or more receivers.

   Since the configuration information may be needed at a particular
   receiver versus many receivers (depending on the multimedia stream
   being unicast e.g. Video on Demand, or multicast e.g. Broadcast or
   IPTV), we need two types of signaling protocols - one to deliver the
   configuration information to many receivers via multicasting
   (described in section 5.1), and the other to deliver the
   configuration information to one and only one receiver via
   unicasting (described in section 5.2).

   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_Receiver11,12,13 via the multicast signaling protocol, whereas
   the FEC_Sender2 is serving only FEC_Receiver2 via the unicast
   signaling protocol.

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

                Figure 1 Topology using Sender and Receiver

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

5.1. Signaling Protocol for Multicasting

   This specification describes using Session Announcement Protocol
   (SAP) version 2 [RFC2974] as the signaling protocol to multicast the
   configuration 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.

      SAP messages are carried over UDP over IP with destination UDP
      port being 9875 and source UDP port being any available number,
      as described in RFC2974. The SAP message(s) should contain an
      authentication header using PGP authentication.

   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 4.1, 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 (as
   explained in the section 5.1.1). This enables the receiver to
   receive the SAP message(s) and obtains the FEC Framework
   Configuration Information for each FEC Framework Instance.

   Using the configuration information, the receiver becomes aware of
   available FEC protection options, corresponding multicast trees (S,G
   or *,G addresses) etc. The receiver may subsequently 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 scope of this document.

   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, it is worth
   discussing the 'Payload' and 'Payload Type' fields. The 'Payload'
   field is used to carry the FEC Framework Configuration Information.
   Subsequently, the optional 'Payload Type' field, which is a MIME
   content type specifier, is used to 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 is encoded in SDP
     format and carried in the SAP payload. Similarly, it would be
     application/xml if the FEC Framework Configuration Information was
     encoded in XML format.

   Section 5.1.1 describes the sender procedure, whereas the section
   5.1.2 describes the receiver procedure in the context of config
   signaling using RFC2974.

5.1.1. Sender Procedure

   The sender signals the FEC framework configuration for each FEC
   framework instance in a periodic SAP announcement message [RFC2974].
   The SAP announcement message is sent to a well known multicast IP
   address and UDP port, as specified in [RFC2974]. The announcement is
   multicast with the same scope as the session 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 must carry the FEC Framework Configuration Information for a
   single FEC Framework Instance. The SAP message is then sent over UDP
   over IP.

     While it is possible to aggregate multiple 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,
     this specification does not recommend it since there is no length
     field in the SAP header to identify SAP message boundary. Hence,
     this specification recommends single SAP announcement message to
     be sent in a UDP datagram.

   The IP packet carrying the SAP message must be sent to destination
   IP address of one of the following depending on the selected scope:

      - (if IPv4 global scope
        is selected for the FEC stream), or

      - FF0X:0:0:0:0:0:2:7FFE (if IPv6 multicasting is selected for the
        FEC stream, where X is the 4-bit scope value), or

      - the highest multicast address (, for example) in
        the relevant administrative scope zone (if IPv4 administrative
        scope is selected for the FEC stream)

   As defined in RFC2974, the IP packet carrying SAP message must use
   destination UDP port being 9875 and source UDP port bein any
   available number. The default IP TTL value (or Hop Limit value)
   should be 255 at the sender, though the sender implementation may
   allow it to be any other value to implicitly create the multicast
   boundary for SAP announcements. 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 to
   ensure 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 method
   explained in [RFC2974], this document recommends a simpler method in
   which the user specifies the time interval in the range of 1-200
   seconds with suggested default value being 60 seconds. In this
   method, the 'time interval' may be signaled in the SAP message
   payload e.g. within the FEC Framework Configuration Information.

     Note that SAP doesn't allow the time-interval to be signaled in
     the SAP header. Hence, the usage of simpler method requires the
     time-interval to be included in the FEC Framework Configuration
     Information, if the default time interval (=60 seconds) for SAP
     message repeations is not used. For example, the usage of "r="
     (repeat time) field in SDP may convey the time-interval value, if
     SDP encoding format is used.

   The time interval must be chosen to ensure that SAP announcement
   messages are sent out before the corresponding multicast routing
   entry e.g. (S,G) or (*,G) (corresponding to the SAP multicast
   tree(s)) on the router(s) times out. (It is worth noting that the
   default time-out period for the multicast routing entry is 210
   seconds, per the PIM specification [RFC4601], though the time-out
   period may be set to another value as allowed by the router

     A SAP implementation may also support the complex method for
     determining the SAP announcement time interval, and provide the
     option to select it.

   The sender may choose to delete the announced FEC Framework
   Configuration Information, as defined in section 4 of RFC2974. The
   explicit deletion is useful if the sender no longer desires to send
   anymore 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 [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 also send the SAP deletion message to
   delete the previous SAP announcement message (i.e., with the
   previous 'Message Identifier Hash' value).

5.1.2. Receiver Procedure

   The receiver must listen on UDP port 9875 for packets arriving with
   IP destination address of either (if IPv4 global scope
   session is used for the FEC stream), or FF0X:0:0:0:0:0:2:7FFE (if
   IPv6 is selected, where X is the 4-bit scope value), or the highest
   IP address (, for example) in the relevant
   administrative scope zone (if IPv4 administrative scope is selected for the FEC stream). These IP addresses
   are mandated for SAP usage by RFC2974 [RFC2974].

   The receiver, upon receiving a SAP announcement message, creates an
   entry, if it doesn't already exist, in a local database and passes
   the FEC Framework Configuration Information from the SAP Payload
   field to the 'FEC Framework' module. Each entry also maintains a
   time-out value, which is (re)set to five times the time-interval
   value, which is either the default = 60 seconds, or the value
   signaled by the sender.

     Note that SAP doesn't allow the time-interval to be signaled in
     the SAP header. Hence, the time-interval should be included in the
     FEC Framework Configuration Information. For example, the usage of
     "r=" (repeat time) field in SDP to convey the time-interval value,
     if SDP encoding format is used.

   The time-out value associated with each entry is reset when the
   corresponding announcement (please see section 5 of [RFC2974]) is
   received. If the time-out value for any entry reaches zero, then
   that entry must be deleted from the database, as described in
   section 4 of [RFC2974]. The receiver, upon receiving a SAP delete
   message, must delete the matching SAP entry in its database, as
   described in section 4 of [RFC2974].

   The deletion of SAP entry must result in the receiver no longer
   using the relevant FEC Framework Configuration Information for the
   corresponding instance, and must no longer subscribe to any related
   FEC streams.

5.2. Signaling Protocol for Unicasting

   This document describes leveraging any signaling protocol that is
   already used by the unicast application, for exchanging the FEC
   Framework Configuration 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 unicast 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 makes it 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 is
   explained the FEC Framework document [RFC6363].

   While enhancing an application's signaling protocol to exchange FEC
   parameters is one method (briefly explained above), an alternative
   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 is not covered by this document, of course.

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

5.2.1. 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 an offer/answer model with SDP, described in
   [RFC3264], to exchange the information between two nodes to
   establish unicast sessions between them. This document extends the
   usage of this model for exchanging the FEC Framework Configuration
   Information, explained in section 4. Any SDP specific enhancements
   to accommodate the FEC Framework are covered in the SDP Elements
   specification [RFC6364].

5.2.2. RTSP

   Real-Time Streaming Protocol (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 defines 'FEC Protection Needed'
   option-tag (please see section 7 for IANA Considerations) and
   prescribes including it in the Require (or Proxy-Require) header of
   SETUP (method) request message, so as to request for FEC protection
   for the data.

   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 Instance) 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-protection-needed

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

   The requesting node (Node1) may then send a new SETUP message to
   convey the selected FEC protection to Node2, and proceed with
   regular RTSP messaging.

   Suffice to say, if the requesting node (Node1) received '551 Option
   not supported' response from Node2, then the requesting node (Node1)
   may send the SETUP message without using the Require header.

6. Security Considerations

   This document recommends SAP message(s) be authenticated to ensure
   sender authentication, as described in section 5.1.

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

7. IANA Considerations

   This document requests IANA to register a new RTSP Option tag
   (option-tag) listed below in the RTSP/1.0 Option Tags table of the
   "Real Time Streaming Protocol (RTSP)/1.0 Parameters" registry
   available from http://www.iana.org/, and provides the following
   information in compliance with section 3.8.1 in [RFC2326]:

     . Name of option-tag = FEC-protection-needed

     . Description        = See section 5.2.2

     . Change of Control  = IETF

8. Acknowledgments

   Thanks to Colin Perkins for pointing out the issue with the time-
   interval for the SAP messages. Additionally, thanks to Vincent Roca,
   Ali Begen, Mark Watson, Ulas Kozat and David Harrington for greatly
   improving this document.

   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.

   [RFC6363] Watson, M., "Forward Error Correction (FEC) Framework",
             RFC6363, March 2011.

   [RFC6364]   Begen, A., "Session Description Protocol Elements for
             FEC Framework ", RFC6364, October 2011.

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

9.2. Informative References

   [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

   [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