AVT                                                             A. Begen
Internet-Draft                                               B. VerSteeg
Intended status:  Standards Track                                  Cisco
Expires:  October 14,  November 20, 2010                                April 12,                                 May 19, 2010

        Port Mapping Between Unicast and Multicast RTP Sessions
              draft-ietf-avt-ports-for-ucast-mcast-rtp-01
              draft-ietf-avt-ports-for-ucast-mcast-rtp-02

Abstract

   This document presents a port mapping solution that allows RTP
   receivers to choose their own receive ports for an auxiliary unicast session
   in RTP applications using both unicast and multicast services.  The
   solution requires multiplexing RTP and RTCP on a single port on both
   endpoints in the unicast session.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  5
   3.  Port Mapping . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Steps for Establishing a Unicast Session . . . . . . . . .  8
     3.2.  Implications of NATs . . . . . . . . . . . . . . . . . . .  9
     3.3.  Message Flows  . . . . . . . . . . . . . . . . . . . . . .  9 10
     3.4.  Keeping the NAT Binding(s) Alive . . . . . . . . . . . . . 11 12
     3.5.  SDP Description  . . . . . . . . . . . . . . . . . . . . . 11 12
   4.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 13 14
     4.1.  PortMappingRequest (PMReq) . . . . . . . . . . . . . . . . 13 14
     4.2.  PortMappingResponse (PMRes)  . . . . . . . . . . . . . . . 14 15
   5.  Procedures for Cookie Construction . . . . . . . . . . . . . . 15 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16 17
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17 18
     7.1.  Registration of FMT Values . . . . . . . . . . . . . . . . 17 18
   8.  Contributors and Acknowledgments . . . . . . . . . . . . . . . 18 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 20
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 19 20
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 19 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 22

1.  Introduction

   In (any-source or source-specific) multicast RTP applications,
   destination ports, i.e., the ports on which the multicast receivers
   receive the RTP and RTCP packets, are defined declaratively.  In
   other words, the receivers cannot choose their receive ports and the
   sender(s) use the pre-defined ports.

   In unicast RTP applications, the receiving end needs to choose its
   receive
   ports for RTP and RTCP since these ports are local resources and only
   the receiving end can determine which ports are available to use.
   The receiving may convey its request to the sending end through
   different ways, one of which is the Offer/Answer Model [RFC3264] for
   the Session Description Protocol (SDP) [RFC4566].  However, the
   Offer/Answer Model requires offer/answer exchange(s) between the
   endpoints, and the resulting delay may not be desirable in delay-
   sensitive real-time applications.  Furthermore, the Offer/Answer
   Model may be burdensome for the endpoints that are concurrently
   running a large number of unicast sessions with other endpoints.

   In this specification, we consider an RTP application that uses one
   or more unicast and multicast RTP sessions together.  While the
   declaration and selection of the ports are well defined and work well
   for multicast and unicast RTP applications, respectively, the usage
   of the ports introduces complications when a receiving end mixes
   unicast and multicast RTP sessions within the same RTP application.

   An example scenario is where the RTP packets are distributed through
   source-specific multicast (SSM) and a receiver sends unicast RTCP
   feedback to a local repair server (also functioning as a feedback
   target) [RFC5760] asking for a retransmission of the packets it is
   missing, and the local repair server sends the retransmission packets
   over a unicast RTP session [RFC4588].

   Another scenario is where a receiver wants to rapidly acquire a new
   primary multicast RTP session and receives one or more RTP burst
   packets over a unicast session before joining the SSM session
   [I-D.ietf-avt-rapid-acquisition-for-rtp].  Similar scenarios exist in
   applications where some part of the content is distributed through
   multicast while the receivers get additional and/or auxiliary content
   through one or more unicast connections, as sketched in Figure 1.

   In this document, we discuss this problem and introduce a solution
   that we refer to as Port Mapping.  This solution allows receivers to
   choose their desired UDP ports for RTP and RTCP receive ports for in every unicast
   session when they are running RTP applications using both unicast and
   multicast services. services, and offer/answer exchange is not available.  This
   solution is not applicable in cases where TCP is used as the
   transport protocol in the unicast sessions.  For such scenarios,
   refer to [RFC4145].

          -----------
         |  Unicast  |................
         |  Source   |.............  :
         | (Server)  |            :  :
          -----------             :  :
                                  v  v
          -----------          ----------             -----------
         | Multicast |------->|  Router  |---------->|Client RTP |
         |  Source   |        |          |..........>|Application|
          -----------          ----------             -----------
                                   | :
                                   | :                -----------
                                   | :..............>|Client RTP |
                                   +---------------->|Application|
                                                      -----------

         -------> Multicast RTP Flow
         .......> Unicast RTP Flow

     Figure 1: RTP applications simultaneously using both unicast and
                            multicast services

   In the remainder of this document, we refer to the RTP endpoints that
   serve other RTP endpoints over a unicast session as the Servers.  The
   receiving RTP endpoints are referred to as Clients.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Port Mapping

   We present the details of the port mapping solution in the context of
   an illustrative example.

   Consider an SSM distribution network where a distribution source
   multicasts RTP packets to a large number of clients, and one or more
   retransmission servers function as feedback targets to collect
   unicast RTCP feedback from these clients [RFC5760].  The
   retransmission servers also join the primary multicast session to
   receive the multicast packets and cache them for a certain time
   period.  When a client detects missing packets in the primary
   multicast session, it requests a retransmission from one of the
   retransmission servers by using an RTCP NACK message [RFC4585].  The
   retransmission server pulls the requested packet(s) out of the cache
   and retransmits them to the requesting client.

   The pertaining RTP and RTCP flows are sketched in Figure 2.  Between
   the client and server, there may be one or more NAT devices
   [RFC4787].

     --------------                                 ---     ----------
    |              |-------------------------------|   |-->|P1        |
    |              |-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-|   |.->|P2        |
    |              |                               |   |   |          |
    | Distribution |      ----------------         |   |   |          |
    |    Source    |     |                |        |   |   |          |
    |              |---->|P1              |        |   |   |          |
    |              |.-.->|P2              |        |   |   |          |
    |              |     |                |        |   |   |          |
     --------------      |              P3|<.=.=.=.|   |=.=|*c1       |
                         |              P3|<~~~~~~~|   |~~~|*c1       |
    PRIMARY MULTICAST    |                |        |   |   |          |
    RTP SESSION with     |                |        |   |   |          |
    UNICAST FEEDBACK     |                |        | N |   |          |
                         | Retransmission |        | A |   |  Client  |
    - - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|-
                         |     Server     |        | T |   |          |
    AUXILIARY UNICAST    |                |        |   |   |          |
    RTP SESSION          |              P4|<~~~~~~~|   |~~>|*c2       |
                         |              P4|........|   |..>|*c2       |
                         |              P4|<.=.=.=.|   |=.>|*c2       |
                         |                |        |   |   |          |
                          ----------------          ---     ----------

    -------> Multicast RTP Flow
    .-.-.-.> Multicast RTCP Flow
    .=.=.=.> Unicast RTCP Reports
    ~~~~~~~> Unicast RTCP Feedback Messages (if any)
    .......> Unicast RTP Flow

    Figure 2: Example scenario showing an SSM distribution with support
                     for retransmissions from a server

   In this figure, we have the following multicast and unicast ports:

   o  Ports P1 and P2 denote the destination RTP and RTCP ports in the
      primary multicast session, respectively.  The clients listen to
      these ports to receive the multicast RTP and RTCP packets.  Ports
      P1 and P2 are defined declaratively.

   o  Port P3 denotes the RTCP port on the feedback target running on
      the retransmission server to collect the RTCP feedback messages,
      and RTP RTCP receiver and extended reports from the clients in the
      primary multicast session.  Port P3 is defined declaratively.

   o  Port P4 denotes the port on the retransmission server used for the
      unicast session.  The server multiplexes RTP and RTCP traffic on
      this single port [I-D.ietf-avt-rtp-and-rtcp-mux] [RFC5761] in the unicast session.  Port P4 is
      defined declaratively. declaratively and MUST be different from port P3.

   o  Ports *c1 and *c2 are chosen by the client. *c1 denotes the port
      on the client used to send the unicast RTCP feedback in the
      primary multicast session. *c2 denotes the port on the client used
      in the unicast session.  The client muxes multiplexes RTP and RTCP
      traffic on this single port [I-D.ietf-avt-rtp-and-rtcp-mux] [RFC5761] in the unicast session.
      Ports c1 and c2 do not have to be different ports.

   Once the unicast session is established, the server needs to remember
   the public IP address and public port of the client as part of the
   session state information.  The public ports of the client are
   denoted by c1' and c2'.

   In addition to the ports, we use the following notation:

   o  DS:  IP address of the distribution source

   o  G:  Destination multicast address

   o  S:  IP address of the retransmission server

   o  C:  IP address of the client

   o  C':  Public IP address of the client (as seen by the server)

   We assume that the information declaratively defined is available as
   part of the session description information and is provided to the
   clients.  The Session Description Protocol (SDP) [RFC4566] and other
   session description methods can be used for this purpose.

3.1.  Steps for Establishing a Unicast Session

   The steps to establish a unicast session are provided below:

   1.  The client ascertains server address (S) and port numbers (P3 and
       P4) from the session description.

   2.  The client determines its receive port numbers (*c1 and *c2).

   3.  The client sends a message to the server via a new RTCP message,
       called PortMappingRequest.  This message MUST be sent from port
       c2 to port P4.  The server learns client's public IP address (C')
       and its public RTP/RTCP port (c2') from the received message.

   4.  The server generates an opaque encapsulation (called Cookie) that
       conveys client's addressing information using a reversible
       transform only known to the server.

   5.  The server sends the Cookie back to the client using a new RTCP
       message, called PortMappingResponse.  This message MUST be sent
       from port P4 to port c2'.

   6.  The client includes the Cookie when necessary in the subsequent
       messages sent to the server.  Note that the unicast session is
       not established during the steps required to retrieve a Cookie
       from the server.  The unicast session is only established after
       the server has received a feedback message (along with a valid
       Cookie) from the client for which it needs to react by sending
       unicast data (See Figure 3).  Until a unicast session is
       established, neither the server nor the client needs to send RTCP
       reports for the unicast session.

   7.  Normal flows ensue, with the server using the addressing
       encapsulated in the opaque Cookie.  The client is responsible for
       keeping the NAT binding alive for the duration of the unicast
       session.

3.2.  Implications of NATs

   There may be one or more NAT devices between the client and server.
   Without an external mechanism such as STUN [RFC5389], the client
   cannot determine whether there are any NATs between itself and the
   server.  Such NAT devices would block all incoming traffic unless the
   client sent traffic of the same transport protocol to the server
   first.  Thus, the client has always to assume that there is at least
   one NAT device and send periodic packets to keep the NAT binding
   alive [I-D.ietf-avt-app-rtp-keepalive].  Since the client multiplexes
   RTP and RTCP on a single port, it has to keep a single NAT binding
   alive for each unicast session.  See Section 3.4 for further details.

   If the NAT device fails for some reason and then restarts, the public
   IP address and/or port assigned to a particular client may change.
   This will invalidate the previously acquired cookies and may result
   in a failure in the unicast session.  Upon detecting the failure, the
   client must acquire new cookies.  Applications using this method must
   be aware of the potential temporary interruptions.

   The NAT device may have endpoint-independent mappings [RFC4787],
   meaning that it assigns the same public IP address and port for the
   packets sent from the same internal IP address and port, even when
   the client is talking to different destinations.  Oppositely, the NAT
   device may have endpoint-dependent mappings in which case the public
   IP address and port of the outgoing packets may differ when they are
   sent to different destinations.  In practice, however, it is a
   difficult task to determine the type of a NAT device
   [I-D.ietf-behave-nat-behavior-discovery].

3.3.  Message Flows

   Figure 3 shows the message flows, where each message is appended with
   the (Source Address, Source Port, Destination Address, Destination
   Port) information.  The optional messages are indicated in
   parentheses and they may or may not be present in certain scenarios.

     ------------   ----------------                     ------
    |Distribution| | Retransmission |                   |      |
    |   Source   | |     Server     |                   |Client|
    |    (DS)    | |       (S)      |                   |  (C) |
     ------------   ----------------                     ------
               |    |                                  -    |
               |    |                                 | |   |
    (DS,*,G,P1)|--->|-------- RTP Multicast --------->|
    (DS,*,G,P1)|--->|------- (RTP Multicast) -------->| |-->|
    (DS,*,G,P2)|.-.>|.-.-.-.- RTCP Multicast .-.-.-.->|
    (DS,*,G,P2)|.-.>|.-.-.-. (RTCP Multicast) -.-.-.->| |-->|
                    |                                 | |   |
                    |                                 | |   |
                    |<=.=. RTCP
                    |<=.= (RTCP Receiver Reports =.=.=| Reports) .=.=| |<..|(C,c1,S,P3)
                    |   (for the multicast session)   | |   |
                    :                                 | |   :
                    :                                 | |   :
                    :                                 | |   :
                    :                                 | |   :
                    |<~~~~~ PortMappingRequest ~~~~~~~| |<~~|(C,c2,S,P4)
                    |                                 |N|   |
       (S,P4,C',c2')|~~~~~~ PortMappingResponse ~~~~~>|A|~~>|
                    |            (Cookie)             |T|   |
                    |                                 | |   |
                    |<~~~~ RTCP NACK with Cookie ~~~~~| |<~~|(C,c1,S,P3)
                    |                                 | |   |
                    |*********************************|*|***|
                    |*   UNICAST SESSION ESTABLISHED  | |  *|
                    |*********************************|*|***|
                    |                                 | |   |
       (S,P4,C',c2')|...... RTP Retransmissions .....>| |..>|
                    |                                 | |   |
                    |                                 | |   |
                    |<=.=. RTCP Receiver Reports =.=.=| |<..|(C,c2,S,P4)
                    |    (for the unicast session)    | |   |
                    |                                 | |   |
       (S,P4,C',c2')|=.=.=. RTCP Sender Reports =.=.=>| |..>|
                    |    (for the unicast session)    | |   |
                    |                                 | |   |
                                                       -

    -------> Multicast RTP Flow
    .-.-.-.> Multicast RTCP Flow
    .=.=.=.> Unicast RTCP Reports
    ~~~~~~~> Unicast RTCP Feedback Messages
    .......> Unicast RTP Flow

        Figure 3: Message flows for establishing a unicast session

   In the example above, the compound RTCP packet carrying the NACK
   message also carries the Cookie since the server must know which port
   the client is expecting to receive the RTP retransmission packet(s)
   and RTCP sender reports on.  If an RTCP message from the client will
   not trigger any transmission from the server (e.g., RTCP receiver and
   extended reports), it does not have to include the Cookie.

3.4.  Keeping the NAT Binding(s) Alive

   Editor's note:  We need to determine the best option to keep the NAT
   bindings alive [I-D.ietf-avt-app-rtp-keepalive].

   Editor's note:  Are RTCP receiver/extended reports enough to keep the
   binding alive?

   TBD.  No, keep-alive messages need to be sent after the
   request is made and until the cookie is no longer needed and the
   client does not start sending RTCP reports until the unicast session
   is actually established which might happen a long time after the
   cookie is requested.

3.5.  SDP Description

   The SDP describing the scenario given in Figure 2 can be written as:

        v=0
        o=ali 1122334455 1122334466 IN IP4 nack.example.com
        s=Local Retransmissions
        t=0 0
        a=group:FID 1 2
        a=rtcp-unicast:rsi
        m=video 41000 RTP/AVPF 98
        i=Primary Multicast Stream
        c=IN IP4 233.252.0.2/255
        a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
        a=rtpmap:98 MP2T/90000
        a=multicast-rtcp:42000
        a=rtcp:43000 IN IP4 192.0.2.1
        a=rtcp-fb:98 nack
        a=mid:1
        m=video 51000 RTP/AVPF 99
        i=Unicast Retransmission Stream
        c=IN IP4 192.0.2.1
        a=sendonly
        a=rtpmap:99 rtx/90000
        a=rtcp-mux
        a=fmtp:99 apt=98; rtx-time=5000
        a=mid:2

       Figure 4: SDP describing an SSM distribution with support for
                    retransmissions from a local server

   In this SDP, the source stream is multicast from a distribution
   source (with a source IP address of 198.51.100.1) to the multicast
   destination address of 233.252.0.2 (G) and port 41000 (P1).  The
   associated RTCP packets are multicast in the same group to port 42000
   (P2).  A retransmission server including feedback target
   functionality with an IP address of 192.0.2.1 (S) and port of 43000
   (P3) is specified with the 'rtcp' attribute.  The server uses port
   51000 (P4) for the unicast sessions.

4.  Message Formats

   Editor's note:  Should we stick with 4585 packet format?

   Editor's note:  Must the client use the same SSRC (and the same but
   unique CNAME) both in the multicast and unicast sessions so the
   reports received in two sessions can be linked?

   The common packet format for the RTCP feedback messages is defined in
   Section 6.1 of [RFC4585].  A feedback message has a fixed-length
   field for version, padding, feedback message type (FMT), payload type
   (PT), length, SSRC of packet sender, SSRC of media sender as well as
   a variable-length field for feedback control information (FCI).

   In the PortMappingRequest and PortMappingResponse messages, the PT
   field is set to RTPFB (205), and the respective FMT fields are set to
   PMReq (7) and PMRes (8).  Depending on the specific scenario, it may
   be desirable to send these messages in a reduced-size RTCP packet
   [RFC5506].  However, unless support for [RFC5506] has been signaled,
   compound RTCP packets MUST be used by following [RFC3550] rules.

   Editor's note:  Should the server always respond to the PMReq message
   as soon as possible?  If the request is rejected, the server could
   send a PMRes message with the Cookie field empty.

   Following the rules specified in [RFC3550], all integer fields in the
   messages defined below are carried in network-byte order, that is,
   most significant byte (octet) first, also known as big-endian.
   Unless otherwise noted, numeric constants are in decimal (base 10).
   Any Reserved field SHALL be set to zero and ignored.

4.1.  PortMappingRequest (PMReq)

   Editor's note:  How do we set the media source sender SSRC field in the
   following message?  Is it application specific (e.g.,
   retransmissions, RAMS, etc.)?  The suggested approach is to use client's SSRC in
   both packet sender and media sender SSRC fields.

      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=2|P|  FMT=7  |     PT=205    |           Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     SSRC of Packet Sender                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     SSRC of Media Source                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     32-bit Random Number                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 5: Syntax for the PortMappingRequest (PMReq) message

   Editor's note:  What else do we need to transmit in the PMReq
   message?

4.2.  PortMappingResponse (PMRes)

   The PortMappingResponse message is used by the server to send a
   Cookie to the client, and by the client to include a Cookie in the
   RTCP packets as necessary.  We might need to rename this message.

   Editor's note:  How do we set the packet sender SSRC and media source sender
   SSRC fields in the following message?  Are they application specific
   (e.g., retransmissions, RAMS, etc.)?  See the earlier editor's note.

      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=2|P|  FMT=8  |     PT=205    |           Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     SSRC of Packet Sender                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     SSRC of Media Source                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                             Cookie                            :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 6: Syntax for the PortMappingResponse (PMRes) message

   Editor's note:  What else do we need to transmit in the PMRes
   message?  In particular, should the server indicate the expiration
   date for the Cookie?

5.  Procedures for Cookie Construction

   Editor's notes:

   The Cookie may contain

   o  A 32-bit value randomly generated by the client [RFC4086]

   o  Client's IP address and port (Note port.  Note that the PMReq and NACK
      messages are sent from different client ports (and maybe from
      different public IP addresses as well), thus the server cannot use
      this information to check whether a cookie is used by the true
      owner of that cookie) cookie.

   o  Client's CNAME

   o  A timestamp to protect against replay attacks (Should the server
      tell the client about the expiration date so that the client may
      request a new cookie before the current one expires?)

   o  HMAC [RFC2104] of the above information (where only the server
      knows the HMAC secret)

   Details are TBC.

6.  Security Considerations

   Editor's notes:

   o  Cookie expiration via timestamping.  This could be important for
      clients behind the same NAT (The clients may still generate the
      same random number)

   o  Stealing cookies.  Can CNAME be used to avoid this for the clients
      behind the same NAT?

   o  Modifying cookies.  Can somebody manipulate the cookies to
      redirect the traffic?

7.  IANA Considerations

   The following contact information shall be used for all registrations
   in this document:

   Ali Begen
   abegen@cisco.com

   170 West Tasman Drive
   San Jose, CA 95134 USA

   Note to the RFC Editor:  In the following, please replace "XXXX" with
   the number of this document prior to publication as an RFC.

7.1.  Registration of FMT Values

   Within the RTPFB range, the following format (FMT) values are
   registered:

        Name:           PMReq
        Long name:      Port Mapping Request
        Value:          7
        Reference:      [RFCXXXX]

        Name:           PMRes
        Long name:      Port Mapping Response
        Value:          8
        Reference:      [RFCXXXX]

8.  Contributors and Acknowledgments

   Many individuals in the AVT and MMUSIC WGs have contributed to this
   work, reviewed earlier versions of this specification and provided
   feedback.  The authors thank each of them.

9.  References

9.1.  Normative References

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

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

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

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              July 2006.

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast
              Sessions with Unicast Feedback", RFC 5760, February 2010.

   [I-D.ietf-avt-rtp-and-rtcp-mux]

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port",
              draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress),
              August 2007. RFC 5761, April 2010.

   [I-D.ietf-avt-app-rtp-keepalive]
              Marjou, X. and A. Sollaud, "Application Mechanism for
              maintaining alive the Network Address Translator (NAT)
              mappings associated to RTP flows.",
              draft-ietf-avt-app-rtp-keepalive-07 (work in progress),
              December 2009.

9.2.  Informative References

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [I-D.ietf-avt-rapid-acquisition-for-rtp]
              Steeg, B., Begen, A., Caenegem, T., and Z. Vax, "Unicast-
              Based Rapid Acquisition of Multicast RTP Sessions",
              draft-ietf-avt-rapid-acquisition-for-rtp-08
              draft-ietf-avt-rapid-acquisition-for-rtp-09 (work in
              progress), March April 2010.

   [I-D.ietf-behave-nat-behavior-discovery]
              MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using STUN", draft-ietf-behave-nat-behavior-discovery-08
              (work in progress), September 2009.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              July 2006.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5506]  Johansson, I. and M. Westerlund, "Support for Reduced-Size
              Real-Time Transport Control Protocol (RTCP): Opportunities
              and Consequences", RFC 5506, April 2009.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

Authors' Addresses

   Ali Begen
   Cisco
   170 West Tasman Drive
   San Jose, CA  95134
   USA
   181 Bay Street
   Toronto, ON  M5J 2T3
   CANADA

   Email:  abegen@cisco.com

   Bill VerSteeg
   Cisco
   5030 Sugarloaf Parkway
   Lawrenceville, GA  30044
   USA

   Email:  billvs@cisco.com