Token-Based Port Mapping Between Unicast and Multicast RTP Sessions
draft-begen-avt-token-for-portmapping-03

Versions: 00 01 02 03                                                   
AVT                                                             A. Begen
Internet-Draft                                                   D. Wing
Intended status:  Standards Track                                  Cisco
Expires:  May 12, 2011                                    T. VanCaenegem
                                                          Alcatel-Lucent
                                                        November 8, 2010


  Token-Based Port Mapping Between Unicast and Multicast RTP Sessions
                draft-begen-avt-token-for-portmapping-03

Abstract

   This document presents a port mapping solution that allows RTP
   receivers to choose their own ports for an auxiliary unicast session
   in RTP applications using both unicast and multicast services
   (almost) without the need for retrieving pre-authorization.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 12, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  5
   3.  Token-Based Port Mapping . . . . . . . . . . . . . . . . . . .  6
     3.1.  Token Request and Retrieval  . . . . . . . . . . . . . . .  6
     3.2.  Unicast Session Establishment  . . . . . . . . . . . . . .  6
   4.  The portmapping-req Attribute  . . . . . . . . . . . . . . . . 11
   5.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Port Mapping Request . . . . . . . . . . . . . . . . . . . 13
     5.2.  Port Mapping Response  . . . . . . . . . . . . . . . . . . 13
     5.3.  Token Verification . . . . . . . . . . . . . . . . . . . . 14
   6.  Procedures for Token Construction  . . . . . . . . . . . . . . 15
   7.  Validating Tokens  . . . . . . . . . . . . . . . . . . . . . . 16
   8.  SDP Example  . . . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Address Pooling NATs . . . . . . . . . . . . . . . . . . . . . 19
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
     11.1. Registration of SDP Attributes . . . . . . . . . . . . . . 21
     11.2. Registration of FMT Values . . . . . . . . . . . . . . . . 21
     11.3. SFMT Values for Port Mapping Messages Registry . . . . . . 21
     11.4. RAMS Response Code Space Registry  . . . . . . . . . . . . 22
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     13.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26





















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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
   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 in every unicast
   session when they are running RTP applications using both unicast and
   multicast services, and offer/answer exchange is not available.  This
   solution is not applicable in cases where TCP is used as the



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   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.





















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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].














































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3.  Token-Based Port Mapping

   Token-based Port Mapping consists of two steps:  (i) Token request
   and retrieval, and (ii) unicast session establishment.  These are
   described below.

3.1.  Token Request and Retrieval

   This first step is required to be completed only once.  Once a Token
   is retrieved from a particular server, it can be used for all the
   unicast sessions the client will be running with this particular
   server.  By default, Tokens are server specific.  However, the client
   can use the same Token to communicate with different servers if these
   servers are provided with the same key used to generate the Token.
   The Token becomes invalid if client's public IP address changes or
   when the server expires the Token.  In these cases, the client has to
   request a new Token.

   The Token is essentially an opaque encapsulation that conveys
   client's IP address information (as seen by the server) using a
   reversible transform only known to the server.  When a request is
   received, the server creates a Token for this particular client, and
   sends it back to the client.  Later, when the client wants to
   establish a unicast session, the Token will be validated by the
   server, making sure that the IP address information matches.  This is
   effective against DoS attacks, e.g., an attacker cannot simply spoof
   another client's IP address and start a unicast transmission towards
   random clients.

3.2.  Unicast Session Establishment

   We illustrate the second step with an 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 multicast session to receive the multicast packets and cache them
   for a certain time period.  When a client detects missing packets in
   the 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 [RFC4588].

   The pertaining RTP and RTCP flows are sketched in Figure 2.  Between
   the client and server, there can be one or more Network Address Port
   Translators (NAPT - hereafter simply called NAT) devices [RFC4787].





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     --------------                                 ---     ----------
    |              |-------------------------------|   |-->|P1        |
    |              |-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-|   |.->|P2        |
    |              |                               |   |   |          |
    | Distribution |      ----------------         |   |   |          |
    |    Source    |     |                |        |   |   |          |
    |              |---->|P1              |        |   |   |          |
    |              |.-.->|P2              |        |   |   |          |
    |              |     |                |        |   |   |          |
     --------------      |              P3|<.=.=.=.|   |=.=|*c0       |
                         |              P3|<~~~~~~~|   |~~~|*c1       |
    MULTICAST RTP        |                |        |   |   |          |
    SESSION with         |                |        |   |   |          |
    UNICAST FEEDBACK     |                |        | N |   |          |
                         | Retransmission |        | A |   |  Client  |
    - - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|-
                         |     Server     |        | T |   |          |
                         |                |        |   |   |          |
    PORT MAPPING         |              PT|<~~~~~~~|   |~~>|*cT       |
                         |                |        |   |   |          |
    - - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|-
                         |                |        |   |   |          |
    AUXILIARY UNICAST    |                |        |   |   |          |
    RTP SESSION          |                |        |   |   |          |
                         |              P3|........|   |..>|*c1       |
                         |              P3|=.=.=.=.|   |=.>|*c1       |
                         |              P4|<.=.=.=.|   |=.=|*c2       |
                         |                |        |   |   |          |
                          ----------------          ---     ----------


    -------> Multicast RTP Flow
    .-.-.-.> Multicast RTCP Flow
    .=.=.=.> Unicast RTCP Reports
    ~~~~~~~> Unicast RTCP Feedback Messages
    .......> 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
      multicast session, respectively.  The clients listen to these
      ports to receive the multicast RTP and RTCP packets.  Ports P1 and
      P2 are defined declaratively.





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   o  Port P3 denotes the RTCP port on the feedback target running on
      the retransmission server to collect the RTCP feedback messages,
      and RTCP receiver and extended reports from the clients in the
      multicast session.  This is also the port that the retransmission
      server uses to send the RTP packets and RTCP sender reports in the
      unicast session.  Port P3 is defined declaratively.

   o  Port P4 denotes the RTCP port on the retransmission server used to
      collect the RTCP receiver and extended reports for the unicast
      session.  Port P4 is defined declaratively and MUST be different
      from port P3.

   o  Ports *c0, *c1 and *c2 are chosen by the client. *c0 denotes the
      port on the client used to send the RTCP reports for the multicast
      session. *c1 denotes the port on the client used to send the
      unicast RTCP feedback messages in the multicast session and to
      receive the RTP packets and RTCP sender reports in the unicast
      session. *c2 denotes the port on the client used to send the RTCP
      receiver and extended reports in the unicast session.  Ports c0,
      c1 and c2 MAY be the same port or different ports.  However, there
      are two advantages of using the same port for both c0 and c1:

      1.  Some NATs only keep bindings active when a packet goes from
          the inside to the outside of the NAT (See REQ-6 of Section 4.3
          of [RFC4787]).  When the retransmission server sends unicast
          packets for a long period of time, this can exceed that
          timeout.  If c0=c1, the occasional (periodic) RTCP receiver
          reports sent from port c0 (for the multicast session) will
          ensure the NAT does not time out the public port associated
          with the incoming unicast traffic to port c1.

      2.  Having c0=c1 conserves NAT port bindings.

      Thus, it is strongly RECOMMENDED that the client uses the same
      port for c0 and c1.

   o  Ports PT and cT denote the ports through which the Token request
      and retrieval occur at the server and client sides, respectively.
      Port PT is declared on a per unicast session basis, although its
      value MAY be the same for all unicast sessions sourced by the
      server.  This way, a Token once requested and retrieved by a
      client from port PT remains valid across different unicast
      sessions.  Port PT MAY be equal to port P3.  Port cT MAY also be
      equal to ports c0 and c1.

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





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   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.

   The following steps summarize the Token-based solution:

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

   2.  The client determines its port numbers (*c0, *c1 and *c2).

   3.  If the client does not have a valid Token:

       A.  The client first sends a message to the server via a new RTCP
           message, called Port Mapping Request to port PT.  This
           message is sent from port cT on the client side.  The server
           learns client's public IP address (C') from the received
           message.  The client can send this message anytime it wants
           (e.g., during initialization), and does not normally ever
           need to re-send this message (See Section 7).

       B.  The server generates an opaque encapsulation (i.e., the
           Token) that conveys client's IP address information using a
           reversible transform only known to the server.  For details,
           see Section 6.

       C.  The server sends the Token back to the client using a new
           RTCP message, called Port Mapping Response.  This message
           MUST be sent from port PT to port cT.

   4.  The client provides the Token to the server using a new RTCP
       message, called Token Verification, whenever the client sends an
       RTCP feedback message for triggering or controlling a unicast
       session.  Note that the unicast session is only established after
       the server has received a feedback message (along with a valid
       Token) from the client for which it needs to react by sending
       unicast data.  Until a unicast session is established, neither



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       the server nor the client needs to send RTCP reports for the
       unicast session.

   5.  Normal flows ensue as shown in Figure 2.  Note that in the
       unicast session the RTP and RTCP packets MUST be multiplexed on
       the (same) port c1.  If the client uses the same port for both c0
       and c1, the RTCP reports sent for the multicast session keep the
       P3->c1(=c0) binding alive.  If the client uses different ports
       for c0 and c1, the client needs to periodically send an explicit
       keep-alive message [I-D.ietf-avt-app-rtp-keepalive] to keep the
       P3->c1 binding alive during the lifetime of the unicast session
       if the unicast session's lifetime is likely to exceed the NAT's
       timeout value.






































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4.  The portmapping-req Attribute

   This new SDP attribute is used declaratively to indicate the port for
   obtaining a Token.  Its presence indicates that a Token MUST be
   included in the feedback messages sent to the server triggering or
   controlling a unicast session.

   The formal description of the 'portmapping-req' attribute is defined
   by the following ABNF [RFC5234] syntax:

         portmapping-req-attribute = "a=portmapping-req:" port CRLF

   Here, 'port' is defined as specified in Section 9 of [RFC4566].  The
   'portmapping-req' attribute is used as a session-level or media-level
   attribute.




































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5.  Message Formats

   This section defines the formats of the RTCP transport-layer feedback
   messages that are exchanged between a server and a client for the
   purpose of Token-based port mapping.  Three RTCP messages are
   defined:

   1.  Port Mapping Request

   2.  Port Mapping Response

   3.  Token Verification

   These are all payload-independent RTCP feedback messages with a
   common format defined in Section 6.1 of [RFC4585], also sketched in
   Figure 3.

      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   |       PT      |          length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  SSRC of packet sender                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  SSRC of media source                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :            Feedback Control Information (FCI)                 :
     :                                                               :

     Figure 3: The common packet format for the RTCP feedback messages

   Each feedback message has a fixed-length field for version, padding,
   feedback message type (FMT), packet type (PT), length, SSRC of packet
   sender, SSRC of media source as well as a variable-length field for
   feedback control information (FCI).

   In the new messages defined in this section, the PT field is set to
   RTPFB (205) and the FMT field is set to Port Mapping (7).  Individual
   Port Mapping messages are identified by a sub-field called Sub
   Feedback Message Type (SFMT).  Any Reserved field SHALL be set to
   zero and ignored.

   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 stated, numeric constants are in decimal (base 10).





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5.1.  Port Mapping Request

   The Port Mapping Request message is identified by SFMT=1.  This
   message is a unicast feedback message transmitted by the client to a
   dedicated server port to request a Token.  In the Port Mapping
   Request message, the client MUST set both the packet sender SSRC and
   media source SSRC fields to its own SSRC since the Port Mapping
   Request message is not necessarily linked to any specific media
   source.  The FCI field has the structure depicted in Figure 4.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    SFMT=1     |                    Reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             Random                            |
     |                             Nonce                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 4: The FCI field of Port Mapping Request message

   o  Random Nonce (64 bits):  Mandatory field that contains a random
      nonce value generated by the client following the procedures of
      [RFC4086].  This nonce MUST be taken into account by the server
      when generating a Token for the client to enable better security
      for clients that share the same IP address.  If the Port Mapping
      Request message is transmitted multiple times for redundancy
      reasons, the random nonce value MUST remain the same in these
      duplicated messages.

5.2.  Port Mapping Response

   The Port Mapping Response message is identified by SFMT=2.  This
   message is sent by the server and delivers the Token to the client.
   In the Port Mapping Response message, the packet sender SSRC and
   media sender SSRC fields are both set to the client's SSRC since the
   Port Mapping Response message is not necessarily linked to any
   specific media source.  The FCI field has the structure depicted in
   Figure 5.












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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    SFMT=2     |                    Reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                              Token                            :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Expiry Time                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 5: FCI field syntax for the Port Mapping Response message

   o  Token (128 bits):  Mandatory element that contains the Token
      generated by the server.

   o  Expiry Time (32 bits):  Mandatory element that contains the expiry
      time of the Token.  The expiry time is expressed in seconds from
      the time the Token was generated.  An expiry time of zero
      indicates that the accompanying Token is not valid.

5.3.  Token Verification

   The Token Verification message is identified by SFMT=3.  This message
   contains the Token and MUST accompany any other RTCP feedback message
   sent by the client to trigger or control a unicast session.  Examples
   include the RAMS-R and RAMS-T messages
   [I-D.ietf-avt-rapid-acquisition-for-rtp] as well as the NACK messages
   [RFC4585].  In the Token Verification message, the client MUST set
   both the packet sender SSRC and media source SSRC fields to its own
   SSRC since the media source SSRC may not be known.  The client MUST
   NOT send a Token Verification message with a Token that has expired.
   The FCI field has the structure depicted in Figure 6.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    SFMT=3     |                    Reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                              Token                            :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 6: FCI field syntax for the Token Verification message

   o  Token (128 bits):  Mandatory element that contains the Token
      previously acquired by the client.






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6.  Procedures for Token Construction

   Editor's notes:

   The Token SHOULD be calculated by the server by taking into account:

   o  Client's IP address as seen by the server

   o  The nonce generated by the client and inserted in the Port Mapping
      Request message

   o  A timestamp to protect against replay attacks

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

   The server conveys the expiration time in the clear to the client in
   the Port Mapping Response message.  Thus, the client can request a
   new Token before the current one expires.

   Details are TBC.






























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7.  Validating Tokens

   Upon receipt of an RTCP feedback message along with the Token
   Verification message that contains a Token, the server MUST validate
   the Token.  The server considers a Token valid if the source IP
   address of the RTCP feedback message matches the IP address in the
   Token and the Token has not expired yet.

   The IP address is encoded into the Token by the server, using an
   algorithm known only to the server.  This, combined with the
   expiration time provides protection against DoS attacks so that a
   client using a certain IP address cannot cause one or more RTP
   packets to be sent to another client with a different IP address.

   When the server detects that the Token is invalid, it SHOULD NOT
   silently discard client's message since this adds an undesired delay.
   Instead, it is RECOMMENDED that applications define an application-
   specific error response.  In applications that have not defined an
   error response, the server MUST reply back to the client with a Port
   Mapping Response message (that goes from port P3 on the server to
   port c1 on the client) where the Token field carries the invalid
   Token sent by the client and the Expiry Time field is set to zero
   (indicating that the Token is invalid).

   For applications using [I-D.ietf-avt-rapid-acquisition-for-rtp], this
   draft defines a new 4xx-level response code in the RAMS Response Code
   Space Registry.
























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8.  SDP Example

   The declarative SDP describing the scenario given in Figure 2 is
   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=Multicast Stream
        c=IN IP4 233.252.0.2/255
        a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1   ; Note 1
        a=rtpmap:98 MP2T/90000s
        a=multicast-rtcp:41500                                 ; Note 1
        a=rtcp:42000 IN IP4 192.0.2.1                          ; Note 2
        a=rtcp-fb:98 nack                                      ; Note 2
        a=mid:1
        m=video 42000 RTP/AVPF 99                              ; Note 3
        i=Unicast Retransmission Stream
        c=IN IP4 192.0.2.1
        a=sendonly
        a=rtpmap:99 rtx/90000
        a=rtcp-mux                                             ; Note 4
        a=rtcp:42500                                           ; Note 5
        a=fmtp:99 apt=98; rtx-time=5000
        a=portmapping-req:30000                                ; Note 6
        a=mid:2

       Figure 7: SDP describing an SSM distribution with support for
                    retransmissions from a local server

   In this description, we highlight the following notes:

   Note 1:  The source stream is multicast from a distribution source
   with a source IP address of 198.51.100.1 (DS) 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 41500
   (P2).

   Note 2:  A retransmission server including feedback target
   functionality with an IP address of 192.0.2.1 (S) and port of 42000
   (P3) is specified with the 'rtcp' attribute.  The feedback
   functionality is enabled for the RTP stream with payload type 98
   through the 'rtcp-fb' attribute [RFC4585].



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   Note 3:  The port specified in the second "m" line (for the unicast
   stream) does not mean anything in this scenario as the client does
   not send any RTP traffic back to the server.

   Note 4:  The server multiplexes RTP and RTCP packets on the same port
   (c1 in Figure 2).

   Note 5:  The server uses port 42500 (P4) for the unicast sessions.

   Note 6:  The "a=portmapping-req" line indicates that a Token needs to
   be retrieved first before a unicast session associated to the
   multicast session can be established and that the Port Mapping
   Request message needs to be sent to port 30000 (PT).






































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9.  Address Pooling NATs

   Large-scale NAT (LSN) devices have a pool of public IPv4 addresses
   and map internal hosts to one of those public IPv4 addresses.  As
   long as an internal host maintains an active mapping in the NAT, the
   same IPv4 address is assigned to new connections.  However, once all
   of the host's mappings have been deleted (e.g., because of timeout),
   it is possible that a new connection from that same host will be
   assigned a different IPv4 address from the pool.  When that occurs,
   the Token will be considered invalid by the server, causing an
   additional round trip for the client to acquire a fresh Token.

   Any traffic from the host which traverses the NAT will prevent this
   problem.  As the host is sending RTCP receiver reports at least every
   5 seconds (Section 6.2 of [RFC3550]) for the multicast session it is
   receiving, those RTCP messages will be sufficient to prevent this
   problem.


































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10.  Security Considerations

   The Token, which is generated based on a client's IP address and
   expiration date, provides protection against denial-of-service (DoS)
   attacks.  An attacker using a certain IP address cannot cause one or
   more RTP packets to be sent to a victim client who has a different IP
   address.  However, if the attacker acquires a valid Token for a
   victim and can spoof the victim's source address, this approach
   becomes vulnerable to replay attacks.

   In networks where multicast services are enabled, however, source
   address validation is already a necessary requirement to prevent an
   attacker from spoofing a multicast (IGMP/MLD) Join message and
   attracting multicast traffic to a victim.  As such an attack is even
   easier to execute, it is expected that source address validation will
   be available.  Yet, if the source address validation capability is
   known not to perform properly, the server can reduce the probability
   of a Token being stolen and used to initiate an attack by expiring
   the Token in a short amount of time.  In the extreme case, the server
   can expire the token after its first use.































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11.  IANA Considerations

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

   Ali Begen
   abegen@cisco.com


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

11.1.  Registration of SDP Attributes

   This document registers a new attribute name in SDP.


        SDP Attribute ("att-field"):
        Attribute name:     portmapping-req
        Long form:          Port for requesting Token
        Type of name:       att-field
        Type of attribute:  Either session or media level
        Subject to charset: No
        Purpose:            See this document
        Reference:          [RFCXXXX]
        Values:             See this document

11.2.  Registration of FMT Values

   Within the RTPFB range, the following format (FMT) value is
   registered:


     Name:       Port Mapping
     Long name:  Port Mapping Between Unicast and Multicast RTP Sessions
     Value:      7
     Reference:  [RFCXXXX]

11.3.  SFMT Values for Port Mapping Messages Registry

   This document creates a new sub-registry for the sub-feedback message
   type (SFMT) values to be used with the FMT value registered for Port
   Mapping messages.  The registry is called the SFMT Values for Port
   Mapping Messages Registry.  This registry is to be managed by the
   IANA according to the Specification Required policy of [RFC5226].

   The length of the SFMT field in the Port Mapping messages is a single
   octet, allowing 256 values.  The registry is initialized with the



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   following entries:


  Value Name                                               Reference
  ----- -------------------------------------------------- -------------
  0     Reserved                                           [RFCXXXX]
  1     Port Mapping Request                               [RFCXXXX]
  2     Port Mapping Response                              [RFCXXXX]
  3     Token Verification                                 [RFCXXXX]
  4-254                          Assignable - Specification Required
  255   Reserved                                           [RFCXXXX]


   The SFMT values 0 and 255 are reserved for future use.

   Any registration for an unassigned SFMT value needs to contain the
   following information:

   o  Contact information of the one doing the registration, including
      at least name, address, and email.

   o  A detailed description of what the new SFMT represents and how it
      shall be interpreted.

11.4.  RAMS Response Code Space Registry

   This document adds the following entry to the RAMS Response Code
   Space Registry.


  Code  Description                                        Reference
  ----- -------------------------------------------------- -------------
  405   Invalid Token                                      [RFCXXXX]

   This response code is used when the Token included by the RTP_Rx in
   the RAMS-R message is invalid.















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12.  Acknowledgments

   The approach presented in this document came out after discussions
   with various individuals in the AVT and MMUSIC WGs, and the breakout
   session held in the Anaheim meeting.  We thank each of these
   individuals.













































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13.  References

13.1.  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.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

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

13.2.  Informative References

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

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

   [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-16 (work in
              progress), October 2010.

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



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

   [I-D.ietf-avt-app-rtp-keepalive]
              Marjou, X. and A. Sollaud, "Application Mechanism for
              keeping alive the Network Address Translator (NAT)
              mappings associated to RTP flows.",
              draft-ietf-avt-app-rtp-keepalive-09 (work in progress),
              September 2010.

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

































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Authors' Addresses

   Ali Begen
   Cisco
   181 Bay Street
   Toronto, ON  M5J 2T3
   Canada

   Email:  abegen@cisco.com


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134
   USA

   Email:  dwing@cisco.com


   Tom VanCaenegem
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerpen,   2018
   Belgium

   Email:  Tom.Van_Caenegem@alcatel-lucent.com
























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