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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 RFC 7825

Network Working Group                                        J. Goldberg
Internet-Draft                                                     Cisco
Intended status: Standards Track                           M. Westerlund
Expires: August 28, 2008                                        Ericsson
                                                                 T. Zeng
                                                 Nextwave Wireless, Inc.
                                                       February 25, 2008


   An Network Address Translator (NAT) Traversal mechanism for media
           controlled by Real-Time Streaming Protocol (RTSP)
                     draft-ietf-mmusic-rtsp-nat-06

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 28, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document defines a solution for Network Address Translation
   (NAT) traversal for datagram based media streams setup and controlled
   with Real-time Streaming Protocol version 2 (RTSP 2.0).  It uses
   Interactive Connectivity Establishment (ICE) adapted to use RTSP as a



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   signalling channel, defining the necessary extra RTSP extensions and
   procedures.

Requirements Language

   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 RFC 2119 [RFC2119].


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  RTSP Extensions  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  ICE Transport Lower Layer  . . . . . . . . . . . . . . . .  6
     3.2.  ICE Candidate Transport Header Parameter . . . . . . . . .  7
     3.3.  ICE Password and Username Transport Header Parameters  . . 10
     3.4.  ICE Feature Tag  . . . . . . . . . . . . . . . . . . . . . 10
     3.5.  Status Codes . . . . . . . . . . . . . . . . . . . . . . . 11
       3.5.1.  150 ICE connectivity checks in progress  . . . . . . . 11
       3.5.2.  480 ICE Processing Failed  . . . . . . . . . . . . . . 11
     3.6.  Server Side SDP Attribute for ICE Support  . . . . . . . . 11
     3.7.  ICE Features Not Required in RTSP  . . . . . . . . . . . . 12
       3.7.1.  ICE-Lite . . . . . . . . . . . . . . . . . . . . . . . 12
       3.7.2.  ICE-Mismatch . . . . . . . . . . . . . . . . . . . . . 12
       3.7.3.  ICE Remote Candidate Transport Header Parameter  . . . 12
   4.  Detailed Solution  . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Session description and RTSP DESCRIBE (optional) . . . . . 13
     4.2.  Setting up the Media Resources . . . . . . . . . . . . . . 14
     4.3.  RTSP SETUP Request . . . . . . . . . . . . . . . . . . . . 14
     4.4.  Gathering Candidates . . . . . . . . . . . . . . . . . . . 15
     4.5.  RTSP Server Response . . . . . . . . . . . . . . . . . . . 16
     4.6.  Server to Client ICE Connectivity Checks . . . . . . . . . 16
     4.7.  Client to Server ICE Connectivity Check  . . . . . . . . . 17
     4.8.  Client Connectivity Checks Complete  . . . . . . . . . . . 17
     4.9.  Server Connectivity Checks Complete  . . . . . . . . . . . 17
     4.10. Releasing Candidates . . . . . . . . . . . . . . . . . . . 17
     4.11. Steady State . . . . . . . . . . . . . . . . . . . . . . . 18
     4.12. re-SETUP . . . . . . . . . . . . . . . . . . . . . . . . . 18
   5.  ICE and Proxies  . . . . . . . . . . . . . . . . . . . . . . . 18
     5.1.  Media Handling Proxies . . . . . . . . . . . . . . . . . . 18
     5.2.  Signalling Only Proxies  . . . . . . . . . . . . . . . . . 19
     5.3.  Non-supporting Proxies . . . . . . . . . . . . . . . . . . 19
   6.  RTP and RTCP Multiplexing  . . . . . . . . . . . . . . . . . . 20
   7.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  RTSP Feature Tags  . . . . . . . . . . . . . . . . . . . . 21



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     8.2.  Transport Protocol Specifications  . . . . . . . . . . . . 21
     8.3.  RTSP Transport Parameters  . . . . . . . . . . . . . . . . 21
     8.4.  RTSP Status Codes  . . . . . . . . . . . . . . . . . . . . 22
     8.5.  SDP Attribute  . . . . . . . . . . . . . . . . . . . . . . 22
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
     9.1.  ICE and RTSP . . . . . . . . . . . . . . . . . . . . . . . 22
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     11.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
   Intellectual Property and Copyright Statements . . . . . . . . . . 26







































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1.  Introduction

   Real-time Streaming Protocol (RTSP)
   [RFC2326][I-D.ietf-mmusic-rfc2326bis] is a protocol used to setup and
   control one or more media streams delivering media to receivers.  It
   is RTSP's functionality of setting up media streams that get into
   serious issues with Network Address Translators (NAT) [RFC3022].
   Commonly the media will be totally blocked by the NAT unless extra
   provisions are taken by the protocol.  There is a clear and present
   need for NAT traversal mechanism for the media setup using RTSP.

   RTSP 1.0 [RFC2326] has suffered from the lack of a standardized NAT
   traversal mechanism for a long time, however due to quality of the
   RTSP 1.0 specification, the work has had to wait on the recently
   defined RTSP 2.0 [I-D.ietf-mmusic-rfc2326bis].  RTSP 2.0 is similar
   to RTSP 1.0 in many respects but significantly for this work, it
   contains a well defined extension mechanism so allowing a NAT
   traversal extension to be defined that is backwards compatible with
   RTSP 2.0 peers not supporting the extension.  This extension
   mechanism was not possible in RTSP 1.0 as it would break RTSP 1.0
   syntax so causing compatibility issues.

   There have been a number of suggested ways of resolving the NAT-
   traversal of media for RTSP of which a large number are already used
   in implementations.  The evaluation of these NAT traversal solutions
   in[I-D.ietf-mmusic-rtsp-nat-evaluation] has shown that there are many
   issues to consider, so after extensive evaluation, we selected a
   mechanism based on Interactive Connectivity Establishment (ICE).
   This was mainly two reasons: Firstly the mechanism supports RTSP
   servers behind NATs and secondly the mechanism solves the security
   threat that uses RTSP servers as Distributed Denial of Service (DDoS)
   attack tools.

   The NAT problem for RTSP signalling traffic itself is beyond the
   scope of this document and is left for future study should the need
   arise, because it is a less prevalent problem than the NAT problem
   for RTSP media streams.


2.  Solution Overview

   This overview assumes that the reader has some familiarity with how
   ICE [I-D.ietf-mmusic-ice] works, as it primarily points out how the
   different ICE steps are accomplished in RTSP.

   1.   RTSP server can indicate it has support for ICE via an SDP
        [RFC4566] attribute in, for example, the SDP returned in RTSP
        DESCRIBE message.  This allows RTSP clients to only send the new



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        ICE interchanges with servers that support ICE so as to limit
        the overhead on current non-ICE supporting RTSP servers.  If
        RTSP DESCRIBE is used the normal capability determination
        mechanism can be used, i.e.  "Supported" header and the defined
        feature tag.

   2.   RTSP client reviews the session description returned, for
        example by an RTSP DESCRIBE message, to determine what media
        resources that need to be setup.  For each of these media
        resources where the transport protocol supports Session
        Traversal Utilities for (NAT) (STUN)
        [I-D.ietf-behave-rfc3489bis] based connectivity checks, the
        client gathers candidate addresses.  See section 4.1.1 in
        [I-D.ietf-mmusic-ice].  The client also installs the STUN
        servers on each of the local candidates.

   3.   RTSP client sends a SETUP request with both a transport
        specification with a lower layer indicating ICE and a new RTSP
        Transport header parameter listing the ICE candidates for each
        media resource.  RTSP proxies in non-ICE transport
        specifications should be treated at lower priority than those
        transport specifications supporting ICE.

   4.   After receiving the list of candidates from a client, the RTSP
        server gathers its own candidates.  If the server has a public
        IP address then a single candidate per address family (e.g.
        IPv4 and IPv6) can be included to reduce the number of
        combinations and speed up the completion.

   5.   The server sets up the media and if successful responds to the
        SETUP request with a 200 OK response.  In that response the
        server selects the transport specification using ICE and
        includes its candidates in the server candidate parameter.

   6.   If the server is behind a NAT then it starts the connectivity
        checks following the procedures described in Section 5.7 and 5.8
        of [I-D.ietf-mmusic-ice].  If the server has a public IP address
        with a single candidate then it can refrain from server
        initiated connectivity checks and rely on triggered checks.

   7.   The client receives the SETUP response and learns the candidate
        address to use for the connectivity checks, and then initiates
        its connectivity check, following the procedures in Section 6 of
        [I-D.ietf-mmusic-ice].

   8.   When a connectivity check from the client reaches the server it
        will result in a triggered check from the server.  This is why
        servers with a public IP address can wait until this triggered



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        check to send out any checks for itself so saving resources and
        mitigating the DDoS potential from server connectivity checks.

   9.   When the client has concluded its connectivity checks and has
        corresponding received the server connectivity checks on the
        promoted candidates for all components of all media streams, it
        can issue a PLAY request.  If the connectivity checks have not
        concluded successfully then the client may send a new SETUP
        request assuming it has any new information or believes the
        server may be able to do more that can result in successful
        checks.

   10.  When the RTSP servers receives a PLAY request it checks to see
        the connectivity checks has concluded successfully and only then
        can play the stream.  If there is a problem with the checks then
        the server sends to the client either a 150 (ICE connectivity
        checks in progress) response to show that it is still working on
        the connectivity checks or a 480 (ICE Processing Failed)
        response to indicate a failure of the checks.  If the checks are
        successful then the server sends a 200 OK response and starts
        delivering media.

   The client may release unused candidates when the ICE processing has
   concluded and a single candidate per component has been promoted.

   The client shall continue to use STUN to send keep-alive for the used
   bindings.  This is important as often RTSP media sessions only
   contain media traffic from the server to the client so the bindings
   in the NAT needs to be refreshed by the client to server traffic
   provided by the STUN keep-alive.


3.  RTSP Extensions

   This section defines the necessary RTSP extensions for performing ICE
   with RTSP.  Note that these extensions are based on the SDP
   attributes in the ICE specification unless expressly indicated.

3.1.  ICE Transport Lower Layer

   A new lower layer "D-ICE" for transport specifications is defined.
   This lower layer is datagram clean except that the protocol used must
   be demultiplexiable with STUN messages (see STUN
   [I-D.ietf-behave-rfc3489bis]).  With datagram clean we mean that it
   must be capable of describing the length of the datagram, transport
   that datagram (as a binary chunk of data) and provide it at the
   receiving side as one single item.  This lower layer can be any
   transport type defined for ICE which does provide datagram transport



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   capabilities.  Though only UDP is defined at present, however TCP
   with framing may be specified and used in the future.

   This lower layer uses ICE to determine which of the different
   candidates shall be used and then when the ICE processing has
   concluded, uses the selected candidate to transport the datagrams
   over this transport.

   This lower layer transport can be combined with all upper layer media
   transport protocols that are possible to demultiplex with STUN and
   which use datagrams.  This specification defines the following
   combinations:

   o  RTP/AVP/D-ICE

   o  RTP/AVPF/D-ICE

   o  RTP/SAVP/D-ICE

   o  RTP/SAVPF/D-ICE

   This list can easily be extended with more transport specifications
   after having performed the evaluation that they are compatible with
   D-ICE as lower layer.

   The lower-layer "D-ICE" has the following rules for the inclusion of
   transport parameters:

   unicast:  As ICE only supports unicast operations, thus it is
      REQUIRED that one include the unicast indicator parameter, see
      section 16.46 in [I-D.ietf-mmusic-rfc2326bis].

   candidates:  The "candidates" parameter SHALL be included as this
      specify at least one candidate to try to establish a working
      transport path with.

   dest_addr:  This parameter SHALL NOT be included as "candidates" is
      used instead to provide the necessary address information.

   ICE-Password:  This parameter SHALL be included.

   ICE-Userfrag:  This parameter SHALL be included.

3.2.  ICE Candidate Transport Header Parameter

   This section defines a new RTSP transport parameter for carrying ICE
   candidates related to the transport specification they appear within,
   which may then be validated with an end-to-end connectivity check



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   using STUN [I-D.ietf-behave-rfc3489bis].  Transport parameters may
   only occur once in each transport specification.  For transport
   specification using "D-ICE" as lower layer, this parameter needs to
   be present.  The parameter can contain one or more ICE candidates.
   In the SETUP response there is only a single transport specification,
   and if that uses the "D-ICE" lower layer this parameter also needs to
   present including the server side candidates.

   tr-parameter  =/ SEMI ice-trn-par
   ice-trn-par   = "candidates" EQUAL DQ SWS ice-candidate
                                      *(SEMI ice-candidate) SWS DQ
   ice-candidate = foundation SP
                   component-id SP
                   transport SP
                   priority SP
                   connection-address SP
                   port SP
                   cand-type
                   [SP rel-addr]
                   [SP rel-port]
                   *(SP extension-att-name SP extension-att-value)

   foundation            = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   component-id          = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   transport             = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   transport-extension   = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   priority              = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   cand-type             = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   candidate-types       = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   rel-addr              = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   rel-port              = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   extension-att-name    = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   extension-att-value   = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   ice-char              = <See section 15.1 of [I-D.ietf-mmusic-ice]>
   connection-address    = <See [RFC4566]>
   port                  = <See [RFC4566]>
   EQUAL                 = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
   DQ                    = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
   SWS                   = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
   SEMI                  = <Defined in [I-D.ietf-mmusic-rfc2326bis]>

   <connection-address>: is the IP address of the candidate, allowing
   for IPv4 addresses, IPv6 addresses and Fully qualified domain names
   (FQDN), taken from [RFC4566].  The connection address SHOULD be on
   the same format (explicit IP or FQDN) as in the dest_addr parameter
   used to express default for the matching candidate.  An IP address
   SHOULD be used, but an FQDN MAY be used in place of an IP address.
   In that case, when receiving an offer or answer containing an FQDN in



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   an a=candidate attribute, the FQDN is looked up in the DNS first
   using an AAAA record (assuming the agent supports IPv6), and if no
   result is found or the agent only supports IPv4, using an A. If the
   DNS query returns more than one IP address, one is chosen, and then
   used for the remainder of ICE processing.

   <port>: is the port of the candidate taken from RFC 4566 [RFC4566].

   <transport>: indicates the transport protocol for the candidate.  The
   ICE specification only defines UDP.  However, extensibility is
   provided to allow for future transport protocols to be used with ICE,
   such as TCP or the Datagram Congestion Control Protocol (DCCP)
   [RFC4340].

   <foundation>: is an identifier that is equivalent for two candidates
   that are of the same type, share the same base, and come from the
   same STUN server, and is composed of one to thirty two <ice-char>.
   The foundation is used to optimize ICE performance in the Frozen
   algorithm.

   <component-id>: identifies the specific component of the media stream
   for which this is a candidate and os a positive integer between 1 and
   256.  It MUST start at 1 and MUST increment by 1 for each component
   of a particular candidate.  For media streams based on RTP,
   candidates for the actual RTP media MUST have a component ID of 1,
   and candidates for RTCP MUST have a component ID of 2.  Other types
   of media streams which require multiple components MUST develop
   specifications which define the mapping of components to component
   IDs.  See Section 14 for additional discussion on extending ICE to
   new media streams.

   <priority>: is a positive integer between 1 and (2**31 - 1).

   <cand-type>: encodes the type of candidate.  The ICE specification
   defines the values "host", "srflx", "prflx" and "relay" for host,
   server reflexive, peer reflexive and relayed candidates,
   respectively.  The set of candidate types is extensible for the
   future.

   <rel-addr> and <rel-port>: convey transport addresses related to the
   candidate, useful for diagnostics and other purposes. <rel-addr> and
   <rel-port> MUST be present for server reflexive, peer reflexive and
   relayed candidates.  If a candidate is server or peer reflexive,
   <rel-addr> and <rel-port> is equal to the base for that server or
   peer reflexive candidate.  If the candidate is relayed, <rel-addr>
   and <rel-port> is equal to the mapped address in the Allocate
   Response that provided the client with that relayed candidate (see
   Appendix B.3 for a discussion of its purpose).  If the candidate is a



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   host candidate <rel-addr> and <rel-port> MUST be omitted.

3.3.  ICE Password and Username Transport Header Parameters

   The ICE password and username for each agent needs to be transported
   using RTSP.  For that purpose new transport header parameters are
   defined.

   There MUST be an "ICE-Password" and "ICE-Userfrag" parameter for each
   media stream.  If two SETUP requests in the same RTSP session have
   identical ICE-Userfrag's, they MUST have identical ICE-Password's.
   The ICE-Userfrag and ICE-Password attributes MUST be chosen randomly
   at the beginning of a session.  The ICE-Userfrag attribute MUST
   contain at least 24 bits of randomness, and the ICE-Password
   attribute MUST contain at least 128 bits of randomness.  This means
   that the ICE-Userfrag attribute will be at least 4 characters long,
   and the ICE-Password at least 22 characters long, since the grammar
   for these attributes allows for 6 bits of randomness per character.
   The attributes MAY be longer than 4 and 22 characters respectively,
   of course, up to 256 characters.  The upper limit allows for buffer
   sizing in implementations.  Its large upper limit allows for
   increased amounts of randomness to be added over time.

   The ABNF [RFC5234] for these parameters are:

   tr-parameter     =/ SEMI ice-password-par
   tr-parameter     =/ SEMI ice-userfrag-par
   ice-password-par = ICE-Password" HCOLON password
   ice-userfrag-par = ICE-Userfrag" HCOLON ufrag
   password         = <Defined in [I-D.ietf-mmusic-ice]>
   ufrag            = <Defined in [I-D.ietf-mmusic-ice]>
   HCOLON           = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
   SEMI             = <Defined in [I-D.ietf-mmusic-rfc2326bis]>

3.4.  ICE Feature Tag

   A feature tag is defined for usage in the RTSP capabilities mechanism
   for ICE support for media transport using datagrams: "setup.ice-d-m".
   This feature tag indicates that one support all the mandatory to
   support functions of this specification.  It is applicable to all
   types of RTSP agents; clients, servers and proxies.

   The RTSP client should send the feature tag "setup.ice-d-m" in the
   "Supported" header in all SETUP requests that contain the "D-ICE"
   lower layer transport.






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3.5.  Status Codes

   ICE needs two new RTSP response codes to indicate correctly progress
   and errors.

   +------+----------------------------------------------+-------------+
   | Code | Reason                                       | Method      |
   +------+----------------------------------------------+-------------+
   | 150  | Server still working on ICE connectivity     | PLAY        |
   |      | checks                                       |             |
   | 480  | ICE Connectivity check failure               | PLAY, SETUP |
   +------+----------------------------------------------+-------------+

        Table 1: New Status codes and their usage with RTSP methods

3.5.1.  150 ICE connectivity checks in progress

   The 150 response code indicates that ICE connectivity checks are
   still in progress and haven't concluded.  This response SHALL be sent
   within 200 milliseconds of receiving a PLAY request that currently
   can't be fulfilled because ICE connectivity checks are still running.
   Subsequently, every 3 seconds after the previous sent one, a 150
   reply shall be sent until the ICE connectivity checks conclude either
   successfully or in failure, and a final response for the request can
   be provided.

3.5.2.  480 ICE Processing Failed

   The 480 client error response code is used in cases when the request
   can't be fulfilled due to a failure in the ICE processing, such as
   that all the connectivity checks have timed out.  This error message
   can appear either in response to a SETUP request to indicate that no
   candidate pair can be constructed or to a PLAY request that the
   server's connectivity checks resulted in failure.

3.6.  Server Side SDP Attribute for ICE Support

   If the server supports the media NAT traversal for RTSP controlled
   sessions, as described in this RFC, then the Server SHALL include the
   "a=rtsp-ice-d-m" SDP attribute in any SDP (if used) describing
   content served by the server.  This is an session level attribute.

   rtsp-ice-d-m-attr = "a=" "rtsp-ice-d-m"








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3.7.  ICE Features Not Required in RTSP

   A number of ICE signalling features are not needed with RTSP and are
   discussed below.

3.7.1.  ICE-Lite

   The ICE-Lite attribute shall not be used in the context of RTSP.  The
   ICE specification describes two implementations of ICE: Full and
   Lite, where hosts that are not behind a NAT are allowed to implement
   only Lite.  For RTSP, the Lite implementation is insufficient because
   it does not cause the media server to send a connectivity check,
   which are used to protect against making the RTSP server a denial of
   service tool.  This document defines another variation implementation
   of ICE, called ICE-RTSP.  It has its own set of simplifications
   suitable to RTSP.  Conceptually, this implementation of ICE-RTSP is
   between ICE-FULL and ICE-LITE for a server and simpler than ICE-FULL
   for clients.

3.7.2.  ICE-Mismatch

   The ice-mismatch parameter indicates that the offer arrived with a
   default destination for a media component that didn't have a
   corresponding candidate attribute.  This is not needed for RTSP as
   the ICE based lower layer transport specification either is supported
   or another alternative transport is used.  This is always explicitly
   indicated in the SETUP request and response.

3.7.3.  ICE Remote Candidate Transport Header Parameter

   The Remote candidate attribute is not needed for RTSP for the
   following reasons.  Each SETUP results in a independent ICE
   processing chain which either fails or results in promoting a single
   candidate pair to usage.  If a new SETUP request for the same media
   is sent this needs to use a new userfragment and password to avoid
   any race conditions or uncertainty for which processing round the
   STUN requests relate to.


4.  Detailed Solution

   This section describes in detail how the interaction and flow of ICE
   works with RTSP messages.








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4.1.  Session description and RTSP DESCRIBE (optional)

   The RTSP server should indicate it has support for ICE by sending the
   "rtsp-ice-d-m" SDP attribute in the response to the RTSP DESCRIBE
   message if SDP is used.  This allows RTSP clients to only send the
   new ICE interchanges with servers that support ICE so limiting the
   overhead on current non-ICE supporting RTSP servers.  When not using
   RTSP DESCRIBE it is still recommended to use the SDP attribute for
   session description.

   A Client can also use the DESCRIBE request to determine explicitly if
   both server and any proxies support ICE.  The client includes the
   "Supported" header with its supported feature tags, including
   "setup.ice-d-m".  Any proxy upon seeing the "Supported" header will
   include the "Proxy-Supported" header with the feature tags it
   supports.  The server will echo back the "Proxy-Supported" header and
   its own version of the Supported header so enabling a client to
   determine if all involved parties support ICE or not.  Note that even
   if a proxy is present in the chain that doesn't indicate support for
   ICE, it may still work.































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   For example:
        C->S: DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/2.0
              CSeq: 312
              User-Agent: PhonyClient 1.2
              Accept: application/sdp, application/example
              Supported: setup.ice-d-m

        S->C: RTSP/2.0 200 OK
              CSeq: 312
              Date: 23 Jan 1997 15:35:06 GMT
              Server: PhonyServer 1.1
              Content-Type: application/sdp
              Content-Length: 367
              Supported: setup.ice-d-m

              v=0
              o=mhandley 2890844526 2890842807 IN IP4 192.0.2.46
              s=SDP Seminar
              i=A Seminar on the session description protocol
              u=http://www.example.com/lectures/sdp.ps
              e=seminar@example.com (Seminar Management)
              t=2873397496 2873404696
              a=recvonly
              a=rtsp-ice-d-m
              a=control: *
              m=audio 3456 RTP/AVP 0
              a=control: /audio
              m=video 2232 RTP/AVP 31
              a=control: /video

4.2.  Setting up the Media Resources

   The RTSP client reviews the session description returned, for example
   by an RTSP DESCRIBE message, to determine what media resources that
   need to be setup.  For each of these media resources where the
   transport protocol supports ICE connectivity checks, the client shall
   gather candidate addresses as described in section 4.1.1 in
   [I-D.ietf-mmusic-ice] according to standard ICE rather than the ICE-
   Lite implementation.

4.3.  RTSP SETUP Request

   The RTSP client will then send one or more SETUP requests to
   establish the media streams required for the desired session.  For
   each media stream where it desires to use ICE it will include a
   transport specification with "D-ICE" as the lower layer.  This
   transport specification SHOULD be placed first in the list to give it
   highest priority.  It is RECOMMENDED that additional transport



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   specifications are provided as a fallback in case of non ICE
   supporting proxies.  For example (Note that some lines are broken in
   contradiction with the defined syntax due to space restrictions in
   the documenting format:
   C->S: SETUP rtsp://server.example.com/fizzle/foo/audio RTSP/2.0
         CSeq: 302
         Transport: RTP/AVP/D-ICE; unicast; candidates = "
                    1 1 UDP 2130706431 10.0.1.1 8998 typ host;
                    2 1 UDP 1694498815 192.0.2.3 45664 typ srflx
                            raddr 10.0.1.1 rport 9002",
                    RTP/AVP/UDP; unicast; dest_addr=":6970"/":6971",
                    RTP/AVP/TCP;unicast;interleaved=0-1
         Accept-Ranges: NPT, UTC
         User-Agent: PhonyClient/1.2
         Supported: setup.ice-d-m


   The client will be initiating and thus the controlling party in the
   ICE processing.

4.4.  Gathering Candidates

   Upon receiving a SETUP request the server can determine what media
   resource should be delivered and which transport alternatives that
   the client supports.  If one based on D-ICE is first on the list of
   supported transports, the below applies, otherwise another transport
   method is preferred and supported.

   The transport specification will provide which media protocol is to
   be used and based on this and the clients candidates, the server
   determines the protocol and if it supports ICE with that protocol.
   The server shall then gather its candidates according to section
   4.1.1 in [I-D.ietf-mmusic-ice].  Servers that have an address that is
   generally reachable by any clients within the address scope the
   server intends to serve MAY be specially configured (high-
   reachability configuration).  This special configuration has the goal
   of reducing the server side candidate to preferably a single one per
   address family.  Instead of gathering all possible addresses
   including relayed and server reflexive addresses, the server uses a
   single address per address family that it knows it should be
   reachable by a client behind one or more NATs.  The reason for this
   special configuration is two fold: Firstly it reduces the load on the
   server in address gathering and in ICE processing during the
   connectivity checks.  Secondly it will reduce the number of
   permutations for candidate pairs significantly thus potentially
   speeding up the conclusion of the ICE processing.  Note however that
   using this option on a server that doesn't fulfill the requirement of
   being reachable is counter-productive and it is important that this



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   is correctly configured.

4.5.  RTSP Server Response

   The server determines if the SETUP request is successful from the
   other perspectives and will return a 200 OK response, otherwise
   returning an error code from the list in Table 4 in
   [I-D.ietf-mmusic-rfc2326bis].  At that point the server, having
   selected a transport specification using the "D-ICE" lower layer,
   will need to include that transport specification in the response
   message.  The transport specification shall include the candidates
   gathered in SectionSection 4.4 in the "candidates" transport header
   parameter as well as the server's username and password.  In the case
   that there are no valid candidate pairs with the combination of the
   client and servers candidates, a 480 (ICE Processing Failed) error
   response shall be returned which must include the servers'
   candidates.  The return of a 480 error may allow both the server and
   client to release its candidates.

   S->C: RTSP/2.0 200 OK
         CSeq: 302
         Session: 12345678
         Transport: RTP/AVP/D-ICE; unicast; candidates = "
                    1 1 UDP 2130706431 192.0.2.56 50234 typ host"
         Accept-Ranges: NPT
         Date: 23 Jan 1997 15:35:06 GMT
         Server: PhonyServer 1.1
         Supported: setup.ice-d-m

4.6.  Server to Client ICE Connectivity Checks

   The server shall start the connectivity checks following the
   procedures described in Section 5.7 and 5.8 of [I-D.ietf-mmusic-ice]
   unless it is configured to use the high-reachability option.  If it
   is then it can suppress its own checks until the servers checks are
   triggered by the client's connectivity checks.

   The server SHALL use a single pacer for all STUN transactions within
   a single RTSP session, i.e across all media streams that are part of
   the same RTSP session.

   When a connectivity check from the client reaches the server it will
   result in a triggered check from the server as specified in section
   7.2.1.4 of [I-D.ietf-mmusic-ice].  This is why servers with a high
   reachability address can wait until this triggered check to send out
   any checks for itself so saving resources and mitigating the DDoS
   potential.




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4.7.  Client to Server ICE Connectivity Check

   The client receives the SETUP response and learns the candidate
   address to use for the connectivity checks.  The client shall
   initiate its connectivity check, following the procedures in Section
   6 of [I-D.ietf-mmusic-ice].

   Aggressive nomination SHALL be used with RTSP.  This doesn't have the
   negative impact that it has in offer/answer as media playing only
   starts after issuing a PLAY request.

4.8.  Client Connectivity Checks Complete

   When the client has concluded its connectivity checks and has
   correspondingly received the server connectivity checks on the
   promoted candidates for all the media components, it can issue a PLAY
   request.  If the client has locally determined that its checks have
   failed it may try providing an extended set of candidates and update
   the server candidate list by issuing a new SETUP request for the
   media stream.

   If the client concluded its connectivity checks succesfully and
   therefore sent a PLAY request but the server have not concluded
   successfully, the server will respond with a 480 (ICE Processing
   Failed).  Upon receiving the 480 (ICE Processing Failed) response,
   then the client may send a new SETUP request assuming it has any new
   information that can be included in the candidate list.

4.9.  Server Connectivity Checks Complete

   When the RTSP server receives a PLAY request, it checks to see that
   the connectivity checks have concluded successfully and only then
   will it play the stream.  If there is a problem with the checks then
   the server sends to the client either a new 150 (ICE connectivity
   checks in progress) response to show that it is still working on the
   connectivity checks or a new 480 response to indicate a failure of
   the checks.  If the checks are successful then the server sends a 200
   OK response and starts delivering media.  The new RTSP errors add to
   the list in Table 4 in [I-D.ietf-mmusic-rfc2326bis] as below:

4.10.  Releasing Candidates

   Both server and client may release its non nominated candidates as
   soon as a 200 PLAY response has been issued/received.







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4.11.  Steady State

   The client will continue to use STUN to send keep-alive for the used
   bindings.  This is important as normally RTSP play mode sessions only
   contain traffic from the server to the client so the bindings in the
   NAT needs to be refreshed by the cleint to server traffic provided by
   the STUN keep-alive.

4.12.  re-SETUP

   If the client decides to change any parameter related to the media
   stream SETUP it will send a new SETUP request.  In this new SETUP
   request the client SHALL include a new different username and
   password to use in the ICE processing.  This request will also cause
   the ICE processing to start from the beginning again.

   If the RTSP session is in playing state at the time of sending the
   SETUP request, the ICE connectivity checks SHALL use Regular
   nomination.  Any ongoing media delivery continues on the previously
   nominated candidate pairs until the new pairs have been nominated for
   the individual candidate.  Once the nomination of the new candidate
   pair has completed, all unused candidates may be released.


5.  ICE and Proxies

   RTSP allows for proxies which can be of two fundamental types
   depending if they relay and potentially cache the media or not.
   Their differing impact on the RTSP NAT traversal solution including
   backwards compatibility is explained below.

5.1.  Media Handling Proxies

   An RTSP proxy that relays or caches the media stream for a particular
   media session can be considered to split the media transport into two
   parts: A media transport between the server and the proxy according
   to the proxies need, and delivery from the proxy to the client.  This
   split means that the NAT traversal solution will need to be run on
   each individual media leg according to need.

   It is RECOMMENDED that any media handling proxy support the media NAT
   traversal defined within this specification.  This is for two
   reasons: Firstly to enable clients to perform NAT traversal for the
   media between the proxy and itself and secondly to allow the proxy to
   be topology independent so able to support performing NAT traversal
   for non-NAT traversal capable clients present in the same address
   domain.




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   For a proxy to support the media NAT traversal defined in this
   specification a proxy will need to implement the solution fully and
   be ready as both a controlling and a controlled ICE peer.  The proxy
   also SHALL include the "setup.ice-d-m" feature tag in any applicable
   capability negotiation headers, such as "Proxy-Supported".

5.2.  Signalling Only Proxies

   A signalling only proxy handles only the RTSP signalling and does not
   have the media relayed through proxy functions.  This type of proxy
   is not likely to work unless the media NAT traversal solution is in
   place between the client and the server, because the DoS protection
   measures usually prevent media delivery to other addresses other than
   from where the RTSP signalling arrives at the server.

   The solution for the Signalling Only proxy is that it must forward
   the RTSP SETUP requests including any transport specification with
   the "D-ICE" lower layer and the related transport parameters.  A
   proxy supporting this functionality SHOULD indicate its capability by
   always including the "setup.ice-d-m" feature tag in the "Proxy-
   Supported" header.

5.3.  Non-supporting Proxies

   A media handling proxy that doesn't support the ICE media NAT
   traversal specified here is assumed to remove the transport
   specification and use any of the lower prioritized transport
   specifications if provided by the requester.  The specification of
   such a non ICE transport enables the negotiation to complete,
   although with a less prefered method as a NAT between the proxy and
   the client will result in failure of the media path.

   A non-media handling transport proxy is expected to ignore and simply
   forward all unknown transport specifications, however, this can only
   be guaranteed for proxies following the published RTSP 2.0
   specification.

   Unfortunately the usage of the "setup.ice-d-m" feature tag in the
   proxy-require will have contradicting results.  For a non ICE
   supporting media handling proxy, the inclusion of the feature tag
   will result in aborting the setup and indicating that it isn't
   supported, which is desirable if you want to provide other fallbacks
   or other transport configurations to handle the situation.  For non-
   supporting non-media handling proxies the result will also result in
   aborting the setup, however, setup might have worked if the proxy-
   require tag wasn't present.  This variance in results makes usage of
   proxy-require not recommended.  We recommend instead the usage of the
   Supported header to force proxies to include the feature tags they



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   support in the proxy-supported which will provide a positive
   indication when all proxies in the chain between the client and
   server support the functionality.  Even if not explicitly indicating
   support, any SETUP response including a transport specification with
   "D-ICE" will be implicit indication that the proxy chain supports at
   least passthrough of this media.


6.  RTP and RTCP Multiplexing

   [I-D.ietf-avt-rtp-and-rtcp-mux] specifies how and when RTP and RTCP
   can be multiplexed on the same port.  This multiplexing is highly
   recommended to combine with ICE as it makes RTP and RTCP only need a
   single component per media stream instead of two, so reducing the
   load on the connectivity checks.

   To enable signalling for the usage of RTP and RTCP multiplexing a new
   RTSP transport header parameter is defined.  The formal syntax (ABNF
   [RFC5234]) of this parameter is the following:

   tr-parameter  =/ SEMI rtcp-mux-par
   rtcp-mux-par  = "rtp-rtcp-mux"
   SEMI          = <Defined in [I-D.ietf-mmusic-rfc2326bis]>
   EQUAL         = <Defined in [I-D.ietf-mmusic-rfc2326bis]>

   The "rtp-rtcp-mux" parameter MAY be included in any transport
   specification that use RTP where RTP and RTCP multiplexing is desired
   and indicates in a SETUP request that multiplexing is requested.  If
   the SETUP response also includes the parameter then RTP and RTCP
   multiplexing SHALL be used for that transport specification.  A SETUP
   request may indicate address information for both RTP and RTCP for
   backwards compatibility reasons.  If RTP and RTCP multiplexing is
   used then only the information specified for RTP SHALL be used.

   For capability exchange, an RTSP feature tag for RTP and RTCP
   multiplexing is defined: "setup.rtp-mux".

   RTSP servers and clients that supports "D-ICE" lower layer transport
   in combination with RTP SHALL also implement RTP and RTCP
   multiplexing as specified in this section and
   [I-D.ietf-avt-rtp-and-rtcp-mux].


7.  Open Issues

   Below is listed the known open issues and questions that needs to be
   resolved:




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   1.  Need a descriptive section on how ICE works for RTSP folks.

   2.  No solution has been specified for how RTSP server's can initiate
       a ICE restart.  Either to add candidates or to reinitate the
       connectivity checks in response to lost bindings.  Basically
       required to find a solution for this.

   3.  Does we need to support multiple components?

   4.  Is the role and processing the most optimal one that can be used?


8.  IANA Considerations

   This document request registration in a number of registries, both
   for RTSP and SDP.

8.1.  RTSP Feature Tags

   This document request that two RTSP feature tags are registered in
   the "RTSP feature tag" registry:

   setup.rtp-mux  See Section Section 6.

   setup.ice-d-m  See Section Section 3.4.

8.2.  Transport Protocol Specifications

   This document needs to register a number of transport protocol
   combinations are registered in RTSP's "Transport Protocol
   Specifications" registry.

   "RTP/AVP/D-ICE":

   "RTP/AVPF/D-ICE":

   "RTP/SAVP/D-ICE":

   "RTP/SAVPF/D-ICE":

8.3.  RTSP Transport Parameters

   This document requests that 4 transport parameters are registered in
   RTSP's "Transport Parameters":







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   "candidates":  See Section Section 3.2.

   "ICE-Password":  See Section Section 3.3.

   "ICE-Userfrag":  See Section Section 3.3.

   "rtp-rtcp-mux":  See Section Section 6.

8.4.  RTSP Status Codes

   This document requests that 2 assignments are done in the "RTSP
   Status Codes" registry.  The suggested values are:

   150:  See Section Section 3.5.1.

   480:  See Section Section 3.5.2.

8.5.  SDP Attribute

   The registration of one SDP attribute is requested:
      SDP Attribute ("att-field"):

        Attribute name:     rtsp-ice-d-m
        Long form:          ICE for RTSP datagram media NAT traversal
        Type of name:       att-field
        Type of attribute:  Session level only
        Subject to charset: No
        Purpose:            RFC XXXX
        Reference:          RFC XXXX
        Values:             No values defined.
        Contact:            Magnus Westerlund
                            E-mail: magnus.westerlund@ericsson.com
                            phone: +46 8 404 82 87


9.  Security Considerations

   ICE [I-D.ietf-mmusic-ice] provides an extensive discussion on
   security considerations which applies here as well.

9.1.  ICE and RTSP

   A long-standing risk with transmitting a packet stream over UDP is
   that the host may not be interested in receiving the stream.  On
   today's Internet many hosts are behind NATs or operate host firewalls
   which do not respond to unsolicited packets with an ICMP port
   unreachable error.  Thus, an attacker can construct SDP with a
   victim's IP address and cause a flood of media packets to be sent to



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   a victim.  The addition of ICE, as described in this document,
   provides protection from the attack described above.  By performing
   the ICE connectivity check, the media server receives confirmation
   that the RTSP client wants the media.  While this protection could
   also be implemented by requiring the IP addresses in the SDP match
   the IP address of the RTSP signaling packet, such a mechanism does
   not protect other hosts with the same IP address (such as behind the
   same NAT), and such a mechanism would prohibit separating the RTSP
   controller from the media playout device (e.g., an IP-enabled remote
   control and an IP-enabled television).


10.  Acknowledgements

   The authors would like to thank Remi Denis-Courmont for suggesting
   the method of integrating ICE in RTSP signalling, Dan Wing for help
   with the security section and numerous other issues.


11.  References

11.1.  Normative References

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

   [I-D.ietf-behave-rfc3489bis]
              Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for (NAT) (STUN)",
              draft-ietf-behave-rfc3489bis-15 (work in progress),
              February 2008.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [I-D.ietf-mmusic-rfc2326bis]
              Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, "Real Time Streaming Protocol 2.0
              (RTSP)", draft-ietf-mmusic-rfc2326bis-17 (work in
              progress), February 2008.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate



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              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

11.2.  Informative References

   [I-D.ietf-mmusic-rtsp-nat-evaluation]
              Westerlund, M., "The evaluation of different NAT traversal
              Techniques for media controlled by  Real-time Streaming
              Protocol (RTSP)", draft-ietf-mmusic-rtsp-nat-evaluation-00
              (work in progress), July 2007.

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

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.


Authors' Addresses

   Jeff Goldberg
   Cisco
   11 New Square, Bedfont Lakes
   Feltham,, Middx  TW14 8HA
   United Kingdom

   Phone: +44 20 8824 1000
   Fax:
   Email: jgoldber@cisco.com
   URI:












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   Magnus Westerlund
   Ericsson
   Torshamsgatan 23
   Stockholm,   SE-164 80
   Sweden

   Phone: +46 8 719 0000
   Fax:
   Email: magnus.westerlund@ericsson.com
   URI:


   Thomas Zeng
   Nextwave Wireless, Inc.
   12670 High Bluff Drive
   San Diego, CA  92130
   USA

   Phone: +1 858 480 3100
   Fax:
   Email: thomas.zeng@gmail.com
   URI:





























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Full Copyright Statement

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