Internet Engineering Task Force                             R. Finlayson
Internet-Draft                                       Live Networks, Inc.
Intended status: Informational                          October 18, 2014
Expires: April 21, 2015

    Notifying RTSP Clients and Proxy Servers About Streams: The RTSP
                           "REGISTER" Command


   We define a new RTSP command - named "REGISTER" - that allows a
   server (or a third party) to notify a RTSP client or a RTSP proxy
   server about a RTSP stream (given by a "rtsp://" URL).  This also
   provides a mechanism whereby a client (or proxy server) on the public
   Internet can access a RTSP server that is behind a firewall or NAT.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  An Example  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Optional Transport Parameters . . . . . . . . . . . . . . . .   4
   5.  Comparison with the "ANNOUNCE" command  . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Requirements Language

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

2.  Introduction

   The Real Time Streaming Protocol (RTSP) [RFC2326] is used primarily
   by a client to access a server's multimedia stream - specified by a
   "rtsp://" URL.  The client is assumed to know the stream's URL in
   advance; the RTSP protocol does not specify any mechanism by which a
   client can be informed of the stream's URL.

   This document defines an optional new RTSP command - named "REGISTER"
   - that allows a server (or a third party) to notify a RTSP client or
   a RTSP proxy server about a RTSP stream (given by a "rtsp://" URL).
   (Note that this command is defined only when sent to a RTSP client
   (or a proxy server, which is effectively both a client and a server).
   Unlike most other RTSP commands, it is not defined when sent to a
   (pure) RTSP server.)

   Possible applications of the "REGISTER" command include:

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   Initialization of a proxy server
           A RTSP proxy server acts as both a client (to access a stream
           from a 'back-end' server) and as a server (to serve this
           stream to one or more 'front-end' clients).  The "REGISTER"
           command can be used to inform a proxy server about a stream
           to deliver from a 'back-end' server.

           This is particularly useful if the proxy server is on the
           public Internet, but the 'back-end' server is on a private
           network (behind a NAT), without a public IP address.  In this
           case the private 'back-end' server can use the "REGISTER"
           command to inform the proxy server about one of its streams.
           In addition, optional parameters - described below - allow
           the proxy server to access this stream via the same TCP
           connection that was used to send the "REGISTER" command, and
           perhaps also to deliver RTP and RTCP packets over this same
           TCP connection (using RTP/RTCP-over-TCP encapsulation, i.e.,
           'interleaving').  Note that this basic approach to NAT
           traversal - TCP tunneling - is just one of the many possible
           NAT traversal techniques for RTSP outlined in
           [I-D.ietf-mmusic-rtsp-nat-evaluation].  Other more general
           (but also much more complex) approaches are also possible -
           for example [I-D.ietf-mmusic-rtsp-nat].

   Initialization of a 'pure' client
           The "REGISTER" command can also be used to inform a 'pure'
           RTSP client (i.e., a RTSP client that does not also act as a
           (proxy) server) about a stream to be played.  This is less
           useful than for a proxy server, because for most (pure) RTSP
           clients, the stream to be played is usually entered by a
           human user (e.g., in entered in a GUI dialog box, or on the
           command-line).  However, it might still be useful for systems
           that have many RTSP clients to choose from, and/or wish to
           control the time at which a RTSP client should request a

3.  An Example

   The following protocol exchange illustrates how a 'back-end' media
   server (on a private network) can inform a proxy server (on the
   public Internet) about one of its streams - in a way that allows the
   proxy server (and thus 'front-end' clients on the public Internet) to
   access the stream.

   Server ( -> Client (a proxy server:
       REGISTER rtsp:// RTSP/1.0
       CSeq: 1
       Transport: reuse_connection;

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   Client (a proxy server: -> Server (
       RTSP/1.0 200 OK
       CSeq: 1

   Because of the "reuse_connection" parameter, the proxy server
   ( can then use the existing RTSP (TCP) connection to
   access the 'back-end' stream "rtsp://" using the
   usual RTSP "DESCRIBE", "SETUP" and "PLAY" commands.  In addition,
   because of the "preferred_delivery_protocol=interleaved" parameter,
   the "SETUP" command can request RTP/RTCP-over-TCP streaming, using
   the standard "interleaved=" mechanism ([RFC2326], section 10.12).
   Finally, the "proxy_url_suffix=proxiedCamera" parameter tells the
   proxy server that it can provide access to the stream using the URL:

4.  Optional Transport Parameters

   Three optional parameters (noted in the example above) can be
   specified in the "Transport:" header of a "REGISTER" command:

           This parameter is a flag, with no value.
           If this parameter is present, the receiving client SHOULD
           reuse the TCP connection (on which it received the "REGISTER"
           command) for subsequent commands on the Request-URI.

           Possible values: "udp", "interleaved".  Default value: "udp".
           This parameter specifies the delivery protocol that the
           receiving client SHOULD request when it subsequently sends
           "SETUP" commands for the Request-URI.  The default value,
           "udp", specifies that the client should request normal RTP/
           RTCP-over-UDP streaming.  The value "interleaved" specifies
           that the client should request interleaved RTP/RTCP-over-TCP
           streaming (using the RTSP TCP connection), as described in
           [RFC2326], section 10.12.

           A text string with syntax <path-noscheme> (as defined in
           [RFC3986] or [I-D.ietf-mmusic-rfc2326bis]).
           This parameter is meaningful only if the receiving client is
           a proxy server.  It specifies a suffix that the proxy server
           SHOULD use in the "rtsp://" URL that (front-end) clients use
           to access the stream.  If the proxy server is already

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           supplying a stream with this same URL suffix, then it SHOULD
           return an error (e.g., "451 Invalid parameter") in response
           to the "REGISTER" request.

5.  Comparison with the "ANNOUNCE" command

   RTSP 1.0[RFC2326] defined a command - named "ANNOUNCE" - that could
   be used to notify a server about a new stream.  The "ANNOUNCE"
   command worked by including - as part of the command - a SDP[RFC4566]
   description for the stream.  A subsequent RTSP command would then be
   used to 'inject' media (as RTP/RTCP packets) into the server, which
   would then make this media available to other RTSP clients.
   Unfortunately the RTSP 1.0 standard did not fully specify how this
   would work; in particular, which RTSP command should be used to
   inject media into the server.  Although the "RECORD" command appeared
   to be the most logical choice, most implementations of this mechanism
   used the "PLAY" command instead.  Because this mechanism was not
   fully specified in RTSP 1.0 (and not extensively used), both the
   "ANNOUNCE" and "RECORD" commands were removed from the RTSP 2.0

   Nonetheless, some commercial RTSP servers did implement this
   mechanism, which was useful for serving streams that originate from a
   private network (behind a NAT).  Such a stream could be "ANNOUNCE"d
   and then injected into a RTSP server located on the public Internet.
   However, implementing this mechanism added a lot of complexity to the
   RTSP server, because it needed to be able to process incoming SDP
   descriptions and RTP/RTCP streams, in addition to its usual
   functionality of generating outgoing SDP descriptions and RTP/RTCP
   streams.  In contrast, the "REGISTER" command, described in this
   document, is simpler and much easier to implement, because it
   requires merely adding familiar RTSP client functionality to the
   server - i.e., implementing a RTSP proxy server.

   The "REGISTER" command also simplifies the media source application,
   because it does not have to rely upon having to "ANNOUNCE" and
   'inject' its data into a separate server.  Instead, the media source
   application can simply be a RTSP server of its own.  RTSP clients on
   the same network can access the stream directly (just like any other
   RTSP stream), but if the media source application also wishes to have
   its stream relayed via a separate server (e.g., across a NAT), then
   it can do so simply by adding a small piece of extra code that sends
   a "REGISTER" command to this server.

6.  IANA Considerations

   If this proposal were adopted as an IETF standard, then two IANA
   registries would need to be updated:

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   o  "RTSP Commands".  (This registry is currently defined for
      RTSP 1.0[RFC2326], and proposed for
      RTSP 2.0[I-D.ietf-mmusic-rfc2326bis].)

   o  "Transport Parameters" (This registry was not defined for
      RTSP 1.0, but has been proposed for RTSP 2.0.)

7.  Security Considerations

   The addition of this new command adds no additional threat to
   existing RTSP clients (or servers), because they will simply reject
   any incoming "REGISTER" command (a command that they do not handle)
   with an error code.

   However, for a proxy server, the "REGISTER" command causes it to
   access (and then potentially serve) a new stream, which is more
   powerful functionality than the normal server functionality of
   serving existing streams (by implementing the regular RTSP
   "DESCRIBE", "SETUP", "PLAY" etc. commands).  Therefore any proxy
   server on the public Internet that implements the "REGISTER" command
   SHOULD implement access control for this command, even if it does not
   implement any access control for "DESCRIBE", "SETUP", "PLAY" etc.

8.  References

8.1.  Normative References

              Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, "Real Time Streaming Protocol 2.0
              (RTSP)", draft-ietf-mmusic-rfc2326bis-28 (work in
              progress), October 2011.

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

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

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8.2.  Informative References

              Westerlund, M. and T. Zeng, "The Evaluation of Different
              Network Address Translator (NAT) Traversal Techniques for
              Media Controlled by Real-time Streaming Protocol (RTSP)",
              draft-ietf-mmusic-rtsp-nat-evaluation-13 (work in
              progress), February 2014.

              Goldberg, J., Westerlund, M., and T. Zeng, "A Network
              Address Translator (NAT) Traversal Mechanism for Media
              Controlled by Real-Time Streaming Protocol (RTSP)", draft-
              ietf-mmusic-rtsp-nat-20 (work in progress), February 2014.

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

Author's Address

   Ross Finlayson
   Live Networks, Inc.
   650 Castro St., suite 120-196
   Mountain View, CA  94041


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