INTERNET-DRAFT                                              John Lazzaro
June 24, 2004                                                CS Division
Expires: December 24, 2004                                   UC Berkeley

    Framing RTP and RTCP Packets over Connection-Oriented Transport


Status of this Memo

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Copyright Notice

Copyright (C) The Internet Society (2004).  All Rights Reserved.


     This memo defines a method for framing Real Time Protocol (RTP) and
     Real Time Control Protocol (RTCP) packets onto connection-oriented
     transport (such as TCP).  The memo also defines how to specify the
     framing method in a session description.

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                            Table of Contents

1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . .   2
2. The Framing Method  . . . . . . . . . . . . . . . . . . . . . . .   2
3. Undefined Properties  . . . . . . . . . . . . . . . . . . . . . .   3
4. Session Descriptions for RTP/AVP over TCP . . . . . . . . . . . .   4
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
6. Congestion Control  . . . . . . . . . . . . . . . . . . . . . . .   6
A. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
B. Security Considerations . . . . . . . . . . . . . . . . . . . . .   6
C. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .   7
D. References  . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     D.1 Normative References  . . . . . . . . . . . . . . . . . . .   7
E. Authors' Address  . . . . . . . . . . . . . . . . . . . . . . . .   8
F. Intellectual Property Rights Statement  . . . . . . . . . . . . .   8
G. Full Copyright Statement  . . . . . . . . . . . . . . . . . . . .   8
H. Change Log for <draft-ietf-avt-rtp-framing-contrans-02.txt> . . .  10

1.  Introduction

The Audio/Video Profile (AVP, [1]) for the Real-Time Protocol (RTP, [2])
does not define a method for framing RTP and Real Time Control Protocol
(RTCP) packets onto connection-oriented transport protocols (such as
TCP).  However, earlier versions of RTP/AVP did define a framing method,
and this method is in use in several implementations.

In this memo, we document the method and show how a session description
[4] may specify the use of the method.

1.1 Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in BCP 14, RFC 2119 [11].

2.  The Framing Method

Figure 1 defines the framing method.

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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
|             LENGTH            |  RTP or RTCP packet ...       |

     Figure 1 -- The bitfield definition of the framing method.

A 16-bit unsigned integer LENGTH field, coded in network byte order
(big-endian), begins the frame.  If LENGTH is non-zero, an RTP or RTCP
packet follows the LENGTH field.  The value coded in the LENGTH field
MUST equal the number of octets in the RTP or RTCP packet.  Zero is a
valid value for LENGTH, and codes the null packet.

This framing method does not use frame markers (i.e. an octet of
constant value that would precede the LENGTH field).  Frame markers are
useful for detecting errors in the LENGTH field.  In lieu of a frame
marker, receivers SHOULD monitor the RTP and RTCP header fields whose
values are predictable (for example, the RTP version number).

3.  Undefined Properties

The framing method does not specify properties above the level of a
single packet.  In particular, Section 2 does not specify:

   The number of RTP or RTCP streams on the connection.

      The framing method is commonly used for sending a single
      RTP or RTCP stream over a connection.  However, Section
      2 does not define this common use as normative, so that
      (for example) a memo that defines an RTP SSRC multiplexing
      protocol may use the framing method.

   Bi-directional issues.

      Section 2 defines a framing method for use in one direction
      on a connection.  The relationship between framed packets
      flowing in defined direction and in the reverse direction is
      not specified.

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   Packet loss and reordering.

      The reliable nature of a connection does not imply that a
      framed RTP stream has a contiguous sequence number ordering.
      For example, if the connection is used to tunnel a UDP stream
      through a network middlebox that only passes TCP, the sequence
      numbers in the framed stream reflect any packet loss or
      reordering on the UDP portion of the end-to-end flow.

   Out-of-band semantics.

      Section 2 does not define the RTP or RTCP semantics for closing
      a TCP socket, or of any other "out of band" signal for the

Memos that normatively include the framing method MAY specify these
properties.  For example, Section 4 of this memo specifies these
properties for RTP sessions specified in session descriptions.

4.  Session Descriptions for RTP/AVP over TCP

In this section, we show how session descriptions may specify RTP
streams that use the framing method.

Figure 2 shows the syntax of a media (m=) line [4] of a session

      "m=" media SP port ["/" integer] SP proto 1*(SP fmt) CRLF

       Figure 2 -- Syntax for an SDP media (m=) line (from [4]).

To specify an RTP/AVP [1] [2] stream that uses the framing method over
TCP, the <proto> token MUST be set to "TCP/RTP/AVP".  To specify a
Secure Real Time Protocol [6] stream that uses the framing method over
TCP, the <proto> token MUST be set to "TCP/RTP/SAVP".

The <fmt> tokens that follow <proto> MUST be unique unsigned integers in
the range 0 to 127.  The <fmt> tokens specify an RTP payload type
associated with the stream.

In all other respects, the session description syntax for the framing
method is identical to [3].

The TCP <port> on the media line exclusively receives RTP packets.  If a

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media stream uses RTCP, a second connection exclusively receives the
RTCP packets.  The port for the RTCP connection is chosen using the
algorithms defined in [4] and in related documents.

The TCP connections MAY carry bi-directional traffic, following the
semantics defined in [3].  Both directions of a connection MUST carry
the same type of packets (RTP or RTCP).  The packets MUST exclusively
code the RTP or RTCP streams specified on the media line(s) associated
with the connection.

The RTP stream MUST have an unbroken sequence number order.  RTCP stream
packets MUST appear as defined in [2], with no lost or re-ordered
packets.  IETF standards-track documents MAY loosen these restrictions
on packet loss and packet ordering.

The out-of-band semantics for the connection MUST comply with [3].

5.  Example

The session descriptions in Figure 3-4 define a TCP RTP/AVT session.

o=first 2520644554 2838152170 IN IP4
t=0 0
c=IN IP4
m=audio 9 TCP/RTP/AVP 11

       Figure 3 -- TCP session description for first participant.

o=second 2520644554 2838152170 IN IP4
t=0 0
c=IN IP4
m=audio 16112 TCP/RTP/AVP 10 11

       Figure 4 -- TCP session description for second participant.

The session descriptions define two parties that participate in a

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connection-oriented RTP/AVP session.  The first party (Figure 3) is
capable of receiving stereo L16 streams (static payload type 11).  The
second party (Figure 4) is capable of receiving mono (static payload
type 10) or stereo L16 streams.

The "setup" attribute in Figure 3 specifies that the first party is
"active" and initiates connections, and the "setup" attribute in Figure
4 specifies that the second party is "passive" and accepts connections

The first party connects to the network address ( and port
(16112) of the second party.  Once the connection is established, it is
used bi-directionally: the first party sends framed RTP packets to the
second party on one direction of the connection, and the second party
sends framed RTP packets to the first party in the other direction of
the connection.

The first party also initiates an RTCP TCP connection to port 16113
(16112 + 1, as defined in [4]) of the second party.  Once the connection
is established, the first party sends framed RTCP packets to the second
party on one direction of the connection, and the second party sends
framed RTCP packets to the first party in the other direction of the

6.  Congestion Control

The RTP congestion control requirements are defined in [1].  As noted in
[1], all transport protocols used on the Internet need to address
congestion control in some way, and RTP is not an exception.

In addition, the congestion control requirements for the Audio/Video
Profile are defined in [2].  The basic congestion control requirement
defined in [2] is that RTP sessions should compete fairly with TCP flows
that share the network.  As the framing method uses TCP, it competes
fairly with other TCP flows by definition.

A.  Acknowledgements

This memo, in part, documents discussions on the AVT mailing list about
TCP and RTP.  Thanks to all of the participants in these discussions.

B.  Security Considerations

Implementors should carefully read the Security Considerations sections
of the RTP [1] and RTP/AVP [2] documents, as most of the issues

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discussed in these sections directly apply to RTP streams framed over
TCP.  Implementors should also review the Secure Real-time Transport
Protocol (SRTP, [6]), an RTP profile that addresses the security issues
discussed in [1] [2].

Below, we discuss security issues that are unique to the framing method.

Attackers may send framed packets with large LENGTH values, to exploit
security holes in applications.  For example, a C implementation may
declare a 1500-byte array as a stack variable, and use LENGTH as the
bound on the loop that reads the framed packet into the array.  This
code would work fine for friendly applications that use Etherframe-sized
RTP packets, but may be open to exploit by an attacker.

In addition to security issues related to RTP packet transport, there
are also security issues that concern the session descriptions of
connection-oriented media sessions.  The security considerations section
of [3] describe these issues in detail.

C.  IANA Considerations

[4] defines the syntax of session description media lines.  We reproduce
this definition in Figure 2 of Section 4 of this memo.  In Section 4, we
define two new token values for the <proto> field of media lines:
"TCP/RTP/AVP" and "TCP/RTP/SAVP".  Section 4 specifies the semantics
associated with the <proto> field tokens, and Section 5 shows an example
of its use in a session description.

D.  References

D.1 Normative References

[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson.
"RTP: A transport protocol for real-time applications", RFC 3550, July

[2] Schulzrinne, H., and S. Casner.  "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 3551, July 2003.

[3] Yon, D. and G. Camarillo.  Connection-Oriented Media Transport in
the Session Description Protocol (SDP),

[4] Handley, M., Jacobson, V., and C. Perkins.  "SDP: Session
Description Protocol", draft-ietf-mmusic-sdp-new-18.txt.

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

[6] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman.
"The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.

E.  Authors' Address

John Lazzaro
UC Berkeley
CS Division
315 Soda Hall
Berkeley CA 94720-1776

F.  Intellectual Property Rights Statement

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
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procedures with respect to rights in RFC documents can be found in BCP
78 and BCP 79.

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The IETF invites any interested party to bring to its attention any
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standard.  Please address the information to the IETF at ietf-

G.  Full Copyright Statement

Copyright (C) The Internet Society (2004).  This document is subject to
the rights, licenses and restrictions contained in BCP 78, and except as
set forth therein, the authors retain all their rights.

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This document and the information contained herein are provided


Funding for the RFC Editor function is currently provided by the
Internet Society.

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H.  Change Log for <draft-ietf-avt-rtp-framing-contrans-02.txt>

[Note to RFC Editors: this Appendix, and its Table of Contents listing,
should be removed from the final version of the memo]

Section 4-5 and Appendix C have been rewritten to conform to the changes
in draft-ietf-mmusic-sdp-comedia-07.txt.  Added a Congestion Control
section, and rewrote Security Considerations.

The "Status of this Memo", "Intellectual Property Rights Statement", and
"Full Copyright" now use the RFC 3667/3668 conventions.  The document
passes idnits, modulo bugs in the script.

The major unresolved issue concerns the inclusion of "TCP/RTP/SAVP" as a
<proto> token to support SRTP.  We do this in Section 4.  Does this
inclusion bring up the same sort of controversies that resulted in TLS's
removal from draft-ietf-mmusic-sdp-comedia-07.txt?  If so, should we
face into those controversies, or should we remove "TCP/RTP/SAVP" from
this document?  Comments welcome.

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