Internet DRAFT - draft-lazzaro-avt-rtp-framing-contrans


INTERNET-DRAFT                                              John Lazzaro
October 20, 2003                                             CS Division
Expires: April 20, 2004                                      UC Berkeley

    Framing RTP and RTCP Packets over Connection-Oriented Transport


Status of this Memo

This document is an Internet-Draft and is subject to all provisions of
Section 10 of RFC2026.

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Copyright Notice

Copyright (C) The Internet Society (2003).  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 and TLS).  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 or TLS  . . . . . . . .   4
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
A. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
B. Security Considerations . . . . . . . . . . . . . . . . . . . . .   6
C. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .   6
D. References  . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     D.1 Normative References  . . . . . . . . . . . . . . . . . . .   7
E. Author Address  . . . . . . . . . . . . . . . . . . . . . . . . .   7
F. Intellectual Property Rights Statement  . . . . . . . . . . . . .   7
G. Full Copyright Statement  . . . . . . . . . . . . . . . . . . . .   8

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
and TLS).  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 or TLS

[3] defines how to specify connection-oriented media streams in session
descriptions.  In this section, we show how to use [3] with the framing

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]).

[4] defines "TCP" as the <proto> token that specifies TCP transport, and
[3] defines "TLS" as the <proto> token that specifies TLS transport.  We
now define how to declare that an RTP/AVP stream that uses the framing
method appears on the TCP or TLS connection.

At least two <fmt> tokens MUST follow <proto>.  The first <fmt> token
MUST be "RTP/AVP".  Subsequent <fmt> tokens MUST be unique unsigned
integers in the range 0 to 127, that specify an RTP payload type
associated with the stream.

The TCP or TLS <port> on the media line exclusively receives RTP
packets.  If a media stream uses RTCP, a second connection exclusively

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receives the RTCP packets.  The port for the RTCP connection is chosen
using the algorithms defined in [4] and in related documents.

The TCP or TLS 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
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

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type 10) or stereo L16 streams.

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

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

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

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.

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.

[4] defines "TCP" as a token value for the <proto> field of media lines,
and [3] defines "TLS" as a token value for the <proto> field of media
lines.  [3] and [4] permit other memos to define tokens for the <fmt>
fields that follow "TCP" or "TLS" on a media line.

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This memo defines <fmt> tokens for use the "TCP" and "TLS" tokens.  At
least two <fmt> tokens MUST follow <proto>.  The first <fmt> token MUST
be "RTP/AVP".  Subsequent <fmt> tokens MUST be unique unsigned integers
in the range 0-127.  Section 4 of this memo specifies the semantics
associated with the <fmt> tokens.

D.  References

D.1 Normative References

[1] Schulzrinne, H., and S. Casner.  "RTP Profile for Audio and Video
Conferences with Minimal Control", work in progress,

[2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson.
"RTP: A transport protocol for real-time applications", work in
progress, draft-ietf-avt-rtp-new-12.txt.

[3] Yon, D.  "Connection-Oriented Media Transport in SDP", work in
progress, draft-ietf-mmusic-sdp-comedia-05.txt.

[4] Handley, M., Jacobson, V., and C. Perkins.  "SDP: Session
Description Protocol", work in progress,

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

E.  Author 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 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
might not be available; neither does it represent that it has made any
effort to identify any such rights.  Information on the IETF's

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procedures with respect to rights in standards-track and standards-
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G.  Full Copyright Statement

Copyright (C) The Internet Society (2002-2003).  All Rights Reserved.

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This document and the information contained herein is provided on an "AS


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Internet Society.

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