Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection-Oriented Transport
draft-ietf-avt-rtp-framing-contrans-06

INTERNET-DRAFT                                              John Lazzaro
September 5, 2005                                            CS Division
Expires: March 5, 2006                                       UC Berkeley

    Framing RTP and RTCP Packets over Connection-Oriented Transport

              <draft-ietf-avt-rtp-framing-contrans-06.txt>

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Copyright (C) The Internet Society (2005).  All Rights Reserved.

                                Abstract

     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 session
     descriptions may specify RTP streams that use the framing method.

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                            Table of Contents

1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . .   2
2. The Framing Method  . . . . . . . . . . . . . . . . . . . . . . .   3
3. Packet Stream Properties  . . . . . . . . . . . . . . . . . . . .   3
4. Session Descriptions for RTP/AVP over TCP . . . . . . . . . . . .   4
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
6. Congestion Control  . . . . . . . . . . . . . . . . . . . . . . .   7
7. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
8. Security Considerations . . . . . . . . . . . . . . . . . . . . .   7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .   8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1 Normative References . . . . . . . . . . . . . . . . . . .   8
Authors' Address . . . . . . . . . . . . . . . . . . . . . . . . . .   9
Intellectual Property Rights Statement . . . . . . . . . . . . . . .   9
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . .   9
Change Log for <draft-ietf-avt-rtp-framing-contrans-06.txt>  . . . .  11

1.  Introduction

The Audio/Video Profile (AVP, [RTP3550]) for the Real-Time Protocol
(RTP, [RFC3551]) 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 framing method that was defined by earlier
versions of RTP/AVP.  In addition, we introduce a mechanism for a
session description [SDP] to signal the use of the framing method.  Note
that session description signalling for the framing method is new, and
was not defined in earlier versions of RTP/AVP.

1.1 Terminology

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 BCP 14, RFC 2119
[RFC2119].

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2.  The Framing Method

Figure 1 defines the framing method.

 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).  See
Appendix A.1 of [RTP3550] for additional guidance.

3.  Packet Stream Properties

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

   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
      connection.

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

In one respect, the framing protocol does indeed specify a property
above the level of a single packet.  If a direction of a connection
carries RTP packets, the streams carried in this direction MUST support
the use of multiple SSRCs in those RTP packets.  If a direction of a
connection carries RTCP packets, the streams carried in this direction
MUST support the use of multiple SSRCs in those RTCP packets.

4.  Session Descriptions for RTP/AVP over TCP

Session management protocols that use the Session Description Protocol
[SDP] in conjunction with the Offer/Answer Protocol [RFC3264] MUST use
the methods described in [COMEDIA] to set up RTP/AVP streams over TCP.
In this case, the use of Offer/Answer is REQUIRED, as the setup methods
described in [COMEDIA] rely on Offer/Answer.

In principle, [COMEDIA] is capable of setting up RTP sessions for any
RTP profile.  In practice, each profile has unique issues that must be
considered when applying [COMEDIA] to set up streams for the profile.

In this memo, we restrict our focus to the Audio/Video Profile (AVP,
[RFC3551]).  Below, we define a token value ("TCP/RTP/AVP") that signals
the use of RTP/AVP in a TCP session.  We also define the operational
procedures that a TCP/RTP/AVP stream MUST follow.

We expect that other standards-track memos will appear to support the
use of the framing method with other RTP profiles.  The support memo for
a new profile MUST define a token value for the profile, using the style
we used for AVP.  Thus, for profile xyz, the token value MUST be

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"TCP/RTP/xyz".  The memo SHOULD adopt the operational procedures we
define below for AVP, unless these procedures are in some way
incompatible with the profile.

The remainder of this section describes how to setup and use an AVP
stream in a TCP session.  Figure 2 shows the syntax of a media (m=) line
[SDP] of a session description:

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

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

The <proto> token value "TCP/RTP/AVP" specifies an RTP/AVP [RTP3550]
[RFC3551] stream that uses the framing method over TCP.

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 [COMEDIA].

The TCP <port> on the media line carries RTP packets.  If a media stream
uses RTCP, a second connection carries RTCP packets.  The port for the
RTCP connection is chosen using the algorithms defined in [SDP] or by
the mechanism defined in [RFC3605].

The TCP connections MAY carry bi-directional traffic, following the
semantics defined in [COMEDIA].  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.

As noted in [RTP3550], the use of RTP without RTCP is strongly
discouraged.  However, if a sender does not wish to send RTCP packets in
a media session, the sender MUST add the lines "b=RS:0" AND "b=RR:0" to
the media description (from [RFC3556]).

If the session descriptions of the offer AND the answer both contain the
"b=RS:0" AND "b=RR:0" lines, a TCP flow for the media session MUST NOT
be created by either endpoint in the session.  In all other cases,
endpoints MUST establish two TCP connections for an RTP AVP stream, one
for RTP and one for RTCP.

As described in [RFC3264], the use of the "sendonly" or "sendrecv"

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attribute in an offer (or answer) indicates that the offerer (or
answerer) intends to send RTP packets on the RTP TCP connection.  The
use of the "recvonly" or "sendrecv" attributes in an offer (or answer)
indicates that the offerer (or answerer) wishes to receive RTP packets
on the RTP TCP connection.

5.  Example

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

v=0
o=first 2520644554 2838152170 IN IP4 first.example.net
s=Example
t=0 0
c=IN IP4 192.0.2.105
m=audio 9 TCP/RTP/AVP 11
a=setup:active
a=connection:new

       Figure 3 -- TCP session description for first participant.

v=0
o=second 2520644554 2838152170 IN IP4 second.example.net
s=Example
t=0 0
c=IN IP4 192.0.2.94
m=audio 16112 TCP/RTP/AVP 10 11
a=setup:passive
a=connection:new

       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
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
[COMEDIA].

The first party connects to the network address (192.0.2.94) and port

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(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 [SDP]) 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 connection.

6.  Congestion Control

The RTP congestion control requirements are defined in [RTP3550].  As
noted in [RTP3550], 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 [RFC3551].  The basic congestion control
requirement defined in [RFC3551] 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.

7.  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.

8.  Security Considerations

Implementors should carefully read the Security Considerations sections
of the RTP [RTP3550] and RTP/AVP [RFC3551] documents, as most of the
issues discussed in these sections directly apply to RTP streams framed
over TCP.

Session descriptions that specify connection-oriented media sessions
(such as the example session shown in Figures 3-4 of Section 5) raise
unique security concerns for streaming media.  The Security
Considerations section of [COMEDIA] describes these issues in detail.

Below, we discuss security issues that are unique to the framing method
defined in Section 2.

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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.  Thus, an
implementation needs to handle packets of any length, from a NULL packet
(LENGTH == 0) to the maximum-length packet holding 64K octets (LENGTH =
0xFFFF).

9.  IANA Considerations

[SDP] 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 a new token value for the <proto> field of media
lines: "TCP/RTP/AVP".  Section 4 specifies the semantics associated with
the <proto> field token, and Section 5 shows an example of its use in a
session description.

10.  References

10.1 Normative References

[RTP3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson.
"RTP: A transport protocol for real-time applications", RFC 3550, July
2003.

[RFC3551] Schulzrinne, H., and S. Casner.  "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3551, July 2003.

[COMEDIA] Yon, D. and G. Camarillo.  Connection-Oriented Media
Transport in the Session Description Protocol (SDP),
draft-ietf-mmusic-sdp-comedia-10.txt.

[SDP] Handley, M., Jacobson, V., and C. Perkins.  "SDP: Session
Description Protocol", draft-ietf-mmusic-sdp-new-25.txt.

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

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

[RFC3605] C. Huitema.  "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.

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[RFC3556] S. Casner.  "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, July
2003.

Authors' Address

John Lazzaro
UC Berkeley
CS Division
315 Soda Hall
Berkeley CA 94720-1776
Email: lazzaro@cs.berkeley.edu

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Change Log for <draft-ietf-avt-rtp-framing-contrans-06.txt>

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

Changes were made in response to Last Call comments from the General
Area review team:

[1] Format of references were changed from [1] style to [RFC2119] style.

[2] Uses of capitalization for emphasis (such as the DOES in the last
paragraph of Section 3) were deleted, to avoid confusion with RFC 2119
uses of capitalization.

[3] In Section 1, we clarify which parts of the memo were originally
specified in earlier versions of RTP/AVP, and which are new:

  In this memo, we document the framing method that was defined by
  earlier versions of RTP/AVP.  In addition, we introduce a mechanism
  for a session description [SDP] to signal the use of the framing
  method.  Note that session description signalling for the framing
  method is new, and was not defined in earlier versions of RTP/AVP.

[4] Updated boilerplate.

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