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rfc:rfc4571

Network Working Group J. Lazzaro Request for Comments: 4571 UC Berkeley Category: Standards Track July 2006

             Framing Real-time Transport Protocol (RTP)
              and RTP Control Protocol (RTCP) Packets
                over Connection-Oriented Transport

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This memo defines a method for framing Real-time Transport Protocol
 (RTP) and RTP 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.

Table of Contents

 1. Introduction ....................................................2
    1.1. Terminology ................................................2
 2. The Framing Method ..............................................2
 3. Packet Stream Properties ........................................3
 4. Session Descriptions for RTP/AVP over TCP .......................3
 5. Example .........................................................5
 6. Congestion Control ..............................................6
 7. Acknowledgements ................................................6
 8. Security Considerations .........................................6
 9. IANA Considerations .............................................7
 10. Normative References ...........................................7

Lazzaro Standards Track [Page 1] RFC 4571 RTP & RTCP over Connection-Oriented Transport July 2006

1. Introduction

 The Audio/Video Profile (AVP, [RFC3550]) for the Real-time Transport
 Protocol (RTP, [RFC3551]) does not define a method for framing RTP
 and RTP 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].

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 bit field 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 it 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 [RFC3550] for additional guidance.

Lazzaro Standards Track [Page 2] RFC 4571 RTP & RTCP over Connection-Oriented Transport July 2006

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 the following:
 Bi-directional issues
    Section 2 defines a framing method for use in one direction on a
    connection.  The relationship between framed packets flowing in a
    defined direction and in the reverse direction is not specified.
 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 synchronization sources (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.

Lazzaro Standards Track [Page 3] RFC 4571 RTP & RTCP over Connection-Oriented Transport July 2006

 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 "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 [RFC3550]
 [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.

Lazzaro Standards Track [Page 4] RFC 4571 RTP & RTCP over Connection-Oriented Transport July 2006

 As noted in [RFC3550], 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, an RTCP 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"
 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 Figures 3 and 4 define a TCP RTP/AVP
 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 the 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 the 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).

Lazzaro Standards Track [Page 5] RFC 4571 RTP & RTCP over Connection-Oriented Transport July 2006

 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
 (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 in 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 in 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 [RFC3550].  As
 noted in [RFC3550], 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 [RFC3550] and RTP/AVP [RFC3551] documents, as
 most of the issues discussed in these sections directly apply to RTP
 streams framed over TCP.

Lazzaro Standards Track [Page 6] RFC 4571 RTP & RTCP over Connection-Oriented Transport July 2006

 Session descriptions that specify connection-oriented media sessions
 (such as the example session shown in Figures 3 and 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.
 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. Normative References

 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
           Jacobson, "RTP: A Transport Protocol for Real-Time
           Applications", STD 64, RFC 3550, July 2003.
 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
           Video Conferences with Minimal Control", STD 65, RFC 3551,
           July 2003.
 [COMEDIA] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
           Session Description Protocol (SDP)", RFC 4145, September
           2005.
 [SDP]     Handley, M., Jacobson, V., and C. Perkins.  "SDP: Session
           Description Protocol", RFC 4566, July 2006.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

Lazzaro Standards Track [Page 7] RFC 4571 RTP & RTCP over Connection-Oriented Transport July 2006

 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
           with Session Description Protocol (SDP)", RFC 3264, June
           2002.
 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
           in Session Description Protocol (SDP)", RFC 3605, October
           2003.
 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
           Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC
           3556, July 2003.

Author's Address

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

Lazzaro Standards Track [Page 8] RFC 4571 RTP & RTCP over Connection-Oriented Transport July 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 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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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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 Administrative Support Activity (IASA).

Lazzaro Standards Track [Page 9]

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