draft-ietf-avt-rtp-framing-contrans-04.txt   draft-ietf-avt-rtp-framing-contrans-05.txt 
INTERNET-DRAFT John Lazzaro INTERNET-DRAFT John Lazzaro
December 9, 2004 CS Division January 7, 2004 CS Division
Expires: June 9, 2005 UC Berkeley Expires: July 7, 2005 UC Berkeley
Framing RTP and RTCP Packets over Connection-Oriented Transport Framing RTP and RTCP Packets over Connection-Oriented Transport
<draft-ietf-avt-rtp-framing-contrans-04.txt> <draft-ietf-avt-rtp-framing-contrans-05.txt>
Status of this Memo Status of this Memo
By submitting this Internet-Draft, I certify that any applicable patent By submitting this Internet-Draft, I certify that any applicable patent
or other IPR claims of which I am aware have been disclosed, and any of or other IPR claims of which I am aware have been disclosed, and any of
which I become aware will be disclosed, in accordance with RFC 3668. which I become aware will be disclosed, in accordance with RFC 3668.
By submitting this Internet-Draft, I accept the provisions of Section 3 By submitting this Internet-Draft, I accept the provisions of Section 3
of RFC 3667 (BCP 78). of RFC 3667 (BCP 78).
skipping to change at page 1, line 35 skipping to change at page 1, line 35
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
This memo defines a method for framing Real Time Protocol (RTP) and This memo defines a method for framing Real Time Protocol (RTP) and
Real Time Control Protocol (RTCP) packets onto connection-oriented Real Time Control Protocol (RTCP) packets onto connection-oriented
transport (such as TCP). The memo also defines how to specify the transport (such as TCP). The memo also defines how session
framing method in a session description. descriptions may specify RTP streams that use the framing method.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Framing Method . . . . . . . . . . . . . . . . . . . . . . . 3 2. The Framing Method . . . . . . . . . . . . . . . . . . . . . . . 3
3. Undefined Properties . . . . . . . . . . . . . . . . . . . . . . 3 3. Packet Stream Properties . . . . . . . . . . . . . . . . . . . . 3
4. Session Descriptions for RTP/AVP over TCP . . . . . . . . . . . . 4 4. Session Descriptions for RTP over TCP . . . . . . . . . . . . . . 4
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Congestion Control . . . . . . . . . . . . . . . . . . . . . . . 6 6. Congestion Control . . . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . . . 6 8. Security Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 7 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1 Normative References . . . . . . . . . . . . . . . . . . . 7 10.1 Normative References . . . . . . . . . . . . . . . . . . . 8
Authors' Address . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Address . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property Rights Statement . . . . . . . . . . . . . . . 8 Intellectual Property Rights Statement . . . . . . . . . . . . . . . 9
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . . 9 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . . 10
Change Log for <draft-ietf-avt-rtp-framing-contrans-04.txt> . . . . 10 Change Log for <draft-ietf-avt-rtp-framing-contrans-05.txt> . . . . 11
1. Introduction 1. Introduction
The Audio/Video Profile (AVP, [1]) for the Real-Time Protocol (RTP, [2]) 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 does not define a method for framing RTP and Real Time Control Protocol
(RTCP) packets onto connection-oriented transport protocols (such as (RTCP) packets onto connection-oriented transport protocols (such as
TCP). However, earlier versions of RTP/AVP did define a framing method, TCP). However, earlier versions of RTP/AVP did define a framing method,
and this method is in use in several implementations. and this method is in use in several implementations.
In this memo, we document the method and show how a session description In this memo, we document the method and show how a session description
skipping to change at page 3, line 27 skipping to change at page 3, line 27
A 16-bit unsigned integer LENGTH field, coded in network byte order 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 (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 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 MUST equal the number of octets in the RTP or RTCP packet. Zero is a
valid value for LENGTH, and codes the null packet. valid value for LENGTH, and codes the null packet.
This framing method does not use frame markers (i.e. an octet of This framing method does not use frame markers (i.e. an octet of
constant value that would precede the LENGTH field). Frame markers are constant value that would precede the LENGTH field). Frame markers are
useful for detecting errors in the LENGTH field. In lieu of a frame useful for detecting errors in the LENGTH field. In lieu of a frame
marker, receivers SHOULD monitor the RTP and RTCP header fields whose marker, receivers SHOULD monitor the RTP and RTCP header fields whose
values are predictable (for example, the RTP version number). values are predictable (for example, the RTP version number). See
Appendix A.1 of [1] for additional guidance.
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. 3. Packet Stream Properties
The framing method is commonly used for sending a single In most respects, the framing method does not specify properties above
RTP or RTCP stream over a connection. However, Section the level of a single packet. In particular, Section 2 does not
2 does not define this common use as normative, so that specify:
(for example) a memo that defines an RTP SSRC multiplexing
protocol may use the framing method.
Bi-directional issues. Bi-directional issues.
Section 2 defines a framing method for use in one direction Section 2 defines a framing method for use in one direction
on a connection. The relationship between framed packets on a connection. The relationship between framed packets
flowing in defined direction and in the reverse direction is flowing in defined direction and in the reverse direction is
not specified. not specified.
Packet loss and reordering. Packet loss and reordering.
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reordering on the UDP portion of the end-to-end flow. reordering on the UDP portion of the end-to-end flow.
Out-of-band semantics. Out-of-band semantics.
Section 2 does not define the RTP or RTCP semantics for closing 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 a TCP socket, or of any other "out of band" signal for the
connection. connection.
Memos that normatively include the framing method MAY specify these Memos that normatively include the framing method MAY specify these
properties. For example, Section 4 of this memo specifies these properties. For example, Section 4 of this memo specifies these
properties for RTP sessions specified in session descriptions. properties for RTP/AVP sessions specified in session descriptions.
In one respect, the framing protocol DOES 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 4. Session Descriptions for RTP/AVP over TCP
In this section, we show how session descriptions may specify RTP Session management protocols that use the Session Description Protocol
streams that use the framing method. [4] in conjunction with the Offer/Answer Protocol [6] MUST use the
methods described in [3] to set up RTP/AVP streams over TCP. In this
case, the use of Offer/Answer is REQUIRED, as the setup methods
described in [3] rely on Offer/Answer.
Figure 2 shows the syntax of a media (m=) line [4] of a session In principle, [3] is capable of setting up RTP sessions for any RTP
description: profile. In practice, each profile has unique issues that must be
considered when applying [3] to set up streams for the profile.
In this memo, we restrict our focus to the Audio/Video Profile (AVP,
[2]). 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
[4] of a session description:
"m=" media SP port ["/" integer] SP proto 1*(SP fmt) CRLF "m=" media SP port ["/" integer] SP proto 1*(SP fmt) CRLF
Figure 2 -- Syntax for an SDP media (m=) line (from [4]). Figure 2 -- Syntax for an SDP media (m=) line (from [4]).
The <proto> token value "TCP/RTP/AVP" specifies an RTP/AVP [1] [2] The <proto> token value "TCP/RTP/AVP" specifies an RTP/AVP [1] [2]
stream that uses the framing method over TCP. stream that uses the framing method over TCP.
The <fmt> tokens that follow <proto> MUST be unique unsigned integers in 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 the range 0 to 127. The <fmt> tokens specify an RTP payload type
associated with the stream. associated with the stream.
In all other respects, the session description syntax for the framing In all other respects, the session description syntax for the framing
method is identical to [3]. method is identical to [3].
The TCP <port> on the media line exclusively receives RTP packets. If a The TCP <port> on the media line carries RTP packets. If a media stream
media stream uses RTCP, a second connection exclusively receives the uses RTCP, a second connection carries RTCP packets. The port for the
RTCP packets. The port for the RTCP connection is chosen using the RTCP connection is chosen using the algorithms defined in [4] or by the
mechanism defined in [7].
algorithms defined in [4] and in related documents.
The TCP connections MAY carry bi-directional traffic, following the The TCP connections MAY carry bi-directional traffic, following the
semantics defined in [3]. Both directions of a connection MUST carry semantics defined in [3]. Both directions of a connection MUST carry
the same type of packets (RTP or RTCP). The packets MUST exclusively 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 code the RTP or RTCP streams specified on the media line(s) associated
with the connection. with the connection.
The RTP stream MUST have an unbroken sequence number order. RTCP stream As noted in [1], the use of RTP without RTCP is strongly discouraged.
packets MUST appear as defined in [2], with no lost or re-ordered However, if a sender does not wish to send RTCP packets in a media
packets. IETF standards-track documents MAY loosen these restrictions session, the sender MUST add the lines "b=RS:0" AND "b=RR:0" to the
on packet loss and packet ordering. media description (from [8]).
The out-of-band semantics for the connection MUST comply with [3]. 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 [6], 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 5. Example
The session descriptions in Figure 3-4 define a TCP RTP/AVT session. The session descriptions in Figure 3-4 define a TCP RTP/AVT session.
v=0 v=0
o=first 2520644554 2838152170 IN IP4 first.example.net o=first 2520644554 2838152170 IN IP4 first.example.net
s=Example s=Example
t=0 0 t=0 0
c=IN IP4 192.0.2.105 c=IN IP4 192.0.2.105
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Considerations section of [3] describes these issues in detail. Considerations section of [3] describes these issues in detail.
Below, we discuss security issues that are unique to the framing method Below, we discuss security issues that are unique to the framing method
defined in Section 2. defined in Section 2.
Attackers may send framed packets with large LENGTH values, to exploit Attackers may send framed packets with large LENGTH values, to exploit
security holes in applications. For example, a C implementation may security holes in applications. For example, a C implementation may
declare a 1500-byte array as a stack variable, and use LENGTH as the 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 bound on the loop that reads the framed packet into the array. This
code would work fine for friendly applications that use Etherframe-sized code would work fine for friendly applications that use Etherframe-sized
RTP packets, but may be open to exploit by an attacker. 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 9. IANA Considerations
[4] defines the syntax of session description media lines. We reproduce [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 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: define a new token value for the <proto> field of media lines:
"TCP/RTP/AVP". Section 4 specifies the semantics associated with the "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 <proto> field token, and Section 5 shows an example of its use in a
session description. session description.
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[3] Yon, D. and G. Camarillo. Connection-Oriented Media Transport in [3] Yon, D. and G. Camarillo. Connection-Oriented Media Transport in
the Session Description Protocol (SDP), the Session Description Protocol (SDP),
draft-ietf-mmusic-sdp-comedia-10.txt. draft-ietf-mmusic-sdp-comedia-10.txt.
[4] Handley, M., Jacobson, V., and C. Perkins. "SDP: Session [4] Handley, M., Jacobson, V., and C. Perkins. "SDP: Session
Description Protocol", draft-ietf-mmusic-sdp-new-22.txt. Description Protocol", draft-ietf-mmusic-sdp-new-22.txt.
[5] Bradner, S. "Key words for use in RFCs to Indicate Requirement [5] Bradner, S. "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[6] Rosenberg, J. and H. Schulzrinne. "An Offer/Answer Model with
SDP", RFC 3264, June 2002.
[7] C. Huitema. "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.
[8] S. Casner. "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, July
2003.
Authors' Address Authors' Address
John Lazzaro John Lazzaro
UC Berkeley UC Berkeley
CS Division CS Division
315 Soda Hall 315 Soda Hall
Berkeley CA 94720-1776 Berkeley CA 94720-1776
Email: lazzaro@cs.berkeley.edu Email: lazzaro@cs.berkeley.edu
Intellectual Property Rights Statement Intellectual Property Rights Statement
skipping to change at page 10, line 5 skipping to change at page 11, line 5
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE. PARTICULAR PURPOSE.
Acknowledgement Acknowledgement
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
Change Log for <draft-ietf-avt-rtp-framing-contrans-04.txt> Change Log for <draft-ietf-avt-rtp-framing-contrans-05.txt>
[Note to RFC Editors: this Appendix, and its Table of Contents listing, [Note to RFC Editors: this Appendix, and its Table of Contents listing,
should be removed from the final version of the memo] should be removed from the final version of the memo]
Updated Figures 3 and 4 to use new comedia "connection" attribute. Changes were made in response to Magnus's comments on AVT.
Changed section numbering to conform to draft-rfc-editor- [Issue 1] I am concerned that the draft is written in a bit to AVP
rfc2223bis-08.txt. centric. I know that the draft only registers, and apparently there is
not enough consensus and interest to define any other profile for the
moment. However the format and its basic signalling properties would be
the same independent of the profile in use.
[Response 1] Section 4 has been rewritten to be less AVP centric. See
the first four paragraphs of Section 4.
---
[Issue 2] Section 2, second paragraph: I think the last sentence could
benefit a informative reference to RTP section A.1 for checks that can
be used to verify correct alignment.
[Response 2] New text similar to that recommended by Magnus has been
added to the final paragraph of Section 2.
---
[Issue 3] Section 3, the first Undefined property: "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."
The expected property must be that any contrans supports usage of
multiple SSRCs. The behavior to expect needs to be the same for RTP
over UDP and RTP over TCP. What comes in on the TCP connection, can be
the same as what can come in over UDP port in unicast mode from a single
source. The difference between TCP and UDP is really only that you
can't receive from multiple sources to the same port as I understand the
signalling.
I would like to rephrase and move the paragraph. It should define the
expected properties in this case for clarity.
[Response 3] Section 3 has been renamed as "Packet Stream Properties".
It begins with the list of unspecified properties, which no longer
includes the property discussed in Issue 3. Following this list of
unspecified properties is the following text:
In one respect, the framing protocol does 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.
---
[Issue 4] Section 4. I think the signaling section should clearly
define that the basic procedure for establishing the TCP connection that
the RTP framing is sent over is using COMEDIA. This should be in the
first paragraph.
For example a sentence like: The transport of RTP/AVP over TCP when
signaled using SDP and the offer/answer method [RFC3264] SHALL establish
its TCP connection as defined by comedia [xx]. The RTP/AVP over TCP is
identified in SDP using the "proto" identifier "TCP/RTP/AVP".
For this SDP "proto" identifier the fmt list ...
I would also like to point out that due to comedia it doesn't seem that
this framing method can be used in any non Offer/Answer usage. Or have
I missed something in the comedia draft?
[Response 4] Done, see the first four paragraphs of Section 4.
---
[Issue 5] Section 4, second last paragraph: "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."
This paragraph is in contradiction with a statement in section 3. I
also think that it is wrong to make this requirement on the packets
entering the TCP connection.
[Response 5] I deleted the offending paragraph. So, the statement in
Section 3 on the topic holds for RTP/AVP by default.
---
[Issue 6] What is meant with the following sentence in section 4: "The
out-of-band semantics for the connection MUST comply with [3]." I don't
think it is clear what is meant with out-of-band semantics.
[Response 6] I deleted the offending sentence. Note that the new text
at the start of Section 4 (described in "Response 4" earlier) makes the
point I was trying to make in this sentence.
---
[Issue 7] Section 4. Does this section also need to define the usage of
the "a=rtcp" SDP attribute under this profile. Because I think there
are advantages of being able to define another TCP port for RTCP than
the RTP port + 1.
[Response 7] Normative reference to RFC 3605 was added to the document,
which is referenced in the second-to-last paragraph of Section 4.
---
[Issue 8] Section 4, which method are used to indicate the non-presence
of RTCP when using this transport?
[Response 8] A mechanism was added, described in the final 3 paragraphs
of Section 4. If a different mechanism is desired, please submit
replacement paragraphs that describe the candidate mechanism, so that
upon WG approval, we can quickly insert it into the document.
---
[Issue 9] Section 8. The Length field consideration. I think one can
be a bit more direct in the recommendation. I would like to add this
following sentence to the end of the paragraph: "Thus, a implementation
needs to handle packets of any length from the NULL packet (Length=0) to
max length 64K packet (Length=0xFFFF).
[Response 9] Done.
---
[Issue 10] The lack of recommendations on how to register more
identifiers for other profiles and what they would need to consider.
[Response 10] I believe this is now covered in the early part of Section
4.
---
 End of changes. 

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