Internet Engineering Task Force AVT WG
INTERNET-DRAFT Ladan Gharai
draft-ietf-avt-tfrc-profile-02.txt USC/ISI
17 August 2004
Expires: February 2005
RTP Profile for TCP Friendly Rate Control
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo specifies a profile called "RTP/AVPCC" for the use of the
real-time transport protocol (RTP) and its associated control
protocol, RTCP, with the TCP Friendly Rate Control (TFRC). TFRC is
Gharai [Page 1]
INTERNET-DRAFT Expires: February 2005 August 2004
an equation based congestion control scheme for unicast flows
operating in a best effort Internet environment. This profile
provides RTP flows with the mechanism to use congestion control in
best effort IP networks.
1. Introduction
[Note to RFC Editor: All references to RFC XXXX are to be replaced
with the RFC number of this memo, when published]
This memo defines a profile called "RTP/AVPCC" for the use of the
real-time transport protocol (RTP) [RTP] and its associated control
protocol, RTCP, with the TCP Friendly Rate Control (TFRC) [TFRC].
TFRC is an equation based congestion control scheme for unicast flows
operating in a best effort Internet environment and competing with
TCP traffic.
Due to a number of inherent TFRC characteristics, the RTP/AVPCC
profile differs from other RTP profiles [AVP] in the following ways:
o TFRC is a unicast congestion control scheme, therefore by
extension the RTP/AVPCC profile can only be used by unicast RTP
flows.
o A TFRC sender relies on receiving feedback from the receiver
either once per round-trip time (RTT) or per data packet. For
certain flows (depending on RTTs and data rates) these TFRC
requirements can result in control traffic that exceeds RFC 3550's
bandwidth and/or timing recommendations for control traffic. The
RTP/AVPCC profile recommends modifications to these
recommendations in order to satisfy TFRCs timing needs for control
traffic in a safe manner.
This memo primarily addresses the means of supporting TFRC's
exchange of congestion control information between senders and
receivers via the following modifications to RTP and RTCP: (1) RTP
data header additions; (2) extensions to the RTCP Receiver Reports;
and (3) modifications to the recommended RTCP timing intervals. For
details on TFRC congestion control readers are referred to [TFRC].
The current TFRC standard, RFC3448, only targets applications with
fixed packet size. TFRC-PS is a variant of TFRC for applications with
varying packet sizes. The RTP/AVPCC profile is applicable to both
congestion control schemes.
Gharai [Page 2]
INTERNET-DRAFT Expires: February 2005 August 2004
2. Relation to the Datagram Congestion Control Protocol
The Datagram Congestion Control Protocol (DCCP) is a minimal general
purpose transport-layer protocol with unreliable yet congestion-
controlled packet delivery semantics and reliable connection setup
and teardown. DCCP currently supports both TFRC and TCP-like
congestion control. In addition DCCP supports a host of other
features, such as: use of Explicit Congestion Notification (ECN) and
the ECN Nonce, reliable option negotiation and Path Maximum Transfer
Unit (PMTU). Naturally an application using RTP/DCCP as its
transport protocol will benefit from the protocol features supported
by DCCP.
In contrast the RTP Profile for TFRC only provides RTP applications a
standardized means for using the TFRC congestion control scheme,
without any of the protocol features of DCCP. However there are a
number of benefits to be gained by the development and
standardization of a RTP Profile for TFRC:
o Media applications lacking congestion control can incorporate
congestion controlled transport without delay by using the
RTP/AVPCC profile. The DCCP protocol is currently under
development and widespread deployment is not yet in place.
o Use of the RTP/AVPCC profile is not contingent on any OS level
changes and can be quickly deployed, as the AVPCC profile is
implemented at the application layer.
o AVPCC/RTP/UDP flows face the same restrictions in firewall
traversal as do UDP flows and do not require NATs and firewall
modifications. DCCP flows, on the other hand, do require NAT
and firewall modifications, however once these modifications
are in place, they can result in easier NAT and firewall
traversal for RTP/DCCP flows in the future.
o Use of the RTP/AVPCC profile with various media applications will
give researchers, implementors and developers a better
understanding of the intricate relationship between media
quality and equation based congestion control. Hopefully this
experience with congestion control and TFRC will ease the
migration of media applications to DCCP once DCCP is deployed.
Overall, the RTP/AVPCC profile provides an immediate means for
congestion control in media streams, in the time being until DCCP is
deployed.
Additionally, there are also a number of differences in the exchange
of congestion control information between DCCP with CCID3 and the
Gharai [Page 3]
INTERNET-DRAFT Expires: February 2005 August 2004
RTP/AVPCC profile:
o A RTP/AVPCC sender transmits the round trip time and
the send timestamp to the RTP/AVPCC receiver. In addition
to congestion control the send timestamp can be used by the
receiver for jitter calculations.
In contrast DCCP with CCID3 transmits a quad round trip
counter to the receiver.
o A RTP/AVPCC receiver only provides the RTP/AVPCC sender
with the loss event rate as computed by the receiver.
In contrast DCCP with CCID3, provides 2 other options for the
transport of loss event rate. A sender may choose to receive
loss intervals or an Ack Vector. These two options provide the
sender with the necessary information to compute the loss event
rate.
o Sequence number: DCCP supports a 48 bit and a 24 bit sequence
number. RTP supports a 16 bit sequence number.
3. Conventions Used in this Document
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 RFC 2119 [2119].
4. RTP and RTCP Packet Forms and Protocol Behavior
The section "RTP Profiles and Payload Format Specifications" of RFC
3550 enumerates a number of items that can be specified or modified
in a profile. This section addresses each of these items and states
which item is modified by the RTP/AVPCC profile:
RTP data header: The standard format of the fixed RTP data
header is used (one marker bit).
Payload types: This profile does not define new payload types,
and has no payload type restrictions.
RTP data header additions: A 16 bit fixed field is added to
the RTP data header for the transport of the quad RTT
counter to the TFRC receiver.
RTP data header extensions: No RTP header extensions are
Gharai [Page 4]
INTERNET-DRAFT Expires: February 2005 August 2004
defined, but applications operating under this profile
MAY use such extensions. Thus, applications SHOULD NOT
assume that the RTP header X bit is always zero and SHOULD
be prepared to ignore the header extension. If a header
extension is defined in the future, that definition MUST
specify the contents of the first 16 bits in such a way
that multiple different extensions can be identified.
RTCP packet types: No additional RTCP packet types are defined
by this profile specification.
RTCP report interval: This profile is restricted to unicast
flows, therefore at all times there is only one active sender
and one receiver. Sessions operating under this profile MAY
specify a separate parameter for the RTCP traffic bandwidth
rather than using the default fraction of the session
bandwidth. In particular this may be necessary for data
flows were the the RTCP recommended reduced minimum interval
is still greater than the RTT.
SR/RR extension: A 16 octet RR extension is defined for the RTCP
RR packet.
SDES use: Applications MAY use any of the SDES items described
in the RTP specification.
Security: <TBC> See Section 9.
String-to-key mapping: No mapping is specified by this profile.
Congestion: This profile specifies how to use RTP/RTCP with TFRC
congestion control.
Underlying protocol: The profile specifies the use of RTP over
unicast UDP flows only, multicast MUST NOT be used.
Transport mapping: The standard mapping of RTP and RTCP to
transport-level addresses is used.
Encapsulation: This profile is defined for encapsulation
over UDP only.
5. The TFRC Feedback Loop
TFRC depends on the exchange of congestion control information
between a sender and receiver. In this section we reiterate which
items are exchanged between a TFRC sender and receiver as discussed
in [TFRC]. We note how the RTP/AVPCC profile accommodates these
Gharai [Page 5]
INTERNET-DRAFT Expires: February 2005 August 2004
exchanges.
5.1. Data Packets
As stated in [TFRC] a TFRC sender transmits the following information
in each data packet to the receiver:
o A sequence number, incremented by one for each data packet
transmitted.
o A timestamp indicating the packet send time and the sender's
current estimate of the round-trip time, RTT. This information
is then used by the receiver to compute the TFRC loss intervals.
- or -
A course-grained timestamp incrementing every quarter of a
round trip time, which is then used to determine the TFRC loss
intervals.
The standard RTP sequence number suffices for TFRCs functionality.
For the computation of the loss intervals the RTP/AVPCC profile
extends the RTP data header as follows: a 32 bit field to transmit a
send timestamp and an additional 32 bit field, present only when the
RTT changes, to transmit the RTT. The presence of the RTT is
indicated by the R bit in the RTP header (see Section 6).
5.2. Feedback Packets
As stated in [TFRC] a TFRC receiver provides the following feedback
to the sender at least once per RTT or per data packet received
(which ever time interval is larger):
o The timestamp of the last data packet received, t_i.
o The amount of time elapsed between the receipt of the last
data packet at the receiver, and the generation of this feedback
report, t_delay. This is used by the sender for RTT computations
(see Section 9).
o The rate at which the receiver estimates that data was received
since the last feedback report was sent, x_recv
o The receiver's current estimate of the loss event rate, p.
To accommodate the feedback of these values the RTP/AVPCC profile
defines a 16 octet extension to the RTCP Receiver Reports (see
Section 7).
Gharai [Page 6]
INTERNET-DRAFT Expires: February 2005 August 2004
6. RTP Data Header Additions
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M|R| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| send time-stamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP header and additions with R=0, no RTT included.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M|R| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| send time-stamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTT |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: RTP header and additions with R=1, RTT included.
7. Receiver Report Extensions
Gharai [Page 7]
INTERNET-DRAFT Expires: February 2005 August 2004
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| RC | PT=RR=201 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fraction lost | cumulative number of packets lost |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended highest sequence number received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| t_i |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| t_delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data rate at the receiver (x_recv) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| loss event rate (p) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 3: RTCP Receiver Report extensions.
8. RTCP Timing Intervals
The RTP/AVPCC profile recommends the use of the TFRC timing feedback
requirements for the RTCP timing intervals, only in instances where
control traffic bandwidth does not exceed RFC 3550's recommended 5%
of data traffic.
A TFRC sender requires feedback from its receiver at least once per
RTT or per packet received (based on the larger time interval). These
requirements are to ensure timely reaction to congestion.
In some instances TFRC's timing requirements may result in timing
intervals for RTCP traffic that are smaller than RFC 3550's
recommended scaled reduced minimum timing interval of 360 divided by
session bandwidth in kilobits/second or t(s) = 360/X(kbps).
For example, Figure 4 depicts two AVPCC flows and their relationship
Gharai [Page 8]
INTERNET-DRAFT Expires: February 2005 August 2004
with RTCP's reduced minimum interval: t(ms) = 360/X (Mbps). The two
flows have data rates of 2 Mbps and 4 Mbps with RTTs of 70 ms and 130
ms, respectively.
The 4 Mbps flow's TFRC feedback requirements of 130 ms falls within
RFC 3550's recommended reduced minimum interval for RTCP traffic.
However the 2 Mbps flow's TFRC feedback requirement of once per 70 ms
is more frequent than the 180 ms recommended by RFC 3550.
However in this case, it is safe to use TFRC's 70 ms interval, as at
the rate of roughly one 88 octet RTCP compound packet per 70 ms, the
feedback traffic for the 2 Mbps flow amounts to 10 kbps, that is less
than 1% of the data flow and well with the 5% recommended by RFC
3550.
Bandwidth (Mbps)
^
| \
| \
| \
| \ 360
| \ t(ms)= -------
| \ X (Mbps)
| \
| \_
4 | \__ x
| \___
2 | x \____
| \_________
+---------------------------------->
70 130
Time (ms)
Figure 4: Relationship between RFC 3550 recommended reduced minimum
interval and session bandwidth (Mbps).
9. Open Issues
There are a number of open issues on the AVPCC on which we are
soliciting input from the community:
o RFC 3550 recommends that the percentage of control traffic
Gharai [Page 9]
INTERNET-DRAFT Expires: February 2005 August 2004
relative to data, be fixed at 5%. For some flows, the feedback
traffic for AVPCC may exceed this recommendation. Should AVPCC
mandate a strict limit on the percentage of control traffic
bandwidth? At what point is feedback too much feedback?
(i.e., does it make sense for control traffic be 50% of data
traffic?)
What are the implications of this limit, in terms of congestion
control, for flows which cannot abide by the limit? This is
particularly the case for low bandwidth flows, under 1 Mbps, and
RTTs of say less than 10 ms.
o Security: Is it possible for the AVPCC to use the security
mechanisms of SRTP as defined in RFC 3711 or is it necessary
to define alternative security profile and mechanisms?
o RTT calculations by the sender: As an alternative to including
t_i and t_delay in each RTCP packet, could the sender use the LSR
and DLSR fields of the Receiver Reports to calculate the RTT?
This is mainly a question of: is the frequency of the RTTs
computed from Sender Reports sufficient for the sender to react
to changes in the RTT and congestion?
o How does this profile relate to other RTP profiles?
10. IANA Considerations
The RTP profile for TCP Friendly Rate Control extends the profile for
audio- visual conferences with minimal control and needs to be
registered for the Session Description Protocol [SDP] as "RTP/AVPCC".
SDP Protocol ("proto"):
Name: RTP/AVPCC
Long form: RTP Profile for TCP Friendly Rate Control
Type of name: proto
Type of attribute: Media level only
Purpose: RFC XXXX
Reference: RFC XXXX
11. Security Considerations
See Section 9 (Open Issues).
Gharai [Page 10]
INTERNET-DRAFT Expires: February 2005 August 2004
12. Acknowledgments
This memo is based upon work supported by the U.S. National Science
Foundation (NSF) under Grant No. 0334182. Any opinions, findings and
conclusions or recommendations expressed in this material are those
of the authors and do not necessarily reflect the views of NSF.
13. Author's Address
Ladan Gharai <ladan@isi.edu>
USC Information Sciences Institute
3811 N. Fairfax Drive, #200
Arlington, VA 22203
USA
Normative References
[RTP] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
Internet Engineering Task Force, RFC 3550 (STD0064), July
2003.
[AVP] H. Schulzrinne and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control," RFC 3551 (STD0065),
July 2003.
[2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", Internet Engineering Task Force,
RFC 2119, March 1997.
[2434] T. Narten and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", Internet Engineering Task
Force, RFC 2434, October 1998.
[TFRC] M. Handley, S. Floyed, J. Padhye and J. widmer,
"TCP Friendly Rate Control (TRFC): Protocol Specification",
Internet Engineering Task Force, RFC 3448, January 2003.
[SDP] M. Handley and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
Gharai [Page 11]
INTERNET-DRAFT Expires: February 2005 August 2004
Informative References
14. IPR Notice
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 procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
15. 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.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
Gharai [Page 12]
INTERNET-DRAFT Expires: February 2005 August 2004
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Gharai [Page 13]
