Internet Engineering Task Force AVT WG INTERNET-DRAFT Ladan Gharai draft-ietf-avt-tfrc-profile-00.txt USC/ISI 21 June 2004 Expires: December 2004 RTP Profile for TCP Friendly Rate Control Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 (2003). 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 a equation based congestion control scheme for unicast flows operating in a best effort Internet environment. Gharai [Page 1]
INTERNET-DRAFT Expires: December 2004 June 2004 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 a 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 AVPCC profile differs from other RTP profiles in the following ways: o TFRC is a unicast congestion control scheme, therefore by extension the AVPCC profile can only be used by unicast RTP flows. o A TFRC sender relies on receiving feedback from the receiver at least once per round-trip time (RTT) in order to adjust its send rate (for flows with send rates of less than one packet per RTT time, a feedback packet should be sent for every data packet received). For certain flows (depending on RTTs and data rates) this TFRC requirement can result in control traffic that exceeds RTP's bandwidth recommendations for control traffic. RTP restricts control traffic to a fixed fraction of session bandwidth, so as to prevent RTCP feedback implosion in multicast scenarios. As AVPCC can only be used by unicast flows, TFRCs increased use of traffic does not effect scalability or cause traffic implosion. o TFRC is highly sensitive and dependent on accurate and current computations of the RTT. The sender uses the RTT to compute a TCP-friendly send rate, while the receiver needs the current RTT to compute its loss event rate. Therefore it is imperative that both the senders and receiver have access to current and accurate RTT measurements. This memo primarily addresses the means of supporting TFRC's exchange of 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) relaxation of the recommended RTCP timing intervals. For details on TFRC congestion control readers are referred to [TFRC]. Gharai [Page 2]
INTERNET-DRAFT Expires: December 2004 June 2004 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 AVPCC profile is applicable to both congestion control schemes. 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) to name a few. 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 AVPCC profile. The DCCP protocol is currently in its early stages of development and widespread deployment is not yet in place. o Use of the 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 can traverse firewalls as they are essentially UDP flows and therefore do not require any special changes to NATs and firewalls. o Use of the 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. In short, the AVPCC profile provides an immediate means for congestion control in media streams, in the time being until DCCP is Gharai [Page 3]
INTERNET-DRAFT Expires: December 2004 June 2004 deployed. 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 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: Two 32bit additional fixed fields are added to the RTP data header for the transport of packet send time and the current RTT to the TFRC receiver. RTP data header extensions: No RTP header extensions are 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. Gharai [Page 4]
INTERNET-DRAFT Expires: December 2004 June 2004 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: The RTP default security services are also the default under this profile. 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. Transport mapping: The standard mapping of RTP and RTCP to transport-level addresses is used. Encapsulation: This profile leaves to applications the specification of RTP encapsulation in protocols other than UDP. 5. The TFRC Feedback Loop TFRC depends on the exchange of 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 AVPCC profile accommodates these 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. This timestamp is used by the receiver to (1) compute the loss event rate and (2) is returned to the sender for RTT computation. The standard RTP header includes a 32 bit timestamp. For real-time data this timestamp indicates the sampling instance of the first octet of the packet. For stored media Gharai [Page 5]
INTERNET-DRAFT Expires: December 2004 June 2004 it represents the presentation time of the packet. Packets belonging to the same video frame/audio sample share the same RTP timestamp value. While it would be preferable to use the RTP timestamp for TFRC's calculations, it can lead to incorrect RTT and loss event rate calculations. For example ... o The sender's current estimate of the round-trip time, RTT. The standard RTP sequence number suffices for TFRCs functionality. To transmit the send timestamp and the RTT to the receiver the AVPCC profile extends the RTP data header by two 32bit fields in order to accommodate their transmission (see Section 5). 5.2. Feedback Packets As stated in [TFRC] a TFRC receiver provides the following feedback to the sender at least once per RTT: o The timestamp of the last data packet received. This is the timestamp is used by the sender to estimate RTT and is only needed if the sender does not save timestamps of transmitted data packets. o The amount of time elapsed between the receipt of the last data packet at the receiver, and the generation of this feedback report. This is used for estimation of the RTT. o The rate at which the receiver estimates that data was received since the last feedback report was sent. o The receiver's current estimate of the loss event rate, p. To accommodate the feedback of these values the AVPCC profile defines a 16 octet extension to the RTCP Receiver Reports (see Section 6). 6. RTP Data Header Additions Gharai [Page 6]
INTERNET-DRAFT Expires: December 2004 June 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|X| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | timestamp (packet send time) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTT | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | contributing source (CSRC) identifiers | | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: 7. Receiver Report Extensions Gharai [Page 7]
INTERNET-DRAFT Expires: December 2004 June 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) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | timestamp_i | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | t_delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data rate at the receiver (x_recv) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | loss event rate (p) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ Figure 2: 8. RTCP Timing Intervals RFC3550 recommends that control traffic be limited to a small and known fraction of the session bandwidth. Specifically it recommends that the fraction of session bandwidth be added for RTCP be fixed at 5%. Based on this fixed bandwidth allotment and the number of senders and receivers the interval between RTCP feedback packets is calculated. In addition to recommended restrictions on control traffic bandwidth, RFC3550 also recommends an average minimum interval of 5 seconds between sending RTCP packets, however this minimum interval can be scaled to a reduced minimum. Computed in seconds of 360 divided by session bandwidth in kilobits/second. These restrictions on the fraction of control traffic bandwidth and Gharai [Page 8]
INTERNET-DRAFT Expires: December 2004 June 2004 the frequency of feedback is to ensure scalability to large multicast groups and prevent control traffic implosion. The TFRC algorithm requires feedback from receivers at least once per RTT. For data rates less than 5Mbps (depending on the RTT) this may require transmitting RTCP packets at higher frequency than recommended by the scaled minimum interval. This increased frequency may or may not results in a control traffic in excess of 5% of the session bandwidth. The AVPCC profile defines the control traffic bandwidth as a separate parameter of the session to accommodate TFRCs feedback requirements. +--------------------------+----------+---------+-----------+------------+ | Session Bandwidth (B) | 10 kbps | 72 kbps | 5000 kbps | 10000 kbps | | Minimum Interval (360/B)| 36 sec | 5 sec | 72 msec | 36 msec | | RTCP Bandwidth | - | - | ~7 kpbs | ~14 kpbs | +--------------------------+----------+---------------------+------------+ Figure 3: Session bandwidth and RTCP minimum intervals. RTCP bandwidth is computed assuming compound packet sizes of 60bytes. 9. IANA Considerations <TBC> 10. Security Considerations <TBC> 11. 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. 12. Author's Address Gharai [Page 9]
INTERNET-DRAFT Expires: December 2004 June 2004 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, 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. Informative References 13. 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. Gharai [Page 10]
INTERNET-DRAFT Expires: December 2004 June 2004 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. 14. Full Copyright Statement Copyright (C) The Internet Society 2003. All Rights Reserved. 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 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 11]
