Internet Engineering Task Force Piers O'Hanlon INTERNET DRAFT UCL December 18, 2008 Ken Carlberg G11 RTCP Extended Report for ECN Marked Packets Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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) 2008 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document describes a Real-Time Control Protocol (RTCP) Extended Report (XR) containing information derived from the reception of Explicit Congestion Notification (ECN) marked packets. This document is symbiotic with the approach described in [rtp-ecn], which presents one approach in establishing end-to-end ECN support for real-time sessions. O'Hanlon, Carlberg Expires June 18, 2009 [Page 1] Internet Draft ECN Extension Report December 18, 2008 1 Introduction Explicit Congestion Notification (ECN) is a dual-layer means of conveying the presence of congestion on an end-to-end manner without dropping packets. The network layer indicates in hop-by-hop IP packets whether or not endpoints support ECN. If yes, then if congestion exists along the downstream path, the IP packet is marked to indicate the congested condition to the endpoint. At the upper layer has the dual responsibility of initially negotiating support for ECN as well as conveying the congested condition to the source endpoint. The initial realization of ECN was described in [rfc2481], and later obsoleted by [rfc3168]. In both cases, TCP was used as the upper layer transport protocol used to negotiate support for ECN during the establishment of an end-to-end connection and convey through the use of TCP acks the presence of congestion along the downstream path. The architecture presented [rfc3168] also opened the design to allow other upper layer protocols to be substitued for TCP. 1.1. Applicability This metric is believed to be applicable to all RTP applications which utilise ECN for congetsion control or other purposes. Additionally it may be utilised by monitoring systems. 2. Design Approach Protocols such as SCTP and DCCP are natural candidates for support of ECN due to the stateful behavior. However, UDP is stateless and not a viable candidate for accomplishing the state negotiation outlined in [rfc3168]. To compensate for this stateless feature, [rtp-ecn] proposes utilizing this RTCP XR extension to provide for an RTP minimal congestion control functionality. By employing Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF) [RFC 4585], it is possible to provide suitable timely feedback at the level necessary for base-line congestion control mechanisms. This newly proposed XR follows the guidelines defined in [rfc3550] and [rfc3611]. 3 RTCP Block Extended Report: ECN This block type permits detailed reporting upon the ECN marking of individual packets. As detailed above the ECN marking may be employed in a variety of ways. The information may also be utilised by monitoring systems. This reporting format utilises an approach closely aligned that in the Section 4.1 [rfc3611] Loss RLE report Block. The main difference with the ECN report block is that it reports both bits of the ECN field. O'Hanlon, Carlberg Expires June 18, 2009 [Page 2] Internet Draft ECN Extension Report December 18, 2008 The reason for this is so that the ECN statistics may be complete, by conveying all three codepoints; Congestion Experienced (CE), ECN-Capable Transport (ECT), and Not-ECT. The ECN Report Block has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BT=TBD | rsvd. | T | block length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of source | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | begin_seq | end_seq | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | chunk 1 | chunk 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : ... : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | chunk n-1 | chunk n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block type (BT): 8 bits A Loss RLE Report Block is identified by the constant TBD. rsvd.: 4 bits This field is reserved for future definition. In the absence of such definition, the bits in this field MUST be set to zero and MUST be ignored by the receiver. thinning (T): 4 bits The amount of thinning performed on the sequence number space. Only those packets with sequence numbers 0 mod 2^T are reported on by this block. A value of 0 indicates that there is no thinning, and all packets are reported on. The maximum thinning is one packet in every 32,768 (amounting to two packets within each 16-bit sequence space). block length: 16 bits The length of this report block, including the header, in 32- bit words minus one. SSRC of source: 32 bits The SSRC of the RTP data packet source being reported upon by this report block. begin_seq: 16 bits The first sequence number that this block reports on. O'Hanlon, Carlberg Expires June 18, 2009 [Page 3] Internet Draft ECN Extension Report December 18, 2008 end_seq: 16 bits The last sequence number that this block reports on plus one. chunk i: 16 bits There are three chunk types: run length, bit vector, and terminating null, defined in [RFC3611] (Section 4). If the chunk is all zeroes, then it is a terminating null chunk. Otherwise, the left most bit of the chunk determines its type: 0 for run length and 1 for bit vector. 4. SDP Attribute The use of SDP to signal XR blocks is specified in [RFC3611], which provides for ease of extension. This section defines such an extension to provide for signalling of the ECN report block. An additional value, "ecn-rle", is defined for the existing "xr-format" parameter in RTCP XR attribute. rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF (as defined in RFC3611) xr-format = xr-format / ecn-rle ecn-rle = "ecn-rle" ["=" max-size] 5. IANA Considerations This document creates a new block type within the IANA "RTCP XR Block Type Registry" called the ECN Metrics Block, and a new parameter xr-ecn within the "RTCP XR SDP Parameters Registry". 6. Security Considerations The proposed RTCP XR report block introduces no new security considerations beyond those described in [RFC3611]. This block may provide per-packet statistics of downstream flows to the upstream source node. 7. References 7.1. Normative References [rfc2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, IETF, March 1997. [rfc3550] Schulzrinne, S., et al, "RTP: A Transport Protocol for Real-Time O'Hanlon, Carlberg Expires June 18, 2009 [Page 4] Internet Draft ECN Extension Report December 18, 2008 Applications", RFC 3550, IETF, July 2003 [rfc3611] Friedman, T., et al, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, IETF, November 2003 [rfc4585] Ott, J., et al, "Extended RTP Profile for Real-time Transport Control Protocol (RTCP) -Based Feedback (RTP/AVPF)", RFC 4585, IETF, July 2006 7.2 Informative References [rtp-ecn] Carlberg, K., P. O'Hanlon, "Explicit Notification Extension (ECN) Extension for RTP", Internet Draft, Work in Progress, Dec 2008 [rfc2481] Ramakrishnan, S., S. Floyd, "A Proposal to Add Explicit Congestion Notification (ECN) to IP", RFC2481, IETF, January 1999 [rfc3168] Ramakrishnan, S., et al, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC3168, IETF, September 2001 Author's Addresses Piers O'Hanlon University College London Gower Street London, UK EMail: p.ohanlon@cs.ucl.ac.uk Ken Carlberg G11 123a Versailles Circle Baltimore, MD, USA email: carlberg@g11.org.uk Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt O'Hanlon, Carlberg Expires June 18, 2009 [Page 5] Internet Draft ECN Extension Report December 18, 2008 made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. O'Hanlon, Carlberg Expires June 18, 2009 [Page 6]