Network Working Group C. Alexander, Ed. Internet-Draft J. Babiarz Expires: January 12, 2006 Nortel July 11, 2005 RTP Payload Format for ECN Probing draft-alexander-rtp-payload-for-ecn-probing-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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. This Internet-Draft will expire on January 12, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This memo defines a Real Time Transport Protocol (RTP) payload format for use when probing for congestion using Explicit Congestion Detection (ECN). This payload format is intended for use with the probing mechanisms described in draft "Real-time ECN Use Cases". While defined in terms of the specific application of admission control, it is desirable to overlay this format with other probing mechanisms so as to reduce the number of probing packet formats. Alexander & Babiarz Expires January 12, 2006 [Page 1] Internet-Draft RTP Payload for ECN Probing July 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. RTP Payload Format for Real-Time ECN Admission Control . . . . 6 4.1 Registration . . . . . . . . . . . . . . . . . . . . . . . 6 4.2 IP Header Fields . . . . . . . . . . . . . . . . . . . . . 6 4.3 RTP Header Fields . . . . . . . . . . . . . . . . . . . . 6 4.4 Payload Format . . . . . . . . . . . . . . . . . . . . . . 6 4.4.1 Version . . . . . . . . . . . . . . . . . . . . . . . 7 4.4.2 Explicit Congestion Notification (ECN) . . . . . . . . 7 4.4.3 Initial RTP Sequence Number (IRSN) . . . . . . . . . . 7 4.4.4 Reserved . . . . . . . . . . . . . . . . . . . . . . . 7 5. Considerations for Payload Format . . . . . . . . . . . . . . 8 5.1 Extensibility Considerations . . . . . . . . . . . . . . . 8 5.2 Flexibility Considerations . . . . . . . . . . . . . . . . 8 6. Considerations for Direct Feedback . . . . . . . . . . . . . . 9 6.1 Feedback via RTP . . . . . . . . . . . . . . . . . . . . . 9 6.2 Feedback via RTCP . . . . . . . . . . . . . . . . . . . . 9 7. MIME Registration . . . . . . . . . . . . . . . . . . . . . . 10 7.1 audio/ecnprobe . . . . . . . . . . . . . . . . . . . . . . 10 7.2 video/ecnprobe . . . . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 11.1 Normative References . . . . . . . . . . . . . . . . . . . 15 11.2 Informative References . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property and Copyright Statements . . . . . . . . 17 Alexander & Babiarz Expires January 12, 2006 [Page 2] Internet-Draft RTP Payload for ECN Probing July 2005 1. Introduction This memo defines a new RTP payload format for use with applications requiring congestion detection along the data path and verification of data path connectivity, for example, admission control of a real- time session. The format described herein is intended for use with the mechanisms described in "Congestion Notification Process for Real-Time Traffic" [2], which defines the use of the Explicit Congestion Notification (ECN) bits in the Internet Protocol (IP) header as a means to detect congestion in the network for real-time inelastic flows. The new format can be used to provide the capabilities described in "Real-time ECN Use Cases" [3], although it may also be used in other contexts. The new RTP payload format defined herein is called "ecnprobe". Packets utilizing this payload are carried as RTP traffic through the IP network. Packets carrying this payload are treated the same as any other RTP packet with the exception of play-back by the receiving device. The advantages of using this payload format are: 1. congestion detection can be performed using a simple probing mechanism without having to extend other protocols; 2. the payload format allows for limited detection of devices making inappropriate changes to the ECN markings in the network; 3. the packet carrying the payload can vary in size from the minimum necessary to carry the payload, to a size padded to mimic a specific codec. Applications will use this payload format to create and send RTP probe packets through the IP network to determine the highest state of congestion along the path taken by the packets. In all uses, applications receiving this payload MUST NOT attempt to play it as actual media. This memo only defines the new payload format. Examples of its usage can be found in [3]. Alexander & Babiarz Expires January 12, 2006 [Page 3] Internet-Draft RTP Payload for ECN Probing July 2005 2. Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC2119 [7] and indicate requirement levels for compliant implementations. Alexander & Babiarz Expires January 12, 2006 [Page 4] Internet-Draft RTP Payload for ECN Probing July 2005 3. Definitions The following terms are used in this document: Cheater: A device in the network that makes inappropriate changes to the ECN markings in the network. A cheating device might re-mark the ECN bits in the IP header in order to hide congestion from an endpoint (i.e., by lowering the ECN congestion marking), or might force an endpoint to think congestion is present when it is not (i.e., by raising the ECN congestion marking). Due to the nature of ECN and how conformant network devices mark ECN for real-time inelastic flows, it is possible to detect the presence of cheater devices which lower the ECN marking, but not those that raise it. Probe Packet: The Probe Packet is an RTP packet utilizing the "ecnprobe" payload format defined herein. Receiver: The Receiver is simply an endpoint device which receives one or more Probe Packets from the Sender. As defined here, the Responder does not respond directly to the Probe Packet. Sender: The Sender is an endpoint device which generates one or more Probe Packets. Alexander & Babiarz Expires January 12, 2006 [Page 5] Internet-Draft RTP Payload for ECN Probing July 2005 4. RTP Payload Format for Real-Time ECN Admission Control The "ecnprobe" payload is transported in RTP packets. However, it is not part of an RTP stream. It therefore has no requirements to use similar properties of the media to be admitted. 4.1 Registration The new RTP payload format is defined as "ecnprobe", with a MIME type of "audio/ecnprobe" for audio and a MIME type of "video/ecnprobe" for video. The payload type for RTP packets carrying this payload is determined dynamically through methods outside the scope of this document. 4.2 IP Header Fields DSCP: The DSCP set in the IP header is a critical component of the ECN method as outlined in [2]. It should be set appropriately for the session media for which admission control is being performed. ECN: Unless attempting to detect for the presence of Cheaters along the media path, an application MUST set the two-bit ECN field in the IP header to '10', which indicates that it is an ECN-capable transport, with no congestion experienced. If attempting to detect for the presence of Cheaters, the ECN field SHOULD be set as required by the detection method being used. 4.3 RTP Header Fields Payload Type: The payload type field MUST be filled with a value determined dynamically. 4.4 Payload Format The "ecnprobe" payload format is shown in Figure 1. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version|ECN| Initial RTP Sequence Number | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: ecnprobe Payload Format It consists of five fields: Version, ECN, Initial RTP Sequence Number (ISRN), and Reserved. Alexander & Babiarz Expires January 12, 2006 [Page 6] Internet-Draft RTP Payload for ECN Probing July 2005 4.4.1 Version The Version field designates the version of the payload format. This field is provided for future extensibility of the payload to carry additional information. The following value is defined by this document for the Version field: 0: Initial version defined by this document. 4.4.2 Explicit Congestion Notification (ECN) This field contains a two-bit ECN value. If an application is trying to detect Cheaters, the Sender SHOULD set this field to the two-bit ECN value used in the IP header when sending the Probe Packet. 4.4.3 Initial RTP Sequence Number (IRSN) In order to perform cheater detection and/or compliance testing in a unidirectional fashion, the receiving endsystem needs to know which packets to use for cheater detection and/or compliance testing. During probing, this is less important as the probe payload will contain the actual ECN value set in the IP header. But during media exchange, the receiver must know exactly which packets are intended for cheater detection and/or compliance testing. The IRSN value represents the initial sequence number used on the outgoing probe and/or media packets. This value is used as described in [2] to allow both ends to know which packets in the media stream are marked for cheater detection. In the event of lost or out-of-order packets, the receiver needs only to check the sequence number of the incoming packet against the calculated sequence number that it expects to find in packets being used for cheater detection and/or compliance testing. 4.4.4 Reserved This field contains 10 bits reserved for future use. Alexander & Babiarz Expires January 12, 2006 [Page 7] Internet-Draft RTP Payload for ECN Probing July 2005 5. Considerations for Payload Format There were two main considerations driving the new payload format defined in this memo: extensibility and flexibility. 5.1 Extensibility Considerations While the intended use for this payload format is for admission control using ECN, the payload format need not be limited to that application. Even for admission control applications which will use it, the payload format also need not be limited to the mechanisms described in this memo. With that in mind, the four-bit Version field is included to allow for extensibility for future applications/ implementations. 5.2 Flexibility Considerations In addition to extensibility, another consideration is the flexibility to allow the initial definition of the payload to be used in as wide a range of implementations as possible. While the default and minimum size of the payload is 4 octets, an application MAY pad to the payload in order to simulate a specific codec. In this case, the application needs to ensure that the packets carrying the padded payload are sent at the appropriate rate corresponding to the codec being simulated. Alexander & Babiarz Expires January 12, 2006 [Page 8] Internet-Draft RTP Payload for ECN Probing July 2005 6. Considerations for Direct Feedback The payload format is currently defined with no explicit mechanism to provide feedback on the Probe Packet(s) from the Receiver to the Sender. This section discusses the options that have been considered, and describes why they are not included along with the payload format definition. 6.1 Feedback via RTP The payload format was originally envisioned to be used with either a unidirectional probe or a bidirectional probe. This document specifies a unidirectional probe. A bidirectional probe flows from the Sender to the Receiver, with the Receiver then generating its own probe in response, with the payload additionally carrying the ECN value from the IP header of the received probe packet. While this is feasible, the unidirectional probing model results in a simpler implementation. 6.2 Feedback via RTCP Also considered was an approach whereby feedback is provided via RTCP from the Receiver to the Sender. While possible, implementing feedback via RTCP in real time as described in "Admission Control Use Case for Real-time ECN" [3] would necessitate violation of the rules governing the RTCP transmission interval described in RFC3550 [1]. The RTCP transmission interval deliberately paces RTCP transmissions to be no more frequent than every 5 seconds, but for an admission control application, the transmission interval would potentially need to be much shorter. Alexander & Babiarz Expires January 12, 2006 [Page 9] Internet-Draft RTP Payload for ECN Probing July 2005 7. MIME Registration This section registeres MIME types for audio/ecnprobe and video/ ecnprobe. 7.1 audio/ecnprobe MIME media type name: audio MIME subtype name: ecnprobe Required Parameters: none Optional Parameters: none Encoding considerations: This type is only defined for transfer via RTP [1] or Secure RTP [5]. Security considerations: See "Security Considerations" (Section 8). Interoperability considerations: none Published specification: This document. Applications which use this media: The "ecnprobe" application subtype is used to perform ECN probing and data path connectivity for admission control, although it is not limited solely to this application. Additional information: 1. Magic number(s): N/A 2. File extensions(s): N/A 3. Macintosh file type code(s): N/A 7.2 video/ecnprobe MIME media type name: video MIME subtype name: ecnprobe Required Parameters: none Optional Parameters: none Alexander & Babiarz Expires January 12, 2006 [Page 10] Internet-Draft RTP Payload for ECN Probing July 2005 Encoding considerations: This type is only defined for transfer via RTP [1] or Secure RTP [5]. Security considerations: See "Security Considerations" (Section 8). Interoperability considerations: none Published specification: This document. Applications which use this media: The "ecnprobe" application subtype is used to perform ECN probing and data path connectivity for admission control, although it is not limited solely to this application. Additional information: 1. Magic number(s): N/A 2. File extensions(s): N/A 3. Macintosh file type code(s): N/A Alexander & Babiarz Expires January 12, 2006 [Page 11] Internet-Draft RTP Payload for ECN Probing July 2005 8. Security Considerations Security considerations for the use of ECN for real-time inelastic flows is covered in [2]. The main consideration to account for here is that when the payload is carrying any relevant information for admission control, the payload SHOULD be secured, e.g., using "The Secure Real-time Transport Protocol (SRTP)" [5] or "Security Architecture for the Internet Protocol" [6]. If an application is attempting to detect Cheaters and the payload is not secured, a cheating device will be able to inspect and modify the ECN field in the payload, thereby circumventing the detection method. Alexander & Babiarz Expires January 12, 2006 [Page 12] Internet-Draft RTP Payload for ECN Probing July 2005 9. IANA Considerations The Version field described in "Version" (Section 4.4.1) will need to be administered. This field should be administered on a first come, first served basis. IANA is requested to make MIME type registrations as specified above in "MIME Registration" (Section 7). Alexander & Babiarz Expires January 12, 2006 [Page 13] Internet-Draft RTP Payload for ECN Probing July 2005 10. Acknowledgements The authors acknowledge a great many inputs, including the following: Francois Audet, Amy Pendleton, Tom Taylor, John Rutledge, Jeremy Matthews, Marvin Krym, Stephen Dudley, and Kwok Ho Chan. Alexander & Babiarz Expires January 12, 2006 [Page 14] Internet-Draft RTP Payload for ECN Probing July 2005 11. References 11.1 Normative References [1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003. [2] Babiarz, J., Chan, K., and V. Firoiu, "Congestion Notification Process for Real-Time Traffic", Internet-Draft draft-babiarz-tsvwg-rtecn-04.txt (Work in Progress), July 2005. [3] Alexander, C., Ed., Babiarz, J., and J. Matthews, "Real-time ECN Use Cases", Internet-Draft draft-alexander-rtecn-use-cases-00.txt (Work in Progress), July 2005. 11.2 Informative References [4] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [5] Baugher, M., Carrara, E., McGrew, D., Naslund, M., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [6] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [7] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. Authors' Addresses Corey W. Alexander (editor) Nortel MS 08704A30 2370 Performance Drive Richardson, TX 75082 US Phone: +1 972 684-8320 Fax: +1 972 684-1838 Email: coreya@nortel.com Alexander & Babiarz Expires January 12, 2006 [Page 15] Internet-Draft RTP Payload for ECN Probing July 2005 Jozef Babiarz Nortel MS 04331C04 3500 Carling Avenue Ottawa, Ontario K2H 8E9 CA Phone: +1 613 763-6098 Fax: +1 613 763-2231 Email: babiarz@nortel.com Alexander & Babiarz Expires January 12, 2006 [Page 16] Internet-Draft RTP Payload for ECN Probing July 2005 Intellectual Property Statement 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. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Copyright Statement Copyright (C) The Internet Society (2005). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Alexander & Babiarz Expires January 12, 2006 [Page 17]