Network Working Group C. Alexander, Ed. Internet-Draft J. Babiarz Expires: August 15, 2005 Nortel February 11, 2005 RTP Payload Format for ECN Probing draft-alexander-rtp-payload-for-ecn-probing-00.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 3 of RFC 3667. 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 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. This Internet-Draft will expire on August 15, 2005. 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 [3]. 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 Alexander & Babiarz Expires August 15, 2005 [Page 1] Internet-Draft ECN Probing February 2005 of probing packet formats. 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 Sender Congestion Indication (SCI) . . . . . . . . . . 7 4.4.3 Responder Congestion Indication (RCI) . . . . . . . . 7 4.4.4 Sender Congestion Indication (SCI) Sequence Number . . 7 4.4.5 Reserved . . . . . . . . . . . . . . . . . . . . . . . 7 5. Considerations for New Payload Format . . . . . . . . . . . . 8 5.1 Extensibility Considerations . . . . . . . . . . . . . . . 8 5.2 Flexibility Considerations . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1 Normative References . . . . . . . . . . . . . . . . . . . 12 9.2 Informative References . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12 Intellectual Property and Copyright Statements . . . . . . . . 14 Alexander & Babiarz Expires August 15, 2005 [Page 2] Internet-Draft ECN Probing February 2005 1. Introduction This memo defines a new RTP payload format for use with applications requiring congestion detection along the data path, verification of data path connectivity, or measurement of Round Trip Time (RTT) between two endpoints, 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 Detection (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 "Admission Control Use Case for Real-time ECN" [3], although it may additionally be used in other contexts. The new RTP payload format defined herein is called "admcntl". 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 new payload format are: 1. congestion detection can be performed using a simple probing mechanism without having to extend other protocols; 2. the payload format accommodates both one-way and two-way/loop-back mechanisms; 3. the payload format allows for limited detection of devices making inappropriate changes to the ECN markings in the network; 4. 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. Depending on the probing mechanism by an implementation, some of the fields defined may not be used. 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 August 15, 2005 [Page 3] Internet-Draft ECN Probing February 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 RFC 2119 [9] and indicate requirement levels for compliant implementations. Alexander & Babiarz Expires August 15, 2005 [Page 4] Internet-Draft ECN Probing February 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. Request Probe Packet: In the context of a one-way probing mechanism, the Request Probe Packet is an RTP packet utilizing the "admcntl" payload format defined herein. In the context of a two-way probing mechanism, it is also an RTP packet using the "admcntl" payload format, but it is limited by definition to flow only from the Sender to the Responder. Responder: In the context of a two-way/loop-back probing mechanism, the Responder is an endpoint device which receives a Request Probe Packet from the Sender, and generates a Response Probe Packet in response. In the context of a one-way probing mechanism, the Responder is simply an endpoint device which receives a Request Probe Packet from the Sender. In this context, however, the Responder does not actually respond with a Response Probe Packet. Response Probe Packet: In the context of a two-way probing mechanism, the Response Probe Packet is an RTP packet utilizing the "admcntl" payload. It differs from the Request Probe Packet in that it is limited by definition to flow only from the Responder to the Sender. A Response Probe Packet has no definition in the context of a one-way probing mechanism. Sender: In the context of both a one-way and a two-way/loop-back probing mechanism, the Sender is an endpoint device which generates a Request Probe Packet. Alexander & Babiarz Expires August 15, 2005 [Page 5] Internet-Draft ECN Probing February 2005 4. RTP Payload Format for Real-Time ECN Admission Control The "admcntl" 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 "admcntl", with a MIME type of "audio/admcntl" for audio and a MIME type of "video/admcntl" for video. The RTP 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 01, 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, and communicated to all application devices involved. 4.4 Payload Format The "admcntl" 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|SCI|RCI| SCI Sequence Number | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: admcntl Payload Format It consists of five fields: Version, SCI, RCI, SCI Sequence Number, and Reserved. Alexander & Babiarz Expires August 15, 2005 [Page 6] Internet-Draft ECN Probing February 2005 4.4.1 Version The Version field designates the version of the payload format. At the time of this writing, this field MUST be set to zero. This field is provided for future extensibility of the payload to carry other information. 4.4.2 Sender Congestion Indication (SCI) This field contains a two-bit ECN value associated with the Sender. It has meaning in both one-way and two-way/loop-back probing mechanisms. For both one-way and two-way/loop-back probing mechanisms, the Sender MUST set this field in the Request Probe Packet to the two-bit ECN value it uses to send the packet. For a two-way/loop-back probing mechanism, the Responder MUST set this field in the Response Probe Packet to the two-bit ECN value retrieved from the ECN field in the IP header of the associated Request Probe Packet. 4.4.3 Responder Congestion Indication (RCI) This field contains a two-bit ECN value associated with the Responder. It has meaning only with two-way/loop-back probing mechanisms. For two-way/loop-back probing mechanisms, the Responder MUST set this field in the Response Probe Packet to the two-bit ECN value it uses to send the packet. For a one-way probing mechanism, this field is unused. 4.4.4 Sender Congestion Indication (SCI) Sequence Number This field contains a 16-bit sequence number. It has meaning only with two-way/loop-back probing mechanisms. For two-way/loop-back probing mechanisms, the Responder MUST set this field in the Response Probe Packet to the 16-bit sequence number in the RTP header of the associated Request Probe Packet. For a one-way probing mechanism, this field is unused. 4.4.5 Reserved This field contains eight bits which are reserved for future use. Alexander & Babiarz Expires August 15, 2005 [Page 7] Internet-Draft ECN Probing February 2005 5. Considerations for New 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 the new 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 and/or implementations. 5.2 Flexibility Considerations In addition to planning the payload format for extensibility, another flexibility consideration is to allow the initial definition of the payload to be used in as wide a range of implementations as possible. In the simplest form of the two-way/loop-back mechanisms, probing for ECN only requires the two-bit SCI field in which to return the received ECN marking. However, the definition of this single field does not limit the use of this payload format only to two-way/loop-back mechanisms. By adding the RCI field for a two-way/loop-back mechanism, and extending the definition of the SCI field for one-way probing mechanisms, both mechanisms can utilize these fields to perform limited detection of Cheaters. The SCI Sequence Number field is also intended to allow for such detection to be performed. The minimum length of the payload is 4 octets, although it MAY be padded to simulate a specific codec. In this case, the application also needs to ensure that the packets carrying the padded payload are sent at the appropriate rate corresponding to the codec being mimicked. Alexander & Babiarz Expires August 15, 2005 [Page 8] Internet-Draft ECN Probing February 2005 6. 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]. Alexander & Babiarz Expires August 15, 2005 [Page 9] Internet-Draft ECN Probing February 2005 7. IANA Considerations The Version field in the admcntl payload format will need to be administered. This field should be administered on a first come, first served basis. Additional details will be provided in future revisions of this memo. Alexander & Babiarz Expires August 15, 2005 [Page 10] Internet-Draft ECN Probing February 2005 8. Acknowledgements The authors acknowledge a great many inputs, including the following: John Rutledge, Jeremy Matthews, Marvin Krym, Stephen Dudley, and Kwok Ho Chan. Alexander & Babiarz Expires August 15, 2005 [Page 11] Internet-Draft ECN Probing February 2005 9. References 9.1 Normative References [1] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, January 2005. [2] Babiarz, J., Chan, K. and V. Firoiu, "Congestion Notification Process for Real-Time Traffic, draft-babiarz-tsvwg-rtecn-03", Internet-Draft Work in Progress, February 2005. [3] Alexander, C., Ed., Babiarz, J. and J. Matthews, "Admission Control Use Case for Real-time ECN, draft-alexander-rtecn-admission-control-use-case-00", Internet-Draft Work in Progress, February 2005. 9.2 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)", Internet-Draft Work in Progress, March 2004. [6] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", Internet-Draft Work in Progress, November 1998. Authors' Addresses Corey W. Alexander (editor) Nortel MS 08704A30 2370 Performance Drive Richardson, TX 75287 US Phone: +1 972 684-8320 Fax: +1 972 684-1838 Email: coreya@nortel.com Alexander & Babiarz Expires August 15, 2005 [Page 12] Internet-Draft ECN Probing February 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 August 15, 2005 [Page 13] Internet-Draft ECN Probing February 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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