Internet Engineering Task Force Flemming Andreasen MMUSIC Working Group David Oran INTERNET-DRAFT Dan Wing Expires: August 2004 Cisco Systems February, 2004 RTP No-Op Payload Format 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 cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-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 document defines an no-op payload format for the Real-time Transport Protocol (RTP), and a mechanism to request an immediate RTCP report. This can be used to verify RTP connectivity and to keep Network Address Translator (NAT) bindings and Firewall pinholes open. INTERNET-DRAFT RTP No-Op Payload February 2004 TABLE OF CONTENTS 1. Introduction.....................................................2 1.1 Notational Conventions............................................2 2. RTP Payload Format for No-Op.......................................3 2.2 Registration......................................................3 2.3 Use of RTP Header Fields..........................................3 2.4 Payload Format....................................................3 2.5 Sender Operation..................................................4 2.6 Mixer, Translator Operation.......................................4 2.7 Receiver Operation................................................4 2.8 Indication of No-OP Capability using SDP..........................5 3. MIME Registration..................................................5 3.1. audio/no-op......................................................5 4. Security Considerations............................................6 5. Acknowledgements...................................................6 6. Authors' Addresses.................................................6 7. Normative References...............................................6 8. Informative References.............................................6 Intellectual Property Statement.......................................7 Full Copyright Statement..............................................7 Acknowledgement.......................................................8 1. Introduction This memo defines a new RTP payload format called "no-op". This payload behaves like a normal RTP payload, except that it isn't played by the receiver. This new payload format is useful for: * bearer continuity testing, such as at the beginning of a call; * keepalives to keep NAT bindings open when RTP media traffic is not otherwise being transmitted; For testing the RTP path, an RTP sender may transmit several No-Op payload packets with the Request Immediate RTCP bit set to 0, followed by one No-Op payload packet with the Request Immediate RTCP bit set to 1. This would cause the RTP receiver to send an RTCP report indicating the quality of the RTP path. The RTP sender could then decide to continue with call setup, abort the session, or perform some other action. 1.1 Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Andreasen, Oran, Wing [Page 2] INTERNET-DRAFT RTP No-Op Payload February 2004 2. RTP Payload Format for No-Op The no-op payload format is carried as part of the RTP stream, and MUST use the same sequence number space, SSRC, and timestamp base as the regular media. 2.2 Registration The RTP payload format is designated as "no-op" and the MIME type as "audio/no-op". The default clock rate is 8000 Hz, but other rates MAY be used. In accordance with current practice, this payload format does not have a static payload type number, but uses a RTP payload type number established dynamically and out-of-band. 2.3 Use of RTP Header Fields Timestamp: The RTP timestamp reflects the measurement point for the current packet. The receiver calculates jitter for RTCP receiver reports based on all packets with a given timestamp. Note: The jitter value should primarily be used as a means for comparing the reception quality between two users or two time-periods, not as an absolute measure. Marker bit: The RTP marker bit has no special significance for this payload type. 2.4 Payload Format The 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | padding (OPTIONAL) | | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The payload contains at least 4 bytes. The first 32 bits are defined as follows: bit 0: "R", "Request immediate RTCP", is used to request transmission of an immediate RTCP report (see section 2.7). bits 1-31: Reserved, and all bits MUST be 0. Additional padding bytes MAY be appended up to the negotiated ptime value in SDP (see section 2.6). These bytes MUST contain all 0 Andreasen, Oran, Wing [Page 3] INTERNET-DRAFT RTP No-Op Payload February 2004 bits. Padding may be useful to generate RTP packets that are the same size as another payload (such as a normal voice payload). 2.5 Sender Operation A source MAY send normal RTP audio and the no-op payload format for the same time instants (but with different sequence numbers of course). This might be done in conjunction with this payload format's "Request Immediate RTCP" opcode. 2.6 Mixer, Translator Operation An RTP mixer or unicast-to-unicast RTP translator SHOULD forward RTP No-Op payload packets normally. A unicast-to-multicast RTP translator SHOULD replicate RTP No-Op payload packets normally. A multicast-to-unicast RTP translator SHOULD NOT replicate an RTP No-Op packet with the Request Immediate RTCP bit set, because the receivers won't be able to prevent flooding of the multicast RTP sender. 2.7 Receiver Operation Upon receipt of an RTP packet with the No-Op payload format and the Send Immediate RTCP Report bit set to 0, the receiver performs normal RTP receive operations on it -- incrementing the RTP receive counter, calculating jitter, and so on. The receiver then discards the packet -- it is not used to play out data. Upon receipt of an RTP packet with the No-Op payload format and the Send Immediate RTCP Report bit set to 1, the receiver performs the steps above and: * if listening on a multicast IP address, the receiver MUST not send an immediate RTCP report, and the receiver MUST follow the normal RTCP transmission rules [RFC3550, sections 6.2 and 6.3]. * if listening on a unicast IP address and sending RTP traffic, the receiver prepares to send an RTCP sender report, and * if listening on a unicast IP address and receiving RTP traffic, the receiver prepares to send an RTCP receiver report. In all cases, before actually sending its RTCP report, the RTCP bandwidth limits and randomization interval MUST be observed [RFC3550, sections 6.2 and 6.3], most especially when multiple SSRCs are in the same session. Andreasen, Oran, Wing [Page 4] INTERNET-DRAFT RTP No-Op Payload February 2004 2.8 Indication of No-OP Capability using SDP Senders and receivers may indicate support for the No-Op payload format, for example, by using the Session Description Protocol ([SDP]). If successful completion of RTP No-Op is required before completing call establishment -- such as to verify the existence or quality of the bearer path -- No-Op preconditions can be used [Andreasen]. The default packetization interval for this payload type is 20ms (ptime:20) but alternate values can be advertised in SDP using the ptime attribute value [SDP]. 3. MIME Registration 3.1. audio/no-op MIME media type name: audio MIME subtype name: no-op Required parameters: none Optional parameters: none Encoding considerations: This type is only defined for transfer via RTP [1]. Security considerations: See the "Security Considerations" section in this document. Interoperability considerations: none Published specification: This document. Applications which use this media: The "no-op" audio subtype is used to maintain network state or verify network connectivity, when a more traditional RTP payload type cannot be used. Additional information: 1. Magic number(s): N/A 2. File extension(s): N/A 3. Macintosh file type code: N/A Andreasen, Oran, Wing [Page 5] INTERNET-DRAFT RTP No-Op Payload February 2004 4. Security Considerations Without security of the RTP stream (via SRTP [SRTP], IPsec, or other means), it is possible for an attacker to spoof RTP packets, including this new packet type. As this new RTP payload type includes a method to request immediate transmission of RTCP, this could be used to cause endpoints to flood the network with RTCP reports. Thus, the RTCP transmissions MUST NOT exceed the bandwidth recommendations described in section 6.3 of [RFC3550]. 5. Acknowledgements Thanks to Henning Schulzrinne for suggesting using RTCP as a feedback mechanism. 6. Authors' Addresses Flemming Andreasen Cisco Systems, Inc. 499 Thornall Street, 8th Floor Edison, NJ 08837 USA EMail: fandreas@cisco.com David Oran Cisco Systems, Inc. 7 Ladyslipper Lane Acton, MA 01720 USA EMail: oran@cisco.com Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 USA EMail: dwing@cisco.com 7. Normative References [RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", http://www.ietf.org/rfc/rfc3550.txt. 8. Informative References [Andreasen] F. Andreasen, "No-Op Preconditions", Work In Progress. Andreasen, Oran, Wing [Page 6] INTERNET-DRAFT RTP No-Op Payload February 2004 [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple Capability Declaration", October 2002, http://www.ietf.org/rfc/rfc3407.txt [SDP] M. Handley and V. Jacobson, "SDP: Session Description Protocol", April 1998, http://www.ietf.org/rfc/rfc2327.txt. Intellectual Property Statement 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. 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Andreasen, Oran, Wing [Page 7] INTERNET-DRAFT RTP No-Op Payload February 2004 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Andreasen, Oran, Wing [Page 8]