AVT F. Andreasen Internet-Draft D. Oran Expires: November 10, 2005 D. Wing Cisco Systems, Inc. May 9, 2005 A No-op payload format for the Realtime Transport Protocol draft-wing-avt-rtp-noop-02 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 November 10, 2005. Copyright Notice Copyright (C) The Internet Society (2005). 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. Requirements Language Andreasen, et al. Expires November 10, 2005 [Page 1] Internet-Draft RTP No-Op Payload Format May 2005 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 [1]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. RTP Payload Format for No-Op . . . . . . . . . . . . . . . . . 3 2.1 Registration . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Use of RTP Header Fields . . . . . . . . . . . . . . . . . 4 2.3 Payload Format . . . . . . . . . . . . . . . . . . . . . . 4 2.4 Sender Operation . . . . . . . . . . . . . . . . . . . . . 4 2.5 Mixer, Translator Operation . . . . . . . . . . . . . . . 5 2.6 Receiver Operation . . . . . . . . . . . . . . . . . . . . 5 2.7 Indication of No-OP Capability using SDP . . . . . . . . . 6 3. Example SDP Offer/Answer . . . . . . . . . . . . . . . . . . . 6 4. MIME Registration . . . . . . . . . . . . . . . . . . . . . . 6 4.1 audio/no-op . . . . . . . . . . . . . . . . . . . . . . . 7 4.2 video/no-op . . . . . . . . . . . . . . . . . . . . . . . 7 4.3 text/no-op . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1 Normative References . . . . . . . . . . . . . . . . . . . 9 8.2 Informational References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . 11 Andreasen, et al. Expires November 10, 2005 [Page 2] Internet-Draft RTP No-Op Payload Format May 2005 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. It is also explicitly designed to interact constructively with the RTCP feedback profile [5]. This new payload format is useful for: o media session reception quality assessment, such as at the beginning of a session; o keepalives to keep NAT bindings and/or firewall pinholes open when RTP media traffic is not otherwise being transmitted. In addition it has a number of uses whose utility is speculative but for which it is easy pressed into service: o measurement-based admission control by probing available bandwidth, and o synthetic load generation for performance testing and other minimally-intrusive instrumentation. Unlike Comfort noise [9], which is specific to voice RTP streams, RTP No-Op is applicable to any kind of RTP stream including video, audio, realtime text, or any other media type that would benefit from the capabilities listed above. This gives RTP No-Op an advantage as a NAT keepalive mechanism. Certain functions and RTP payload types can use RTP No-Op without re-inventing their own payload-specific NAT keepalive mechanism -- such as video muting, Clearmode [10], and text [11]. Some audio codecs have their own 'silence' packets. However, some codecs only send such 'silence' packets if the noise floor changes; G.729b [12] is an example of such a codec. RTP No-Op allows the RTP stack itself, rather than the codec, to send periodic packets as a keepalive mechanism. 2. RTP Payload Format for No-Op 2.1 Registration The RTP payload format is designated as "no-op" and the MIME types are "audio/no-op", "video/no-op", and "text/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 out-of-band, e.g. through SDP [3]. Andreasen, et al. Expires November 10, 2005 [Page 3] Internet-Draft RTP No-Op Payload Format May 2005 2.2 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. 2.3 Payload Format The payload format is shown below. 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 invocation of RTCP feedback by timely transmission of an RTCP report (see Section 2.6). bits 1-31: Reserved; contents are ignored. Additional padding bytes MAY be appended up to the negotiated ptime value in SDP (see Section 2.7). These bytes are ignored. Padding may be useful to generate RTP packets that are the same size as a normal media payload. 2.4 Sender Operation When an endpoint is in 'recvonly' or 'inactive' the endpoint isn't allowed to send RTP data. However, to keep a NAT binding alive, the endpoint will need to send packets over the RTP port. RTP No-Op is ideally suited to this. In comparison, if one participant in an audio multicast conference is in 'recvonly' or 'inactive', yet occasionally sends RFC3389 comfort noise packets in order to keep its NAT binding open, these comfort noise packets are interpreted as audio packets by receivers and mixers which can cause undesirable behavior -- such as selection of the primary speaker or the playout Andreasen, et al. Expires November 10, 2005 [Page 4] Internet-Draft RTP No-Op Payload Format May 2005 of comfort noise when no audio should be played. To keep NAT and firewall bindings active, endpoints MUST occasionally send a packet to their RTP peer. The type of RTP packet depends on the negotiated RTP payload: o if both ends support RFC3389 and the negotiated RTP payload is appropriate for RFC3389, an RFC3389 packet SHOULD be transmitted. o if both ends support RTP No-Op and the negotiated RTP payload is inappropriate for RFC3389, RTP No-Op SHOULD be transmitted. 2.5 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 unless: 1. all receivers are known to be operating under the bandwidth limitations rules of [5], and 2. the restriction of applicability to "small groups" in [5] is observed Otherwise the sender may be flooded with RTCP reports. 2.6 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 "Se""nd Immediate RTCP Report" bit set to 1, the receiver adjusts counters as described above and then also performs the following steps (with reference to the definitions and procedures of the AVPF profile [5]): 1. ascertains whether the associated RTP session is operating under the AVPF RTP profile (or one derived from it via combination with another RTP profile). If not the receiver takes no further action on this packet. If so, it continues as follows. 2. generates a feedback "Event" which in turn may trigger the generation of a "FB message". 3. sends the FB message as an "early RTCP packet" assuming the bandwidth constraints for feedback messages are satisfied. Andreasen, et al. Expires November 10, 2005 [Page 5] Internet-Draft RTP No-Op Payload Format May 2005 4. Otherwise, takes no further action 2.7 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 [3]. If the payload format is being used for connectivity verification (e.g. in conjunction with [4]) senders and receivers MUST advertise the AVPF profile (or a profile used in combination with it). 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 [3]. 3. Example SDP Offer/Answer Offer: v=0 o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com s c=IN IP4 host.atlanta.example.com t=0 0 m=audio 49170 RTP/AVP 0 33 a=rtpmap:0 PCMU/8000 a=rtpmap:33 no-op/8000 m=video 51372 RTP/AVP 31 36 a=rtpmap:31 H261/90000 a=rtpmap:36 no-op/90000 Answer: v=0 o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com s c=IN IP4 host.biloxi.example.com t=0 0 m=audio 49174 RTP/AVP 0 33 a=rtpmap:0 PCMU/8000 a=rtpmap:33 no-op/8000 m=video 49170 RTP/AVP 32 36 a=rtpmap:32 MPV/90000 a=rtpmap:36 no-op/8000 4. MIME Registration This section registers MIME types for audio/no-op, video/no-op, and text/no-op. Andreasen, et al. Expires November 10, 2005 [Page 6] Internet-Draft RTP No-Op Payload Format May 2005 4.1 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 [2]. Security considerations: See Section 5, "Security Considerations", in this document. Interoperability considerations: none Published specification: This document. Applications which use this media: The "no-op" application 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 4.2 video/no-op MIME media type name: video MIME subtype name: no-op Required parameters: none Optional parameters: none Encoding considerations: This type is only defined for transfer via RTP [2]. Security considerations: See Section 5, "Security Considerations", in this document. Interoperability considerations: none Andreasen, et al. Expires November 10, 2005 [Page 7] Internet-Draft RTP No-Op Payload Format May 2005 Published specification: This document. Applications which use this media: The "no-op" application 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 4.3 text/no-op MIME media type name: text MIME subtype name: no-op Required parameters: none Optional parameters: none Encoding considerations: This type is only defined for transfer via RTP [2]. Security considerations: See Section 5, "Security Considerations", in this document. Interoperability considerations: none Published specification: This document. Applications which use this media: The "no-op" application 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 5. Security Considerations Without security of the RTP stream (via SRTP [8], IPsec [7], 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 Andreasen, et al. Expires November 10, 2005 [Page 8] Internet-Draft RTP No-Op Payload Format May 2005 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 [2]. 6. IANA Considerations IANA is requested to make a MIME type registration as specified above in Section 4 7. Acknowledgments Thanks to Henning Schulzrinne for suggesting using RTCP as a feedback mechanism. 8. References 8.1 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [3] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [4] Andreasen, F., "Connectivity Preconditions for Session Description Protocol Media Streams", draft-andreasen-mmusic-connectivityprecondition-02 (work in progress), February 2005. [5] Ott, J. and S. Wenger, "Extended RTP Profile for RTCP-based Feedback(RTP/AVPF)", draft-ietf-avt-rtcp-feedback-11 (work in progress), August 2004. [6] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Multimedia Session Establishment Protocols", draft-ietf-mmusic-ice-04 (work in progress), February 2005. 8.2 Informational References [7] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [8] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Andreasen, et al. Expires November 10, 2005 [Page 9] Internet-Draft RTP No-Op Payload Format May 2005 Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [9] Zopf, R., "Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)", RFC 3389, September 2002. [10] Kreuter, R., "RTP Payload Format for a 64 kbit/s Transparent Call", RFC 4040, April 2005. [11] Hellstrom, G., "RTP Payload for Text Conversation", RFC 2793, May 2000. [12] International Telecommunications Union, "G.729 Annex B", November 1999, . 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 Andreasen, et al. 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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. Andreasen, et al. Expires November 10, 2005 [Page 11] Internet-Draft RTP No-Op Payload Format May 2005 Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Andreasen, et al. Expires November 10, 2005 [Page 12]