AVT D. Singer Internet-Draft Apple Computer Inc. Intended status: Standards Track H. Desineni Expires: July 5, 2007 Qualcomm January 2007 Transmission Time offsets in RTP streams draft-ietf-avt-rtp-toffset-05.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 July 5, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Singer & Desineni Expires July 5, 2007 [Page 1] Internet-Draft RTP Transmission Offsets January 2007 Abstract This document describes a method to inform RTP clients when RTP packets are transmitted at a time other than their 'nominal' transmission time. It also provides a mechanism to provide improved inter-arrival jitter reports from the clients, that take into account the reported transmission times. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 3. Transmission offset . . . . . . . . . . . . . . . . . . . . . 5 4. Extended Jitter Reports . . . . . . . . . . . . . . . . . . . 8 5. Signaling (setup) information . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 8. RFC Editor Considerations . . . . . . . . . . . . . . . . . . 12 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13 10. Normative References . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property and Copyright Statements . . . . . . . . . . 16 Singer & Desineni Expires July 5, 2007 [Page 2] Internet-Draft RTP Transmission Offsets January 2007 1. Introduction In the RTP specification [RFC3550] network jitter calculations are based on the presumption that packets are transmitted essentially in accordance with their RTP timestamps. This must be true, of course, on average over longer time intervals, as the client is playing the packets out according to those timestamps. However, for individual packets, this may under some circumstances not be true: o When the data rate of the stream is bursty, such as with video where I-frames may be significantly larger than P or B frames, traffic smoothing may need to be applied to maintain an appropriate data rate. o In video which has forward decode dependencies, frames may need to be transmitted in decoding order (the sequence number order) but with, of course, presentation timestamps. Under these circumstances the transmission time of a frame sent early in sequence does not correspond to its RTP timestamp. o When re-transmissions are sent, the re-transmitted packet clearly has a different actual transmission time from the original, even though they share the same timestamp. Under some circumstances it can help the receiver, or intermediate network elements, to know the actual transmission time of the packet. This RTP header extension element allows the communication of this information. The RTP specification does not define a transmission timestamp, and nor does this specification. This specification merely provides information on the relationship between the relative transmission times and relative RTP timestamps. This specification allows the transmitter to indicate to the receiver any known variation between the spacing of transmission times and the spacing of RTP timestamps; any unreported variation introduced at or after the point of measurement of the transmission time will be treated as network jitter by the receiver. The definition of the point where the transmission time is measured or defined is left to the transmitter, though it should, of course, be consistent from packet to packet. This information can also be of use to report the inter-arrival jitter caused by the network, excluding that introduced by the source. A new RTCP packet is defined to enable this reporting. Singer & Desineni Expires July 5, 2007 [Page 3] Internet-Draft RTP Transmission Offsets January 2007 2. Requirements notation 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 [RFC2119]. Singer & Desineni Expires July 5, 2007 [Page 4] Internet-Draft RTP Transmission Offsets January 2007 3. Transmission offset Classically a pair of RTP packets with timestamps S2 and S1 are transmitted with a time interval between them of (S2 - S1). This specification permits sending an offset value O in each packet, O1 and O2. One characteristic of these offsets is that the original transmission interval can be deduced to be (S2 + O2) - (S1 + O1). More precisely, the offset is defined as follows (with the function RtoN converting from RTP to NTP times, and NtoR doing the reverse): o Take an RTP stream that has a recent RTCP sender report relating RTP timestamp S0 to NTP timestamp N0; o consider a packet sent after that with RTP timestamp S1. Nominally this is sent at N1 = (N0 + RtoN(S1 - S0)); o If it was actually sent at a different time, Na, then the offset value O1 is O1 = NtoR(Na - N1). The transmission time is signalled to the receiver in-band using The IETF Generic RTP header extension [hdrext]. The payload of this extension (the transmitted value) is a 24-bit signed integer. When added to the RTP timestamp of the packet, it represents the "effective" RTP transmission time of the packet, on the RTP timescale. The reported transmission time T1 of a packet with timestamp S1 and an offset of O1, from the above equations, is T1 = S1+O1 (though of course the transmission time values only have meaning when two or more are compared). The form of the transmission offset extension block is as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID | len=2 | transmission offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The length field takes the value 2 to indicate that 3 bytes follow. The sign of the offset value depends greatly on the choice of the initial mapping of RTP to NTP times. In general, without scanning a stream entirely it is not possible to ensure that this mapping would keep all the offsets positive; therefore this specification allows negative values. Imagine a stream with the following timestamps and sizes in bytes: Singer & Desineni Expires July 5, 2007 [Page 5] Internet-Draft RTP Transmission Offsets January 2007 200 2kb 300 4kb 400 2kb 500 12kb 600 ...effective end of stream This has 20KB spread over 400 time units, i.e. on average 1 kb per 20 time-units. We traffic-smooth this, and establish that given a transmission time of x for the first packet, we would transmit the following packets at the given intervals later: x + 000 2kb x + 040 4kb x + 120 2kb x + 160 12kb x + 400 ...effective end of stream The choice of x is esssentially arbitrary: only relative values of timestamps matter. Now, let's say I claim on the first packet that it went out *at* its RTP timestamp, i.e. with an offset of 0, i.e. x is 200. Then the offset values are 0 -60 -80 -140 This is because in this case, I traffic-smooth this because conceptually I think of sending the small packets 'early'. But since only the relative values are significant, it is just as valid to say x is 400, whereupon the offset values are 200 140 120 60 In a stream where this extension is not in effect (i.e. not declared or negotiated), the actual transmission offset is therefore unknown. However, when the extension is in effect for the stream, it MAY be omitted in those packets for which the offset is 0 (zero); that is, packets sent at their nominal time do not need this extension tagging. Therefore the implied transmission time of an un-tagged RTP packet depends on whether the extension is in effect for the stream (and therefore the transmission offset is 0) or not (whereupon the transmission offset is unknown). The jitter calculations performed by an RTP client MUST NOT use these Singer & Desineni Expires July 5, 2007 [Page 6] Internet-Draft RTP Transmission Offsets January 2007 transmission offsets. In general, the sender (or intermediate network elements doing RTP analysis) cannot always know whether the offsets have been taken into account or not, and for consistency therefore, the jitter calculation should continue to operate on the 'raw' reception times. However, see the section on extended jitter reports, below. There are no extension attributes defined for this extension. It is structurally possible to have more than one extension of the same type in a packet. However, this extension is only defined for the source to report, and intermediate network nodes that are not the source of the RTP session MUST NOT add this extension (whether or not it was previously present) and MUST NOT alter the existing transmission offset value in a packet, if the extension is already present. (Of course, it is clear that network elements that terminate an RTP flow, and are the source for a new RTP flow, can add a transmission offset extension header to the RTP packets of the new flow, if desired.) Singer & Desineni Expires July 5, 2007 [Page 7] Internet-Draft RTP Transmission Offsets January 2007 4. Extended Jitter Reports The inter arrival jitter computed as defined in Sec 6.4.1 of RFC3550 provides inter-arrival jitter reports which include any source- introduced jitter (transmission time offsets). If it is desired to indicate the actual network jitter, excluding the source-introduced jitter, the new RTCP packet type defined here may be used. It has the following form: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ hdr |V=2|P| RC | PT=IJ=195 | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | interarrival jitter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . | interarrival jitter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ If present, this RTCP packet must be placed after a receiver report (inside a compound RTCP packet), and MUST have the same value for RC as the receiver report. The content is exactly that number of interarrival jitter calculations, calculated using the same formula as for sender and receiver reports, but taking into account the transmission offsets for the streams (if any); that is, using the values T1=S1+O1, T2 etc. as defined above, instead of S1, S2 etc.. (If no transmission offset information is given for a stream, then the value of interarrival jitter in this packet and in the receiver report will be identical). Precisely, the replacement equation for the equation in the RTP specification is, where Ri is the arrival time: D(i,j) = (Rj - Ri) - ((Sj + Oj) - (Si + Oi)) = (Rj - (Sj + Oj)) - (Ri - (Si + Oi)) Singer & Desineni Expires July 5, 2007 [Page 8] Internet-Draft RTP Transmission Offsets January 2007 5. Signaling (setup) information The URI for declaring this header extension in an extmap attribute is "urn:ietf:params:rtp-hdrext:toffset". There is no additional setup information needed for this extension (no extensionattributes). Singer & Desineni Expires July 5, 2007 [Page 9] Internet-Draft RTP Transmission Offsets January 2007 6. Security Considerations The given transmission offsets are only informative and it is hard to see security considerations from associating them with media streams. Singer & Desineni Expires July 5, 2007 [Page 10] Internet-Draft RTP Transmission Offsets January 2007 7. IANA Considerations The RTCP packet type used for the adjusted interarrival jitter needs to be registered, in accordance with section 15 of [RFC3550]. The abbreviation is "IJ", the full name is "Extended inter-arrival jitter report", the suggested value is 195, and the specification is this document. The name toffset needs to be registered into the rtp-hdrext section of the urn:ietf: namespace, referring to RFCxxxx. Singer & Desineni Expires July 5, 2007 [Page 11] Internet-Draft RTP Transmission Offsets January 2007 8. RFC Editor Considerations The reference to an Internet Draft needs to be updated to the RFC when it is published (which should be before this draft). RFCxxxx in the IANA considerations needs to be updated with the number of this RFC. Singer & Desineni Expires July 5, 2007 [Page 12] Internet-Draft RTP Transmission Offsets January 2007 9. Acknowledgments Ron Frederick, Colin Perkins, and Steve Casner all contributed substantially to this document, and their help and contributions helped turn an idea into a specification. Singer & Desineni Expires July 5, 2007 [Page 13] Internet-Draft RTP Transmission Offsets January 2007 10. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, STD 0064, July 2003. [hdrext] Singer, D., "A general mechanism for RTP Header Extensions", ID draft-ietf-avt-rtp-hdrext-08, October 2006. Singer & Desineni Expires July 5, 2007 [Page 14] Internet-Draft RTP Transmission Offsets January 2007 Authors' Addresses David Singer Apple Computer Inc. 1 Infinite Loop Cupertino, CA 95014 US Phone: +1 408 996 1010 Email: singer@apple.com Harikishan Desineni Qualcomm 5775 Morehouse Drive San Diego, CA 92121 US Phone: +1 858 845 8996 Email: hd@qualcomm.com Singer & Desineni Expires July 5, 2007 [Page 15] Internet-Draft RTP Transmission Offsets January 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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. 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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 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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Singer & Desineni Expires July 5, 2007 [Page 16]