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Network Working Group                                          D. SingerRequest for Comments: 5450                           Apple Computer Inc.Category: Standards Track                                    H. Desineni                                                                Qualcomm                                                              March 2009Transmission Time Offsets in RTP StreamsStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (c) 2009 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents in effect on the date of   publication of this document (http://trustee.ietf.org/license-info).   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Abstract   This document describes a method to inform Real-time Transport   Protocol (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.Singer & Desineni           Standards Track                     [Page 1]

RFC 5450                RTP Transmission Offsets              March 2009Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Requirements Notation . . . . . . . . . . . . . . . . . . . . .33.  Transmission Offset . . . . . . . . . . . . . . . . . . . . . .34.  Extended Jitter Reports . . . . . . . . . . . . . . . . . . . .55.  Signaling (Setup) Information . . . . . . . . . . . . . . . . .66.  Security Considerations . . . . . . . . . . . . . . . . . . . .67.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .78.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . .79.  Normative References  . . . . . . . . . . . . . . . . . . . . .71.  Introduction   In the Real-time Transport Protocol (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 not be true   under some circumstances, such as:   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 that 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 retransmissions are sent, the retransmitted 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; nor   does this specification.  This specification merely provides   information on the relationship between the relative transmission   times and relative RTP timestamps.Singer & Desineni           Standards Track                     [Page 2]

RFC 5450                RTP Transmission Offsets              March 2009   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 RTP Control Protocol (RTCP) packet is defined to   enable this reporting.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].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 Network Time Protocol (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 signaled to the receiver in-band using the   general mechanism for RTP header extensions [RFC5285].  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 withSinger & Desineni           Standards Track                     [Page 3]

RFC 5450                RTP Transmission Offsets              March 2009   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 KB):   200    2 KB   300    4 KB   400    2 KB   500   12 KB   600   ...effective end of stream   This has 20 KB 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   2 KB   x + 040   4 KB   x + 120   2 KB   x + 160  12 KB   x + 400 ...effective end of stream   The choice of x is essentially 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,   meaning that x is 200.  Then the offset values are:      0    -60    -80   -140Singer & Desineni           Standards Track                     [Page 4]

RFC 5450                RTP Transmission Offsets              March 2009   This is because in this case, I traffic-smooth by conceptually   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 to be tagged with   this extension.  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   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.  Therefore, for   consistency, the jitter calculation should continue to operate on the   'raw' reception times.  However, seeSection 4 on extended jitter   reports, below.   There are no extensionattributes 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.  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.)4.  Extended Jitter Reports   The inter-arrival jitter computed as defined in Section 6.4.1 ofRFC3550 provides inter-arrival jitter reports that include any source-   introduced jitter (transmission time offsets).  If it is desired toSinger & Desineni           Standards Track                     [Page 5]

RFC 5450                RTP Transmission Offsets              March 2009   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            |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |                      inter-arrival jitter                     |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       .                                                               .       .                                                               .       .                                                               .       |                      inter-arrival 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   (reception report count) as the receiver report.  The content is   exactly that number of inter-arrival 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, the formula uses 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 inter-arrival jitter in this   packet and in the receiver report will be identical).   Precisely, the replacement equation for the equation in the RTP   specification is as follows, where Rj is the most recent arrival   time:   D(i,j) = (Rj - Ri) - ((Sj + Oj) - (Si + Oi))          = (Rj - (Sj + Oj)) - (Ri - (Si + Oi))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).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           Standards Track                     [Page 6]

RFC 5450                RTP Transmission Offsets              March 2009   The underlying security considerations of [RFC3550] should be taken   into account.   It is possible that malicious senders (or systems tampering with   packets in transit) could send offsets that are implausible, could   confuse the receiver, or result in calculated jitter values that   might mislead the sender.  Both the sender and receiver of the   transmission offsets and jitter values should take care that such   behavior does not result in denial of service or other problems.7.  IANA Considerations   The RTCP packet type used for the adjusted inter-arrival jitter has   been registered, in accordance withSection 15 of [RFC3550].  IANA   has added a new value to the RTCP Control Packet types subregistry of   the Real-Time Transport Protocol (RTP) Parameters registry, according   to the following data:   abbrev.  name                                  value   Reference   -------  ------------------------------------  ------  ---------   IJ       Extended inter-arrival jitter report  195RFC 5450   Additionally, IANA has registered a new extension URI to the RTP   Compact Header Extensions subregistry of the Real-Time Transport   Protocol (RTP) Parameters registry, according to the following data:      Extension URI: urn:ietf:params:rtp-hdrext:toffset      Description:   Transmission Time offsets      Contact:       singer@apple.com      Reference:RFC 54508.  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.9.  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", STD 64,RFC 3550, July 2003.   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP              Header Extensions",RFC 5285, July 2008.Singer & Desineni           Standards Track                     [Page 7]

RFC 5450                RTP Transmission Offsets              March 2009Authors' 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.comSinger & Desineni           Standards Track                     [Page 8]