Transmission Time Offsets in RTP Streams
RFC 5450

Network Working Group                                          D. Singer
Request for Comments: 5450                           Apple Computer Inc.
Category: Standards Track                                    H. Desineni
                                                                Qualcomm
                                                              March 2009

                Transmission Time Offsets in RTP Streams

Status 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.

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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 2009

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . . . 3
   3.  Transmission Offset . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Extended Jitter Reports . . . . . . . . . . . . . . . . . . . . 5
   5.  Signaling (Setup) Information . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Normative References  . . . . . . . . . . . . . . . . . . . . . 7

1.  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