■■ Lost packets reported as a fraction of the total and as a cumulative
number.
■■ Highest sequence number received.
■■ Inter-arrival jitter and other parameters.Listening-only participants will send Receiver Reports (RR) to applications
that monitor the quality of service, while speaking participants will also send
Sender Reports (SR).
A special Source Description RTCP packet (SDES) conveys information
about the user:
■■ Canonical Name (CNAME) to identify the participants in the conference
■■ Username (such “John Doe, Bit Recycler, Megacorp”)
■■ Phone number
■■ Geographic user location
■■ Name of the application that is using RTP/RTCPRTP also uses protocol-specific devices such as translators and mixers.
An RTP translator connects two different transport networks, such as IP v.4
and IP v.6 networks. Translators also may change the media encoding as
required, allowing two endpoints that have no common codecs to be able to
communicate.
An RTP mixer receives media streams from several sources, combines them,
and forwards the combined stream. An RTP mixer will add its own SSRC iden-
tifier to the existing identifiers in the component streams.RTP Payloads and Payload Format Specifications
The Audio/Visual Profile (AVP) for RTP [18] specifies payloads registered
with the Internet Assigned Numbers Authority (IANA) and specifies such
items as the name, clock rate, or frame size of audio codecs and encoding inde-
pendent parameters (such as the audio left, right, center, surround, front, and
rear).
RTP payloads are grouped for specific applications (such as for audio/video
conferencing). Payload types specify specific codecs, such as for MPEG-4
streams, DV format video, or Enhanced Variable Rate Codec (EVRC)
Speech [19].
The highly structured and open approach of the RTP payload and format
specifications has led to a rich portfolio of standard payloads for the most-
used audio and video codecs.
RTP also allows dynamic payloads, which are defined at the initiation of a
session.92 Chapter 5