Media Transport Using RTP
The RTP document, RFC 3550 [17] consists of the following:
■■ RTP for media packet transport
■■ The RTCP to monitor the quality of service and generate reports to the
networkRTP uses UDP for transport over IP.
A complete treatment of RTP would require a book by itself. RTP has many
capabilities, including minimal control for multimedia conferences.
We will provide here in a nutshell only the most relevant aspects required to
understand the environment for SIP. We refer readers to the Web site of the
IETF AVT working group (WG) at http://ietf.org/html.charters/
avt-charter.htmlfor more information.
Audio and video packets are encapsulated in RTP packets that provide the
following information carried in the RTP header:
■■ Packet sequence number—Allows the user to reorder packets on arrival
and to detect the loss of packets.
■■ Timestamps—Allows jitter to be detected (that is, packet arrival time
variations across the network).
■■ Synchronization (media) source—Allows the identification of the sources
of the packet streams (such as specific microphones or specific video
cameras).
■■ Contributing media source—Allows the identification of a specific media
source from several others that have been mixed together (see the fol-
lowing discussion), for example in a centralized multiparty conference.It is important to realize that RTP is an application layer protocol and does
not provide any QoS guarantees at all. However, it does allow transmission
impairments such as packet loss or jitter to be detected.
RTCP uses data at the receiver to convey back to senders in the network that
monitor QoS to perform fault diagnosis, and report long-term statistic data.
Information conveyed by RTCP includes the following:
■■ The Network Time Protocol (NTP) timestamps can be used to assess
absolute round-trip delay.
■■ The RTP timestamps can be used in conjunction with NTP timestamps,
(for example, to assess the local RTP clock rate).
■■ Synchronization (media) source identifier (SSRC).
■■ Packet and byte counts.Real-Time Internet Multimedia 91