SDP 协议简单解析

SDP—Session Description Protocol

The Session Description Protocol, defined by RFC 2327 [1], was developed by the IETF MMUSIC working group. It is more of a description syntax than a protocol in that it does not provide a full-range media negotiation capability. The original purpose of SDP was to describe multicast sessions set up over the Internet's multicast backbone, the MBONE. The first application of SDP was by the experimental Session Announcement Protocol (SAP) [2] used to post and retrieve announcements of MBONE sessions. SAP messages carry a SDP message body, and was the template for SIP's use of SDP. Even though it was designed for multicast, SDP has been applied to the more general problem of describing general multimedia sessions established using SIP.

As seen in the examples of Chapter 3, SDP contains the following information about the media session:

• IP Address (IPv4 address or host name);

• Port number (used by UDP or TCP for transport);

• Media type (audio, video, interactive whiteboard, and so forth);

• Media encoding scheme (PCM A-Law, MPEG II video, and so forth).

In addition, SDP contains information about the following:

• Subject of the session;

• Start and stop times;

• Contact information about the session.

Like SIP, SDP uses text coding. An SDP message is composed of a series of lines, called fields, whose names are abbreviated by a single lower-case letter, and are in a required order to simplify parsing. The set of mandatory SDP fields is shown in Table 2.1. The complete set is shown in Table 7.1.

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