Session Description Protocol

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The Session Description Protocol (SDP), as defined in RFC 2327, is “intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation.”

SDP as a protocol is carried in Session Initiation Protocol (SIP) packets. When a SIP call is initiated between two IP telephones, the INVITE message contains a SDP section. In effect, the full protocol stack would look like:

Ethernet { IP { UDP { SIP { SDP }}}}

SDP descriptions include:

  • Session identifier and name
  • Time(s) the session is active
  • The media descriptions proposed for the session
  • Information to receive those media (addresses, ports, formats, and so on)
  • Information about the bandwidth to be used by the conference
  • Contact information for the person responsible for the session

In general, SDP conveys information to allow a receiving telephone to judge the compatibility of the call. In practice, not all phones validate all portions of the SDP description. This leads to vendor incompatibilities and lost calls.

Common Fields in SDP

SDP, as the name implies, provides information about multimedia sessions. The information conveyed is broken into three categories.

  • Session: This provides information about the name, identifier, and owners of the session. In telephony this could include the name of the calling party, the IP address information of the source station (telephone or gateway), and connection information (port number and bandwidth requirements).
  • Time: The time description is included in telephony calls, but is set to 0, meaning the session is active now. Call features could use this information for scheduled calls or wake-up calls.
  • Media: This provides the details of the actual media being passed between the parties. The initiator of a call could include multiple media descriptors and allow the recipient to select one. Media descriptors include sampling rate, encoding scheme, and samples per packet.

SDP Media Codes

SDP Media Codes

Within the media descriptions of SDP are a series of media format codes. Each identifies content for a multimedia session. A single call can have multiple media types defined. This can be used as an auto-negotiation capability in the IP telephony environment (e.g., a phone lists all possible options, the receiver picks the compatible one). In a multimedia call, multiple descriptors could define the audio, video, and data content of a session, all in one message exchange.

The media formats are listed in RFC 1890 and example payload types and descriptions for common encoding schemes are shown on the visual.