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Collaboration, Voice & AV · Intermediate

Session Description Protocol (SDP)

A text format (RFC 8866) carried in SIP bodies that describes media streams: IP address, port, codecs and attributes.

Definition

SDP lists a connection address (c=), media lines (m=) with port, transport and payload types, and attributes (a=) such as rtpmap, ptime, direction (sendrecv, sendonly, recvonly, inactive) and crypto keys. SIP uses SDP within the offer/answer model to agree on media parameters. SDP itself does not transport anything; it only describes the sessions.

Example

m=audio 24680 RTP/AVP 0 18 101 offers G.711 mu-law, G.729 and telephone-event on UDP port 24680.

Exam tip

One-way audio often traces back to a wrong c= address in SDP, such as a private IP sent through NAT.

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