SDP (Session Description Protocol)
SDP is the format inside SIP messages that describes the media of a call: codecs, IP addresses, ports and encryption parameters. Learn what it contains.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
SDP is a text format, defined in RFC 8866, carried in the body of SIP messages to describe the media streams of a session, including codecs, addresses, ports and security attributes.
SIP decides that a call should happen; SDP describes what the call will consist of. An INVITE carries an SDP body listing the media the caller can handle, and the answer carries the callee's choice from that list. This offer/answer exchange is specified in RFC 3264, and the SDP format itself in RFC 8866.
An SDP body is a short set of lines, each starting with a letter. The c= line gives the IP address where media should be sent. Each m= line names a media type, a port and a list of payload types. a=rtpmap lines map those payload types to codec names and clock rates, for example 0 PCMU/8000 or 111 opus/48000/2. Other attributes carry direction (sendrecv, sendonly), DTMF support as telephone-event, SRTP keys in crypto lines, and ICE candidates.
Because SDP carries the addresses the far end will send audio to, mistakes here are the root of most one-way audio problems. A client behind NAT that advertises its private address in c= cannot receive media from outside. STUN, ICE and server-side NAT handling exist to correct that line.
Reading the SDP in a detailed SIP log is the fastest way to see which codec was chosen and where each side expects audio.
Sources and further reading
- RFC 8866: SDP: Session Description Protocolwww.rfc-editor.org
- RFC 3264: An Offer/Answer Model with the Session Description Protocol (SDP)www.rfc-editor.org
This article is published by SpaGreen Creative, the maker of SIPdia. Product names mentioned belong to their owners. Where it describes other software, treat the steps as general guidance and check the official documentation linked above.
