SIP Fundamentals

SDP (Session Description Protocol)

SDP (RFC 8866) is the body format inside a SIP INVITE that describes the offered media: codecs, IP addresses, ports, encryption, DTMF mode. It uses a simple line-oriented key=value syntax.

Annotated example

v=0                                # protocol version
o=- 12345 1 IN IP4 192.0.2.1       # origin: session-id, version, address
s=-                                # session name (often empty)
c=IN IP4 192.0.2.1                 # connection: where to send media
t=0 0                              # timing (0 0 = unbounded)
m=audio 16384 RTP/AVP 0 8 9 101    # media: type, port, profile, payload-types
a=rtpmap:0 PCMU/8000               # PT 0  = G.711 mu-law
a=rtpmap:8 PCMA/8000               # PT 8  = G.711 A-law
a=rtpmap:9 G722/8000               # PT 9  = G.722 wideband
a=rtpmap:101 telephone-event/8000  # PT 101 = RFC 4733 DTMF
a=fmtp:101 0-16                    # DTMF events 0-16 supported
a=sendrecv                         # bidirectional

Offer/Answer

RFC 3264 defines the offer/answer model: the caller sends an SDP offer in INVITE; the callee replies with a refined SDP answer in 200 OK. The answer must intersect the offer's codec list (it cannot add new codecs).

Common SDP issues

• Codec mismatch: caller offers only Opus, callee only does G.711 → 488 Not Acceptable Here.
• Wrong c= line: private IP in c= breaks media behind NAT — see NAT traversal.
• DTMF mismatch: in-band vs RFC 4733 vs SIP INFO. Use a=rtpmap:101 telephone-event/8000 unless you have a strong reason.

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