Module 1: Foundations

SDP Structure — Offer/Answer Model

Learn about sdp structure — offer/answer model

Duration: 8 minutes
Level: Advanced
Lesson: 50

SDP: The Blueprint for Media

While SIP handles signaling (call setup, teardown, transfer), it says nothing about media. That's SDP's job. The Session Description Protocol (RFC 4566) describes what media to send, where to send it, and how to encode it. SDP rides inside SIP messages as the body — the Content-Type is application/sdp.

Every audio issue you'll ever debug eventually comes down to what's in the SDP. Learning to read SDP fluently is a core NOC skill.

Anatomy of an SDP

Here's a real-world SDP from an INVITE:

v=0
o=- 12345 67890 IN IP4 10.0.1.50
s=-
c=IN IP4 10.0.1.50
t=0 0
m=audio 10000 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=sendrecv
a=ptime:20

Line by Line

v=0 — Protocol version. Always 0. Not negotiated, not interesting.

o=- 12345 67890 IN IP4 10.0.1.50 — Origin. The session identifier (12345) and version (67890) are important for re-INVITEs. When the session version increments, it signals a change. The IP here is informational — it's not necessarily where media goes.

s=- — Session name. Almost always a hyphen (meaningless). Legacy field.

c=IN IP4 10.0.1.50 — Connection address. This is where media goes. This is the most operationally critical line in the SDP. If this contains a private IP (10.x, 192.168.x, 172.16-31.x) but the endpoint is behind NAT, you've found the source of one-way audio.

t=0 0 — Timing. Start time 0, stop time 0 = permanent session. Not relevant for calls.

m=audio 10000 RTP/AVP 0 8 101 — Media line. The second most important line. Breaks down as:

  • audio — media type
  • 10000 — port number (where to send RTP)
  • RTP/AVP — transport (RTP Audio Video Profile)
  • 0 8 101 — payload type numbers (codec identifiers)

a=rtpmap:0 PCMU/8000 — Maps payload type 0 to G.711 μ-law at 8000 Hz. Payload types 0-95 are static (well-known), but rtpmap still explicitly declares them.

a=rtpmap:8 PCMA/8000 — Payload type 8 = G.711 A-law.

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