📝SDP

SDP = Session Description Protocol. RFC 4566 (2006).

SDP is a textual format to describe sessions (usually RTP).

SDP usually refers to what participant is expected to receive. i.e., it’s a request for others to send data.

Fields:

v
protocol version (0)
o
originator information
s
session description
c
connection info
t
timestamps (NTP) for start and end of session
m
media (type, RTP port, profile, payload types) m=video 5004 RTP/AVP 96 97 (RTP/AVP—audio/video profile)
a

additional attribute

a=rtpmap:
define new RTP Payload Type (a=rtpmap:97 H264/90000)
a=fmtp:
format parameters (a=fmtp:97 profile-level-id=42001f)
a=rtcp:
set RTCP port explicitly (a=rtcp:53020). If not specified, RTP port+1 is assumed (RFC 3605)
a=rtcp-mux
RTP/RTCP multiplexing (same port) (RFC 5761)
a=rtcp-rsize

Reduced-Size RTCP (RFC 5506)

  • drop some fields, making packets smaller, so there is higher chance they are delivered
  • should be used with RTP/AVPF (RTP Profile for RTCP-based feedback) RFC 4585
a=recvonly, a=sendonly, a=sendrecv

intention (RFC 4566). usage in SDP negotiation (RFC 3264)

  • sendrecv is the default value

Symmetric RTP (RFC 4961)—RTP packets are send and received on the same port (important for NAT traversal?)

See also:

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