[VOIPSEC] SDP format for SRTP in SIP

[Geoff Devine]

> Can any tell me what is the SDP format for SRTP in SIP.
> i.e. how the ethreal capter looks like for SRTP while negotiating.


In the SDP, requesting RTP looks like:
m=audio 3456 RTP/AVP 0

SRTP looks like:
m=audio 3456 RTP/SAVP 0 

This can be found in IANA:
http://www.iana.org/assignments/rtp-parameters

Key exchange isn't part of SRTP.  
I happen to be in the SDESCRIPTIONS camp (keys passed "in the clear"
embedded in the SDP and encrypt SIP) rather than the MIKEY (end-to-end
key exchange protocol) camp.  If you can't trust all the SIP proxies to
keep the keying information confidential, you probably want to use MIKEY
at added complexity and cut-thru latency.  PacketCable, where the core
is trusted, uses a method similar to SDESCRIPTIONS and it makes
CALEA/Lawful Intercept much easier to deal with.

Geoff