"m=audio 18646 RTP/AVP 0 8 101 13"

That is the codec list.
0 = PCMU (G711 u-law)
8 = PCMA (G711 a-law)
101 = DTMF RFC2833
13 = Comfort Noise

Numbers 96-127 are dynamic. Anything else is static and defined by IANA (http://www.iana.org/assignments/rtp-parameters).

A dynamic payload type needs a rtpmap line to map the number to the codec name. That is not required for static payload types, because they have a preset rtpmap.

FreeSWITCH doesn't add the a=rtpmap lines for static types by default. The advantage of doing so is it makes your packets smaller - less bandwidth usage, and less likely to get a fragmented packet.

Some (very few) devices are broken though and require the rtpmap lines for all codecs. These have broken SIP implementations because the RFC states the rtpmap is optional for static types. There is a compatibility option if you're using one of those and really do require it, but changes are you don't: http://wiki.freeswitch.org/wiki/Variable_verbose_sdp

-Steve

On 25 May 2011 01:56, Liam Farr <liam@intersys.co.nz> wrote:

Hi,

My outbound SIP invites are going out without a codec list? Could someone point me in the right direction on how to set / insert this?

send 972 bytes to udp/[1.2.3.4]:5060 at 00:28:52.144565:

------------------------------------------------------------------------
INVITE sip:0800000000@1.2.3.4:5060 SIP/2.0
Via: SIP/2.0/UDP 9.8.7.6;rport;branch=z9hG4bK2QeQtFFa7Z6Hr

Max-Forwards: 69
From: "Liam - Test" <sip:12345678@9.8.7.6>;tag=ryFQea59e6Upe
To: <sip:0800000000@1.2.3.4:5060>

Call-ID: cec20463-0108-122f-689e-c99dbf5a2f43
CSeq: 12805602 INVITE
Contact: <sip:12345678@9.8.7.6:5060>
User-Agent: FreeSWITCH-mod_sofia/1.0.7-svn-exported

Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER, NOTIFY
Supported: timer, precondition, path, replaces
Allow-Events: talk, hold, refer
Content-Type: application/sdp

Content-Disposition: session
Content-Length: 189
X-FS-Support: update_display
Remote-Party-ID: "Liam - Test" <sip:12345678@9.8.7.6>;party=calling;screen=yes;privacy=off

v=0
o=FreeSWITCH 1306264686 1306264687 IN IP4 9.8.7.6
s=FreeSWITCH
c=IN IP4 9.8.7.6
t=0 0
m=audio 18646 RTP/AVP 0 8 101 13
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
------------------------------------------------------------------------

Thanks

Liam

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