Some CUBE SIP Profile and SDP Questions

[TONY SMITH]

Hi,

We may have to apply a SIP profile to fix up media addresses in SDP, replacing internal address with the NAT external.  In drafting the profile I've come across a few things I'm not clear on, could anyone comment on these?

(1) What's the significance of there being more than one "c=IN" field in the SDP?  For example it seems that the CUBE's outgoing Invites always have two, one before and one after the "m=audio" field, same IP address in each.

v=0
o=CiscoSystemsSIP-GW-UserAgent 590 872 IN IP4 x.x.x.x
s=SIP Call
c=IN IP4 x.x.x.x
t=0 0
m=audio 25636 RTP/AVP 8 18 101
c=IN IP4 x.x.x.x
a=rtpmap:8 PCMA/8000
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=ptime:20

A SIP profile modifying "Audio-Connection-Info" re-writes the second instance but not the first.

It's tempting to think that the second "c=" is the effective one for audio, coming after the "m=audio" field.  But that's not supported by the fact that some messages have only one "c=", and it appears before the "m=audio".

(2) Leading on from that, is there a reference document somewhere giving the field definitions recognised with SIP profiles? 

Thanks, Tony S

[Mike_Brezicky]

The first c= is the generic Connection-Info part of the session description, vs the second c= as the Audio-Connection-Info part of the media description. In most deployments I have seen, the first is redundant when the second is present.

In some cases it actually causes audio issues with ISP's. See here for a good breakdown - https://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-border-element/116010-dup-c-lines-problem-solution-00.html

[TONY SMITH]

So referring to that document it looks like I can modify the first entry with "Connection-Info"

request ANY sdp-header Connection-Info modify "x.x.x.x" "y.y.y.y"
request ANY sdp-header Audio-Connection-Info modify "x.x.x.x" "y.y.y.y"

I need both because the messages that only have one entry have it before the "m=audio" and presumably that takes effect for audio.

Leading me to the other question, have you seen any document giving a reference list of field and header names used in SIP profiles?  I have only been able to find examples, not reference material.  So for example if I wanted to modify the "o=" SDP field I can't look that up in any document but would have to find an example.

Thanks, Tony S

[Mike_Brezicky]

correct on the modify statements.

You can look at the SDP RFC doc - https://tools.ietf.org/html/rfc2327
Pulled from it:
Session description
v= (protocol version)
o= (owner/creator and session identifier).
s= (session name)
i=* (session information)
u=* (URI of description)
e=* (email address)
p=* (phone number)
c=* (connection information - not required if included in all media)
b=* (bandwidth information)
One or more time descriptions (see below)
z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)
Zero or more media descriptions (see below)

Time description
t= (time the session is active)
r=* (zero or more repeat times)

Media description
m= (media name and transport address)
i=* (media title)
c=* (connection information - optional if included at session-level)
b=* (bandwidth information)
k=* (encryption key)
a=* (zero or more media attribute lines)

[TONY SMITH]

My point, which I'm probably not making clearly enough is that I can't find a reference to say what Cisco's special name is for each of these fields.  For example there's nothing that I can see in the RFC that would positively say that for the "o=" field we need to use "Session-Owner", and not for example "Originator"

o=<username> <session id> <version> <network type> <address type> <address>

The "o=" field gives the originator of the session (their username and the address
of the user's host) plus a session id and session version number.

[Ritesh Desai]

Hi,

Refer to URL: https://community.cisco.com/t5/collaboration-voice-and-video/understanding-sip-traces/ta-p/3137704

to understand of SIP traces with headers. I‘m sure you’ll find the URL helpful.

Thanks & regards,
Ritesh Desai
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