Subject: [sipcore] UAC or UAS

From: "Klatsky, Carl" <[email-masked]>
Date: Mon, 9 Dec 2013 14:31:06 +0000

Hi SIPcore,

In this exchange between A and B:

B sends A an INVITE with no SDP
A replies with a 200 OK with SDP
B replies with an ACK with SDP

Is A the UAS or UAC in this scenario?  I thought A would be UAS but wanted to check with the SIP community.  Thanks.

Regards,

Carl Klatsky
Comcast

---

From: Brett Tate <[email-masked]>
Date: Mon, 9 Dec 2013 16:25:10 +0000

> Is A the UAS or UAC in this scenario?

A is the UAS.

RFC 3261 section 6 contains the definitions.

"User Agent Client (UAC): A user agent client is a logical entity
   that creates a new request, and then uses the client
   transaction state machinery to send it.  The role of UAC lasts
   only for the duration of that transaction.  In other words, if
   a piece of software initiates a request, it acts as a UAC for
   the duration of that transaction.  If it receives a request
   later, it assumes the role of a user agent server for the
   processing of that transaction."

"User Agent Server (UAS): A user agent server is a logical entity
   that generates a response to a SIP request.  The response
   accepts, rejects, or redirects the request.  This role lasts
   only for the duration of that transaction.  In other words, if
   a piece of software responds to a request, it acts as a UAS for
   the duration of that transaction.  If it generates a request
   later, it assumes the role of a user agent client for the
   processing of that transaction."

If you have other general SIP questions, you might want to post them to sip-implementors.

https://lists.cs.columbia.edu/mailman/listinfo/sip-implementors