Subject: [sipcore] RFC4028 : Clarification of "turned-off' in 7.2. Processing a 2xx Response

From: OKUMURA Shinji <[email-masked]>
Date: Sun, 14 Jun 2020 18:47:06 +0900

Hi,

Section 7.2 says,
    If the 2xx response did not contain a Session-Expires header field,
    there is no session expiration.  In this case, no refreshes need to
    be sent.  A 2xx without a Session-Expires can come for both initial
    and subsequent session refresh requests.  This means that the session
    timer can be 'turned-off' in mid dialog by receiving a response
    without a Session-Expires header field.

I understand the following.

The current specification allows a UAC and proxies to add the SE header to the INVITE request, and a UAS and proxies can also add it to the 2xx response. In addition, IIUC, if a request contains a SE header, a UAS must include a SE header in the response. As a result, only if all entities do not want a session timer, the session timer is 'turned-off'. And in practice, once the session timer starts to be used, it is rarely turned-off.
Because section 7.4 says,
    In a session refresh request sent within a dialog with an active
    session timer, the Session-Expires header field SHOULD be present.
    <snip> As such, a UA should only
    ignore the SHOULD in unusual and singular cases where it is desirable
    to change the session interval mid-dialog.

Regards,
Shinji