Re: [sipcore] 4028bis: memory refresh - proxy assumptions

[Roman Shpount <[e-mail]>]

On Fri, May 15, 2020 at 3:30 AM Christer Holmberg <
[e-mail]> wrote:

> >>>>>1. Defining proxy behavior based on having a transaction in progress:
> This is hard to implement in practice since initial INVITE and UPDATE
> >>>>>might not even go over the same path and the same proxies.
> >>>>
> >>>> Section 8 of RFC 4028 says:
> >>>>
> >>>>   “Proxies that ask for session timers SHOULD record-route, as they
> >>>>   won't receive refreshes if they don't.”
> >>>>
> >>>> In my opinion it should be possible to mandate that a proxy that asks
> for session timers, or modifies a session timer value (S-E or Min-SE), also
> record-route.
> >>>>
> >>> I have mentioned before that even requiring record-route would be
> insufficient. First of all, having a transaction in progress for a proxy is
> something
> >>> that can be ambiguous due to network delays or message re-ordering
> even if it is always on the dialog path. Second, the amount of state
> maintained
> >>> by proxy, if it needs to track transactions vs dialogs increases
> significantly. Third, even if the proxy can be the same "logical proxy", it
> can actually be
> >>> multiple physical servers sharing an address behind a load balancer,
> so UPDATE and INVITE transactions can end up on the different servers.
> >>
> >> You could still configure those proxies to behave in the same way
> (insert/modify the S-E and Min-SE header field values in the same way) when
> it comes to the session-timer.
> >
> > Absolutely correct. Having proxies use the same consistent rules should
> be sufficient.
> >
> > My point is that we cannot define proxy behavior based on having a
> transaction in progress. This is only non-ambiguous and implementable for a
> UA.
>
> If the proxy is not aware of transactions, it could always behave in the
> same way, i.e., always insert/modify the same S-E and Min-SE header field
> values in INVITEs and UPDATEs.
>
> I really see no reason why a proxy should suddenly change session timer
> behavior - especially a proxy that is not session aware, or aware of other
> ongoing transactions.
>
>
Proxy can be dialog aware but not aware of all the transactions in the
dialog. Proxy behavior most likely must be defined in a way that the rules
by which it inserts S-E and Min-SE headers should be consistent
throughout the dialog. I do not see there is a reason for a proxy to
suddenly change session timer behavior during the dialog.

Behavior of UA should be defined in such a way that they can change S-E and
Min-SE headers, since B2BUA scenarios can cause such behavior. Doing this
can be limited in scope since UA are obviously both transaction and dialog
aware.
_____________
Roman Shpount