Re: [sipcore] RFC4028 : Very basic question in 8.1

[Paul Kyzivat <[e-mail]>]

On 7/9/20 9:17 AM, Roman Shpount wrote:
> Shinji,
> 
> This is a negotiation where all parties (UAC, UAS and proxies on path) 
> must agree on the session timer value. Min-SE is the minimum allowed 
> value. SE is the maximum allowed value. If Proxy needs a higher SE 
> interval, it needs to renegotiate maximum allowed session timer value 
> with all the parties upstream. This is why it rejects the request with 
> 422 and a higher Min-SE value.

What is the point of session timer?

It is to use traffic on the session to verify liveness of the session 
and of the parties on the path. This pertains to both UAs and proxies, 
but their needs differ in some respects:

1) proxies can't initiate traffic; they must depend on observing traffic 
generated by the UAs. So they have an interest in requesting that the 
UAs send traffic with some minimum frequency.

2) UAs can test liveness any time they want by sending a request, 
without need of session timers. But if both ends do this independently 
then there may be more traffic than necessary, and possibly glare. ST 
negotiations allow them to find a mutually agreeable keep-alive interval 
and refresher, minimizing overhead.

The negotiation process allows all the parties to express an opinion. It 
works by ratcheting up the minimum value and ratcheting down the maximum 
value to find a mutually agreeable value between. No party is allowed to 
increase the max or decrease the min.

The purpose of Min-SE is to limit the overhead caused by ST refreshes 
when there otherwise would be no traffic. (Aside from this there would 
be no need for Min-SE.) The benefit of ST falls mostly to the proxies, 
while the cost is typically heavier on the UAs. So the Min-SE is 
primarily for the purpose of protecting the UAs from excessive demands 
by proxies.

> BTW, there is no guarantee that session timer negotiation will succeed. 
> In general case it is possible that session timer requirements for all 
> the proxies UAC and UAS cannot be satisfied at the same time.

Yes.

If, at the end of the negotiation, Min-SE and SE are higher than one of 
the UAs desires, it is still free to send extra traffic to test liveness 
more often than the ST refreshes do.

If the Min-SE and SE are higher than one of the proxies desires, then it 
is pretty much out of luck. In some sense this is futile for it to 
complain because the timer refreshes are only part of the traffic. The 
UAs may send other traffic at a rate the proxy doesn't like, and there 
is little it can do about that. It can try the 422 route, but only if it 
would prefer no session over a session with more traffic.

	Thanks,
	Paul