Subject: Re: [Sipping] Re: Comments on revised 'sos' draft

From: William Marshall <[email-masked]>
Date: Mon, 6 Jan 2003 07:24:51 -0500 (EST)

Jonathan wrote:
> It is most certainly not "under-specified".

I'll conceed on the first point (a rejected BYE), but not yet on the second.

Nowhere in RFC3261 does it state that if SIP is being used to control
a 2500-style telehpone handset, and that handset goes to an "onhook" 
state with a session active, that the UA MUST send a BYE.

If the UA does not, due e.g. to knowing that it just called a number
such as 9-1-1, and the PSAP sends a mid-call INVITE with all the
proper Route headers and Contact values and tags, it will certainly
match the existing dialog at the UA, so will not be a new INVITE.
So a 481 is not the proper response.  Is 180-Ringing allowable?

Bill Marshall
[email-masked]

-----original message-----
Date: Sun, 05 Jan 2003 22:41:29 -0500
From: Jonathan Rosenberg <[email-masked]>
To: Henning Schulzrinne <[email-masked]>
Cc: William Marshall <[email-masked]>, [email-masked],
        [email-masked]
Subject: Re: [Sipping] Re: Comments on revised 'sos' draft

inline.

Henning Schulzrinne wrote:
> William Marshall wrote:
> 
>
>> 1) what is the effect of a BYE request generated by a UAC that is
>> rejected by the UAS with a 4xx (e.g. 403-Forbidden)?  Does the UA
>> need to wait for the response to the BYE before tossing call state?
>> Likewise, call-stateful proxies on the path?  There is, of course,
>> a denial-of-service threat here when a non-PSAP rejects a BYE.
> 
> 
> Given the many ways that the caller can terminate a call, this seems 
> like a second-order problem. Can ISUP refuse a teardown request? What 
> happens in that case?

SIP does in fact describe behavior here. According to Section 15.1.1:

The UAC MUST
    consider the session terminated (and therefore stop sending or
    listening for media) as soon as the BYE request is passed to the
    client transaction.

Thus, rejecting the call will not achieve the "desired" effect.

>>
>> 2) what is the effect of a mid-call INVITE when the instrument is
>> in an on-hook state?  Is 180-Ringing an allowable response?
> 
> 
> You mean for PSAP initiated call-back? This would be the same situation 
> as if I had sent a BYE and an INVITE crossed on the wire. It would 
> probably yield an invalid call-ID error.

THis re-INVITE would appear to the UA as a new INVITE (not corresponding 
to an existing dialog) but with tags. Behavior in this case is also well 
specified in 3261. Section 13.3.1 bullet 3 covers this case, and it 
refers the reader to Section 12.2.2. That section says that, unless you 
are specifically re-creating old dialogs beacuse you are a conferencing 
server or something, then you should send a 481.

> 
>>
>> I believe both case are "under-specified" in rfc3261, so the needed
>> behavior is allowed but not mandated.  I think this is more than a BCP.

It is most certainly not "under-specified".

-Jonathan R.
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---

From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 06 Jan 2003 15:41:09 -0500

> state with a session active, that the UA MUST send a BYE.

Of course.

My point was that if it DID send a BYE, the behavior would be to send a 481.

> So a 481 is not the proper response.  Is 180-Ringing allowable?

Sure, the UAS can send a 180. Whether the UA would "alert" the user when 
receiving a re-INVITE is a matter of UI, and not subject to 
specification. The intent of the re-INVITE is standardized, though - to 
modify session characteristics. Thus a UA would decide whether to alert 
the user based on this definition. It would not be correct to interpret 
the re-INVITE as specifically for the purposes of PSAP initiated ring, 
however.

-JOnathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave.
Chief Scientist                             First Floor
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---

From: Paul Kyzivat <[email-masked]>
Date: Mon, 06 Jan 2003 17:11:29 -0500

> the re-INVITE as specifically for the purposes of PSAP initiated ring, 

While there is probably nothing wrong with returning a 180 in this case, 
I believe there is something wrong with then actually ringing the phone 
and waiting for it to to go off hook before sending a final response.

The reason is that this is a reINVITE. There has been a bunch of 
discussion of call flows involving reinvite, and the need for it to 
complete without an extended delay.

Perhaps that decision could be reopened now. I have some other cases 
where it is desirable to permit alerting, with the associated delay, on 
a reinvite. But it will be messy to reopen that can of worms.

	Paul