Subject: [Sip] CANCEL and 100 Trying Question

From: "Fuxbruner, Amihay" <[email-masked]>
Date: Tue, 26 Feb 2002 11:43:36 +0200

Hello,

Probably answered long ago, but I couldn't find it :-(

2543 says UAC sends CANCEL after receiving 1xx only, to prevent the case
where UAS get CANCEL before INVITE.
In case UAC sends CANCEL after receiving 100 from next proxy, still UAS may
get CANCEL before INVITE.

Is this correct?

Thanks,

Amihay Fuxbruner
System Engineering - Signaling R&D
Comverse

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From: Ofir Arkin <[email-masked]>
Date: Tue, 26 Feb 2002 10:57:18 +0000

Amihay,

Nothing to do with the way SIP routing the messages between SIP 
elements. We are talking about Networking here, so stating that the 
CANCEL need to follow the same path as the INVITE is half of the pint :P

This also depends on the medium you are using.

If you are using a closed network which you control the QoS, or at least 
there is not a lot of latency, than the CANCEL should not arrive before 
the INVITE. A contributing fact is that CANCEL will follow the same SIP 
path as the INVITE, and since there are not a lot of IP routing 
possibilities, than, again, the CANCEL should arrive after the INVITE.

In an open environment (Internet) anything can happen, and your messages 
are more prone to error/routing possibilities on their path to the 
target. They may take different routing paths from one SIP element to 
another (IP based). You are not controlling the network, and there are 
no QoS based mechanisms, so it is possible that your CANCEL arrive 
before the INVITE.

Ofir

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Ofir Arkin
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From: Robert Sparks <[email-masked]>
Date: 26 Feb 2002 09:39:58 -0600

CANCEL is a hop-hop message.

In the example you present, the CANCEL will arrive at the 
stateful proxy that generated the 100 after the INVITE that
stimulated that 100. That proxy will in turn generate CANCELs
to each branch that it forwarded the INVITE down that has
received its own provisional response. 

In the simple case of A->P->B, A can send a CANCEL to P
after P provides a 100. P will not send a CANCEL to B until
it sees a provisional response from B (which means B has seen
the INVITE).

So, no - the UAS will not receive a CANCEL before the INVITE it
should be applied to.

RjS

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From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 26 Feb 2002 18:17:09 +0200

Hello,

> get CANCEL before INVITE.

No. CANCEL is a hop-by-hop method, and every proxy in the path will wait
until the 100 from the next hop is received before sending the CANCEL.

Regards,

Gonzalo