[Sip] Reason header simplification

[Rohan Mahy <rohan@cisco.com>]

Hi,

It occurred to me that there is a lot of overlap between the 1-9 reason
codes in the reason header, and the 3 digit SIP reason codes.  I think
these could be consolidated as described below.

Reason: 1 (call completed elsewhere) in a CANCEL could instead become a
CANCEL with a Reason: 200 OK (you are receiving a CANCEL because the
status of the call was a 200 OK, presumably from another branch).

The only difference between Reason 2 (abandoned) and Reason 4 (terminated)
is that 2 is used in CANCEL, and 4 is used for BYE.  We could allocate a
new 4xx response code (say 499) for "terminated" and use this instead.
This response code would also be useful in the call-info package.

Reason: 3 (timed out) is easily expressed with Reason: 408 Timeout

Reason: 5 (transfer complete) isn't needed because a notification to that
effect is generated when using REFER

Reason: 6 (no media) could be represented with a new response code (say
498)

Reason: 7 (interworking) does really have any semantic meaning, so just
use the SIP response code.

Reason: 8 (offer refused) sounds like a 603 Decline or 488 Incompatible
Media to me.  The SIP responses are actually more rich, but not
significantly more complex.

Reason: 9 (update requested) can be implied by the fact that the reason
header is in a 155 response.  the "real" reason is the repairable error


Also I don't like to idea of sending ISUP (Q.850) or ISDN (Q.931) response
codes around inside a SIP network.  I'd rather use standard ISUP-SIP or
ISDN-SIP cause code translation, so that an ordinary SIP UA can just use a
set of well-defined SIP cause codes.

hope this makes sense.

thanks,
-rohan





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