Subject: [Sipping] SDP Rollback in draft-sawada-sipping-sip-offeranswer-01.txt

From: "Siddhartha Bhakta" <[email-masked]>
Date: Tue, 14 Nov 2006 17:36:31 -0000

Hi,

I am not sure (as I am a very new user to SIPPING

group) whether SDP rollback has ever been discussed

regarding this draft or not. I personally feel that

'SDP rollback' is very relevant topics regarding this

draft.

The sec 1.2 indicates the response(488) to reject any

SDP offer. But the missing part is that if any SIP

request carrying SDP offer is rejected by 3xx-6xx then

SDP offer should be rolled-back. The rollback means,

applying last session description.

Rather than specifying different SIP messages

separately, it would be better to have some thumb rule

that shall be quite easy to follow as far as SDP

rollback is concerned. I am proposing following thumb

rule. Please indicate whether that makes sense or not.

The SDP rollback shall be associated with transaction

rollback/rejection.

[1] If the transaction request (carrying SDP offer) is

rejected then SDP offer shall be rolled-back. The

exception is the ACK request. The ACK (of 2xx) can not

be rejected. (The ACK of 3xx-6xx is not the

transaction initiating request).

[2] If any transaction request carries SDP answer

(e.g., ACK or PRACK) then that transaction can not

rejected as there shall be the confusion over whether

SDP shall be rolled-back or not. I suppose, SDP answer

can not be rolled-back. There is no confusion for ACK,

as it can not rejected but for PRACK the restriction

should be there that it can not be rejected if it

carries SDP answer.

This draft says that PRACK(irrespective of whether it

is carrying SDP offer/answer) can not be rejected. I

can not understand the rationale about this statement.

Though, I can appreciate why PRACK MUST not be

rejected if it carries SDP answer.

[3] If any SIP response contains SDP offer then that

SDP can not be rolled-back. If any SIP entity wants to

rollback the SDP offer carried by SIP response, it

should initiate a SIP transaction carrying old SDP to

accomplish it.

There is a special case for re-INVITE. If multiple SDP

offer/answer happens(using PRACK/UPDATE) within

re-INVITE transaction and re-INVITE is rejected by

4xx-6xx then whether all the SDP offer/answer happens

within re-INVITE transaction shall be rolled-back or

not. My suggestion would be that once SDP offer/answer

is completed, that can not rolled-back by means of

transaction rejection. That means, all the completed

SDP offer/answer shall not be rolled-back due to

4xx-6xx of re-INVITE. This decision shall save the

work of SIP UA, SIP SBC, B2BUA etc. that follows SDP

offer/answer state.

Please comment.

Thanks and Regards,

Siddhartha

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