Hi,
RFC3261: "...MUST ignore any session
descriptions in subsequent responses..."
I
think that the common industry understanding of RFC3261 is that 1 offer has 1
answer, even though that 1 answer may be transmitted several times. And the
1st transmission is used (treated as THE answer). While you are speaking
about several answers with 1 matching offer. That is a fundamental
difference.
In your chart SDP4 is a reliable answer. Therefore SDP5
might be interpreted as a new offer, hence UAC could send an answer in PRACK.
Quite similarly to 3PCC cases, where 200 contains the offer and ACK the
answer.
In my opinion it is better not to allow an answer
updating a prior answer of the same offer. If early meadia is a real issue,
implement support for 100rel and send update using UPDATE method. Or UAC
could play ring-back tone locally. That is another cheap and easy solution. I do
not see any reason for hacking.
BR,
Som
From:
sipping-bounces@xxxxxxxx [mailto:sipping-bounces@xxxxxxxx] On Behalf Of
ext gao.yang2@xxxxxxxxxx
Sent: Wednesday, April 14, 2010 11:19
To: OKUMURA Shinji
Cc: sipping-bounces@xxxxxxxx; sipping@xxxxxxxx
Subject: [Sipping] 答复: Re: About offeranswer draft:
Sent: Wednesday, April 14, 2010 11:19
To: OKUMURA Shinji
Cc: sipping-bounces@xxxxxxxx; sipping@xxxxxxxx
Subject: [Sipping] 答复: Re: About offeranswer draft:
Hi Shinji,
sipping-bounces@xxxxxxxx 写于 2010-04-14 15:31:54:
> Hi Gao,
>
> gao.yang2@xxxxxxxxxx
> Wed, 14 Apr 2010 12:21:45 +0800
> >sipping-bounces@xxxxxxxx 写于 2010-04-14 11:17:23:
> >
> >> Hi Gao,
> >>
> >> In the following case,
> >>
> >> UAC UAS
> >> | F1 INVITE (SDP1) | <-- offer
> >> |-------------------->|
> >> | F2 1xx (SDP2) |
> >> |<--------------------|
> >> | F3 1xx (SDP3) |
> >> |<--------------------|
> >> | F4 1xx-rel (SDP4) | <-- answer
> >> |<--------------------|
> >> | F5 1xx-rel (SDP5) |
> >> |<--------------------|
> >> | F6 1xxl (SDP6) |
> >> |<--------------------|
> >> | F7 2xx INV(SDP7) |
> >> |<--------------------|
> >> | F8 ACK |
> >> |-------------------->|
> >> (PRACK transactions are not shown)
> >>
> >> I tried to arrange the rules.
> >> (small letters mean informational)
> >>
> >> UAC,
> >> (Rc1) MUST treat SDP2 as the answer.
> >> (Rc2) MUST ignore SDP5, SDP6 and SDP7.
> >> (Rc3) may treat SDP3 as the answer.
> >
> >[Gao] OK
> >
> >> (Rc4) should treat SDP4 as the answer and confirm the current O/A
> >> status by sending new offer.
> >
> >[Gao] support of this, though this may be modification of current RFC3261.
>
> You will probably think of the following statements,
> RFC3261/13.2.1 Creating the Initial INVITE
> (snip) "The UAC MUST treat the first session
> description it receives as the answer, and MUST ignore any
> session descriptions in subsequent responses to the initial
> INVITE."
>
> No doubt this is one of the cussword built in this document.
> It is a personal interpretation of mine,
> "as the answer" is associated with not "treat" but "receives",
> and "treat" means "not ignore".
>
> Just putting that aside for now, I think there is a consensus that
> Rc4 does not need a modification of current RFC3261.
I see. But if there's memo of such consensus, it would be quite useful in interworking testing.
>
> >> UAS,
> >> (Rs1) should not send SDP5, SDP6 and SDP7.
> >> (Rs2) must not send SDP2 and SDP3 if these are not the same as SDP4.
> >>
> >> Rc3 and Rc4 are new added descriptions.
> >> Rs1 and Rs2 are current descriptions in this draft.
> >>
> >> I think "MUST NOT" is suitable for (Rs1).
> >> Because RFC3261 says
> >> Once the UAS has sent or received an answer to the initial
> >> offer, it MUST NOT generate subsequent offers in any responses
> >> to the initial INVITE. This means that a UAS based on this
> >> specification alone can never generate subsequent offers until
> >> completion of the initial transaction.
> >>
> >
> >[Gao] Yes.
> >
> >> SDP5 and SDP7 are regarded as "subsequent offers".
> >
> >[Gao] as "MUST NOT" is suitable for (Rs1), so there must be no "subsequent
> >offers" in subsequent response.
>
> Certainly UAC MUST ignore SDPs no matter what these are.
>
> Regards,
> Shinji
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