Re: [Sip] draft-ietf-sip-info-events-00: package negotiation

[Eric Burger]

I would offer the likelihood of a UAC only half implementing Info Packages is about 00.000% (five zeros, anyone?). I cannot imagine anyone expending the effort to implement Info Packages but not use them.

On Oct 22, 2008, at 5:36 AM, Christer Holmberg wrote:

If the UAS is a legacy device, the UAC will detect it from the absence of Send/Recv-Info headers. If the UAC is a legacy device, the UAS will detect it from the absence of Send/Recv-Info headers. Since I will soon make it clear a conforming UAC can send a legacy INFO, half negotiation is a non-issue. For example, let's say it's two weeks from now and I've published the update to RFC 3398 to use Info Packages. Either the implementor will implement the full ISUP package, or not. It just does not make sense to do half the job, since 90% of the job gets done when one does the first half. If one side does not do Info Packages, then presumably, in the interests of legacy interoperability (and often because the code is already written), the device falls back to legacy INFO/RFC 3398. 3pcc? By the time you can list one header (Send or Recv), you can list both. Therefore, I would offer it is an implementation failure to list only a Send-Info or Recv-Info header.

Hi,

General issues with the negotiation mechanism:

What happens if the initial INVITE by a UAC includes only Send-Info or

Recv-Info? Does that indicate a "half-legacy" device??? Or does it delay

negotiation for one but not the other? Or does

the presence of one mean the absence of the other is equivalent to an

empty one?

As I think about it, I don't think its really necessary to treat the

absence of these headers as meaning "legacy". Treating the absence of
these headers as equivalent to their presence without

any packages is sufficient to get the proper legacy behavior. The

tricky part is figuring out out when their absence *doesn't* mean that,

which is significant only in the non-legacy case. In

particular, when they aren't present in INVITE it means deferral of the

"offer" to the UAS. And possibly absence in an UPDATE means "don't
renegotiate"???

I don't think the presence, or absence, of the S/R-Info headers should
in any way be connected to legacy in any way. A UA can be legacy no

based on "configuration", as described in the draft.

So, if a UAC only included Send-Info, it means that it only supports

INFO usages, but that is completely separated.

matter if the headers are present, or not present, but that is all done sending of Info packages. Then, it may, or may not, also support legacy

I'm confused by the three way negotiation of info packages in

INVITE/2xx/ACK. Its allowed when the Send-Info/Recv-Info are in the
INVITE, but not when they are deferred to the INVITE response,

or when they are conveyed in UPDATE and its response. If the three way

handshake is needed to seal the deal, then those other options should be

off the table. But the "deferred offer" of these

is really needed, so it can't be off the table. Or is this just an

extra benefit that can be exploited when possible?

Also, I don't see any provision for additional round(s) of negotiation

before 2xx via PRACK and UPDATE, though there is provision for
additional "half-negotiations" via provisional responses

and the final response. Given the parallels with O/A, I'd prefer to see

them more aligned in this regard, though I can be talked out of that
with a good argument.

I agree (I think :). The negotiation whould be 2-way. Then, based on
what the UAC gets from the UAS, it may send a PRACK/UPDATE/re-INVITE
with a new set of supported Info packages, but that is a new 2-way
negotiation transaction.

I'd also like to take advantage of the lessons that have been learned

with O/A and nail down more of the semantics now. One issue that leaps
to mind in that regard is what happens if we start

to negotiate info packages in a reINVITE and then the reINVITE fails?

That's a good point.


set in a 18x. Then we may start using info packages based on that
negotiation long before the re-INVITE is rejected.

I think we should use the same rule as for offer/answer. Because, assume I send you a new set of packages in a re-INVITE, and you send me back a

IMO what is needed is a state model for this stuff, added to the state

for an invite-dialog-usage. This state should determine which packages
may be sent, and which should be received without

error. We should be very clear about exactly which events cause changes

to this state.

Even then there are race conditions that need to be clarified. For

instance: Suppose, as a result of an INVITE/1xx exchange between Alice

and Bob, that packages P & Q are allowed to be sent from Alice to Bob.

Alice then sends an INFO with package Q, and at the same time Bob sends

a 2xx that only allows the receipt of package P. Then Bob receives the
INFO with package Q. What should happen?

Another issue is that if the Send-Info/Recv-Info contents change in

subsequent 1xx responses to the same INVITE, the UAS has no guarantee
that they are received. So any reduction in what can be

received cannot be counted upon until the 2xx/ACK.

True. But, I wonder whether this will really be a real-life problem.


Then the UAC knows.

Regards.

Christer

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And, we could a response code (4xx Info package not supported), which is used by the UAS if it receives an info package which it doesn't support.

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