Subject: RE: [Sip] Doubts in SIP/SDP

From: "Anil Bollineni" <[email-masked]>
Date: Tue, 7 Dec 2004 16:10:40 -0800

Hi,
  Thanks for the response. I have a basic question, considering 3 views.

1. Suppose terminating endpoint does not support 183, and sends the
early media for IVR, should  UAC, play the media treated as if early
media. 

2. Any case the terminating endpoint must support 183 and send the media
after it sends. The 100rel is optional, in this case the sequence of 183
and early media is not guaranteed. Should UAC prepare media irrespective
of call progress message?
3. In case of 100Rel calling UA requires the option 100rel, should
terminating endpoint MUST send always after PRACK is received?

Specifically, if we want deploy a firewall between originating and
terminating endpoints, should we allow early media after INVITE without
183 is being seen?

Thanks,
Anil

-----Original Message-----
From: Dean Willis [mailto:[email-masked]] 
Sent: Tuesday, December 07, 2004 2:05 PM
To: Anil Bollineni
Cc: Syed, Mohammad Rafi; Fatih "Ey|p" NAR;
[email-masked]; [email-masked]
Subject: Re: [Sip] Doubts in SIP/SDP

On Dec 6, 2004, at 4:58 PM, Anil Bollineni wrote:

> Hi,
>    Thanks for the response. Actually I may confuse you. Specifically, 
> if the callee sends the media after 183, and 183 is lost, will the 
> caller should accept the media, because it don't know codec should the

> media expect or caller should decode whatever is coming, by finding 
> the codecs in the list it offers. Sorry I don't know that it is the 
> basic requirement, but in RFC can I find the statements that tell the 
> media starts after the SDP negotiation is completed.
>

I believe the rule is "the caller MUST be ready to receive any media 
described in the SDP of an INVITE immediately after sending that 
INVITE." This is how we get all those neat early-media use-cases.

Note that this use case frequently occurs when placing a call from SIP 
through a PSTN gateway (with ISDN) that terminates on a PSTN IVR, like 
that used by American Airlines. The "answer" SDP arrives in a 200OK 
that is sent by the gateway after the PSTN circuit moves into connected 
state. However, the phone circuit is not in the "connected" state for 
the first round of engagement with the IVR. The gateway in my office 
will only tolerate this for about 32 seconds before it gives up on the 
INVITE, so I have to navigate the first level of the IVR quickly.

--
Dean 

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---

From: Paul Kyzivat <[email-masked]>
Date: Wed, 08 Dec 2004 08:50:17 -0500

> early media for IVR, should  UAC, play the media treated as if early

Dean was pretty clear - you should be prepared to receive media as soon 
as you send the offer, even before you receive *any* signaling in 
response. What you do with it after it is received is your problem, but 
in general you SHOULD render it.

> of call progress message?

Nobody is *obligated* to *send* a 183. So you can't put a MUST on that. 
Nor is there any requirement to *send* media. You send media if and when 
you have media to send, once you have a place to send it.

> terminating endpoints, should we allow early media after INVITE without

Firewalls are evil, though perhaps necessary. In general one hopes they 
do as little harm as possible. Your firewall SHOULD allow early media 
after the invite, before any response. You may decide to be more heavy 
handed, but if so be prepared to have some unhappy users.

	Paul

> Sip-implementors mailing list
> [email-masked]
> http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
>

---

From: Dean Willis <[email-masked]>
Date: Thu, 9 Dec 2004 16:31:13 -0600

BTW, this is a lovely "sip-implementors" list question.

Answers herein are my opinion, and may not mesh with some particularly 
strict interpretations. On the other hand, one can refer to the Postel 
axiom "be liberal with what you accept" (or however he said that . . .)

More detailed procedures are available in 
draft-ietf-sipping-early-media-02.txt, which I believe has been 
approved by the IESG and is in the RFC Editor queue.

> early media for IVR, should  UAC, play the media treated as if early

yes.

> of call progress message?

The UAC should play any media it receives that is consistent with the 
SDP it offered in the INVITE.

The exception to this case is described in the early media draft:

         Note that, in some situations, the UAC does need to receive
         the answer before being able to play any media. UAs in such
         a situation (e.g., QoS, media authorization or media
         encryption is required) use preconditions to avoid media
         clipping.

The use of preconditions is discussed in RFC 3312.

> terminating endpoint MUST send always after PRACK is received?

That's debatable. One might be inclined to suspect that the calling UA 
has issues with "really early media" in this case, and the UAS SHOULD 
hold off until receiving the PRACK. I don't know that MUST applies, as 
one can reasonably expect that SOME UASs might not delay, so the 
calling UA MUST be prepared to deal with the consequences of a called 
UA that responds with media before sending an SDP answer.

> terminating endpoints, should we allow early media after INVITE without

yes.

--
Dean