Subject: [SIP] More questions on draft-rosenberg-sip-app-components-00.txt

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Mon, 11 Dec 2000 03:10:48 -0800

Hi all,

Excellent draft. 

My questions relate to a few specifics of the ever-thorny stream/DTMF
forking mechanism (or as some people may dispairingly call it: "The forking
DTMF problem" :)

I know it is just a draft but these are a few of the issues that concern me:

1) How does a UA know that an added media line actually should be a copy of
an existing stream (voice or DTMF or both) rather than a new sendonly
session? 
2) If there are multiple audio or data streams, how does the UA know which
to fork ?
3) In the Figure 4 example (where there are two proxies requesting the DTMF
stream): How does Proxy B know which stream in the 200 OK (to the second
re-INVITE) is the stream that connects/forks the DTMF stream to itself?
[Especially if the two applications are hosted on the same server.] Does it
need to if the streams are sendonly? [Deletion?]
4) In the above example, how does the proxy differiate between forked
streams and new streams added by the UA ?
5) How many locations would you suggest that a UA be capable of forking to?
5, 10, 20 ?
6) How does a UA protect itself from unwittingly being part of a denial of
service attack on a third party (eg by the proxy asking you to fork 20
streams of G711 to the target server)? Requiring user intervention on
forking streams really defeats their usefulness.
7) How do you know if a UA supports stream forking behaviour ? Do we need
some sort of Allow/Accept field?
8) What happens when AS's in both directions are generating re-INVITE's
towards caller and callee? Without stream identification, things are going
to get fairly complicated for the proxies (require end-to-end request glare
avoidance?)

It seems to me that without SDP media streams identifcation beyond the
standard "n'th m line" routine, things are going to get very brittle? Do we
need to require the SDP Media Stream Identification draft (a=fid:x) for
proper stream forking?

Where do you foresee that these issues will be trashed out? On the list, at
the IETF, in a separate draft or as part of the aforemention draft (which I
doubt) ?

Regards,

Robert.

-- My opinions are my own. I tried selling them once but everybody
	seems to already have one. -- 

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