Subject: [SIP] draft-ietf-sip-rfc2543bis-01.txt: General questions

From: Subramania Sivaram <[email-masked]>
Date: Sat, 14 Oct 2000 19:57:25 +0530

While reading the above doc, I had the following questions. [I am sorry if
these 
are very basic, owing to my rather short association with this working
group.]

- What is maddr & when/ why would it exist in a SIP URI ??

- Section 4.2.6: What exactly are multicast registrations & why are they
sent to
"all SIP servers" multicast address ??

- Sec 4.2.6, Page 32
There is a sentence which says
"a traveler sip:[email-masked] (To) might register under the Request-URI
sip:atlanta.hiayh.org , with the former as the To 
header field and the latter as the Request-URI."

When a request is sent to the SIP proxy at acme.com (for [email-masked]), 
how does it know to forward it to sip:atlanta.hiayh.org ?? 
In general, how does one SIP proxy know which
is the next proxy to send to (in other words how is the proxy sequence
determined) ??

- Sec 6.34.2, There is a sentence which reads
"Since the URIs contained in the Record-Route header fields 
are not useful for the reverse request path..."
Why is this the case ??

- Can any request be forked ?? e.g. a BYE on a premature disconnect from the
caller (only
INVITE seems to be used in most examples).

- What are the possible uses of a SIP proxy ?? As a registrar,
a firewall, what else ?? I believe it can be used for implementing
features e.g. ACD, but most features seem to exist at the UA (is this
correct ??).

Lastly, a general comment. I find that procedures (behaviour of an entity
like a proxy or a UAS etc.- its processing of requests and responses) 
is spread over the complete document, which makes it quite difficult 
to understand, especially because the document is quite large. e.g. the
behaviour of a proxy on receipt of ACK is presented in Sec 4.2.2 ACK in
detail, as well as in different parts of Sec 12. In general, processing
details are distributed between Sections 1, 4, 10, portions of 6, 10, 11, 12

(leaving out security). Are there any plans to re-organise the document 
in the future, so that all procedures are in one place (more like ITU-T
standards, which I feel are easier to read) ??

Thanks,
Sivaram.

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 16 Oct 2000 00:11:54 -0400

> -----Original Message-----
> From: Subramania Sivaram [mailto:[email-masked]]
> Sent: Saturday, October 14, 2000 10:27 AM
> To: [email-masked]; Henning Schulzrinne; Jonathan Rosenberg
> Subject: [SIP] draft-ietf-sip-rfc2543bis-01.txt: General questions
> 
> 

maddr specifies the specific IP address to send to. It exists when the DNS
translation of the address in the URI cannot be relied upon, but a domain
name needs to be there for other purposes, like application logic.

> "all SIP servers" multicast address ??

REGISTERs sent via multicast. Multicast is used since it allows for
autoconfigurations. A well-known multicast group must be used to eliminate a
configured address.

> is the next proxy to send to (in other words how is the proxy sequence

Its a matter of local configuration. The spec defines some standard
approaches, such as using a registration database, or forwarding to the
address in the request URI. ANything else you like is possible.

> are not useful for the reverse request path..."

Because they refer to the called party. If they are used by the called party
in requests back to the caller, they are indicating the wrong person.

> INVITE seems to be used in most examples).

Any request can be forked.

> features e.g. ACD, but most features seem to exist at the UA (is this

Routing. Namespace partitioning. Scale. Logging/billing, etc. See:
http://www.cs.columbia.edu/~jdrosen/papers/devconf_spring2000_proxies.ppt

which explains in details why you want proxies.

-Jonathan R.
---
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.cs.columbia.edu/~jdrosen         PHONE: (973) 952-5000
http://www.dynamicsoft.com

---

From: Subramania Sivaram <[email-masked]>
Date: Mon, 16 Oct 2000 17:04:56 +0530

Sorry Jonathan, Haven't been able to fully understand your brief replies.
Hence, some more questions in-line.

> name needs to be there for other purposes, like application logic.
	[Sivaram]  Any specific scenarios that you have in mind for this
(where DNS translation
	cannot be relied upon etc.) ?? And, why does the spec often say that
maddr is usually
	a multicast address ??

> autoconfigurations. A well-known multicast group must be used to eliminate
	[Sivaram]  Why would I want to send a REGISTER to a multicast
address ?? Is
	it for direct communication between UAs (bypassing proxies) ??

> address in the request URI. ANything else you like is possible.
	[Sivaram]  Are you saying that in the above example alice needs to
register
	at two SIP proxies, one at acme.com & other at atlanta.hiayh.org [so
that the
	SIP proxy at acme.com can forward the request to atlanta.hiayh.org]
??

> in requests back to the caller, they are indicating the wrong person.
	[Sivaram]  I thought record route identified proxies on the path
from the UAC to the UAS [if
	they want to stay on the path of future requests from either end].
If Via can be used to
	ensure replies take the reverse path to the request, why don't
addresses added in Record
	Route work for the reverse direction (i.e. for future requests from
the current UAS to current UAC) ??