Subject: [SIP] IM Questions

From: Bobby Sardana <[email-masked]>
Date: Tue, 05 Sep 2000 14:29:04 -0700

Greetings:

I have been reading the draft [draft-rosenberg-impp-im-00.txt] and have
the following questions:

a. In section 4.1 Message flow, the message F4 has two "Via" headers. Is
this an error since the proxy should have popped the topmost "Via"?
b. The draft does not outline support for multiple users (2+). How will
the communication happen? What will be the message flow?
c. How are dynamic users added to a buddy list? Can a REGISTER message
be generated to perform the dynamic addition?

Any information on the above is appreciated.

Regards,

Bobby Sardana.
MobileRain Technologies, Inc.
[email-masked]

---

From: "farhan" <[email-masked]>
Date: Wed, 20 Sep 2000 07:18:25 +0530

>a. Can a centralized server INVITE buddies on its own? e.g., if a
centralized
>server is managing a buddy list -- 3 users and recognizes that the 3rd one
just
>became "on" and wants to INVITE it to the session. In this case how is the
CSeq
>value propagated?

CSeq value is unique to a combination of Call-ID, To and From. Therefore,
the CSeq doesnt have to be unique among all the messages between all the
buddies and the centralised server.
For instance, if a UA always starts with sequnce 1, there is no problem if
the responding UA too starts a message in the opposite direction.
CSeq is used to differentiate between two separate messages within the same
Call-ID between two sip urls (To: and From:).

>Each IM also carries a CSeq, which is a sequence number plus the name of
the
>method of the request. The CSeq uniquely identifies each IM in the session.
>
>Consider the following scenario (3 users - A, B, & C)
>
>a. A sends MESSAGE to a centralized server for propagation to B - CSeq 1
>b. A and B are communicating with monotonically increasing CSeq numbers.
>c. When C gets invited by the centralized server, if the above mentioned
>criteria is good, what should be the CSeq value in INVITE and how is it
>syncrhonized between A, B, C for the same conference.

Cseq value can still be 1 for the invite to C. the Cseq values are between
the server and (lets say) A are in a different 'sequence space' from the
CSeq values between the server and C.
As agreed upon by sean in a mail of a few hours ago, it is the Call-ID that
remains unique in a session of IMs. the uniqueness of a message is derived
from the combination of Call-ID, CSeq, To: and From:. So, as long as a UA
keeps incrementing the CSeq of it's succesive outgoing  MESSAGEs, there is
no ambiguity at all.

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