Subject: [SIP] some queries

From: "rahul pande" <[email-masked]>
Date: Tue, 25 Jul 2000 14:29:46 GMT

hi all,
    i have some questions to ask :
1) what is the use to the Via header fields ?
2) In one of the articles i had read that "UDP enables SIP to multicast" 
does it mean that TCP cannot be used for this.
3) How do i issue multicast address, what is the mechanism?
4) if i want to send some text/plain message wiil i specify it like:
      m=text <portno> tcp plain in SDP?
5) who is the registration server is it my location server or any other 
entity?
      I will reiterate that i am a novice and please convey to me the whole 
details even if the questions sound foolish.
     thanks,
          rahul

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From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 26 Jul 2000 01:37:10 -0400

> 1) what is the use to the Via header fields ?

for routing responses back towards the UAC.

> does it mean that TCP cannot be used for this.

TCP can never be used for multicast. This has nothing to do with SIP.

> 3) How do i issue multicast address, what is the mechanism?

You mean how do you send a SIP request to a multicast address? The rules
are the same for INVITE except:

1. servers don't transmit final responses other than 2xx, 401, 407 and
6xx
2. the sender really has to CANCEL upon getting the final response
3. servers can optionally suppress sending their 2xx, 401, 407, 6xx if
they see another server send one first.
 
>       m=text <portno> tcp plain in SDP?

Are you talking about instant messaging? This isn't how you would do
that:
http://www.cs.columbia.edu/~jdrosen/papers/draft-rosenberg-impp-im-00.txt

> 5) who is the registration server is it my location server or any other

You can discover a registration server using multicast, one can be
administratively configured, you can learn it through DHCP, or discover
it through SLP. Other possibilities exist as well.

> details even if the questions sound foolish.

You should check out the FAQ and the various tutorials and slides at:
http://www.cs.columbia.edu/~hgs/sip

-Jonathan R.
-- 
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
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