> 1. Totag, fromtag and call id are dialog identifier as
> per RFC3261, why we require all these three parameter?
REGISTER is not a dialog forming request (although some say that it creates a
"pseudo dialog"). The tag requirement was added to RFC 3261 for non dialog
forming requests mostly because it was added for dialog forming requests;
however some may find it beneficial when dealing with forking proxies.
> Why Only one parameter cannot act as dialog identifier?
If you look at section 10, you'll notice that only the Call-ID (not the tags)
are associated with the binding. Thus the Call-ID is the only "pseudo dialog"
identifier.
> 2. Is Call id a unique identifier for particular login.
> If we login after logout, how the call id will be changed.
I assume "login" means REGISTER. If "logout" means sending REGISTER to remove
the binding, there isn't much benefit to continue using the same Call-ID if the
binding was removed. However, these are the RFC 3261 recommendations.
Section 10.2 indicates:
"Call-ID: All registrations from a UAC SHOULD use the same Call-ID
header field value for registrations sent to a particular
registrar.
If the same client were to use different Call-ID values, a
registrar could not detect whether a delayed REGISTER request
might have arrived out of order."
Section 10.2.4:
"A UA SHOULD use the same Call-ID for all registrations during a
single boot cycle. Registration refreshes SHOULD be sent to the same
network address as the original registration, unless redirected."
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