[SIPForum-discussion] capability query and Invitation procedure

abhishek.chattopadhyay at wipro.com abhishek.chattopadhyay at wipro.com
Thu May 21 15:13:04 UTC 2009

Hi john,


This is true that we can use the SIP INVITE method to query the far end
capability. But there can be cases where we need to find out the far end
capability before or during the establishment of a dialog. In all such
cases (since the invite transaction is not complete or not started) the
re-invite construct will not be available, but option can very well
serve the purpose.


In the same lines according to RFC:  Option method can be used both
within and out of a dialog, so in case of a query before initiating a
dialog, a re-invite would not be able to help us.

Further an option method is more capable for a query than an INVITE:

An option can find the capability of a proxy (not the case with invite)

An option with max forward 0, can also be responded without any error
(RE-Invite, Invite handling doesn't allow that)



>From RFC 3261 section 11:
   The SIP method OPTIONS allows a UA to query another UA or a proxy
   server as to its capabilities.  This allows a client to discover
   information about the supported methods, content types, extensions,
   codecs, etc. without "ringing" the other party.  For example, before
   a client inserts a Require header field into an INVITE listing an
   option that it is not certain the destination UAS supports, the
   client can query the destination UAS with an OPTIONS to see if this
   option is returned in a Supported header field.  All UAs MUST support
   the OPTIONS method.
   The target of the OPTIONS request is identified by the Request-URI,
   which could identify another UA or a SIP server.  If the OPTIONS is
   addressed to a proxy server, the Request-URI is set without a user
   part, similar to the way a Request-URI is set for a REGISTER request.

Please refer section 11 for more elaborate explanation.


In practice I have found that when query is made using the option METHOD
with in a dialog is done once we have received an answer for our offer
and oppositely (OPTION) can be received and answered once we have
answered a far end offer, and then appropriately adjusting the session.
However I think that depends on the implementation also, and cannot be
taken or stated as a standard.




From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of sipsipsip sip
Sent: Thursday, May 21, 2009 1:33 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] capability query and Invitation procedure


Hi all,



             I need some clarification on capability query and
Invitation procedure


The Reason why we use capability query is that 


To query for terminal capabilities (endpoint is capable to support
certain media features?). 

Why this cant be done using Invite procedure ??

It can be done using the SDP paramters and negotiations can be done .
Why to use capability query 




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