[SIPForum-discussion] capability query and Invitation procedure

Samir Patni samir_patni at persistent.co.in
Sun May 24 10:15:15 UTC 2009

Hi All, 

Media capablities could be found using INVITE too but sometimes the UAC would be interested to know the signalling capablities of the far end/intermediate network elements. 

1. What all message types are supported by the UAS or some intermediatory network element. 
Accept: application/sdp;level=1, application/x-private, text/html

2. Which message encodings is understood at the far end. 
Accept-Encoding: gzip

3.  preferred languages for reason phrases, session descriptions, or
status responses carried as message bodies in the response. 
Accept-Language: da, en-gb;q=0.8, en;q=0.7

4: What all extentions are supported. 
Supported: 100rel

5: set of methods supported by the UA

Thanks and Regards, 
Samir Patni

-----Original Message-----
From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Keerthi
Sent: Friday, May 22, 2009 2:14 PM
To: abhishek.chattopadhyay at wipro.com
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] capability query and Invitation procedure

abhishek.chattopadhyay at wipro.com wrote:
> 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.
> Regards
> ------------------------------------------------------------------------
> *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
> Regards,
> John
> ------------------------------------------------------------------------
> ------------------------------------------------------------------------
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Hi All,

I agree with your response.
In RFC 3261 says, only on the Server side (UAS), if the UAC receives the 
OPTIONS Request with the Max-Forwards header value with zero (0), then 
the UAC should ignore the OPTIONS request or it should response with the 
200 OK OPTIONS response.

Thanks in Advance..



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