[SIPForum-discussion] Is SDP body is required for RefreshInvites.........?

[Garron, James]

RFC 4028 states that the re-INVITE SHOULD include SDP even if the
details have not changed, and if it does it MUST have the same o-line as
the previous SDP.  I agree with Sriram in that the RFC does not state
that the entire SDP must be identical, but I would expect that not
sending the entire SDP identically could/would cause interoperability
issues with other implementations.

 

>From RFC 4028 Session Timers in the Session Initiation Protocol (SIP);
Sect 7.4

 

   A re-INVITE generated to refresh the session is a normal re-INVITE,
   and an UPDATE generated to refresh a session is a normal UPDATE.  If
   a UAC knows that its peer supports the UPDATE method, it is
   RECOMMENDED that UPDATE be used instead of a re-INVITE.  A UA can
   make this determination if it has seen an Allow header field from its
   peer with the value 'UPDATE', or through a mid-dialog OPTIONS
   request.  It is RECOMMENDED that the UPDATE request not contain an
   offer [4], but a re-INVITE SHOULD contain one, even if the details of
   the session have not changed.  In that case, the offer MUST indicate
   that it has not changed.  In the case of SDP, this is accomplished by
   including the same value for the origin field as did previous SDP
   messages to its peer.  The same is true for an answer exchanged as a
   result of a session refresh request; if it has not changed, that MUST
   be indicated.

 

 

________________________________

From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Sriram Subramanian
Sent: Monday, November 30, 2009 2:31 AM
To: govindraj_h at yahoo.co.in.com
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Is SDP body is required for
RefreshInvites.........?

 

Hi,

       Its actually a good question ?? but not mentioned anywhere in the
RFC's detailing the behaviour ,But i have seen a case in my test bed
which i would like to share .

 

I have an IP phone CISCO 7960 series that actually differentiates
between reinvite and Session refresh invite based on the
"Session-Version" incremental value.The SDP body (Codecs,attribute
states) ,even if u change in a Session Refresh Invite it is not taking
it into account it first checks the 'o' lines in the SDP to see if it is
incremented and then if it is not incremented it simply ignores the
other lines .This was confirmed in my testbed when a call hold scenario
was tried without incrementing the Session Version value in the 'o' line
and the ip phone did not identify it as a Hold scenario.

 

Now coming to your point of mainting  the SDP body in a refresh invite
.As long as you maintain the mandatory headers declared in both RFC 2327
(m line is mandatory as per this RFC) as well as RFC 4566 in the SDP
body even if u miss the optional headers it should not create a problem
mostly.

 

Regards,

Sriram

 

 



 

On Wed, Nov 25, 2009 at 12:05 PM, Govindraj.B.H @ Gkk
<govindraj_h at yahoo.co.in> wrote:

Hi,

      As per my knowledge, the main intention of Session Refresh
Requests is to inform the intermidiate elements about the session
aliveness. Which can be served by normal Invite. Why do we need to send
the session body along with all the session refresh Invite requests
there by increasing the traffic density.

 

    There is a parameter which differentiate the "Re-Invites" with the
"session refresh Invite" request i.e "Session-Version". ( SV increments
for each Re-Invite but remains constant for all the session refresh
requests ). 

 

   Is there any other reason behind sending the body in Session Refresh
Invites.........?

 

  [ If it is the only reason ( Session-Version ) to differentiate the
Re-invites with the Refresh Invites, then there is no point in sending
the Body params along with the session refresh Invites. Still we can
differentiate both the requests. Invite with "Content-Length = 0" is a
Refresh Invite and "C-T = Non-zero" is an Re-Invite. So with this
implementation we can reduce the traffic overhead. ]

 

All, plz provide me your inputs/suggestions for the above discussion.

 

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