Subject: Standby

From: Gonzalo Camarillo <[email-masked]>
Date: Fri, 05 Nov 1999 13:43:37 +0200

Hi,

I have a high-reliable end-user (let's say a softswitch), and my primary
system crashes... the sencondary system takes over, but it has a
different IP address.

I will issue another INVITE changing my "Contact" header.
Should I introduce the old SDP in this 2nd INVITE (I do not want to
change anything in the SDP)?
Will the end-user understand my updated INVITE without SDP?

I could also use a "Replace" header (call control draft)...

This is also applicable to the paper that Elin and Henning wrote about
SIP mobility, where nothing but our "contact" header is changed when we
move and get a new IP address.

In this 2nd scenario is even more important, since we wouldn't have to
transmit the old SDP again over the air interface...

Regards,

Gonzalo
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From: Henning Schulzrinne <[email-masked]>
Date: Fri, 05 Nov 1999 07:34:48 -0500

Interesting issue. I suppose the general question is:

Does an empty message body mean that the old body is still in force?

It is easy for the end system to detect that two SDP messages are the
same (via the o= line), so this is not an issue for processing. The
message size also doesn't seem that big a deal, given that it is smaller
than a typical voice packet.

One good reason to send it again is robustness: We've tried to set up
things so that calls can recover easily from crashes by encouraging
idempotent behavior. (This is one reason for having the re-INVITE be an
INVITE rather than some other request.) In the soft-state tradition,
this means that a "refresh" should have the same and complete
information as the original set-up.

In the current mechanism, any firewalls and such that are in the middle
are now "self-healing". If they conk out and lose state, just send
another INVITE... (This goes back to our old session liveness
discussion.)

Thus, while the answer to the question is probably "yes", I don't think
we'd want to encourage this shortcut since it adds one more special case
to the end system and any intermediate systems that care about message
bodies, without any real gain.

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From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 05 Nov 1999 08:50:59 -0500

> information as the original set-up.

I agree completely. I'd really like to keep the INVITE requests
idempotent. So, every re-INVITE should contain the session description.
The recipient can tell if it changed without parsing through it by
checking the version in the origin field.

-Jonathan R.

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