Subject: RE: [SIP] sip mobility with REGISTER

From: "Roy, Radhika R, ALCOO" <[email-masked]>
Date: Thu, 28 Sep 2000 15:48:39 -0400

Hi, Billy:

The bottom line of your argument is "performance (e.g., frequency of use,
latency, etc.)" problems, if you use anything in the SIP layer to address
mobility.

Let assume for sake of argument that there is no performance problem: Will
the procedure work using REGISTER and re-INVITE?

The second aspect of your assumption is that mobile IP mobile will solve all
problems. No, mobile IP has also problems from performance point of view
especially for real-time communications like voice in addition to fast
hand-offs and frequency of location updates.

I guess that we do need a mobility solution in the IP layer (like mobile IP
and/or cellular IP).

One thing is for sure that there is a change in point of attachment (e.g.,
addresses). Now SIP layer also deal with addresses. It appears to me that
this change of addresses need to be propagated in the upper layer (e.g., SIP
layer) although the lower layer may use mobile IP and/or cellular IP. In
turn, it may trigger reREGISTER or re-INVITE messages.

Please note that the lower layer will deal with media diversion or
re-routing (SIP layer is not involved).

Is that a fair analysis?

Best regards,
Radhika R. Roy
AT&T

-----Original Message-----
From: Billy Biggs [mailto:[email-masked]]
Sent: Thursday, September 28, 2000 3:18 PM
To: Roy, Radhika R, ALCOO
Cc: farhan; sip
Subject: Re: [SIP] sip mobility with REGISTER

  Hi Radhika,

> Yes, REGISTER can be used to update the location as one moves from one
> place to another.

  I think you are confused as to the general purpose of a REGISTER
message.  A REGISTER is intended to be used to inform the proxy of a new
point of contact for a user, not a new location for an active point of
contact.

  In other words, the intent of REGISTER is not to support mobility of a
physical terminal, but rather to support activation and shutdown of
distinct terminals: enabling my phone at home to being receiving calls
to my phone at work.  The two phones are distinct, but they do share my
public address.

  If a specific terminal is about to change IP addresses and you wish to
retain active sessions, I suggest you implement a Mobile-IP based system
which provides mobility at the IP layer, allowing active sessions to
continue without being updated, or a transport-layer solution which
provides mobility below the IP layer.  Anything that does NOT involve
modifying the IP address used by the terminal for its connections to the
Internet.  This will work without having to send any SIP messages at
all.  It also allows active TCP connections (such as HTTP traffic) to
also remain active during the transition.

  While INVITE and REGISTER may be used to attempt to achieve terminal
mobility, it seems to me that the consensus here is that it is a bad
idea.  This is because:

    1.  A SIP specific solution seems to be overkill and inconsistent.
    2.  High latency of SIP requests.  The re-INVITE may take alot of
	time to propagate and be accepted, and it may fail and have to
        be re-attempted.  Handover times may take many seconds, and may
        fail completely.

> It clearly shows that the involvement of the SIP layer is very
> minimal. Do we need any additional standard work in the SIP layer to
> couple the change in the lower address to invoke the REGISTER message?

  No.  Even to do the hacked solution of REGISTER'ing under the new
address and attempting to re-INVITE any active sessions, you don't need
to add any new modifications to SIP.

-- 
Billy Biggs, 3Com              [email-masked]
http://www.div8.net/billy/          [email-masked]

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---

From: Billy Biggs <[email-masked]>
Date: Thu, 28 Sep 2000 15:02:25 -0500

Radhika,

> The bottom line of your argument is "performance (e.g., frequency of
> use, latency, etc.)" problems, if you use anything in the SIP layer to
> address mobility.

  No, the bottom line of my argument is correctness of the solution.

> the procedure work using REGISTER and re-INVITE?

  Yes it will work, in a sense of technically you can send SIP messages
which can effectively divert an active SIP session from one endpoint to
another.

  However, that solution is not the correct solution for solving the
more general problem of a mobile endpoint which uses communication
services which run over IP.

  If an endpoint decides to change IP addresses, what happens to any
active HTTP connections, or telnet connections, or TCP connections, or
SIP sessions?  A solution which solves the problem only for SIP sessions
seems to be a waste.