Subject: RE: [SIP] sip mobility with REGISTER

From: "Roy, Radhika R, ALCOO" <[email-masked]>
Date: Thu, 28 Sep 2000 17:42:52 -0400

Hi, Billy:

I guess that we agreeing on the fundamental points.

What is the difference then?

SIP is basically transport independent (e.g., it is not tied to any
transport - it can run on IP, it can also run on ATM).

No layer is basically aware about mobility (e.g., CDMA, TDMA, LAN, IP, ATM,
etc.).

To solve some mobility related problems people devise some mechanisms. For
example, mobile IP, cellular IP, wireless ATM, wireless LAN, etc. are the
answers for addressing some kinds of mobility related problems. These are
value-added features. So, no one can mandate what needs to be done in the
value-added features (e.g., "any mobility solution in an IP network MUST NOT
allow the IP address of an active session to change" is NOT applicable).

Hope this will clarify the points.

Best regards,
Radhika R. Roy
AT&T

-----Original Message-----
From: Billy Biggs [mailto:[email-masked]]
Sent: Thursday, September 28, 2000 5:11 PM
To: Roy, Radhika R, ALCOO
Cc: farhan; sip
Subject: Re: [SIP] sip mobility with REGISTER

  Radhika,

> However, there is one fundamental point: Change in point of attachment
> in the mobile environment.  [...]   Depending on this "change of
> address," each layer has to take the appropriate action.  If any layer
> is transparent to this change of address, no action is necessary.

  The IP layer has no concept of mobility, and IP addresses are used
casually in upper-layer protocols.  As well, all protocols which run
over IP assume that the IP addresses used do not change mid-session.
So, any mobility solution in an IP network MUST NOT allow the IP address
of an active session to change.

  This means that IP mobility can only be implemented through address
changes in the lower layers, tunnelling through a 3rd party with a fixed
IP address, or informing the routers of new route information for a
specific address.

  Because of this architecture, applications built on IP are guarenteed
that they do not need to worry about mobility, since it is unsupported
by the IP layer they are built on.

  Since SIP is built on IP, by definition no changes need to be made to
SIP to support terminal mobility.

  The mechanisms SIP provides for doing SIP "mobility" are addressing a
different problem: that of user mobility, not endpoint mobility.  While
I agree that you can get away with updating a session endpoint in SIP, I
don't believe this solution should be in any way associated with the
concept of endpoint mobility.

-- 
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 18:03:41 -0500

Radhika,

  I believe that I understand what you are asking, however I think that
it would be easier to discuss if you gave some more specific
requirements about what you are trying to accomplish.

> To solve some mobility related problems people devise some mechanisms.
> For example, mobile IP, cellular IP, wireless ATM, wireless LAN, etc.
> are the answers for addressing some kinds of mobility related
> problems. These are value-added features. So, no one can mandate what
> needs to be done in the value-added features (e.g., "any mobility
> solution in an IP network MUST NOT allow the IP address of an active
> session to change" is NOT applicable).

  Alright, so I'll drop my argument about it being a bad idea to change
IP addresses mid-stream and consider some hypothetical mobility
solution.  This solution seems to involve the endpoint changing its IP
address and then attempting to migrate all active connections over to
its new IP address.

  As I have mentioned before, if an endpoint wants to handoff a session
to another SIP user agent, no extensions to SIP are required.  In fact,
I believe this has already been discussed on the mailing list.  So,
already I think we've got you covered.

  Your original questions in this thread were:

> Do we need any additional standard work in the SIP layer to couple the
> change in the lower address to invoke the REGISTER message?

  Well, since the endpoint is changing its own IP address, it seems that
it would already have enough knowledge to un-REGISTER the old address
and REGISTER again with the new one.  I don't think we need any
additional standards work.

> It appears that the location update (with single register with a
> single or multiple registers with multiple domains) is the basic
> problem to solve (Does this work need to be done in the SIP WG or
> other WGs?).  According, REGISTER and/or INVITE messages may be
> impacted.

  I don't see any changes that need to be made to the REGISTER or INVITE
messages to support switching the addresses involved in a session.

> transport - it can run on IP, it can also run on ATM).

  This is true, however remember that a SIP URI is defined in terms of
IP addresses, so while tunnelling SIP messages across other networks is
feasible (and remembering this state in Via headers is also feasible),
the endpoints themselves are very much tied to an IP network.