Subject: RE: [SIP] sip mobility with REGISTER

From: "Roy, Radhika R, ALCOO" <[email-masked]>
Date: Thu, 28 Sep 2000 13:39:11 -0400

Hi, Farhan:

I guess that you are right.

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

The only thing that you need is the support of the lower layer to resolve
the address as you move from place to place. In turn, the lower layer needs
to make the SIP layer aware that it need to re-REGISTER as the point of
attachment is changed. Accordingly, an re-INVITE message may be sent.

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?

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.

That is all! I hope that other members will also provide comments on this.

Best regards,
Radhika R. Roy
AT&T

-----Original Message-----
From: farhan [mailto:[email-masked]]
Sent: Thursday, September 28, 2000 12:45 PM
To: sip
Subject: [SIP] sip mobility with REGISTER

i maybe a little naive here. but isn't REGISTER already covering a great
deal of mobility issues?

moblity will be, for a fact handled at lower levels. for instance,
bluetooth. On my Windows CE PDA, running off a bluetooth pcmcia card from
ericssons, I have little problem of mobility - bluetooth handles everything.

in the coming ICE age, i dont think we are going to be faced with exclusive
sip-only network. as per the RFC, sip is an application level protocol thus,
the transport layer issues are, well, transport level issues. The issue is
aren't we really talking about using a general purpose networking layer to
run SIP? If we are, then it is understood that SIP, http, and a host of
other application layer protocols for services being delivered over the
network will not have to reinvent their own mobility schemes which may also
work in separate ways. I mean, if my sip UA is in turn going to invoke a web
page that creates a http persistent session with cookies, i wouldn't want to
loose the continuity of the page as i move from one room to another while
the sip session goes on. Thus, it is essential to have the mobility issues
handled at a lower level. SIP is and should be an application layer
protocol. Lets

in anycase, what specific mobility issues are not handlable by the REGISTER
request? Are we also discussiing when REGISTER should be invoked? I dont
think that even that is a good idea to put on paper. Different types of
networks will have different approaches and needs. A mobile network may
derive their 'q' from signal strength and the registrar may have to decide
on a criterion different from a provider like hotsip which maybe searching
for a least-cost route (prefer a sip UA to a sip gateway).

- farhan

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---

From: Billy Biggs <[email-masked]>
Date: Thu, 28 Sep 2000 14:17:36 -0500

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]

---

From: farhan <[email-masked]>
Date: Thu, 28 Sep 2000 11:44:16 +0530

> point of contact for a user, not a new location for an active point of

we dont 'intend' things in a protocol. we just use them. a protocol
specifies how two entities will interact in a predictible manner. it
does not and should not enter into the realm of why they want to do
that. if that were the case, you are essentially arguing against
innovative application of existing protocols. for instance, the purpose
of http is to transfer documents and not read email - hotmail is out.
the purpose of UDP is not real time communication but unreliable
delivery- rtp is out.

> Even to do the hacked solution of REGISTER'ing under the new

i dont forsee reinvites or re-registers due to mobile ip issues. mobile
ip should gaurenttee seamless and continuous coverage. it is completely
an issue of the network (or maybe even a lower layer - like in
bluetooth).

lets define the issue of mobility. when we are talking within sip
persepective, we are essentially talking of app level mobility. correct
me if i am wrong.

app level mobility means that i should be able to switch between varying
apps. from my desktop UA to my cell phone to my answering machine at
voicemail.com seamlessly and on a single id. something that is being
promised by likes of hotsip. the mobility is NOT in terms of physical
space but application layer. for that end, REGISTER is perfect. With
remarkable clarity and insight, REGISTER talks about third party
registerations, multiple registerations and forking locators. All of
them fully and completely cover mobility.

- farhan