Subject: [mobile-ip] a question about BU

From: "Yu \(Ryan\) Chen" <[email-masked]>
Date: Fri, 11 May 2001 10:10:38 -0400

Hi,
I am interested in route optimization of MIP. How ever, I got a question and
asked help for clarification.
I understand that the BU message are sent to CN when the MN changes its
location. My question is: Can CN resume any open TCP connection immediately
at the new IP address of MN with some mechanism or it has to rely on the
previous FA to keep the old TCP connection open. In other word, is route
optimization effective on existing TCP connections or only new connections?

Thanks in advance for any help.
Ryan

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From: Thierry Ernst <[email-masked]>
Date: Fri, 11 May 2001 16:31:32 +0200

> optimization effective on existing TCP connections or only new connections?

The CN does not need to resume the TCP connection.    The MN has two
active addresses: the home address used as an identifier and the careof
address used for routing.  The careof address is transparent to TCP
which always uses the home address in the TCP header.

The IP header re-route the packet to the MN's careof address (source
routing via MN's careof address).

Active TCP connections are not broken. This is what for MIP has been
designed this way.

Hope this helps,
Thierry

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From: Xiaofeng Xu <[email-masked]>
Date: Fri, 11 May 2001 09:34:04 -0500

If the TCP connection is built on MN's home address, the BU won't affect it,
but will bind it to new CoA.

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From: "Yu \(Ryan\) Chen" <[email-masked]>
Date: Fri, 11 May 2001 11:51:09 -0400

Thanks for all the replies. However, it seems that I didn't state the
question clear enough.
As we know, route optimization is proposed to solve the triangle routing
problem of MIP. Sure CN need not break existing TCP connections if it use
the home address to connect with MN. but then any existing connections will
not benefit from BU, am I right?
There are actually two case:
1. The TCP connection is based on home address of MN, then no additional
effort needed for open connetions.
2. The TCP connection is based on previous COA of MN, then the previous FA
should forward datagrams to new COA to keep open connections.
However, both of above cases still suffer from triangle routing problem.
So my question is: suppose the CN received a BU, no matter its connection
with MN based on home address or previous COA, is there some way that the CN
could use the new COA for any existing TCP connections but not either break
them or routing via HA or previous FA? I know TCP connections are based on
peer IP and ports and it looks like impossible.
So if it is impossible, is that mean BU only benefit new connections of MN?

Best regards,
Ryan

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From: "Charles E. Perkins" <[email-masked]>
Date: Fri, 11 May 2001 10:11:32 -0700

Hello again Ryan,

> not benefit from BU, am I right?

No, this isn't right, because all TCP connections (that you wish to
keep, anyway) should be based on the home address.

> effort needed for open connetions.

This should be all of the cases of interest.

> should forward datagrams to new COA to keep open connections.

If a node has TCP protocol control blocks based on the care-of address,
then that should amount to a policy determination that the TCP connection
does not have to survive reconnection.

> However, both of above cases still suffer from triangle routing problem.

Case (1) should not have the problem, because even though IP uses the
care-of address (thus avoiding triangle routing), TCP uses the home
address (thus avoiding broken connections).

> So if it is impossible, is that mean BU only benefit new connections of MN?

It is possible to let TCP use the home address and to let IP use the
care-of address.  Then the Binding Update will benefit all connections
of the mobile node.

Regards,
Charlie P.

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From: "Yu \(Ryan\) Chen" <[email-masked]>
Date: Fri, 11 May 2001 14:21:26 -0400

Hi, Charlie,

Thank you for the help. I refered to the route optimization draft again and
I see what made me confused. So actually the CN tunnels the following
datagrams to the MN
itself instead of sending it to the HA after it receives BU message. Am I
right?
If it is right, BU relieves the triangle routing problem but will require
the capability of IP encapsulation on CN, doesn't it?

BR,
Ryan

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From: "Charles E. Perkins" <[email-masked]>
Date: Fri, 11 May 2001 11:38:36 -0700

Hello Ryan,

> itself instead of sending it to the HA after it receives BU message. Am I

Yes, this is correct.

> the capability of IP encapsulation on CN, doesn't it?

Yes, the correspondent node has to be able to do encapsulation
in order to accept the Binding Update.

If you can suggest some improved wording to make this clearer in the
Route Optimization specification, that would be useful to avoid any
similar confusion for future readers.  Would at least a statement
somewhere in the specification to say that correspondents have to at
least do encapsulation would be helpful?

Regards,
Charlie P.

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From: "Yu \(Ryan\) Chen" <[email-masked]>
Date: Fri, 11 May 2001 15:28:01 -0400

Charlie,

In section 3.1 of the draft, it says:
" When sending an IP datagram, if the sending node has a binding cache
   entry for the destination node, it SHOULD tunnel the datagram to the
   mobile node's care-of address using the encapsulation techniques used
   by home agents, and described in [9, 10, 3, 4]. "
I would recommend to add: "Note the sending node should not send the
datagram to the MN's COA directly unless it does not need maintain any
open connections."
To many applications does not require mobility support, for example,
applications
based on UDP, it may be useful to let the CN use the new COA directly.
However,
it may be out of the scope of the draft.
BTW, return to my original question, is there a way that CN could send the
datagrams
to the new COA directly instead of tunneling without breaking open TCP
connections?
It seems to me that it requires tearing TCP layer from IP layer. Any
comments?

Cheers,
Ryan

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From: Pekka Nikander <[email-masked]>
Date: Sat, 12 May 2001 05:25:28 +0100

> datagrams to the new COA directly instead of tunneling without breaking
> open TCP connections?

Homeless Mobile IPv6 proposes one such extension for Mobile IPv6.
See draft-nikander-mobileip-homelessv6-01.txt.

I am not aware of any work that would be applicable to IPv4.

--Pekka Nikander