Subject: [nemo] Re: some inquiries

From: Alexandru Petrescu <[email-masked]>
Date: Fri, 03 Oct 2003 20:23:42 +0200

KAI CHEONG WONG wrote:
> I am an university student in australia. Iam doing a research on the 
> problem of NEMO basic potocol whch is the drawback of double crossing
> over tunnel. It happens when LFN decides to communicate with VMN in
> the same subnetwork such as in the train. It is because the LFN dont
> know the VMN is on the same link, that means LFN will send packets to
> the MR which forwards packets to its HA crossing the tunnel. After
> the HA received the packets, it will send back to the MR crossing the
> same tunnel again. It leads to the inefficiency.

This inefficiency is indeed not very good.  However, if one considers
that a MN that visits the train performs classical Mobile IPv6-based RO
with the CN that resides fixed in that train, then the inefficiency of
double encapsulation happens only before the RO procedure is performed.
 After RO, MN and CN talk directly, no tunnels.

In addition to this, SAS (source address selection) employed correctly
between MN and CN could help towards not needing RO at all; (of course
this has the drawback that CN can not talk to MN's Home Address).

I would also add that, when I read the generic train description you
provide, I can't see that the tunnels involved (between MN and its HA,
and between MR and its HA) would "cross over" each other, they are
encapsulated one into the other.

Two tunnels "cross" each other when one (and only one) endpoint of one
tunnel is in the IP path formed by the the two endpoints of the other
tunnel.  This is a definition I'm trying, my oppinion only, feel free to
invent other definitions.

In your scenario, tunnels would "cross" each other only if the train
moving network could be at home when inside the VMN, which is
unrealistic of course.

But in other scenarios, this problem _could_ be realistic:

Assume that a car contains a mobile network and an MR to connect to the
Internet via GPRS and WLAN hotspots only.  Inside this car, there is
another mobile network, that has another smaller MR that connects with
Bluetooth to the car bigger mobile network, and that offers WLAN access
to its LFN. This smaller mobile network is served by a HA deployed in
the car.  This grouping of mobile networks can move freely wherever GPRS
or WLAN coverage exists.  The smaller mobile network can separate, get
out of the car and connect to a train, or go inside driver's house.  All
entities can connect to the Internet and be reached at their permanent
home addresses, if the current nemo base solution is used.

But, move the car to an area where is no GPRS nor WLAN coverage, such as
a remote montaineous location, driver's cottage connected to the
Internet with a satellite dish.  There is only a Bluetooth AP in this
small hose.  In this case, the only way to connect everything to
the Internet, is to first connect the smaller mobile network via
Bluetooth to the house Bluetooth AP (instead of the car's BT AP) and the
car bigger mobile network to the small mobile network with WLAN (instead
of to a hotspot area AP).  This can be done and all entities in both
mobile networks can be connected to the Internet but, with current
solution, they can not be reached at their permanent home address.

(wondering about comercialization of this scenario, look at Carey doing
GPRS outside and WLAN inside the luxury sedan).

> However, i have read the draft-ietf, it seems doesnt mention about 
> that problem.

I'm not sure that the current network mobility solution as described by
the draft should treat this problem.  Maybe this can be solved by other
means, RO, SAS and other tools can be employed.

At most this could be mentioned as an acknowledged limitation related to
a particular scenario of deployment.  But then again, maybe this is more
related to "usages" or potential scenarios, or more to an applicability
statement.  One simple solution draft can not solve everything to
everybody.  It already does a good job for most scenarios, I believe.

> 1, Why there exist the bi-directional tunnel between the MR and its 
> HA insteads of uni-directional tunnel?

Bidirectional is better than uni-directional.  It avoids outgoing
packets always taking different paths than incoming packets, which might
break somehow TCP.  It also avoids many other bad things.  Moreover, it
allowed people to design a three-party scheme to perform Route
Optimization with a higher level of security (when no sec infrastructure
can be relied upon).

> Can MNN such as VMN send the packets to CN using VMN's home address 
> as the source address and send it directly through the foreign link 
> which its attaching ?

MN's home address can be filtered out by the foreign link because not
being topologically correct.  So, no, VMN should not use its HoA, but
its CoA.

> 2, What is the FA of the VMN which attaching to the mobile network?

With Mobile IPv6, the FA concept of Mobile IPv4 is eliminated, in that
the tunnel between MH and its HA finishes at MH (instead of FA, or
"co-located CoA"), in that an address can be obtained from the RA of an
AR, or in that a DHCPv6 server can exist anywhere in the visited domain
(not necessarily in the FA).

In other words, it is sufficient for MR to send RA's to the fixed link
in the train, no need of FA.

> 3,  What is the solution(based on NEMO basic protocol) which should 
> clearly address the problem of double crossing over tunnel?

Choose.  Base your choice on the basic protocol, but without touching it
too much.  You have an idea?

Alex
GBU

---

From: Alexandru Petrescu <[email-masked]>
Date: Fri, 03 Oct 2003 20:23:42 +0200

Alex
GBU