Subject: [mobile-ip] MN-to-MN communications in MIPv4

From: Lewis Adam-CAL022 <[email-masked]>
Date: Mon, 3 Dec 2001 11:45:14 -0600

I thought I read the answer to this in RFC2002, but after going back to confirm it I was unable to find it again.  It is my assumption that two mobile nodes connected to the same FA should be able to by-pass normal IP routing (e.g. the need to tunnel through a HA) and route directly through the FA.  I make this assumption since it would be a logical optimization since the FA has all the necessary information needed to do so (e.g. home IP address, link layer address, etc.).  The only supporting text I was able to find on this in the RFC was on page 10.

"Similarly, a mobile node and a prospective or current foreign agent MUST be able to exchange datagrams without relying on standard IP routing mechanisms; that is, those mechanisms which make forwarding decisions based upon the network-prefix of the destination address in the IP header. This requirement can be satisfied if the foreign agent and the visiting mobile node have an interface on the same link. In this case, the mobile node and foreign agent simply bypass their normal IP routing mechanism when sending datagrams to each other, addressing the underlying link-layer packets to their respective link-layer addresses. Other placements of the foreign agent relative to the mobile node MAY also be possible using other mechanisms to exchange datagrams between these nodes, but such placements are beyond the scope of this document."

Adam

Adam C. Lewis 
Senior Engineer 
Advanced Technology & Strategy 
Global Technology Development Group 
Commercial, Government, and Industrial Solutions Sector 

1301 E. Algonquin Road 
IL02/Rm. 509 
Schaumburg, IL 60196 
Email: [email-masked] 
Phone: (847) 576-8489 Fax: (847) 576-9018

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From: "Roy Jose" <[email-masked]>
Date: Wed, 5 Dec 2001 21:57:21 +0530

I think there is potential routing problem here. Suppose MN1 and MN2 are
visitors on an FA (FA1).  Now suppose MN2 does FA handoff and moves to FA2.
FA1 will have a visiting entry for MN2 till the lifetime expiration. Now if
FA1 tries to by-pass traffic to MN2 through it's visiting interface, packets
won't reach MN2. This problem won't exist if HA send BU's to previous FA.
But, I don't think many people would implement Route Optimizations on HA.

-Roy

> I thought I read the answer to this in RFC2002, but after going back to
confirm it I was unable to find it again.  It is my assumption that two
mobile nodes connected to the same FA should be able to by-pass normal IP
routing (e.g. the need to tunnel through a HA) and route directly through
the FA.  I make this assumption since it would be a logical optimization
since the FA has all the necessary information needed to do so (e.g. home IP
address, link layer address, etc.).  The only supporting text I was able to
find on this in the RFC was on page 10.
>
> "Similarly, a mobile node and a prospective or current foreign agent MUST
be able to exchange datagrams without relying on standard IP routing
mechanisms; that is, those mechanisms which make forwarding decisions based
upon the network-prefix of the destination address in the IP header. This
requirement can be satisfied if the foreign agent and the visiting mobile
node have an interface on the same link. In this case, the mobile node and
foreign agent simply bypass their normal IP routing mechanism when sending
datagrams to each other, addressing the underlying link-layer packets to
their respective link-layer addresses. Other placements of the foreign agent
relative to the mobile node MAY also be possible using other mechanisms to
exchange datagrams between these nodes, but such placements are beyond the
scope of this document."

---

From: "Subrata Goswami" <[email-masked]>
Date: Wed, 5 Dec 2001 11:04:06 -0800

Please bear with me a little while.  If MN1 and MN2 wants to communicate
and both are in the same foreign net, then the following are the options
available for them to pass IP packets without involving the HA.

1. The FA is also the gateway router of the foreign subnet.
2. FA does proxy ARP (assuming Ethernet link layer) for the MN's.
3. The MN's respond to Home Address ARPs (again assuming Ethernet).

Am I missing anything here ?

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of Roy Jose
Sent: Wednesday, December 05, 2001 8:27 AM
To: [email-masked]
Subject: Re: [mobile-ip] MN-to-MN communications in MIPv4

-Roy

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From: "Alper E. YEGIN" <[email-masked]>
Date: Wed, 5 Dec 2001 16:51:21 -0800

Roy,

Having gone through this exercise twice (two implementations),
yes, this is the major problem with this innocent-looking-free-optimization
:)

It gets home agent out of the loop. Therefore stale routing information on
foreign
agent can send the packets to blackhole after mobile moves off link.

Also, any ingress filtering and control the home network would impose on
incoming packets would be "skipped" if we let this happen.

alper

---

From: "Roy Jose" <[email-masked]>
Date: Thu, 6 Dec 2001 14:49:26 +0530

Hi Subrata,

MN or FA are not supposed to send any ARP replies. FA is the default gateway
for MN's always:) Btw, I am not sure if you got my point right. What I meant
was previous FA(FA1) will have stale information(ARP/route) for the MN(MN2)
even after FA handoff and that will cause problems.

Thanks,
Roy

---

From: "Subrata Goswami" <[email-masked]>
Date: Thu, 6 Dec 2001 09:36:45 -0800

Hi Roy, MN and FA can do ARP, only restriction is MN can
only do unicast ARP reply if it not co-located.

"While the mobile node is away from home, it MUST NOT transmit any
   broadcast ARP Request or ARP Reply messages.  Finally, while the
   mobile node is away from home, it MUST NOT reply to ARP Requests
   in which the target IP address is its own home address, unless the
   ARP Request is sent by a foreign agent with which the mobile node
   is attempting to register or a foreign agent with which the mobile
   node has an unexpired registration.  In the latter case, the mobile
   node MUST use a unicast ARP Reply to respond to the foreign agent.
   Note that if the mobile node is using a co-located care-of address
   and receives an ARP Request in which the target IP address is this
   care-of address, then the mobile node SHOULD reply to this ARP
   Request.  Note also that, when transmitting a Registration Request on
   a foreign network, a mobile node may discover the link-layer address
   of a foreign agent by storing the address as it is received from the
   Agent Advertisement from that foreign agent, but not by transmitting
   a broadcast ARP Request message."

I have been able loacte in RFC 2002 where it says FA has to be
the default router for the MN.  RFC2002 says,

	  "For
         datagrams sent by a mobile node, the foreign agent may serve as
         a default router for registered mobile nodes."

Given the possibility in IPv4 that the FA may not be the default gateway
for the MN, the FA may not even be able to collect any accounting data.

Will address the stale-data issue in a later email.

Subrata

-----Original Message-----
Sent: Thursday, December 06, 2001 1:19 AM

Hi Subrata,

Thanks,
Roy