Subject: (mobile-ip) Multicast support in IETF RFC 2002

From: Jun Wei <[email-masked]>
Date: Tue, 4 Nov 1997 18:10:47 -0800 (PST)

In IETF RFC 2002, it says:

   In order to receive multicasts, a mobile node MUST join the 
   multicast group in one of two ways.  First, a mobile node MAY
   join the group via a (local) multicast router on the visited
   subnet.  This option assumes that there is a multicast router
   present on the visited subnet.  If the mobile node is using a
   co-located care-of address, it SHOULD use this address as the 
   source IP address of its IGMP [5] messages.  Otherwise, it
   MUST use its home address.

We concern the case that mobile node does not have a co-located
care-of address, and there is a local multicast router on the
visiting network. If we use mobile node's home address as IGMP
source address, it would not work. Because the multicast router
would think the IGMP reports (by mobile node) came from its home
network.

We consider a simple way for receiving multicast locally in
visiting subnet. That is to use care-of address as the source IP
address in IGMP reports, whether or not the care-of address is
co-located on mobile node. As far as IGMP reports concern, the
multicast router does not care from which node in that subnet
IGMP reports come from. As long as there is one node sends IGMP
report for certain multicast group, the multicast router will
forward concerned multicast packets into that subnet. The mobile
node's care-of address is local to the visiting network, thus it
will serve the purpose for IGMP report if mobile node use it as
IGMP source address even it is not co-located on mobile nodes. 
It is not a problem for IGMP even if there will be multiple IGMP
reports with same source address from different (mobile) nodes
when multiple of them happen to be interested in the same
multicast group.. 

We like to hear your comment on this. 

Thanks for your time.

Jun Wei

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From: Xinhua Zhao <[email-masked]>
Date: Tue, 04 Nov 1997 21:16:59 -0800

> when multiple of them happen to be interested in the same

That's a pretty neat idea.

My only concern with this approach is: as I recall, the care-of address 
provided by a foreign agent could be on an interface other than the 
interface on the same subnet as the visiting mobile host. At least,
RFC2002 does not exclude the use of such care-of addresses. In this case, 
using this care-of address as source address in IGMP report will have 
the same problem as using its home address. So, do we need to restrict
the care-of address provided by a foreign agent to be always on the
subnet a mobile host is visiting?

> We like to hear your comment on this. 

Xinhua Zhao
MosquitoNet Project, Stanford University
http://MosquitoNet.Stanford.EDU/~zhao
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From: vishwanath <[email-masked]>
Date: Wed, 5 Nov 1997 15:59:35 +0000 (GMT)

On Tue, 4 Nov 1997, Jun Wei wrote:

>->    present on the visited subnet.  If the mobile node is using a

First of all does it mean here that the mobile node should not use the
care of address assigned by a F.A. ...? And why at all home address should
come into picture here when we talk about a local multicat router in the
visited subnet..?

> would think the IGMP reports (by mobile node) came from its home

Good Query..

> when multiple of them happen to be interested in the same

Mobile node may be using a care of address assigned by an F.A. or a DHCP
server.. I think it least matters when it wants to join a multicast
group.. All that matters is what source address it should use..? Well if
at all it uses the care of address as the source address, then it is not
efficient, because, mobile host will then have to join the multicast
group each time it enters a new subnet.. as it will be unable to receive
the multicasts from previously visited network.
	I think using the home address as the source address will be more
advantagious even though there are overheads of bi-directional tunnelling,
as we will be able to achieve the complete transparency in mobility.

> We like to hear your comment on this. 

                	              \/////
        	                      o/ oio \o
 	___________________________oOo__\ - /__oOo________________________

	Vishwanath S. Edavayyanamath, Indian Institute of Science,
	Protocol Engineering Lab, Electrical Communication Engineering,
        Bangalore - 560 012, India.
	Ph: (+91)-80-3092282, Fax: (+91)-80-334 7991
        E-mail : [email-masked]
	WEB : http://pclab.ece.iisc.ernet.in/vishu/ 	   / o 
	_________________________________________________/_ ////_/><....

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From: Jun Wei <[email-masked]>
Date: Mon, 10 Nov 1997 12:12:41 -0800 (PST)

On Wed, 5 Nov 1997, vishwanath wrote:

   group each time it enters a new subnet..

A mobile host will not need to join the multicast group each time
it enters a new subnet. What it does is to reply to multicast
router's IGMP query periodically(even it is not mobile). When a
moible host moves to a visiting network, it is not necessary to
rejoin as it will receive IGMP query and reply(assume there is a 
multicast router in the visiting network). Only if the delay is
a concern, one can have mobile host to rejoin the multicast group
each time it moves to a new subnet. But this way, one will have
to have mobile host to send IGMP report each time it moves.

   as it will be unable to receive
   the multicasts from previously visited network.

This is a scoping problem. If the multicast group has limited
scope, and the mobile host happens to move out of range, nothing
will help, except to have home agent forward via bi-directional
tunneling. However, having home agent forward to each individual
mobile host is basically unicast, which may not be desirable for
some multicast applications, especially for those multicast
scoped for globe internet.

Using bi-directional tunnelling also imposes another problem: 
duplicate copies of multicast packets forwarded into one subnet
when there are more than one mobile hosts in that subnet
interested in the same multicast group.
   
                   	              \/////
           	                      o/ oio \o

Jun Wei

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From: Jun Wei <[email-masked]>
Date: Mon, 10 Nov 1997 12:30:40 -0800 (PST)

I agree with you. However, at least, we can always *configure*
foreign agent care-of address as the interface on the same subnet
as the mobile host visiting. (RFC 2002 only requires that foreign
agent MUST configure a care-of address).  I'm not sure when one
wants to configure it as the one not on that subnet. I like to
hear the reason. 
   
   Xinhua Zhao

 Jun

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From: Gerald Maguire <[email-masked]>
Date: Mon, 10 Nov 1997 23:02:31 +0100 (MET)

There is no need to have a (mobile) host specific address - all we
care about is that _someone_ on this (multicast) link is interested in
the multicast. The first to reply is sufficient.  The reply will use
as its Destination IP address the group address and can use as the
source IP address the IP address of the multicast router which sent
the query (which it copied from the source IP address in the IGMP
query) address.

Is there something wrong with this simple solution?

The solution works whether the host is mobile or not. However, it does
mean that you can't distinguish (at the network layer) who responded,
but if you care - you could look at the link layer header. As the
question of who sent the response is not relevant to IGMP, it does not
matter that you would have to use link layer information to determine
who actually sent the response.

Chip

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From: Jun Wei <[email-masked]>
Date: Mon, 10 Nov 1997 14:19:17 -0800 (PST)

On Mon, 10 Nov 1997, Gerald Maguire wrote:

What if there is no other host interested in that multicast group
in that subnet?

   
   Chip

 Jun

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From: Changpeng Fan <[email-masked]>
Date: Tue, 11 Nov 1997 11:11:47 +0100

Jun Wei wrote:

> However, at least, we can always *configure*

This can surely happen when it is easier or more efficient
for the foreign agent to capture and process packets on one
interface than on the other one. We have actually such a
scenario -- the foreign agent sits between an Ethernet and
an ATM network.

Changpeng