Subject: (mobile-ip) Agent advertisements and links

From: Venkata Alapati <[email-masked]>
Date: Fri, 20 Mar 1998 08:46:14 -0600

I am new to Mobile-IP and  was reading the RFC 2002 IP Mobility
	support. 
	
	Could some one please explain me what is meant by
	a link when he mentions that Home Agent advertisements are sent 
	out on the links if the Agent is attached to multiple links ?

	Is this link a data link connection between the mobile node and
      the Agent ? or is it just a router link between the router and the

	Agent ? If it is a router link, how can these advertisements 
	reach the mobile node when they have to be transmitted over the
	air (for wireless).

Thanks very much for your time and help,
Somi Alapati.

-----------------------------------------------------------------------------
IETF Mobile IP Working Group Mailing List - Archives:  software.watson.ibm.com
Unsubscribe:	unsubscribe mobile-ip		(as message body, not subject)
Direct all administrative requests to [email-masked]

---

From: Gerald Maguire <[email-masked]>
Date: Fri, 20 Mar 1998 17:04:11 +0100 (MET)

(Data) Link is used in this context to indicate that the advertisement is
sent on each network which the home agent is attached to and over
which it wishes to offer its services as a home agent.

		-------		 |
		|      |---------|
		|  HA  |-------| |
		|      |-----| | |
		-------	     | | |
			     | | |
			     | | |
			     | | |

Assume that the vertical lines in the figure above are each ethernet
segments, in this case the HA is attached to 3 ethernet segments. If
we assume that each segment has one subnet associated with it and that
the home agent wishes to provide services to mobiles attached to these
three segments, then the home agent needs to send advertisements on
each of these three segements.

A mobile which is attached to one of these segments can hear the
advertisement sent on this link and thus know that it is at home.

Consider what happens if one of the segments were to be replaced by a
wireless LAN, then any mobile which can receive the advertisement on
this link will know that it is at home. Thus we see that there is no
difference between the wired and wireless link case.

Note that a single physical interface may be used to attach to
multiple data links, for example the physical DS-SS wireless LAN interface
could be used for multiple spreading codes or a wired ethernet could
be used for multiple subnets which just happen to share the same
physical cable (but yet are distinct data links). However, as a client
is probably only listening to one data link at a time - it needs to
hear an advertisement on this link to know that is at home.

If a client is listening to multiple links, then it could get an
advertisement on any of them, and it is free to select which one it
would like to use (i.e., to which it advertisement it will act
upon). Therefore, for an mobile client with multiple links it might be
possible to be at home on one link while not at home on other links;
it is up to the mobile if it wants to be home.
	Consider for instance a mobile which has a cellular network
interface and an IrDA access point. This client is currently sitting in
front of an IrDA access point which is attached to a network which is
attach to the home agent. The client is also in one of the cells of
the cellular system - which also is attached to its home agent. The
client might hear advertisements on both networks, it will have to
make a policy decision as to which network interface and data link it
wants to use.

Chip