Subject: New section on mobility

From: Phil Karn <[email-masked]>
Date: Fri, 10 Mar 2000 13:38:34 -0800

Here's the text I just added to kick off the mobility section. Since the
deadline is looming, I'm going to submit the LINK document as it currently
stands, and update (again) the copy on my web page. --Phil

Internet users are increasingly mobile. Not only are many Internet
nodes laptop computers, but pocket organizers and mobile embedded
systems are also becoming nodes on the Internet. These nodes may
connect to many different access points on the Internet over time, and
they expect this to be largely transparent to their activities.
Except when they are not connected to the Internet at all, and for
performance differences when they are connected, they expect that
everything will "just work" regardless of their current Internet
attachment point or local subnetwork technology.

Mobility can be provided at any of several layers in the Internet
protocol stack, and there is ongoing debate as to which are the most
appropriate and efficient. Mobility is already an feature of certain
application layer protocols; the Post Office Protocol (POP) [RFC1939]
and the Internet Message Access Protocol (IMAP) [RFC2060] were created
specifically to provide mobility in the receipt of electronic mail.

Mobility can also be provided at the IP layer [RFC2002]. This
mechanism provides greater transparency, viz., IP addresses that
remain fixed as the nodes move, but at the cost of potentially
significant network overhead and increased delay because of the
non-optimum network routing and tunneling involved.

Some subnetworks may provide internal mobility, transparent to IP, as
a feature of their own internal routing mechanisms. To the extent that
these simplify routing at the IP layer, reduce the need for mechanisms
like Mobile IP, or exploit mechanisms unique to the subnetwork, this
is generally desirable. This is especially true when the subnetwork
covers a relatively small geographic area and the users move rapidly
between the attachment points within that area.

However, if the subnetwork is physically large and connects to other
parts of the Internet at multiple geographic points, care should be
taken to optimize the wide-area routing of packets between nodes on
the external Internet and nodes on the subnet. This is generally done
with "nearest exit" routing strategies. Because a given subnetwork may
be unaware of the actual physical location of a destination on another
subnetwork, it simply routes packets bound for the other subnetwork to
the nearest gateway between the two. This implies some awareness of IP
addressing and routing within the subnetwork. The subnetwork may wish
to use IP routing internally for wide area routing and restrict
subnetwork-specific routing to constrained geographic areas where the
effects of suboptimal routing are minimized.