Network layer-based mobility management schemes can be categorized as falling into one of three basic types per [IOAN93]. They will be summarized in the following subsections.
The first mobility management scheme is called the Permanent Address assignment Scheme [IOAN93]. In PAS the mobile host network layer address remains constant, with the routing system adapting itself to changes in the host's location.
To support PAS, each router in the mobile internetwork must maintain a host route (routing table entry consisting of host address and next hop for that host's current location) for each mobile host in the internetwork. As each mobile host moves throughout the internetwork, all routers must update their routing tables accordingly.
Figure 1.8 depicts a mobile host, Mobile 2, changing its point of access to the mobile internetwork from Area X to Area Z. Accommodating this change in PAS network topology entails changes to the routing tables in Router X, Router Y and Router Z; these routing tables are displayed immediately prior to the movement of Mobile 2 in Table 1.2 and immediately following the movement of Mobile 2 in Table 1.3. Because Router W has Router X as the "next hop" for both Area X and Area Z, its routing table remains unchanged by the movement of Mobile 2.
PAS is problematic because it requires every router in the mobile internetwork to have a current host route entry for every mobile host-which might number in the millions. This simple scheme clearly does not scale well and is thus inappropriate for large mobile networks. Too bad-it's easy for the mobile hosts and transparent to their correspondents.
The second basic mobility management scheme is called the Temporary Address assignment Scheme [IOAN93], which is the logical opposite of PAS. In TAS a mobile host adopts a network layer address consistent with its current subnet location. When the host moves, its network layer address changes to reflect its new location.
In TAS, the onus of work supporting mobility is placed on the mobile host and its applications rather than the network infrastructure. TAS is a form of the Application Awareness approach to mobility.
Figure 1.9 depicts a mobile host, Mobile X.1, moving from
Area X to Area Z; in the process its temporary address changes from
X.1 to Z.2. Because this new address reflects its new location in Area
Z, packets addressed to Mobile Z.2 will be forwarded to it via
conventional routing. The routing tables for Routers W, X, Y and Z are
depicted before the move in Table 1.7 and following the move in Table
![[*]](/latex2html-98.1p1/icons.gif/cross_ref_motif.gif){kind=icon}
. In all routing tables, the entry for host X.1 has been deleted
and a new entry for host Z.2 has been created.
Unfortunately, peer applications will not be able to create associations with the mobile host for a while because its network layer address changed. Until peer applications know to use the Z.2 address rather than the X.1 address, the Mobile host will be unable to receive packets.
Obviously, TAS is trivial for routers to support-they don't need to do anything special.1.13 However, maintenance of accurate DNS name server information is extremely difficult with TAS.1.14 Also, mechanisms are required to ensure global network address uniqueness.1.15
Changing network addresses impacts TCP and other transport layer protocols which assume permanent1.16 network layer addresses. Under TAS, sessions would have to be torn-down whenever a host move required a change of address. This is the same problem created by the Directory Lookup approach to mobility and conflicts with our desire for transparent mobility.
The third basic scheme for mobility management is called the Embedded Network Scheme [IOAN93]. ENS embeds a virtual network of mobile hosts and support infrastructure (mobility routers and other assistant entities) in the midst of a larger internetwork. These mobility routers serve to provide mobility in the virtual network; other elements in the data network infrastructure, such as routers, can remain ignorant about host mobility.
Figure 1.10 depicts a mobile host, Mobile X.1, moving from
Area X to Area Z. Only the routing tables for the "special"
mobility-aware routers-Routers X* and Z*-have been changed to
reflect this change in network topology. The routing tables for all
routers are depicted in Table prior to the mobile's movement and
Table following the mobile's movement.
By decoupling mobility management from the conventional routing mechanisms, ENS provides a more efficient means of routing to mobile hosts and supports constant network addresses. ENS typically involves either data packet encapsulation or the use of secondary temporary addresses by the "special" routers to provide mobility. ENS embodies the Administrative Redirection approach to mobility in its transparency to the rest of the world.
CDPD services are provided via an ENS-type of mobility management; this mobility is transparent to existing routers and hosts. The Mobile IP definition and Novell's Mobile IPX are also implementations of the ENS concept. The primary difference between these systems is in the way that the assisting entities are defined.
In CDPD and Mobile IP, two cooperating assisting entities provide mobility management. One of them-the Mobile Home Function in CDPD, the mobility router in Mobile IP-serves as the public local router for packets destined to the mobile host. The second assistant-the Mobile Serving Function in CDPD, the foreign agent in Mobile IP-receives packets which have been redirected by the first assistant and forward them on to the mobile via the local data link.
Packet encapsulation or tunneling is used by CDPD and Mobile IP to redirect packets from the first to the second assisting entity, as depicted in Figure 1.11. The details of this mobility management scheme is presented in Chapter 4 (for CDPD) and Chapter 10 (for Mobile IP).
Novell's Mobile IPX differs in that it uses a single assisting entity, called the home router, to forward packets to a mobile node (host). The home router advertises an "internal" network number which the mobile adopts as its network number field in a so-called "permanent" address; the node number field for this permanent address is sequentially assigned by the home router to registering mobiles.
Whenever a mobile relocates, it determines its location-dependent (physical) IPX address in the usual way (by prepending a local router's network number to its hardwired MAC identifier, as we discussed in Section 1.5.1). The mobile then notifies its home router of its new location. All the home router has to do is maintain a table of permanent address to physical address mappings.
Whenever an IPX packet destined for the mobile's permanent address arrives at the home router (based on standard IPX packet routing), the home router replaces that destination address (the mobile's permanent address) with the mobile's current physical address and redirects the packet onward as depicted in Figure 1.12. The mobile then internally replaces the substituted physical address with its permanent address to avoid confusion by its higher layers and applications. This destination address substitution may be compared with the packet encapsulation technique used by CDPD and Mobile IP.
