The Mobile IP Working Group of the Internet Engineering Task Force (IETF) is addressing the requirement of mobility in today's Internet. Mobile IP enables a mobile node to send and receive packets over the Internet using its home address regardless of its point of attachment. In essence, Mobile IP extends the existing Internet Protocol to allow a portable computer to be moved from one network to another without changing its IP address and without losing existing connections. In this section we will discuss:
The current base specification for Mobile IP is an ``Internet Draft". Internet Drafts are draft documents that may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet Drafts as reference material or to cite them other than as ``work in progress."
The Mobile IP Working Group of the Internet Engineering Task Force (IETF) is the culmination of efforts by many individuals interested in the problem of mobile routing of hosts. The first meetings were in the form of BOF (Birds of a Feather) sessions held at the Atlanta (July, 1991), Santa Fe (November, 1991), and San Diego (March, 1992) IETF meetings. In June, 1992, a proposed charter for a formal Working Group was submitted to the IETF and at the same time a mailing list was set up for conduct of the group's business. Following a revision of the charter, the Working Group was officially formed in June 30, 1992.
The IETF Mobile IP Working Group (mobileip WG) is chartered to develop or adopt architectures and protocols to support mobility within the Internet. In the near-term, protocols for supporting transparent host ``roaming" among different subnetworks and different media (e.g., LANs, dial-up links, and wireless communication channels) are to be developed and entered into the Internet standards track. The work is expected to consist mainly of new and/or revised protocols at the (inter)network layer, but may also include proposed modifications to higher-layer protocols (e.g., transport or directory). However, it is a requirement that the proposed solutions allow mobile hosts to interoperate with existing Internet systems. In the longer term, the group may address, to the extent not covered by the mobile host solutions, other types of internet mobility, such as mobile subnets (e.g., a local network within a vehicle), or mobile clusters of subnets (e.g., a collection of hosts, routers, and subnets within a large vehicle, like a ship or spacecraft, or a collection of wireless, mobile routers that provide a dynamically changing internet topology).
In this section we provide an overview of the current base specification for Mobile IP. The terminology is similar to CDPD and is summarized in Table 1. The Mobile IP approach is analogous to postal service delivery: whenever you move to a new location, you ask your home post office to forward your mail to your new address via the local post office there. Thus, a mobile node first leaves its home network and connects to a foreign network. An agent on the home network then intercepts packets sent to the mobile node and forwards them to an agent on the foreign agent. This agent then delivers packets locally to the mobile node visiting that network.
Mobile nodes are supported by two service entities.
These entities interact in the following ways: 1. Agent Discovery Home agents and foreign agents may advertise their availability via broadcast on each link for which they provide service. A newly arrived mobile node can likewise broadcast a solicitation on the link to learn if any prospective agents are present. The advertisement is an extension of router advertisement (RFC 1256). It allows a mobile node to determine its point of attachment (moved to a new foreign network, or returned to its home network). Advertisements contain:
2. Registration When the mobile node is away from home, it registers its care-of address with its home agent. Depending on its method of attachment, the mobile node will register either directly with its home agent, or through a foreign agent which forwards the registration to the home agent. Home and foreign agents may reject registration requests; this option is necessary to combat registration attacks by the ``bad guys." Registration attacks can be of at least three types: Forgery, whereby bogus mobile node location is sent to the home agent; Modification, whereby a valid registration request is altered to send the mobile node's traffic elsewhere; and Replay which involves storing a valid registration request for later malicious diversion of mobile node traffic. To prevent an attacker from changing a mobility binding the following precautions are taken:
3. Tunneling/Encapsulation Tunneling is used for transportation of mobile node packets from the home network to the foreign network. There are two endpoints: The home agent which encapsulates and transmits; and the care-of address entity which receives and decapsulates. The original packet becomes a payload in the new packet sent to the care-of address.There are various options for implementing this: IP-in-IP (draft), GRE, and Minimal Encapsulation are among the encapsulation options.
The following steps outline the operation of the Mobile IP protocol:
When away from home, Mobile IP uses protocol tunneling to hide a mobile node's home address from intervening routers between its home network and its current location. The tunnel terminates at the mobile node's care-of address. The care-of address must be an address to which datagrams can be delivered via conventional IP routing. At the care-of address, the original datagram is removed from the tunnel and delivered to the mobile node. Mobile IP provides two alternative modes for the acquisition of a care-of address:
The mode of using a co-located care-of address has the advantage that it allows a mobile node to function without a foreign agent, for example, in networks that have not yet deployed a foreign agent. It does, however, place additional burden on the IPv4 address space because it requires a pool of addresses within the foreign network to be made available to visiting mobile nodes. It is difficult to efficiently maintain pools of addresses for each subnet that may permit mobile nodes to visit. It is important to understand the distinction between the care-of address and the foreign agent functions. The care-of address is simply the endpoint of the tunnel. It might indeed be an address of a foreign agent (a foreign agent care-of address), but it might instead be an address temporarily acquired by the mobile node (a co-located care-of address). A foreign agent, on the other hand, is a mobility agent that provides services to mobile nodes.
[c]../gif/fig1.gif In Figure 2, the visiting mobile node transmits a packet to the corre- spondent node. Routing of this packet is done in the conventional way, with no need to involve either the home or foreign agent.
[c]../gif/fig2.gif
The Mobile IP protocol is outlined in steps below, under four basic procedural categories. In our discussions MN denotes ``Mobile Node," HA denotes ``Home Agent," and FA denotes ``Foreign Agent". 1. Network Attachment During this phase, foreign and home agents advertise their presence via agent advertisement messages. The mobile node may also optionally solicit an agent advertisement message from them. 1. MN - attaches to a new foreign network. 2. MN - solicits an agent advertisement (if necessary). 3. FA - sends advertisement. 2. Registration Now that the mobile node is on a foreign network, it obtains a care-of address on this network, and registers its new care-of address with its home agent, possibly via the foreign agent. 4. MN - requests registration from FA. 5. FA - forwards registration request to HA. 6. HA - sends registration reply to FA. 7. FA - forwards registration reply to MN. 8. HA - proxy ARPs for MN. 3. Data Transfer to the Mobile Node Data sent to the mobile node's home address are now intercepted and tunneled by the home agent to the mobile node's care-of address. These are then received at the tunnel endpoint (foreign agent for example) and delivered to the mobile node. 9. HA - intercepts, encapsulates, and forwards packets to FA (arrow 2 in Figure 1). 10. FA - decapsulates and forwards to MN (arrow 3 in Figure 2). 4. Data Transfer From the Mobile Node Data from the mobile node are delivered to their destination using stan- dard IP routing mechanisms, not necessarily passing through the home agent. 11. MN - Encapsulates and forwards packets to Destination (Figure 2).