INTRODUCTION
Mobile IP was developed to enable computers to maintain Internet connectivity while moving from one Internet attachment point to another. Mobile IP is particularly suited to wireless connection, although it can work with wired connection as well.
The term mobile suggests that the user's point of attachment changes dynamically and all connections are automatically maintained despite the change. This is in contrast with the term nomadic, which indicates that the user's Internet connection is terminated each time the user moves and a new connection is initiated when the user dials back in, having assigned a new, temporary, IP address.
OPERATION OF MOBILE IP
The general terms concerning the way in which the mobile IP deals with the problem of dynamic IP are shown in the following figure:
Mobile IP Scenario
A mobile node is assigned to a particular network, which is its home network.
The IP address on home network is static and is known as its home address. The mobile node can move to another network, which is considered a foreign network. With the new connection, the mobile node registers with a network node, typically a router, on the foreign network known as a foreign agent. Then it gives care-of address of the mobile node to agent on home network known as home agent. The care- of address identifies the foreign agent’s location. Typically, one or more routers on a network will implement the roles of both home and foreign agents.
When IP datagrams are exchanged over connection between the mobile node and another host (see the above figure), the following operations occurs:
1. Server X transmit an IP datagram, for mobile node A, with A’s home address in the IP
header. The IP datagram is routed to A’s home network.
2. The home agent intercepts the IP datagram, encapsulates the entire datagram inside a
new IP datagram, which has the A’s care-of address in the header, and retransmits it to the foreign agent. The use of an outer IP datagram with a different destination IP address is called tunneling.
3. The foreign agent strips off the outer IP header, encapsulates the original IP datagram in
a network-level PDU, and delivers the original datagram to A across the foreign network.
4. When A sends IP traffic to X, it uses X’s IP address, which in this case is fixed, because
X is not a mobile node.
5. The IP datagram from A to X travels directly across the Internet to X, using X’s IP
address.
The above mentioned operations are supported by the following capabilities of mobile IP:
- Discovery: a mobile node uses a discovery procedure to identify prospective home and
foreign agents;
- Registration: a mobile node uses an authenticated registration procedure to inform home
agent of its care-of address;
- Tunneling: is used to forward IP datagrams from a home address to a care-of address
The following figure illustrates the underlying protocol support for mobile IP capability.
Protocol Support for Mobile IP
The registration protocol communicates between an application on the mobile node and an application in the home agent, and uses a transport-level protocol.
Discovery makes use of the existing ICMP (Internet Control Message Protocol) by adding the appropriate extensions to the ICMP header.
Tunneling is performed at the IP level.
Mobile IP is specified in a number of RFC’s. The following table lists some useful terminology from RFC 2002:
Mobile IP Terminology (RFC 2002)
DISCOVERY
The discovery process in Mobile IP is similar to the router process defined in the Internet Control Message Protocol (ICMP). The mobile node is responsible for ongoing discovery process. It must determine if it is attached to its home network or to a foreign network. Transition from home network to foreign network can occur at any time without notification to the network layer. A mobile node listens for the agent advertisement messages and compares network portion of the router's IP address with the network portion of home address. If these network portions do not match, then the mobile node is on a foreign network.
The agent advertisement extension follows the ICMP router advertisement fields and has the following structure:
Mobile IP Agent Advertisement Message
- Type: 16 indicates that this is an agent advertisement;
- Length: (6 + 4N), where N is the number of care-of addresses advertised;
- Sequence number: the count of agent advertisement messages sent since the agent was
initialized;
- Lifetime: the longest lifetime, in seconds, that this agent is willing to accept a
registration request from a mobile node;
- R: registration with this foreign agent is required;
- B: busy;
- H: this agent acts as a home agent on this network;
- F: this agent acts as a foreign agent on this network;
- M: this agent can receive minimal encapsulated (explained later) tunneled IP datagrams;
- G: this agent can receive GRE encapsulated (explained later) tunneled IP datagrams;
- V: this agent supports Van Jacobson header compression (RFC 1144);
- Care-of address: the care-of address supported by this agent in this network.
The optional prefix-length extension is used by the mobile node to compare the network portion of its
own IP address with the network portion of the router.
Agent Solicitation
Foreign agents are expected to issue agent advertisement messages periodically. If a mobile node needs agent information immediately, it can issue an ICMP router solicitation message. Any agent receiving this message will then issue an agent advertisement.
Move Detection
A mobile node may move from one network to another due to some handoff mechanism without IP level being aware. The agent discovery process is intended to enable the agent to detect such a move The agent may use one of two algorithms to detect the move:
- use of lifetime field: the mobile node uses lifetime field as a timer for agent
advertisements;
- use of network prefix: the mobile node checks if any newly received agent
advertisement messages are on the same network as the node's current care-of address
Co-Located Addresses
If mobile node moves to a network that has no foreign agents, or all foreign agents are busy, it can act as its own foreign agent by using co-located care-of address. This is an IP address obtained by the mobile node associated with mobile node's current network interface.
The means to acquire co-located address:
- temporary IP address through an Internet service, such as DHCP (Dynamic Host
Configuration Protocol);
- may be owned by the mobile node as a long-term address for use while visiting a given
foreign network.
REGISTRATION PROCESS
The registration process has four steps:
1. The mobile node sends registration request to the foreign agent requesting forwarding
service;
2. The foreign agent relays request to the home agent;
3. The home agent accepts or denies the request and sends registration reply to the foreign
agent;
4. The foreign agent relays reply to the mobile node.
Registration Operation Messages
The registrations operation uses two types of messages, carried in UDP segments (see the next figure):
the registration request message and the registration reply message.
The registration request message has the following fields:
- Type: 1 indicates a registration request;
- S: simultaneous binding;
- B: broadcast datagrams;
- D: decapsulation by mobile node;
Mobile IP Registration Messages
- M: the home agent should use minimal encapsulation;
- V: Van Jacobson header compression;
- G: GRE encapsulation;
- Lifetime: the number of seconds before the registration is considered expired;
- Home address: the home IP address of the mobile node;
- Home agent: the IP address of the mobile node’s home agent;
- Care-of-address: the IP address at this end of the tunnel;
- Identification: a 64 number generated by the mobile node;
- Extensions: authentication extension (so far defined).
The registration reply message includes the following fields:
- Type: 3 a registration replay;
- Code: indicates the result of the registration request (see the next table);
- Lifetime: the number of seconds before the registration is considered expired;
- Home address: the home IP address of the mobile node;
- Home agent: the IP address of the mobile node’s home agent;
- Identification: a 64 number used for matching registrations requests;
- Extensions : authentication extension (so far defined).
Code Values for a Mobile IP Registration Reply
Registration Procedure Security
A key concern with the registration procedure is security. Mobile IP is designed to resist to two types of attacks:
1. A node pretending to be a foreign agent sends a registration request to a home agent
to divert mobile node traffic to itself;
2. A malicious agent replays old registration messages to cut mobile node from
network.
For message authentication, each registration request and reply contain an authentication extension (see the next figure) with the following fields:
Mobile IP Authentication Extension
- Type: used to designate the type of this authentication extension;
- Length: 4 plus the number of bytes in the authenticator;
- Security parameter index (SPI): an index that identifies a security context between a
pair of nodes;
- Authenticator: a code used to authenticate the message.
The default authentication algorithm uses keyed-MD5 to produce a 128-bit message digest.
Types of Authentication Extensions
Three types of authentication extension have been defined:
- Mobile-home: provides for authentication of registration messages between the
mobile node and the home agent; must be present;
- Mobile-foreign : may be present when a security association exists between the
mobile node and the foreign agent;
- Foreign-home: may be present when a security association exists between foreign a
agent and a home agent.
The authenticator protects the identification field in the request and reply messages. If the mobile node and the home agent maintain synchronization, also, for authentication purpose, time stamps can be used. Alternatively, the mobile node could generate values using a pseudorandom number generator. If the home agent knows the algorithm, then it knows what identification value to expect next.
TUNNELING
Once a mobile agent is registered with a home agent, the home agent intercepts IP datagrams sent to mobile node's home address, in order to forward these datagram via tunneling. The home agent informs other nodes on the home network that IP datagrams to mobile node should be delivered to this agent.
The home agent steals the identity of the mobile node in order to capture packets destined for that node, that are transmitted across the home network
To forward an IP datagrams to a care-of address via tunneling, the home agent puts the entire IP
datagram into an outer IP datagram. This is a form of encapsulation, just as placing an IP header in front of a TCP segment for encapsulating the TCP segment in an IP datagram
Mobile IP Encapsulation Options
There are three encapsulation options for mobile IP:
- IP-within-IP encapsulation: the simplest approach defined in RFC2003;
- Minimal encapsulation: defined in RFC 2004
- Generic routing encapsulation (GRE): defined in RFC 1701
IP-within-IP encapsulation: The entire IP datagram becomes payload in new IP datagram (see the next figure (a)). The original, inner IP header is unchanged except TTL, decremented by 1.
The outer header is a full IP header. Two fields are copied from the inner header: the version number is 4 (IPv4), and the type of the service requested for the outer IP datagram that is the same as that requested for the inner IP datagram.
In the inner IP header, the source and destination address refers to the host that is sending the original datagram, and the destination address is the home address of the intended recipient. In the outer IP header, the source and destination
Mobile IP Encapsulation
addresses refer to the entry and exit points of the tunnel. Thus, the source address typically is the IP address of the home agent, and the destination address is the care-of address for the intended destination.
Minimal encapsulation: This encapsulation results in less overhead and can be used if the mobile node, home agent and foreign agent all agree to do so. The new header is inserted between original IP header and original IP payload (the above (b) figure). It includes the following fields:
- Protocol: copied from the destination address field in the original IP header;
- S: if 0/1, the original source address is absent/present and the length of this header is
8/12 bytes;
- Header checksum: computed over all fields of this header;
- Original destination address: copied from the source address field in the original IP
header;
- Original source address: copied from the destination address field in the original IP
header.
The following fields are modified in the original IP header to form new outer IP header:
- Total length: incremented by the size of the minimal forwarding header (8 or 12);
- Protocol: 55 – the protocol number assigned to minimal IP encapsulation;
- Header check sum: computed over all fields of this header;
- Source address: the IP address of the encapsulator;
- Destination address: the IP address of the exit point of the tunnel.
For a minimal encapsulation, the home agent (encapsulator) prepares the encapsulated data- gram with the format given in the figure from above (b). This datagram is the tunneled across
the Internet to the care-of address. Here the fields in the minimal forwarding headers are restored to the original IP header and the forwarding header is removed from the datagram. The total length field in the IP header is decremented by the size of the minimal forwarding header (8 or 12) and the header checksum field is recomputed.
LITERATURE
PERK97 Perkins, C. Mobile IP
IEEE Communications Magazine, May 1997
PERK98 Perkins, C. Mobile Networking Through Mobile IP IEEE Internet Computing, January-February 1998
STAL02 Stallings, W. Wireless Communications and Networks Upper Saddle River, NJ: Prentice Hall 2002
SOLO98 Solomon, J. Mobile IP: The Internet Unplugged Upper Saddle River, NJ
IP Routing for Wireless/Mobile Hosts: IEFT Working Group on Mobile IP (Current RFCs and Internet drafts).
REVIEW QUESTIONS
1. Explain the distinction between a mobile user and a nomadic user.
2. What is tunneling?
3. List and briefly define the capabilities provided by mobile IP.
4. What is the relationship between mobile IP discovery and ICMP?
5. What are the two different types of destination address that can be assigned to a mobile node while it is attached to a foreign network?
6. Under what circumstances would a mobile node chose to use each of the types of the address referred to Question 5?
