Also discuss how tunnelling works for mobile Ip using IP in IP encapsulation.

Subject: Mobile Communication and Computing

Topic: Mobile Network

Difficulty: High

Agent Discovery

A mobile node has to find a foreign agent when it moves away from its home network. To solve this problem, mobile IP describes two methods: agent advertisement and agent solicitation.

Agent advertisement For this method, foreign agents and home agents advertise their presence periodically using special agent advertisement messages, which are broadcast into the subnet. Mobile IP does not use a new packet type for agent advertisement; it uses the router advertisement packet of ICMP, and appends an agent advertisement message. The agent advertisement packet according to RFC 1256 with the extension for mobility is shown below.

The TTL field of the IP packet is set to 1 for all advertisements to avoid forwarding them. The Type is set to 9, the code can be 0, if the agent also routes traffic from non-mobile nodes, or 16, if it does not route anything other than mobile traffic. The number of addresses advertised with this packet is in #addresses while the addresses themselves follow as shown.

Lifetime denotes the length of time this advertisement is valid. Preference levels for each address help a node to choose the router that is the most eager one to get a new node. The extension for mobility has the following fields defined: type is set to 16, length depends on the number of COAs provided with the message and equals 6 + 4*(number of addresses).
The sequence number shows the total number of advertisements sent since initialization by the agent.

By the registration lifetime the agent can specify the maximum lifetime in seconds a node can request during registration.

The following bits specify the characteristics of an agent in detail.

The R bit (registration) shows, if a registration with this agent is required even when using a co-located COA at the MN.

If the agent is currently too busy to accept new registrations it can set the B bit.

The following two bits denote if the agent offers services as a home agent ( H ) or foreign agent ( F ) on the link where the advertisement has been sent.

Bits M and G specify the method of encapsulation used for the tunnel. While IP-in-IP encapsulation is the mandatory standard,

M can specify minimal encapsulation and

G generic routing encapsulation.

In the first version of mobile IP (RFC 2002) the V bit specified the use of header compression according to RFC 1144.

Now the field r at the same bit position is set to zero and must be ignored.

The new field T indicates that reverse tunneling is supported by the FA.

The following fields contain the COAs advertised. A foreign agent setting the F bit must advertise at least one COA. A mobile node in a sub-net can now receive agent advertisements from either its home agent or a foreign agent. This is one way for the MN to discover its location.