In many situations, mobile computers use wireless links to connect to the network. Wireless links are particularly vulnerable to passive eavesdropping, active replay attacks, and other active attacks.
Because Mobile IP recognizes its inability to reduce or eliminate this vulnerability, Mobile IP uses a form of authentication to protect Mobile IP registration messages from these types of attack. The default algorithm that is used is MD5, with a key size of 128 bits. The default operational mode requires that this 128-bit key precede and succeed the data to be hashed. The foreign agent uses MD5 to support authentication. The foreign agent also uses key sizes of 128 bits or greater, with manual key distribution. Mobile IP can support more authentication algorithms, algorithm modes, key distribution methods, and key sizes.
These methods do prevent Mobile IP registration messages from being altered. However, Mobile IP also uses a form of replay protection to alert Mobile IP entities when they receive duplicates of previous Mobile IP registration messages. If this protection method were not used, the mobile node and its home agent might become unsynchronized when either of them receives a registration message. Hence, Mobile IP updates its state. For example, a home agent receives a duplicate deregistration message while the mobile node is registered through a foreign agent.
Replay protection is ensured either by a method known as nonces, or timestamps. Nonces and timestamps are exchanged by home agents and mobile nodes within the Mobile IP registration messages. Nonces and timestamps are protected from change by an authentication mechanism. Consequently, if a home agent or mobile node receives a duplicate message, the duplicate message can be thrown away.
The use of tunnels can be a significant vulnerability, especially if registration is not authenticated. Also, the Address Resolution Protocol (ARP) is not authenticated, and can potentially be used to steal another host's traffic.