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3. Registration

Mobile IP registration provides a flexible mechanism for mobile nodes to:

  • Request and receive mobility services
  • Inform their home agent of their current care-of address
  • Renew registrations before they expire
  • Deregister when returning home

3.1. Registration Overview​

The registration process involves three parties:

  1. Mobile Node: Initiates registration requests
  2. Foreign Agent (optional): Relays registration messages
  3. Home Agent: Accepts or denies registration requests

Registration can be performed via a foreign agent or directly when using co-located care-of addresses.

3.2. Authentication​

All registration messages MUST be authenticated. Mobile IP uses the Mobile-Home Authentication Extension for this purpose. Authentication prevents malicious nodes from hijacking mobile node sessions.

3.3. Registration Request​

The mobile node sends a Registration Request message to its home agent via the foreign agent (or directly if using a co-located care-of address).

Key fields in Registration Request:

  • Type: 1 (Registration Request)
  • Lifetime: Requested registration duration in seconds
  • Care-of Address: The termination point of the tunnel
  • Home Address: The mobile node's permanent IP address
  • Home Agent: The IP address of the mobile node's home agent
  • Identification: A 64-bit number used for replay protection

3.3.1. Mobile Node Considerations​

Mobile nodes MUST:

  • Set the Lifetime field to the desired registration period
  • Include a valid Mobile-Home Authentication Extension
  • Use proper identification values to prevent replay attacks

3.3.2. Foreign Agent Considerations​

When receiving a Registration Request, the foreign agent:

  • Checks for validity
  • May deny the request if resources are unavailable
  • Relays valid requests to the home agent
  • Adds a Foreign-Home Authentication Extension if shared security exists

3.3.3. Home Agent Considerations​

The home agent:

  • Authenticates the Registration Request
  • Checks if the mobile node is authorized
  • Creates or updates a mobility binding
  • Sends a Registration Reply

3.4. Registration Reply​

The home agent sends a Registration Reply to grant or deny the request.

Key fields in Registration Reply:

  • Type: 3 (Registration Reply)
  • Code: Indicates acceptance or reason for denial
  • Lifetime: Granted registration lifetime (may be less than requested)
  • Home Address: The mobile node's permanent IP address
  • Home Agent: The IP address of the home agent

3.4.1. Mobile Node Considerations​

Upon receiving a Registration Reply, the mobile node:

  • Verifies the authentication
  • Checks the Code field for acceptance or rejection
  • Updates its registration state
  • Reschedules re-registration based on the granted Lifetime

3.4.2. Foreign Agent Considerations​

The foreign agent:

  • Authenticates the Reply if foreign-home authentication exists
  • Updates its visitor list if the registration was accepted
  • Relays the Reply to the mobile node

3.4.3. Home Agent Considerations​

The home agent:

  • Sets the Code field appropriately
  • MAY reduce the requested Lifetime
  • MUST include the Mobile-Home Authentication Extension

3.5. Registration Extensions​

Extensions provide additional functionality and security to registration messages.

3.5.1. Computing Authentication Extension Values​

Authentication values are computed using keyed MD5 [23] over specified portions of the registration message.

3.5.2. Mobile-Home Authentication Extension​

This extension is REQUIRED in all Registration Requests and Replies between the mobile node and home agent.

Format:

  • Type: 32
  • Length: 4 plus the number of bytes in the Authenticator
  • SPI: Security Parameters Index (4 bytes)
  • Authenticator: Variable length (typically 128 bits for MD5)

3.5.3. Mobile-Foreign Authentication Extension​

This extension MAY be included when the mobile node and foreign agent share a security association.

3.5.4. Foreign-Home Authentication Extension​

This extension MAY be included when the foreign agent and home agent share a security association.

3.6. Denials of Registration Requests​

Registration may be denied for various reasons, indicated by the Code field in the Registration Reply:

  • Code 65: Registration accepted
  • Code 128: Reason unspecified
  • Code 129: Administratively prohibited
  • Code 130: Insufficient resources
  • Code 131: Mobile node failed authentication
  • Code 133: Requested Lifetime too long
  • Code 134: Poorly formed Request
  • Code 135: Poorly formed Reply
  • Code 136: Requested encapsulation unavailable

3.7. Registration Message Options​

Mobile IP supports various optional features through extension mechanisms, allowing for protocol evolution while maintaining backward compatibility.

3.8. State Management​

3.8.1. Mobility Bindings​

The home agent maintains mobility bindings that associate the mobile node's home address with its current care-of address and remaining lifetime.

3.8.2. Visitor List​

Foreign agents maintain visitor lists tracking mobile nodes currently registered through them, including their home addresses, home agents, and registration lifetimes.

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