Mobile IPv4 lets a device keep using its home IP address while it is away from its home network. It does this by registering a care-of address with its home agent: the agent stores that relationship as a mobility binding, then tunnels packets addressed to the home address to the device’s current location.
The key distinction is that a tunnel changes how a datagram is delivered, not the address the correspondent uses for the mobile node. The original datagram remains inside an outer IP packet until the tunnel endpoint removes that outer header.
What is a Mobile IPv4 binding?
A mobility binding is the home agent’s time-limited record linking a mobile node’s home address to a care-of address. It tells the agent where to send traffic for the node while it is registered away from home. Depending on the registration, the agent can retain more than one simultaneous care-of address.
The mobile node tells its home agent its current location through Mobile IPv4’s registration procedure. The home agent must be able to intercept datagrams addressed to the node on its home network while the node is registered elsewhere. These are protocol requirements in RFC 3344, Section 4 and Section 4.2.3 (August 2002); RFC 5944 is the revised Mobile IPv4 specification.
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How does Mobile IPv4 register a care-of address?
The mobile node sends a Registration Request that identifies the home address, home-agent address, care-of address, requested lifetime, and a 64-bit identification value. The request’s flags indicate which optional behaviors the node wants. The home agent records the binding for the granted registration lifetime.
Care-of address choices
| Mode | Where the tunnel ends | Who decapsulates |
|---|---|---|
| Foreign-agent care-of address | At the foreign agent on the visited network | The foreign agent removes the outer header and forwards the inner datagram to the mobile node. |
| Co-located care-of address | At an address assigned to the mobile node itself | The mobile node removes the outer header directly. |
A care-of address identifies the tunnel’s delivery endpoint; it does not replace the mobile node’s home address as the address used by correspondents.
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Registration flags
- S: Retain simultaneous bindings.
- B: Request forwarding of broadcast datagrams.
- D: Indicate that the mobile node performs decapsulation.
- M: Request minimal encapsulation.
- G: Request GRE encapsulation.
- T: Request reverse tunneling.
A requested lifetime of zero asks to deregister the binding. A lifetime of 0xffff represents infinity in RFC 3344. These values describe Mobile IPv4 registration semantics, not a guarantee that every agent will grant the request.
How does a Mobile IPv4 tunnel carry a datagram?
Encapsulation places the original IP datagram inside a new delivery packet. The original packet is the inner datagram: its source and destination remain those of the original communication. The added outer IP header directs the encapsulated packet to the care-of address so it can cross the network to the tunnel endpoint.
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RFC 3344 requires home agents and foreign agents to support IP-in-IP encapsulation. It also defines minimal encapsulation and GRE as optional alternatives. The registration flags M and G request those optional methods; they do not change the purpose of the tunnel, which is to deliver the original datagram to the mobile node’s current location.
What happens on the forward path?
- A correspondent sends a datagram to the mobile node’s home address, just as it would when the node is at home.
- The home agent intercepts that datagram while the node is registered away, consults its binding, and tunnels the datagram to the registered care-of address.
- At a foreign-agent care-of address, the foreign agent checks that the inner destination matches a current entry in its visitor list. If it matches, the agent decapsulates the packet and forwards the original datagram to the mobile node.
- With a co-located care-of address, the mobile node receives the tunnel packet and decapsulates it itself.
The visitor-list check matters: a foreign agent must not forward a tunneled datagram whose inner destination does not match a current visitor-list entry. RFC 3344 specifies this rule to prevent a possible routing loop.
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What is reverse tunneling, and when is it used?
Reverse tunneling carries traffic sent by the mobile node back through the home agent instead of allowing that traffic to follow an ordinary direct route. Direct routing may work, but a network’s ingress filtering, topology, or policy can make a path through the home agent necessary or preferable.
When reverse tunneling is used, the mobile node’s outbound datagram is carried in a tunnel to the home agent, which then forwards it onward. RFC 2344 requires the home agent to validate a reverse-tunneled packet: the outer source must match a registered care-of address, and the inner source must match the mobile node. The T registration flag requests reverse tunneling.
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How do the main Mobile IPv4 options differ?
| Choice | What changes | What stays the same |
|---|---|---|
| Foreign-agent versus co-located care-of address | The tunnel endpoint and decapsulator: the foreign agent handles decapsulation in the first mode; the mobile node handles it in the second. | The binding still maps the home address to a care-of address. |
| IP-in-IP, minimal, or GRE encapsulation | The method used to carry the original datagram through the tunnel. RFC 3344 requires IP-in-IP support; minimal and GRE are optional. | The tunnel delivers the original datagram to the registered endpoint. |
| Forward-only versus reverse tunneling | Whether the mobile node’s outbound traffic is also sent through the home agent. | Forward tunneling delivers traffic addressed to the mobile node’s home address. |
| Single versus simultaneous bindings | Whether the home agent retains one care-of address or keeps simultaneous bindings as requested with the S flag. | The binding associates the home address with registered delivery location information. |
| Broadcast forwarding requested or not | Whether the home agent forwards broadcast datagrams for that registration; the B flag requests this behavior. | Ordinary unicast forwarding still follows the registered binding. |
What happens when a binding expires or broadcast traffic is involved?
The home agent forwards broadcast datagrams only for registrations that request them with the B flag. If a binding expires, the home agent deletes it without sending a new Registration Reply. A node therefore cannot rely on an expired binding to keep receiving tunneled traffic; it must have an active registration for the agent to use that location record.
Mobile IPv4 describes protocol behavior, not product support. The RFCs do not establish that a particular consumer router, VPN product, operating system, or training service implements Mobile IPv4; support must be checked for the specific product and version.
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