In the original IPv4 model, there was a strong if implicit assumption that each IP host would stay put. One role of an IPv4 address is simply as a unique endpoint identifier, but another role is as a locator: some prefix of the address (eg the network part, in the class-A/B/C strategy, or the provider prefix) represents something about where the host is physically located. Thus, if a host moves far enough, it may need a new address.
When laptops are moved from site to site, it is common for them to receive a new IP address at each location, eg via DHCP as the laptop connects to the local Wi-Fi. But what if we wish to support devices like smartphones that may remain active and communicating while moving for thousands of miles? Changing IP addresses requires changing TCP connections; life (and application development) might be simpler if a device had a single, unchanging IP address.
One option, commonly used with smartphones connected to some so-called “3G” networks, is to treat the phone’s data network as a giant wireless LAN. The phone’s IP address need not change as it moves within this LAN, and it is up to the phone provider to figure out how to manage LAN-level routing, much as is done in 188.8.131.52 Roaming.
But Mobile IP is another option, documented in RFC 5944. In this scheme, a mobile host has a permanent home address and, while roaming about, will also have a temporary care-of address, which changes from place to place. The care-of address might be, for example, an IP address assigned by a local Wi-Fi network, and which in the absence of Mobile IP would be the IP address for the mobile host. (This kind of care-of address is known as “co-located”; the care-of address can also be associated with some other device – known as a foreign agent – in the vicinity of the mobile host.) The goal of Mobile IP is to make sure that the mobile host is always reachable via its home address.
To maintain connectivity to the home address, a Mobile IP host needs to have a home agent back on the home network; the job of the home agent is to maintain an IP tunnel that always connects to the device’s current care-of address. Packets arriving at the home network addressed to the home address will be forwarded to the mobile device over this tunnel by the home agent. Similarly, if the mobile device wishes to send packets from its home address – that is, with the home address as IP source address – it can use the tunnel to forward the packet to the home agent.
The home agent may use proxy ARP (7.9.1 ARP Finer Points) to declare itself to be the appropriate destination on the home LAN for packets addressed to the home (IP) address; it is then straightforward for the home agent to forward the packets.
An agent discovery process is used for the mobile host to decide whether it is mobile or not; if it is, it then needs to notify its home agent of its current care-of address.
7.13.1 IP-in-IP Encapsulation
There are several forms of packet encapsulation that can be used for Mobile IP tunneling, but the default one is IP-in-IP encapsulation, defined in RFC 2003. In this process, the entire original IP packet (with header addressed to the home address) is used as data for a new IP packet, with a new IP header (the “outer” header) addressed to the care-of address.
A value of 4 in the outer-IP-header
Protocol field indicates that IPv4-in-IPv4 tunneling is being used, so the receiver knows to forward the packet on using the information in the inner header. The MTU of the tunnel will be the original MTU of the path to the care-of address, minus the size of the outer header. A very similar mechanism is used for IPv6-in-IPv4 encapsulation (that is, with IPv6 in the inner packet), except that the outer IPv4
Protocol field value is now 41. See 8.13 IPv6 Connectivity via Tunneling.
IP-in-IP encapsulation presents some difficulties for NAT routers. If two hosts A and B behind a NAT router send out encapsulated packets, the packets may differ only in the source IP address. The NAT router, upon receiving responses, doesn’t know whether to forward them to A or to B. One partial solution is for the NAT router to support only one inside host sending encapsulated packets. If the NAT router knew that encapsulation was being used for Mobile IP, it might look at the home address in the inner header to determine the correct home agent to which to deliver the packet, but this is a big assumption. A fuller solution is outlined in RFC 3519.