Packet-switched networks use the Internet Protocol (IP) to deliver packets from a source host to a destination host using addressing structures that are based solely on the respective host addresses. The most widespread addressing structures in current use are IP Version 4 (IPv4), and its successor, IP Version 6 (IPv6). Whereas IPv4 uses 32-bit addresses, IPv6 uses 128-bit addresses, divided into a prefix of 64 bits and a suffix of 64 bits. Because IP is designed to be flexible enough for use across networks having different technologies for link-layer implementation, a protocol is provided for resolving IP addresses to data link addresses. In IPv4 networks, this protocol is Address Resolution Protocol (ARP), and in IPv6 networks, it is Neighbor Discovery Protocol (NDP).
Although the earliest IP networks involved fixed nodes, there has been an increasing need to support users who wish to maintain their internet connections while moving, and, indeed, while roaming between different subnets or different wireless systems. Mobile IP is a protocol that permits a Mobile Node (MN) to maintain an unchanging home IP address (HoA) despite changing its point of attachment to the internet. In Mobile IP, a router on the MN's home network, assumes the representation of the MN while the MN is attached to a foreign network.
The MN, when it connects to the foreign network, is assigned a Care-of Address (CoA). The MN informs its Home Agent of the CoA in a binding update message. This causes a bidirectional tunnel to be set up between the Home Agent and the MN (at its CoA). Now, if a correspondent node (i.e., any node other than the MN under discussion) wishes to send packets to the MN, it will address the packets to the HoA. These packets will be delivered to the Home Agent, which will encapsulate them and forward them via the bidirectional tunnel to the MN at its CoA. Conversely, the MN may use the bidirectional tunnel for sending packets to the correspondent node. Alternatively, the MN may send packets directly to the correspondent node, although the precise procedures for doing so differ between Mobile IPv4 and Mobile IPv6.
Proxy Mobile IP (PMIPv6) is a more recent standard that provides functionality similar to Mobile IP, but that obviates any need for the host implemented in the MN to include any protocol modifications adapted to support mobility. Instead, all mobility support is implemented on the fixed side of the network, including keeping track of the location of the MN. In particular, the MN is able to keep its HoA, and does not need to be configured with an additional CoA. (However, the CoA is used elsewhere in the network, as explained below.)
PMIPv6 introduces two special functionalities for mobility support: These are the Mobility Access Gateway (MAG) and the Local Mobility Anchor (LMA). The Mobility Access Gateway (MAG), which is supported on an access router, manages the mobility-related signaling for an MN that is attached to its access link. The MAG tracks the MN's coming to and going from the access link, and supports the MN's mobility by exchanging signaling messages with the MN's Local Mobility Anchor (LMA). The LMA is the Home Agent for the MN in a Proxy Mobile IPv6 domain, i.e., in a network that implements the PMIPv6 protocol.
The PMIPv6 protocol is initiated when a mobile host, executing in an MN, enters a PMIP domain and attaches to an access link. A brief summary of the ensuing transactions is presented below. More details can be found in S. Gundavalli, Ed., RFC 5213, “Proxy Mobile IPv6,” IETF Network Working Group (August 2008), http://tools.ietf.org/html/rfc5213, particularly at Section 3, “Proxy Mobile IPv6 Protocol Overview”, pp. 9-15.
After the MN attaches to an access link, the MAG on that access link acquires the identity of the MN and determines whether the MN is authorized for entry, i.e., whether it is authorized for the pertinent network-based mobility-management service. If the authorization is confirmed, the MN will be able to obtain an address configuration on the connected interface. Subsequently, support by the network will assure that the entire PMIP domain appears to the MN as a single link.
The address configuration obtained by the MN includes at least one home address (HoA) and the default-router address on the connected link. The home network prefix is a prefix assigned to the link between the MN and the MAG. The HoA is an address from the MN's home network prefix. The MN will be able to use this address as long as it is attached to the access network that is within the scope of the PMIP domain.
One difference between Mobile IPv6 and PMIPv6 is that in Mobile IPv6, the home agent is apprised of the home address of the MN. In PMIP, by contrast, the mobility entities are required to know the MN's home network prefixes, but do not necessarily know the exact address that the MN configured on its interface from its home network prefix. However, there is a stable identifier of the MN, referred to as the Mobile Node Identifier (MN-Identifier), that the PMIP mobility entities can acquire and can use for identifying the MN. The MN-Identifier is typically the Network Access Identifier (NAI), or some other identifier such as the Media Access Control (MAC) address.
As in ordinary IP and in Mobile IP, the protocol used to obtain address configuration information is Dynamic Host Configuration Protocol (DHCP). Network devices, acting as DHCP clients, obtain network parameters, including addresses, from DHCP servers. Thus, for example, a DHCP client at the MN may obtain address information from a DHCP server implemented at the MAG, or alternatively, it may obtain the address information from a DHCP server implemented at the LMA, which forwards the information through a DHCP relay implemented at the MAG.
The mobile node connecting to the PMIP domain may be an IPv4 node or an IPv6 node, or it may have dual capability. According to which IP version is applicable, the MN will be configured with an IPv4 or IPv6 address (or even dual addresses). In an IPv6 transport network, IPv4 in IPv6 tunneling may be provided to deliver IPv4 traffic. If an IPv4 transport network is used between the MAG and the LMA, IPv6 in IPv4 tunneling is implemented.
As in Mobile IP, the PMIP network also establishes a bidirectional tunnel between the LMA and the MAG. A global address, referred to as the LMA Address (LMAA) is configured on the interface of the LMA as one transport endpoint of the tunnel. This is the address to which the MAG sends Proxy Binding Update (PBU) messages, which are discussed below.
Another global address, referred to as the Proxy Care-of Address (Proxy-CoA), is configured on the egress interface of the MAG. This address is the other transport endpoint of the tunnel between the LMA and the MAG. To the LMA, this is the care-of address of the MN. The LMA registers this address in the Binding Cache entry for the MN.
In this regard, a “binding” associates the MN's home network prefix (as assigned to a given interface of the MN) with the MN's current Proxy-CoA. Such associations typically have limited lifetimes. The remaining lifetime of a given such association may be included in the information referred to as the “binding.” Bindings are established through Proxy Binding messages exchanged between the MAG and the LMA. More specifically, the MAG requests the establishment of a binding by sending a PBU to the LMA. The LMA responds by sending a Proxy Binding Acknowledgement (PBA) to the MAG.
As noted above, IPv6 uses 128-bit addresses, whereas IPv4 uses 32-bit addresses. Because of the high demand for IP addresses, the total space of possible public IPv4 addresses is prone to exhaustion. One way to mitigate that problem is to locally share public IPv4 addresses. That is, a plurality of nodes within, e.g., a private network may share a single public IP address which is visible to the outside world, but within the private network use private IP addresses that are not globally routable on the Internet. With such an arrangement, public IP addresses may be shared without confusion. At the interface between the private and public networks, a network entity such as a Network Address Translation (NAT) box or gateway translates between the private and public address space.
For the above reasons, among others, it will at least sometimes be desirable to allocate one or more private home addresses to an IPv4 mobile node attaching to a Proxy Mobile IPv6 network. The PMIPv6 specification RFC 5213 does not specify a procedure for making such an address allocation. This procedure may, however, be requested by some deployment scenarios.