Heterogeneous Address Space Connectivity Problem

In the foreseeable future, three types of address spaces will coexist in the Internet, they are IP, IPv6, and reusable-IP. Table 1 describes the connectivity between all combinations of the three address space types. In a connection, the initiator is the ``caller'' host that sends the first packet to start the connection; the responder is the ``callee'' host that answers the in-coming connection. For example, to connect a reusable-IP (R-IP) initiator to an IP responder, it is well known that NAT [25] can be used and it works well in practice. Similarly, NAT-PT [27] can be used to connect an IPv6 initiator to an IP responder. On the other hand, to connect an IP initiator to an IPv6 responder (case (a), Table 1), or to connect an IP initiator to a reusable-IP responder (case (b), Table 1) is hard because the responder does not have any IP address and the initiator cannot address the responder directly. Solving these problems is the key challenge in maintaining the bi-directional connectivity abstraction of the Internet.

Table 1: Taxonomy of address space connectivity

		Responder
		IP	IPv6	R-IP
	IP	Trivial	(a) Hard	(b) Hard
Initiator	IPv6	NAT-PT	Trivial	Reduces to (b)
	R-IP	NAT	Reduces to (a)	Reduces to (b)

We emphasize that the problems underlying case (a) and case (b) are essentially identical, except that case (a) requires additional packet header format conversion which has been well documented in [18,27]. Thus for simplicity, for the remainder of this paper, we only consider case (b), where an IP initiator is connecting to a reusable-IP responder. The results can be mapped to case (a).

Note that because there are multiple coexisting instances of the reusable-IP address space, connecting a reusable-IP initiator to a reusable-IP responder in a different instance of the address space is non-trivial. However, under NAT, this is equivalent to the initiator's NAT gateway (which is an IP host) connecting to the reusable-IP responder. Therefore, this case can be reduced to case (b) as indicated in Table 1. Similarly, connecting a reusable-IP initiator to an IPv6 responder reduces to case (a), and connecting an IPv6 initiator to a reusable-IP responder reduces to case (b).

In summary, the key difficulty in achieving bi-directional connectivity across heterogeneous address spaces is to provide connectivity from IP hosts to non-IP hosts.