3.10 IP Futures

There are three problems with IP. They are a result of the phenomenal growth of the Internet over the past few years . (See Exercise 1.2 also.)

Over half of all class B addresses have already been allocated. Current estimates predict exhaustion of the class B address space around 1995, if they continue to be allocated as they have been in the past.
32-bit IP addresses in general are inadequate for the predicted long- term growth of the Internet.
The current routing structure is not hierarchical, but flat, requiring one routing table entry per network. As the number of networks grows, amplified by the allocation of multiple class C addresses to a site with multiple networks, instead of a single class B address, the size of the routing tables grows.

CIDR (Classless Interdomain Routing) proposes a fix to the third problem that will extend the usefulness of the current version of IP (IP version 4) into the next century. We discuss it in more detail in Section 10.8.

Four proposals have been made for a new version of IP, often called IPng, for the next generation of IP. The May 1993 issue of IEEE Network (vol. 7, no. 3) contains overviews of the first three proposals, along with an article on CIDR. RFC 1454 [Dixon 1993] also compares the first three proposals.

SIP, the Simple Internet Protocol. It proposes a minimal set of changes to IP that uses 64-bit addresses and a different header format. (The first 4 bits of the header still contain the version number, with a value other than 4.)
PIP. This proposal also uses larger, variable-length, hierarchical addresses with a different header format.
TUBA, which stands for "TCP and UDP with Bigger Addresses," is based on the OSI CLNP (Connectionless Network Protocol), an OSI protocol similar to IP. It provides much larger addresses: variable length, up to 20 bytes. Since CLNP is an existing protocol, whereas SIP and PIP are just proposals, documentation already exists on CLNP. RFC 1347 [Callon 1992] provides details on TUBA. Chapter 7 of [Perlman 1992] contains a comparison of IPv4 and CLNP. Many routers already support CLNP, but few hosts do.
TP/IX, which is described in RFC 1475 [Ullmann 1993]. As with SIP, it uses 64 bits for IP addresses, but it also changes the TCP and UDP headers: 32-bit port number for both protocols, along with 64-bit sequence numbers, 64-bit acknowledgment numbers , and 32-bit windows for TCP.

The first three proposals use basically the same versions of TCP and UDP as the transport layers .

Since only one of these four proposals will be chosen as the successor to IPv4, and since the decision may have been made by the time you read this, we won't say any more about them. With the forthcoming implementation of CIDR to handle the short-term problem, it will take many years to implement the successor to IPv4.