IPv6 to the rescue?
In a high-tech world of 18-month product lifecycles, IP is an anomaly. The Internet Protocol predates networked personal computers, Apple Macintosh, and Microsoft Windows. But the current version of IP (version 4), published in 1981 as a Department of Defense standard in RFC 791, needs a major facelift. Even so, IPv4 managed to scale up with the Internet as it grew from a small network linking dozens of systems across the U.S. research community to a global network now approaching 100 million nodes.
If you've heard about IPv6, it may be in the context of the IPv4 address space squeeze. IPv4 uses 32-bit addresses, putting an upper limit of about 4 billion ([2.sup.32]) on the number of nodes that can be addressed. Network addresses are scarce and getting scarcer as the Internet continues to grow, but the more pressing problem for IPv4 is how to cope with the explosive growth of Internet routing tables. As more and more networks are linked to the Internet, the number of routes a backbone router must track is in some cases exceeding 140,000 entries, and further growth will hamper the ability of the backbones to carry Internet traffic.
IPv6 solves the address space problem with 128-bit addresses, and the routing table problem with address aggregation. And, with a streamlined header and some refined design ideas, IPv6 fixes network autoconfiguration, mobile IP, IP security, and other issues that have dogged IP networking for years, such as fragmentation, source routing, and sending very large packets (jumbograms).
IPv4 allocates network addresses from three categories: Class A addresses use only the first eight bits of the address to identify the network, leaving the other 24 bits for addressing nodes; Class B addresses use the first 16 bits to identify the network and 16 bits to address nodes; Class C networks are identified by the first 24 bits, with only eight bits for nodes. This class structure severely limits IPv4's ability to handle growth. A temporary remedy, Classless InterDomain Routing (CIDR) breaks the class system by binding groups of Class C networks together and making them behave as if they are a single network entity for the purposes of routing tables.
IPv6's 128-bit addresses handle network complexity flexibly, by aggregating it. RFC 2374 describes a global aggregation architecture for IPv6 network addresses, in which all node addresses consist of two parts: the high-order 64 bits identify the network, the low-order 64 bits identify the node. IPv6 network addresses comprise a format prefix (the type of IPv6 address), a top-level aggregation entity (likely to be a country or major connectivity provider), eight bits reserved for future growth, a next-level aggregation entity (likely to be a large corporation or ISP), and a site-level aggregation entity (likely to be assigned internally by the next-level entity). The resulting aggregated addresses are more efficiently routed across backbones. An upper limit of no more than 8,192 top-level aggregators pares backbone routing tables.
Comparing the IPv4 and IPv6 header formats reveals more change. The figure shows how the IPv6 header eliminates the Header Length (IHL), Identification, Flags, Fragment Offset, Header Checksum, as well as the Options and Padding fields. Because IPv6 headers are all the same length, the header length field is unnecessary. IPv6 prohibits fragmentation except between end nodes, so the related Identification, Flags, and Fragment Offset fields can go. IPv6 options are handled in separate extension headers that follow the IPv6 header, so they no longer clutter the main header. The IPv4 Type of Service field evolves into the Traffic Class field, and the Time to Live field becomes the Hop Limit field. The Flow Label field helps IPv6 support the concept of flows: sequences of packets that require similar routing treatment--for example, audio or video streams.
The simplified, standard-sized IPv6 header makes routing simpler, especially for packets with special options. IPv4 forces routers to sense and handle special packets, like those using IPsec encryption and authentication options, as part of the standard routing process. IPv6 routers can ignore end-to-end options and need process only those relevant to the routing process.
IS IPV6 A SURE THING?
IPv6 address and header changes help poise IP for continued growth into the next century, but other modifications help make IPv6 easier to use and more efficient. Although the Dynamic Host Configuration Protocol (DHCP) automatically configures approved nodes, IPv6's stateless autoconfiguration lets unknown nodes query the nearest autoconfiguration server for basic network configuration.
IPv6 competes with Y2K and the Euro conversion for resources and also faces reluctance on the part of a significant portion of the North American ISP community. Unless something better comes along, IPv6 is the protocol most likely to be in the Internet's future--but compelling applications, like plug-and-play networking and streaming audio and video, are needed to speed IPv6 onto a desktop near you.
Loshin is author of IPv6 Clearly Explained, AP Professional 1999.
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|Title Annotation:||Internet/Web/Online Service Information; new Internet protocol|
|Date:||Dec 1, 1998|
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