he primary role of routers is to forward packets toward their final destinations. To this purpose, a router must decide for each incoming packet where to send it next. More exactly, the forwarding decision consists of finding the address of the next-hop router as well as the egress port through which the packet should be sent. This forwarding information is stored in a forwarding table that the router computes based on the information gathered by routing protocols. To consult the forwarding table, the router uses the packet's destination address as a key; this operation is called address lookup. Once the forwarding information is retrieved, the router can transfer the packet from the incoming link to the appropriate outgoing link, in a process called switching.The exponential growth of the Internet has stressed its routing system. While the data rates of links have kept pace with the increasing traffic, it has been difficult for the packet processing capacity of routers to keep up with these increased data rates. Specifically, the address lookup operation is a major bottleneck in the forwarding performance of today's routers. This article presents a survey of the latest algorithms for efficient IP address lookup. We start by tracing the evolution of the IP addressing architecture. The addressing architecture is of fundamental importance to the routing architecture, and reviewing it will help us to understand the address lookup problem.
The Classful Addressing SchemeIn IPv4, IP addresses are 32 bits long and, when broken up into 4 groups of 8 bits, are normally represented as four decimal numbers separated by dots. For example, the address 10000010_01010110_00010000_01000010 corresponds in dotted-decimal notation to 130.86.16.66.One of the fundamental objectives of the Internet Protocol is to interconnect networks, so routing on a network basis was a natural choice (rather than routing on a host basis). Thus, the IP address scheme initially used a simple two-level hierarchy, with networks at the top level and hosts at the bottom level. This hierarchy is reflected in the fact that an IP address consists of two parts, a network part and a host part. The network part identifies the network to which a host is attached, and thus all hosts attached to the same network agree in the network part of their IP addresses.Since the network part corresponds to the first bits of the IP address, it is called the address prefix. We will write prefixes as bit strings of up to 32 bits in IPv4 followed by a *. For example, the prefix 1000001001010110* represents all the 2 16 addresses that begin with the bit pattern 1000001001010110. Alternatively, prefixes can be indicated using the dotted-decimal notation, so the same prefix can be written as 130.86/16, where the number after the slash indicates the length of the prefix.With a two-level hierarchy, IP routers forwarded packets based only on the network part, until packets reached the destination network. As a result, a forwarding table only needed to store a single entry to forward ...
