An Internet router performs the following functions:
See chapter 3 (Link Layer).
See chapter 4 (Internet Layer - Protocols) and chapter 5 (Internet Layer - Forwarding) for more information.
A router vendor will have many choices on power, complexity, and features for a particular router product. It may be helpful to observe that the Internet system is neither homogeneous nor fully connected. For reasons of technology and geography it is growing into a global interconnect system plus a fringe of LANs around the edge. More and more these fringe LANs are becoming richly interconnected, thus making them less out on the fringe and more demanding on router requirements.
Routers in the global interconnect system generally require:
These routers need routing algorithms that are highly dynamic, impose minimal processing and communication burdens, and offer type-of-service routing. Congestion is still not a completely resolved issue (see Section [5.3.6]). Improvements in these areas are expected, as the research community is actively working on these issues.
These routers need to be highly reliable, providing 24 hours a day, 7 days a week service. Equipment and software faults can have a wide-spread (sometimes global) effect. In case of failure, they must recover quickly. In any environment, a router must be highly robust and able to operate, possibly in a degraded state, under conditions of extreme congestion or failure of network resources.
Internet routers normally operate in an unattended mode. They will typically be operated remotely from a centralized monitoring center. They need to provide sophisticated means for monitoring and measuring traffic and other events and for diagnosing faults.
Long-haul lines in the Internet today are most frequently full duplex 56 KBPS, DS1 (1.544 Mbps), or DS3 (45 Mbps) speeds. LANs, which are half duplex multiaccess media, are typically Ethernet (10Mbps) and, to a lesser degree, FDDI (100Mbps). However, network media technology is constantly advancing and higher speeds are likely in the future.
The requirements for routers used in the LAN fringe (e.g., campus networks) depend greatly on the demands of the local networks. These may be high or medium-performance devices, probably competitively procured from several different vendors and operated by an internal organization (e.g., a campus computing center). The design of these routers should emphasize low average latency and good burst performance, together with delay and type-of-service sensitive resource management. In this environment there may be less formal O&M but it will not be less important. The need for the routing mechanism to be highly dynamic will become more important as networks become more complex and interconnected. Users will demand more out of their local connections because of the speed of the global interconnects.
As networks have grown, and as more networks have become old enough that they are phasing out older equipment, it has become increasingly imperative that routers interoperate with routers from other vendors.
Even though the Internet system is not fully interconnected, many parts of the system need to have redundant connectivity. Rich connectivity allows reliable service despite failures of communication lines and routers, and it can also improve service by shortening Internet paths and by providing additional capacity. Unfortunately, this richer topology can make it much more difficult to choose the best path to a particular destination.