2.3 Router Characteristics

An Internet router performs the following functions:

1. Conforms to specific Internet protocols specified in this document, including the Internet Protocol (IP), Internet Control Message Protocol (ICMP), and others as necessary.

2. Interfaces to two or more packet networks. For each connected network the router must implement the functions required by that network. These functions typically include:
   • Encapsulating and decapsulating the IP datagrams with the connected network framing (e.g., an Ethernet header and checksum),

   • Sending and receiving IP datagrams up to the maximum size supported by that network, this size is the network's Maximum Transmission Unit or MTU,

   • Translating the IP destination address into an appropriate network-level address for the connected network (e.g., an Ethernet hardware address), if needed, and

   • Responding to network flow control and error indications, if any.

   See chapter 3 (Link Layer).

3. Receives and forwards Internet datagrams. Important issues in this process are buffer management, congestion control, and fairness.
   • Recognizes error conditions and generates ICMP error and information messages as required.

   • Drops datagrams whose time-to-live fields have reached zero.

   • Fragments datagrams when necessary to fit into the MTU of the next network.

   See chapter 4 (Internet Layer - Protocols) and chapter 5 (Internet Layer - Forwarding) for more information.

4. Chooses a next-hop destination for each IP datagram, based on the information in its routing database. See chapter 5 (Internet Layer - Forwarding) for more information.

5. (Usually) supports an interior gateway protocol (IGP) to carry out distributed routing and reachability algorithms with the other routers in the same autonomous system. In addition, some routers will need to support an exterior gateway protocol (EGP) to exchange topological information with other autonomous systems. See chapter 7 (Application Layer - Routing Protocols) for more information.

6. Provides network management and system support facilities, including loading, debugging, status reporting, exception reporting and control. See chapter 8 (Application Layer - Network Management Protocols) and chapter 10 (Operation and Maintenance) 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.

• The global interconnect system is composed of a number of wide-area networks to which are attached routers of several Autonomous Systems (AS); there are relatively few hosts connected directly to the system.

• Most hosts are connected to LANs. Many organizations have clusters of LANs interconnected by local routers. Each such cluster is connected by routers at one or more points into the global interconnect system. If it is connected at only one point, a LAN is known as a stub network.

Routers in the global interconnect system generally require:

• Advanced Routing and Forwarding Algorithms

  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.

• High Availability

  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.

• Advanced O&M Features

  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.

• High Performance

  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.