3.5 Summary

This chapter has begun to look at some of the issues involved in building scalable and heterogeneous networks by using switches and routers to interconnect links and networks. The most common use of switching is to interconnection Ethernet segments. These L2 switches, often called bridges, use techniques such as source address learning to improve forwarding efficiency and spanning tree algorithms to avoid looping. These switches are extensively used in data centers, campuses, and corporate networks.

To deal with heterogeneous networks, the Internetworking Protocol (IP) was invented and forms the basis of today's routers. IP tackles heterogeneity by defining a simple, common service model for an internetwork, which is based on the best-effort delivery of IP datagrams. An important part of the service model is the global addressing scheme, which enables any two nodes in an internetwork to uniquely identify each other for the purposes of exchanging data. The IP service model is simple enough to be supported by any known networking technology, and the ARP mechanism is used to translate global IP addresses into local link-layer addresses.

A crucial aspect of the operation of an internetwork is the determination of efficient routes to any destination. Internet routing algorithms solve this problem in a distributed fashion; this chapter introduced the two major classes of algorithms—link-state and distance-vector—along with examples of their application (RIP and OSPF).

Both switches and routers need to forward packets from inputs to outputs at a high rate and, in some circumstances, grow to a large size to accommodate hundreds of ports. Building switches that both scale and offer high performance at acceptable cost is complicated by the problem of contention; as a consequence, switches and routers often employ special-purpose hardware rather than being built from general-purpose workstations.

Further Reading

The seminal paper on bridges, in particular the spanning tree algorithm, is the article by Perlman below. Not surprisingly, countless papers have been written on various aspects of the Internet; the paper by Cerf and Kahn is the one that originally introduced the TCP/IP architecture and is worth reading for its historical perspective. Finally, McKeown's paper, one of many on switch design, describes an approach to switch design that uses cells internally but has been used commercially as the basis for high-performance routers forwarding variable-length IP packets.

  • Perlman, R. An algorithm for distributed computation of spanning trees in an extended LAN. Proceedings of the Ninth Data Communications Symposium, pages 44-53, September 1985.

  • Cerf, V., and R. Kahn. A protocol for packet network intercommunication. IEEE Transactions on Communications COM-22(5):637-648, May 1974.

  • McKeown, N. The iSLIP scheduling algorithm for input-queued switches. IEEE Transactions on Networking 7(2):188-201, April 1999.

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