Chapter 3: Switching

The main topic here is how LAN switching works; first, in 3.1, avery generic approach to switching (much of which also applies to IP routing), and then in 3.2 a specific application of switching to Ethernet (also called bridging).These two sections are the main part of the chapter.

Switching assumes that each packet has an abstract destination address; switches must make sure each packet is eventually delivered to that address.

This chapter also includes Section 3.3 on ATM ("asynchronous transfermode"), which is an important and very different approach to LANs and switching.

We'll omit section 3.4 on switching hardware.

Section 3.1

The main issue for switches, which just connect different physical LANs,is to figure out which way packets get forwarded; that is, to look at theaddress and decide to where that switch will send the packet. Each switchmakes a decision only for the next hop of the route, but, with a littlecoordination, the packets get routed along the correct global path.

There are two main approaches to switching: datagram switching and virtual circuits. There is also a third technique, source routing, that is seldom used in practice. The Internet is built using datagram switching, as areswitched Ethernets. ATM is a technique based on virtual circuits; the ideawas to support the requirements for both data and voice. The essentialidea behind datagram switching is that the switches do not maintainany state information about endpoint-to-endpoint connections; they keepa huge table of every destination and look up the next hop to thatdestination for every packet. This means they spend a lot of time on lookup,and the per-packet overhead is very high, but there is zero setup overheadfor sending a packet. Virtual circuit mechanisms, on the other hand, requirethat a "virtual circuit" be set up for every connection, which is a lotof setup overhead. The tradeoff is that then the tables used to forwardeach individual packet can be kept very small and compact (because theycontain entries only for the active connections, not for every destinationin the world), and per-packet overhead is negligible.

Study questions:

Why would datagram switching work better with relatively large packets?(Think about where the overhead is.) Why would virtual circuits be a betterchoice if packets were small?

Exercises:

1    Give VC tables, given the connections
2    Give datagram tables
3    datagram tables with a "default" entry
4    From VC tables, figure out the connections

Section 3.2 - Ethernet bridging/switching

Just read 3.2.1, on how Ethernet bridges work. ("Bridge" is an older term for "switch".) The basic idea is datagram routing, and every switch maintains a table of all the known Ethernet hosts in the LAN (an Ethernet switchgenerally has enough memory for at least 10,000 entries). The twist hereis how bridges build these tables: by entering the source of eachpacket they see. Initially the tables are empty. This strategy dependson a crucial feature: if an Ethernet bridge doesn't know how to delivera packet, it has a fallback delivery strategy of broadcasting it on alllinks. That strategy is not available on the Internet at large,and so we'll see in chapter 4 that IP routing -- which also uses datagramrouting -- has to use a much more upfront algorithm for building the routingtables.

Nowadays, it is routine for bridges to join Ethernet to Token Ring.

Study Questions

What are the limitations on how big a switched Ethernet can grow? Why can't the entire Internet be one big switched Ethernet?

Exercises

13    Learning bridge example

Section 3.3 - ATM

Actually you are just to read 3.3.1 on the general strategy of cells. Cells are tiny packets, relative to the IP/Ethernet world. In the latter, packet sizes start at 512 bytes and go up to 1500 bytes, or more; in ATM, cells (packets) are 53 bytes (48 bytes of data following 5 bytes of header). The IP world, in general, believes in large packets. Cells are an important example of the merits of small packets. Most (but not all) IP people have been relatively uninterested in ATM.

ATM is usually installed today as a LAN alternative, but its originalproponents really had in mind using it to replace IP entirely; the Internetwould become a global ATM network and ATM's routing and congestion algorithmswould rule the world. At the present time, this seems unlikely to cometo pass.

Study questions

People have said that "ATM" stands for "A Tariffing Mechanism" and "A Technological Mistake". Explain these. Can you propose another?
Why are cells good for voice traffic?

Exercises

28    Why does cell switching always use virtual circuits?