Computer Networks · free preview
Switching and Forwarding
A switch is a multi-input, multi-output device that joins links into larger networks and gives every host the full speed of its own link, forwarding is the per-packet decision of which output a packet takes while routing is the background process that computes the tables behind that decision, datagram switches carry the full destination address in every packet with no per-connection state, virtual circuit switches establish state at setup and then forward on short link-local identifiers rewritten hop by hop, and source routing lets the sending host spell out the path explicitly.
1. The Switch
The Switch
2. Datagrams
Datagrams
3. Virtual Circuits
Virtual Circuits
4. Source Routing
Source Routing
5. Forwarding versus routing
A switch crashes and reboots with empty memory. Which functions must be rebuilt before traffic flows correctly again, and which were never stored there to begin with?
6. The datagram trade
Why can a datagram network offer no bandwidth guarantee to a flow, while a virtual circuit network can?
7. Link-local identifiers
A virtual circuit's packets cross four switches, and the identifier in the packet header is different on every link. Why is this rewriting allowed, and what does it buy?
8. Setup delay
Two hosts want to exchange one short message. Which switching model gets the first data bit to the destination with the least delay, and why?
9. Failure behavior
A link fails inside a network carrying both datagram traffic and established virtual circuits. Compare what happens to each.
10. Source routing
In source routing, how does a packet with a three-switch path instruct each switch, and what does each switch do to the header?
11. Cell size
Asynchronous transfer mode networks switched fixed-size cells of how many bytes total, header included?
12. Table entry
A virtual circuit table entry records four fields: the incoming interface, the incoming identifier, the outgoing interface, and what fourth field?
13. Signalled circuits
A virtual circuit established on demand by host signalling, rather than by an administrator, is called a switched virtual circuit, abbreviated how?
14. Star benefit
On a switched star topology, each host has its own link to the switch, so unlike a shared-medium network, adding a host does not reduce what resource available to existing hosts?
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