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交换与转发
交换机是一种多输入多输出设备，它将多条链路连接成更大的网络，并让每台主机都能独享其链路的全部速率；转发是对每个分组决定其从哪个输出端口发出的逐包决策，而路由则是计算该决策背后所用表项的后台过程；数据报交换机在每个分组中都携带完整的目的地址，不维护任何每连接状态；虚电路交换机在建立连接时创建状态，之后仅依据逐跳改写的短链路本地标识符进行转发；而源路由则让发送主机显式地指明整条路径。
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?