Unit 1, Packet switching

接下来讲计算机网络的三个architectural ideas，三个原则：packet switching, layering and encapsulation.

Packet switching

Data is broken down into self-contained packets, which are forwarded hop-by-hop based on the information in the packet header (source and destination address).

每一个switch在forward的时候不记录从src到dst的完整route，只从forward table上寻找下一个hop点，并将它和dst地址发给下一个switch。

A sequence of packets is defined as a “flow”, e.g. datagrams transported in a voice call. A collection of datagrams that belong to the same end-to-end communication, e.g. a TCP connection.

The packets are sent independently, in a discrete, self-contained manner. The switch doesn’t have to know how many parts left, or how many connection states are active.

Switch这种传输机制的好处是简单（不用掌握flow of packets）高效，同时传送packets，不会造成partially idle等浪费。那么，如何为不同的user分配这个link的capacity呢？这就用到了Statistical Multiplexing技术。

How much share to be allocated depends on how much others are using it.