1.4.1 How do loss and delay occur?
packets queue in router buffers 包在缓存中的排队模式
- packet arrival rate to link (temporarily) exceeds output link
capacity - packets queue, wait for turn
Packet loss
- queue (aka buffer) preceding link in buffer has finite capacity
- packet arriving to full queue dropped (aka lost)
- lost packet may be retransmitted by previous node, by source end
system, or not at all
（丢失的包可以由先前的节点或源端系统重新传输，也可能无法被传输）
Throughput（吞吐量）
throughput: rate (bits/time unit) at which bits transferred between
sender/receiver
发送方/接收方之间传输比特的速率(比特/时间单位)
• instantaneous: rate at given point in time
• average: rate over longer period of time
bottleneck link：link on end-end path that constrains end-end throughput.
限制终端吞吐量的终端路径上的链接
per-connection end- end throughput: min(R c ,R s ,R/10)
▪ in practice: R c or R s is often bottleneck