概况
protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt
network edge（网络边缘）:
hosts: clients and servers
servers often in data centers
network core（网络核心）:
interconnected routers
网络边缘
access networks（接入网）
wired, wireless communication links
传输时延=L/R L分组长度 R传输速率
physical media（物理媒介）
twisted pair (TP)(guided media): 双绞线，距离最长100m
coaxial cable(guided media):同轴电缆，能用做导引型共享媒体
fiber optic cable(guided media):光纤。高速低错误率
Physical media: radio(unguided media)陆地无线电信道
网络核心
分组交换
存储转发传输
packet-switching分组交换机: hosts break application-layer messages into packets
store and forward存储转发传输: entire packet must arrive at router before it can be transmitted on next link
端到端传输时延=NL/R (assuming zero propagation delay忽略传播时延)L分组长度 R传输速率 N链路条数
排队时延和分组丢失
queueing delay排队时延 ，缓存空间已满时，到达的分组或者已经排队的分组可能会被丢弃(loss)
网络核心两个重要的功能:
routing路由: determines source-destination route taken by packets
每个路由器有转发表，根据路由选择协议(routing protocol)决定最短路径配置转发表
forwarding转发: move packets from router’s input to appropriate router output
电路交换(curcuit switching)
电路交换：预留了端系统间沿路径通信所需要的资源 通过FDM（频分复用）或TDM（时分复用）实现
端到端传输时间和链路数量无关
分组交换和电路交换的对比
分组交换的缺点：端对端时延可变且不可预测
分组交换的优点：
- 带宽共享比电路交换好
- 简单高效，成本低
潮流向着分组交换方向发展
时延，丢包和吞吐量 delay, loss, throughput in networks
四种时延类型：
- 节点处理时延（nodal processing delay）
- check bit errors
- determine output link
- typically < msec(微秒)
- 排队时延(queuing delay)
- time waiting at output link for transmission
- depends on congestion level of router
- 传输时延（transmission delay）
- 传播时延(propagation delay)
总时延为上面四个时延相加
## 传播时延和传输时延比较
丢包
- 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
Throughput 吞吐量
- throughput: rate (bits/time unit) at which bits transferred between sender/receiver
- instantaneous: rate at given point in time
- average: rate over longer period of time
取决于瓶颈链路的速率（最小速率）
protocol layers, service models
优点
- 概念化
- 模块化
潜在的缺点
- 一层可能冗余较低层的功能
- 某层的功能可能需要在其他层出现的信息
Internet protocol stack
- application应用层: supporting network applications(FTP, SMTP, HTTP)
- transport运输层: process-process data transfer(TCP, UDP)
- network网络层: routing of datagrams from source to destination(IP, routing protocols)
- link链路层: data transfer between neighboring network elements（Ethernet, 802.111）(WiFi), PPP
- physical物理层: bits “on the wire
ISO/OSI reference model
- presentation表示层: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions
- session会话层: synchronization, checkpointing, recovery of data exchange
Encapsulation 封装
