《计算机网络自顶向下方法》第一章笔记
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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:

  1. 带宽共享比电路交换好
  2. 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:

  1. 节点处理时延(nodal processing delay)
  • check bit errors
  • determine output link
  • typically < msec(微秒)
  1. 排队时延(queuing delay)
  • time waiting at output link for transmission
  • depends on congestion level of router
  1. 传输时延(transmission delay)
  2. 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

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