计算机网络Chapter3 Transport-layer 传输层

MoMeaks 12/20/2021 计算机网络

# Transport-layer 传输层

Multiplexing at sender：多路复用在发送方

gathering data chunks at the source host from different sockets.
gathering data chunks at the source host from different sockets, encapsulating each data chunk(collection of bits) with header information (that will later be used in demultiplexing) to create segments
passing the segments to the network layer.
在源主机上从不同的socket收集数据块。
在源主机上从不同的套接字收集数据块，用头信息(稍后将用于解复用)封装每个数据块(位的集合)来创建段
将网段传递给网络层。

Demultiplexing at receiver：接收方多路分解

use header info to deliver received segments to correct socket

How demultiplexing works:

host receives IP datagrams 数据报 (message, data)
- i. each datagram has source IP address, destination IP address
- ii. each datagram carries one transportlayer segment (packets)
- iii. each segment has source, destination port number
host uses IP addresses & port numbers to direct segment to appropriate socket

**校验：**注意反码运算 1‘s compomect 最高位溢出则结果加一

校验和为结果值的反码

面向连接传输（可靠传输 reliable data transport）

seq:请求确认信息，ack:确认信息并依赖于ack

客户机主动发起连接释放报文段，停止发送数据，主动关闭TCP连接The client initiates a connection to release the packet segment, stops sending data, and closes the TCP connection：

FIN = 1,seq = u
服务器收到连接释放报文段后确认The server acknowledges the connection release packet after receiving the segment：

ACK = 1,seq = v,ack=u+1
服务器通知客户端释放连接The server notifies the client to release the connection：

FIN = 1, ACK=1,seq=w,ack=u+1
客户端收到连接释放报文后，发出确认After receiving the connection release packet, the client sends an acknowledgement：

ACK=1,seq=u+1,ack=w+1

TCP利用滑动窗口机制实现流量控制。

⭐️在通信过程中，接收方根据自己接收缓存的大小，动态地调整发送方的发送窗口大小，即接收窗口rwnd （接收方设置确认报文段的窗口字段来将rwnd通知给发送方），发送方的发送窗口取接收窗口rwnd和拥塞窗口cwnd的最小值min

接收窗口rwnd：

拥塞窗口cwnd：

作用：（为什么要通知窗口大小）