计算机网络Chapter3 Transport-layer 传输层
# Transport-layer 传输层
# 3.2 multiplexing and demultiplexing
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
# 3.3 connectionless transport: UDP
**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
# 3.4 connection-oriented transport: TCP
面向连接传输（可靠传输 reliable data transport）
# TCP 连接建立（三次握手）
seq:请求确认信息，ack:确认信息并依赖于ack
- 客户机发起：SYN=1,seq=x
- 服务器响应并返回确认信息：SYN=1,ACK=1,ack=x+1,seq=y
- 客户端收到并返回确认信息：ACK=1,ack=y+1,seq=x+1
# 数据传输
# TCP连接释放（四次握手）
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
# 流量控制
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）
- 防止接收方内存溢出，分组丢失 Prevent receiver memory overflow and packet loss
# 3.5 TCP congestion control TCP拥塞控制
- 慢开始 Slow start
- 拥塞避免 Congestion avoidance
快重传 fast retransmission
快恢复 quick recovery
