Transmission Control Protocol

Feature

• connection-oriented

Structure

TCP segment

header

20 bytes

Pipeline (sliding window)

Go-Back-N (GBN)

the receiver doesn't need a buffer

Scenario

• out of order / packets gap
  • discard all the following packets

window size

suppose seq number is $n$ bit

$$W_{\text{Sender}}+W_{\text{Receiver}}\le 2^n,\text{where }{W}_{\text{Sender}}=1$$

Selective Repeat

the receiver need a buffer to store packets,

window size

suppose seq number is $n$ bit

$$W_{\text{Sender}}+W_{\text{Receiver}}\le 2^n,\text{where }{W}_{\text{Sender}}=W_{\text{Receiver}}$$

flow control

防止缓冲区溢出

1. 建立连接时交换buffer大小
2. tcp头部包含free buffer space

congestion control

TCP拥塞控制通过调整发送窗口大小来控制网络中的数据流量，避免网络拥塞。

拥塞窗口

• cwnd (congestion window): 发送方根据网络状况维护的窗口
• 发送窗口 = min(cwnd, 接收窗口rwnd)

1. slow start

• 初始cwnd = 1 MSS
• 每收到一个ACK，cwnd增加1 MSS（即每次翻倍）
• cwnd呈指数增长：1, 2, 4, 8, 16, ...
• 当cwnd达到慢启动阈值ssthresh时，进入拥塞避免阶段

2. congestion avoidance

• 每个RTT，cwnd增加1 MSS
• cwnd呈线性增长
• 检测到拥塞时：
  • ssthresh = cwnd / 2
  • cwnd = 1 MSS（如果是超时）
  • cwnd = ssthresh（如果是3个重复ACK，进入快速恢复）

Tahoe

3 duplicate ACKs trigger fast retransmit:

• ssthresh = cwnd / 2
• cwnd = 1
• 发送方立即重传丢失的报文段 & goto slow start

Reno

3 duplicate ACKs trigger fast retransmit:

• ssthresh = cwnd / 2
• cwnd = ssthresh
• 发送方立即重传丢失的报文段 & goto congestion avoidance