docs: wip tcp congestion control

src/en/guide/concepts/network/2-tcp-udp.md

@@ -66,4 +66,83 @@ The reason why a four-way handshake is needed is because TCP is a **full-duplex*
 
 ::: details
 If the client sends a `SYN` command, and drops the line before the server returns the `SYN` command, the server will try to resend the `SYN-ACK` command. Under Linux, the default is to retry 5 times, the interval time starts to double from 1s, i.e., `1s, 2s, 4s, 8s, 16s`, so the timeout time is `1s + 2s + 4s+ 8s+ 16s + 32s = 63s`. After the timeout, TCP will disconnect.
+:::
+
+
+#### How does TCP ensure the reliability of transmission?
+
+::: tip TCP ensures the reliability of data transmission through the following features
+
+- Sequence numbers and acknowledgments
+- Timeout retransmission
+- Flow control
+- Congestion control
+- Checksum
+
+:::
+
+##### Sequence numbers and acknowledgment signals
+
+TCP uses sequence numbers to sort and deduplicate packets, and through the ACK acknowledgment mechanism, it ensures that packets are successfully delivered, ensuring the integrity of the data.
+
+#### Timeout retransmission
+
+TCP can retransmit packets until an ACK acknowledgment is received when packets are lost or delayed, through the timeout retransmission mechanism.
+
+::: warning What is RTT? What is RTO?
+
+::: details
+RTT stands for Round Trip Time, which refers to the time required for data to be transmitted from the sender to the receiver and back. RTT is used to measure network latency, that is, the total time that data passes through during transmission.
+
+RTO stands for Retransmission Timeout, which refers to the time that the sender waits for acknowledgment (ACK) after sending data in network communication. If the sender does not receive an acknowledgment within the RTO time, it will assume that the data is lost and resend the data.
+
+The calculation of RTO is usually based on the measurement of RTT. The sender estimates the RTO of the next data transmission based on the previous RTT. In general, the value of RTO will be larger than RTT to ensure that data can be successfully retransmitted even in cases of high or unstable network latency.
+
+The calculation method of RTO can vary depending on the specific network protocol or implementation, but its purpose is to ensure the reliable transmission of data to cope with problems such as packet loss, delay, and congestion in the network.
+:::
+
+::: warning What is the impact of RTO length on retransmission?
+
+::: details 
+If the RTO is too long, the retransmission time will be greatly extended, reducing transmission efficiency. If the RTO is too short, it may lead to frequent retransmissions, exacerbating network congestion, and further triggering more retransmissions.
+:::
+
+
+::: tip Common retransmission mechanisms
+
+::: details
+
+- Timeout retransmission
+
+![timeout retransmission](/assets/image/article/network/timeout-retransmission.png)
+
+There are two situations for timeout retransmission, the timeout caused by the loss of the sent data packet, and the timeout caused by the loss of the returned ACK data packet. Under the timeout retransmission mechanism, a timer is started every time a data packet is sent. If the timer expires, retransmission will be triggered. If the retransmission fails, the next timeout time will be twice the current value. ==The disadvantage of timeout retransmission is that the cycle is long, which may reduce efficiency.==
+
+- Fast retransmission
+
+![tcp](/assets/image/article/network/tcp-3-retry-new.png)
+
+Under the fast retransmission mechanism, after a data packet is lost, the receiving end immediately returns a duplicate acknowledgment packet for each out-of-order packet received, informing the sending end of the missing packet. When the sender receives three duplicate acknowledgment packets, it will immediately retransmit the missing packet.
+
+==Under the fast retransmission mechanism, we can see that the packets `3,4,5` all return `ACK=2`, so we need to retransmit the packets `3,4,5`. Is there a way to not need to retransmit packets `3,4,5`?==
+
+- SACK (Selective Acknowledgment)
+
+![SACK](/assets/image/article/network/sack.png)
+
+The SACK method ([RFC 2018](https://www.ietf.org/rfc/rfc2883.txt)) adds a buffer to the tcp option field to record the transmitted packets, so that the sender can see the packets that have not been successfully transmitted, allowing the sender to only transmit the missing packets without needing to retransmit other packets.
+
+- D-SACK (Duplicate SACK)
+
+D-SACK ([RFC 2883](https://datatracker.ietf.org/doc/html/rfc2018)) mainly solves the problem of ACK loss. D-SACK uses the first segment of SACK as a flag to mark the packets that have been ACKed. When the receiving end receives a duplicate packet, it will write the packet into the D-SACK flag, telling the sender that the packet has been received, and the ACK may have been lost.
+:::
+
+### UDP
+
+### Common interview questions
+
+::: tip What is a SYN Flood attack?
+
+::: details
+
 :::

src/guide/concepts/network/2-tcp-udp.md

@@ -33,7 +33,7 @@ title: "TCP/UDP"
 
 ![TCP Stateful](/assets/image/article/network/tcp_state.png)
 
-#### TCP 三次握手
+#### TCP 三次握手/四次挥手
 
 ![TCP Connection](/assets/image/article/network/tcp-connect.png)
 
@@ -67,4 +67,105 @@ TCP 建立连接的过程就是同步序列号的过程，SYN (Synchronize Seque
 
 ::: details
 假如客户端发送 `SYN` 指令，在服务器返回 `SYN` 指令之前掉线了，服务器会尝试重发 `SYN-ACK` 指令，linux 下默认重试 5 次，间隔时间从 1s 开始翻倍增长，即 `1s, 2s, 4s, 8s, 16s`, 因此超时时间为  `1s + 2s + 4s+ 8s+ 16s + 32s = 63s`。在超时之后 TCP 才会断开连接。
-:::
+:::
+
+#### TCP 如何保证传输的可靠性？
+
+::: tip TCP 通过以下几个特性保证数据传输的可靠性
+
+- 序列号和确认应答
+- 超时重传
+- 流量控制
+- 拥塞控制
+- 校验和
+
+:::
+
+##### 序列号和确认应答信号
+
+TCP 通过序列号可以对数据包进行排序和去重，同时通过 ACK 应答机制确保数据包成功送达，保证了数据的完整性。
+
+#### 超时重传
+
+TCP 通过超时重传机制可以在数据包丢失或者延迟时，重新发送数据包直到收到 ACK 应答。
+
+::: warning 什么是RTT？什么是RTO？
+
+::: details
+RTT代表往返时间（Round Trip Time），是指数据从发送方传输到接收方并返回所需的时间。RTT用于衡量网络延迟，即数据在传输过程中经过的总时间。
+
+RTO代表重传超时时间（Retransmission Timeout），是指在网络通信中，发送方发送数据后等待确认（ACK）的时间。如果发送方在RTO时间内未收到确认，它会假设数据丢失，并重新发送该数据。
+
+RTO的计算通常基于RTT的测量。发送方根据前一次的RTT来估计下一次发送数据的RTO。一般情况下，RTO的值会比RTT大一些，以确保在网络延迟较高或不稳定的情况下也能够成功重传数据。
+
+RTO的计算方法可以根据具体的网络协议或实现而有所不同，但其目的是为了确保数据的可靠传输，以应对网络中的丢包、延迟和拥塞等问题。
+:::
+
+::: warning RTO 长短对重传有什么影响？
+
+::: details 
+若 RTO 过长则重传时间会大大延长，降低传输效率。若 RTO 过短则可能会导致频繁重传，加剧网络的拥堵，进一步触发更多的重传。
+:::
+
+
+::: tip 常见的重传机制
+
+::: details
+
+- 超时重传
+
+![timeout retransmission](/assets/image/article/network/timeout-retransmission.png)
+
+超时重传有两种情况，发送的数据包丢失导致的超时，和返回的 ACK 数据包丢失导致的超时。超时重传机制下，在每次发送数据包的时候都会启动一个定时器，如果定时器到期则会触发重传。若重传失败，则下一次超时时间为当前值的两倍。==超时重传的缺点是周期较长，可能会降低效率。==
+
+- 快速重传
+
+![tcp](/assets/image/article/network/tcp-3-retry-new.png)
+
+在快速重传机制下，在数据包丢失后，接收端每接收一个失序的数据包就立即返回重复的确认报文段，告知发送端缺少的报文段。当发送方收到三个重复的确认报文段后，会立即重传缺失的报文段。
+
+==在快速重传的机制下，我们可以看到 `3,4,5` 的报文段返回的都是 `ACK=2`, 因此还需要重传 `3,4,5` 报文段。有没有方法可以不需要重传 `3,4,5` 报文段？==
+
+- SACK (Selective Acknowledgment, 选择性确认)
+
+![SACK](/assets/image/article/network/sack.png)
+
+SACK 方法 ([RFC 2018](https://www.ietf.org/rfc/rfc2883.txt)) 在 tcp 的 option 字段添加了缓冲区，用于记录已传输的数据包，这样发送方可以看到未成功传输的数据包，使得发送方可以仅传输缺失的数据包而不需要额外重传其他数据包。
+
+- D-SACK (Duplicate SACK, 重复选择性确认)
+
+D-SACK ([RFC 2883](https://datatracker.ietf.org/doc/html/rfc2018)) 主要解决了 ACK 丢失的问题， D-SACK 使用 SACK 第一个段作为标志位，用于标记已经 ACK 的数据包。 当接收端接收到重复的报文段时会将该报文段写入 D-SACK 的标志位，告诉发送方已接收到报文段， ACK 可能已丢失。
+:::
+
+#### 流量控制
+
+TCP 使用滑动窗口来控制流量，使得发送端可以根据接收端的接收能力控制发送数据的速度。
+
+![发送窗口(窗口大小为 7)](/assets/image/article/network/tcp-send-window.png)
+![接收窗口(窗口大小为 7)](/assets/image/article/network/tcp-receive-window.png)
+
+::: warning 
+接收端在应答 `ACK` 的时候会将当前可用的窗口大小写入 tcp 头部，发送端根据窗口大小调整发送速率。
+:::
+
+#### 拥塞控制
+
+::: tip 算法
+- 慢启动
+- 拥塞避免
+- 快重传
+- 快恢复
+:::
+
+![拥塞控制](/assets/image/article/network/tcp-congestion-control.png)
+
+### UDP
+
+### 常见面试题
+
+::: tip 什么是 SYN Flood 攻击？
+
+::: details
+
+:::
+