"Linux Network Development Required Course" 7_TCP and UDP

TCP/IP Hierarchy

• Application layer: each application can define (use) various such protocols
• Transport layer: ensure that the sent data can reach the target host and complete the data transmission
• Network layer: fill in the packet address and select the data transmission path
• Data link layer: Integrate links of different connection methods to shield network differences
• Physical layer: specific connection method, wired, wireless, optical fiber...

How TCP/IP works

Features of the TCP/IP Hierarchy

• The upper layer relies on the ability to adjoin the lower layer, and the lower layer only serves the direct adjacency to the upper layer
• The upper layer does not know the working mechanism of the lower layer, and the lower layer does not care about the data content transmitted by the upper layer
• Without cross-layer services, the roles in the hierarchy are indispensable

In-depth understanding of the network layer (IP layer)

• IP addressing: IP addresses are network-layer addresses used to identify hosts on the network
• Routing control: Control how data reaches the host (eg: which routers need to be forwarded)
• Connectionless: Packets are passed on the network based on IP addresses (no connection to the destination host is required)

 Mac 地址：数据链路层所使用的硬件地址
Mac 地址与网络无关，出厂时写入到网络设备中
当主机从网络上每收到一个数据帧时，首先检查数据中的 MAC 地址。如果是发往本机的数据帧则收下，之后进行其他的处理；否则就将此帧丢弃，不再进行其他的处理。

IP address and MAC address

• The IP address is dynamic and does not belong to a specific hardware (the MAC address belongs to the specific hardware)
• The IP address is the address used by the network layer (for delivering packets across the network)
• The MAC address is the address used by the data link layer (to determine which host in the destination network is receiving data)
• The router records the mapping relationship between the host IP address and the MAC address in the network (the ARP protocol is completed)

IP Routing Control

• In order to send data to the target host, all hosts maintain a routing table
• The routing table records which router the IP packet should be sent to next

IP data forwarding

• IP packet forwarding uses a "best effort" policy

  • "Best effort" means "will work hard", but does not guarantee results
  • When forwarding, the data legitimacy will be checked through additional information, but no retransmission will be performed if an exception occurs
  • Forwarding in units of packets, no guarantee of arrival (after sending, it will sink into the sea of networks)

Does the TCP/IP network hierarchy provide reliable data transfer?

Transmission Control Protocol

• TCP provides reliable data transmission over the protocol implementation

  • TCP does not have the concept of "packet", which realizes streaming (data such as water, no head and no tail)
  • There is a service status inside TCP, which can accurately know whether the data has been sent successfully , whether it has been received...
  • TCP can perform blocking control in behavior (when the network environment deteriorates, it can adjust the data transmission speed)

TCP connection establishment

https://www.bilibili.com/video/BV1kV411j7hA/?aid=418245176&bvid=BV1kV411j7hA&cid=343076847&page=1

Q: After the three interactions are over, the connection is established, why is it three times?
A: Prevent old duplicate connections from causing connection confusion issues

 比如在网络环境比较复杂的情况，客户端可能会连续发送多次请求。如果只设计成两次握手的情况，服务端只能一接收请求，然后返回请求信息，也不知道客户端是否请求成功。这些过期请求的话就会造成网络连接的混乱。
所以设计成三次握手的情况，客户端在接收到服务端SEQ+1的返回消息之后，就会知道这个连接是历史连接，所以会发送报文给服务端，告诉服务端。
所以TCP设计成三次握手的目的就是为了避免重复连接。
然后可以设计成四次握手？五次握手？不可以？答案是也是可以的，不过为了节省资源，三次握手就可以符合实际情况，所以就没必要设计成四次握手、五次握手等等情况

Inherent flaws in TCP (DDoS attacks)

• After the client receives SYN,ACK , it will not reply to the last ACK message
• This will cause the server to consume resources without actually communicating
• When multiple puppet clients make "connection requests" to the server at the same time, the server resources will be exhausted

TCP connection disconnected

Q: After the four interactions are over, the connection is disconnected, why is it four times?
Answer: Because TCP is full-duplex communication

 第一次挥手:当主动方发送断开连接的请求（即FIN报文）给被动方时，仅仅代表主动方不会再发送数据报文了，但主动方仍可以接收数据报文。
第二次挥手:被动方此时有可能还有相应的数据报文需要发送，因此需要先发送ACK报文，告知主动方“我知道你想断开连接的请求了”。这样主动方便不会因为没有收到应答而继续发送断开连接的请求（即FIN报文）
第三次挥手:被动方在处理完数据报文后，便发送给主动方FIN报文；这样可以保证数据通信正常可靠地完成。发送完FIN报文后，被动方进入LAST_ACK阶段（超时等待）。
第四挥手:如果主动方及时发送ACK报文进行连接中断的确认，这时被动方就直接释放连接，进入可用状态。

The characteristics of UDP: completely inherit the working method of the network layer

• No need to connect, directly specify the IP address and port to send data
• Listen to a fixed port, as long as there is data, all receive
• Regardless of network conditions, as long as there is data, all can be sent
• Don't care if the data arrives

Where to use UDP

• Insensitive to data, occasions requiring real-time (such as: live broadcast, real-time games)
• Occasions with a better network environment (eg: IoT home)
• Occasions that require deep customization of the protocol (such as: "UDP protocol without packet loss")

Difference between TCP and UDP

• TCP: Did the bottle go to the mouth? Did the bottle go to the mouth? Did the bottle go to the mouth? OK, here's some water for you.
• UDP: Give it to you, give it to you, I don't care if you drink it or not.