TCP/IP Study Notes (9)-TCP protocol Overview

Finally, I saw the TCP protocol. This is the most important and exciting part of the TCP/IP explanation. It takes a lot of effort to read it. The previous TFTP and BOOTP are both simple protocols, so you don't need to take notes.
 
TCP and UDP are on the same layer-transport layer, but the most difference between TCP and UDP is that TCP provides a reliable data transmission service, which is connection-oriented, that is, the two hosts that use TCP communication first need to go through a "call" process, wait until the communication preparation is complete before data transmission and end the call. Therefore, TCP is much more reliable than UDP. UDP sends data directly, and no matter whether the recipient is receiving the message, even if UDP cannot be delivered, it will not generate ICMP error packets, this was reiterated many times.
 
Extract the simple working principle of TCP to ensure reliability as follows:
 
Application Data is divided into data blocks that TCP considers to be the most suitable for sending. This is completely different from UDP, and the length of the datagram generated by the application will remain unchanged. The unit of information transmitted to an IP address by TCP is a segment or segment (segment) (see Figure 1-7 ). In section 1 8.4, we will see how TCP determines the length of the packet segment.
When TCP sends a segment, it starts a timer and waits for the destination to confirm receiving the segment. If a confirmation message cannot be received in time, the message segment will be resold. In chapter 2, we will understand the self-adaptive timeout and retransmission policies of TCP.
When TCP receives data from the other end of the TCP connection, it sends a confirmation message. This confirmation is not sent immediately and will usually be postponed by a few minutes, which will be discussed in section 1 9.3.
TCP will maintain its header and data validation. This is an end-to-end test to detect any changes in data during transmission. If verification and error are received, t p discards the packet segment and does not confirm receipt of the packet segment (meaning that the initiator times out and resends the packet segment ).
Since the TCP packet segment is transmitted as an IP datagram, the arrival of the IP datagram may be out of order, so the arrival of the TCP packet segment may also be out of order. If necessary, TCP sorts the received data again and delivers the received data to the application layer in the correct order.
TCP can also provide traffic control. Each side of a TCP connection has a fixed buffer space. The TCP receiving end only allows the other end to send data that can be accepted by the receiving end buffer. This will prevent the buffer overflow of the slow host caused by the fast host.
From this section, we can see that the way to maintain reliability in TCP is timeout and re-transmission, which makes sense, although TCP can also use a variety of ICMP packets to process these, however, this is not reliable. The most reliable method is to resend the datagram as long as it is not confirmed until it is confirmed by the other party.
 
The same as the UDP header, the TCP Header has the sending port number and the receiving port number. However, the TCP header information is obviously more than that of UDP. As you can see, TCP provides all the necessary information required for sending and confirming. This is detailed in the P171-173. It can be imagined that the process of sending TCP data should be as follows.
 
Establish connection between the two parties
The sender sends the TCP datagram to the receiver and waits for the Peer to confirm the TCP datagram. If no, the sender resends the datagram. If yes, the sender sends the next datagram.
The receiver waits for the sender's datagram. If the received datagram is correct and verified, the receiver sends an ACK (confirmed) datagram and waits for the next TCP datagram to arrive. Wait until FIN is received (send complete datagram)
Abort connection
To establish a TCP connection, the system may establish a new process (the worst is also a thread) for data transmission.

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