Shulou(Shulou.com)06/01 Report--

[TOC]

1.TCP protocol and UDP protocol

They are the transport layer protocols of the TCP/IP suite.

​ TCP (Transmission Control Protocol), that is, Transmission Control Protocol

​ UDP (User Datagram Protocol), the user Datagram protocol

Key words: connection, reliability, at present, most of the data transmission uses TCP protocol, in the occurrence of network congestion and other special cases, will wait and go, high reliability

​ full-duplex service

2.TCP message segment format

Source port number (16bits): send port

Destination port number (16bits): receive port

Serial number (32bits): the sender numbers the data so as to facilitate the reorganization of the receiver

Confirmation number (32bits): used to confirm the information of the sender

URG (1bit): emergency pointer bit, 1 indicates on, emergency pointer on (16bits)

ACK (bit): confirm bit

PSH (1bit): when PSH=1, it is sent directly to the application layer without caching; (the data is first put in the cache and then extracted in the cache)

These two are not often triggered.

RST (bit): use it when re-establishing a connection

SYN: (1bit) synchronous connection bit, SYN=1 when connecting

FIN (1bit); disconnect

Window size (16bits): indicates that the size of the data received is variable

Checksum (16bits): checksum function

Emergency pointer (16bits): whether the data is urgent

Options: as reserved

Among them, ACK,SYN,FIN is the most important (involving three waves and four handshakes)

3.TCP three-way handshake

First, PC1 sends the message segment 1 of [SYN=1 (application for connection)] [Seq (serial number) = x]

Then after receiving this message segment 1, PC2 replies [ACK=1 (confirm knowledge) and SYN=1 (also apply for connection)] message segment 2 [Seq (serial number) = y _ mai Ack (confirmation number) = x _ blank 1 ~ #, indicating PC1's confirmation of the receipt of the previous message segment 1.

Finally, after receiving the message segment 2 of PC2, PC1 sends [ACK=1 (acknowledgement)] message segment 3 to PC2 for the second time [Seq (serial number) = Xue1jinAck (acknowledgement number) = yaq1jinyq1 indicates PC1's acknowledgement of the previous message segment 2]

Thus, the TCP connection is established.

4.TCP waved four times.

PC1 initiates a disconnection request, that is, to send [FIN (disconnect) = 1] message segment 1

After receiving segment 1, PC2 will send [ACK (acknowledgement) = 1] segment 2.

At this time, there are two situations, one is that PC2 is not transmitting data to PC1, and the other is that PC2 still has data that is being transmitted. Here, we first send [ACK (acknowledgement) = 1] message segment 2, and then send [FIN (disconnect) = 1 Ack (confirmation awareness) = 1] message segment 3.

When PC1 receives message segment 3, it will send [ACK (confirmation) = 1] message segment 4

At this point, the TCP connection is closed

Introduction of 5.UDP Protocol

6. Common protocols and their ports

TFTP is similar to FTP, simple File transfer Protocol

NTP network time protocol, very important

FTP file server

TELNET remote connection, sent in clear text (cannot be used on WAN)

STMP Mail sending Protocol

HTTP website protocol

POP3 Mail receiving Protocol

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