Table of contents
2.1. Features of UDP protocol:
1. The tcpdump command
We can use the tcp-sock code we wrote before to capture packets and practice it
tcpdump universal formula:
tcpdum -i any port [port number] -s 0 -w 123.dat
-w means to put the data obtained by capturing the packet into the file 123.dat
Premise: When the tcpdump command is executed, the root user needs to be used
At this time, we go to the browser to send a request to the server, and the requested data can be obtained on the 123.dat file
After we send the request, we close tcpdump, and we can find that 17 data packets have been obtained
When we open the 123.dat file at this time, we will find that it is full of things we cannot understand, because all the captured things are binary
At this time, we can use the wireshark tool to let him analyze it for us
We can see the analyzed content:
2. UDP protocol
foreword
The transport layer is responsible for the transmission between end-to-end, and the end refers to the port (port); both the TCP protocol and the UDP protocol are protocols of the transport layer
In the transport layer, the source port and destination port need to be agreed.
2.1. Features of UDP protocol:
Connectionless, unreliable, datagram-oriented
2.2, UDP protocol format
16-bit UDP length :
What is saved is the total length of UDP data (udp header part + data), the unit is byte;
Consider: The maximum number that can be represented by 16 bits is 65536, that is to say, the maximum total length of udp is 15536 bytes
A single piece of udp data will not exceed 65536 bytes, if it exceeds, what should I do?
Combined with the packet capture tool: analyze the UDP protocol, and use the udp_socket programming we wrote before
Open wire shark to analyze the data we captured
We can open one of the UDP messages for analysis:
Compared with the format of the UDP protocol, we can better understand the content sent:
16-bit checksum:
In order to verify whether the data is distorted during transmission,
The sender will mark the checksum, and the receiver will pass the checksum verification
2.3. Application of UDP:
