Basic protocols running on Ethernet

Basic protocols running on Ethernet 1. Introduction A network listening tool was compiled some time ago. To analyze captured data packets, you must be familiar with various network protocols. I am suffering from the fact that there are few articles on the Internet that focus on this aspect, and most of the books are not carefully explained. A small number of books devoted to network protocols are too large to meet our requirements. Therefore, we will write out common Ethernet protocols to share with you. 2. Ethernet features CSMA/CD technology is used for Ethernet, that is, carrier-Connected Multi-Access access with collision detection. In CSMA/CD technology, if there is no data on the network, the data can be transmitted at any time. If the network is used, it will be sent again when the network is idle. Data transmitted over Ethernet is not sent to all NICs. Www.2cto.com 3. In Ethernet, data is exchanged in units of data structures called frames. The header of the Ethernet subnet contains the address of the receiving end and the sending end, and the packet part of the subnet contains the data type. The last place is the error checksum and correction code. The synchronization code and delimiter have been removed from the packets we have captured. The following is a definition of the header section: Ethernet accept Header Format acceptor MAC address (48 bits) sender MAC address (48 bits) type field (16 bits) Note: here we mention a bit, one of which is binary data. Generally, a byte is composed of eight digits. Type field meaning (refer to RFC1700) type (hexadecimal) protocol 0800IP0806ARP8035 RARP809B Appletalk8137-8138 Novell814C SNMP 4. IP datagram composition when the type field of the Ethernet data metric is hexadecimal 0800, indicates that the part of the ethereum packet is an IP datagram. The header part stores important information such as version, header length, survival time, and IP addresses of the receiving and sending terminals. Www.2cto.com IP datagram header definition field bits description version 4 indicates the IP version. The current version number is 4 header length 4 indicates the length of the header to 32 bits as a unit of service type 8 priority and reliability and other service quality requirements of the value of the datagram length of 16 IP datagram, in 8 bits (bytes) identify IP datagram number for Unit 16 mark 3 mark whether there is a broken block data block offset 13 when the broken block is generated should be earlier than the reached data block size survival time 8 storage control IP datagram life long and Short value Protocol 8 indicates the upper-layer protocol header verification code 16 checks the header error using the verification code sending end address 32 sending end IP address Destination Address 32 receiving end IP address 5 TCP protocol Transmission Control Protocol (TCP) is the protocol used at the transport layer. When the value of the Protocol field of the IP datagram is 6, it indicates that the part of the packet of this IP datagram is a TCP datagram. TCP Data header definition short name length (BIT) the Port Number of the sender and client program. The port number of the receiver port is 16. The port number of the receiver and the server program is 32. The data of the message segment is located in the byte of all data blocks. the value confirmation number 32 indicates that the receiver offsets 4 header lengths for the data blocks received by the sender, use 32 bits as a unit Reserved 6 extended (Reserved) control bits 6 represents the size of the bit window 16 window indicating the meaning of a field (the amount of data that can be sent at a time) check Code 16 error check use emergency Data Pointer 16 insert signal and other emergency data end position options and fill 32 select equal www.2cto.com 6, UDP datagram when IP Datagram Protocol field value is 17, the part of the IP datagram packet carries the UDP datagram. The following is the structure of the UDP data header. UDP datagram sending end port (16 bits) receiving end port (16 bits) datagram length (16 bits) Verification Code (16 bits) data