TCP
network communication mode
Role: guide the communication of network equipment;
OSI seven-layer model:
7. Application layer: the application layer protocol (http, FTP, Telnet.) generates corresponding data for the application;
6. Presentation layer: convert the data generated by the application layer into a language that network devices can understand
5. Session layer: Generate an independent session for application data, avoiding mutual interference between different data
4. Transport layer: Encapsulate a layer of transport layer headers for application data, which is used to improve transmission services for data;
TCP--reliable, slow transmission efficiency;
UCP--unreliable, high transmission efficiency;
Text, file - using TCP encapsulation
Voice, video - using UDP encapsulation
3. Network layer: Encapsulate the network layer header for data:
Encapsulate sip and dip;
sip: the ip address of the sender;
dip: the ip address of the destination;
2. Data link layer: Encapsulate the data link layer header for data
Generally, smac and dmac will be packaged;
smac: the mac address of the sender;
The mac address of the dmac receiver;
1. Physical layer: Convert the packaged data into a signal suitable for transmission on the physical link for transmission;
application layer
HTTP (Hypertext Transfer Protocol, Hypertext Transfer Protocol): used to access various pages on the web server.
FTP (File Transfer Protocol, File Transfer Protocol): Provides a way for file transfer, which allows data to be transferred from one host to another.
DNS (Domain Name Service, domain name resolution service): used to realize the conversion from the host domain name to the IP address.
transport layer
TCP (Transmission Control Protocol, Transmission Control Protocol): Provides reliable connection-oriented communication services for applications. Currently, many popular applications use TCP.
UDP (User Datagram Protocol, User Datagram Protocol): Provides connectionless communication, and does not guarantee the reliability of transmitted data packets.
Network layer
IP (Internet Protocol, Internet Protocol): Encapsulate the data at the transport layer into packets and complete the forwarding from the source site to the destination site, providing connectionless and unreliable services.
IGMP (Internet Group Management Protocol, Internet Group Management Protocol): A protocol responsible for managing IP multicast members. It is used to establish and maintain multicast group membership between an IP host and its directly adjacent multicast routers.
ICMP (Internet Control Message Protocol, Internet Control Message Protocol): Based on the IP protocol, control messages are sent in the network to provide feedback on various problems that may occur in the communication environment. Through this information, managers can diagnose the problems and take appropriate measures to solve them.
data link layer
PPP (Point-to-Point Protocol, Point-to-Point Protocol): A point-to-point mode data link layer protocol, mostly used in wide area networks.
Ethernet (Ethernet Protocol): A multi-access broadcast data link layer protocol, which is currently the most widely used LAN technology.
PPPoE (Point-to-Point Protocol over Ethernet, PPP protocol over Ethernet): PPPoE provides the function of connecting multiple hosts in a network to a remote access concentrator through a simple bridge access device (access device). Common applications include home broadband dial-up Internet access.
TCP/IP five-layer model
5 Application layer: equivalent to the functions of layer 5~7 in the OSI model; data
4 transport layer segments
3 network layer packets
2 data link layer frame
1 physical layer bit stream
Data sending: the process of encapsulating the data generated by the application layer layer by layer from top to bottom based on the TCP/IP five-layer model, and then sending it out
Data reception: After the network device receives the data, it performs the process of decapsulating layer by layer based on the TCP/IP five-layer model until it sees the application layer data
Data forwarding: the data is sent from the sending end to the receiving end, and the passing network equipment is responsible for forwarding the data
The router works at the network layer and forwards according to the network layer header;
The switch works at the data link layer and forwards according to the header of the data link layer;
MAC address (physical address)
Function: uniquely identify a network card (unique globally)
Features: The equipment is inherent and cannot be changed later;
Format: hexadecimal;
Size: 48bit
Classification: Classification according to communication type:
Unicast mac: used to uniquely identify a network card to achieve 1-to-1 communication, the 8th bit of the binary angle is fixed at 0;
Multicast mac: used to identify a group of devices and realize one-to-many communication, the 8th bit of the binary angle is fixed at 1;
Broadcast mac: used to implement 1-to-all communication, the mac address is F (hexadecimal angle), and it is all 1 in binary angle
Network layer header (ip header)
Encapsulation protocol: ip protocol
Key content:
Version: v4, v6, default is v4
TOS: Class of service, used to classify traffic and realize QOS;
Fragmentation fields: fragmentation id, flags, offset;
Fragmentation: When the size of the data that the network device needs to send exceeds the interface MTU (the maximum sending unit, the default is 1500B), the data needs to be divided into several small data and sent out, and the receiving end reassembles after receiving it;
Fragment id: used to mark whether a piece of data is fragmented data, if so, the fragment id is not 0;
flags: More segments are used to identify whether the fragment is the last fragment, if not, it is 1, if it is, it is 0, when the receiving end receives a fragment with the flags bit of 0, it can start reassembly;
Offset: It is used to identify the reassembly sequence of the fragments, in case the fragments are in the process of transmission, the sequence is interchanged, resulting in disordered data packets.
eg:
4500B data, fragment 1 1500B
2 1500B
3 1500B
Offset: 0 1499 Shard 1
1500 2999 Shard 2
3000 4499 Shard 2
TTL (time to live)
Range: 0~255, the initial value is 255;
Function: 1. Identify how many layer-3 devices (routers, layer-3 switches, firewalls) the data packet has passed through;
2. Prevent unlimited forwarding of data packets,
Principle: When a data packet is sent, the TTL is 255, and it will be decremented by 1 every time it is forwarded by a layer-3 device
When the TTL in the data packet is reduced to 0, it loses the qualification to be sent;
agreement number
Role: Used to identify the upper layer protocol type,
It can be realized that when the device processes the network layer header, it knows the type of the upper layer protocol, and calls the corresponding protocol in advance to process the data:
TCP--6
UDP--17
sip: the ip address of the sender
dip: the IP address of the receiving end