Technical Principles of layer-4 switches

The technical principles of layer-4 switches are generally divided into two categories: WAN switches and LAN switches. The features and usage of these two types of switches are described below. The fourth layer of the OSI model is the transport layer. The main task of the transport layer is to take charge of end-to-end communication tasks, layer-4 switch technology can judge and analyze the target port based on the protocol port information of the data frame. In layer-4, the TCP and UDP headers contain portnumbers, which uniquely differentiate which application protocols (such as HTTP and FTP) are contained in each packet ). The endpoint system uses this information to distinguish packet data, especially the port number, so that a computer system at the receiving end can determine the type of the IP packet it receives and hand it over to appropriate high-level software. A combination of port numbers and device IP addresses is usually called "socket )". The port numbers between 1 and 255 are retained. They are called "well-known" ports, that is, these ports are the same in all TCP/IP protocol stack implementations of the host. In addition to the "familiar" port. Standard UNIX services are allocated between port 256 and port 1024. custom applications generally allocate port numbers over port 1024. You can find the latest list of allocated port Numbers on RFc1700 "Assigned Numbers. The additional information provided by the TCP/UDP port number can be used by the network switch, which is the basis for layer-3 switching. With the development of the communication industry and the advancement of information technology in the national economy, the Ethernet switch market has steadily increased. Ethernet features high performance, high flexibility, relatively simple, and easy to implement. Therefore, Ethernet technology has become the most important LAN networking technology today, and Ethernet switches have become the most popular switches. In a broad sense, layer-4 switches are divided into two types: WAN switches and LAN switches. WAN switches are mainly used in the telecom field to provide basic communication platforms. LAN switches are used in local networks to connect terminal devices, such as PCs and network printers. According to the current complex network architecture, network switches are divided into access layer switches, aggregation layer switches, and core layer switches. Among them, all the core layer switches adopt the modular chassis design, and have basically designed the 1000Base-T module. The ethernet switch that supports 1000Base-T in the access layer is basically a fixed-port switch. It uses 10/100 M Port as the main port and provides a 1000Base-T uplink port in a fixed port or expansion slot mode. The 1000Base-T switch of the aggregation layer has two types of designs: the Chassis type and the fixed port type. It can provide multiple 1000Base-T ports, and generally other ports such as 1000Base-X. The access layer and aggregation layer switches form a complete small and medium LAN Solution. From the perspective of transmission media and transmission speed, LAN switches can be divided into Ethernet switches, Fast Ethernet switches, Gigabit Ethernet switches, FDDI switches, ATM switches, and wildcard ring switches, these switches are applicable to Ethernet, fast Ethernet, FDDI, ATM, and ring-based network. Enterprise-level switches, department-level switches, and working group switches are available for large-scale applications. The standards of various manufacturers are not completely consistent. Generally, enterprise-level switches are rack-mounted, and department-level switches can be rack-mounted or fixed-configuration switches. The working group-level switches are generally fixed configurations and have simple functions. On the other hand, from the perspective of application scale, enterprise-level switches support large enterprise applications with more than 500 information points as enterprise-level switches, and department-level switches support Switches of medium enterprises with less than 300 information points, switches that support less than 100 information points are working group-level switches. According to the architecture features, LAN switches are also divided into three types: rack-mounted, fixed configuration with expansion slot, and fixed configuration without expansion slot. A rack-mounted switch is a slot-type switch that provides excellent scalability and supports different network types, such as Ethernet, fast Ethernet, Gigabit Ethernet, ATM, licensing ring, and FDDI, however, the price is relatively high. Many high-end switches use a rack-mounted structure. A fixed-configuration switch with expansion slot is a switch with a fixed port and a small number of expansion slots. This type of switch supports a fixed-port network, you can also expand other network types to support other types of networks. The prices of these vswitches are centered. A fixed configuration switch without expansion slot only supports one type of network (generally Ethernet) and can be used in LAN in a small enterprise or office environment. It is the cheapest and most widely used. According to the layer-7 network model of OSI, switches can be divided into layer-2 switches, layer-3 switches, and layer-4 switches until Layer-7 switches. Layer 2 Switches Based on MAC addresses are the most common and used for the network access layer and aggregation layer. Layer-3 switches based on IP addresses and protocols are widely used in the core layer of the network, and a small number of switches are also used in the aggregation layer. Some layer-3 switches also have layer-4 switching function. You can determine the target port based on the protocol port information of the data frame. A layer-4 or above vswitch is called a Content-Type Switch and is mainly used in Internet data centers. Based on the manageability of vswitches, layer-4 vswitches can be divided into manageable vswitches and unmanageable vswitches. The difference between these vswitches is that they support network management protocols such as SNMP and RMON. Manageable switches Facilitate network monitoring and traffic analysis. However, the cost is relatively high. The management switch should be selected for large and medium-sized networks at the aggregation layer. The access layer depends on application requirements, and the core layer switches are all managed switches. Based on whether a switch can be stacked, the switch can be divided into two types: stackable layer-4 switch and non-stackable switch. A major purpose of stack technology is to increase the port density. Layer-4 switch technology analysis layer-2 switch and layer-3 Switch are both end-to-end exchange processes based on port addresses, although this vswitch Technology Based on MAC addresses and IP addresses, it can greatly improve the data transmission rate between nodes, but cannot determine or dynamically limit the port exchange process and data traffic according to the port host's requirements, that is, there is a lack of layer-4 intelligent application exchange requirements. The layer-4 switch not only performs end-to-end switching, but also determines or limits the exchange traffic based on the Application Characteristics of the port host. To put it simply, the layer-4 switch is based on the packet exchange process at the transport layer. It is a new type of LAN switch based on the Application exchange requirements at the TCP/IP application layer. The layer-4 switch supports all protocols below the layer-4 of TCP/UDP. It can identify the packet header length of at least 80 bytes and distinguish the application type of data packets based on the TCP/UDP port number, in this way, access control at the application layer and service quality assurance are achieved. Therefore, a layer-4 switch is not so much a hardware network device as a software network management system. That is to say, the layer-4 switch is a kind of network management switching equipment that focuses on software technology and supplemented by hardware technology. It is worth noting that some people still have some vague concepts to varying degrees, the layer-4 switch adds the ability to identify the layer-4 protocol port on the layer-3 switch, and only adds some value-added software on the layer-3 switch, therefore, it does not work on the transmission layer, but still performs the exchange operation on the third layer. It is only more sensitive to the third layer exchange. It denies the key technology and role of the fourth layer exchange from the root. We know that the layer-2 802.1p field of the data packet or the layer-3 IPToS field can be used to distinguish the priority of the data packet itself. We say that the layer-4 switch is based on the layer-4 Data Packet Exchange, this means that it can analyze the data packet application type based on the layer-4 TCP/UDP port number, that is, the layer-4 switch not only has all the switching functions and performance of the layer-3 switch, it also supports smart functions that are impossible for layer-3 switches to control network traffic and service quality. The layer-4 switch supports important technologies. As described above, layer-2 switching devices rely on the MAC address and VLAN tag information of the 802.1Q protocol to complete the link layer switching process, layer-3 switching/routing is to use IP address information for network path selection to complete the switching process, the layer-4 switching device uses the packet header information of the transport layer to help information exchange and transmission. That is to say, the specific content described in the switch information of the fourth layer is essentially a protocol or process contained in each IP packet, such as HTTP for WEB transmission and FTP for file transmission, telnet for terminal communication, SSL for secure communication, and other protocols. In this way, in an IP network, the layer-4 Exchange Protocol is actually TCP (for connection-based conversations, such as FTP) and UDP (for connectionless communication, for example, SNMP or SMTP. Because the packet headers of TCP and UDP packets not only include the domain "port number", it also specifies the type of network data of the packets being transmitted, using this information (Port Number) related to a specific application, you can complete a large number of quality services related to network data and information transmission and exchange, because they are the main technology widely used by layer-4 switches. Layer-2 switches perform end-to-end data exchange based on the MAC address table of The Link Layer through routes. layer-3 switches perform end-to-end Switching Based on IP addresses. layer-4 switches can not only complete end-to-end switching, based on the application characteristics of the port host, it can determine or limit its exchange traffic. Therefore, the industry has a "Application Switch" for the layer-4 switch. Today, with the rapid development of network exchange, 4-layer switches are worth buying.