Technical Description of Multilayer Switching in Communication Network
The lower four layers (physical layer, data link layer, network layer, and transmission layer) of the Open System Interconnection Reference Model (OSI-RM) proposed by the International Organization for Standardization (ISO) are the communication layer, so the bottom layer can be analyzed one by one:
(1) The first floor
For the physical layer, traditional circuit switching belongs to this layer.
(2) The second floor
For the data link layer, in the traditional sense, the real exchange belongs to this layer. In this layer, a hardware-based forwarding mechanism is adopted, which can forward various data link layer protocols, including Ethernet in local area network (LAN) and high-speed token ring network (FD-DI) and wide area network (WAN). Frame Relay (FR) and Asynchronous Transfer Mode (ATM) exchanged through VC, etc., the classic LAN multi-port bridge also belongs to this layer. This layer supports simple network segmentation and can significantly improve network performance. The popularity of Layer 2 switching has brought about the development of Layer 3 and Layer 4 switching.
(3) The third floor
It is the network layer, and the main task is to find a suitable route for the packet. Traditional routers use software and a general-purpose CPU to forward datagrams, so the delay is relatively large and the forwarding speed is relatively slow, and the third-layer switching is proposed to address this problem. The so-called layer 3 switching is not just using the function of layer 3, but combining the routing of layer 3 with the switching function of layer 2 to realize the rapid grouping of the network. Correspondingly, the goal of the layer 3 switch is to have both features, and usually adopts ASIC (SupplicaTIon Specific Integrated Circuit) to solidify commonly used software functions in the hardware, forming a complete subset of routers. In the future, Layer 3 switches will have more functions and become more complete routers. For example, in addition to the forwarding function, it will also have the function of automatically dividing the data flow level and service level, and provide some form of QoS, etc., which will be another important feature of the layer 3 switch.
Since the mid-1990s, major companies in the world have conducted research on Layer 3 switching, and have put forward many different solutions and launched many products. The more influential ones are: Cisco's tag switching (TAG Swiching), Ipsilon's IP switching, Toshiba's cell switching router (CSR: Cell Switching Router) and IBM's ARIS (Aggregate Route based IP Switching), and IETF's multi-protocol label switching (MPLS: MulTI-Protocol Label Switching) and so on.
TAG Switching is a router at the edge of switching. It maps the Layer 3 address of each input frame into a simple tag, and then converts the tagged frame into ATM cells, and then maps it to VC, marking it on the network core ATM switch. Switching, the router keeps the marking information table (routing table) to find the third layer routing. Finally, the marked cells are sent to the destination router, and the destination router removes the information marks, converts the cells into frames, and sends them to the final destination. In this process, the performance of forwarding frames can be improved by exchanging tags (small data units) and performing a simple tag query only once.
IP switching technology is a technology that combines the second layer switching function and the third layer routing function. It is another type of multi-layer switching. Similar to CSR, it is an application of data flow-driven IP switching. That is to say, they can arrange the resources of the switch according to the arrival of independent service flows, and realize the exchange process by label allocation and mapping the data flow to the signaling information (IFMP and FANP) on the VC. These are carried out independently of a single IP data flow, maintaining the scalability of the Internet model and forwarding all services in a step-by-step manner at the third layer, and introducing a specific control protocol to convert IP data The flow is transferred to an end-to-end straight-through path.
ARIS is a converged path-based IP switching technology. Its basic function is to establish and maintain a switching path according to the destination of the converged IP data stream. ARIS uses the information of the routing protocol and connects the information with the exchange path from the network entrance to the exit, thus achieving the above function. Specifically: ARIS is a control protocol that runs on the integrated switch / router device network. Because ISR is a layer 2 switch that supports IP routing (such as ATM), ARIS can use the information in the routing protocol (by OSPF , BGP) maps IP packets to the layer 2 switching path, and becomes the first IP switching mechanism that introduces a convergence switching path through ISR equipment and the network. Between multiple different sources and the same destination, ARIS can establish a multi-point to point switching path, thereby reducing the consumption of switch resources.
For Layer 3 switching technology, because each company uses its own standards, it is difficult to connect with each other. Therefore, the IETF is developing standards for MPLS to unify the standards of Layer 3 switching technologies of various companies.
MPLS is a technology that perfectly combines Layer 2 switching functions with Layer 3 routing functions. Based on IP routing and control protocols, MPLS provides connection-oriented switching. It is called multi-protocol because MPLS can support various protocols at the network layer, such as IPv4, IPv6, IPX, CLNP, etc. At the same time, MPLS also supports various protocols at the second layer, supporting any entity that can be at the network layer The second layer of media that transports network layer packets is not specific to a certain link technology. MPLS completes the transmission of information in a very concise manner, and is similar to TAG and ATM exchange. It introduces a fixed-length short label (Label) as the basis for data forwarding in the MPLS network.
The MPLS network is composed of a label switching router (LSR) and a label edge router (LER), and distributes labels between LER and LSR through a label distribution protocol (LDP). That is to say, when each MPLS device runs a layer 3 routing protocol (such as OSPF and BGP), it will use the label distribution protocol according to the calculated route to allocate labels to the information flow and establish a data transmission channel, that is, a label switching path (LSP), so when service data (IP packets, frame relay frames, or ATM cells) reach the LER at the edge of the MPLS network, the LER will first correspond the data stream to a fixed-length label according to a certain principle. The corresponding principle not only considers the destination address information of the data flow, but also considers the information about QoS, and then labels the data. In this way, in the future forwarding process, the LSR simply forwards the data in the form of table lookup and exchange based on the Label carried by the data stream. When the data reaches the exit LER, the LER removes the label and restores the original data stream. Way to transfer data.
In short, multiple services, traffic classification, traffic planning, display routing, different levels of service and IP QoS (based on ATM), etc., are the key features of MPLS. The goal is to solve the problem of network expansion and provide multiple network services on the same infrastructure. With the emergence of MPLS, a virtual private network (VPN) that supports real-time communication on the shared Internet will also become a reality in a short time.
(4) The fourth floor
Layer 4 switching allows communication data to be prioritized according to applications, so managers can limit certain applications to use a certain amount of bandwidth for important application communications. Essentially, Layer 4 switching provides the method proposed by the Open Systems Consortium (COS) in the network.
It is the transport layer, which expands the exchange of the second and third layers and supports fine-grained network adjustment and prioritization of communication flows. The fourth layer of information used, such as the port number of the TCP / UDP application, is used to To further determine the forwarding method of traffic.
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