Store and Forward is one of the most widely used technologies in the computer network field. The controller of the Ethernet switch first caches the data packets coming from the input port, checks whether the data packets are correct, and filters out the conflict packet errors.After confirming that the packet is correct, get the destination address, find the output port address you want to send through the lookup table, and then send the packet.Because of this, the store and forward mode has a large delay in data processing, which is its disadvantage. However, it can detect errors in packets entering the switch, and can support the exchange between input/output ports at different speeds, which can effectively improve network performance.Its other advantage is that this switching mode supports the conversion between different speed ports, and maintains the cooperation between high-speed ports and low-speed ports.The implementation method is to store 10Mbps low-speed packets and forward them to the port at 100Mbps.
Store and forward is a traditional forwarding method, which is the earliest used forwarding method.The figure shows the structure of a data frame. Data enters the switch from left to right in the figure, that is, from the "Preamble" starting field. First, the switch starts the receiving process, starts receiving frames, andFrom the "Preamble" field to the final CRC, when the complete frame is received, the received packet is put into the cache, and then the switch starts the forwarding process. The forwarding strategy is determined according to the DMAC shown in the received frame, that is, the target MAC address. If it exists in the MAC address table, it is forwarded to the corresponding port. If it does not exist, it is sent to all ports.
Store and forward procedures
Store and forward characteristics
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(1) Error checking function
CRC is used to verify the previous data to prevent errors.Since the store and forward process starts only after the whole frame is received, error verification can be performed when the CRC field is received. The switch calculates the CRC of the received data and compares the calculated value with the value of the received CRC field. If they are the same, the data is not damaged. If they are different, the data is damaged.
(2) Auto Cache
(3) Policy function
That is, the function of ACL access control list, which controls data through policies,The control layers involved in ACLs range from Layer 2 to Layer 7 of OSI. Since store and forward store the entire frame, it is conceivable that if the switch has the ability to process multi-layer data, it can execute ACLs. After all, the target referenced by ACLs already exists in the received frame.
Store forward exchange mode
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Storage and forwarding technology can be divided into message switching technology and packet switching technology.
(1) Message exchange technology
When the message exchange technology is used in the case that the two communication parties are adjacent contacts, the source directly delivers the message to the destination.
When the communication parties of the message exchange technology are not in the same physical network, the prepared message will be transmitted to the receiver through the intermediate node through a certain routing mechanism.At this point, the intermediate node is no longer just a connection, but also has the ability to store and process data.
In the message exchange system, due to the large amount of data to be exchanged and processed at one time, it is necessary to put forward higher requirements for the storage and processing capacity of communication processing, which greatly increases the communication cost.Moreover, this kind of system is easy to cause blockage, and its flexibility and reliability will decline.At the same time, there is a great possibility of errors in the data exchange process, and it is difficult to detect.
(2) Packet switching technology
Packet switching technology is a data exchange mode that splits the payload data in a message, adds certain control information and message number, forms a packet, and then transmits the packet as a unit.
The intermediate node in this switching system is no longer just a simple connection link, but needs to judge whether the packets are right or wrong and make routing choices.
In order to avoid network congestion in packet switching technology, a certain flow control mechanism is adopted.Packet grouping and reassembly of messages will take a lot of time, but it only occurs at the receiving and sending places, and packet processing efficiency is high, so the overall efficiency is also high.However, because some communication control data needs to be added in the communication process, it will cause a lot of repeated additional overhead;In the transmission process, packets may be lost, broken, out of order, etc. Relevant personnel need to take certain strategies to solve these problems.
Packet switching technology can be divided into datagram switching technology and virtual circuit switching technology according to different implementation mechanisms.
① Datagram exchange technology
The datagram switching technology is similar to the message switching technology. The difference is that the former transmits packets every time, while the latter transmits the entire message.
This is a "connectionless" data exchange technology. It has a certain routing algorithm to dynamically select the transmission path of datagrams in the process of data transmission, and this is an unordered transmission.
② Virtual Circuit Switching Technology
Virtual circuit switching technology is different from datagram switching technology. It is a connection oriented switching technology.The reason why it is called "virtual circuit" is that in the process of data exchange, special dynamic logic circuits will be established between the two communication parties.Different from the physical circuit in the circuit switching system, this logic circuit is not used exclusively, but is similar to the channel multiplexing technology, which multiplexes the intermediate nodes for data exchange.However, since the time of each data exchange is very short, we can regard the transmission of all packets as simultaneous.Each successful virtual circuit needs to assign an identification number and code it into the group to avoid confusion.The packets are transmitted orderly through the established dedicated channel, so each packet no longer contains control data, reducing auxiliary overhead.
Virtual circuit switching technology is a kind of switching technology with strong reliability and high system efficiency.It is to establish a dedicated logical connection in the working process, and each node can establish multiple virtual circuit connections with any node.Because there is no auxiliary information, the system has little additional overhead, and the intermediate node has a certain error detection mechanism.
Packets in datagram switching technology and virtual circuit switching technology need to choose a communication path according to a certain routing algorithm, but each packet in the former needs to find a route separately, while all packets in a communication process in virtual circuit switching technology only need to select a route once.Obviously, routing calculation based on datagram mode is more frequent than that based on virtual circuit mode.[1]
Technical Principles
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Store and forwardtechnical requirementSwitchAfter receiving all data packets, decide how to forward them. Direct forwarding starts before the switch receives the whole frame, which can effectively reduce the switching delay.However, if the switch has started data forwarding before it has fully received and checked the correctness of the data packet, then in the environment of low communication quality, the switch will forward all the complete data packets and wrong data packets, which actually brings a lot of garbage communication packets to the entire switching network.Therefore, the pass through forwarding technology is suitable for the network environment with better network link quality and fewer error packets.
Advantages and disadvantages
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Advantages: Very reliable, because it stores the data frame of the input port in the switch cache first, and then performs CRC check.If an error is detected in the frame, discard the frame; otherwise, get the destination address of the frame, obtain the output port by looking up the MAC address table, and then forward the data frame;And the store and forward switching mode also supports the conversion between ports with different speeds, facilitating the protocol work between high-speed ports and low-speed ports.
Disadvantages: According to some characteristics of store and forward, we can see that the features provided by store and forward are very attractive and perfect, but the fatal weakness of store and forward is speed.The data processing delay of store, forward and exchange mode is large. The main reason is that the input and output terminals have to go through serial parallel conversion. This tedious process will affect the response speed, which also results in the phenomenon of high delay.
