Cut through

In computer networking, cut-through switching, also called cut-through forwarding is a method for packet switching systems, wherein the switch starts forwarding a frame (or packet) before the whole frame has been received, normally as soon as the destination address and outgoing interface is determined. Compared to store and forward, this technique reduces latency through the switch and relies on the destination devices for error handling. Pure cut-through switching is only possible when the speed of the outgoing interface is at least equal or higher than the incoming interface speed. Adaptive switching dynamically selects between cut-through and store and forward behaviors based on current network conditions. Cut-through switching is closely associated with wormhole switching. When cut-through switching is used in Ethernet the switch is not able to verify the integrity of an incoming frame before forwarding it. A cut-through switch will forward corrupted frames, whereas a store and forward switch will drop them. The technology was developed by Kalpana, the company that introduced the first Ethernet switch. The primary advantage of cut-through Ethernet switches, compared to store-and-forward Ethernet switches, is lower latency. Cut-through Ethernet switches can support an end-to-end network delay latency of about 10 microseconds. End-to-end application latencies below 3 microseconds require specialized hardware such as InfiniBand. Fragment free is a variation on cut-through switching that partially addresses this problem by assuring that collision fragments are not forwarded. Fragment free will hold the frame until the first 64 bytes are read from the source to detect a collision before forwarding. This is only useful if there is a chance of a collision on the source port. The theory here is that frames that are damaged (usually by collisions) are often shorter than the minimum valid ethernet frame size of 64 bytes. With a fragment-free buffer the first 64 bytes of each frame, updates the source MAC and port if necessary, reads the destination MAC, and forwards the frame.