Preallocation of Buffers to control Congestion

By permanently allocating buffers to each virtual circuit in each IMP, there will always be a place to store any incoming packet until it can be forwarded. First consider the case of a stop-and-wait IMP-IMP protocol. One buffer per virtual circuit per IMP is sufficient for simplex circuits, and one for each direction is sufficient for full duplex circuits. When a packet arrives, the acknowledgement is not sent back to the sending IMP until the packet has been forwarded. Thus an acknowledgment means that the receiver not only received the packet correctly, but also has a free buffer and is willing to accept another one. If the IMP-IMP protocol allows multiple outstanding packets, each IMP will have to dedicate a full window's worth of buffers to each virtual circuit to completely eliminate the possibility of congestion. Because dedicating a complete set of buffers to an idle virtual circuit is expensive, some subnets may use it only where low delay and high bandwidth are essential, for example, on virtual circuits carrying digitized speech.