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Bolt Beranek and Newman Inc. 2. IMP Processes [ u [ [. ~. ' . [ [ This section considers the algorithms that the IMP uses in performing its functions as a message-switching center and interface between Host <strong>com</strong>puters. Figure 2-l helps summarize some of the terms we will be using. The Host sends the IMP a message up to 8095 bits long. The message has a leader specifying its destination. The source IMP accepts the message in packets up to 1008 bits long. Each packet has a header to allow for the transmission from IMP to IMP. Figure 2-1 demonstrates how message l is transferred from IMP to IMP in these packets, numbered 1-l, l-2, and l-3. When a packet is successfully received at each IMP, an acknowledge or ack is sent back to the previous IMP. Inter-IMP acks are shown returning for each packet. Finally the message arrives at the destination IMP where it is reassembled: that is, the packets are re<strong>com</strong>bined into the original message. The message is sent to the destination Host and when it has been accepted, a Ready For Next Message (RFNM) is sent back to the source Host. A RFNM is a unique, one-packet message and it is acknowledged. Several points are worth noting. First, acks are not actually separate transmissions, but are piggy-backed in returning packets to cut down on overhead. Next, packets on the inter-IMP lines are checksummed in the modem interface hardware and the IMP employs a positive acknowledgment retransmission scheme. That is, if a packet is in error, it is not acknowledged. Then it is retransmitted until an acknowledge is received. Further, because of dynamic routing, an IMP may send the several packets of a L [ 2-1 2/73
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Page 55 and 56: DATA STRUCTURES LOCAL DATA TA, TX,
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INITIALIZATION Initialization is ac
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IMP ROUTING MESSAGE FORMAT ... ~ST
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