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A Technical History of the ARPANET -
A Technical Tour

IMP-to-IMP

Communication

The Data-Link and Network layers of the OSI Seven Layer Model, are somewhat overlapped in the ARPANET architecture.  The IMP-IMP layer in coordination with the IMP-Host layer provide the Host-Host layer with a virtual circuit, while the IMP-IMP layer works with datagrams (Datagram service is available to the Hosts but it is rarely used).  While the IMPs do communicate with each other via packets (fragmented from the messages from the Host-IMP layer), they encapsulate the packets into frames before sending.  A frame is created by sending a SYN (SYNchronize), a DLE (Data Line Escape), an STX (Start of TeXt), the packet, a DLE, an ETX (End of TeXt), and finally an SYN.  The IMP will continue to send SYNs until it is ready to send another frame. More communication issues are taken care of in the packet. 

(See Flow Control for information on the sliding window protocol, and PDU for more information on the packet)

Figure 1: Frame format (Control, routing, and other special packets
will have a different format)

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