The Cypress Link Level Protocol
Douglas E. Comer · Purdue e-Pubs (Purdue University System) · 1986
This document describes the Cypress network link-level protocol including details of packet format, framing, and character stuffing. GeneralCypress is a packet-switched network described in [CNY86].It sends data over serial lines in packets, where a packet consists of from 3 to 578 octets.Each octet contains exactly 8 bits.At the lowest level, the leased lines usually operate in bit-serial mode using synchronous modems.In most instances, computers acting as packet switches communicate using asynchronous serial interfaces and require an asynchronous-to-synchronous converter between the computer and modem.However, we assume that the low-level hardware details are hidden from the Cypress protocol software by a device driver that controls the interface device.We also assume (based on experiments with an early protoype network) that error rates on the leased lines are low.The Cypress link-level protocol has been designed around best-effort delivery and has been optimized for the transmission of IP datagrams.IP packets require only a 2-octet header in addition to the datagram being sent.There is no checksum for either the Cypress header or the data and there is as little intelligence at the link level as possible.