RFC 2143 (rfc2143) - Page 2 of 5
Encapsulating IP with the Small Computer System Interface
Alternative Format: Original Text Document
RFC 2143 Encapsulating IP with the SCSI May 1997 This document outlines a protocol for connecting hosts running the TCP/IP protocol suite over a Small Computer System Interface (SCSI) bus. Despite the limitation in the furthest distance between hosts, SCSI permits close clusters of workstations to communicate between each other at speeds approaching 360 megabits per second. The proposed introduction of newer SCSI implementations such as serial SCSI will bring much faster communication at greater distances. 2. Background to the Small Computer System Interface (SCSI) SCSI defines a physical and data link protocol for connecting peripherals to hosts. Devices connect autonomously to a bus and send synchronous or asynchronous messages to other devices. Devices are identified by a numeric identifier (ID). For the original SCSI protocol, devices are given a user-selectable SCSI ID between 0 and 7. Wide SCSI specifies a range of SCSI IDs between 0 and 15. The most typical SCSI configuration comprises of a host adapter and one or more SCSI- capable peripherals responding to asynchronous messages from the host adapter. The most critical aspect of the protocol with respect to its use as a data link for the Internet protocols is that a SCSI device must act as an "initiator" (generator of SCSI commands/requests) or a "target" (a device which responds to SCSI commands from the initiator). This model is correct for a master/slave relationship between host adapter and devices. The only time an initiator receives a message addressed to it is in response to a command issued by it in the past and a target device always generates a response to every command it receives. Clearly this is unsuitable for the peer-to-peer model required for multiple host adapters to asynchronously send SCSI commands to one another without surplus bus traffic. Furthermore, some host adapters may refuse to accept a message from another adapter as it expects to only initiate SCSI commands. This restriction to the protocol requires that SCSI adapters used for IP encapsulation support what is known as "target mode", with software device driver support to pass these messages up to higher layer modules for processing. Elliston Experimental
