RFC 2143 (rfc2143) - Page 2 of 5

Encapsulating IP with the Small Computer System Interface

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