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+Network Working Group                                        B. Elliston
+Request for Comments: 2143                             Compucat Research
+Category: Experimental                                          May 1997
+
+
+       Encapsulating IP with the Small Computer System Interface
+
+Status of this Memo
+
+   This memo defines an Experimental Protocol for the Internet
+   community.  This memo does not specify an Internet standard of any
+   kind.  Discussion and suggestions for improvement are requested.
+   Distribution of this memo is unlimited.
+
+Table of Contents
+
+   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . .  1
+   2.   Brief background to the Small Computer System Interface  .  2
+   3.   Link Encapsulation . . . . . . . . . . . . . . . . . . . .  3
+   4.   An Address Resolution Protocol . . . . . . . . . . . . . .  4
+   5.   Scalability  . . . . . . . . . . . . . . . . . . . . . . .  4
+   6.   Possible applications  . . . . . . . . . . . . . . . . . .  5
+   7.   Security considerations  . . . . . . . . . . . . . . . . .  5
+   8.   References . . . . . . . . . . . . . . . . . . . . . . . .  5
+   9.   Author's Address . . . . . . . . . . . . . . . . . . . . .  5
+
+1.  Introduction
+
+   As the capacity of local area networks increases to meet the demands
+   of high volume application data, there is a class of network
+   intensive problems which may be applied to small clusters of
+   workstations with high bandwidth interconnection.
+
+   A general observation of networks is that the bit rate of the data
+   path typically decreases as the distance between hosts increases.  It
+   is common to see regional networks connected at a rate of 64Kbps and
+   office networks connected at 100Mbps, but the inverse is far less
+   common.
+
+   The same is true of peripheral and memory interconnection.  Memory
+   close to a CPU's core may run at speeds equivalent to a gigabit
+   network.  More importantly, devices such as disks may connect a
+   number of metres away from its host at speeds well in excess of
+   current local area network capacity.
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+Elliston                      Experimental                      [Page 1]
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+RFC 2143             Encapsulating IP with the SCSI             May 1997
+
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+   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.
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+Elliston                      Experimental                      [Page 2]
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+RFC 2143             Encapsulating IP with the SCSI             May 1997
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+3.  Link Encapsulation
+
+   The ANSI SCSI standard defines classes of peripherals which may be
+   interconnected with the SCSI protocol.  One of these is the class of
+   "communication devices" [1].
+
+   The standard defines three message types capable of carrying a
+   general-purpose payload across communication devices.  Each of these
+   are known as the "SEND MESSAGE" message type, but the size and and
+   structure of the message header differs amongst them.  The three
+   forms of message header are six (6), ten (10) and twelve (12) bytes
+   long.
+
+   It was decided that the ten byte header offers the greatest
+   flexibility for encapsulating version 4 IP datagrams for the
+   following reasons:
+
+      a. The transfer length field is 16 bits in size which is perfectly
+         matched to the datagram length field in IP version 4.
+         Implementations of IP will run efficiently as datagrams will
+         never be fragmented over SCSI networks.
+
+      b. The SCSI "stream select" field, which was designed to permit
+         a device to specify the stream of data to which a block
+         belongs, may be used to encode the payload type (in a similar
+         manner to the Ethernet frame type field).  For consistency, the
+         lowest four bits of the "stream select" field should match the
+         set of values assigned by the IEEE for Ethernet protocol types.
+
+   Encapsulating an IP datagram within a SCSI message is
+   straightforward:
+
+      +------------------+-----------------------------------+
+      | SCSI header      | IP datagram                       |
+      +------------------+-----------------------------------+
+
+   The fields of the SCSI header should be completed as follows:
+
+        Byte  0:    0x2A (SEND_MESSAGE(10) opcode)
+        Byte  1:    Logical unit number encoded into top 3 bits | 0x00
+        Byte  2:    0x00
+        Byte  3:    0x00
+        Byte  4:    0x00
+        Byte  5:    Protocol type encoded into lowest 4 bits | 0x00
+        Byte  6:    0x00
+        Bytes 7/8:  IP datagram length, big endian representation
+        Byte  9:    0x00
+
+
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+Elliston                      Experimental                      [Page 3]
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+RFC 2143             Encapsulating IP with the SCSI             May 1997
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+4.  An Address Resolution Protocol
+
+   When IP decides that the next hop for a datagram will be onto a SCSI
+   network supported by a SCSI IP network interface implementation, it
+   is necessary to acquire a data link address to deliver the datagram.
+
+   Network interfaces such as Ethernet have well-known methods for
+   acquiring the media address for an Internet protocol address, the
+   most common being the Address Resolution Protocol (ARP).  In existing
+   implementations, the forwarding host "yells" using a broadcast media
+   address and expects the named host to respond.
+
+   The SCSI protocol does not provide a broadcast data link address.  An
+   acceptable solution to the address resolution problem for a SCSI
+   network is to simulate a broadcast by performing a series of round-
+   robin transmissions to each target.  Depending on the SCSI protocol
+   being used, this would require upward of seven independent bus
+   accesses.  This is not grossly inefficient, however, if combined with
+   an effective ARP caching policy.  A further possible optimisation is
+   negative ARP caching, whereby incomplete ARP bindings are not queried
+   for some period in the future.
+
+5.  Scalability
+
+   While the utility of a network architecture based around a bus
+   network which can span less than ten metres and support only eight
+   hosts may be questionable, the flexibility of IP and in particular,
+   IP routing, improves the scalability of this architecture.
+
+   Consider a network of eight hosts connected to a SCSI bus in which
+   each host acts as a multi-homed host with a second host adapter
+   connecting another seven hosts to it.  When configured with IP packet
+   routing capability, each of the 64 total hosts may communicate with
+   one another at high speed in a packet switched manner.
+
+   Depending on the I/O bus capabilities of certain workstations, it may
+   also be possible to configure a multi-homed host with a greater
+   number of SCSI host adapters, permitting centralised star
+   configurations to be constructed.
+
+   It should be apparent that for little expense, massively parallel
+   virtual machines can be built based upon the IP protocol running over
+   the high-bandwidth SCSI protocol.
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+Elliston                      Experimental                      [Page 4]
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+RFC 2143             Encapsulating IP with the SCSI             May 1997
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+6.  Possible Applications
+
+   Research has been made into the capability of "networks of
+   workstations", and their performance compared to supercomputers.  An
+   observation that has been made thus far is that bottlenecks exist in
+   the channels by which executable code is transported between hosts
+   for execution.  A very high-speed network architecture based around
+   the Internet protocol would permit a seamless transition of existing
+   application software to a high-bandwidth environment.
+
+   Other applications that have been considered are server clusters for
+   fault-tolerant NFS, World-Wide Web and database services.
+
+7.  Security Considerations
+
+   Transmitting IP datagrams across a SCSI bus raises similar security
+   issues to other local area networking architectures.  The scale of
+   security problems relating to protocols above the data link layer
+   should be obvious to a reader current in Internet security.
+
+8.  References
+
+   [1]  ANSI X3T9 Technical Committee, "Small Computer System
+        Interface - 2", X3T9.2, Project 375D, Revision 10L, September
+        1993.
+
+9.  Author's Address
+
+   Ben Elliston
+   Compucat Research Pty Limited
+   Box 7305 Canberra Mail Centre
+   Canberra ACT 2610
+   Australia
+
+   Phone: +61 6 295 1331
+   Fax:   +61 6 295 1855
+   Email: ben.elliston@compucat.com.au
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+Elliston                      Experimental                      [Page 5]
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