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Versions: 00 rfc2143                                                    

Network Working Group                                        B. Elliston
INTERNET-DRAFT                                         Compucat Research
Category: Experimental                                        March 1997

        Encapsulating IP with the Small Computer System Interface

Status of this Memo
   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

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Table of Contents

  1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . . 1
  2.   Brief background to the Small Computer System Interface  . . 1
  3.   Link Encapsulation . . . . . . . . . . . . . . . . . . . . . 1
  4.   An Address Resolution Protocol . . . . . . . . . . . . . . . 1
  5.   Scalability  . . . . . . . . . . . . . . . . . . . . . . . . 1
  6.   Possible applications  . . . . . . . . . . . . . . . . . . . 1
  7.   Security considerations  . . . . . . . . . . . . . . . . . . 1
  8.   References . . . . . . . . . . . . . . . . . . . . . . . . . 1
  9.   Author's Address . . . . . . . . . . . . . . . . . . . . . . 1

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

   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.

   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

   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.

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

   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

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

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.

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

9.  Author's Address

   Ben Elliston
   Compucat Research Pty Limited
   Box 7305 Canberra Mail Centre
   Canberra ACT 2610

   Phone: +61 6 295 1331
   Fax:   +61 6 295 1855
   Email: ben.elliston@compucat.com.au