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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
 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.

Elliston Experimental [Page 1] 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

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
 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 [Page 2] RFC 2143 Encapsulating IP with the SCSI May 1997

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
    | 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

Elliston Experimental [Page 3] RFC 2143 Encapsulating IP with the SCSI May 1997

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.

Elliston Experimental [Page 4] RFC 2143 Encapsulating IP with the SCSI May 1997

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

Elliston Experimental [Page 5]

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