IP over SCSI
LLamers at aol.com
LLamers at aol.com
Tue Oct 22 10:38:05 PDT 1996
* From the SCSI Reflector (scsi at symbios.com), posted by:
* LLamers at aol.com
Subject: IP over SCSI internet-draft
From: bje at fir.canberra.edu.au (Ben Elliston)
Date: 17 Oct 96 13:00:45 GMT
Here is a draft for a proposed protocol for encapsulating IP in SCSI.
Please send any comments to the contact me via e-mail using the address
given in Section 9 (Author's Address). After making suggested changes,
I will be sending this on to the RFC editor.
-- cut here --
Network Working Group B. Elliston
Request for Comments: nnnn Compucat Research
Category: Experimental October 1996
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 . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Possible applications . . . . . . . . . . . . . . . . . . . 6
7. Security considerations . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Author's Address . . . . . . . . . . . . . . . . . . . . . . 9
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
protocol, devices are given a user-selectable SCSI ID between 0 and 7.
The most typical SCSI configuration comprises of a host adapter and one or
more SCSI-capable peripherals responding to asynchronous messages from the
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'' .
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
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: 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. Two
possible solutions to the address resolution problem for a SCSI network are:
a. Using a simple formula to compute the SCSI hardware ID
based on the host portion of the IP protocol address. A current
implementation of ARP over SCSI on the Linux operating system
derives a 3-bit SCSI ID from a (potentially) 32-bit host address
using the formula:
scsi_id = ((ipaddr & ~netmask) - 1) % 8
The operator syntax is adopted from the C programming language:
& is the bitwise AND operator
~ is the bitwise NOT operator (one's complement)
% is the modulo operator
It should be obvious from the above that the choice of SCSI ID
would be dependent on the IP protocol address of the network
interface, or vice-versa and must be statically selected.
b. Simulating a broadcast by performing round-robin transmissions
to each target. In the SCSI-1 protocol, where only eight SCSI
IDs exist, this would require only seven transmissions (the
sender need not not query itself). This is only marginally
inefficient and is even less so when combined with an effective
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
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.
 ANSI X3T9 Technical Committee, "Small Computer System
Interface - 2", X3T9.2, Project 375D, Revision 10L, September
9. Author's Address
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 at compucat.com.au
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