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RFC2143 - Encapsulating IP with the Small Computer System Interface

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

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.

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.

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

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.

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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