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Network Working Group                                        B. EllistonRequest for Comments: 2143                             Compucat ResearchCategory: Experimental                                          May 1997Encapsulating IP with the Small Computer System InterfaceStatus 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 Contents1.   Introduction . . . . . . . . . . . . . . . . . . . . . . .1   2.   Brief background to the Small Computer System Interface  .  23.   Link Encapsulation . . . . . . . . . . . . . . . . . . . .34.   An Address Resolution Protocol . . . . . . . . . . . . . .45.   Scalability  . . . . . . . . . . . . . . . . . . . . . . .46.   Possible applications  . . . . . . . . . . . . . . . . . .57.   Security considerations  . . . . . . . . . . . . . . . . .58.   References . . . . . . . . . . . . . . . . . . . . . . . .59.   Author's Address . . . . . . . . . . . . . . . . . . . . .51.  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.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   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.Elliston                      Experimental                      [Page 2]

RFC 2143             Encapsulating IP with the SCSI             May 19973.  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:    0x00Elliston                      Experimental                      [Page 3]

RFC 2143             Encapsulating IP with the SCSI             May 19974.  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 19976.  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.auElliston                      Experimental                      [Page 5]