Encapsulating IP Using SCSI

Mr. Elliston is working on a protocol for using SCSI devices to network Linux clusters in order to transfer data at high speeds.
Design

At the beginning of the project, I specified some overall goals. These goals have had a major impact on the scope of my “IP over SCSI” project. Some people found items worthy of criticism—and on occasion, they were right. The main thing is to realize that some of the issues raised just didn't fit the scope of the current project. They will be addressed in a later implementation. The goals I set were:

  • Take a purist's approach and develop a means of carrying IP datagrams across a SCSI bus. This means that the limitations of SCSI such as the number of addressable stations would have to be accepted and that larger networks would need to be constructed using conventional strategies such as situating IP gateways between these small SCSI networks. This has an interesting consequence which will be discussed later.

  • Develop a protocol that is simple to specify and easy to implement.

  • Implement the protocol within the Linux kernel as a modular network interface (in the sense that it can be loaded and unloaded using the kernel module tools). My reasons for using Linux are fairly obvious: PC-based SCSI adapters are much more readily available than SCSI adapters for any other system, and the Linux kernel source code is freely available to study and modify. Furthermore, most of the kernel developers are happy to correspond via e-mail to explain chunks of source code or areas where documentation is lacking.

  • Implement the network interface in such a way that it would operate correctly regardless of the model of a SCSI host adapter. This may introduce further performance penalties, but is obviously desirable for most applications.

Implementation

Given these design goals, I developed a network driver which had the following attributes:

  • The Linux SCSI mid-layer was utilized to satisfy the requirement of interfacing to host adapters regardless of manufacturer. This undoubtedly raises some performance issues, but I have not yet identified them. The SCSI mid-layer does not acknowledge the requirement to initialize a host adapter into target mode, which means, unfortunately, that every Linux low-level SCSI device driver will require modification if it is indeed to be capable of ever supporting target mode or supporting IP over SCSI. I have modified the Adaptec 1522 device driver in this way, but there appears to be a lot of work involved in modifying all of the Linux SCSI drivers.

  • IP datagrams can be transmitted from any SCSI host adapter to any other host adapter. These adapters may potentially be in the same host or, more likely, in independent hosts.

Applications

When I designed IP over SCSI, my intentions were to permit a number of closely situated machines running Linux to communicate using their existing base of software applications without modification, but at much higher speeds. This has minimal value, however, as networks such as Ethernet seem to serve most people's needs.

Other applications, which have not yet been fully exploited, could benefit a great deal from high-speed interconnectivity between hosts. I was recently a witness to a demonstration of the PVM (parallel virtual machine) manager running a massive computation on 31 Pentium-based Linux machines, and we observed that the bottleneck was the network used to transmit units of “work” and the subsequent results between the machines.

I, therefore, see that IP over SCSI has a number of immediate applications:

  • A high-speed networking facility for general-purpose applications (e.g., using it as a dedicated network for file sharing with NFS in a research laboratory), while Ethernet could be used for all other applications.

  • A means of connecting existing, closely situated machines for high-speed applications such as FTP mirroring or Web search engines.

  • Clustering and coarse-grained hypercubes constructed using the inter-networking concept: small networks of hosts connected via SCSI and interconnected to all other such networks via one or more SCSI interconnections. Here, each multi-homed host, equipped with multiple SCSI adapters, acts as an IP gateway between the connected networks. Structures such as these are conceivable:

.----+---+---+---+---+---.
     |   |   |   |   |
     |   B   C   D   E
     A
     |   F   G   H   I
     |   |   |   |   |
.----+---+---+---+---+---.

Here, hosts [B-E] can communicate with hosts [F-I], despite the fact that a SCSI-1 bus, for example, is unable to support a total of nine hosts.

Getting more creative:

A---B---C---D---E---.
|
F---G---H---I---J---.
|
K---L---M---N---O---.
|
P---Q---R---S---T---.
|
U---V---W---X---Y---.

This arrangement can naturally be extended to three dimensions by, at the bare minimum, adding a third SCSI interface to the gateway hosts {A,F,K,P,U}.

______________________

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