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Article 6563 of comp.protocols.tcp-ip:
From: martillo@cpoint.UUCP (Joachim Carlo Santos Martillo)
Subject: TCP/IP versus OSI
Message-ID: <2145@cpoint.UUCP>
Date: 15 Mar 89 12:37:56 GMT
Reply-To: martillo@cpoint.UUCP (Joachim Carlo Santos Martillo)
Organization: Clearpoint Research Corp., Hopkinton Mass.
The following is an article which I am going to submit to Data
Communications in reply to a column which William Stallings
did on me a few months ago. I think people in this forum might
be interested, and I would not mind some comments.
Round 2 in the great TCP/IP versus OSI Debate
I. INTRODUCTION
When ISO published the first proposal for the ISO reference
model in 1978, DARPA-sponsored research in packet switching
for data communications had already been progressing for
over 10 years. The NCP protocol suite, from which the X.25
packet-switching protocol suite originated, had already been
rejected as unsuitable for genuine resource-sharing computer
networks. The major architectural and protocol development
for internetting over the ARPANET was completed during the
1978-79 period. The complete conversion of DARPA-sponsored
networks to internetting occurred in January, 1983, when
DARPA required all ARPANET computers to use TCP/IP. Since
then, with an effective architecture, with working protocols
on real networks, researchers and developers within the ARPA
Internet community have been refining computer networking
and providing continually more resource sharing at lower
costs. At the same time, with no obvious architecture, with
theoretical or idealized networks and while actively
ignoring the work being done in the ARPA Internet context,
the ISO OSI standards committees were developing basic
remote terminal and file transfer protocols. The ISO OSI
protocol suite generally provides potentially much less at
much more cost than the ARPA Internet suite already
provides. No one should be surprised that many computer
networking system architects wish to debate the merits of
the OSI reference model and that many relatively pleased
business, technical and academic users of the ARPA Internet
protocol suite would like such a debate to be actively
pursued in the media.
______________________________________________________________
| |
| Background |
| |
|Since June, 1988 William Stallings and I have been engaging|
|in a guerilla debate in the reader's forum and the EOT|
|feature on the technical and economic merits of OSI versus|
|ARPANET-style networking. Enough issues have been raised to|
|require a complete article to continue the discussion. The|
|debate is of major interest because managers are now making|
|strategic decisions which will affect the development, cost|
|and functionality of corporate networks over the whole|
|world. A valid approach to the debate deals with the|
|technical, economic and logistic issues but avoids ad|
|hominem attacks. I apologize for those comments in my forum|
|letter which might be construed as personal attacks on|
|William Stallings. |
| |
|Since I have not yet published many papers and my book is|
|only 3/4s finished, I should introduce myself before I|
|refute the ideas which Stallings presented in the September|
|EOT feature. I am a system designer and implementer who is|
|a founder and Project Director at Constellation Technologies|
|which is a Boston-based start-up consulting and|
|manufacturing company specializing in increasing the|
|performance, reliability and security of standard low-level|
|communications technologies for any of the plethora of|
|computer networking environments currently available. |
| |
|I am not an "Arpanet Old Network Boy." My original|
|experience is in telephony. I have implemented Signaling|
|System 6, X.25, Q.921 and Q.931. During a one-year research|
|position at MIT, I worked on TFTP and helped develop the X|
|network transparent windowing protocol. Later I developed|
|PC/NTS which uses IEEE 802.2 Type 2 to provide PC-Prime|
|Series 50 connectivity over IEEE 802.3 (Ethernet) networks.|
|My partner Tony Bono and I have attended various IEEE and|
|CCITT standards-related committees in various official|
|capacities. |
_____________________________________________________________|
II. THE DEBATE
Part of the problem with debating is the lack of a mutually
agreeable and understood set of concepts in which to frame
the debate. I have yet to meet a communications engineer
who had a sense of what a process might be. Having taught
working software and hardware engineers at Harvard
University and AT&T and having attended the international
standards committees with many hardware, software and
communications engineers, I have observed that overall
system design concepts in computer networking need a lot
more attention and understanding than they have been
getting. Normally in the standardization process, this lack
of attention would not be serious because official standards
bodies usually simply make official already existing de
facto standards like Ethernet 2.0 which had already proven
themselves. In the case of OSI, the ISO committee, for no
obvious reasons, chose to ignore the proven ARPA Internet de
facto standard.
______________________________________________________________
| |
| Architecture, |
| Functional Specification, |
| Design Specification |
| | |
|Nowadays, we read a lot of hype about CASE, object-oriented|
|program techniques and languages designed to facilitate or|
|to ease the development of large software projects. These|
|tools generally duck the hardest and most interesting system|
|design and development problem which is the design under|
|constraint of major systems which somebody might actually|
|want to buy. The hype avoids the real issue that student|
|engineers are either simply not taught or do not learn|
|system design in university engineering programs. If|
|software engineers generally knew how to produce acceptable|
|architectures, functional specifications and design|
|specifications, the push for automatic tools would be much|
|less. In fact, the development of CASE tools for automatic|
|creation of systems architectures, functional specifications|
|and design specifications requires understanding exactly how|
|to produce proper architectures and specifications. But if|
|engineers knew how to produce good architectures and|
|specifications for software, presumably student engineers|
|would receive reasonable instruction in producing|
|architectures and specifications, and then there would be|
|much less need for automatic CASE tools to produce system|
|architectures, functional specifications or design|
|specifications. |
| |
|Just as an architectural description of a building would|
|point out that a building is Gothic or Georgian, an|
|operating system architecture might point out that the|
|operating system is multitasking, pre-emptively time-sliced|
|with kernel privileged routines running at interrupt level.|
|A system architecture would describe statically and|
|abstractly the fundamental operating system entities. In|
|Unix, the fundamental operating system entities on the user|
|side would be the process and the file. The functional|
|specification would describe the functionality to be|
|provided to the user within the constraints of the|
|architecture. A functional specification should not list the|
|function calls used in the system. The design specification|
|should specify the model by which the architecture is to be|
|implemented to provide the desired functionality. A little|
|pseudocode can be useful depending on the particular design|
|specification detail level. Data structures, which are|
|likely to change many times during implementations, should|
|not appear in the design specification. |
| |
|Ancillary documents which treat financial and project|
|management issues should be available to the development|
|team. In all cases documents must be short. Otherwise,|
|there is no assurance the all members of the development or|
|product management teams will read and fully comprehend|
|their documents. Detail and verbiage can be the enemy of|
|clarity. Good architectures and functional specifications|
|for moderately large systems like Unix generally require|
|about 10-20 pages. A good high-level design specification|
|for such a system would take about 25 pages. If the|
|documents are longer, something may be wrong. The key is|
|understanding what should not be included in such documents.|
|The ISO OSI documents generally violate all these|
|principles. |
_____________________________________________________________|
As a consequence, the ISO OSI committee and OSI boosters
have an obligation to justify their viewpoint in debate and
technical discussion with computer networking experts and
system designers. Unfortunately, the debate over the use of
OSI versus TCP/IP has so far suffered from three problems:
o a lack of systems level viewpoint,
o a lack of developer insight and
o an hostility toward critical appraisal either
technically or economically of the proposed ISO
OSI standards.
The following material is an attempt to engage in a critical
analysis of OSI on the basis of system architecture,
development principles and business economics. Note that in
the following article unattributed quotations are taken from
the itemized list which Stallings used in EOT to attempt to
summarize my position.
III. INTERNETWORKING: THE KEY SYSTEM LEVEL START POINT
The most powerful system level architectural design concept
in modern computer networking is internetworking.
Internetworking is practically absent from the OSI reference
model which concentrates on layering, which is an
implementation technique, and on the virtual connection,
which would be a feature of a proper architecture.
Internetworking is good for the same reason Unix is good.
The Unix architects and the ARPA Internet architects, after
several missteps, concluded that the most useful designs are
achieved by first choosing an effective computational or
application model for the user and then figuring out how to
implement this model on a particular set of hardware.
Without taking a position on success or failure, I have the
impression that the SNA and VMS architects by way of
contrast set out to make the most effective use of their
hardware. As a consequence both SNA and VMS are rather
inflexible systems which are often rather inconvenient for
users even though the hardware is often quite effectively
used. Of course, starting from the user computational or
application model does not preclude eventually making the
most effective use of the hardware once the desired
computational or application model has been implemented.
______________________________________________________________
| |
| Internetworking |
| |
|The internetworking approach enables system designers and|
|implementers to provide network users with a single, highly|
|available, highly reliable, easily enlarged, easily|
|modifiable, virtual network. The user does not need to know|
|that this single virtual network is composed of a multitude|
|of technologically heterogeneous wide area and local area|
|networks with multiple domains of authority.|
|Internetworking is achieved by means of a coherent system|
|level view through the use of an obligatory internet|
|protocol with ancillary monitoring protocol, gateways,|
|exterior/internal gateway protocols and hierarchical domain|
|name service. |
| |
|In the internetworking (not interworking) approach, if two|
|hosts are attached to the same physical subnetwork of an|
|internetwork, the hosts communicate directly with each|
|other. If the hosts are attached to different physical|
|subnetworks, the hosts communicate via gateways local to|
|each host. Gateways understand and learn the internetwork|
|topology dynamically at a subnetwork (not host level) and|
|route data from the source subnetwork to destination|
|subnetwork on a subnetwork hop by subnetwork hop basis. The|
|detail of information required for routing and configuration|
|is reduced by orders of magnitude. In the ARPA Internet,|
|gateways learn topological information dynamically and|
|provide reliability as well as availability by performing|
|alternate routing of IP datagrams in cases of network|
|congestion or network failures. |
| |
|An authoritative domain, Within the ARPA Internet, can|
|conceal from the rest of the internetwork a lot of internal|
|structural detail because gateways in other domains need|
|only know about gateways within their own domain and|
|gateways between authoritative domains. Thus, logical|
|subnetworks of an internetwork may also themselves be|
|catenets (concatenated networks) with internal gateways|
|connecting different physical subnetworks within each|
|catenet. For example, to send traffic to MIT, a gateway at|
|U.C. Berkeley only need know about gateways between MIT and|
|other domains and need know nothing about the internal|
|structure of the MIT domain's catenet. |
_____________________________________________________________|
The ARPA Internet is one realization of the internetworking
model. While I am not particularly enamored of some of the
ARPA protocol features (nor of Unix features by the way),1
the ARPA Internet works well with capacity for expansion.
SINet (described in "How to grow a world-class X.25
network," Data Communications, May 1988) is based on the
CSNet subnetwork within the ARPA Internet.
____________________
1 The use of local-IP-address, local-TCP-port, remote-IP-
address, remote-TCP-port quadruples to uniquely identify a
given TCP virtual circuit is an impediment to providing
greater reliability and availability for a non-gateway
multihomed host. A even larger problem with TCP/IP could
lie in the possibly non-optimal partitioning of
functionality between TCP, IP and ICMP.
____________________
______________________________________________________________
| |
| WANs and LANs |
| |
|OSI actually has an architecture. Like the ARPANET, OSI|
|predicates the existence of a communications subnet|
|consisting communications subnet processors (or subnet|
|switches) and communications subnet access processors (or|
|access switches). Access switches are also known as IMPs|
|(Interface Message Processors) or PSNs (Packet Switch Nodes)|
|in the ARPANET context. PSPDN (Packet-Switched Public Data|
|Network) terminology usually designates access switches|
|simply as packet switches. The communication subnet may be|
|hierarchical and may contain adjunct processors other than|
|subnet and access switches. The internal architecture of|
|the communications subnet is quite distinct from the|
|architecture presented to end-point hosts. The|
|communications subnet may use protocols completely different|
|from the protocols used for communication between two end-|
|point hosts. An end-point host receives and transmits data|
|to its attached access switch via a subnet access protocol.|
|The communications subnet is responsible for taking a packet|
|received at an access switch and transporting the packet to|
|the access switch attached to the destination end-point|
|host. The existence of such a well-defined communications|
|subnet is the hall mark of a Wide-Area Network (WAN). |
|
|Unfortunately, from the standpoint of making computer|
|networking generally and inexpensively available, access and|
|subnet switches are expensive devices to build which need|
|fairly complicated control software. DECNET gets around|
|some of these problems by incorporating the communications|
|subnet logic into end-point hosts. As a consequence,|
|customers who wish to run DECNET typically have to purchase|
|much more powerful machines than they might otherwise use.|
|For the situation of a communications subnet which need|
|support connectivity for only a small number of hosts, LAN|
|developers found a more cost effective solution by|
|developing a degenerate form of packet switches based on|
|hardware-logic packet filtering rather than software|
|controlled packet switching. These degenerate packet|
|switches are installed in the end-point hosts, are accessed|
|often via DMA2 as LAN controllers and are attached to|
|extremely simplified communications subnets like coaxial|
|cables. Direct host-to-switch (controller) access,|
|degenerate packet-switching (packet-filtering) and|
|simplified communications subnets are the distinguishing|
|features of LANs. |
| |
|While ISO was ignoring the whole internetworking issue of|
|providing universal connectivity between end-point hosts|
|attached to different physical networks within internetworks|
|composed of many WANs and even more LANs concatenated|
|together, and while the IEEE was confusing all the issues by|
|presenting as an end-to-end protocol a communications subnet|
|protocol (IEEE 802.2) based on a communications subnet|
|access protocol (X.25 level 2), the ARPA Internet community|
|developed an internet architecture capable of providing the|
|universal connectivity and resource sharing which business,|
|technical and academic users really want and need. |
______________________________________________________________
____________________
2 Some machines like the Prime 50 Series do not use genuine
DMA but instead use inefficient microcoded I/O. IBM
machines generally use more efficient and somewhat more
expensive internal switching.
____________________
The backbone of the ARPA Internet is the ARPANET. The
ARPANET is a packet switched subnetwork within the ARPA
Internet. The ARPANET communications subnet access protocol
is 1822. CSNet was set up as an experiment to demonstrate
that the ARPA Internet architecture and suite of protocols
would function on a packet network whose communications
subnet access protocol is X.25. Using an X.25-accessed
packet network instead of an 1822-accessed packet network
makes sense despite the glaring deficiencies of X.25,3
because X.25 controllers are available for many more systems
than 1822 controllers and because many proprietary
networking schemes like SNA and DECNET can use X.25-accessed
packet networks but cannot use a packet network accessed by
1822.
Yet, calling SINet a world class X.25 network is as
reasonable as calling the ARPANET a world class 1822
network.4 Schlumberger has produced a world class TCP/IP
network whose wires can be shared with SNA and DECNET hosts.
Schlumberger has shown enthusiasm for the flexible,
effective ARPANET suite of protocols but has given no
support in the development of SINet to the idea that
business should prepare to migrate to OSI based networks.
I would be an OSI-enthusiast if ISO had reinvented
internetworking correctly. Unfortunately, the ISO OSI
reference model which first appeared in 1978 clearly ignored
all the ARPA community work on intercomputer networking and
resource sharing which was easily accessible in the
literature of the time. Instead of building the OSI network
on an internetworking foundation, ISO standardized on the
older less effective host-to-packet-switch-to-packet-data-
subnet-to-packet-switch-to-host (NCP) model which the DARPA
____________________
3 For example, X.25 does flow control on the host to packet
switch connection on the basis of packets transmitted rather
than on the basis of consumption of advertised memory
window. The exchange of lots of little packets on an X.25
connection can cause continual transmission throttling even
though the receiver has lots of space for incoming data.
4 Or as much sense as calling Ethernet LANs DMA-based
networks because the packet switches (an Ethernet controller
is a degenerate case of a packet switch) on the LAN are
typically accessed by DMA.
____________________
had abandoned 5 years earlier because of lack of flexibility
and other problems.
______________________________________________________________
| |
| Pieces of the ARPA Internet Conceptually |
| |
| |
| |
| |
| |
| |
| (No Graphics) |
| |
| |
______________________________________________________________
Nowadays, mostly in response to US vendors and DARPA, pieces
of the ARPA Internet architecture have resurfaced in the OSI
reference model quite incoherently rather than as a
consequence of an integrated correct architectural
viewpoint. Connectionless-mode transmission is described in
ISO/7498/DAD1 which is an addendum to ISO 7498 and not a
core document. Because connectionless-mode transmission is
defined in an addendum, the procedure apparently need not be
implemented, and UK GOSIP, for example, explicitly rejects
the use of the connectionless transmission mode. The
introduction to the 1986 ISO 7498/DAD1 explicitly states, as
follows, that ISO was extremely reluctant to incorporate a
genuine datagram based protocol which could be used for
internetworking.
ISO 7498 describes the Reference Model of Open
Systems Interconnection. It is the intention of
that International standard that the Reference
model should establish a framework for coordinating
the development of existing and future standards
for the interconnection of systems. The assumption
that connection is a fundamental prerequisite for
communication in the OSI environment permeates the
Reference Model and is one of the most useful and
important unifying concepts of the architecture
which it describes. However, since the
International Standard was produced it has been
realized that this deeply-rooted connection
orientation unnecessarily limits the power and
scope of the Reference Model, since it excludes
important classes of applications and important
classes of communication network technology which
have a fundamentally connectionless nature.
An OSI connectionless-mode protocol packet may undergo
something like fragmentation, but from the literature, this
form of segmentation as used in OSI networks is hardly
equivalent to ARPA Internet fragmentation. Stallings states
the following in Handbook of Computer-Communications
Standards, the Open Systems Interconnection (OSI) Model and
OSI-Related Standards, on p. 18 (the only reference to
anything resembling fragmentation in the book).
Whether the application entity sends data in
messages or in a continuous stream, lower level
protocols may need to break up the data into blocks
of some smaller bounded size. This process is
called segmentation.
Such a process is not equivalent to ARPA Internet
fragmentation. In the ARPA Internet fragmentation is the
process whereby the gateway software operating at the IP
layer converts a single IP packet into several separate IP
packets and then routes the packets. Each ARPA IP fragment
has a full IP header. It is not obvious that each OSI
segment has a complete packet header. The ARPA fragmentation
procedure is not carried out by lower protocol layers. A N-
layer packet in OSI is segmented at layer N-1 while the
packet is routed (relayed) at layer N+1.
This partitioning of basic internetworking procedures across
layer 2 (N-1), layer 3 (N) and layer 4 (N+1) violates the
following principles described in ISO/DIS 7498: Information
Processing Systems -- Open Systems Interconnection -- Basic
Reference Model.
P1: do not create so many layers as to make the system
engineering task of describing and integrating the
layers more difficult than necessary [ISO uses
three layers where one could be used];
P2: create a boundary at a point where the description
of services can be small and the number or
interactions across the boundary are minimized [by
putting per-packet relaying in layer 4 at least
two interactions across the boundary are required
per packet];
P5: select boundaries at a point which past experience
has demonstrated to be successful [the ARPA
Internet layering boundaries which combine the
addressing, fragmentation and routing in one layer
has proven successful];
P6: create a layer where there is a need for a
different level of abstraction in the handling of
data, e.g. morphology, syntax, semantics
[fragmentation, routing, and network addressing
are all seem quit naturally to be part of network
layer semantics as the ARPA Internet example
shows];
P9: allow changes of functions or protocols to be made
within a layer without affecting other layers [I
would think changing the manner of addressing at
layer 3 would affect relaying at layer 4].
Even if OSI N-1 segmentation and N+1 relaying could be used
in the same way as fragmentation and routing in the ARPA
Internet, it takes a lot more apparatus than simply
permitting the use of the ISO connectionless "internet"
protocol to achieve internetworking.
The OSI documents almost concede this point because ISO
7498/DAD 1, ISO/DIS 8473 (Information Processing Systems --
Data Communications -- Protocol for Providing
Connectionless-Mode Network Service) actually provide for N-
layer segmentation (actually fragmentation) and N-layer
routing right in the network layer in addition to the OSI
standard N-1 segmentation and N+1 relaying. Providing such
functionality directly in the network layer actually seems
in greater accordance with OSI design principles, but if ISO
is really conceding this point, ISO should go back and
redesign the system rather than leaving this mishmash of N-1
segmentation, N segmentation, N routing and N+1 relaying.
The current connectionless-mode network service is still
insufficient for internetworking because the gateway
protocols are not present and the connectionless-mode error
PDUs (Protocol Data Units) do not provide the necessary ICMP
functionality. The documents also indicate a major
confusion between an internetwork gateway, which connects
different subnetworks of one catenet (concatenated network),
and a simple bridge, which connects several separate
physical networks into a single network at the link layer,
or an interworking unit, which is a subnet switch connecting
two different communications subnets either under different
administrative authorities or using different internal
protocols.5 Tanenbaum writes the following about the
____________________
5 This confusion is most distressing from a security
standpoint. The November 2 ARPA Internet (Cornell) virus
attack shows that one of the major threats to network
security is insider attack which is a problem with even the
most isolated corporate network. Because many ARPA Internet
network authorities were assuming insider good behavior,
ARPA Internet network administrators often did not erect
security barriers or close trapdoors. Nevertheless,
gateways have far more potential than bridges or
interworking units to provide reasonable firewalls to hinder
and frustrate insider attack. MIT/Project Athena which
makes judicious use of gateways and which does not assume
insider good behavior was relatively unaffected by the
virus. Any document which confuses gateways, bridges and
interworking units is encouraging security laxity.
____________________
connectionless-mode network service in Computer Networks, p.
321.
In the OSI model, internetworking is done in the
network layer. In all honesty, this is not one of
the areas in which ISO has devised a model that has
met with universal acclaim (network security is
another one).6 From looking at the documents, one
gets the feeling that internetworking was hastily
grafted onto the main structure at the last minute.
In particular, the objections from the ARPA
Internet community did not carry as much weight as
they perhaps should have, inasmuch as DARPA had 10
years experience running an internet with hundreds
of interconnected networks, and had a good idea of
what worked in practice and what did not.
Internetworking, the key concept of modern computer
networking, exists within the OSI reference model as a
conceptual wart which violates even the OSI principles. If
ISO had not tacked internetworking onto the OSI model, ISO
was afraid that DARPA and that part of the US computer
industry with experience with modern computer networking
would have absolutely rejected the OSI reference model as
unusable.
____________________
6 Actually, I find ISO 7498/2 (Security Architecture) to be
one of the more reasonable ISO documents. I would disagree
that simple encryption is the only form of security which
should be performed at the link layer because it seems
sensible that if a multilevel secure mini is replaced by a
cluster of PCs on a LAN, multilevel security might be
desirable at the link layer. Providing multilevel security
at the link layer would require more than simple encryption.
Still, ISO 7498/2 has the virtue of not pretending to solve
completely the network security problem. The document gives
instead a framework indentifying fundamental concepts and
building blocks for developing a security system in a
networked environment.
____________________
IV. "GREATER RICHNESS" VERSUS DEVELOPER INSIGHT
In view of this major conceptual flaw which OSI has with
respect to internetworking, no one should therefore be
surprised that instead of tight technical discussion and
reasoning, implementers and designers like me are
continually subjected to vague assertions of "greater
richness" of the OSI protocols over the ARPA Internet
protocols. In ARPA Internet RFCs, real-world practical
discussion is common. I would not mind similar developer
insight or even hints about the integration of these OSI
protocol interpreters into genuine operating systems
participating in an OSI interoperable environment.
The customers should realize "greater richness" costs a lot
of extra money even if a lot of the added features are
useless to the customer. "Greater richness" might
necessitate the use of a much more powerful processor if
"greater richness" forced much more obligatory but
purposeless protocol processing overhead. "Greater richness"
might also represent a bad or less than optimal partitioning
of the problem.
A. OSI NETWORK MANAGEMENT AND NETVIEW
Netview has so much "greater richness" than the network
management protocols and systems under development in the
ARPA Internet context that I have real problems with the
standardization of Netview into OSI network management as
the obligatory user interface and data analysis system.
Netview is big, costly, hard to implement, and extremely
demanding on the rest of the network management system. As
OSI network management apparently subsumes most of the
capabilities of Arpanet ICMP (Internet Control Monitoring
Protocol) which is a sine qua non for internetworking, I am
as a developer rather distressed that full blown OSI network
management (possibly including a full implementation of
FTAM) might have to run on a poor little laser printer with
a dumb ethernet interface card and not much processing
power.
B. FTAM IS DANGEROUS
The "greater richness" of FTAM seems to lie in the ability
to transmit single records and in the ability to restart
aborted file transfer sessions. Transmission of single
records seems fairly useless in the general case since
operating systems like Unix and DOS do not base their file
systems on records while the records of file systems like
those of Primos and VMS have no relationship whatsoever to
one another. Including single record or partial file
transfer in the remote transfer utility seems is a good
example of bad partitioning of the problem. This capability
really belongs in a separate network file system. A network
file system should be separate from the remote file transfer
system because the major issues in security, performance,
data encoding translation and locating objects to be
transferred are different in major ways for the two systems.
The ability to restart aborted file transfers is more
dangerous than helpful. If the transfer were aborted in an
OSI network, it could have been aborted because one or both
of the end hosts died or because some piece of the network
died. If the network died, a checkpointed file transfer can
probably be restarted. If a host died on the other hand, it
may have gradually gone insane and the checkpoints may be
useless. The checkpoints could only be guaranteed if end
hosts have special self-diagnosing hardware (which is
expensive). In the absence of special hardware and ways of
determining exactly why a file transfer aborted, the file
transfer must be restarted from the beginning. By the way,
even with the greater richness of FTAM, it is not clear to
me that a file could be transferred by FTAM from IBM PC A to
a Prime Series 50 to IBM PC B in such a way that the file on
PC A and on PC B could be guaranteed to be identical.
C. X.400: E-MAIL AS GOOD AS THE POSTAL SERVICE
As currently used and envisioned, the X.400 family message
handling also has "greater richness." X.400 seems to
include binary-encoded arbitrary message-transmission,
simple mail exchange and notification provided by a
Submission and Delivery Entity (SDE). In comparison with
ARPA SMTP (Simple Mail Transfer Protocol), X.400 is overly
complicated with hordes of User Agent Entities (UAEs),
Message Transfer Agent Entities (MTAEs) and SDEs scurrying
around potentially eating up -- especially during periods of
high traffic -- lots of computer cycles on originator,
target and intermediate host systems because the source UAE
has to transfer mail through the local MTAE and intermediate
MTAEs on a hop-by-hop basis to get to the target machine.7
____________________
7 I have to admit that if I were implementing X.400, I would
probably implement the local UAE and MTAE in one process.
The CCITT specification does not strictly forbid this
design, but the specification does seem to discourage
strongly such a design. I consider it a major flaw with a
protocol specification when the simplest design is so
strongly counterindicated. It does seem to be obligatory
that mail traffic which passes through an Intermediate
System (IS) must pass through an MTAE running on that IS.
____________________
The design is particularly obnoxious because X.400 increases
the number of ways of getting mail transmission failure by
using so many intermediate entities above the transport
layer. The SMTP architecture is, by contrast, simple and
direct. The user mail program connects to the target system
SMTP daemon by a reliable byte stream (like a TCP virtual
circuit) and transfers the mail. Hop-by-hop transfers
through intermediate systems are possible when needed. One
SMTP daemon simply connects to another the same way a user
mail program connects to an SMTP daemon.
The relatively greater complexity and obscurity of X.400
arises because a major purpose of X.400 seems to be to
intermingle intercomputer mail service and telephony
services like telex or teletex to fit the computer
networking into the PTT (Post, Telegraph & Telephone
administration) model of data communications (not an
unreasonable goal for a CCITT protocol specification but
probably not the best technical or cost-effective design for
the typical customer). Mail gateways are apparently
supposed to handle document interchange and conversion.
Document interchange and conversion is a really hard problem
requiring detailed knowledge at least of word processor file
formats, operating system architecture, data encoding, and
machine architecture.
It may be impossible to develop a satisfactory network
representation which can handle all possible document
content, language and source/target hardware combinations as
well as provide interconversion with tradition telephonic
data transmission encodings. The cost of development of such
a system might be hard to justify, and a customer might have
a hard time justifying paying the price a manufacturer would
probably have to charge for this product. A network file
system or remote file transfer provides a much more
reasonable means of document sharing or interchange than
tacking an e-mail address into a file with a complicated
internal structure, sending this file through the mail
system and then removing the addressing information before
putting the document through the appropriate document or
graphics handler.
A NETASCII-based e-mail system corresponds exactly to the
obvious mapping of the typical physical letter, which does
not usually contain complicated pictorial or tabular data,
to an electronic letter and is sufficient for practically
all electronic mail traffic. Special hybrid systems can be
developed for that extremely tiny fraction of traffic for
which NETASCII representations may be insufficient and for
which a network file system or FTP may be insufficient. A
correct partitioning of the electronic mail should be kept
completely separate from telephony services, document
interchange and document conversion.
______________________________________________________________
| |
| X.400 Mail Connections |
| |
| |
| |
| |
| |
| |
| (No Graphics) |
| |
| |
______________________________________________________________
D. ARPA SMTP: DESIGNING MAIL AND MESSAGING RIGHT
The MIT environment at Project Athena, where IBM and DEC are
conducting a major experiment in the productization of
academic software, provides an instructive example of the
differences between e-mail, messaging and notification. The
mail system used at MIT is an implementation of the basic
SMTP-based ARPA Internet mail system. More than four years
ago the ARPA Internet mail system was extremely powerful
and world-spanning. It enabled then and still enables
electronic mail to reach users on any of well over 100,000
hosts in N. America, Europe, large portions of E. Asia and
Israel. The Citicorp network (described in "How one firm
created its own global electronic mail network," Data
Communications, June 1988, p. 167), while probably
sufficient for Citicorp's current needs, connects an
insignificant number of CPUs (47), provides no potential for
connectivity outside the Citicorp domain of authority and
will probably not scale well with respect to routing or
configuration as it grows.
The MIT environment is complex and purposely (apparently in
the strategies of DEC and IBM) anticipates the sort of
environment which should become typical within the business
world within the next few years. MIT is an authoritative
domain within the ARPA Internet. The gateways out of the
MIT domain communicate with gateways in other domains via
the Exterior Gateway Protocol (EGP). Internally, currently
used internal gateway protocols are GGP, RIP and HELLO. The
MIT domain is composed of a multitude of Ethernet and other
types of local area networks connected by a fiber-optic
backbone physically and by gateway machines logically. This
use of gateways provides firewalls between the different
physical networks so that little sins (temporary network
meltdowns caused by Chernobyl packets) do not become big
sins propagating themselves throughout the network. The
gatewayed architecture of the MIT network also permits a
necessary traffic engineering by putting file system, paging
and boot servers on the same physical network with their
most likely clients so that this sort of traffic need not be
propagate throughout the complete MIT domain.
Difficult to reach locations achieve connectivity by means
of non-switched telephone links. Since MIT has its own
5ESS, these links may be converted to ISDN at some point.
While there are some minis and mainframes in the network,
the vast majority of hosts within the MIT network are
personal workstations with high resolution graphics displays
of the Vaxstation and RT/PC type and personal computers of
the IBM PC, PC/XT and PC/AT type. A few Apollos, Suns,
Sonys and various workstations of the 80386 type as well as
Lisp Machines and PCs from other manufacturers like Apple
are also on the air. Most of the workstations are public.
When a user logs in to such a workstation, after appropriate
Kerberos (MIT security system) authentication, he has full
access to his own network files and directory as well as
access to those resources within the network which he has
the right to use.
To assist the administration of the MIT domain within the
ARPA Internet, several network processes might be
continually sending (possibly non-ASCII) event messages to a
network management server which might every few hours
perform some data analysis on received messages and then
format a summary mail message to send to a network
administrator. This mail message would be placed in that
network administrator's mailbox by his mail home's SMTP
daemon which then might check whether this network
administrator is reachable somewhere within the local domain
(maybe on a PC with a network interface which was recently
turned on and then was dynamically assigned an IP address by
a local authoritative dynamic IP address server after
appropriate authentication). If this administrator is
available, the SMTP daemon might notify him via the
notification service (maybe by popping up a window on the
administrator's display) that he has received mail which he
could read from his remote location via a post office
protocol.
I have seen the above system being developed on top of the
basic "static" TCP/IP protocol suite by researchers at MIT,
DEC and IBM over the last 4 years. X.400 contains a lot
this MIT network functionality mishmashed together but I as
a customer or designer prefer the much more modular MIT mail
system. It is an extensible, dynamically configurable
TCP/IP-based architecture from which a customer could chose
those pieces of the system which he needs. The MIT system
requires relatively little static configuration. Yet by
properly choosing the system pieces, coding an appropriate
filter program and setting up a tiny amount of appropriate
configuration data, a customer could even set up a portal to
send e-mail to a fax machine. In comparison, X.400 requires
complicated directory services and an immense amount of
static configuration about the end user and end user machine
to compensate for the internetworking-deficient or
internetworking-incompatible addressing scheme. The need for
such a level of static configuration is unfortunate for
system users because in the real world a PC or workstation
might easily be moved from one LAN to another or might be
easily replaced by a workstation or PC of another type.
An MIT-style mail system could also be much cheaper to
develop and consequently could be much less costly to
purchase than an X.400 mail system simply because it
represents a much better partitioning of the problem. One
or two engineers produced each module of the MIT mail system
in approximately 6 months. Because of complexity and
obscurity, the development of X.400 products (I saw an
example at Prime) is measured in staff years. The executive
who chooses X.400 will cost his firm an immense amount of
money which will look utterly wasted when his firm joins
with another firm in some venture and the top executives of
both firms try to exchange mail via their X.400 mail
systems. Simple mail exchange between such systems would
likely be very hard to impossible because the different
corporations could easily have made permissible but
incompatible choices in their initial system set-up. At the
very last complete reconfiguration of both systems could be
necessary. Had the firms chosen an ARPA Internet mail
system like the MIT system, once both firms had ARPA
Internet connectivity or set up a domain-to-domain gateway,
mail would simply work.
______________________________________________________________
| |
| SMTP Mail Connections |
| |
| |
| |
| |
| |
| |
| (No Graphics) |
| |
| |
______________________________________________________________
V. IS THE TCP/IP PROTOCOL SUITE "STATIC?"
Because of the mail system development in progress at MIT,
DEC and IBM, the X development which I and others have done
and which is still continuing, SUN NFS (Network File System)
development, IBM AFS (Andrew File System) development,
Xenix-Net development, Kerberos development, and the other
plethora of protocol systems being developed within the ARPA
Internet context (including the VMTP transaction processing
system and commercial distributed database systems like
network Ingress), I am at the very least puzzled by Mr.
Stallings' assertion that "[it] is the military standards
that appear on procurement specifications and that have
driven the development of interoperable commercially
available TCP/IP products."
______________________________________________________________
| |
| Partitioning the Problem |
| |
|The X window system is an example of a clearly and well|
|partitioned system. In windowing, the first piece of the|
|problem is virtualizing the high-resolution raster graphics|
|device. Individual applications do not want or need to know|
|about the details of the hardware. Thus, to provide|
|hardware independence, applications should only deal with|
|virtual high-resolution raster-graphics devices and should|
|only know about its own virtual high resolution raster-|
|graphics devices (windows). The next piece of the problem|
|is to translate between virtual high-resolution raster|
|graphics devices and the physical high-resolution raster|
|graphics device (display). The final part of the problem|
|lies in managing the windows on the display. This problem,|
|with a little consideration clearly differentiates itself|
|from translating between virtual and physical high-|
|resolution raster-graphics devices. |
| | |
|In the X window system, communication between the|
|application and its windows is handled by the X library and|
|those libraries built on top of the basic X library.|
|Virtual to physical and physical to virtual translation is|
|handled by the X server. X display management is handled by|
|the X window manager. | |
| | |
|After partitioning the problem, careful consideration of|
|display management leads to the conclusion that if all|
|windows on a display are treated as "children" of a single|
|"root" window, all of which "belong" in some sense to the|
|window manager, then the X window manager itself becomes an|
|ordinary application which talks to the X server via the X|
|library. As a consequence, developers can easily implement|
|different display management strategies as ordinary|
|applications without having to "hack" the operating system.|
|The server itself may be partitioned (under operating|
|systems which support the concept) into a privileged portion|
|which directly accesses the display hardware and a non-|
|privileged portion which requests services from the|
|privileged part of the server. Under Unix, the privileged|
|part of the server goes into the display, mouse and keyboard|
|drivers while the non-privileged part becomes an ordinary|
|application. In common parlance, X server usually refers to|
|the non-privileged part of the X server which is implemented|
|as an ordinary application. |
| |
|The last step in realizing the X window system is choosing|
|the communications mechanism between the X server and|
|ordinary applications or the display manager. Because the|
|problem was nicely partitioned, the communications problem|
|is completely extrinsic to the windowing problem as lives as|
|an easily replaceable interface module. The initial choice|
|at MIT was to use TCP/IP virtual circuits, which provided|
|immediate network transparency, but in fact because X only|
|requires sequenced reliable byte-streams so that DECNET VCs|
|or shared-memory communications mechanisms can easily|
|replace TCP/IP virtual circuits according to the|
|requirements of the target environment. Systems built on|
|well-partitioned approaches to solving problems often show|
|such flexibility because of modularity of the approach and|
|because a successful partitioning of the problem will often|
|in its solution increase the understanding of the original|
|problem that developers can perceive greater tractability|
|and simplicity in the original and related problems than|
|they might have originally seen. |
_____________________________________________________________|
It seems somewhat propagandistic to label the TCP/IP
protocol suite static and military. New RFCs are
continually being generated as Paul Strauss has pointed out
in his September article. Such new protocols only become
military standards slowly because the military
standardization of new protocols and systems is a long
tedious political process which once completed may require
expensive conformance and verification procedures. After
all, neither the obligatory ICMP nor the immensely useful
UDP (User Datagram Protocol) have associated military
standards. Often, after reviewing those products generated
by market forces, the US military specifies and acquires
products which go beyond existing military standards. By the
way, hierarchical domain name servers and X are used on
MILNET.
VI. ENTERPRISE NETWORKING AND SOPHISTICATED APPLICATIONS:
SELLING INTERCOMPUTER NETWORKING
The military are not the only users "more interested in
sophisticated applications than in a slightly enhanced
version of Kermit." The whole DEC enterprise networking
strategy is postulated on this observation. Stallings
ignored my reference to network file systems as a
sophisticated networking application. Yet, in several
consulting jobs, I have seen brokers and investment bankers
make extensive use of network file systems. I also believe
network transparent graphics will be popular in the business
world. At Saloman Brothers both IBM PCs and SUN
workstations are extensively used. With X, it is possible
for a PC user to run a SUN application remotely which uses
the PC as the output device. This capability seems highly
desirable in the Saloman Brothers environment.
Unfortunately "OSI is unlikely ever to provide for [such]
resource sharing because it is industry-driven." Wayne Rash
Jr., a member of the professional staff of American
Management Systems, Inc. (Arlington, Virginia) who acts as
a US federal government microcomputer consultant, writes the
following in "Is More Always Better," Byte, September 1988,
p. 131.
You've probably seen the AT&T television ads about
this trend [toward downsizing and the development
of LAN-based resource-sharing systems]. They
feature two executives, one of whom is equipping
his office with stand-alone microcomputers. He's
being intimidated by another executive, who tells
him in a very nasty scene, "Stop blowing your
budget" on personal computers and hook all your
users to a central system. This is one view of
workgroup computing, although AT&T has the perverse
idea that the intimidator is the forward thinker in
the scene.
AT&T and to an even greater extent the similarly inclined
European PTTs have major input into OSI specification.
VII. BIG AND SMALL PLAYERS CONSTRAIN OSI
The inclinations of AT&T and the PTTs are not the only
constraints under which the OSI reference model was
developed. A proprietary computer networking system, sold
to a customer, becomes a cow which the manufacturer can milk
for years. Complete and effective official standards make it
difficult for a company to lock a customer into a
proprietary system. A customer could shop for the cheapest
standard system, or could chose the offering of the
manufacturer considered most reliable. It is proverbial
that no MIS executive gets fired for choosing IBM. Small
players have genuine reason to fear that a big player like
Unisys, which no longer has a major proprietary computer
networking installed base8, or AT&T, which never had a major
proprietary computer networking installed base9, might try
to establish themselves in the minds of customers as the
ultimate authority for the supply of true OSI connectivity.
Thus, small players fear that a complete and effective
official standard might only benefit the big players.
Players like AT&T or Unisys fear IBM might hi-jack the
standard. IBM would prefer to preserve its own proprietary
base and avoid competing with the little guys on a
cost/performance basis in what could turn into a commodity
marker.
No such considerations were operative in the development of
the ARPA Internet suite of protocols. DARPA had a specific
need for intercomputer networking, was willing to pay top
dollar to get the top experts in the intercomputer
networking field to design the system right and was less
concerned by issues of competition (except perhaps for turf
battles within the U.S. government). By contrast, almost
all players who have input into the ISO standardization
process have had reasons and have apparently worked hard to
limit the effectiveness of OSI systems.
With all the limitations, which have been incorporated into
the OSI design and suite of protocols, the small players
have no reason to fear being overwhelmed by big players like
Unisys or AT&T. The big players have the dilemma of either
being non-standard or of providing an ineffective,
incomplete but genuine international standards. Small
vendors have lots of room to offer enhanced versions perhaps
drawing from more sophisticated internetworking concepts. In
any case, most small vendors, as well as DEC and IBM, are
hedging their bets by offering both OSI and TCP/IP based
products. IBM seems well positioned with on-going projects
at the University of Michigan, CMU, MIT, Brown and Stanford
and with IBM's creditability in the business world to set
the standard for the business use of TCP/IP style
____________________
8 BNA and DCA seem hardly to count even to the Unisys
management.
9 Connecting computer systems to the telephone network is
not computer networking in any real sense.
____________________
networking. By contrast, no major manufacturer really seems
to want to build OSI products, and with the current state of
OSI, there is really no reason to buy OSI products.
VIII. MAP: FOLLOWING THE OSI MODEL
MAP shows perfectly the result of following the OSI model to
produce a computer networking system. GM analysts sold MAP
to GM's top management on the basis of the predicted cost
savings. Since GM engineers designed, sponsored and gave
birth to MAP, I am not surprised that an internal GM study
has found MAP products less expensive than non-MAP compliant
products. If the internal study found anything else, heads
would have to roll. Yet, as far as I know, neither IBM nor
DEC have bought into the concept although both companies
would probably supply MAP products for sufficient profit.
Ungermann-Bass and other similar vendors have also announced
a disinclination to produce IEEE 802.4 based products.
Allen-Bradley has chosen DECNET in preference to a MAP-based
manufacturing and materials handling system. This defection
of major manufacturers, vendors and customers from the MAP
market has to limit the amount of MAP products available for
customers to purchase.
Nowadays, GM can purchase equipment for its manufacturing
floor from a limited selection of products, which are the
computer networking equivalent of bows and arrows, whereas
in the past GM was stuck with rocks and knives. Bows and
arrows might be sufficient for the current GM applications;
however, if my firm had designed MAP, GM would have the
networking equivalent of nuclear weapons, for the MAP
network would have been built around an internet with a
genuine multimedium gatewayed easily modifiable environment
so that in those locations where token-bus noise resistance
is insufficient and where higher bandwidths might be needed,
fiber media could be used. With the imminent deluge of
fiber-based products, MAP looks excessively limited.
(Actually, the MAP standards committees have shown some
belated awareness that fiber might be useful in factories.)
IX. EXTENDING OSI VIA PROTOCOL CONVERTERS: QUO VADIT?
Interestingly enough, even when OSI systems try to overcome
OSI limitations via protocol conversion to provide access to
some of the sophisticated resource sharing to which ARPA
Internet users have long been accustomed, the service is
specified in such a way as to place major limitations on
performance of more sophisticated applications. Just like
IBM and other system manufacturers, I have no problems with
providing to the customer at sufficient profit exactly
those products which the customer specifies. Yet, if
contracted for advice on a system like the NBS TCP/IP-to-OSI
protocol converter IS (Intermediate System), described in
"Getting there from here," Data Communications, August 1988,
I might point out that such a system could easily double
packet traffic on a single LAN, decrease network
availability and reliability, prevent alternate routing, and
harm throughput by creating a bottleneck at the IS which
must perform both TCP/IP and OSI protocol termination.
X. CONCLUSION
Official standardization simply by itself does not make a
proposal good. Good standards generally were already good
before they became official standards. The IEEE and other
standards bodies generate lots of standards for systems
which quickly pass into oblivion. OSI was generated de
novo, apparently with a conscious decision to ignore the
already functioning ARPA Internet example. Unless a major
rethinking of OSI (like redesigning OSI on the solid
foundation of the internetworking concept) takes place in
the near future, I must conclude that the ARPA Internet
suite of protocols will be around for a long time and that
users of OSI will be immensely disappointed by the cost,
performance, flexibility and manageability of their
networks.
I. Introduction 1
II. The Debate 2
III. Internetworking: The Key System Level Start Point 4
IV. "Greater Richness" Versus Developer Insight 14
A. OSI Network Management and Netview 14
B. FTAM is Dangerous 14
C. X.400: E-Mail as Good as the Postal Service 15
D. ARPA SMTP: Designing Mail and Messaging Right 18
V. Is the TCP/IP Protocol Suite "Static?" 22
VI. Enterprise Networking and Sophisticated Applications:
Selling Intercomputer Networking 24
VII. Big and Small Players Constrain OSI 24
VIII. MAP: Following the OSI Model 26
IX. Extending OSI Via Protocol Converters: Quo vadit? 26
X. Conclusion 27