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\noindent
David Golombek\\
6.033 Section 6, David Wetherall\\
Paper \#4: Complexity as a Good Thing:\\Or, When Extra Design is Worth It.\\\\

Ethernet's design is truly a classic, such that when it is finally
retired from all active use (in a few hundred years), it will be
regarded as milestone in networking.  Its simplicity and efficiency
across the board, from conceptual design to ease of implementation
mark it as a classic.  Yet Ethernet's Day is passed, for we now need
greater datat throughput and larger numbers of hosts on single
networks.  Autonet, characterized by real-time self-configuration,
high data throughput, and high percentage packet delivery is one of several
new alternatives to Ethernet.  Although these features involve a
significant rise in complexity over Ethernet, Autonet's added
functionality easily matches this price.

While at the Jet Propulsion Laboratory, home of hundreds of scientists
all trying to work on the same computer network, I first encountered
one of the problems of ethernet.  While cleaning one evening, it
appears a janitor managed to remove the terminator on an Ethernet.
No user or system administrator was around at the time, and
configuration of the Labs is poor enough that it took a couple hours
the next morning to sort out what the problem was and where the
problem had occurred.  Autonet solves this problem by doing dynamic
reconfiguration of itself.  Each switch contains a Motorolla 68000
microprocessor running the Autopilot software designed with Autonet,
which polls hardware registers and the status of nearby switches.
When errors are detected, the switch can rebuild a new routing table
minus the dead host and redistribute this new map in an atomic
fashion, such that all hosts are updated at the same time.  No
interaction on the part of the user is neccessary, save to physically
fix broken hardware as appropriate.

The original 3 Mbit/second bandwidth of Ethernet seemed like a very
reasonable number when the system was designed in the mid 70s, but
with the rise of X and other bandwidth consuming applications, this,
and even the later update to 10 Mbps Ethernet, were simply too slow.
Any viable replacement to Ethernet needs to provide a significant
gain.  Autonet offers another order of magnitude, bringing throughput
to 100 Mbps based upon current hardware.  This number is not set in
stone, but depends upon the medium of transport and the speed of the
switches, both of which can be improved in the future without serious
change to the protocol.  Although it might be nice to immediately
switch from Ethernet to a many-Gigabit protocol, this step appears too
much for the forseeable future, and thus a temporary improvement of an
order of magnitude is acceptible.  This increase is obtained via the
low latency of the crossbar switches used, and the ability to route
several transactions simultaneously by only running them on the
shortest path from source to destination, freeing up all other parts
of the network.  

Ethernet has never promised more than a best-effort at delivering
packets to the destination machine.  Thus, line noise, high
congestion, and simple computer overloading often lead to dropping of
packets, which must be dealt with in software.  This was considered a
very workable solution, and was very elegant to implement and deal
with.  Autonet, however, has broken this tradition, and attempts to
give a much higher percentage successful completion of packet
delivery.  It does this by having large FIFO's on each of the
switches, such that packets are never lost on initial reception before
the crossbar can be configured, and using sophisticated flow control
and lockout algorithms.  Despite initial skepticism of this approach,
the fact tha working Autonet networks exist and are acceptable to
users indicates that it has indeed been implemented successfuly,
leading to more efficient use of the existing lines.   Although this
system seems like it might not handle heavy load as gracefully as some
others, not enough information is presented in the paper to fully
determine.  

Ethernet clearly needs to be replaced in terms of bandwidth available.
Autonet meets this goal rather clearly, offering an order of magnitude
increase.  But it also offers dynamic reconfiguration, a feature long
desired by network administrators around the world, and
almost-guaranteed packet delivery, which leads to higher network
efficiency (again increasing data throughput).  Although Ethernets
design is classic for its simplicity, the increase in complication of
Autonet is overshadowed by the benefits it offers.  Arguments against
the level of complication can be offset by the mere fact that the
system is up and running, and doing so as claimed.

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