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Possible Cell Virus Attacks ATM Network


ATM Network Reliability Group
San Antonio Rd., Mountain View, CA
April 1, 1992

For Immediate Release:

Possible Cell Virus Attacks ATM Network

The ATM Network Reliability Group today announced the immediate formation of the Cell Behaviour Task Force to address serious problems in several recently installed networks based on Asynchronous Transfer Mode (ATM).

Sometimes known as cell-switching, ATM has been touted as the technology to break the long-standing barriers between packet and circuit based communications services. Utilizing very small packets, called cells, with simple headers that can be processed and routed in hardware, the new networks are able to interleave packet and circuit data on the same transmission lines without resorting to wasteful and expensive time-division multiplexing techniques.

However, eleven similar incidents on four separate networks (identities
withheld by request) have raised doubts about the reliability of the new
technology. In each case, a large ATM switch suffered congestive overload and failure, apparently due to uncontrolled replication of normally benign control messages known as OAM cells.

These single-celled messages provide in-band control functions for virtual circuits, including hop-by-hop and end-to-end functions such as path connectivity, delay measurement, etc. There are two distinct varieties, type 4 and type 5, which normally make up only a very small fraction of the population of cell traffic in a typical ATM switch. For reference, a relatively small ATM switch with a gigabit capacity can handle roughly two million cells per second of which no more than 200 per second, or 0.01%, would be OAM cells.

The failed switches suffered from an extreme overload of OAM cells, which apparently began replicating themselves using the multicast capabilities of the switch through an as yet undiscovered control mechanism. The rapid influx of OAM cells quickly overwhelmed the control processors that normally handle such messages and blocked the outgoing control paths so that no warning could be given.

Memory dumps later showed buffers and queues full of mostly identical OAM-like cells, with two interesting features. First, the buffers did not contain standard type 4 or type 5 cells but a combination of parts of each. Second, slight one- and two-bit mutations of the most common message appeared later in the queues, which were then further replicated.

The intriguing behavioral possibilities were first noted by Dr. R. Jones, a
former high-school biology teacher, who overhead discussions during lunch at the Straw Hat Pizza Parlor near the Reliability Group's Mountain View offices.

Now a home-computer enthusiast, he helped develop the sensitive masking technique used to analyze the cell buffers, based on his experiences with similar tests for blood cell typing. He also suggested two of the more important investigative areas being explored. First, the original replicated cells may only have been a symptom of an earlier problem for which OAM cells were being automatically generated in an attempt to fix the problem.

The OAM cells may themselves have caused more problems, with a cascading effect similar to a biological auto-immune reaction. The second area, which has caused more concern, is the possibility of infection of neighboring switches,
though so far only individual switches have failed.

The actual cause of the problem is still unknown. The most commonly held opinion is that a combination of line errors and software or hardware bugs conspired to bring about the initial duplication. A NJ-based member of the investigating team has suggested that variant encodings of the Application Signalling Bit in the cell header might also be responsible.

But some have suggested that the problem seems more similar to computer virus attacks and the Internet "worm" in particular, and are particularly intrigued by its dual-inheritance and mutation aspects. Experts discount the possibility of a true, replicating, single-cell virus in the ATM network, observing that the 48-octet payload of an ATM cell is too small to contain a sufficiently complex program. An investigator who wished to remain nameless maintained that the team's greatest difficulty was interpreting the ATM standards themselves, and hinted that the whole affair might come down to a misunderstanding on the part of the switch manufacturer.

Until the problem is solved, however, the task force is recommending that communications customers use caution when buying or using ATM network services, especially for critical functions such as remote banking, and cash machine operations in particular.

Principal Investigator,

Tracy Mallory





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