Frame relay security: DLCI corruption is no big deal

We recently discussed that the PVCs in frame relay and ATM networks preclude the need for special tunneling software in IP virtual private networks (VPNs) that use these connection-oriented network protocols. But what happens if the data link connection identifier (DLCI) gets corrupted somewhere and has a "wrong" circuit number?

No problem. The cyclic redundancy check (CRC) function in the frame relay protocol checks for bit errors in the DLCI. If it discovers any, the frame is discarded. So, even if a bit error on the line changed a DLCI so that your data would end up be delivered to the wrong location, the protocol makes sure that the information is discarded before it is passed to a higher level. In ATM, the header error check (HEC) field checks the integrity of the virtual path identifier (VPI) and virtual circuit identifier (VCI) to provide the same function.

Regardless of the technology used, you must also consider the effects of a misdelivered frame if the two safeguards already discussed fail. The odds are virtually nil that your data is going to be in a meaningful form within the frame relay frame or ATM cell. Rather, it will be a segment of information from a higher-layer protocol, such as TCP or SNA. That higher layer protocol will see the misdelivered information as a protocol error and will discard it long before it reaches the "presentation layer," where it is actually viewed.

So this only leaves one remaining question. We have (reasonably) assumed the frame relay or ATM network uses PVCs. Next time: What if you are using a switched virtual circuit (SVC) service?

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Steven Taylor, consultant and broadband packet evangelist, and Joanie Wexler, an independent networking technology editor and writer, team up to bring you this analysis and commentary. Taylor specializes in education and market analysis, and Wexler adds incisive reporting and research. For more detailed information on most of the topics discussed in this newsletter, connect to www.webtorials.com, the first Web site dedicated exclusively to market studies and technology tutorials in the Broadband Packet areas of Frame Relay, ATM, and IP. Feedback and additional topic ideas are welcome. Please contact taylor@webtorials.com or joanie_wexler@mindspring.com.

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Archive of Network World on Frame Relay newsletters