On Oct 19 8:12, William Allen Simpson wrote: % There has been a recent bit of scare mongering from Lucent about PPP % over SONet/SDH over on the IETF-PPP list. % Has anyone (that has deployed) been having incident reports?
-- End of excerpt from William Allen Simpson
Bill, There are known operational incidents where an adversary sent an IP packet designed to set the SONET scrambling algorithm ["f(x)"] to all zeros. This caused the SONET device to lose communications syncronisation and the circuit to go down, requiring a manual reset. Truly an ugly situation. I've known about this issue ever since the PPP/SONET spec was published, but have withheld public comment until someone else mentioned the issue on a public list. Cisco originally was going to implement a scrambler in its PPP/SONET implementation (IOS folks like gmc definitely understood the issue), but the cisco hardware types would not accept free clue and didn't include the scrambler. Basically all the PPP/SONET implementations I know about can be taken out with a single well-known (and easily calculated) IP datagram. Sigh. I have not seen the Lucent proposal myself, so I'm not sure what it looks like.
From time to time I've talked with a couple of folks at a small networking startup in Mountain View about this issue. My suggestions to them have been of the form below:
Add a scrambler algorithm to the PPP/SONET spec. A reasonable approach to that scrambler might be something of the general form: X^^A + X^^B + C Where: ^^ is the exponentiation operator A,B are prime numbers with O(10) or larger. (A > B) C is a prime number other than 1. Adding additional exponential terms might generally strengthen the algorithm, but intuitively I believe that the above form is a reasonable tradeoff between implementation complexity and strength. It is crucial that the selected algorithm be checked against the SONET scrambler. An early proposal for ATM-layer scrambling (since fixed) had the property that it fought with the SONET-layer scrambler. At this point, an implementation would want to retain backward compatibility with the deployed systems. Hence, I'd suggest that implementers put in a knob letting administrators select either "no PPP scrambling" or "PPP scrambling". Clearly *I* don't want to buy products that have the vulnerability noted at the top. Ran rja@home.net PS: I'm not on the PPP or PPPSDH lists, so if folks on those lists want me to see any followups, those folks will need to Cc: me directly. PPS: The followups list definitely will need trimming. Please edit appropriately if you followup...