I think if your goal is to see if they know that your shouldn't blindly filter ICMP for IPv6, and you're specifically looking for knowledge of PMTUD, then a better question would be "Please list the problems that could occur if all ICMPv6 traffic is blocked between two host systems." Which should get you a minimum of neighbor discovery, and up into PMTUD for those who have some knowledge on the subject. If you just say ICMP your answers will be all over the place since blocking of ICMP outright for endpoints is rampant today in the IPv4 world. They might even know the answer but not think of it because of the lack of context. I generally try to stay away from any question that has a definitive answer, as that will only tell you if they happened to read and retain that piece of information somewhere along the way. In my experience, people who have an "OK" understanding of Layer-3, might not always have a good understanding of what happens below that. A better approach might be to have an open ended question that asks them to describe what events will take place for a pair of host systems to communicate in as much detail as they can. If you're asking the question you can leave it intentionally vague and use the questions they ask to evaluate their ability to work through problems; if it needs to be asked by HR then you can narrow it down to include more detail. A good applicant should be able to explain the ARP process at a minimum. If they can't they have no business being in networking in a question like this. I know it sounds trivial, but you'd be surprised how many "experts" I've met who go blank at a question like this. Even more telling than a correct answer is an incorrect answer. I'm always on the look-out for IT people who like to make stuff up; I have no tolerance for that. On Thu, Jul 5, 2012 at 1:02 PM, William Herrin <bill@herrin.us> wrote:
Hi folks,
I gave my HR folks a screening question to ask candidates for an IP expert position. I've gotten some "unexpected" answers, so I want to do a sanity check and make sure I'm not asking something unreasonable. And by "unexpected" I don't mean naively incorrect answers, I mean oh-my-God-how-did-you-get-that-cisco-certification answers.
The question was:
You implement a firewall on which you block all ICMP packets. What part of the TCP protocol (not IP in general, TCP specifically) malfunctions as a result?
My questions for you are:
1. As an expert who follows NANOG, do you know the answer? Or is this question too hard?
2. Is the question too vague? Is there a clearer way to word it?
3. Is there a better screening question I could pass to HR to ask and check the candidate's response against the supplied answer?
Thanks, Bill Herrin
-- William D. Herrin ................ herrin@dirtside.com bill@herrin.us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004
-- Ray Soucy Epic Communications Specialist Phone: +1 (207) 561-3526 Networkmaine, a Unit of the University of Maine System http://www.networkmaine.net/