Quiz 5 Key CNT 5106C 22 March 2011 Instructions: Write your last name, then first initial in upper righthand corner of answer sheet, and write “Quiz 5” centered at top. Answer all questions. 1. What is HOL blocking in routers? Where can it occur and why? 2. a) What is NAT and what benefits does it provide? b) How does NAT route datagrams from an outside source to a destination that is behind it? Explain. KEY 1. HOL is head of line blocking, where the first packet in the queue of an input line cannot be move to the proper output line queue, preventing other packets in the input queue to be moved to their output queues (which may not be busy at that time). It occurs in input queues, and it can occur because the fabric is busy or the output line itself is busy as another packet is being sent there. 2. a) NAT is network address translation, and it provides several benefits. The main ones are that many interfaces (e.g., end hosts) can share the same routable IP address, and interfaces behind the NAT cannot be accessed directly unless they initiate a connection. Interfaces behind the NAT use unroutable IP addresses, so the NAT uses it routable IP
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