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Unformatted text preview: Computer Networks Link initialization Saad Mneimneh Computer Science Hunter College of CUNY New York- hello- hello/ack- ack three-way handshake 1 Introduction All ARQ algorithms assume some sort of initialization of the DLC link. We did not explicitly emphasize this fact before to keep the illustration simple. However, for the ARQ to operate correctly, we require that initially: no frames are traveling on the link SN and RN are both equal to 0 at both ends of the link One might think that this problem of initialization is trivial: simply initialize the DLC at each end of the link when the link is first physically placed, and then use an ARQ algorithm forever. Unfortunately, the link can fail. good news (link view): the ARQ algorithm guarantees that frames not received correctly before the failure would be received after the failure. bad news (network view): when a link fails for a long period of time, it be- comes necessary for the transport and/or network layers to take over and set up alternative paths for packets that were not delivered. When the failed link eventually returns to operation, the higher layers restore the path through the link, thus causing the DLCs at both ends to possibly deliver duplicate packets. Therefore, both DLCs should view the link as being segmented into an alternation of up and down periods. The DLCs must then be properly initialized at the beginning of each up period. At the end of an up period, however, there may be frames in 1 the process of being communicated that are not received. These frames will not be received upon the start of another up period because the DLCs will be reinitialized....
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