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Data Link Layer
===============

This is an assignment for Level-3/Term-2.


Implementation Guideline
------------------------

[x] Adopt the code from your TCP assignment. Replace 'struct msg' with 'struct
    pkt'. Also replace 'struct pkt' with 'struct frm'. In each frame, keep an
    additional variable to designate the type of the frame.

[x] Make necessary changes in the procedure names to replace the word 'layer5'
    with 'layer3' and 'layer3' with 'layer1'. For example,
    tolayer3(calling_entity,packet) will be renamed as
    tolayer1(calling_entity,frame); tolayer5(calling_entity,message) will be
    renamed as tolayer3(calling_entity,packet).

[x] Unidirectional/bidirectional data transfer. Please follow the TCP assignment
    guideline to achieve this.

[x] Implementation of piggybacked acknowledgement: To achieve piggybacking with
    minimal change in your code, we have come up with the following protocol
    which differs from the ideal implementation of piggybacking.

[x] Error control. You will implement the error detection mechanism using CRC
    (Cyclic Redundancy Check) checksum. The generator polynomial should be taken
    as input in your code. Your code must print the input bit string to the CRC
    algorithm, generator polynomial, and the calculated CRC while computing CRC
    on the sender’s side. On the receiver side, print the input bit string to
    the CRC algorithm, generator polynomial and an error message if a data
    transmission error has occurred.


Protocol for Piggybacked Acknowledgement
----------------------------------------

After B receives a data frame Fab from A, B does not send an acknowledgement
immediately. B sets a state variable called “OutstandingACK” to 1. If B receives
a packet from its upper layer (layer 3) while OutstandingACK = 1, then B creates
a data-ack frame Fba and piggybacks the acknowledgement of the previously
received frame Fab in this frame. Otherwise, B creates a data frame.

However, it is possible that B does not receive any data frame to piggyback the
acknowledgement. In this case, A will eventually resend frame Fab. Upon
receiving a duplicate frame, B will send the outstanding acknowledge frame