CITS3002 Computer Networks  
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A Sample selective-repeat Protocol, continued

We have most work to perform when a frame arrives at the Physical Layer - either some DL_DATA or an DL_ACK.

As with earlier protocols, we must first determine if the frame has been corrupted. A more complex selective-repeat protocol may incorporate DL_NACKs. 

EVENT_HANDLER(physical_ready)
{
    FRAME     frame;
    int       link, checksum;
    size_t    len;

    len     = sizeof(FRAME);
    CHECK(CNET_read_physical(&link, &frame, &len));

    checksum          = frame.checksum;
    frame.checksum    = 0;

    if(CNET_ccitt((unsigned char *)&frame, len) != checksum) {
        return;       // bad checksum, simply ignore frame
    }
    if(frame.kind == DL_ACK) {
        if(between(ackexpected, frame.seqno, nextdatatosend)) {

            while(between(ackexpected, frame.seqno, nextdatatosend)) { 
                --nbuffered;          // buffer now available

                CNET_stop_timer(timers[ackexpected % NRBUFS]);
                timers[ackexpected % NRBUFS] = NULLTIMER;
                inc(&ackexpected);
            }
            CNET_enable_application(ALLNODES);
        }
    }



CITS3002 Computer Networks, Lecture 3, Data Link Layer protocols, p31, 13th March 2024.