Consider Go-Back-N Sliding Window Protocol. (a) We do not have a timer associated with ACK frames, what does this imply, i.e., what assumption do we make? (b) For a damaged frame, the protocol decides to do nothing. Argue whether the receiver sends an NAK to the sender could accelerate the transmission speed. (c) Suppose that the sender receives an ACK whose sequence number is outside of the sending window. What does that mean and how does the sender handle the situation? (d) Suppose that the frame sequence number is between 0..MAX_SEQ, where MAX_SEQ is the maximum sequence number. Explain why for this protocol, the window size on the senders side is set to MAX_SEQ?

*Source Code*

/* Protocol 5 (pipelining) allows multiple outstanding frames. The sender * may transmit up to MAX_SEQ frames without waiting for an ack. In addition, * unlike the previous protocols, the network layer is not assumed to have * a new packet all the time. Instead, the network layer causes a * network_layer_ready event when there is a packet to send. * * To compile: cc -o protocol5 p5.c simulator.o * To run: protocol5 events timeout pct_loss pct_cksum debug_flags * * Written by Andrew S. Tanenbaum * Revised by Shivakant Mishra */ #define MAX_SEQ 7 /* should be 2^n - 1 */ typedef enum {frame_arrival, cksum_err, timeout, network_layer_ready} event_type; #include "protocol.h" static boolean between(seq_nr a, seq_nr b, seq_nr c) { /* Return true if (a <=b < c circularly; false otherwise. */ if (((a <= b) && (b < c)) || ((c < a) && (a <= b)) || ((b < c) && (c < a))) return(true); else return(false); } static void send_data(seq_nr frame_nr, seq_nr frame_expected, packet buffer[]) { /* Construct and send a data frame. */ frame s; /* scratch variable */ init_frame(&s); s.kind = data; s.info = buffer[frame_nr]; /* insert packet into frame */ s.seq = frame_nr; /* insert sequence number into frame */ s.ack = (frame_expected + MAX_SEQ) % (MAX_SEQ + 1); /* piggyback ack */ to_physical_layer(&s); /* transmit the frame */ start_timer(frame_nr); /* start the timer running */ } void protocol5(void) { seq_nr next_frame_to_send; /* MAX_SEQ > 1; used for outbound stream */ seq_nr ack_expected; /* oldest frame as yet unacknowledged */ seq_nr frame_expected; /* next frame expected on inbound stream */ frame r; /* scratch variable */ packet buffer[MAX_SEQ+1]; /* buffers for the outbound stream */ seq_nr nbuffered; /* # output buffers currently in use */ seq_nr i; /* used to index into the buffer array */ event_type event; enable_network_layer(); /* allow network_layer_ready events */ ack_expected = 0; /* next ack expected inbound */ next_frame_to_send = 0; /* next frame going out */ frame_expected = 0; /* number of frame expected inbound */ nbuffered = 0; /* initially no packets are buffered */ while (true) { wait_for_event(&event); /* four possibilities: see event_type above */ switch(event) { case network_layer_ready: /* the network layer has a packet to send */ /* Accept, save, and transmit a new frame. */ from_network_layer(&buffer[next_frame_to_send]); /* fetch new packet */ nbuffered = nbuffered + 1; /* expand the sender's window */ send_data(next_frame_to_send, frame_expected, buffer); /* transmit the frame */ inc(next_frame_to_send); /* advance sender's upper window edge */ break; case frame_arrival: /* a data or control frame has arrived */ from_physical_layer(&r); /* get incoming frame from physical layer */ if (r.seq == frame_expected) { /* Frames are accepted only in order. */ to_network_layer(&r.info); /* pass packet to network layer */ inc(frame_expected); /* advance lower edge of receiver's window */ } /* Ack n implies n - 1, n - 2, etc. Check for this. */ while (between(ack_expected, r.ack, next_frame_to_send)) { /* Handle piggybacked ack. */ nbuffered = nbuffered - 1; /* one frame fewer buffered */ stop_timer(ack_expected); /* frame arrived intact; stop timer */ inc(ack_expected); /* contract sender's window */ } break; case cksum_err: ; /* just ignore bad frames */ break; case timeout: /* trouble; retransmit all outstanding frames */ next_frame_to_send = ack_expected; /* start retransmitting here */ for (i = 1; i <= nbuffered; i++) { send_data(next_frame_to_send, frame_expected, buffer); /* resend 1 frame */ inc(next_frame_to_send); /* prepare to send the next one */ } } if (nbuffered < MAX_SEQ) enable_network_layer(); else disable_network_layer(); } } int main(int argc, char *argv[]) { int timeout_interval, pkt_loss, garbled, debug_flags; long event; if (!parse_first_five_parameters(argc, argv, &event, &timeout_interval, &pkt_loss, &garbled, &debug_flags)) { printf ("Usage: p5 events timeout loss cksum debug "); exit(1); } printf(" Simulating Protocol 5 "); start_simulator(protocol5, protocol5, event, timeout_interval, pkt_loss, garbled, debug_flags); }