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→Протоколы с выборочным повтором
Также можно посылать отрицательное подтверждение (NAK), в этом случае отправитель перепошлёт потерянный кадр до истечения таймаута.
<code> /* Protocol 6 (nonsequential receive) accepts frames out of order, but passes packets to the network layer in order. Associated with each outstanding frame is a timer. When the timer goes off, only that frame is retransmitted, not all the outstanding frames, as in protocol 5. */ #define MAX_SEQ 7 /* should be 2^n - 1 */ #define NR_BUFS ((MAX_SEQ + 1)/2) typedef enum {frame_arrival, cksum_err, timeout, network_layer_ready, ack_timeout} event_type; #include "protocol.h" boolean no_nak = true; /* no nak has been sent yet */ seq_nr oldest_frame = MAX_SEQ+1; /* init value is for the simulator */ static boolean between(seq_nr a, seq_nr b, seq_nr c) { /* Same as between in protocol5, but shorter and more obscure. */ return ((a <= b) && (b < c)) || ((c < a) && (a <= b)) || ((b < c) && (c < a)); } static void send_frame(frame_kind fk, seq_nr frame_nr, seq_nr frame_expected, packet buffer[]) { /* Construct and send a data, ack, or nak frame. */ frame s; /* scratch variable */ s.kind = fk; /* kind == data, ack, or nak */ if (fk == data) s.info = buffer[Файл: Networks 1frame_nr % NR_BUFS]; s.5 Protocol6seq = frame_nr; /* only meaningful for data frames */ s.ack = (frame_expected + MAX_SEQ) % (MAX_SEQ + 1); if (fk == nak) no_nak = false; /* one nak per frame, please */ to_physical_layer(&s); /* transmit the frame */ if (fk == data) start_timer(frame_nr % NR_BUFS); stop_ack_timer(); /* no need for separate ack frame */ } void protocol6(void) { seq_nr ack_expected; /* lower edge of sender's window */ seq_nr next_frame_to_send; /* upper edge of sender's window + 1 */ seq_nr frame_expected; /* lower edge of receiver's window */ seq_nr too_far; /* upper edge of receiver's window + 1 */ int i; /* index into buffer pool */ frame r; /* scratch variable */ packet out_buf[NR_BUFS]; /* buffers for the outbound stream */ packet in_buf[NR_BUFS]; /* buffers for the inbound stream */ boolean arrived[NR_BUFS]; /* inbound bit map */ seq_nr nbuffered; /* how many output buffers currently used */ event_type event; enable_network_layer(); /* initialize */ ack_expected = 0; /* next ack expected on the inbound stream */ next_frame_to_send = 0; /* number of next outgoing frame */ frame_expected = 0; /* frame number expected */ too_far = NR_BUFS; /* receiver's upper window + 1.png*/ nbuffered = 0; /* initially no packets are buffered */ for (i = 0; i < NR_BUFS; i++) arrived[i]= false; while (true) { wait_for_event(&event); /* five possibilities: see event_type above */ switch(event) { case network_layer_ready: /* accept, save, and transmit a new frame */ nbuffered = nbuffered + 1; /* expand the window */ from_network_layer(&out_buf[next_frame_to_send % NR_BUFS]); /* fetch new packet */ send_frame(data, next_frame_to_send, frame_expected, out_buf); /* transmit the frame */ inc(next_frame_to_send); /* advance upper window edge */ break;
=== Верификация протоколов ===
