Transcript ch11Review
Assume that a file is transferred from a node A to a node B. The file has been fragmented in 5 frames. Frame 0 is corrupted, the ACK of frame 1 is corrupted, ACK of frame 4 arrived after the time of frame 4 expired. Show the flow of frames exchanged between the two nodes, and the status of the windows after the transmission/reception of each information/control frame, if the field of sequence number is three-bit-length, and the sender and receiver are using the maximum possible window size, for the following data link control techniques: • • • • • Simplest Stop-And-Wait Stop-and-wait ARQ Go-back-N ARQ Selective- Repeat ARQ Simplest (ideal channel, no error) A B f0 f1 f2 f3 f4 Stop and Wait in ideal channel (no error) A B f0 ACK 1 f1 ACK 0 f1 ACK 0 f0 ACK 1 Stop and Wait ARQ A B f0 Time out f0 ACK 1 accepted f1 ACK 0 accepted f1 ACK 0 discarded f0 ACK 1 accepted f1 Time out f1 ACK 0 accepted discarded Go-Back-N ARQ 0 1 2 3 4 5 6 7 0 A Time out Time out 0 1 2 3 4 5 6 7 0 B f0 f1 f2 f1 f0 ACK 1 f2 ACK 2 f2 f3 0 1 2 3 4 5 6 7 0 … 0 1 2 3 4 5 6 7 8 discarded discarded 0 1 2 3 4 5 6 7 8 f1 ACK 4 discarded discarded 0 1 2 3 4 5 6 7 8 f4 Time out 0 1 2 3 4 5 6 7 0 … ACK 5 f4 ACK 5 0 1 2 3 4 5 6 7 8 discarded Duplex Communication with Stop-and-Wait ARQ Assume that computer A and computer B communicate with each other. Each has 3 frames to send to the other. The three data frames sent by A are denoted as A1, A2, A3 and the frames from B are B1, B2, B3. They use duplex Stop-and-Wait ARQ for the communication, i.e., each data frame has a sequence # field and an Ack # field. Suppose each data frame needs 1 ms to transmit and each ack-only frame needs no time to transmit. The time-out value is set to be 5ms. The round trip time is always 2 ms. Use “A1, (0,1)” denote a frame with data A1 and sequence 0, ack 1. Ideal channel without any error/lost or congestion A1 A2 A3 A B B1 B2 B3 Algorithm 11.5 Sender-site algorithm for Stop-and-Wait ARQ (continued) 11.8 Algorithm 11.5 Sender-site algorithm for Stop-and-Wait ARQ 11.9 (continued) Algorithm 11.6 Receiver-site algorithm for Stop-and-Wait ARQ Protocol 11.10 Rn is the sequence number of the next packet expected Stop-and-Wait ARQ Example Assume that computer A sends 3 frames to computer B. The three data frames sent by A are denoted as A1, A2, A3. Suppose each data frame needs 1 ms to transmit and each ack frame needs no time to transmit. The time-out value is set to be 4ms. The round trip time is always 2 ms. Use “A1, 0” denote a frame with data A1 and sequence 0. Use “Ack 1” denote an ack frame with ack number 1. Case 1: Ideal channel without any error/lost or congestion A1 A2 A3 A B Case 2: second frame from A to B is lost A1 A2 A3 A B Case 3: second frame from A to B uses 3 ms A1 A2 A3 A B Case 4: second ack frame from B to A uses 3ms A1 A2 A3 A B