Fundamentals of Computer Networks ECE 478578 Lecture 7

Fundamentals of Computer Networks ECE 478/578 Lecture #7: Reliable Transmission Instructor: Loukas Lazos Dept of Electrical and Computer Engineering University of Arizona

Reliable Transmission strategies under a lossless channel Simplex connection Simplex Stop-and-wait Transmission Strategies under a lossy channel Process of retransmitting frames that cannot be corrected by CRC codes Stop-and-wait Go-back-N Selective repeat 2

Simplex Protocol – Lossless Channel Data flows one direction only Communication channel never loses frames Sender Receiver request reply negotiate ack Receiver is always ready to receive packets Problem: receiver can be flooded if it does not process packets fast enough . . . Solution: Introduce delay at sender 3

Simplex Protocol – Lossy Channel Stop-and-wait 4

Failure of the Stop-and-Wait ACK is lost or arrives after timeout 5

Solution based on Sequence No Use a one-bit sequence number on the header Sender Receiver Fram e 0 ACK Fram e 0 Fram e 1 ACK Fram e 0 6

(In)Efficiency of Stop-and-Wait Example: Consider 1. 5 Mbps link, 45 ms RTT Delay x Bandwidth =67. 5 kb = 8 KB If frames are 1 KB long then max rate 1024 x 8 /0. 045 = 182 Kbps Only 1/8 th of link’s capacity Goal: keep the pipe full, i. e. have maximum # of bits unacknowledged at any given time Our example, we could have 8 frames unacknowledged 7

Go-back-N Protocol Main idea: Leave up to N frames unacknowledged at any given time 8

Go-Back-N Each frame is assigned a Seq. Num Variables at the sender SWS: Send Window Size LAR: Last Acknowledgment Received LFS: Last Frame Sent Rule: LFS – LAR SWS (at most SWS frames un. ACKed) Variables at the receiver RWS: Receive Window Size LAF: Largest Acceptable Frame (in seq #) LFR: Last Frame Received Rule: LAF – LFR RWS 9

Sliding Window Depicted At the sender At the receiver 10

Updating Variables SWS Sender Variables LSFi LARi Si RWS Receiver Variables LFRi LAFi Si = LFRi+1 (Go-back-N only accepts packets in order) Si LAFi (which is guaranteed in our case, for RWS 1) LSFi+1 =LSF + 1, LFRi+1 = LFRi + 1, LAFi+1 = LAFi + 1 Window at sender advanced only if ACK received 11

Example of Go-Back-N SWS = 4 LFS = 4 LAR = 0 Sender Timeout Receiver 2 0 1 4 3 1 2 RWS = 1 LFR = 0 LAF = 1 discarded Max Sequence Number SWS + 1 Link does not re-arrange packets 12

Piggypacking Duplex communication, attach ACK on the reply frame Asymmetry in frame size can cause timeouts Go-Back-N can be turned into Stop-and-Wait Sender Receiver 13

Selective Repeat Window size can be very large for nets with large delay x bandwidth Inefficient to retransmit all N frames if one is lost Selective repeat allows the re-transmission of only the lost packets Accepts out-of-order packets Simply increase the RWS up to SWS (does not make sense to allow for RWS > SWS) 14

Example of Selective Repeat SWS = 4 LFS = 4 LAR = 0 LFS = 4 LAR = 4 Sender Timeout Receiver 2 0 1 3 4 1 RWS = 4 LFR = 0 LAF = 4 Buffered 5 6 7 Max Sequence Number 2 SWS 15