1. Go-Back-N ARQ
packets transmitted continuously (when available) without waiting for ACK, up to N outstanding, unACK’ed packets
a logically different sender timer association with each unACK’ed packet: extension of AB protocol
Receiver:
ACK packet if correctly received and in-order, pass to higher layer
NACK or ignore (possibly discard) corrupt or out-of-order packet
Sender :
if NACK received, or timeout, for packet n, begin resending from n all over again
cumulative ACK: ACK of n implicitly acknowledges up through n

2. Go-Back-N continued no receiver transport buffering with discard
saves resources at receiver
avoids large bursts of packet delivery to higher layers
simplicity in buffering and protocol processing at sender and receiver
tradeoff between buffering/processing complexity and bandwidth

3. Go Back N: Example

5. Selective Repeat ARQ As in go back-N: Unlike Go-Back-N:
packet transmitted when available, up to limit timer associated with each unACK’ed packet
receiver NACK’s or ignores corrupted packets
Unlike Go-Back-N:
out-of-order (but otherwise correct) packet is ACK’ed
receiver: buffers out-of-order packets
sender: on timeout or NACK of packet n, just retransmit n

7. Selective Repeat ARQ (cont)
Notes:
more receiver buffering than Go back-N
more complicated buffer management by both sides
saves bandwidth: no need to retransmit correctly received packets

8. Selective Repeat ARQ: example

10. How Big Can a Window Be? Suppose sequence number space size is N
Q: How big can window be and have SR still work?
Partial answer: N-1 won’t work:
Fundamental problem: sender and receiver
do not have synchronized info
cannot know exact same information

12. Throughput Comparison
p - loss probability
t_trans - pkt transmission time
rtt - round trip time
tput_SW = (1-p)/(rtt+t_trans)
tput_GB = (1-p)/(p rtt + t_trans)
tput_SR = (1-p)/t_trans

13. Throughput Comparison
1GB/sec link
1KB pkt
-> t_trans = 8ms
rtt =1ms

14. Throughput Comparison
1GB/sec link
1KB pkt
-> t_trans = 8ms
rtt =30ms

15. Detecting Errors: checksums
Need to detect errors: bits in packet may be flipped while in transit or stored at intermediate notes.
Approach: add extra bits to packets that will allow us to detect (possibly correct) bit errors

16. Simple example: Parity
Given n-1 bit packet, add n-th bit, choosing value so that total number of 1
bits in packets (including nth bit) is odd (odd parity).
example packet:
At receiver:
count # 1’s in packet, if even, then error!
what if even number of bit flips?
what if odd number of bit flips?
error detection
seq ack data (parity bit)

17. Simple example: Parity
Note:
many codes with more powerful error detection capabilities
CRC-16: 16 bit code, detects 2 random bit errors, 16 errors in sequence.
packet header itself often separately checksummed
checksumming also done at data link layer
hardware support for transport-level checksum: SGI

18. Forward Error Correction: FEC
ARQ protocols operate by detecting errors and retransmitting
retransmission needs round-trip delay to recover
may be too long for deep space, or high-speed, real-time applications
FEC: key idea is to transmit enough redundant data to allow receiver to recover from errors itself! (no sender transmission required)

19. (a) 2-dimensional parity
row parity
d1, … d1,J d1,J+1
d2, … d2,J d2,J+1
dI, … dI,J dI,J+1
dI+1,1 … dI+1,J d1,j+1
seq # ACK# data EDF
column
parity
(b) packet format
(a) 2-dimensional parity
(c) example: no errors
(d) example: single bit error