7 - Reliability

Question 1

Rdt 1.0 (this card is not very good, update it)

Answer 1

Perfectly reliable

Question 2

Rdt2

Answer 2

Underlying channel may flip bits in packets

so use ACKs to recover

• error detection
• receiver feedback

Question 3

rdt 3.0

Answer 3

Send pkt 0
Receive pkt 0
send ACK 0
Receive ACK 0

repeat

If not ACK received, resend last packet

Question 4

What is sliding window protocol?

Answer 4

Send more data at once

first packet bit, t = 0
last bit transmitted t= L/R

first packet bit arrives
last packt bit arrives, send ACK
last bit of 2nd pkt, send ACK
last bit of 3rd, send ACK

ACK arrives send next packet t = RTT + L/R

(3Psize)/(BRTT) = efficiency increased by N (window size)

Question 5

What is pipelined protocols?

Answer 5

pipelining - sender allows multiple, unacknowledged pkts

go-back-N
selective repeat

Question 6

Go back N

Answer 6

Receiver only sends cumulative ACKs (ie no gaps)

Sender maintains timer for oldest unacked packet
- retransmit all unacked packets

Question 7

Selective Repeat

Answer 7

Receiver acks individual packets
Sender maintains timer for each unacked packet
- only resend unacked packet

Question 8

What is the advantage of GbN?

Answer 8

Simplicity of receiver buffering, need NOT buffer any out of order packets

Question 9

What is a disadvantage of GbN?

Answer 9

subsequent retransmission might be lost, necessitating further transmissions

Question 10

how large must window size be?

Answer 10

Less than or equal to half the size of the sequence number space for SR protocol

Question 11

Parity Bit

Answer 11

A bit that is added to ensure that num of 1 bits is always even (or odd)

Question 12

What is the internet checksum?

Answer 12

Binary data viewed as 16bit symbols where b is the 1’s complement of the rest of data

If it’s 1111…1 no error, else error

Question 13

1’s Complement

Answer 13

Convert all 0s to 1s and all 1s to 0s