Chapter 3 - Part2

Question 1

What is the Stop and Wait Protocol

Answer 1

sender sends one packet then waits for a receiver response

• resends packet if no response (time-out)

Question 2

How is reliable services implemented (for app/transport layer)

Answer 2

Using sockets between the application and transport layer to ensure reliable data transfer protocol

• Works even if the network layer has an unreliable channel (link)

Question 3

What are some mechanisms provided by the stop-and-wait protocol?

Answer 3

• ACKS
• Retransmissions
• Checksums
• Timeout
• Sequence #’s

Question 4

What kind of protocol is Sequence numbering found in? TCP, UDP, or both?

Answer 4

TCP ONLY.

Question 5

For Stop and Wait Protocol, what happens when there is no loss?

Answer 5

Sender: sends pkt 0

Receiver: rcv pkt 0, sends ack 0

Question 6

For Stop and Wait Protocol, what happens when there is packet loss in the middle of transmission?

Answer 6

Timeout happens and it resends the packet that is lost.

Question 7

For Stop and Wait Protocol, what happens when there is a packet with bit error?

Answer 7

Receiver detects the error through checksum.

Receiver sends ACK for the last packet received correctly

Question 8

For Stop and Wait Protocol, what happens when ACK gets lost in the middle of transmission?

Answer 8

Sender doesn’t receive the ACK.

Sender sends the same packet.

Receiver receives DUPLICATE packet
- receiver ignores packet but sends ACK again

Question 9

For Stop and Wait Protocol, what happens when there is a premature timeout / delayed ACK?

Answer 9

Timeout occurs. Sender resends packet.

Receiver receives DUPLICATE packet and sends ACK again.

Sender receives 2 back-to-back ACKS, ignores it.

Question 10

Cons of a stop-and-wait operation?

Answer 10

Waste of time during RTT periods, utilization is very low since it only sends 1 packet at a time.

Question 11

How to solve the Stop-and-wait operation?

Answer 11

Through Pipelined Protocols:
- increases utilization
- More packets per RTT

Question 12

What are the 2 Pipelined Protocols?

Answer 12

1. Go-Back-N
2. Selective Repeat

Question 13

What is Go-Back-N? Key points / window

Answer 13

Key points:
- No buffers for out-of-order packets (Discards them)
- Uses cumulative ACKs (ACK for last in-order packet)
– Ex: receiving ACK2 assumes ACK0 and ACK1 are received
- Retransmits the lost packet and all subsequent packets

Window:
- Only slides when cumulative ACKs are received.
- Sends all packets in a window for lost packet

Question 14

What is Selective Repeat? key points / window

Answer 14

Key points:
- Buffers out of order packets before delivering them to app layer
- Uses individual ACKs
- Sender only retransmits the packets not received & timer for each unacked packet

Window:
-Slides forward as ACKs are received