3.4

Question 1

What is the USP protocol? (Protocol A)

Answer 1

Unrestricted Simple Protocol

Sender (Repeats forever all steps)

• Get App Layer data from socket
• Construct Transport Layer segment with header
• Give segment to Network Layer to transmit

Receiver (Repeats forever all steps)

• Wait for segment from Network Layer
• Process segment, remove Transport Layer header
• Direct message to appropriate AL socket

Question 2

What is the SAW protocol? (Protocol B)

Answer 2

Stop and Wait Protocol

Sender (Repeats forever all steps)

• Get App Layer data from socket
• Construct Transport Layer segment with header
• Give segment to Network Layer to transmit
• Wait for Ack from other end (via NL)

Receiver (Repeats forever all steps)

• Wait for segment from Network Layer
• Process segment, remove Transport Layer header
• Direct message to appropriate AL socket
• Construct Ack segment
• Give Ack to NL to transmit

Question 3

What is the PNA protocol? (Protocol C)

Answer 3

Positive Negative ACKS
(handles corrupted data, but not acks)

Sender (Repeats forever all steps)

• Get App Layer data from socket
• Construct Transport Layer segment with header
• Give segment to Network Layer to transmit

Receiver (Repeat all steps for ever)
-Wait for segment from NL
-Process segment, compute checksum
-If valid checksum
Then remove TL header
Deliver data to AL_Socket
Construct Ack
Give Ack to NL to transmit

Else Discard Segment
Construct Nak
Give Nak to NL to transmit

Question 4

What is the PAR protocol? (Protocol D)

Answer 4

Positive Ack with retransmission
(handles both corrupted data and acks)

Sender  (Repeat forever)
Construct TL segment with header (including seqnum)
give segment to NL to transmit
set timer
wait for event
If valid ack cancel timer
Get next Al from socket
update seqnum

Receiver
-Wait for segment from NL
-Process segment, compute checksum
-If valid checksum
Then 
  If seqnum = expected
  Then remove TL header
  Deliver data to AL_Socket
  Construct Ack
  Give Ack to NL to transmit
  update expected seqnum
  Else Discard Segment
  Construct Nak
  Give Nak to NL to transmit
Else discard segment