Tutorial 2 Solution

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Tutorial 2 Solution
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• Q1. Consider a channel that can lose packets but
  has a maximum delay that is known. Modify
  protocol rdt2.1 to include sender timeout and
  retransmit. Argue why the protocol can work
  correctly.

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rdt2.1 in action no error
sender
receiver
wait for call 0
pkt 0
wait for pkt 0
ACK
wait for ACK/NAK
wait for call 1
pkt 1
wait for pkt 1
ACK
wait for ACK/NAK
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rdt 2.1 in action corrupt packet
sender
receiver
wait for call 0
corrupt
pkt 0
wait for pkt 0
NAK
re-transmision
wait for ACK/NAK
pkt 0
ACK
wait for call 1
pkt 1
wait for pkt 1
ACK
wait for ACK/NAK
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rdt 2.1 in action corrupt ACK
sender
receiver
wait for call 0
pkt 0
wait for pkt 0
corrupt
ACK
re-transmission
wait for ACK/NAK
pkt 0
wait for pkt 1
discard pkt
ACK
wait for call 1
pkt 1
ACK
wait for ACK/NAK
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rdt 2.1 in action data packet is lost
sender
receiver
wait for call 0
lost
pkt 0
wait for pkt 0
?
wait for ACK/NAK
?
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rdt 2.1 in action ACK/NAK is lost
sender
receiver
wait for call 0
pkt 0
wait for pkt 0
ACK
lost
wait for ACK/NAK
?
wait for pkt 1
?
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Modifications

• Changes of the sender
• Timer events
• When to start the timer
• How long is the timeout?
• What will the sender do for a timeout event
• Changes of the receiver?
• Check how the original receiver responds to new
  events (i.e., lost packets or ACKs)

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timer gt 2 maximum delay
Start timer
Stop timer
Start timer
Stop timer
Stop timer
Stop timer
Start timer
Start timer
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• How the Receiver Responds
• Suppose the timeout is caused by a lost data
  packet, i.e., a packet on the sender-to-receiver
  channel. The receiver never received the previous
  transmission and, if the timeout retransmission
  is received, it look exactly the same as if the
  original transmission is being received.
• Suppose an ACK is lost. The sender will
  eventually retransmit the packet on a timeout.
  But a retransmission is exactly the same action
  that is take if an ACK is garbled. Thus the
  sender's reaction is the same with a loss, as
  with a garbled ACK.

No changes are needed!
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timer in action data packet is lost
sender
receiver
wait for call 0
lost
pkt 0
start timer
wait for ACK/NAK
timeout
start timer
pkt 0
re-tranmission
wait for pkt 0
ACK
stop timer
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timer in action ACK/NAK is lost
sender
receiver
wait for call 0
pkt 0
start timer
wait for pkt 0
ACK
lost
wait for ACK/NAK
timeout
start timer
pkt 0
re-tranmission
stop timer
wait for pkt 1
discard pkt
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• Q2. Redesign rdt2.1 without using NAK packets. In
  other words, the receiver in the new protocol can
  only use ACK packets. Draw the FSM of the new
  protocol.

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rdt2.2 a NAK-free protocol

• same functionality as rdt2.1, using ACKs only
• instead of NAK, receiver sends ACK for last pkt
  received OK
• receiver must explicitly include seq of pkt
  being ACKed
• duplicate ACK at sender results in same action as
  NAK retransmit current pkt

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rdt2.2 sender, receiver fragments
rdt_send(data)
sndpkt make_pkt(0, data, checksum) udt_send(sndp
kt)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isACK(rcvpkt,1) )
udt_send(sndpkt)
sender FSM fragment
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt,0)
rdt_rcv(rcvpkt) (corrupt(rcvpkt)
has_seq1(rcvpkt))
L
receiver FSM fragment
udt_send(sndpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
has_seq1(rcvpkt)
extract(rcvpkt,data) deliver_data(data) sndpkt
make_pkt(ACK1, chksum) udt_send(sndpkt)