NUS.SOC.CS2105

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Application
Transport
you are here
Network
Link
Physical
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Application
Transport (process-to-process)
Network (unreliable, host-to-host)
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Transport (process-to-process)
Network (unreliable, host-to-host)
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Transport (process-to-process)
Network (unreliable, host-to-host)
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src port dst port
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Connection-Oriented Socket

• (Src Port, Src Addr,
• Dest Port, Dest Addr)

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Connectionless Socket

• (Dest Addr, Dest Port)

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UDP Segment Header
src port dst port length checksum
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UDP Checksum
0110011001100000
0101010101010101
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UDP Checksum
1011101110110101
1000111100001100
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UDP Checksum
0100101011000010
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UDP detects errors

• (no recovery)

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Reliable Data Transfer

• rdt1.0, rdt2.0, rdt3.0

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Application
Application
Transport
Transport
rdt_send()
deliver_data()
Network
Network
udt_send()
rdt_recv()
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Finite State Machine
Event
Action
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hungry
get up eat
sleep
awake
stomach full
go back to bed
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rdt 1.0

• underlying channel is reliable

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rdt 1.0 sender
rdt_send(data)
wait for call
pkt make_pkt(data) udt_send(pkt)
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rdt 1.0 receiver
rdt_recv(pkt)
wait for call
extract(pkt, data) deliver_data(data)
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send data
recv pkt
send pkt
deliver
wait for call
wait for call
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rdt 2.0

• channels with bit-errors

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ARQ AutomaticRepeat reQuest

1. Error detection
2. Receiver feedback (ACK NAK)
3. Retransmission

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rdt 2.0 sender
wait for call
wait for ACK/NAK
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rdt 2.0 receiver
wait for call
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(No Transcript)
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Stop-and-Wait
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What if ACK/NAK is corrupted?
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Idea Retransmit if ACK/NAK is corrupted
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(No Transcript)
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rdt 2.1

• channels with bit-errors

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rdt 2.1 sender
wait for call 0
wait for ACK/NAK 0
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wait for call 0
wait for ACK/NAK 0
wait for ACK/NAK 1
wait for call 1
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rdt 2.1 receiver
wait for 0
wait for 1
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wait for call 0
wait for 0
wait for ACK/NAK 0
wait for 1
wait for ACK/NAK 1
wait for call 1
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wait for call 0
wait for 0
wait for ACK/NAK 0
wait for 1
wait for ACK/NAK 1
wait for call 1
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rdt 2.2

• NAK-free

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No NAK

• Send ACK of last correctly received packet instead

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rdt 3.0

• lossy channels with bit-errors

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How to tell if a packet is loss?

• (cant know for sure)

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Use Timeout

• (may lead to dup. packets)

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(No Transcript)
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rdt 3.0 sender
wait for call 0
wait for ACK 0
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Alternating-bit Protocol
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R 6000 kbpsL 1500 bytesRTT 300 ms