rdt2.0 has a fatal flaw!

What happens if ACK/NAK corrupted?

• sender doesn’t know what happened at receiver!

• san’t just retransmit: possible duplicate

What to do?

• sender ACKs/NAKs receiver’s ACK/NAK? What if sender ACK/NAK lost?

• retransmit, but this might cause retransmission of correctly received pkt!

Handling duplicates:

• sender adds sequence number to each pkt

• sender retransmits current pkt if ACK/NAK garbled

• receiver discards (doesn’t deliver up) duplicate pkt

Sender sends one packet,

then waits for receiver

response

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