We want to have a reliable protocol built on top of an unreliable channel.

• Complexity of reliable data transfer protocol will depend on characteristics of unreliable channel (i.e. lost, corrupt, reordered data, etc.)

• Neither the sender nor receiever know the “state” of the other
  • e.g. neither know whether a particular message was received unless communicated via a message

The interfaces for reliable data transfer are as follows:

• rdt_send - pass data from application to deliver
• udt_send - called by rdt to transfer packet of unreliable channel to receiever
• deliver_data - called by rdt to deliver data to upper layer
• rdt_rcv - called when packet arrives on receiever side of channel

We consider unidirectional data transfer, but control info will flow in both directions.

Sender/receiver actions can be specified by finite state machines.

rdt1.0 - reliable transfer over a reliable channel

Assumption is that underlying channel is perfectly reliable.

• no bit errors
• no loss of packets

Very simple FSMs for sender/receiver: