Go-Back-N and Selective Repeat: Reliable Data Transfer Protocols

What is Go-Back-N (GBN) in reliable data transfer?

A pipelined approach where the sender can send multiple packets before needing an acknowledgment, but must retransmit all packets from the lost one onward.

What type of acknowledgments (ACKs) does Go-Back-N use?

Cumulative ACKs, which acknowledge all packets up to a certain sequence number.

To which packet does the timeout in Go-Back-N apply?

The oldest unacknowledged packet.

What action does the GBN sender take upon a timeout?

Retransmits the oldest unacknowledged packet and all subsequent packets.

What does the GBN receiver do with out-of-order packets?

It discards them and waits for the missing packet to arrive.

What is Selective Repeat (SR) in reliable data transfer?

An approach where separate timers are used for each packet, allowing the receiver to buffer out-of-order packets.

How does the SR sender respond when data is received from above?

It constructs a packet and sends the data if there is room in the window; otherwise, it returns data to the application.

What happens when a packet's timer goes off in Selective Repeat?

Only that specific packet is retransmitted.

What does the SR sender do when an ACK is received?

If the ACKed packet is in the window, it marks it as received and may move the window forward.

What does the SR receiver do upon receiving a correctly sequenced packet?

It sends a selective ACK and delivers the packet's data along with any buffered packets with consecutive sequence numbers.

What does the SR receiver do with packets that are prior to the window?

It acknowledges them but discards their data since it has already been delivered.

What action does the SR receiver take when a corrupted packet is received?

It discards and ignores the corrupted packet.

What is the dilemma faced by the SR receiver when the sequence number space is small?

It cannot distinguish between new data and a retransmission if the sequence numbers are recycled.

What assumption is made about the network in reliable data transfer?

That the network will not reorder packets, as out-of-order packets may indicate packet loss.

What is done to prevent confusion with recycled sequence numbers in SR?

Ensure that the window size is no more than half the size of the sequence number space.

What is the significance of having a large sequence number space in reliable data transfer?

It reduces the likelihood of confusion between old and new packets in the event of packet reordering.