Reliable Data Transfer: Go-back-n

Go-back-n Sender

A go-back-n sender uses a sending buffer that can store an entire sliding window of segments.

• The segments are sent with a sending sliding window that we looked at in the last lesson.
• The sender must wait for an acknowledgment once its sending buffer is full.
• When a go-back-n sender receives an acknowledgment, it removes all the acknowledged segments from the sending buffer.

Retransmission Timer

A go-back-n sender uses a retransmission timer to detect segment losses. It maintains one retransmission timer per connection. Here’s how it works:

1. This timer is started when the first segment is sent.
2. When the go-back-n sender receives an acknowledgment, it restarts the retransmission timer, but only if any unacknowledged segments exist in its sending window.
3. When the retransmission timer expires, the go-back-n sender assumes that all of the unacknowledged segments currently stored in its sending buffer have been lost. It thus retransmits all the unacknowledged segments in the buffer and restarts its retransmission timer.

The general operation of go-back-n is illustrated in the figure below.

Advantages of Go-back-n

The main advantage of go-back-n is that it can be easily implemented, and it can also provide good performance when only a few segments are lost. But when there are many losses, the performance of go-back-n quickly drops for two reasons:

• The go-back-n receiver does not accept out-of-sequence segments.
• The go-back-n sender retransmits all unacknowledged segments once it has detected a loss.

Since the go-back-n protocol does not accept out of order segments, it can waste a lot of bandwidth if segments are frequently lost.

The Selective Repeat protocol attempts to remedy this by accepting out of order packets and only retransmitting packets that are corrupted or lost in the network.

Selective Repeat

• Uses a sliding window protocol just like go-back-n.

• The window size should be less than or equal to half the sequence numbers available. This avoids packets being identified incorrectly. Here’s an example: suppose the window size is greater than half the buffer size.

  • Segment ‘1’ is lost, hence the receiver expects a segment with sequence number 1 to be retransmitted.
  • Meanwhile, the window wraps around back to sequence number ‘1.’
  • The sender sends a new packet with sequence number 1 and the receiver perceives it to be the original one that it was expecting.

• Senders retransmit unacknowledged packets after a timeout or if a NAK (negative acknowledgment/not acknowledged) is received.

• The receiver acknowledges all correct packets.

• The receiver stores correct packets until they can be delivered in order to the upper application layer.

Note: NAKS Negative acknowledgements can be realized using positive acknowledgements (ACKs) of the last correctly received segment!

Comparing to go-back-n

In comparison to the go-back-n protocol, selective repeat conserves bandwidth, which is, the rate of data transfer across a path.

Quick Quiz!

In the next lesson, we’ll start with UDP, a transport layer protocol.