- A protocol that doesn't detect and correct lost and/or damaged messages is said to be unreliable
- Detect errors (e.g., using sequence numbers, parity checks, CRC)
- Retransmit messages in the event of an error (i.e., be tenacious)
Common ARQ Schemes
- Stop-and-Wait ARQ
- Go-Back-N ARQ
- Selective-Repeat ARQ
Stop-and-Wait ARQ
- Based On:
- Stop-and-Wait flow control
- Frames have a 1-bit sequence number
- ACKs include sequence numbers
- Receiver sends an ACK 1-N if Frame N is receiveed
- Transmitter waits for an ACK N before sending the Frame N
- Transmitter retransmits Frame N if it does not receive ACK 1-N before the timeout-timer counts down
Stop-and-Wait ARQ (cont.)
Go-Back-N ARQ
- Based On:
- Sliding window flow control
- If the transmitter sends all of the frames in its window and does not receive an ACK before the timeout-time counts down it will retransmit all of the frames in the window
- Receiver sends a NAK N if Frame N has an error (and then discards subsequent incoming frames until it receives Frame N)
- If the transmitter receives a NAK N it retransmits Frame N and all subsequent frames that haven't been acknowledged
Go-Back-N ARQ (cont.)
Example of a Lost/Corrupted Frame
An Analysis of Go-Back-N ARQ
- Suppose:
- The windows size is large
- The product of delay and bandwidth is large
- A large number of packets are in the pipeline
- An error in a packet causes a large number of packets to be retransmited, many of which needen't be
Selective-Repeat ARQ
- Based On:
- Go-Back-N ARQ
- Each frame must be acknowledged individually
- The transmitter only retransmits frames for which a NAK is received
