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ARQ Protocols (Automatic Repeat ReQuest)

ARQ protocols are used for error control. Lost packets are automatically retransmitted after a timeout. The receiver has to return acknowledgements when packets are received correctly.  Types of ARQ protocols are:

GO-Back-N
As noted above, a timer is used to recover from lost data. If a timeout occurs the sender resends all packets that have been sent, but not yet been acknowledged. If a packet is received correctly and in order, it is delivered to the user and an acknowledge is sent. In all other cases the receiver discards the packet.

Selective Repeat
Selective-repeat protocols avoid unnecessary retransmissions by having the sender retransmit only those packets that it suspects were lost. This individual retransmissions require that the receiver individually acknowledges correctly received packets.

The Animation shows how Go-Back-N and Selective Repeat work. Click onto any packet that should get lost. Additionally you have to provide the correct acknowledgement number at the receiver.

Data Packet  Cumulative Acknowledge  Individual Acknowledge