• Channel may flip bits in packet (use How to calculate checksum of packet? ).

  • error detection

  • feedback: control msgs (ACK, NAK) from receiver to sender

  • Send acknowledgements (ACKs) receiver explicitly tells sender that pkt is OK

  • Send negative ack (NAKs) receiver tells sender that pkt had errors sender retransmits pkt

Fun question:

What if ACK / NAK is corrupted?