TCP Dup ACK and Selective Acknowledgement

• Duplicate ACKs occur when some data from a group of packets was lost
• Receiver will send again ACKs for those group of data to tell sender what part is lost
• The ACKs that were already received by sender previously are the duplicate ACKs
• The ACKs that will be missing from this group of ACKs will be the indicator on what packet is missing
• Several dup ACKs may be an indicator of packet loss
• Further Reading:
  • What Causes Network Packet Loss?
  • How TCP Works - Selective Acknowledgment (SACK)