scope:

This note defines an extension of the Selective Acknowledgement (SACK) Option [RFC2018] for TCP. RFC 2018 specified the use of the SACK option for acknowledging out-of-sequence data not covered by TCP's cumulative acknowledgement field. This note extends RFC 2018 by specifying the use of the SACK option for acknowledging duplicate packets. This note suggests that when duplicate packets are received, the first block of the SACK option field can be used to report the sequence numbers of the packet that triggered the acknowledgement. This extension to the SACK option allows the TCP sender to infer the order of packets received at the receiver, allowing the sender to infer when it has unnecessarily retransmitted a packet. A TCP sender could then use this information for more robust operation in an environment of reordered packets [BPS99], ACK loss, packet replication, and/or early retransmit timeouts.