Description: The TCP senders current estimate of the number of unacknowledged data octets in the network.

While not in recovery (e.g., while the receiver is not reporting missing data to the sender), this is precisely the same as 'Flight size' as defined in RFC 2581, which can be computed as SND.NXT minus SND.UNA. [RFC793]

During recovery, the TCP sender has incomplete information about the state of the network (e.g., which segments are lost vs reordered, especially if the return path is also dropping TCP acknowledgments). Current TCP standards do not mandate any specific algorithm for estimating the number of unacknowledged data octets in the network.

RFC 3517 describes a conservative algorithm to use SACK

information to estimate the number of unacknowledged data octets in the network. tcpEStatsPerfPipeSize object SHOULD be the same as 'pipe' as defined in RFC 3517 if it is implemented. (Note that while not in recovery the pipe algorithm yields the same values as flight size).

If RFC 3517 is not implemented, the data octets in flight SHOULD be estimated as SND.NXT minus SND.UNA adjusted by some measure of the data that has left the network and retransmitted data. For example, with Reno or NewReno style TCP, the number of duplicate acknowledgment is used to count the number of segments that have left the network. That is, PipeSize=SND.NXT-SND.UNA+(retransmits-dupacks)*CurMSS