Yes - it may be possible to remove the ACKNOW flag; The FreeBSD stack has that behavior (sending empty ACKs) in various cases during loss recovery, retransmissions though, so it is not limited to PRR.
When initially implementing PRR I used other loss recovery codepaths as scaffolding, and that's probably why the ACKNOW is still in there. Richard Scheffenegger Senior Solution Architect NAS & Networking NetApp +43 1 3676 811 3157 Direct Phone +43 664 8866 1857 Mobile Phone richard.scheffeneg...@netapp.com -----Original Message----- From: owner-freebsd-...@freebsd.org <owner-freebsd-...@freebsd.org> On Behalf Of jaeyong yoo Sent: Mittwoch, 19. Februar 2025 00:12 To: freebsd-net@freebsd.org Subject: Sending empty segment upon receiving partial ACK [Sie erhalten nicht häufig E-Mails von y.jaey...@gmail.com. Weitere Informationen, warum dies wichtig ist, finden Sie unter https://aka.ms/LearnAboutSenderIdentification ] EXTERNAL EMAIL - USE CAUTION when clicking links or attachments Hi freebsd-net, I am observing somewhat strange pcap behavior. Scenario: A --> B A is the only sender of the data and B is the only receiver. Note that we use PRR. When B is sending partial ACKs to A, there are cases when A sends out just an empty segment with the same sequence number to B. Which seems to be pure overhead. After digging through the code, I think this could be triggered by the following sequence: 1. https://urldefense.com/v3/__https://github.com/freebsd/freebsd-src/blob/main/sys/netinet/tcp_input.c*L2892__;Iw!!Nhn8V6BzJA!WM32oNanemi8Gf4F5Tc0D4GS8cYXuQEbRH9qrKnSgt2VnUMbwkPJJ0N-p0RuifaZryRFAVXjavn-tkAYEVnLMGWdc1s$ during prr-partial ack processing, it calls tcp_output with ACKNOW flag. 2.https://urldefense.com/v3/__https://github.com/freebsd/freebsd-src/blob/main/sys/netinet/tcp_output.c*L415__;Iw!!Nhn8V6BzJA!WM32oNanemi8Gf4F5Tc0D4GS8cYXuQEbRH9qrKnSgt2VnUMbwkPJJ0N-p0RuifaZryRFAVXjavn-tkAYEVnLuyuyDvs$ in tcp_output, it determines "len" how much to send and when ACKed bytes in partial ack is small enough, this "len" becomes zero. 3. https://urldefense.com/v3/__https://github.com/freebsd/freebsd-src/blob/main/sys/netinet/tcp_output.c*L702__;Iw!!Nhn8V6BzJA!WM32oNanemi8Gf4F5Tc0D4GS8cYXuQEbRH9qrKnSgt2VnUMbwkPJJ0N-p0RuifaZryRFAVXjavn-tkAYEVnL6dNE6CQ$ As the flag is set to "ACKNOW", with zero length, it anyway sends out a segment with 0 length. My question is, is there some check before sending out like checking if the length is zero? Thanks, Jaeyong
