Date: Tue, 18 Feb 2025 12:11:48 -0500 From: jaeyong yoo <y.jaeyong@gmail.com> To: freebsd-net@freebsd.org Subject: Sending empty segment upon receiving partial ACK Message-ID: <CANud0THEOnkWbuOn413AV_auSrY7-SysXThrTyigZMxujEdqEg@mail.gmail.com>
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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://github.com/freebsd/freebsd-src/blob/main/sys/netinet/tcp_input.c#L2892 during prr-partial ack processing, it calls tcp_output with ACKNOW flag. 2.https://github.com/freebsd/freebsd-src/blob/main/sys/netinet/tcp_output.c#L415 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://github.com/freebsd/freebsd-src/blob/main/sys/netinet/tcp_output.c#L702 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
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