Sending empty segment upon receiving partial ACK

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