Re: Re: [PATCH] tcp: Add an extra check for consecutive failed keepalive probes

On Fri, Jan 10, 2025 at 10:58 AM lizhe [off-list ref] wrote:

Hi, Neal


If the TCP_USER_TIMEOUT option is not enabled, and attempts to send TCP keepalive probes continuously fail,

then who limits the number of increments to icsk->icsk_probes_out?

The code that I pasted in my previous message limits the number of
increments to icsk->icsk_probes_out. :-)

The code is right here in the lines surrounding line 809 of tcp_timer
in Linux v6.12, which can also be viewed here more conveniently:

https://elixir.bootlin.com/linux/v6.12/source/net/ipv4/tcp_timer.c#L809

Adding this code is feasible. If not added, the system would continuously send keepalive probes without any limit.

If these probes continually fail, the process would persist indefinitely because there would be no measure in place to restrict the increments of icsk->icsk_probes_out++.

It's not true that the system would continuously send keepalive probes
without any limit. The packetdrill test I pasted in my previous
message verifies that Linux TCP stops sending keepalives after the
number of probes configured by net.ipv4.tcp_keepalive_probes or
TCP_KEEPCNT. If you think I'm still missing something, please provide
a tcpdump trace or packetdrill test showing the behavior you are
claiming. :-)

thanks,
neal