netdev - Fw: [Bug 219849] New: Issue with Missing Final ACK in TCP Connections to gracefully close the TCP connection

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Message-ID: <20250309160544.31536ac4@hermes.local>
Date: Sun, 9 Mar 2025 16:05:44 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: netdev@...r.kernel.org
Subject: Fw: [Bug 219849] New: Issue with Missing Final ACK in TCP
 Connections to gracefully close the TCP connection –
 Regression in 5.15.123 (15251e78)?

Begin forwarded message:

Date: Fri, 07 Mar 2025 19:35:26 +0000
From: bugzilla-daemon@...nel.org
To: stephen@...workplumber.org
Subject: [Bug 219849] New: Issue with Missing Final ACK in TCP Connections to gracefully close the TCP connection – Regression in 5.15.123 (15251e78)?

https://bugzilla.kernel.org/show_bug.cgi?id=219849

            Bug ID: 219849
           Summary: Issue with Missing Final ACK in TCP Connections to
                    gracefully close the TCP connection – Regression in
                    5.15.123 (15251e78)?
           Product: Networking
           Version: 2.5
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: IPV4
          Assignee: stephen@...workplumber.org
          Reporter: marko.pacaric@...cedes-benz.com
        Regression: No

Created attachment 307779
  --> https://bugzilla.kernel.org/attachment.cgi?id=307779&action=edit  
Wireshark_cutout

Dear Linux Community,

I’m currently investigating a complex issue related to TCP connection
termination and would appreciate your insights to help clarify the situation.

Background

We have a complex network setup involving multiple hops and a special
implementation on the mobile provider side.

Given this setup, we rely heavily on TCP connections being gracefully
closed—following the standard FIN → FIN-ACK → ACK sequence.

The Problem

For the past few months, we have been struggling with an issue where the final
ACK from our client is not being sent, leaving the connection stuck in
CLOSE_WAIT or LAST_ACK state on the receiver's side. This behavior is causing
significant issues in our system.

What We've Tried

1.       Application-Level Investigation
o    Initially, we suspected an issue in our implementation.
o    We rebuilt several applications, but the final ACK was consistently
missing.
o    We even tested with different programming languages and libraries—same
issue.
2.       Kernel & TCP Stack Configuration
o    We modified various TCP stack parameters, but none of the changes resolved
the problem.
Findings

To isolate the issue, we started testing with different Linux kernel versions:

·         Our current used version5.15.123
(KERNEL.PLATFORM.2.0.r10-05800-kernel.0) → Final ACK is missing, connections
remain in FIN_WAIT_2.
·         5.15.104 (KERNEL.PLATFORM.2.0.r10-04600-kernel.0) → Final ACK is
sent, connections close correctly.
Through systematic downgrading, we identified that 5.15.104 is the last version
where the issue does not occur. We then analyzed the commits between these
versions and found that the issue seems to be introduced by the following
commit:

🔗 Commit 15251e783a4b
https://git.codelinaro.org/clo/la/kernel/msm-5.15/-/commit/15251e783a4b

What did we do now

We build ourselfs a small patch which reverts the commit, so that we can see if
the false behavior is revered.

After applying the following patch, we do no longer observe the missing ACKs:

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 420d3bdeaa1b..0580d8719f37 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -923,7 +923,6 @@ static void tcp_v4_send_ack(const struct sock *sk,
                              &arg, arg.iov[0].iov_len,
                              transmit_time);

-       sock_net_set(ctl_sk, &init_net);
        __TCP_INC_STATS(net, TCP_MIB_OUTSEGS);
        local_bh_enable();
}

Next Steps & Questions

1.       Could this commit be responsible for suppressing the final ACK in
certain network conditions?
2.       Has anyone else observed similar behavior in recent kernel versions?
3.       Are there any known workarounds or patches addressing this issue?
Any insights or suggestions on how to proceed would be greatly appreciated!

Thank you very much,

Marko

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