Re: Asymmetric forwarded TCP TX collapse over one GRE path on FreeBSD 14.3-RELEASE-p8 (PF independent, offloads disabled)

Each real life - 1500, gre 1472.
I checked this. I tried setting mtu 1200 on the client device. I have mss clamping at 1400 in pf.
If it's mtu, it doesn't explain why the identical tunnel on the same machine functions normally.


> 24 февр. 2026 г., в 21:10, Mark Saad <[email protected]> написал(а):
> 
> 
> 
> On Tue, Feb 24, 2026 at 12:19 PM Ivan K. <[email protected] <mailto:[email protected]>> wrote:
>> Hello, everyone. The text below was written by me and “polished” by a neural network, which I have been using to research this problem for three days now.
>> 
>> I am investigating a reproducible GRE issue and would appreciate advice on further debugging.
>> 
>> Versions / hosts
>> - Router: FreeBSD 14.3-RELEASE-p8
>> - VPS0: FreeBSD 14.3-RELEASE-p8
>> - VPS2: FreeBSD 14.3-RELEASE-p8
>> 
>> Topology
>> - Router has WAN on VLAN interface `onp` (parent `igb0`) and LAN on `igb0`.
>> - Two GRE tunnels from the same router:
>>   - GRE to VPS0: healthy
>>   - GRE to VPS2: problematic
>> - There is also an ng tunnel to VPS2 (works better than bad GRE, but not full speed).
>> - Clients behind router tested via both Wi-Fi and wired Ethernet (same problem).
>> - Also tested traffic from a VM on the router: VM path does not reproduce the severe collapse.
>> 
>> Core symptom (asymmetric)
>> - Only TX direction is affected (client uploads through GRE->VPS2).
>> - RX/download direction is not affected.
>> - Through GRE->VPS2, TCP upload collapses to about ~100 Kbit/s per flow.
>> - 5 parallel TCP flows produce roughly 5 x ~100 Kbit/s.
>> - iperf3 TCP shows many retransmissions and very small cwnd (~2.7 KB).
>> - Same client via GRE->VPS0 is fast (normal high throughput).
>> - Traffic generated locally on router through GRE->VPS2 is also fast.
>> 
>> Firewall / policy checks
>> - PF disabled completely (`pfctl -d`) for tests: problem remains.
>> - Therefore this is not PF rule/policy dependent in current testing.
>> - Also checked for ipfw/dummynet:
>>   - `kldstat | egrep 'ipfw|dummynet|pf'` shows only `pf.ko`.
>>   - `ipfw show` returns “Protocol not available”.
>>   - `net.inet.ip.fw.enable` OID not present.
>> 
>> Offload / NIC checks performed
>> - Explicitly disabled offload features (multiple attempts), including:
>>   - `-lro`
>>   - `-tso` / tso-related toggles
>>   - `-rxcsum` / `-txcsum` / v6 checksum toggles
>>   - VLAN hardware offload related toggles where supported
>> - No improvement.
>> - Current interface snapshot during testing:
>>   - `ifconfig igb0` shows options: `VLAN_MTU,JUMBO_MTU,WOL_MAGIC,HWSTATS` (no LRO/TSO flags active)
>>   - `ifconfig onp` shows `options=0`
>> - Reboots of router and VPS2 done after changes: no improvement.
>> - Also tested with minimal manual GRE setup (without tunnel-management scripts, only one GRE up): no improvement.
>> 
>> Queue/counters/sysctl checks
>> - `netstat -Q` shows no queue drops (`QDrops=0`).
>> - `netstat -iW -I ...` during tests shows no obvious interface error growth.
>> - `sysctl net.inet.ip.fastforwarding` does not exist on this release.
>> 
>> UDP vs TCP behavior
>> - UDP over problematic GRE path can run at high throughput (similar order from client and from router), i.e., no analogous dramatic collapse.
>> - TCP over same path collapses hard in TX direction from forwarded clients.
>> 
>> Packet capture findings
>> - Collected captures at multiple points (router and VPS side).
>> - In bad runs, client-side TCP packets are seen, but many segments are retransmitted repeatedly.
>> - Comparing capture points suggests segments often appear on VPS GRE side only much later / after retransmission, consistent with loss on the GRE path for this forwarded TCP case.
>> - In control runs (local router-generated traffic), packet timing/count alignment is normal.
>> 
>> What makes this puzzling
>> - Same router, same OS version, same general config:
>>   - GRE to VPS0 works well.
>>   - GRE to VPS2 fails for forwarded TCP TX only.
>> - PF disabled does not change behavior.
>> - Offloads disabled does not change behavior.
>> - No obvious drops in standard queue/interface counters.
>> 
>> Request
>> Any suggestions on what to inspect next for FreeBSD 14.3-p8 in this scenario:
>> - netisr / forwarding path diagnostics
>> - GRE-specific kernel instrumentation
>> - known igb/VLAN/GRE forwarding edge cases
>> - any tunables or debug knobs that could explain “forwarded TCP TX collapse only on one GRE peer/path”
>> 
>> Thanks.
>> 
> 
> 
> Ivan
>  Just a guess but it sounds like you may have a MTU issue . What is the MTU settings on each of the real interfaces and the gre tunnels ?
> --
> mark saad | [email protected] <mailto:[email protected]>