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

Yes, I wrote about this in my original letter. It is an Intel Gigabit server network card. I disabled everything possible on it with the command:

ifconfig igb0 -mextpg -vlanhwtag -vlanhwfilter -vlanhwtso -vlanhwcsum -rxcsum -txcsum -rxcsum6 -txcsum6 -tso -lro

The options that remained are: VLAN_MTU, JUMBO_MTU, WOL_MAGIC, HWSTATS

> 24 февр. 2026 г., в 23:23, Mark Saad <[email protected]> написал(а):
> 
> 
> 
> On Tue, Feb 24, 2026 at 3:02 PM Ivan K. <[email protected] <mailto:[email protected]>> wrote:
>> 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] <mailto:[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]>
>> 
> 
> Ivan
>   What sort of network cards are you using? This is another long shot but do you have anything that does GRE offload? Some of the intel
> and mellanox cards support some sort of checksum offloads. 
> 
> --
> mark saad | [email protected] <mailto:[email protected]>