Re: ssh client_loop send disconnnect from Dom0 -> DomU (NetBSD 10.0_BETA/Xen)

[Matthias Petermann <mp%petermann-it.de@localhost>]

Hello,

On 25.06.23 03:48, RVP wrote:
> On Sat, 24 Jun 2023, Brian Buhrow wrote:
>
> > In any case, The fact that you're getting regular delays on your pings 
> > suggests
> > there is a delay between the time when the arp cache times out and 
> > when it gets refreshed.
>
> This would be determined by `net.inet.arp.nd_delay' I think (on
> -HEAD).
>
> > As a consequence of that delay, if you have a high speed stream 
> > running when the cache times out,
> > it's possible the send buffer of the sending process, i.e. sshd, is 
> > filling up before that
> > cache gets refreshed and the packets can flow again.
>
> In this case, the kernel would either block the sshd process or
> return EAGAIN--which is handled. The kernel should only return a
> EHOSTDOWN if `net.inet.arp.nd_bmaxtries' * `net.inet.arp.nd_retrans'
> (ie. 3 * 1000ms) has passed without getting an ARP response. Even
> on a LAN, this is pretty unlikely (even with that peculiarly short
> 30-second ARP-address cache timeout). Smells like a Xen+load+timing
> issue (not hand-wavy at all there, RVP!). It would be interesting
> to see the tcpdump capture from the DomU.
>
> -RVP

Over the last day I did some further tests and tried out all the hints I 
got in this thread. Here is s short summary:

1) Run a ping over night from DomU to Dom0 -> no dropouts

2) increased the ARP cache timeout  net.inet.arp.nd_reachable=1200000
   on both, Dom0 and DomU  -> this seemed to have an effect at first,
   but the problem still exists (its not a measured fact but a feeling,
   that it happens now a bit less often and later)

3) Checked send/receive buffer configuration

```
srv-net$ sysctl net.inet.tcp.sendbuf_auto
net.inet.tcp.sendbuf_auto = 1
srv-net$ sysctl net.inet.tcp.recvbuf_auto
net.inet.tcp.recvbuf_auto = 1
srv-net$ sysctl net.inet.tcp.sendbuf_max
net.inet.tcp.sendbuf_max = 262144
srv-net$ sysctl net.inet.tcp.recvbuf_max
net.inet.tcp.recvbuf_max = 262144
```

These samples are from DomU, but Dom0 has an identical configuration.

4) Run the test with tcpdump from DomU -> this is currently ongoing. I 
will followup as soon I have the results.


Kind regards
Matthias

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