[OpenWrt-Devel] Lantiq xrx200: PTM issues

[Martin Schiller]

Hi!

I have discovered the following problem:

If you have established a PPPoE session via VDSL / PTM connection incl.
VLAN tagging and send data with a relatively small send buffer
(SO_SNDBUF), then an ENOBUFS always comes back.

We first noticed this with stagnating data transfers over an OpenVPN
connection.

Also with iputils-ping, since by default the send buffer is relatively
small.

You can also force this with busybox ping by using

echo "5000"> / proc / sys / net / core / wmem_default

minimizes the system default value.

Then you send pings with a packet size of e.g. 4000 bytes and the
second package is already in the pants:

------------------------------------------------------------
root @ OpenWrt: ~ # ping -s4000 10.200.1.142
PING 10.200.1.142 (10.200.1.142): 4000 data bytes
4008 bytes from 10.200.1.142: seq = 0 ttl = 63 time = 20.519 ms
ping: sendto: No buffer space available
root @ OpenWrt: ~ #
------------------------------------------------------------

So it should be easily reproducible for everyone.

Traffic that is only routed through the router is not affected.

The manpage of sendto says:
-----------------------------------------------------------------------
ENOBUFS
     The output queue for a network interface was full. This generally
     indicates that the interface has stopped sending, but may be caused
     by transient congestion. (Normally, this does not occur in Linux.
     Packets are just silently dropped when a device queue overflows.)
-----------------------------------------------------------------------

But all former packets have already been transmitted.

This issue seems to be in there since lede-17.01.

I can't reproduce it with owrt-15.05.

Does anyone have any idea how to solve the problem?

Martin

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