[FS#854] Unstable Internet caused by frequent PPPoE reconnect, network.wan.keepalive=0 has no effect

[LEDE Bugs]

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FS#854 - Unstable Internet caused by frequent PPPoE reconnect, network.wan.keepalive=0 has no effect
User who did this - kochstefan (kochstefan)

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I have noticed this issue the first time at the begin of the year 2016 with OpenWRT. The issue occurs for me every time when connecting to my SSH-server behind the OpenWRT router and load (SSH/SCP server uploads to SSH/SCP downloading remote client) a big file.

So the SSH/SCP client does this:
scp domainname.of.server:/path/to/big/file bigfile
bigfile                                                                                                                                                                                                                          0% 3712KB   0.0KB/s - stalled -

To reproduce: The SSH server behind OpenWRT router needs a slower internet connection as the remote client (server's upload speed is slower than client's download speed). So the upload speed of the SSH servers internet connection is fully exhausted.

I assume that the keepalive message does not get transmitted properly because of exhausted internet connection. The question is, why there is no prioritization or balancing.

The workaround for me was to add "option keepalive '100 30'" to the WAN interface within /etc/config/network.

Recently, I noticed a similar failure again. The issue occurs every time when loading a big file via SCP, too. So I have tried your suggestion of '1024 86400', too. But it seems to be another error (not the keepalive issue):
Tue May  1 14:32:32 2018 daemon.notice pppd[1905]: Modem hangup
Tue May  1 14:32:32 2018 daemon.info pppd[1905]: Connect time 9.8 minutes.
Tue May  1 14:32:32 2018 daemon.info pppd[1905]: Sent 114234224 bytes, received 2755645 bytes.

If no keepalive option is set I get the log messages:
Tue May  1 14:54:03 2018 daemon.info pppd[1874]: No response to 5 echo-requests
Tue May  1 14:54:03 2018 daemon.notice pppd[1874]: Serial link appears to be disconnected.
Tue May  1 14:54:03 2018 daemon.info pppd[1874]: Connect time 5.8 minutes.
Tue May  1 14:54:03 2018 daemon.info pppd[1874]: Sent 4718939 bytes, received 539442 bytes.

With both options '100 30' and '1024 86400' there is no "No response to X echo-requests" message within the log.

pppd was started with:
/usr/sbin/pppd nodetach ipparam wan ifname pppoe-wan lcp-echo-interval 86400 lcp-echo-failure 1024 lcp-echo-adaptive +ipv6 set AUTOIPV6=1 nodefaultroute usepeerdns maxfail 1 [...]

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More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=854#comment4681