linux-kernel - Re: [Patch] fix packet loss and massive ping spikes with PPP multi-link

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Date:	Wed, 31 Mar 2010 11:03:44 +0100
From:	Ben McKeegan <ben@...servers.co.uk>
To:	netdev@...r.kernel.org, linux-ppp@...r.kernel.org
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Alexander E. Patrakov" <patrakov@...il.com>,
	Richard Hartmann <richih.mailinglist@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [Patch] fix packet loss and massive ping spikes with PPP multi-link

>>> Making it runtime per link selectable would be nicer but thats a bit more
>>> work.
>> Doesn't it work already via echoing values to 
>> /sys/module/ppp/generic/parameters/ml_explode in the above code?
> 
> Thats runtime (and why I set 0600 in the permissions for the example) but
> not per link.
> 

I needed to do something similar a while back and I took a very 
different approach, which I think is more flexible.   Rather than 
implement a new round-robin scheduler I simply introduced a target 
minimum fragment size into the fragment size calculation, as a per 
bundle parameter that can be configured via a new ioctl.  This modifies 
the algorithm so that it tries to limit the number of fragments such 
that each fragment is at least the minimum size.  If the minimum size is 
greater than the packet size it will not be fragmented all but will 
instead just get sent down the next available channel.

A pppd plugin generates the ioctl call allowing this to be tweaked per 
connection.  It is more flexible in that you can still have the larger 
packets fragmented if you wish.

We've used a variant of this patch on our ADSL LNS pool for a few years 
now with varying results.  We originally did it to save bandwidth as we 
have a per packet overhead and fragmenting tiny packets such as VoIP 
across a bundle of 4 lines made no sense at all.  We've experimented 
with higher minimum settings up to and above the link MTU, thus 
achieving the equivalent of Richard's patch.

In some cases this has improved performance, others it makes it worse. 
It depends a lot on the lines and traffic patterns, and it is certainly 
not a change we would wish to have on by default.  Any solution going 
into mainline kernel would need to be tunable per connection.  One of 
the issues seems to be with poor recovery from packet loss on low 
volume, highly delay sensitive traffic on large bundles of lines.  With 
Linux at both ends you are relying on received sequence numbers to 
detect loss.  When packets are being fragmented across all channels and 
a fragment is lost, the receiving system is able to spot the lost 
fragment fairly quickly.  Once you start sending some multilink frames 
down individual channels, it takes a lot longer for the receiver to 
notice the packet loss on an individual channel.  Until another fragment 
is successfully received on the lossy channel, the fragments of the 
incomplete frame sit in the queue clogging up the other channels (the 
receiver is attempting to preserve the original packet order and is 
still waiting for the lost fragment).

Original patch attached.   This almost certainly needs updating to take 
account of other more recent changes in multi link algorithm but it may 
provide some inspiration.

Regards,
Ben.

View attachment "mppp-min-frag-size.patch" of type "text/x-diff" (4256 bytes)