fq_codel with 802.15.4 6LoWPAN fragmentation issue

Hi,

when using "fq_codel" as network scheduler, I noticed an issue with IEEE 802.15.4 6LoWPAN fragmentation. The issue is that high payloads IPv6 packets are dropped.

I detected the issue on the sending side which segments the 6LoWPAN packet into several fragments.

In the IEEE 802.15.4 6LoWPAN fragmentation we have a 6LoWPAN interface (MTU 1280) on top of an IEEE 802.15.4 interface (MTU 127). The 6LoWPAN fragmentation algorithm creates in the "ndo_start_xmit" netdev_ops callback function of the 6LoWPAN interface new sk_buffs for the underlaying IEEE 802.15.4 interface (as skb->dev) and sends them via a dev_queue_xmit.

IEEE 802.15.4 is a very slow connection and after each transmit there is an "interframe spacing time" which depends on frequency setting. In my testcase, that's about ~500 us for a payload above 18 bytes. This is done by calling netif_stop_queue for the interface and start a timer after each transmitted frame according to the interframe spacing time. When the timer fires, it wakes the queue again by calling netif_wake_queue.

Now: What I am noticed with fq_codel is that the code at [0] will drop high payloaded IPv6 packets. Of course, if we have an IPv6 packet of more than 1280 bytes payload we have a fragmentation (IPv6 fragmentation) over a fragmentation (802.15.4 6LoWPAN fragmentation).

When I increase the "params->interval" parameter I can send a "longer" IPv6 packet depending how much I increase the "params->interval" parameter.

I assume that the "params->interval" which defaults to MS2TIME(100) is too small. I further assume that at the beginning of fragmentation the interval measurement starts "vars->first_above_time = now + params->interval;" (line 247 at [0]) and while sending several fragments with the above mentioned interframe spacing time, the sending takes too long and the skb will be dropped.

My questions are:

- What's the best way to deal with something like that?

- Is there a way to set the "params->interval" _per interface_? So I can change the default parameter to a suitable value in the "ndo_init" callback of the 6LoWPAN interface.

Thanks in advance.

- Alex

[0] http://lxr.free-electrons.com/source/include/net/codel.h#L248

Sure, fq_codel is a qdisc, each network device gets its own qdisc hierarchy.

You can therefore set different parameter on your interface,

Very simple example :

tc qdisc dev lowpan0 root fq_codel interval XXX

Sorry, one keyword was missing :

tc qdisc replace dev lowpan0 root fq_codel interval XXX

But the parameter that you probably want to change is not the interval. It is probably the target :

tc qdisc replace dev lowpan0 root fq_codel interval 200ms target 25ms

Now, if the fragmentation was done after qdisc dequeue, as in GSO layer, you would not have this issue : all segments of one ipv6 message would be transmitted, no matter what.