On Fri, 2015-01-16 at 13:02 +0100, Alexander Aring wrote: > 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. 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 -- To unsubscribe from this list: send the line "unsubscribe linux-wpan" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
