[Cerowrt-devel] Correctly calculating overheads on unknown connections

[Andy Furniss]

Sebastian Moeller wrote:
> Hi Andy,
>
> On Sep 23, 2014, at 17:10 , Andy Furniss <adf.lists at gmail.com>
> wrote:

> BUT if you look at the kernel code, stab does not automatically
> include the ethernet overhead, so the subtract 14 in the above is
> actually wrong. See
> http://lxr.free-electrons.com/source/net/sched/sch_api.c#L538 where
> “pkt_len = skb->len + stab->szopts.overhead; is used instead of using
> “qdisc_skb_cb(skb)->pkt_len” that as filled properly in
> http://lxr.free-electrons.com/source/net/core/dev.c#L2705 . At least
> to me this clearly looks like the ethernet overhead is not pre-added
> when using stab, but I could be wrong. And on an ADSL link you can
> see this quite well, with the proper overhead values sqm-scripts
> still controls the latency under netperf-wrapper’s RRUL test nicely
> even if the shaping rate equals the line rate, with the overhead to
> small latency goes down the drain ;)

I guess skb->len varies depending on the interface.

Anyway here's a quick test on my desktop PC running a git kernel and tc.

I used to shape remotely pppoa/vc mux dsl so know that for me

ping -s 10 .... = one cell and -s 11 = 2 cells - overhead on IP was 10.

Paste time -

ph4[/mnt/sda8/Qos/stab-tests]# cat stab-hfsc
#set -x
TC=/sbin/tc

$TC qdisc del dev eth0 root &>/dev/null

if [ "$1" = "stop" ]
then
         exit
fi

$TC qdisc add dev eth0 root handle 1: stab overhead -4 linklayer atm 
hfsc default ffff
$TC class add dev eth0 parent 1: classid 1:1 hfsc sc rate 1kbit ul rate 
1kbit
$TC qdisc add dev eth0 parent 1:1 pfifo limit 200
$TC class add dev eth0 parent 1:0 classid 1:ffff hfsc sc rate 80mbit ul 
rate 80mbit

$TC filter add dev eth0 parent 1: protocol ip prio 1 \
     u32 match ip protocol 1 0xff classid 1:1

ph4[/mnt/sda8/Qos/stab-tests]# ./stab-hfsc
ph4[/mnt/sda8/Qos/stab-tests]# ping -s 10 -c 1 noki
PING noki.andys.lan (192.168.0.1) 10(38) bytes of data.
18 bytes from noki.andys.lan (192.168.0.1): icmp_req=1 ttl=64

--- noki.andys.lan ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms

ph4[/mnt/sda8/Qos/stab-tests]# tc -s qdisc ls dev eth0
qdisc hfsc 1: root refcnt 2 default ffff
  Sent 106 bytes 2 pkt (dropped 0, overlimits 0 requeues 0)
  backlog 0b 0p requeues 0
qdisc pfifo 8005: parent 1:1 limit 200p
  Sent 53 bytes 1 pkt (dropped 0, overlimits 0 requeues 0)
  backlog 0b 0p requeues 0
ph4[/mnt/sda8/Qos/stab-tests]# ./stab-hfsc
ph4[/mnt/sda8/Qos/stab-tests]# ping -s 11 -c 1 noki
PING noki.andys.lan (192.168.0.1) 11(39) bytes of data.
19 bytes from noki.andys.lan (192.168.0.1): icmp_req=1 ttl=64

--- noki.andys.lan ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms

ph4[/mnt/sda8/Qos/stab-tests]# tc -s qdisc ls dev eth0
qdisc hfsc 1: root refcnt 2 default ffff
  Sent 106 bytes 1 pkt (dropped 0, overlimits 0 requeues 0)
  backlog 0b 0p requeues 0
qdisc pfifo 8006: parent 1:1 limit 200p
  Sent 106 bytes 1 pkt (dropped 0, overlimits 0 requeues 0)
  backlog 0b 0p requeues 0
ph4[/mnt/sda8/Qos/stab-tests]#

So it seems that overhead -4 is the correct thing to do.

I also tested backlogged (-i 0.2) with -s 10 and 11 and tcpdump showed 
the correct deltas -

ph4[/mnt/sda8/Qos/stab-tests]# tcpdump -nnttti eth0 icmp and dst host noki

snip

00:00:00.424000 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1345, seq 92, length 18
00:00:00.424000 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1345, seq 93, length 18
00:00:00.424000 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1345, seq 94, length 18
00:00:00.424000 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1345, seq 95, length 18
00:00:00.424000 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1345, seq 96, length 18
00:00:00.424001 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1345, seq 97, length 18
00:00:00.423999 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1345, seq 98, length 18
00:00:00.424000 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1347, seq 1, length 19
00:00:00.848000 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1347, seq 2, length 19
00:00:00.848001 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1347, seq 3, length 19
00:00:00.847999 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1347, seq 4, length 19
00:00:00.848000 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1347, seq 5, length 19
00:00:00.847999 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1347, seq 6, length 19
00:00:00.848002 IP 192.168.0.3 > 192.168.0.1: ICMP echo request, id 
1347, seq 7, length 19