hello

I tried to monitor the the machine's traffic rate, ie, bytes per seconds, but I got accumulated bytes

```
time host instance type value
2015-12-09T07:40:44.711518Z "localhost" "eth0" "if_octets" 3143180744
2015-12-09T07:40:49.711476Z "localhost" "eth0" "if_octets" 3143182227
2015-12-09T07:40:54.711829Z "localhost" "eth0" "if_octets" 3143182856
2015-12-09T07:40:59.711906Z "localhost" "eth0" "if_octets" 3143183111
2015-12-09T07:41:04.712183Z "localhost" "eth0" "if_octets" 3143185899
2015-12-09T07:41:09.71305Z "localhost" "eth0" "if_octets" 3143188055
2015-12-09T07:41:14.711672Z "localhost" "eth0" "if_octets" 3143189590
```

which I expected something like this.

```
time host instance type value
2015-12-09T07:40:44.711518Z "localhost" "eth0" "if_octets_per_second" 80744
2015-12-09T07:40:49.711476Z "localhost" "eth0" "if_octets_per_second" 2227
2015-12-09T07:40:54.711829Z "localhost" "eth0" "if_octets_per_second" 3856
2015-12-09T07:40:59.711906Z "localhost" "eth0" "if_octets_per_second" 13111
2015-12-09T07:41:04.712183Z "localhost" "eth0" "if_octets_per_second" 8991
2015-12-09T07:41:09.71305Z "localhost" "eth0" "if_octets_per_second" 0551
2015-12-09T07:41:14.711672Z "localhost" "eth0" "if_octets_per_second" 2321
```



## here is my config

```
FQDNLookup true
BaseDir "/home/kui.yuan/collectd-5.5.0/var/lib/collectd"
PIDFile "/home/kui.yuan/collectd-5.5.0/var/run/collectd.pid"
PluginDir "/home/kui.yuan/collectd-5.5.0/lib/collectd"
TypesDB "/home/kui.yuan/collectd-5.5.0/share/collectd/types.db"

Interval 5

MaxReadInterval 86400
Timeout 2
ReadThreads 5
WriteThreads 5

LoadPlugin syslog
LoadPlugin logfile

<Plugin logfile>
LogLevel info
File "/tmp/collectd.log"
Timestamp true
PrintSeverity false
</Plugin>

LoadPlugin cpu
LoadPlugin interface
LoadPlugin load
LoadPlugin memory
LoadPlugin network

<Plugin cpu>
ReportByCpu false
ReportByState true
ValuesPercentage true
</Plugin>

<Plugin interface>
Interface "eth0"
IgnoreSelected false
</Plugin>

<Plugin network>
Server "localhost" "25826"
</Plugin>
```

## my version
collectd 5.5.0

## my os
CentOS release 6.7 (Final)

what wrong with it?

I didnot get the data as describe in the documentation - https://collectd.org/wiki/index.php/Plugin:Interface

> The Interface plugin collects information about the traffic (octets per second), packets per second and errors of interfaces´╗┐
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