SNMP High Capacity Counters

[Hartmut Brandt]

Eugene Grosbein wrote:
> Hi!
> 
> I've just found that ports/net-snmp (version 5.4) built
> WITH_MFD_REWRITES=yes supports IF-MIB, and in theory should show 64-bit
> ifHC* counters but it does not.
> 
> It seems agent/mibgroup/if-mib/data_access/interface_sysctl.c that obtains
> interface statistics from the kernel.
> The function netsnmp_arch_interface_container_load() has the following code:
> 
>         /* get counters */
>         entry->stats.ibytes.low = ifp->ifm_data.ifi_ibytes;
>         entry->stats.ibytes.high = 0;
>         entry->stats.iucast.low = ifp->ifm_data.ifi_ipackets;
>         entry->stats.iucast.high = 0;
>         entry->stats.imcast.low = ifp->ifm_data.ifi_imcasts;
>         entry->stats.imcast.high = 0;
> 
> So, it always produce 32-bit quantities. My question is:
> does FreeBSD/i386 kernel maintain 64-bit counters for interface statictics
> these days? If yes, since what version?

It does not, because not all architectures have atomic 64-bit increments 
and adds. Implementing 64-bit counters on these architectures would 
require some kind of locking. This was discussed in the past.

You might look at the IF-MIB implementation of bsnmp (it is in the base 
system). It uses periodic polling to detect wraps of the 32-bit 
counters. The poll interval is tuned to the fastest interface in the 
system (given that all interfaces reported the correct speed).

Note, that the netsnmp implementation is plain wrong - if the daemon 
does not support the HC counters it should never pretend to do. This is 
explicitely stated somewhere in the RFCs.

harti