Understanding Linux Network Internals

ICMP Statistics

The ICMP protocol keeps the statistics defined in RFC 2011, storing them in icmp_mib data structures. The kernel maintains statistics on a per-CPU basis, and for each CPU it distinguishes between statistics updated in software interrupt context and those updated outside that context. In other words, for each counter there are two instances per CPU: one of those two instances is used by code running in software interrupt context and the other is used by code not running in software interrupt context. All of those icmp_mib instances are allocated by init_ipv4_mibs in net/ipv4/af_inet.c. icmp_statistics is a two-element array, whose first element represents the per-CPU array of icmp_mib instances used by code that runs in software interrupt context, and whose second element represents the other per-CPU array.

    static int _ _init init_ipv4_mibs(void)
    {
        ...
        icmp_statistics[0] = alloc_percpu(struct icmp_mib);
       icmp_statistics[1] = alloc_percpu(struct icmp_mib);
        ...
    }

The icmp_mib structure consists of an array of unsigned long members, one for each counter defined in RFC 2011 for the ICMP protocol:

    #define SNMP_MIB_DUMMY  _ _ICMP_MIB_MAX
    #define ICMP_MIB_MAX    (ICMP_MIB_MAX + 1)
    struct icmp_mib {
        unsigned long mibs[ICMP_MIB_MAX];
    } _ _SNMP_MIB_ALIGN_ _;

The counters are identified via the enumeration list ICMP_MIB_ XXX, defined in include/linux/snmp.h:

    enum
    {
        ICMP_MIB_NUM = 0,
        ICMP_MIB_INMSG,
        ...
        ICMP_MIB_OUTADDRMASKREPS,
        _ _ICMP_MIB_MAX
    }

Note that the size of the icmp_mib array ...