[OpenFlow] Wildcard Explanation

This article is about flow wildcard for match field. Basically, we can get understood most of them at a glance. But, for  NW_SRC_MASK and NW_DST_MASK they need to do more a little bit math. I only give an example with NW_SRC_MASK because NW_DST_MASK is similar case. Please refer to the following picture:

The position of NW_SRC_MASK is from 8 to 13. If we want to setup a IP subnet mask as 192.168.1.0/24, we should give the value: 001000 (8 bits are wirdcarded). Another example, for instance, 192.168.0.0/16 (16 bits are wirdcarded), the value should be 010000.

/* Flow wildcards. */ enum ofp_flow_wildcards { OFPFW_IN_PORT = 1 << 0, /* Switch input port. */ OFPFW_DL_VLAN = 1 << 1, /* VLAN id. */ OFPFW_DL_SRC = 1 << 2, /* Ethernet source address. */ OFPFW_DL_DST = 1 << 3, /* Ethernet destination address. */ OFPFW_DL_TYPE = 1 << 4, /* Ethernet frame type. */ OFPFW_NW_PROTO = 1 << 5, /* IP protocol. */ OFPFW_TP_SRC = 1 << 6, /* TCP/UDP source port. */ OFPFW_TP_DST = 1 << 7, /* TCP/UDP destination port. */ /* IP source address wildcard bit count. 0 is exact match, 1 ignores the * LSB, 2 ignores the 2 least-significant bits, ..., 32 and higher wildcard * the entire field. This is the *opposite* of the usual convention where * e.g. /24 indicates that 8 bits (not 24 bits) are wildcarded. */ OFPFW_NW_SRC_SHIFT = 8, OFPFW_NW_SRC_BITS = 6, OFPFW_NW_SRC_MASK = ((1 << OFPFW_NW_SRC_BITS) - 1) << OFPFW_NW_SRC_SHIFT, OFPFW_NW_SRC_ALL = 32 << OFPFW_NW_SRC_SHIFT, /* IP destination address wildcard bit count. Same format as source. */ OFPFW_NW_DST_SHIFT = 14, OFPFW_NW_DST_BITS = 6, OFPFW_NW_DST_MASK = ((1 << OFPFW_NW_DST_BITS) - 1) << OFPFW_NW_DST_SHIFT, OFPFW_NW_DST_ALL = 32 << OFPFW_NW_DST_SHIFT, OFPFW_DL_VLAN_PCP = 1 << 20, /* VLAN priority. */ OFPFW_NW_TOS = 1 << 21, /* IP ToS (DSCP field, 6 bits). */ /* Wildcard all fields. */ OFPFW_ALL = ((1 << 22) - 1) };