5.1. Ordering of Flow Specifications

5.1. Ordering of Flow Specifications

More than one Flow Specification may match a particular traffic flow. Thus, it is necessary to define the order in which Flow Specifications get matched and actions being applied to a particular traffic flow. This ordering function is such that it does not depend on the arrival order of the Flow Specification via BGP and thus is consistent in the network.

The relative order of two Flow Specifications is determined by comparing their respective components. The algorithm starts by comparing the left-most components (lowest component type value) of the Flow Specifications. If the types differ, the Flow Specification with lowest numeric type value has higher precedence (and thus will match before) than the Flow Specification that doesn't contain that component type. If the component types are the same, then a type-specific comparison is performed (see below). If the types are equal, the algorithm continues with the next component.

For IP prefix values (IP destination or source prefix), if one of the two prefixes to compare is a more specific prefix of the other, the more specific prefix has higher precedence. Otherwise, the one with the lowest IP value has higher precedence.

For all other component types, unless otherwise specified, the comparison is performed by comparing the component data as a binary string using the memcmp() function as defined by [ISO_IEC_9899]. For strings with equal lengths, the lowest string (memcmp) has higher precedence. For strings of different lengths, the common prefix is compared. If the common prefix is not equal, the string with the lowest prefix has higher precedence. If the common prefix is equal, the longest string is considered to have higher precedence than the shorter one.

The code in Appendix A shows a Python3 implementation of the comparison algorithm. The full code was tested with Python 3.6.3 and can be obtained at <https://github.com/stoffi92/rfc5575bis/tree/master/flowspec-cmp>.