RFC 2597 Assured Forwarding Per-Hop Behaviour Group
RFC 2597 defines four Assured Forwarding groups, denoted by the letters “AF” followed by two digits:
- The first digit denotes the AF class number and can range from 1 through 4 (these values correspond to the three most-significant bits of the codepoint or the IPP value that the codepoint falls under). Incidentally, the AF class number does not in itself represent hierarchy (that is, AF class 4 does not necessarily get any preferential treatment over AF class 1).
- The second digit refers to the level of drop precedence within each AF class and can range from 1 (lowest drop precedence) through 3 (highest drop precedence).
The three levels of drop precedence are analogous to the three states of a traffic light:
- Drop precedence 1, also known as the “conforming” state, is comparable to a green traffic light.
- Drop precedence 2, also known as the “exceeding” state, is comparable to a yellow traffic light (where a moderate allowance in excess of the conforming rate is allowed to prevent erratic traffic patterns).
- Drop precedence 3, also known as the “violating” state, is comparable to a red traffic light.
Policing:
Packets within an AF class are always initially marked to drop precedence of 1 and can only be remarked to drop precedence 2 or 3 by a policer, which meters traffic rates and determines if the traffic is exceeding or violating a given traffic contract.
Then, for example, during periods of congestion on an RFC 2597-compliant node, packets remarked AF33 (representing the highest drop precedence for AF class 3) would be dropped more often than packets remarked AF32; in turn, packets remarked AF32 would be dropped more often than packets marked AF31.
RFC 3246 An Expedited Forwarding Per-Hop Behaviour
