RFC 2597 defines a group of DSCP settings called “Assured Forwarding” Per Hop Behavior (PHB) to be recognized by RFC compliant DSCP routers and switches called DS nodes. The Assured Forwarding PHB class is presented as AF(xy), where x=traffic class, and y=drop precedence. 4 traffic classes, and 3 drop precedences are defined. For example, AF21 = traffic class 2, drop precedence 1. The traffic class values (1-4) have escalating priority values where traffic marked as AF11 has a lower priority than AF41. Conversely, the drop precedence value (1-3) represents an escalating drop preference within the specified class, a descending priority. For example, traffic marked as AF43 is more likely to be dropped than AF41.
The actual DSCP binary and decimal values of Assured Forwarding PHBs are as follows:
- AF11 = 001010 = 10
- AF12 = 001100 = 12
- AF13 = 001110 = 14
- AF21 = 010010 = 18
- AF22 = 010100 = 20
- AF23 = 010110 = 22
- AF31 = 011010 = 26
- AF32 = 011100 = 28
- AF33 = 011110 = 30
- AF41 = 100010 = 34
- AF42 = 100100 = 36
- AF43 = 100110 = 38
Following the logic of IP Precedence, and 802.1p COS, it would be easy to believe a packet marked with a DSCP value of 38 would have a higher priority and less likely to be dropped than a packed marked as 34. However the reverse is true according to RFC 2597 DSCP compliant queueing behavior where a packet marked as AF43 (decimal 38) is more likely to be dropped than AF41 (decimal 34) during periods of congestion. This is because AF43 has a higher drop priority within traffic class 4. Drop priorities values are only compared against traffic within the same class. For example, AF21 is more likely to be dropped than AF43. Although AF43 has a higher drop precedence setting (3) than AF21 (1), the traffic class setting of (4) dominates the class setting of (2) and therefore drop precedence settings are not compared when deciding which packet receives better service.
If traffic within a class exceeds defined CIRs for that class, that traffic can have its drop precedence bit setting incremented. For example, if Email traffic exceeds a defined CIR you can remark the PHB from AF11 to AF12. If a specified traffic class exceeds a PIR (peak information rate) you can remark the PHB to an even higher drop priority of AF13, and/or simply just drop the packet.
The following are recommended baseline markings with DSCP Assured Forwarding PHB:
- Interactive Video: AF41
- Mission Critical Data (locally defined): AF31
- Transactional Data (dlsw, sql, sap): AF21
- Bulk Data (email, ftp, backups): AF11
Cisco Systems: Enterprise QoS Solution Reference Network Design Guide