[GTER] DSCP Assured Forwarding PHB

Rubens Kuhl Jr. rubens.kuhl at neovia.com.br
Wed Jul 4 23:08:17 -03 2007

DSCP Assured Forwarding PHB

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 priorty. For
example, traffic marked as AF43 is more likely to 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 in RFC 2597 DSCP compliant queueing
behavior, a packet marked as AF43 (38) is more likely to be dropped
than AF41 (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 receivies 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.

Recommended baseline markings with DSCP Assured Forwarding PHB:

Interactive Video: AF41
Mission Critical Data (locally defined): AF31
Transactional Data (dlsw, sql, sap): AF21
Bulk (email, ftp, backups): AF11

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