Medianet Reference Guide - Cisco

Chapter 4
Medianet QoS Design Considerations
Drivers for QoS Design Evolution
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.
The full set of AF PHBs are detailed in Figure 4-6.
Figure 4-6
Assured Forwarding PHBs with Decimal and Binary Equivalents
AF PHB
DSCP
AF Class 1
Conforming
DP
AF11
Exceeding
DP
AF12
Violating
DP
AF13
10
001 010
12
001 100
14
001 110
AF Class 2
AF21
AF22
AF23
18
010 010
20
010 100
22
010 110
AF Class 3
AF31
AF32
AF33
26
011 010
28
011 100
30
011 110
AF Class 4
AF41
AF42
AF43
34
100 010
36
100 100
38
100 110
226605
RFC 3246 An Expedited Forwarding Per-Hop Behavior
The Expedited Forwarding PHB is defined in RFC 3246. In short, the definition describes a
strict-priority treatment for packets that have been marked to a DSCP value of 46 (101110), which is also
termed Expedited Forwarding (or EF). Any packet marked 46/EF that encounters congestion at a given
network node is to be moved to the front-of-the-line and serviced in a strict priority manner. It doesn’t
matter how such behavior is implemented—whether in hardware or software—as long as the behavior is
met for the given platform at the network node.
Note
Incidentally, the RFC 3246 does not specify which application is to receive such treatment; this is open
to the network administrator to decide, although the industry norm over the last decade has been to use
the EF PHB for VoIP.
The EF PHB provides an excellent case-point of the value of standardized PHBs. For example, if a
network administrator decides to mark his VoIP traffic to EF and service it with strict priority over his
networks, he can extend his policies to protect his voice traffic even over networks that he does not have
direct administrative control. He can do this by partnering with service providers and/or extranet partners
who follow the same standard PHB and who thus continue to service his (EF marked) voice traffic with
strict priority over their networks.
RFC 3662 A Lower Effort Per-Domain Behavior for Differentiated Services
While most of the PHBs discussed so far represent manners in which traffic may be treated
preferentially, there are cases it may be desired to treat traffic deferentially. For example, certain types
of non-business traffic, such as gaming, video-downloads, peer-to-peer media sharing, and so on might
dominate network links if left unabated.
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Medianet Reference Guide
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