QOS DSCP and AF mechanisms

Well, just some quick notes about DSCP and AF PHB mechanisms before going further into study of 3750 switch QOS.

Background Theory

Differentiated Services (DiffServ) is a new model in which traffic is treated by intermediate systems with relative priorities based on the type of services (ToS) field. Defined in RFC 2474 and RFC 2475 , the DiffServ standard supersedes the original specification for defining packet priority described in RFC 791 . DiffServ increases the number of definable priority levels by reallocating bits of an IP packet for priority marking.
The DiffServ architecture defines the DiffServ (DS) field, which supersedes the ToS field in IPv4 to make per-hop behavior (PHB) decisions about packet classification and traffic conditioning functions, such as metering, marking, shaping, and policing.
The RFCs do not dictate the way to implement PHBs; this is the responsibility of the vendor. Cisco implements queuing techniques that can base their PHB on the IP precedence or DSCP value in the IP header of a packet. Based on DSCP or IP precedence, traffic can be put into a particular service class. Packets within a service class are treated the same way.

Conventions

For more information on document conventions, refer to Cisco Technical Tips Conventions.

Differentiated Services Code Point

The six most significant bits of the DiffServ field is called as the DSCP. The last two Currently Unused (CU) bits in the DiffServ field were not defined within the DiffServ field architecture; these are now used as Explicit Congestion Notification (ECN) bits. Routers at the edge of the network classify packets and mark them with either the IP Precedence or DSCP value in a Diffserv network. Other network devices in the core that support Diffserv use the DSCP value in the IP header to select a PHB behavior for the packet and provide the appropriate QoS treatment.
The diagrams in this section show a comparison between the ToS byte defined by RFC 791 and the DiffServ field.
ToS Byte

P2

P1

P0

T2

T1

T0

CU1

CU0

• IP precedence—three bits (P2 to P0)
  
• Delay, Throughput and Reliability—three bits (T2 to T0)
  
• CU (Currently Unused)—two bits(CU1-CU0)
  

DiffServ Field

DS5	DS4	DS3	DS2	DS1	DS0	ECN	ECN

• DSCP—six bits (DS5-DS0)
  
• ECN—two bits
  

The standardized DiffServ field of the packet is marked with a value so that the packet receives a particular forwarding treatment or PHB, at each network node.
The default DSCP is 000 000. Class selector DSCPs are values that are backward compatible with IP precedence. When converting between IP precedence and DSCP, match the three most significant bits. In other words:

IP Prec 5 (101) maps to IP DSCP 101 000

ToS Byte

1

0

1

T2

T1

T0

CU2

CU0

DiffServ Field

1	0	1	0	0	0	ECN	ECN

The DiffServ standard utilizes the same precedence bits (the most significant bits—DS5, DS4 and DS3) for priority setting, but further clarifies the definitions, offering finer granularity through the use of the next three bits in the DSCP. DiffServ reorganizes and renames the precedence levels (still defined by the three most significant bits of the DSCP) into these categories (the levels are explained in greater detail in this document):

Precedence Level	Description
7	Stays the same (link layer and routing protocol keep alive)
6	Stays the same (used for IP routing protocols)
5	Express Forwarding (EF)
4	Class 4
3	Class 3
2	Class 2
1	Class 1
0	Best effort

With this system, a device prioritizes traffic by class first. Then it differentiates and prioritizes same-class traffic, taking the drop probability into account.
The DiffServ standard does not specify a precise definition of "low," "medium," and "high" drop probability. Not all devices recognize the DiffServ (DS2 and DS1) settings; and even when these settings are recognized, they do not necessarily trigger the same PHB forwarding action at each network node. Each node implements its own response based on how it is configured.

Assured Forwarding

RFC 2597 defines the assured forwarding (AF) PHB and describes it as a means for a provider DS domain to offer different levels of forwarding assurances for IP packets received from a customer DS domain. The Assured Forwarding PHB guarantees a certain amount of bandwidth to an AF class and allows access to extra bandwidth, if available. There are four AF classes, AF1x through AF4x. Within each class, there are three drop probabilities. Depending on a given network's policy, packets can be selected for a PHB based on required throughput, delay, jitter, loss or according to priority of access to network services.
Classes 1 to 4 are referred to as AF classes. The following table illustrates the DSCP coding for specifying the AF class with the probability. Bits DS5, DS4 and DS3 define the class; bits DS2 and DS1 specify the drop probability; bit DS0 is always zero.

Drop	Class 1	Class 2	Class 3	Class 4
Low	001010 AF11 DSCP 10	010010 AF21 DSCP 18	011010 AF31 DSCP 26	100010 AF41 DSCP 34
Medium	001100 AF12 DSCP 12	010100 AF 22 DSCP 20	011100 AF32 DSCP 28	100100 AF42 DSCP 36
High	001110 AF13 DSCP 14	010110 AF23 DSCP 22	011110 AF33 DSCP 30	100110 AF43 DSCP 38

Expedited Forwarding

RFC 2598 defines the Expedited Forwarding (EF) PHB: "The EF PHB can be used to build a low loss, low latency, low jitter, assured bandwidth, end-to-end service through DS (Diffserv) domains. Such a service appears to the endpoints like a point-to- point connection or a "virtual leased line." This service has also been described as Premium service." Codepoint 101110 is recommended for the EF PHB, which corresponds to a DSCP value of 46.
Again, vendor-specific mechanisms need to be configured to implement these PHBs. Refer to RFC 2598 for more information about EF PHB.

Reference : http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml