Well, just some quick notes about DSCP and AF PHB mechanisms before going further into study of 3750 switch QOS.
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.
For more information on document conventions, refer to
Cisco
Technical Tips Conventions.
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
DiffServ Field
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.
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
|
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
