class should be treated independently and should
have allocated bandwidth that is based on the QoS policy. There
are three DSCP values assigned to each of the four AF classes
as illustrated in Figure and the “DSCP Values Assigned to AF
Classes” table. DSCP Values Assigned to AF Classes
AF Class Drop Probability DSCP Value AF
Class 1 AF11 (low) 001 01 0 AF12 (medium) 001 10 0 AF13 (high)
001 11 0 AF Class 2 AF21 (low) 010 01 0 AF22 (medium) 010 10 0
AF23 (high) 010 11 0 AF Class 3 AF31 (low) 011 01 0 AF32
(medium) 011 10 0 AF33 (high) 011 11 0 AF Class 4 AF41 (low)
100 01 0 AF42 (medium) 100 10 0 AF43 (high) 100 11 0
Content 4.1 Introducing Classification and
Marking 4.1.7 Standard PHB Groups Recall
that a PHB is the externally observable forwarding behavior
applied at a DiffServ-compliant node to a DiffServ behavior
aggregate (BA). With the ability of the system to mark packets
according to DSCP setting, collections of packets—each with the
same DSCP setting and sent in a particular direction—can be
grouped into a BA. Packets from multiple sources or
applications can belong to the same BA. The IETF defines a PBH
group as a set of one or more PHBs that can only be
meaningfully specified and implemented simultaneously, due to a
common constraint applying to all PHBs in the set such as a
queue servicing or queue management policy. In other words, a
PHB group refers to the packet scheduling, queuing, policing,
or shaping behavior of a node on any given packet belonging to
a BA, as configured by a service level agreement (SLA) or a
policy map. The three standard PHBs are as follows: -
Default PHB (as defined in RFC 2474)
- AF PHB (as
defined in RFC 2597)
- EF PHB (as defined in RFC 2598)
class-selector PHB (as defined in RFC 2474)
The
default PHB specifies that a packet marked with a DSCP value of
000000 (recommended) receives best-effort service from a
DiffServ-compliant node. If a packet arrives at a
DiffServ-compliant node and the DSCP value is not mapped to any
other PHB, the packet is mapped to the default PHB. The AF PHB
defines four AF classes (n = 1–4: AF1, AF2, AF3, and AF4). Each
class is assigned a specific amount of buffer space and
interface bandwidth. It is allowed to obtain bandwidth from
other AF classes, if bandwidth is available. IP packets using
the AF PHB group are assigned by the customer or the provider
DS domain into one or more of these AF classes according to the
services to which the customer has subscribed. The EF PHB
defines one class, which assigns a fixed amount of bandwidth
only for that class.
Content 4.1
Introducing Classification and Marking 4.1.8
Mapping CoS to Network Layer QoS Figure shows how CoS is
mapped to Layer 3 QoS. IP headers are preserved end to end when
IP packets are transported across a network; data link layer
headers are not preserved. This means that the IP layer is the
most logical place to mark packets for end-to-end QoS. However,
there are edge devices that can mark frames only at the data
link layer, and there are many other network devices that
operate only at the network layer. To provide true end-to-end
QoS, the ability to map QoS markings between the data link
layer and the network layer is essential. Enterprise networks
typically consist of a number of remote sites connected to the
headquarters campus via a WAN. Remote sites typically consist
of a switched LAN, and the headquarters campus network is both
routed and switched. Providing end-to-end QoS through such an
environment requires that CoS markings that are set at the LAN
edge are mapped into QoS markings (such as IP precedence or
DSCP) for transit through Campus or WAN routers. Campus and WAN
routers can also map the QoS markings to new data-link headers
for transit across the LAN. With the mapping, QoS can be
preserved and uniformly applied across the enterprise. Service
providers offering IP services have a requirement to provide
robust QoS solutions to their customers. The ability to map
network layer QoS to link layer CoS allows these service
providers to offer a complete end-to-end QoS solution that does
not depend on any specific link layer technology. Compatibility
between an MPLS transport layer and network layer QoS is also
achieved by mapping between MPLS EXP bits and the IP precedence
or DSCP bits. A service provider can map the customer network
layer QoS marking as is or change it to fit an agreed-upon SLA.
The information in the MPLS EXP bits can be carried end to end
in the MPLS network, independent of the transport media. In
addition, the network layer marking can remain unchanged so
that when the packet leaves the service provider MPLS network,
the original QoS markings remain intact. Thus, a service
provider with an MPLS network can help provide a true
end-to-end QoS solution.
Content 4.1
Introducing Classification and Marking 4.1.9
QoS Service Class Defined QoS service class is defined in
Figure . When you create an administrative policy requiring
QoS, you must determine how network traffic is to be treated.
As part of that policy definition, network traffic must be
associated with a specific service class. QoS classification
mechanisms are used to separate traffic and identify packets as
belonging to a specific service class. QoS marking mechanisms
are used to tag each packet as belonging to the assigned
service class. After the packets are identified as belonging to
a specific service class, QoS mechanisms, such as policing,
shaping, and queuing techniques, can be applied to each service
class to meet the specifications of the administrative policy.
Packets belonging to the same service class are given the same
treatment for QoS. A QoS service class, being a logical
grouping, can be defined in many ways: - Organization
or department (for example, marketing, engineering, and
sales)
- A specific customer or set of customers
- Specific applications or sets of applications (for
example, Telnet, FTP, voice, Session Announcement Protocol
[SAP], Oracle, and video)
- Specific users or sets of
users (based on MAC address, IP address, and LAN port, for
example)
- Specific network destinations (for example,
tunnel interfaces and virtual private networks [VPNs])
Figure is an example of how to define QoS service class.
A network administrator wants to apply QoS to the corporate
network to better control bandwidth allocation of different
network applications. Before QoS can be applied, an
administrative QoS policy is first defined as follows:
- Voice traffic is to be given a strict priority over all
other traffic types and a bandwidth of 35 percent.
-
Business applications (FTP, Telnet client TN3270, and Oracle)
should be given priority over web traffic and have a guaranteed
bandwidth of 20 percent.
- Web traffic should consume
no more than 30 percent of any WAN link.
As a result
of this policy, three QoS service classes have been defined:
- Voice class: Is to be treated with a strict
high-priority service.
- Business applications
class: Requires a guaranteed bandwidth of 20 percent and is
to be given priority over web traffic.
- Web
class: Allowed to consume only up to 30 percent of any WAN
link.
The remaining 15 percent is for management,
signaling, and routing. These percentages are not necessarily
recommended values, but are simply representative of this
particular example.
Content 4.1
Introducing Classification and Marking 4.1.10
Implementing QoS Policy Using a QoS Service Class
Specifying an administrative policy for QoS requires that a
specific set of service classes be defined. QoS mechanisms are
uniformly applied to these individual service classes to meet
the requirements of the administrative policy. Because the
application of QoS mechanisms is applied to different service
classes and used to differentiate among applications, users,