Content Overview In any network where
networked applications require differentiated levels of
service, traffic must be sorted into different classes to which
quality of service (QoS) is applied. Classification, marking,
and queuing are critical functions of any successful QoS
implementation. Classification allows network devices to
identify traffic as belonging to a specific class with the
specific QoS requirements determined by an administrative QoS
policy. After network traffic is sorted, individual packets are
marked so that other network devices can apply QoS features
uniformly to those packets in compliance with the defined QoS
policy. Queuing dispatches the packets according to their
markings. This module introduces classification, marking, and
queuing using various methods of implementation. Web
Links Cisco IOS Quality of Service Solutions
Configuration Guide, Release 12.4
http://www.cisco.com/en/US/products/ps6350/products
_configuration_guide_book09186a0080435d50.html
This
publication provides an excellent resource for reference and
future study. It provides the theory, practical examples and
solutions for every aspect of Cisco IOS QoS. DiffServ -- The
Scalable End-to-End QoS Model
http://www.cisco.com/en/US/products/ps6610/products
_white_paper09186a00800a3e2f.shtml Branch QoS Design
At-a-Glance
http://www.cisco.com/application/pdf/en/us/guest
/tech/
tk759/c1482/cdccont_0900aecd80295aa8.pdf
Network-Based Application Recognition and Distributed
Network-Based Application Recognition
http://www.cisco.com/en/US/products/ps6350/products
_configuration_guide_chapter09186a0080455985.html
Configuring NBAR
http://www.cisco.com/univercd/cc/td/doc/product/
software/ios122/122cgcr/fqos_c/fqcprt1/qcfnbar.htm
Cisco AutoQoS White Paper
http://www.cisco.com/application/pdf/en/us/guest/tech/
tk759/c1482/cdccont_0900aecd8019f3e0.pdf Network Based
Application Recognition Performance Analysis
http://www.cisco.com/application/pdf/en/us/guest/tech/
tk759/c1482/cdccont_0900aecd80295a9b.pdf Understanding
Quality of Service on Catalyst 6000 Family Switches
http://www.cisco.com/application/pdf/en/us/guest/tech/
tk759/c1482/cdccont_0900aecd80295ac3.pdf QoS Configuration
and Monitoring Whitepapers
http://www.cisco.com/en/US/tech/tk543/tk759/
tech_white_papers_list.html
Content 4.1
Introducing Classification and Marking 4.1.1
Classification Classification is the process of identifying
traffic and categorizing that traffic into classes.
Classification uses a traffic descriptor to categorize a packet
within a specific group to define that packet. Traffic
descriptors that are typically used include: - Incoming
interface
- IP precedence
- Differentiated
services code point (DSCP)
- Source or destination
address
- Application
After the packet has
been classified or identified, the packet is then accessible
for QoS handling on the network. Using classification, network
administrators can partition network traffic into multiple
classes of service (CoS). When traffic descriptors are used to
classify traffic, the source implicitly agrees to adhere to the
contracted terms and the network promises QoS. Various QoS
mechanisms, such as traffic policing, traffic shaping, and
queuing techniques, use the traffic descriptor of the packet
(that is, the classification of the packet) to ensure adherence
to that agreement. Classification should take place at the
network edge, typically in the wiring closet, within IP phones,
or at network endpoints. Cisco recommends that classification
occur as close to the source of the traffic as possible. Figure
summarizes these points. Note
The term
classification is interchangeable with the term
packet classification.
Content 4.1
Introducing Classification and Marking 4.1.2
Marking Marking is related to classification. Marking
allows network devices to classify a packet or frame at the
edge based on a specific traffic descriptor. QoS classification
tools categorize packets by examining the contents of the
frame, cell, and packet headers; whereas marking tools allow
the QoS tool to change the packet headers for easier
classification. Many QoS tools rely on a classification
function to determine to which traffic the tool applies. To
place voice and data traffic in separate queues, for example,
you must use some form of classification to differentiate the
two types of traffic and place the identified traffic in the
proper queue. Marking provides a way for QoS tools to change
bits in the packet header to indicate the level of service this
packet should receive from other QoS tools. For instance, you
can use marking tools to change the marking in voice packets to
ensure that a classification tool can differentiate a voice
packet from a data packet. Without the marking feature, the
frame, packet, or cell remains unchanged. Marking involves
placing a value into one of the small number of well-defined
frame, packet, or cell header fields specifically designed for
QoS marking. By marking a packet, other QoS functions can
perform classification based on the marked field inside a
header. Marking simplifies the network's QoS design, it
simplifies configuration of other QoS tools, and it reduces the
overhead required by each of the other QoS tools to classify
the packets. Although classification and marking tools do not
directly affect the bandwidth, delay, jitter, or loss
experienced by traffic in the network, classification and
marking tools are the building blocks for all other QoS tools.
With these tools, all traffic on the network is identified for
the next QoS tool to act upon. Traffic descriptors that are
typically used include these: - Link layer:
-
CoS (Inter-Switch Link [ISL], 802.1p)
- Multiprotocol
Label Switching (MPLS) experimental (EXP) bits
- Frame
Relay
- Network layer:
Marking can be used
to set information in the Layer 2 frame or Layer 3 packet
headers. Marking a packet or frame with its classification
allows subsequent network devices to easily distinguish the
marked packet or frame as belonging to a specific class. After
the packets or frames are identified as belonging to a specific
class, QoS mechanisms can be uniformly applied to ensure
compliance with administrative QoS policies. Figure summarizes
these points.
Content 4.1 Introducing
Classification and Marking 4.1.3 Classification
and Marking at the Link Layer The 802.1Q standard is an
IEEE specification for implementing VLANs in Layer 2 switched
networks. The 802.1Q specification defines two 2-byte fields
(tag protocol identifier [TPID] and tag control information