Information Technology Reference
In-Depth Information
You can assign each of these data application categories a specific level of QoS. You can
then map the actual applications to these categories using a variety of methods (such as
incoming interface, exit interface, access lists, and so on).
QoS Mechanisms
With the applications requiring different levels of QoS, multiple models and mechanisms
emerged to address the needs. Today, the following models are available for you to de-
ploy QoS:
Key
To p i c
Best Effort:
Best Effort makes the list simply because this is the model every net-
work uses by default. On the positive side, the Best-Effort model requires absolutely
no effort at all on your end to implement. No QoS mechanisms are used, and all traf-
fic is treated on a first come, first served basis. Of course, this does not address the
QoS requirements of most network environments today.
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Integrated Services (IntServ):
The IntServ model works through a method of
reservations. For example, if a user wanted to make an 80Kbps VoIP call over the data
network, the network designed purely to the IntServ model would reserve 80Kbps on
every network device between the two VoIP endpoints using the Resource Reserva-
tion Protocol (RSVP). For the duration of the call, 80Kbps of bandwidth would not
be available for any other use other than the VoIP call. Although the IntServ model is
the only model that provides
guaranteed
bandwidth, it also has scalability issues. If
enough reservations are made, the network simply runs out of bandwidth.
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Differentiated Services (DiffServ):
The DiffServ model is the most popular and
flexible model to use for implementing QoS. In this model, you can configure every
device to respond with a variety of QoS methods based on different traffic classes.
You can specify what network traffic goes into each class and how each class is
treated. Unlike the IntServ model, the traffic is not absolutely guaranteed (since the
network devices do not completely reserve the bandwidth). However, DiffServ gets
so close to guaranteed bandwidth (some Cisco documentation refers to it as “almost
guaranteed” bandwidth), while at the same time addressing the scalability concerns
of IntServ, that it has become the standard QoS model used by most organizations
around the world.
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The QoS model you use is primarily a strategy or mindset of how you design and imple-
ment QoS throughout your network. The QoS mechanisms themselves are a series of tools
that combine together to deliver the levels of service your network traffic needs to sur-
vive. Each of these tools fit into one of the following categories:
Key
To p i c
Classification and Marking:
These tools allow you to identify and mark a packet so
network devices can easily identify it as it crosses the network. Typically the first de-
vice that receives the packet identifies it using tools such as access-lists, incoming in-
terfaces, or deep packet inspection (which looks at the application data itself). These
tools can be processor intense and add delay to the packet, so after the packet is ini-
tially identified, it is then marked. The marking can be in the layer 2 (data link) header
(allowing switches to read it) and/or the layer 3 (network) header so routers can read
it. Then, as the packet crosses the rest of the network, the network devices simply
look at the marking to classify it rather than digging deep in the packet.
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