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530 Chapter 14: SNA Topologies
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Differentiated service--An example of this type of QoS is using the IP Precedence field
of the IP packet to declare that the data in this packet is important and should be given
priority. This basically means that some of the traffic, depending on the IP Precedence
field, can be treated with more urgency than other packets. This also involves queuing
algorithms such as custom and priority queuing.
·
Guaranteed service--This type of service usually means a guaranteed bandwidth from
one end of the network to the other for an application (RSVP protocol, for example). This
means that along the path from end to end, routers need to reserve buffer space, queuing
disciplines to ensure that specific traffic gets a specific service level.
NOTE
It is not uncommon to find businesses defining the levels of service as follows and referencing
them with the maximum amount of delay:
· High availability--150 milliseconds or less response time
· Normal availability--1-second response time
· Low availability--2-second response time
· Best effort--5-second response time
Much of the first part of this section focuses on DLSw+ prioritization, the steps needed to
configure prioritization, and the ports that TCP uses to distinguish traffic. The end of this
section discusses the use of Cisco IOS queuing algorithms--priority queuing, custom queuing,
and weighted fair queuing--to provide differentiated services and meet service-level
requirements.
Prioritizing Traffic
DLSw+ encapsulates all SNA and NetBIOS traffic into TCP packets. This makes it difficult to
identify the traffic. For that reason, DLSw+ supports opening four separate TCP connections
and then places traffic based on priority directly from the input queue into one of these four
pipes. Then, at the output interface, traffic is prioritized among these four TCP connections
based on its TCP port number.
To specify the traffic and activate the four TCP connections (ports 2065, 1981, 1982, and 1983),
a priority keyword must be used on a remote-peer statement.
In Figure 14-17, by default, DLSw+ assigns certain traffic to specific TCP ports as follows:
·
TCP port 2065--Defaults to high priority. This port carries all circuit administration
frames, peer keepalives, and capabilities exchange.
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