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Page 128 of 378
Queue management
553-3001-202
Standard 1.00
ITG Engineering Guidelines
DiffServ/TOS values must first be converted to a decimal value of the
DiffServ/TOS byte in the IP packet header. For example, the 8-bit TOS field
value of 0010 0100 which indicates "Precedence=Priority";
"Reliability=High" is converted to a decimal value of 36 before being entered
in the Control or Voice fields.
If the intranet provides differentiated services based on the DiffServ/TOS
field, then the ITG Trunk and other traffic marked with this DiffServ/TOS
value could be delivered with the goal of meeting this class of traffic's QoS
objectives.
Note: It is not a requirement to have a router which has priority IP
packet routing capability. The ITG can function without priority routing
mechanisms if you design the intranet to minimize traffic congestion
through the WAN backbone links and routers. Refer to "Implement QoS
in IP networks" on page 126.
From "Queuing delay" on page 121, it can be seen that queueing delay is a
major contributor to delay, especially on highly-utilized and low-bandwidth
WAN links. Routers that are TOS-aware and support class-based queuing can
help reduce queueing delay of voice packets when these packets are treated
with preference over other packets. To this end, Class-Based Queueing
(CBQ) can be considered for implementation on these routers, with the ITG
traffic prioritized against other traffic. Classed-based queueing however may
be CPU-intensive and may not scale well when applied on high-bandwidth
links, hence if this is to be implemented for the first time on the intranet do so
selectively. Usually CBQ is implemented at edge routers, or entry routers into
the core.
The global synchronization situation described in "TCP traffic behavior" on
page 127 can be countered using a buffer management scheme which
discards packets randomly as the queue starts to exceed some threshold.
WRED (Weighted Random Early Detection), an implementation of this
strategy, additionally inspects the TOS bits in the IP header when considering
which packets to drop during buffer build up. In an intranet environment
where TCP traffic dominates real-time traffic, WRED can be used to
maximize the dropping of packets from long-lived TCP sessions and
minimize the dropping of voice packets. As in CBQ, check the configuration
April 2000
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