Oversubscribing Atm Ports; Minimizing Latency On The Sar Scheduler; Hrr Scheduler Behavior And Strict-Priority Scheduling - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - QUALITY OF SERVICE CONFIGURATION GUIDE 2010-09-22 Configuration Manual

HRR Scheduler Behavior and Strict-Priority Scheduling

Copyright © 2010, Juniper Networks, Inc.
scheduler to a rate that is less than the aggregate VC rate. This shaping prevents the VC
queue in the SAR scheduler from being congested with strict-priority traffic.
The major difference between relative and true strict priority on ATM line modules is that
relative strict priority shapes the aggregate for the VC to a pre–cell tax rate, whereas true
strict priority shapes the aggregate for the VC to a post–cell tax rate. For example, shaping
the VC to 1 Mbps in the HRR scheduler allows 1 Mbps of frame data, but cell tax adds
anywhere from 100 Kbps to 1 Mbps additional bandwidth, depending on packet size.
Shaping the VC to 1 Mbps in the SAR scheduler allows just 1 Mbps of cell bytes regardless
of packet size.

Oversubscribing ATM Ports

You cannot oversubscribe ATM ports and still achieve low latency with relative
strict-priority scheduling. There are several ways to ensure that ports are not
oversubscribed. The most common is to use a per-VC scheduler by configuring the HRR
scheduler with either ATM VP or VC node shaping (using the atm-vp node or atm-vc
node commands), and setting the sum of the shaping rates less than the port rate. In
these scenarios, the cell residency in the SAR scheduler is minimal, and cell scheduling
does not interfere with relative strict priority.

Minimizing Latency on the SAR Scheduler

There are two methods you can use to control latency on the SAR scheduler. In the first
method, you set the ATM QoS port mode to low-latency mode. In low-latency mode,
the HRR scheduler controls scheduling, buffering in the SAR scheduler is limited, and
latency caused by the SAR scheduler is minimized.
You can also use the default no qos-mode-port mode of SAR operation to minimize the
latency induced by the SAR. In this method, you set qos shaping-mode cell and shape
an OC-3 ATM port to 149 Mbps, or an OC-12 ATM port to 600 Mbps. By throttling the rate
at which the HRR scheduler delivers packets to the SAR, you bound SAR buffering and
latency. This approach retains the flexibility to configure different ATM QoS in the SAR,
including shaped VP tunnels, UBR+PCR, nrtVBR, and CBR services.
To set the SAR mode, use the qos-mode-port command. For more information about
operational modes on ATM interfaces, see "ATM Integrated Scheduler Overview" on
page 151.
NOTE: Controlling latency is not normally required. If you undersubscribe the
port rate in the HRR scheduler, you can obtain latency bounds without
modifying the SAR mode of operation.
The HRR scheduler does not offer native strict-priority scheduling above the first scheduler
level in the hardware; however, you can configure very large weights in the round robin
in the HRR scheduler to obtain approximate strict-priority scheduling. Note that under
conditions of low VC bandwidth and large packet sizes, latency and jitter increase because
of the inherent propagation delay of large packets over a small shaping rate. The following
Chapter 8: Configuring Strict-Priority Scheduling
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