Backpressure And The Integrated Scheduler - Juniper JUNOSE 11.1.X - QUALITY OF SERVICE CONFIGURATION GUIDE 3-21-2010 Configuration Manual

JUNOSe 11.1.x Quality of Service Configuration Guide
NOTE: The term HRR scheduler is used in this chapter to describe the scheduling
performed by the ASIC on the ATM line module. Although the ASIC might differ
depending on the ATM line module, the configuration and performance of the HRR
scheduler are the same. For example, the ERX7xx models, ERX14xx models, and
ERX310 Broadband Services router use the egress forwarding ASIC (EFA); and the
E120 and E320 Broadband Services Routers use the frame forwarding ASIC (FFA)
on the ES2 4G LM.
The HRR scheduler and the SAR scheduler work together as an integrated scheduler
for ATM traffic. The HRR scheduler is configured by default with per-VC and per-IP
interface scheduler nodes, and one best-effort class queue for each IP interface. The
SAR scheduler implements weighted round-robin scheduling with one queue per VC.
The VC queues are grouped into round robins based on the ATM service classes and
the VP tunnels you have configured.
In the default integrated mode, controlled by the ATM application, the SAR scheduler
controls the scheduling via the VC backpressure messages it sends to the HRR
scheduler. When the HRR scheduler receives a backpressure message from the SAR
scheduler, the HRR scheduler disables the node regardless of the node weight or
shaping rate. When the HRR scheduler receives a backpressure release, the scheduler
node is reenabled.

Backpressure and the Integrated Scheduler

ATM packets are initially scheduled through the HRR scheduler and then sent to the
SAR scheduler, from where the cells are scheduled onto the circuit. If a SAR VC queue
begins to fill up, the SAR scheduler issues VC backpressure messages to the HRR
scheduler. The backpressure messages control the amount of traffic the HRR scheduler
sends to the SAR scheduler. The SAR scheduler can also exert port backpressure on
the HRR scheduler.
In default integrated mode, the SAR sends VC backpressure messages as well as port
backpressure messages. Port backpressure messages are sent to the port node in
the hierarchical scheduler.
Backpressure is a critical mechanism that enables the two schedulers in series to
operate as a single integrated scheduler. Backpressure ensures that packets do not
drain over internal data paths at an unmanageable rate from the HRR scheduler to
the SAR scheduler. Without backpressure from the SAR scheduler, the HRR scheduler
does not detect congestion even if the SAR scheduler is completely saturated.
NOTE: The default QoS profile for ATM (atm-default) contains the atm-vc node
command, which creates the scheduler node that is required by the SAR VC
backpressure mechanism. If the SAR scheduler is operating in default integrated
mode, this command must be in QoS profiles that are attached to ATM ports.
Figure 43 on page 161 shows the HRR and SAR schedulers working together to form
the integrated scheduler. When the SAR VC queues start to back up, the SAR exerts
VC backpressure to the corresponding VC node in the HRR scheduler.
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ATM Integrated Scheduler Overview