ATM Integrated Scheduler Overview
The E-series router provides extended ATM QoS functionality through its integrated scheduler. The integrated scheduler consists of two schedulers in series—the hierarchical round robin (HRR) scheduler and the segmentation and reassembly (SAR) scheduler.
The integrated scheduler enables you to configure QoS on your ATM networks using the HRR scheduler that is used on all E-series ASIC-enabled line modules. In addition, you can use the commercial SAR scheduler to configure traditional ATM cell-based QoS.
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.
Figure 43 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.
VC backpressure affects only VC nodes that are in the default traffic-class group. As a consequence, VC nodes that are in named traffic-class groups within the scheduler hierarchy are not affected by VC backpressure.
VP Shaping
VP shaping can be performed either in the SAR or by using the QoS shaping application using QoS profiles. Configuring VP shaping in the SAR enables traffic to be sent out of the port at a rate that closely matches strict ATM contract rates. SAR VP shaping is configured for the physical port using the atm vp-tunnel command.