A port-based bandwidth allocation mechanism must consider the effect that line encapsulation overhead plays relative to the bandwidth allocated per service or subscriber or multiservice site. The service or subscriber or multiservice site level bandwidth definition (at the queue level) operates on a packet accounting basis. For Ethernet, this includes the DLC header, the payload, and the trailing CRC. This does not include the IFG or the preamble. This means that an Ethernet packet consumes 20 bytes more bandwidth on the wire than what the policer or queue accounted for. When considering HDLC encoded PoS or SDH ports on the 7750 SR, the overhead is variable, based on ‛7e’ insertions (and other TDM framing issues). The HDLC and SONET/SDH frame overhead is not included for queues forwarding on PoS and SDH links.
The port-based scheduler hierarchy must translate the frame-based accounting (on-the-wire bandwidth allocation) it performs to the packet-based accounting in the queues. When the port scheduler considers the maximum amount of bandwidth a queue should get, it must first determine how much bandwidth the policer or queue can use. This is based on the offered load the policer or queue is currently experiencing (how many octets are being offered). The offered load is compared to the configured CIR and PIR of the queue or policer. The CIR value determines how much of the offered load should be considered in the within-CIR bandwidth allocation pass. The PIR value determines how much of the remaining offered load (after within-CIR) should be considered for the above-CIR bandwidth allocation pass.
For Ethernet policers or queues (associated with an egress Ethernet port), the packet to frame conversion is relatively easy. The system multiplies the number of offered packets by 20 bytes and adds the result to the offered octets (offeredPackets x 20 + offeredOctets = frameOfferedLoad). This frame-offered-load value represents the amount of line rate bandwidth the policer or queue is requesting. The system computes the ratio of increase between the offered-load and frame-offered-load and calculates the current frame-based CIR and PIR. The frame-CIR and frame-PIR values are used as the limiting values in the within-CIR and above-CIR port bandwidth distribution passes.
For PoS or SDH queues on the 7750 SR, the packet to frame conversion is more difficult to dynamically calculate because of the variable nature of HDLC encoding. Wherever a ‛7e’ bit or byte pattern appears in the data stream, the framer performing the HDLC encoding must place another ‛7e’ within the payload. Because this added HDLC encoding is unknown to the forwarding plane, the system allows for an encapsulation overhead parameter that can be provisioned on a per queue basis. This is provided on a per queue basis to allow for differences in the encapsulation behavior between service flows in different queues. The system multiplies the offered load of the queue by the encapsulation-overhead parameter and adds the result to the offered load of the queue (offeredOctets * configuredEncapsulationOverhead + offeredOctets = frameOfferedLoad). The frame-offered-load value is used by the egress PoS/SDH port scheduler in the same manner as the egress Ethernet port scheduler.
From a provisioning perspective, queues, policers, and service level (and subscriber level) scheduler policies are always provisioned with packet-based parameters. The system converts these values to frame-based on-the-wire values for the purpose of port bandwidth allocation. However, port-based scheduler policy scheduler maximum rates and CIR values are always interpreted as on-the-wire values and must be provisioned accordingly. Figure: Port bandwidth distribution for service and port scheduler hierarchies and Figure: Port bandwidth distribution for direct queue to port scheduler hierarchy show a logical view of bandwidth distribution from the port to the queue level and shows the packet or frame-based provisioning at each step.
