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Transmission Rate with Intelligent Oversubscription on SONET/SDH OC48/STM16 IQE PICs Overview

Junos OS class of service (CoS) enables you to treat traffic differently by providing a minimum bandwidth guarantee, low latency, low packet loss, or a combination of these properties for categories of traffic, called forwarding classes. When traffic reaches an outbound interface, traffic is queued for transmission on the physical media. The forwarding class determines the queuing of traffic and other functions of processing class of service, such as rewriting behavior aggregate markers.

You can control the way the system services queues by configuring schedulers and scheduler maps. After traffic is placed in the appropriate queues, a scheduler defines how an interface should process this traffic from each queue. A scheduler is associated with a particular queue and a forwarding class through a scheduler map.

The parameters in a scheduler that define how to service a queue include transmission rate, transmission priority, buffer size, and a random early detection (RED) algorithm. You can define the order in which packets transmit a queue by configuring a priority and transmission rate for each queue. The buffer size and RED configuration define the storage and dropping of packets for each queue.

Junos OS supports multiple levels of transmission priority, with higher-priority queues being serviced before lower-priority queues, as long as the higher-priority forwarding classes retain enough bandwidth credit. The priority levels are Strict-High, High, Medium-High, Medium-Low, and Low. The priority scheduling of forwarding classes determines the order in which an outbound interface transmits traffic from the queues.

The transmission rate, on the other hand, controls how much bandwidth the traffic associated with a given forwarding class can consume. By default, all queues can exceed their assigned transmission rate if other queues are not fully utilizing their assigned rates, unless you configure the transmission rate with the exact option.

The transmission rate can be a fixed value, such as 1 megabit per second (Mbps), a percentage of the total available bandwidth, or the rest of the available bandwidth. You can limit the transmission bandwidth to the exact value you configure, or allow it to exceed the configured rate if additional bandwidth is available from other queues. This property enables you to ensure that each queue receives the amount of bandwidth appropriate to its level of service.

The SONET/SDH OC48/STM16 IQE PIC is a clear-channel PIC that is designed to provide better scaling and improved queuing, buffering, and traffic shaping along with clear-channel functionality. The PIC is preconfigured with five levels of priorities with three priorities for traffic below the guaranteed rate (CIR) and two priorities for traffic above the guaranteed rate (PIR).

Oversubscription is a state where the transmission rate of the incoming packet is much higher than the rate the Packet Forwarding Engine and system can handle, causing important packets to be dropped. If an oversubscribed link or service experiences an excess of traffic—either bursty or non-bursty—it can result in traffic loss or delay that could potentially affect other services and links. To reduce the risks of oversubscription, the SONET/SDH OC48/STM16 IQE PIC ensures prioritization and allows mission-critical services to be protected during congestion with intelligent dropping of packets. In addition to dropping low priority packets during congestion, the PIC prevents quality deterioration in periods of high traffic with intelligent sharing of excess bandwidth, providing bandwidth optimization.

Previously, the SONET/SDH OC48/STM16 IQE PIC supported a maximum bandwidth optimization by oversubscribing the available bandwidth up to 200 percent. This optimization was achieved by excluding the transmission rate percentage specified for the Strict-High queue from the total 100 percent transmission rate. Therefore, the transmission rate percentage for all the non-Strict-High queues added up to 100 percent. This computation was done after the internal mapping of the excess priority or the excess rate.

At the time of oversubscription, the queues were split into two priority groups:

  • Strict-High

  • High, Medium-High, Medium-Low, and Low

As an enhancement to the intelligent oversubscription feature on SONET/SDH OC48/STM16 IQE PICs, support for maximum bandwidth optimization is increased to 300 percent with an additional priority group being created for all queues marked with low priority.

When the sum of transmission rates for all queues exceeds 100 percent, the interface is in an oversubscribed state. At the time of oversubscription, the queues are split into three priority groups with the intelligent oversubscription feature enhancement:

  • Strict-High

  • High, Medium-High, and Medium-Low

  • Low

The sum of transmission rates for all queues in each of the priority groups is less than or equal to 100 percent, thereby allowing the SONET/SDH OC48/STM16 IQE PICs to support the maximum bandwidth optimization by overconfiguring the available bandwidth up to 300 percent.

Note:

When the sum of transmission rates of all queues in any of the priority groups exceeds 100 percent, the commit fails and an error message is displayed.

When the sum of transmission rates for all queues exceeds 100 percent, the configured transmission rates are scaled down to 100 percent. This is called rebasing and is required for accurate mapping of transmission rates to the weights assigned to each queue. Weights are assigned to a queue based on the queue properties and are used to determine the distribution of available bandwidth and flow of traffic from each queue.

Configuring any of the queues with the remainder option on an oversubscribed SONET/SDH OC48/STM16 IQE PIC is not allowed. When a queue is configured with the remainder option, and the sum of transmission rates for all non-remainder queues is less than or equal to 100 percent, rebasing is not required. However, calculating the remainder transmission rate for a queue configured with the remainder option differs.

Note:

The remainder option is not supported on an oversubscribed SONET/SDH OC48/STM16 IQE PIC. When the sum of transmission rates for all queues exceeds 100 percent, and if one or more queues are configured with the remainder option, a syslog error message is generated and the configuration is ignored.

Previously, the remainder calculation on a SONET/SDH OC48/STM16 IQE PIC did not include the queues specified for Strict-High priority. The remainder transmission rate was calculated by taking into consideration transmission rates for all other queues excluding the sum of transmission rates of all Strict-High queues. With the enhancement to the intelligent oversubscription on a SONET/SDH OC48/STM16 IQE PIC, the remainder transmission rate is calculated by taking into consideration the transmission rates of all other queues irrespective of the priority specified.

The rebased values are only used for assigning weights to queues, which affect the order in which the queues are serviced. If any queue that qualifies for rebasing is configured with the rate-limit option, weights are assigned to queues after applying the configured value of rate-limit for that particular queue.

As an example, sample configurations A and B in Table 1 display the need for rebasing transmission rates and remainder calculation.

Table 1: Rebasing Transmission Rates and Remainder Calculation

Queue

Priority

Transmission rate

Configuration A

q0

Strict-High

100%

q1

High

30%

q2

Medium-High

30%

q3

Medium-Low

30%

q4

Low

20%

q5

Low

20%

q6

Low

20%

q7

Low

remainder

Configuration B

q0

Strict-High

30%

q1

Medium

20%

q2

Low

40%

q3

Low

remainder

In Configuration A, the sum of transmission rates of non-remainder queues (q0, q1, q2, q3, q4, q5, and q6) exceeds 100 percent, leaving the interface in an oversubscribed state. Because configuring the remainder option is not supported on an oversubscribed PIC, and q7 is a remainder queue, Configuration A is ignored, although all the queues qualify for rebasing.

However, if the sum of transmission rates for all the queues exceeded 300 percent, or if the sum of transmission rates for all queues in any of the priority groups exceeded 100 percent, the configuration is ignored.

In Configuration B, the sum of transmission rates of non-remainder queues (q0, q1, and q2) is less than 100 percent. Therefore, rebasing of transmission rates is not required. Remainder calculation for q3 is done by deducting the sum of transmission rates of non-remainder queues from 100, irrespective of the priority specified. In this example, the transmission rate for q3 is (100 - 30 - 20 - 40) 10%.

The support for oversubscribing the bandwidth on a SONET/SDH OC48/STM16 IQE PIC up to 300 percent increases the efficiency of networks and reduces CapEx for network operators. Large service providers have exacting performance requirements, and the impact of traffic disruptions due to congestion on an oversubscribed interface can be significant. The SONET/SDH OC48/STM16 IQE PIC virtually eliminates this risk with intelligent oversubscription capabilities that enable carriers to ensure the performance of mission-critical services on oversubscribed interfaces and routers.