Cisco QoS - 低延迟排队

最初释放基于类的加权公平队列（CB-WFQ），而不支持优先级排队（PQ）系统。虽然实时语音和视频应用程序可以在CB-WFQ系统中接收带宽预留，但系统无法保证延迟和抖动（延迟变化）实时，交互式语音和视频对话的要求。为了解决这种需求，思科很快就揭开了一个名为低延迟排队（LLQ）的新系统，它使用与CB-WFQ相同的模块化QoC CLI（MQC）方法。思科文档有时是指LLQ作为下面的公式：PQ + CB-WFQ。思科为MQC添加了优先级排队支持，以提供以下三种保证：•丢包•延迟•抖动CB-WFQ系统的带宽保证仅保证数据包丢失。CB-WFQ保证使用带宽命令配置，而PQ系统配置为Priate ite命令。PQ具有不同的行为特征，而不是CB-WFQ保证值得有一些时间和注意的保证。我将与上次博客中使用的以下带宽保证示例进行了交谈：策略 - 映射测试策略类FTP带宽百分比10类HTTP带宽百分比20！接口串行0/0/0带宽1536服务策略输出测试 - 策略此QoS策略保证在拥塞期间为FTP和HTTP流量提供一定量的带宽。当T1链路上没有拥塞时，任何一个应用程序都可以使用整个可用的1.536Mbps。 During periods of congestion, FTP traffic is only guaranteed 153.6kbps (router will round the bandwidth number to the next highest increment of 8kpbs – 160kbps). FTP could transmit at much higher rates however because there is no limit on the FTP traffic. Policing and shaping technologies set a ceiling or maximum bandwidth, while the bandwidth command configures minimum guarantees. FTP and HTTP traffic will always receive an amount of bandwidth higher than the bandwidth statement unless the QoS policy provisions 100% of the bandwidth and there is high congestion on the output interface. Both CB-WFQ and LLQ policies can only be applied to egress (output) interfaces. Only classification, marking, and policing technologies can be used on ingress (input) to Cisco routers. The priority queue (PQ) is implicitly policed on the Cisco routers. If voice over IP was added to our policy from the previous page, the configuration changes to the policy would be similar to the following: Policy-map test-policy Class voice priority 264 ! Interface Serial 0/0/0 service-policy output test-policy The PQ will never be able to forward traffic over 264kbps. If there is no congestion on the output interface, the policy will limit voice traffic to 264kbps. I chose this number with the assumption that the customer was sending up to 10 G.729 voice calls over a PPP link. The audio codec is 8kbps, 16kbps for layer 3 and 4 overhead (IP/UDP/RTP), and 2.4kbps for the layer 2 PPP overhead. See my previous blog for more detailed information on the sizing of voice calls. The priority queue does include a small burst interval that is automatically configured based on a 200ms burst interval of the configured bandwidth. The bust usually accommodates for the layer 2 overhead if this was not calculated. The layer 2 overhead of ATM cells may be much larger than the automatically configured burst size. A default voice sample rate of 20ms results in a high cell tax (overhead) when using ATM cell-based services. Although the priority command can be used in multiple policy-map classes, there is only one Priority queue in the L3 software queuing system. Version 3.3 of Cisco’s QoS SRND recommends the allocation of no more than 33% of the interface bandwidth to the priority queue to ensure other application classes get a fair share of the available bandwidth. The 33% recommendation is very generic and can be exceeded on an as needed basis. The Telepresence SRND (2.0) describes situations where much higher bandwidth values can be allocated to the priority queue on an as needed basis. The following policy map configures one priority queue that is 724kbps in size, but the voice class will be implicitly policed to 264kbps and the video class 460kbps. The video class was sized based on one H.264 video call (384kbps) and 20% overhead. Policy-map test-policy Class voice Priority 264 Class video Priority 460 The next blog will continue our coverage of LLQ and the MQC. We will discuss the processing order of the policy-map, and the class-default. REFERENCES Cisco Feature Navigator www.cisco.com/go/fn