Dec 15, 2009

Riverbed Steelhead QoS Class Queue Methods

Optionally, select one of the following queue methods for the class from the drop-down list:
  • SFQ. Shared Fair Queueing (SFQ) is the default queue for all classes. Determines Steelhead appliance behavior when the number of packets in a QoS class outbound queue exceeds the configured queue length. When SFQ is used, packets are dropped from within the queue in a round-robin fashion, among the present traffic flows. SFQ ensures that each flow within the QoS class receives a fair share of output bandwidth relative to each other, preventing bursty flows from starving other flows within the QoS class.
  • FIFO. Transmits all flows in the order that they are received (first in, first out). Bursty sources can cause long delays in delivering time-sensitive application traffic and potentially to network control and signaling messages.
  • MXTCP. Has very different use cases than the other queue parameters. MX-TCP also has secondary effects that you need to understand before configuring:
  1. When optimized traffic is mapped into a QoS class with the MX-TCP queuing parameter, the TCP congestion control mechanism for that traffic is altered on the Steelhead appliance. The normal TCP behavior of reducing the outbound sending rate when detecting congestion or packet loss is disabled, and the outbound rate is made to match the minimum guaranteed bandwidth configured on the QoS class.
  2. You can use MX-TCP to achieve high-throughput rates even when the physical medium carrying the traffic has high loss rates. For example, MX-TCP is commonly used for ensuring high throughput on satellite connections where a lower-layer-loss recovery technique is not in use.
  3. Another usage of MX-TCP is to achieve high throughput over highbandwidth, high-latency links, especially when intermediate routers do not have properly tuned interface buffers. Improperly tuned router buffers cause TCP to perceive congestion in the network, resulting in unnecessarily dropped packets, even when the network can support high throughput rates.
Important: Use caution when specifying MX-TCP. The outbound rate for the optimized traffic in the configured QoS class immediately increases to the specified bandwidth, and does not decrease in the presence of network congestion. The Steelhead appliance always tries to transmit traffic at the specified rate. If no QoS mechanism (either parent classes on the Steelhead appliance, or another QoS mechanism in the WAN or WAN infrastructure) is in use to protect other traffic, that other traffic might be impacted by MX-TCP not backing off to fairly share bandwidth.
When MX-TCP is configured as the queue parameter for a QoS class, the following parameters for that class are also affected:
  • Link share weight. The link share weight parameter has no effect on a QoS class configured with MX-TCP.
  • Upper limit. The upper limit parameter has no effect on a QoS classconfigured with MX-TCP.
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