Riverbed SteelHead 3070-SD, Руководство пользователя

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106 | Configuring QoS Shaping on SteelHead SD
If you set the QoS priority in a traffic rule
• classifies traffic based on the DSCP mark and shapes it according to a fixed bandwidth allocation
designated for each traffic class to ensure that their aggregate bandwidth doesn’t exceed the
configured rate.
• is per packet; it is not flow based.
• applies only on WAN interfaces, not LAN interfaces. It is not site aware.
If all queues have equal size packets, and all queues are full, they are not handled equally. Urgent receives
40% of the bandwidth, High receives 30%, Normal receives 20%, and Low receives 10%. This bandwidth
distribution occurs when you have different traffic classes shared on the same WAN uplink and under
congestion conditions they are competing for the same bandwidth rate. If all your traffic is in the Low
traffic class, then it’s not competing with the other queue classes, so it receives 100% of the bandwidth.
Traffic scheduling and shaping uses a basic single rate to regulate the data transmission rate and drain
the queues. Each round turn traverses only traffic class queues that are in the active bitmap and always
in the order from higher to lower priority queues. The token bucket shaper then throttles the rate at
which packets are transmitted by determining when a packet selected by the DWRR can be sent.
To ensure that each class-based queue doesn’t overflow, when the queue is filled to its maximum
capacity, the newly arriving packets are dropped until the queue has enough room to accept incoming
traffic.
To view the TOS/DSCP and QoS traffic classes table, see Appendix B, “TOS, DSCP, QoS Traffic Class
Table.”
If you set the QoS priority in a traffic rule
You can also configure custom DSCP marks in the SCM when you configure the QoS priority in traffic
rules (Rules > Traffic Rules > New Traffic Rule). Traffic rules allow you to mark traffic to specific DSCP
value. Only DSCP marked traffic is placed in the QoS priority queues. Traffic must have a DSCP marking
value for QoS traffic to be classified. For details, see the
SteelConnect Manager User Guide .
If the QoS priority is configured in a traffic rule, then:
• inbound QoS shaping is done before the QoS mark is applied.
• outbound QoS shaping is done after the QoS mark is applied.
PQ class ID QoS class (bandwidth) Example of
traffic type DSCP values
Urgent (3) Latency Sensitive - 40% VoIP Class Selector CS7(56), CS6(48), EF(46),
VA(44), CS5(40), CS4(32)
High (2) Streaming Media - 30% Video AF4x(34,36,38), AF3x(26,28,30), CS3(24),
AF2x(18,20,22), TOS4(4), CS2(16), TOS1(1)
Normal (1) Best Effort - 20% MAPI CS0, AF1x, TOS2, and if the DSCP value is not
specified
Low (0) Background Traffic - 10% YouTube CS1
Note: The DSCP standards for QoS have been specified and respecified many times. For the latest standards, see
DSCP RFC 2474, RFC 3168, RFC 3260, and RFC 5865.