Samsung Ubigate iBG3026, Руководство по настройке

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CHAPTER 19. QoS
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© SAMSUNG Electronics Co., Ltd.
Traffic Policing versus Traffic Shaping
Policing controls the traffic by dropping packets or marking down their
priority when the configured rate is exceeded. Shaping controls the traffic by
delaying packets using a queuing mechanism when they arrive faster than the
configured rate. Therefore shaping can smooth out the burstiness in a flow,
helping to minimize buffer overruns in intermediate routers. Policing does not
smooth out peaks, but it can control peaks by dropping packets or marking
them down.
Since shaping delays packets, thereby throttling the packet rate instead of
dropping them, adaptive applications(those using TCP) perform better.
Dropping packets can cause exponential back-off of TCP which can affect the
throughput. This problem with policing can be minimized by configuring the
peak parameter of the policer to be a sufficiently large value.
Need for Traffic Policing
Policing has the advantage of providing low latency since it does not queue
packets. This makes policing a good choice for interactive and streaming
voice and video applications. Policing also uses much less resources in the
router than shaping. It is a better and more practical method to provide QoS
for incoming traffic on an interface.
On iBG3026 access routers, policing can be used to limit the maximum rate of
a traffic flow received on a WAN interface. This is especially useful when the
POP/CO router does not do any shaping or policing of traffic before
transmitting it onto the WAN or it is under a different administrative domain
which prevents access to the QoS configuration.
Traffic Policing Functionality on iBG3026
Customers will now be able to rate limit inbound traffic on the WAN links
using policing while doing CBQ for outbound traffic. This means the
iBG3026 can now provide QoS for traffic in both directions, eliminating
dependency on the upstream router. Policing of outbound WAN traffic is also
possible, but using CBQ to shape traffic is recommended because of reasons
mentioned in section ‘Traffic Policing versus Traffic Shaping’.
Additionally, CBQ in iBG3026 OS also provides bandwidth guarantee,
bandwidth borrowing and prioritization. It provides ‘Total Link Access
Control’ instead of just rate limiting.