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Zorluk: Çok zorQoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

An enterprise router connects a branch office to the head office over a sub-rate Metro Ethernet circuit with a Committed Information Rate (CIR) of 20 Mbps on a 100 Mbps physical Ethernet handoff. Users report that during peak hours, TCP file transfers experience severe throughput degradation due to packet drops and retransmissions, while real-time UDP streams undergo frequent drops during bursts. An audit of the egress WAN interface reveals a QoS policy enforcing the 20 Mbps bandwidth limit using traffic policing. Which explanation correctly identifies why traffic shaping should be implemented instead of traffic policing on this egress interface?

  1. Traffic shaping buffers excess out-of-profile packets in queues and transmits them smoothly over time up to the CIR rate, whereas traffic policing immediately drops or remarks packets exceeding the rate limit without buffering.Cevap
  2. B
    Traffic policing buffers non-conforming burst traffic until interface queue capacity opens, whereas traffic shaping immediately drops out-of-profile packets to prevent TCP window size expansion.
  3. C
    Traffic policing dynamically adjusts physical Layer 1 interface signaling speeds to match CIR limits, whereas traffic shaping alters Layer 2 framing headers to drop non-conforming packets.
  4. D
    Traffic shaping operates exclusively at Layer 2 using CoS bits to prioritize bursty data frames, whereas traffic policing operates at Layer 3 by rewriting IP TOS precedence values to drop packets.

Cevap

Traffic shaping buffers excess out-of-profile packets in queues and transmits them smoothly over time up to the CIR rate, whereas traffic policing immediately drops or remarks packets exceeding the rate limit without buffering.
Traffic shaping mitigates bursty traffic drops on sub-rate interfaces by storing out-of-profile packets in software queues and pacing their transmission at the CIR rate over time. Traffic policing does not buffer packets; non-conforming traffic exceeding the CIR is immediately dropped or remarked, leading to high TCP retransmissions and stream degradation during bursts.

Adım Adım Çözüm

1
Analyze the operational behavior of Traffic Policing.
Traffic policing measures the traffic rate against a configured limit (such as CIR). Traffic that exceeds the profile is either immediately dropped or remarked to a lower priority (e.g., lower DSCP value). Policers do not buffer traffic, causing TCP window collapses and packet loss during traffic bursts.
Policing is designed for hard rate enforcement, usually applied inbound or at service provider handoffs.
2
Analyze the operational behavior of Traffic Shaping.
Traffic shaping measures the traffic rate against a configured CIR profile, but excess traffic above the rate limit is retained in software queues and scheduled for delayed transmission. This smoothes out bursty traffic profiles.
Shaping prevents packet loss caused by transient traffic bursts by using memory buffering, making it ideal for sub-rate Ethernet access links.
3
Compare policing versus shaping for the sub-rate WAN scenario.
Because the physical port operates at 100 Mbps while the ISP contract limits throughput to 20 Mbps CIR, bursty transmissions from the router fill the link at 100 Mbps burst rates. Traffic policing drops these bursts immediately. Traffic shaping holds excess packets in queues and meters them out at 20 Mbps, preventing TCP retransmissions and stream drops.
Shaping is recommended egress on sub-rate WAN interfaces to avoid provider policer drops and smooth bursty traffic.

Anahtar Kavram

Difference between QoS Traffic Policing and Traffic Shaping
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QoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

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