4.7 Forward Per-Hop Behavior for QoS Flashcards
Summarize the forwarding per-hop behavior (PHB) for QoS, such as classification, marking, queuing, congestion, policing, and shaping. (27 cards)
What is the function of Per-Hop Behavior (PHB) in QoS?
It determines how packets are handled at each network hop.
PHB is defined by the Differentiated Services Code Point (DSCP) field, which directs network devices on how to treat packets based on their priority and QoS policies.
What are the key parameters of QoS?
- Bandwidth
- Delay
- Jitter
- Loss
Bandwidth: Data transfer rate over the network.
Delay: Time taken for a packet to travel from source to destination.
Jitter: Variation in packet arrival times.
Loss: Percentage of dropped packets during transmission.
What are the types of traffic that typically require high QoS?
- Voice
- Video
- Real-time Data
Voice: Real-time communication needing low latency for clear audio.
Video: Streaming media that requires consistent bandwidth and low delay.
Real-time Data: Applications needing immediate data processing with minimal delay.
Define:
classification
It groups packets based on specific characteristics.
It allows network devices to apply different QoS policies based on packet type, such as voice or video, and prioritize them accordingly.
What are the two main traffic classification methods in QoS.
- Class-based classification
- Behavior-based classification
Class-based classification: Groups packets by attributes like source or application.
Behavior-based classification: Applies QoS based on traffic patterns.
Why is marking important in QoS?
It assigns priority to packets using DSCP.
By marking packets with DSCP values, routers and switches identify and prioritize traffic throughout the network, ensuring critical data is treated with higher priority.
What is Tail Drop?
A congestion control method that drops packets from a full queue.
Tail Drop is a simple congestion control mechanism, but it can lead to significant packet loss, especially during periods of high congestion.
How does Queuing impact QoS in networking?
It prioritizes traffic in buffers.
Queuing ensures that high-priority traffic, like voice and video, is transmitted first, reducing delays for critical applications.
What does Policing do in QoS?
Enforces traffic limits by discarding or re-marking excess traffic.
This prevents network congestion by controlling the rate at which traffic is transmitted, ensuring traffic does not exceed predefined limits.
True or False:
Policing discards packets that exceed the configured traffic rate.
True
Policing ensures traffic adheres to rate limits by either dropping packets or re-marking them as lower priority.
Define:
Congestion Management
It involves techniques like queuing to prevent network congestion.
Effective congestion management helps prevent delays and packet loss by controlling how traffic is queued and scheduled during periods of high load.
Why is congestion avoidance important in QoS?
It prevents network degradation
Techniques like Random Early Detection (RED) proactively manage congestion by dropping packets early to avoid tail drop.
How does Random Early Detection (RED) help in congestion avoidance?
By dropping packets early to avoid tail drop.
RED helps prevent global synchronization and reduces the likelihood of congestion collapse by probabilistically dropping packets before queues become full.
What is the role of Priority Queuing in managing network traffic?
It sends high-priority traffic first.
It prioritizes real-time traffic like VoIP and video, ensuring that critical applications experience minimal delays during congestion.
What is the key difference between Policing and Shaping in QoS?
- Policing drops or re-marks excess traffic.
- shaping delays it.
Shaping buffers traffic until the network can accommodate it, while policing directly limits traffic rate by discarding or marking excess packets.
True or False:
PHBs are defined by the DSCP field in IP headers.
True
Differentiated Services Code Point (DSCP) values determine how each packet is treated by network devices, specifying its priority and forwarding behavior.
How does Weighted Fair Queuing (WFQ) work in queuing?
It allocates bandwidth based on packet flow weight.
WFQ ensures that bandwidth is fairly distributed among different traffic flows, with higher-priority traffic receiving more resources.
Define:
Class-Based Weighted Fair Queuing (CBWFQ).
It guarantees minimum bandwidth to each class.
CBWFQ uses weighted round-robin scheduling to allocate resources, ensuring that higher-priority traffic gets sufficient bandwidth.
Describe Round-Robin queuing in QoS.
It ensures that each flow gets an equal share of bandwidth.
This method is often used in WFQ and CBWFQ to balance load between different types of traffic.
What is the impact of Jitter on real-time applications?
It causes variations in packet arrival times.
Jitter can negatively affect the quality of real-time applications, such as video conferencing, where consistent timing is crucial for smooth playback.
True or False:
Shaping always results in packet loss when traffic exceeds the configured rate.
False
Shaping delays excess traffic rather than dropping it, ensuring that all packets eventually reach their destination, albeit with potential delays.
What is the time it takes for a packet to reach its destination called?
Delay
Delay is a critical factor in QoS for real-time applications like VoIP, where low delay is essential for communication quality.
How does Traffic Policing help manage network congestion?
It keeps traffic within rate limits.
By enforcing traffic limits, policing prevents network congestion and ensures that critical traffic is not delayed or lost due to excessive load.
Describe the role of Assured Forwarding (AF) in DiffServ.
It defines 12 DSCP values for traffic with varying drop priorities.
AF provides multiple levels of forwarding assurance for different traffic classes, helping manage congestion while ensuring some traffic is prioritized over others.
