QoS Questions

QoS quick summary: 1. Network factors: + Bandwidth: the speed of the link (or the capacity available on the link), usually measured in bits per second (bps) + Delay (or latency): how long a packet takes to get from the sender to the receiver. The more the delay, the slower the network. Delay is usually measured in milliseconds (ms) + Jitter: A measure of the variation in delay between packets. For example, one packet need 50ms to reach B from A while another packet takes 40ms then the jitter is 10ms + Loss: When packets travels to the destination, some of them may get lost. 2. QoS Models: + Best Effort: No QoS policies applied + Integrated Services (IntServ): Resource Reservation Protocol (RSVP) is used to reserve bandwidth + Differentiated Services (DiffServ): Packets are classified and marked individually; policy decisions are made independently by each node in a path. 3. QoS Markings: + IP Precedence: The first three bits of the IP ToS field (8 traffic classes) + Differentiated Service Code Point (DSCP): The first six bits of the IP ToS are used to provide granular classification 4. QoS terms: + Tail drop: When the queue is full, the packet is dropped. This is the default behavior. + Policing: is used to control the rate of traffic flowing across an interface. During a bandwidth exceed (crossed the maximum configured rate), the excess traffic is generally dropped or remarked. The result of traffic policing is an output rate that appears as a saw-tooth with crests and troughs. Traffic policing can be applied to inbound and outbound interfaces. Unlike traffic shaping, QoS policing avoids delays due to queuing. Policing is configured in bytes. + Shaping: retains excess packets in a queue and then schedules the excess for later transmission over increments of time. When traffic reaches the maximum configured rate, additional packets are queued instead of being dropped to proceed later. Traffic shaping is applicable only on outbound interfaces as buffering and queuing happens only on outbound interfaces. Shaping is configured in bits per second. + Marking: allows you to mark (set or change) a value (attribute) for the traffic belonging to a specific class 5. Congestion Management (types of queuing): uses the marking on each packet to determine which queue to place packets in First-in, first-out (FIFO): FIFO entails no concept of priority or classes of traffic. With FIFO, transmission of packets out the interface occurs in the order the packets arrive, which means no QoS Priority Queuing (PQ): schedules traffic such that the higher-priority queues “always” get serviced first Custom Queuing (CQ): provide specific traffic guaranteed bandwidth at a potential congestion point, assuring the traffic a fixed portion of available bandwidth and leaving the remaining bandwidth to other traffic. Weighted fair queueing (WFQ): offers dynamic, fair queueing that divides bandwidth across queues of traffic based on weights. CBWFQ

Question 1

Explanation

The following diagram illustrates the key difference between traffic policing and traffic shaping. Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate (or committed information rate), excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.

Note: Committed information rate (CIR): The minimum guaranteed data transfer rate agreed to by the routing device.

Question 2

Question 3

Explanation

The IP datagram header contains an 8-bit field called ToS (Type of Service). The field has been part of the IP header since the beginning, but it was rarely used until the recent introduction of Differentiated Services (Diff-Serv).

Note:
+ CoS does not exists in an IP header. It appears in the header of a 802.1Q frame only. CoS is used for QoS on a trunk link.
+ DSCP uses the first 6 bits of the TOS field.

Question 4

Explanation

The Resource Reservation Protocol (RSVP) protocol allows applications to reserve bandwidth for their data flows. It is used by a host, on the behalf of an application data flow, to request a specific amount of bandwidth from the network. RSVP is also used by the routers to forward bandwidth reservation requests.

Question 5

Explanation

The following diagram illustrates the key difference between traffic policing and traffic shaping. Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate (or committed information rate), excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.

Note: Committed information rate (CIR): The minimum guaranteed data transfer rate agreed to by the routing device.

Question 6

Explanation

Layer-3 marking is accomplished using the 8-bit Type of Service (ToS) field, part of the IP header. A mark in this field will remain unchanged as it travels from hop-to-hop, unless a Layer-3 device is explicitly configured to overwrite this field. There are two marking methods that use the ToS field:
+ IP Precedence: uses the first three bits of the ToS field.
+ Differentiated Service Code Point (DSCP): uses the first six bits of the ToS field. When using DSCP, the ToS field is often referred to as the Differentiated Services (DS) field.

Reference: http://www.routeralley.com/guides/qos_classification.pdf

Question 7

Explanation

The following diagram illustrates the key difference between traffic policing and traffic shaping. Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate (or committed information rate), excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.

Question 8

Question 9

Explanation

With Priority Queueing (PQ), traffic is classified into high, medium, normal, and low priority queues. The high priority traffic is serviced first, then medium priority traffic, followed by normal and low priority traffic.  -> Therefore we can assign higher priority for voice traffic.

Also with PQ, higher priority traffic can starve the lower priority queues of bandwidth. No bandwidth guarantees are possible -> It is still good because this network is mostly used for data traffic so voice traffic amount is small.

With First In First Out (FIFO) or Weighted Fair Queueing (WFQ), there is no priority servicing so they are not suitable here.

Reference: https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/qos_solutions/QoSVoIP/QoSVoIP.html

Weighted Random Early Detection (WRED) is just a congestion avoidance mechanism. WRED measures the size of the queues depending on the Precedence value and starts dropping packets when the queue is between the minimum threshold and the maximum threshold -> It does not have priority servicing either.

Question 10

Explanation

This module discusses the types of queueing and queueing-related features (such as bandwidth management) which constitute the congestion management QoS features:

Class-based WFQ (CBWFQ): extends the standard WFQ functionality to provide support for user-defined traffic classes. For CBWFQ, you define traffic classes based on match criteria including protocols, access control lists (ACLs), and input interfaces. Packets satisfying the match criteria for a class constitute the traffic for that class.

Priority queueing (PQ): With PQ, packets belonging to one priority class of traffic are sent before all lower priority traffic to ensure timely delivery of those packets.

Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_conmgt/configuration/xe-3s/qos-conmgt-xe-3s-book/qos-conmgt-oview.html

Note: Committed Access Rate (CAR) is only used for bandwidth limitation by dropping excessive traffic.

Question 11

Explanation

The following diagram illustrates the key difference between traffic policing and traffic shaping. Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate (or committed information rate), excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.