Volume 2 - Chapter 15: Quality of Service (QoS) Flashcards
What are the 4 main characteristics of QoS?
Bandwidth
Delay
Jitter
Loss
Describe the QoS characteristic delay.
The time it takes for a packet sent by a host to arrive at its destination (one-way).
Describe the QoS characteristic jitter.
The difference in one-way delay between different packets. For example:
* Packet 1: one-way delay is 350 ms
* Packet 2: one-way delay is 475 ms.
Jitter in this scenario is 25 ms
Loss is expressed as a ____ .
Percentage.
For example, 5 packets out of 200 are lost. The loss is 2.5 percent.
A user’s perception of the performance of an application on a network is refered to as?
Quality of Experience (QoE)
A large file transfer or backup over a network is a considered to be a ____ flow.
non-interactive
A real time application like a VOIP call or streaming service is considered to be a ____ flow.
interactive
Describe the general QoS requirements to support VOIP.
- Delay (one-way): 150 ms or less
- Jitter: 30 ms or less
- Loss: 1% or less
Describe the general QoS requirements to support video.
- Bandwidth: 384 Kbps to 20+ Mbps
- Delay (one-way): 200–400 ms
- Jitter: 30–50 ms
- Loss: 0.1%–1%
Describe the purpose of classification/marking in regards to QoS.
To classify a packet based on information in the header, then mark the message by changing information in the header.
Where in the topology is it recommended to mark traffic in regards to QoS?
As close to the source of the traffic as possible.
For example, on a switchport where the host is connected.
Describe the size and purpose of the DSCP field in regards to QoS.
Differentiated Services Code Point (DSCP) is a 6 byte field in the IP header used for marking packets.
True or False:
All fields in the IP and TCP header are available for QoS classification.
True, QoS can classify traffic on all information in the IP and TCP headers. Here are just a few for example:
* Source IP
* Destination IP
* Source port
* Checksums
Describe the purpose of NBAR in regards to QoS.
Cisco Network Based Application Recognition (NBAR) is used to categorize traffic beyond application ports.
For example, classifying Webex traffic differently than Youtube
The NBAR term of defining characteristics of different applications is referred to as?
application signatures.
Describe the size and purpose of the IPP Type of Service (ToS) field
IP Precence is a 3-bit field in the IPv4 header which is used for QoS marking.
It is the predecessor to DSCP and only supports 8 possible markings.
Describe the size and purpose of the DSCP Type of Service (ToS) field
DSCP is a 6-bit field in the IPv4 header which is used for QoS marking.
It supports up to 64 values for marking.
The IPv6 DSCP field is refered to as?
The IPv6 Traffic Class byte
Functionally the same as the IPv4 DSCP field.
Describe the difference between QoS and CoS
Quality of Service refers to classifying and marking traffic based on information in the IP header.
Class of Service referes to classifying and marking traffic based on information in the Ethernet header.
Describe the size and purpose of the PCP field
Priority Code Point (PCP) is a 3-bit field in the Ethernet header which is used for QoS marking.
Where is the location of the PCP field in the Ethernet header?
The PCP is located in the last 3-bits of the 4-bit 802.1Q field.
As such, PCP Class of Service can only be used on trunks (switch-to-switch) links
Describe the size and purpose of the TID ToS field
The TID (Traffic Identifier) is a 3-bit field in the 802.11 header used for QoS marking.
Describe the size and purpose of the EXP ToS field
The EXP (Experimental Bits) is a 3-bit field in the MPLS label used for QoS marking of WAN traffic.
The point in a network where QoS/CoS markings are considered to be trusted is refered to as?
The trust boundry.
For example, a switch may be a trust boundry. QoS markings added from the switch are trusted by the rest of the network.
However, if the QoS markings are added by a host connected to the switch, they will be ignored. The host is not considered trusted.
True or False:
An IP phone is considered to be a generally common trust boundry.
True; DSCP and CoS markings can be set on a IP-phone by a voice admin.
This way, the traffic can be marked as close to the source of the traffic as possible, without the host itself creating the markings.
The phone can also mark PC and voice traffic with different markings.
Which DSCP value as defined by DiffServ best matches the following?
Used for packets that need low latency, low delay, and low jitter
Expedited Forwarding (EF)
What is the DSCP value for Expedited Forwarding?
Decimal 46
What DSCP value is most commonly used for voice traffic?
Expedited Forwarding (EF)
Which DSCP type as defined by DiffServ best matches the following?
- Includes 12 individual DSCP markings
- Has 4 queues, each with 3 drop priority classes
Assured Forwarding (AF)
What is the AF DSCP value that has the best queue and best drop priority?
AF41
What is the AF DSCP value that has the worst queue and best drop priority?
AF11
What is the AF DSCP value that has the worst queue and worst drop priority?
AF13
What is the AF DSCP value that has the best queue and worst drop priority?
AF43
2 packets are marked with the following Assured Forwarding DSCP markings:
* Packet 1: AF21
* Packet 2: AF22
Describe how QoS will treat these packets.
Both packets will be put into the same queue (queue 2), but packet 1 will receive preferred treatment for drops
2 packets are marked with the following Assured Forwarding DSCP markings:
* Packet 1: AF33
* Packet 2: AF22
Describe how QoS will treat these packets.
Packet 1 will be put into the better queue (queue 3) over packet 2 (queue 2).
However, packet 2 will receive preferred treatment for drops
In regards to QoS, define the purpose of the Class Selector (CS).
The Class Selector (CS) DSCP values represent the older 3-bit IPP values.
Describe how the Class Selector (CS) values fit within the current DSCP field.
The DSCP values append 3 zeros after the 3-bit IPP value.
What would be the DSCP value that includes a Class Selector value of CS5?
The decimal DSCP value would be 40:
1 0 1 0 0 0
As per RFC 4954, what is the recommended DSPC marking for voice payload?
DSCP EF (Expedited Forwarding)
As per RFC 4954, what is the recommended DSPC marking for Interactive Video?
AF4x
As per RFC 4954, what is the recommended DSPC marking for High Priority (low latency traffic)?
AF2x
As per RFC 4954, what is the recommended DSPC marking for Streaming Video?
AF3x
As per RFC 4954, what is the recommended DSPC marking for Standard Data?
CS0
What term best describes the following?
A QoS toolset used to “hold” packets while they wait for an outgoing interface to be available.
Queuing
Which QoS queuing strategy best matches the following?:
A single output queue that sends out the first packet to be received.
First-In First-Out (FIFO)
Which QoS queuing strategy best matches the following?:
Multiple queues that are each cycled through in order; where each queue gets time to be processed.
Round-Robin Scheduling
Also know as Prioritization
Describe the difference between Round-Robin and Class-Based Weighted Fair Queuing (CBWFQ).
Both strategies provide each queue with a guaranteed minimum amount of time.
However, CBWFQ gives priority to certain queues so they get more processing time.
Which queuing strategy best matches the following?:
* Queue 1: Allocated 50% of time
* Queue 2: Allocated 30% of time
* Queue 3: Allocated 20% of time
Class-Based Weighted Fair Queuing (CBWFQ)
Which QoS queuing strategy best matches the following?:
Multiple queues that are each cycled through in order; where traffic in a special queue always gets processed first
Low Latency Queuing (LLQ)
Which queuing strategy best matches the following?:
* Voice Queue: Allocated 50% of time
* Queue 1: Allocated 20% of time
* Queue 2: Allocated 15% of time
* Queue 3: Allocated 15% of time
Low Latency Queuing (LLQ)
In regards to QoS, describe the function of Queue Starvation.
When there is more priority traffic in bits-per-second than in interface can handle. Preventing other non-priority queues from ever getting processing time.
In regards to QoS, describe the function of Policing.
To limit the amount of traffic being placed into high-priority queues (LLQ)
Describe how policing/policers limit traffic into high-priority queues.
By placing a cap on the amount of bandwidth in the high-priority queue. If a certain cap is reached (20% for example), the the excess traffic is discarded.
What tool can be used to limit the amount of voice/video traffic to prevent policers from discarding excess traffic?
Call Admission Control (CAC) tools.
What type of queuing method should be used for data classes and non-interactive voice/video?
Class-Based Weighted Fair Queuing
What type of queuing method should be used for interactive voice and video?
Low-Latency Queuing (LLQ) with a priority queue
True or False, voice and video should be put into different queues so policing can apply to them differently?
True
In regards to QoS, describe the purpose of Shaping and Queuing.
The monitor and keep the current bit rate of traffic passing through and interface below a configured threshold.
Describe the difference between shaping and queuing.
With shaping, if a packet pushes the interface past the shaping rate; then the packet will be queued
With policing, if a packet pushes the interface past the policing rate; then the packet will be discarded
Policer tools allow for temporary ____ that push traffic past the configured policing rate.
bursts
Where are policing and shaping tools most commonly used?
At the edge of the network; usually the WAN connection.
A mutually agreed WAN service speed is refered to as?
A Committed Information Rate (CIR)
Describe the purpose of a shaper’s time interval.
To establish a percentage of time that an interface is allow to transmit data at its maximum speed.
A network engineer has a CIR of 200 Mbps on their WAN link, but their outgoing interface uses 1 Gbps.
What would be the shaping behavior used with a 1 second time interval?
The outgoing interface would sent at the full 1 Gbps interface rate, but only for 200 ms before resting for 800 ms. This results in a 200 Mbps average
What is the recommended shaping interval to support voice and video.
10 ms maximum
In regards to QoS, describe the function of tail-drop
Tail drop occurs in a network with medium to high congestion. When multiple queues are full, traffic will be dropped.
When does tail drop typically occur?
When a TCP window is shrunk due to a lost segment.
What term best describes the following?
A tool used to drop a percentage of data depending on how full a queue is.
Congestion Avoidance Tools.
