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ag-CCNA > 17 - IP Services II - QoS > Flashcards

17 - IP Services II - QoS Flashcards

1
Q

What four characteristics of network traffic does QoS let you manage?

A

Bandwidth
Delay
Jitter
Loss

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2
Q

What two types of Delay are there?

A

One way delay

Round trip delay

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3
Q

What is Jitter?

A

The variation in one way delay between consecutive packets sent to the same application

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4
Q

What causes most loss in networks (more than faulty cabling)?

A

Device queues getting too full then discarding packets

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5
Q

What is QoE?

A

Quality of Experience

Typically the users perception of their use of the application on the network

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6
Q

What was historically called ‘batch traffic’?

A

Non interfactive data / data applications e.g. Data backup or file transfers

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7
Q

What are the general steps taken for VoIP packets?

A
  1. Codec processes (digitizes) sound to create binary code for a certain time period e.g. 20ms
  2. Data placed into IP packet
  3. Packet transmitted over network
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8
Q

What characteristics of QoS would you balance for VoIP?

A

Don’t neccessarily need much Bandwidth.

But interactive voice needs better level of quality for Delay, Jitter and Loss

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9
Q

What are the recommended guidelines for QoS for interactive voice?

A

Delay (one-way): <150ms
Jitter: <30ms
Loss: <1%

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10
Q

What are the recommended guidelines for QoS for video?

A

Bandwidth: 384kbps to 20+Mbps
Delay (one-way): 200-400ms
Jitter: 30-50ms
Loss: 0.1 - 1%

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11
Q

What are 4 key areas of QoS tools?

A

Classification and Marking
Queuing
Shaping and Policing
Congestion Avoidance

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12
Q

What is Classification and Marking for QoS?

A

The QoS tool that classifies packets based on their header contents and then marks the message by changing some bits in specific header fields

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13
Q

What roles do queues playing in Classification?

A

Packets can be classified into different types of queues to then be sent as determined appropriately by scheduling / prioritization

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14
Q

What are the rough steps for a router forwarding a packet with respect to classification and queuing?

A
  1. Router makes forwarding decisions
  2. Classification logic applied to put packets in appropriate output queue
  3. Router waits for outgoing interface to be available
  4. Queuing tools scheduling logic chooses the next packet prioritizing one over the other
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15
Q

What is the recommended strategy for Matching in terms of complexity?

A

Do complex matching early in the life of a packet, then marking the packet to allow devices that process the packet later in its life to use much simpler classification logic

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16
Q

What does marking a packet do?

A

QoS tool changes header fields to set values

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17
Q

What is the DSCP field?

A

Differentiated Services Code Point

6 bit field for QoS marking

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18
Q

Can you perform classification with an ACL?

A

Yes but not every classification can be made with an ACL due to more complex requirements

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19
Q

What is NBAR?

A

Cisco Network Based Application Recognition

Matches packets for classification in a large variety of ways that are useful for QoS

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20
Q

How might you use NBAR?

A

Classify traffic type for an Amazon Video and give it a unique DSCP marking

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21
Q

What is CoS?

A

Class of Service
3 bit field in the third byte of the 802.1Q header

A way of managing traffic by grouping similar types of traffic and treating each as a class with a certain level of service priority

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22
Q

Why does marking a QoS field in the IP header work well?

A

Because the IP header exists for the entire trip from source to destination host

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23
Q

What is an IPv4 ToS field?

A

Type of Service field
1 byte

Used to put the DSCP (6 bits) marking value

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24
Q

What was formerly used in the ToS field before DSCP?

A

3 bits of IPP (IP Precedence)

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25
Q

What is the DSCP field contained within in an IPv6 header?

A

Traffic Class byte

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26
Q

What is PcP?

A

Priority Code Point

Another name for the CoS field in the 802.1q header

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27
Q

What is a downside of QoS fields such as CoS?

A

Because they are in the 802.1q header, they only exist when 802.1q trunking is used on a link

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28
Q

What is the TID field and where is it used?

A

QoS field for WiFi in 802.11 headers

29
Q

What is the EXP field and where is it used?

A

MPLS WAN in the MPLS label header

30
Q

What is the trust boundary for a network with respect to QoS?

A

The point in the path of a packets flow at which the networking devices can trust the QoS markings

31
Q

True/False: When the access layer includes an IP phone, it is typically the trust boundary instead of the access layer switch

A

True

32
Q

What are three important sets of DSCP values used in DiffServ?

A 
Expedited Forwarding (EF)
Assured Forwarding (AF)
Class Selector (CS)
33
Q

What is the EF DSCP used for and what is its value?

A

Expedited Forwarding. Single DSCP value

Used for packets that need

  • Low Delay (low latency)
  • Low Jitter
  • Low Loss

Value: 46

34
Q

What is the AF DSCP used for and what is its value?

A

Assured Forwarding. 12 DSCP values

4 queues, using the 3 levels of drop priority for congestion avoidance tools

AFXY
- X is queue number
- Y is priority
Lowest is best

35
Q

What is the CS DSCP used for and what is its value?

A

Class Selector. 8 DSCP values

Equivalents of IPP 0 -7 (CS0-7)
Decimal values 0, 8…56

36
Q

How might you DSCP mark video and high priority data?

A

AF4x: Interactive video (conferencing)
AF3x: Streaming video
AF2x: High priority (low latency) data

37
Q

How might you DSCP mark standard data?

A

CS0

38
Q

What are two key things a Queuing system needs to do?

A

Classifier and Scheduler

Needs a classifier function to choose which packets are placed into which queue (reacting to previously marked values or more extensive matches).

Also needs a scheduler to decide which message to take next when interface becomes available

39
Q

What can a scheduler perform for a queuing system?

A

Priorization

40
Q

What is weighted round robin scheduling?

A

Round robin but with the concept of weighting, wherein one queue gets given more preference than another

41
Q

What is CBWFQ

A

Class Based Weighted Fair Queuing

Used by routers to guarantee a minimum amount of bandwidth to each class

42
Q

How does CBWFQ work?

A

Uses weighted round robin queuing but lets engineer define the weightings as a percentage of link bandwidth

43
Q

What is the downside of a round-robin scheduler for delay, jitter and loss for voice and video packets? And how is it remedied?

A

It adds too much delay for voice and video packets. It does not provide low enough delay, jitter or loss. A round robin queue may have to send from other queues before it gets back to the queue the important packet is on.

Add Low Latency Queuing (LLQ) to the scheduler

44
Q

How does LLQ work?

A

Tells a scheduler to treat one or more queues as special priority queues, always taking the next message from one of these special priority queues

45
Q

What is a potential problem with LLQ / priority queuing and what would be the solution?

A

Queue starvation

The speed of the interface is X bits per second but there is always more than this number of bits coming into the priority queue, thus the other queues never get served

Solution is to limit the amount of traffic placed in to the priority queue

46
Q

What is Policing?

A

A way of limiting the amount of traffic placed into the priority queue

47
Q

How do you prevent a Policer discarding too many excess messages beyond the bandwidth limit for a priority queue (which would affect experience)?

A

Limit the amount of voice and video that the network routes out this link

48
Q

What type of queuing method would you use for data classes and non interactive voice and video?

A

A round robin queuing method such as CBWFQ

49
Q

What type of queuing method would you use for interactive voice and video?

A

Priority queue with LLQ

50
Q

Why should you define enough bandwidth for each priority queue?

A

So that the built in policer doesn’t discard messages from the priority queues

51
Q

What is CAC and what should you use it for?

A

Call Admission Control

Avoid adding too much voice or video to the network which would trigger the policer function

52
Q

What do Shapers and Policers do?

A

Monitor the traffic rate through a device versus a configured shaping or policing rate

53
Q

What is the difference in functionality between a Shaper and Policer?

A

Shapers hold packets in queues to delay packets

Policers discard or mark packets

54
Q

How would you summarize the logic of Shaping and Policing?

A
  1. Check packet and if it pushes measured rate past the configured shaping / policing rate
  2. If no, do nothing
  3. If yes:
    If Shaping then delay by queuing
    If Policing then discard or mark differently
55
Q

What is a burst with respect to Policers?

A

Policers allow for a burst beyond the set policing rate for a short time after a period of low activity

56
Q

Where is it generally best to use Policing?

A

Best used at the edge between two networks e.g. PPP Ethernet WAN between two enterprise routers

57
Q

What is the CIR?

A

Committed information rate.

Agreed allowed sending rate in each direction

58
Q

How would you summarize the key features of policing?

A
  • Measures traffic rate over time in comparison to configured rate
  • Allows short bursts after inactivity
  • Enabled in either direction but typically at ingress
  • Can discard packets but can also re-mark for later
59
Q

What does a Shaper do?

A

Slows down messages by queuing them and then servicing shaping queues, but not based on when the physical interface is available but the shaping rate

60
Q

What is a bad side effect of Shaping and how should it be fixed?

A

Because it slows down packets it creates more delay and sometimes jitter

You should configure a shaper’s setting to change the internal operation of the shaper to reduce delay and jitter caused to voice and video traffic

61
Q

What is a Shapers time interval?

A

It’s internal logic that deals with how a shaper averages, over time, sending at a particular rate. It sends as fast as it can then waits, and repeats

62
Q

Why should you configure a short Time interval for a shaper for voice or video packets that need low delay and jitter?

A

To prevent a packet arriving just as the shaper finishes sending data for a time interval, which would cause it to have to wait before the shaper schedules the next packet, even if its in the priority shaping queue

63
Q

How would you summarize the key features of Shapers?

A
  • Measure traffic rate over configured period of time in comparison to configured rate
  • Allows short bursts after inactivity
  • Enabled for egress
  • Slow down packets by queuing and releasing over time at shaping rate
  • Use queuing tools to create and schedule shaping queues
64
Q

What flow control mechanism does TCP use?

A

Windowing

65
Q

What is a TCP window

A

A window size granted to the sender by the receiver which defines the number of bytes a sender can transmit over the TCP connection before receiving an ACK

66
Q

How much does the TCP window grow and shrink by when everything good / when loss is detected

A
  • Receiver doubles window every time receiver acknowledges data eventually growing to size sender never has to stop sending
  • When receiver senses loss of TCP segment, window shrinks by one half
67
Q

What is tail drop?

A

When output queues are full and new packets arriving to be added have to be dropped

68
Q

What do congestion avoidance tools primarily use?

A

TCPs own windowing mechanisms

ag-CCNA (21 decks)

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