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IT infrastructuur > 14. QoS > Flashcards

14. QoS Flashcards

1
Q

What are the 3 leaing causes of quality issues?

A
  • Packet loss
  • Latency and Jitter
  • Lack of bandwith
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2
Q

What is the recommended network latency?

A

Not more than 400 ms

For real-time traffic 150 ms

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3
Q

How can latency be broken down?

A 
Propagation delay (fixed)
Serialization delay (fixed)
Processing delay (fixed)
Dealy variation (variable)
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4
Q

What is propagation delay?

A

The time it takes for a packet to tavel from the source to a destination at the speed of light.

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5
Q

What is the refractive index?

A

The lack of vaccum conditions in a cable slows down the speed of light know by this index

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6
Q

What is serialization delay?

A

The time it takes to place all the bits of a packet onto a link. The hiogher the link speed, the lower this delay.

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7
Q

What is the formula to calculate serialization delay?

A

packet size in bits / line speed in bps = x aantal seconden

Omrekenen naar ms dat x 1000

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8
Q

What is processing delay?

A

Fixed amount of time it takes for a networking device to thae the packet from an input interface and place the packet onto the output queue of the output interface

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9
Q

What factors can influence the processing delay?

A

CPU speed
Type of CEF
Architecture

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10
Q

What is delay variation?

A

Called Jitter, is the difference in latency betweens packets.
If one packet needs 70 ms to traverse the network and the other 50 ms, the jitter is 20 ms

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11
Q

How can Jitter be precented with life time packets?

A

A de-jitter buffer can smooth out the various delays. +/- 30 ms can be taken for this. If a packets arrives after it will be dropped.

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12
Q

How can packet loss be prevented?

A
  • Change link speed
  • Use QoS mechanisms
  • Implement traffic policing to drop low prio packets and allow high prio
  • Implement traffic shaping to delay instead of dropping packets
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13
Q

What are the 3 QoS models?

A
  • Best effort
  • IntServ
  • DiffServ
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14
Q

What is IntServ?

A

Created for real-time apps. The application signals the requirements to the network to reserve resource (bandwith). It uses RSVP to reserve throughout the network and Call Admission Control (CAC) to guarantee that no other IP traffic can use the reserver capacity.

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15
Q

What is the downside of IntServ?

A

It needs to maintain a RSVP path for every single flow.. This does not scale well in big networks

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16
Q

How does IntSer work?

A

It sends a RSVP PATH message with the bandwith that needs to be reserved. The receiver sends back a RSVP reservation request back to reserve again.

17
Q

What is DiffServ?

A

DiffserV divides IP traffic in classes to prioritize.

18
Q

What is packet classification?

A

A QoS mechanism that distinguishes traffic streams. The caegorize IP packets with a specific class. Should take place as close to the source as possible

19
Q

What are the 7 types used for classification?

A 
Internal
Layer 1
Layer 2
Layer 2.5
Layer 3
Layer 4
Layer 7
20
Q

What is NBAR2?

A

A deep packet inspection engine that can identify & classify protocols and application using layer 3-7 data.

21
Q

What are the 2 NBAR2 modes?

A

Protocol Discovery - Gets statistics to define QoS shaping and policing
Modular QoS - traffic matching a network protocol can be classified

22
Q

What is marking?

A

Mechanism that colors packets by changing a field within the packet or a frame header

23
Q

What are the 4 types used for marking?

A
  • Internal - QoS groups
  • Layer 2 - 802.1Q / CoS
  • Layer 2.5 - MPLS bits
  • Layer 3 - DSCP and IPP
24
Q

What is the TPID value?

A

16-bit field assigned with the value 0x8100

25
Q

What is the TCO field?

A

16-bit field composed of:
PCP - priority code point 3
DEI - Drop elegible Indicator 1
VLANID 12

26
Q

What does PCP do?

A

Priority Code Point uses prio 0 -7 to mark certain traffic

27
Q

What does DEI do?

A

Drop Elegible Indicator uses bit 0 for keeping and 1 to mark it to be dropped during congestion

28
Q

What are the PCP code?

A 
BK - background
BE - best effort
EE - excellent effort
CA - critical application
VI - video 100ms
VO - voice 10ms
IC - internetwork control
NC - network control
29
Q

What is the drawback of L2 marking?

A

I cannot traverse a non 802.1Q link or L3

30
Q

What is the ToS field?

A

8-bit field, only 3 bits are used for IPP.

IPP 6 & 7 are reserved

31
Q

What is the DiffServ field?

A

8-bit field, 6 bits for DSCP

32
Q

What are the 4 types of PHB?

A

Class selector
Default forwarding - best effort
Assured forwarding - guaranteed bandwith
Expedited forwarding - low delay

IT infrastructuur (31 decks)

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