QoS

Question 1

3 categories of QoS

Answer 1

1. IntServ (strict)
2. DiffServ (flexible)
3. Best effort (not strict)

Question 2

TCP slow start

Answer 2

when tcp reduces its window size because of dropped or delayed traffic segments

Question 3

WRED / RED

Answer 3

Weighted random early detection:introduces the possibility of discard for packets with specific markings at specific queue depths

–> if the queue is nearly full –> will discard some TCP traffic (will be retransmitted) vs udp won’t be retransmitted

Question 4

LFI

Answer 4

Link Fragmentation and Interleaving:

Qos mechanism that fragments large packets and interleaves smaller packets with the fragments

Question 5

QoS markings

Answer 5

! per hop behaviour

L2
1. CoS (class of service): L2 marking, will not cross a router (on 802.1Q frame)

L3

• ToS DSCP (differentiated services code point) L3 marking
• . IP precedence : L3 marking

Question 6

QoS shaping & policing

Answer 6

shaping: delays traffic rather than dropping, used on slower speed interfaces –> maximise bandwidth
policing: drops traffic rather than delaying it, used on faster speed interfaces –> chops off the extremely high traffic, but retransmission

Question 7

At which OSI layer does QoS operate?

Answer 7

Quality of Service (QoS) occurs at both Layer 2 and Layer 3 of the OSI Model.

Layer 2 Quality of Service (QoS) allows for traffic prioritization and bandwidth management to minimize network delay using Cost of Service (CoS) classification, and DSCP marking under the 802.1p standard.

Layer 3 Quality of Service (QoS) allows for managing the quality of network connections through its packet routing decisions.

Question 8

compression

Answer 8

VOIP payload can be reduced by 50%

–> CRTP compressed RTP

–> WAN accelerator do specifically this

Question 9

Which protocol can be used for traffic engineering?

Answer 9

QoS (Quality of Service) is a feature set used to engineer or prioritize various traffic types based on classifications marked in traffic.

Question 10

What field in an IP packet is used to set QoS settings?

Answer 10

QoS settings are set in what’s known as the ToS (Type of Service) Byte in an IPv4 packet (Traffic Class in IPv6).

Question 11

DSCP

Answer 11

Differentiated Services Code Point (DSCP) is a means of MARKING traffic and of providing QoS in modern Layer 3 IP networks. It uses the 6-bit Differentiated Services (DS) field in the IP header for the purpose of packet classification.

Question 12

DiffServ

Answer 12

PER hOP BEHAVIOUR