Chapter 6: Quality of Service Flashcards
Learning the importance of QoS.
1
Q
Describe PerHop Behaviour
A
- Expedited Forwarding (EF) / Cisco Low Latency Queue / priority percent n / Highest strict priority, used for VoIP
- Assured Forwarding (AF) / Cisco Weighted Fair Queue / bandwidth percent / queues with fixed schedule time or percentage
- Class Selector (CS) / Cisco Class Selector / priority percent n / 8 priority levels
- Best Effort (BE) / Cisco Best Effort / bandwidth remaining percent n queue-length size / remaining bandwidth
2
Q
What is QoS and where is it used?
A
- QoS represents set of techniques necessary to manage network performance.
- Network performance can be affected by Packet Delay, Jitter, and Loss
- During congestion, QoS filter which packets can be dropped
- QoS is usually used when sensitive traffic is present like: voice, video data, online gaming, and web-browsing
- QoS should only be used when link is experiencing congestion
3
Q
Describe what Delay, Jitter and Loss mean.
A
- Delay ( time it takes for packet to travel from source destination, delay may occur due to congestion on the link )
- Jitter ( variation in delay of received packets )
- Loss ( a packed is dropped when congestion occurs, usually when there is no QoS in place)
4
Q
List the queuing algorithms and when are they implemented.
A
- First In First Out (no packet priority, all packets treated equally, traffic sensitive data may be dropped during congestion)
- Weighted Fair Qeueuing (automated scheduling, E1 speed default, classifies ased on header addressing source/destination IP & MAC port numbers and other protocols, applies priority to traffic to classify)
- Class-Based Weighted Fair Queuing (extends WFQ functionality by providing user-defined traffic classes, based on match criteria like protocols ACLs and interfaces, assigns traffic characteristics like bandwidth size, weight, max limit, guarantees bandwidth delivered during congestion)
- Low Latency Queuing (strict priority to CBWFQ on ingress interface, reduces jitter in voice, thus adding priority to packets before sending, which are sent first)
5
Q
List the traffic types.
A
- Voice (predictable, smooth, delay sensitive, no retransmit, high priority)
- Video (bursty, greedy, drop/delay sensitive, UDP)
- Data (TCP/UDP, no tolerance data loss [email, webpages], prefer TCP to resend packets if lost, FTP is greedy, insensitive to drop/delay)
6
Q
List QoS models possible.
A
- Integrated Services (delivers end-to-end QoS, uses resource reservation and admission control, as well as connection -oriented approach like telephone, uses RSVP to signal QoS of traffic needs)
- Differentiated Services (simple/scalable, provides QoS almost guarantee, low latency to critical traffic, cost-effecctive, non end-end but rather enforces QoS on each hop)
7
Q
How is QoS marked on L2 and L3?
A
- On L2, 802.1Q supports VLAN tagging, 2 fields are added (QoS prioritization is 802.1p, it uses first 3 bits [8 levels of priority] in tag control filed [TCI])
- On L3, IPv4 & IPv6 have tags 8 bit (IPv4 - ToS field, IPv6 - Traffic Class field)
8
Q
Q: List some QoS Implementation Techniques.
A
- Trust Boundaries (traffic classified and marked close to source, for IP phones, APs, videoconference, typically requires re-marking)
- Congestion Avoidance (monitors network traffic loads to anticipate and avoid, achieved by packed dropping, monitor average depth of queue, doesn’t really work well with UDP)
- Police Shaping and Policing (Shaping lists a boundary and schedules, policing drops or remarks excess traffic)
9
Q
Identify five principle causes of network delay.
A
- link aggregation causing congestion2. serialization delay
- propogation delay
- jitter buffer
- link queuing
10
Q
What type of network usually uses QoS?
A
converged, multiservice network meaning it carries packets from multiple applications including voice video and data
11
Q
List 3 possible congestion scenarios
A
- Aggregation
- Speed Mismatch
- LAN to WAN
