14. QoS Flashcards
What are the 3 leaing causes of quality issues?
- Packet loss
- Latency and Jitter
- Lack of bandwith
What is the recommended network latency?
Not more than 400 ms
For real-time traffic 150 ms
How can latency be broken down?
Propagation delay (fixed) Serialization delay (fixed) Processing delay (fixed) Dealy variation (variable)
What is propagation delay?
The time it takes for a packet to tavel from the source to a destination at the speed of light.
What is the refractive index?
The lack of vaccum conditions in a cable slows down the speed of light know by this index
What is serialization delay?
The time it takes to place all the bits of a packet onto a link. The hiogher the link speed, the lower this delay.
What is the formula to calculate serialization delay?
packet size in bits / line speed in bps = x aantal seconden
Omrekenen naar ms dat x 1000
What is processing delay?
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
What factors can influence the processing delay?
CPU speed
Type of CEF
Architecture
What is delay variation?
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
How can Jitter be precented with life time packets?
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.
How can packet loss be prevented?
- 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
What are the 3 QoS models?
- Best effort
- IntServ
- DiffServ
What is IntServ?
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.
What is the downside of IntServ?
It needs to maintain a RSVP path for every single flow.. This does not scale well in big networks
How does IntSer work?
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.
What is DiffServ?
DiffserV divides IP traffic in classes to prioritize.
What is packet classification?
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
What are the 7 types used for classification?
Internal Layer 1 Layer 2 Layer 2.5 Layer 3 Layer 4 Layer 7
What is NBAR2?
A deep packet inspection engine that can identify & classify protocols and application using layer 3-7 data.
What are the 2 NBAR2 modes?
Protocol Discovery - Gets statistics to define QoS shaping and policing
Modular QoS - traffic matching a network protocol can be classified
What is marking?
Mechanism that colors packets by changing a field within the packet or a frame header
What are the 4 types used for marking?
- Internal - QoS groups
- Layer 2 - 802.1Q / CoS
- Layer 2.5 - MPLS bits
- Layer 3 - DSCP and IPP
What is the TPID value?
16-bit field assigned with the value 0x8100
What is the TCO field?
16-bit field composed of:
PCP - priority code point 3
DEI - Drop elegible Indicator 1
VLANID 12
What does PCP do?
Priority Code Point uses prio 0 -7 to mark certain traffic
What does DEI do?
Drop Elegible Indicator uses bit 0 for keeping and 1 to mark it to be dropped during congestion
What are the PCP code?
BK - background BE - best effort EE - excellent effort CA - critical application VI - video 100ms VO - voice 10ms IC - internetwork control NC - network control
What is the drawback of L2 marking?
I cannot traverse a non 802.1Q link or L3
What is the ToS field?
8-bit field, only 3 bits are used for IPP.
IPP 6 & 7 are reserved
What is the DiffServ field?
8-bit field, 6 bits for DSCP
What are the 4 types of PHB?
Class selector
Default forwarding - best effort
Assured forwarding - guaranteed bandwith
Expedited forwarding - low delay
