QoS and Congestion Flashcards
Monitoring the system for QoS
Determine where and when congestion occurs. Pass info to where action can be taken. Adjust system operation to correct problem.
QoS Skip
use Label Switching or MPLS for good QoS, Integrated services, or Differentiated services.
A flow
Stream of packets from source to destination
Why do we need flow
Due to bandwidth, jitter and delay
Retransmissions
used to restore lost data partially
When an application can enhance network service
Network requirements are less demanding than application requirements
Buffering packets at receiver
Reduce jitter
Jitter
Variations in delay or packet arrival times. Easily noticed on audio or video.
Why can video sustain lost frames
they go unnoticed if they aren’t in abundance
Networks support different categories of QoS
Constant bitrate, Realtime variable bitrate, Non realtime variable bitrate, Available bitrate.
Guarantee high QoS
design a network with enough capacity for traffic. Overprovisioning and expensive. Low latency and no loss. Best performance.
A network with less requirements can still match this using QoS techniques
Issues to address to help QoS
What application requires from a network. How to control traffic entering network. How to make resource reservations at router. If extra traffic can be accommodated safely on network.
Load shedding
removes excess load
Link Bandwidth
Network must apportion bandwidth between different flows.
Queue Space
Defines which packets should be dropped
When too much traffic is offered
congestion sets in and performance degrades sharply.
Goodput
Rate at which useful packets enter the network
It is impossible to avoid congestion completely in a ______ network.
In a datagram network.
If all routers had infinite memory
Congestion would get worse.
Bufferbloat
Network devices have more memory than needed. Senders need timely information about congestion, as packets are stored in router buffer instead of being dropped.
How to solve bufferbloat
Load shedding, or providing additional capacity
Congestion Control
Behavior of all the hosts and routers.
Traffic Management
Making sure the network is able to carry the offered traffic
Flow Control
Relates to traffic between specific sender and receiver.
Approaches for building a good network
Build network provisioned for traffic it needs to carry. Resources can be added dynamically when there is congestion. Links and routers must be upgraded. Routes can be modified for changing traffic patterns. Divert traffic from heavily used routes
Traffic Aware routing
Enable mobile listeners to route packets around hotspots
What to do if you cant increase capacity of a network
Lighten the load
Virtual Circuit Network
If a new connection congests the network, it could be rejected. Illustration of admission control.
Admission control
Forbids senders from sending traffic if the networks capacity cannot accommodate it
Strategies to drop packets that networks are unable to deliver
Traffic Shaping and Traffic Policing
Traffic Shaping
Limits transmissions from a particular sender
Traffic Policing
Dropping traffic from a particular sender if it exceeds some rate
Divert traffic from hotspots
By factoring in load when calculating routes
How to factor in load when calculating routes
Setting link weight to be a function of the fixed link bandwidth, propagation delay, observed load or average queueing delay
Traffic Aware Routing Downfall
If routing algorithm sends all data through a particular network segment, can increase traffic leading to congestion.
Traffic Aware routing unsecurity
If routing through unsafe parts of a network
Successful solutions for traffic aware routing
Gallaghers routing method and Multipath routing
Multipath Routing
There may be multiple ways of getting from A to B by taking another path
Gallagher’s routing method
Move traffic between routes slow enough to allow it to converge
Difficulties of traffic aware routing
routing protocols do not change according to load. Instead traffic engineering is used. Now, some software defined networking automates some of these tasks.
