21 - Introducing QoS Flashcards
Applications such as FTP and TFTP are considered __ applications. Both are used to send and receive files. Typically, a user selects a group of files that need to be retrieved and then starts the transfer. Once the download starts, no additional human interaction is required. Even with low bandwidth, the download will complete eventually.
batch
__ applications are applications in which the user waits for a response. Because these applications require human interaction, response times are more important than batch applications, but still are not critical.
Interactive
__ applications such as voice and video applications also involve human interaction. Because of the amount of information that is transmitted, bandwidth is critical. In additional, because these applications are time-critical, a delay on the network can cause a problem.
Real-time
What are the characteristics of data traffic?
- Smooth/bursty
- Benign/greedy
- Drop-insensitive
- Delay-insensitive
- TCP retransmits
What are the characteristics of voice traffic?
• Smooth • Benign • Drop-sensitive • Delay-sensitive • UDP priority • One-way requirements o Latency < 150ms o Jitter < 30ms o Loss <1% o Bandwidth 30-128 kbps
What are the characteristics of video traffic?
• Smooth • Benign • Drop-sensitive • Delay-sensitive • UDP priority • One-way requirements o Latency < 150ms o Jitter < 30ms o Loss < 0.1-1% o Bandwidth 384 kbps – 20+ Mbps
The fundamental purpose of __ is to manage contention for network resource to maximise the end-user experience of a session. Because not all packets are equal, you should not treat them equally.
QoS
When there is __ on a link, less important traffic is delayed or dropped in favour of delay-sensitive, business-important traffic.
contention
QoS gives priority to some sessions over other sessions. Packets of __-sensitive sessions bypass queues of packets belonging to __-sensitive sessions. When queue buffers overflow, packets are dropped on the session that can recover from the loss or those sessions that can be eliminated with minimal business impact.
delay, non-delay
Generally, you can place QoS tools into these different categories:
Classification and marking .
Policing, shaping and re-marking tools –
Congestion management or scheduling tools
Link specific tools
These tools analyse sessions to determine which traffic class they belong to and therefore which treatment the packets in the session should receive. Classification should happen as few times as possible because it takes time and uses resources. For that reason, packets are marked after classification.
Classification and marking
These tools assign different classes of traffic to certain portions of network resources. When traffic exceeds available resources, some traffic might be dropped, delayed, or re-marked to avoid congestion on a link.
Policing, shaping and re-marking tools
With these tools, when traffic exceeds network resources that are available, traffic is queued. Queued traffic will await available resources. Traffic classes that do not handle delay well are better off being dropped unless there is a guarantee delay-free bandwidth for that traffic class.
Congestion management or scheduling tools
With these tools, there are certain types of connections, such as WAN links, that can be provisioned with special traffic handling tools. One such example is fragmentation
Link specific tools
The ___ is a point in the network where packet markings are not necessarily trusted. You can create, remove, or rewrite markings at that point. The borders of a trust domain are the network locations where packet markings are accepted and acted upon.
trust boundary
A __ is a tool that inspects packets within a field to identify the type of traffic that the packet is carrying. Traffic is then directed to a policy enforcement mechanism for that type of traffic.
classifier
___ mechanisms include marking, queue, policing, shaping or any combination of those mechanisms.
Policy enforcement
A __ is a tool that writes a value in the header of the packet, frame, cell, tag, or label. The aim of ___ is to preserve the classification decision that the classifier tool reached. Devices that follow the device where __ was performed do not have to do classification and analysis to determine how to treat the packet.
marker, marking
__, __, __, __, and __ are different terms to describe designated fields in a frame of the packet header. How devices treat packets depends on the field values.
CoS, ToS, DSCP, Class Selector, and TID
___ of traffic determines which type of traffic the packets or frames belong to. Only after you identify traffic can you apply policies to it (marking, shaping, policing).
Classification
Best practice is to identify and mark traffic as __ to the trust boundary as possible
close
3 most common ways to classify traffic are:
- Markings – classification is done on existing L2 or L3 settings
- Addressing – classification is done based on source and destination interface, or L2 dest address, or L3 source and dest address, or source and dest L4 port.
- Application signatures – Classification is done based on application content. This is also called deep packet inspection.
__ and __ are tools that identify and respond to traffic violations. They usually identify traffic violations in a similar manner but differ in their response.
policers, shapers
Performs checks for traffic violation against a configured rate. The action they take in response is either dropping or re-marking the excess traffic
policers
_ are traffic-smoothing tools that work in cooperation with buffering mechanisms. A shaper does not drop traffic, but smooths it out so it never exceeds a configured rate.
shaper
__ occurs when a packet enters a device faster than it can exit. When this happens, __ management tools are activated. Queueing is temporary storage of backed-up packets. You perform queueing to avoid dropping packets.
congestion
Includes queue (or buffering). It uses a logic that re-orders packets into output buffers. It is only activated when congestion occurs.
congestion management
A process of deciding which packet should be send out next. This happens regardless of whether there is congestion on the link,
scheduling
Takes the previous model and adds a queue with strict priority (for real time traffic)
low-latency queueing
Single queue with packets that are sent in the exact order they arrive
FIFO
Set of four queues that are service in strict-priority order. By enforcing strict priority, the lower priority queues are server only when the hight-priority queues are empty
PQ
Set of 16 queues with a round-robin scheduler. This does not provide strict priority for real-time traffic
CQ
An algorithm that divides the interface bandwidth by the number of flows, thus ensuring proper distribution of the bandwidth for all applications. Provides good service for the real-time traffic, but there are no guarantees for a particular flow.
WFQ
Combination of bandwidth guarantee with dynamic fairness of other flows. It does not provide latency guarantee and is only suitable for data traffic management
CBWFQ
A method that is essentially CBWFQ with strict priority. This method is suitable for mixes of data and real-time traffic. _ provides both latency and bandwidth guarantee.
LLQ
Devices can either wait for queues to fill up and then start dropping packets or drop packets before queues fill up. Dropping packets as they arrive is called __. Selective dropping of packets while queues are filling up is called ___.
tail drop, congestion avoidance
When a queue fills up, it drops packets as they arrive. This can result in waste of ___ if TCP traffic is predominant.
bandwidth
Cisco uses __ which drops packets randomly, but “randomness” is skewed by traffic weights.
WRED
QoS features can be applied using the ___and the tasks for configuring the __. The ___ allows you to define a traffic class, create as traffic policy (policy map), and attach the traffic policy to an interface. The traffic policy contains the QoS feature that will be applied to the traffic class
Modular QoS CLI (MQC)
The __ structure allows you to define a traffic class, create a traffic policy, and attach the traffic policy to an interface.
MQC
How is MQC configured?
- Define a traffic class by using the class-map command. A traffic class is used to classify traffic
- Create a traffic policy by using the policy-map command. A traffic policy contains a traffic class and one or more QoS features that will be applied to the traffic class. The QoS features in the traffic policy determine how to treat the classified traffic
- Attach the traffic policy (policy map) to the interface by using the service-policy command
Methods for implementing QoS: requires each interface to be individually configured. Time consuming
Legacy CLI
Methods for implementing QoS: coded at CLI, uses configuration modules. Best method for QoS fine tuning
Modular QoS at the CLI (MQC)
Methods for implementing QoS: applies a possible QoS configuration to the interface. Fasted way to implement QoS
Cisco AutoQoS
Methods for implementing QoS: application for simple QoS configuration
Cisco SDM QoS Wizard
A traffic class contains 3 major elements: a___ name, a series of ___ commands, and, if more than one match command is used in the traffic class, __ on how to evaluate these match commands.
traffic class, match, instructions
