Lecture 07: QoS Concepts, Network Design and Troubleshooting
HKUSPACE CCIT ENA
Syllabus inspired by Cisco Networking Academy CCNA v7.0 (ENSA)
Module Objectives
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Topic Title
Topic Objective
Network Transmission Quality
Explain how network transmission characteristics impact quality.
Traffic Characteristics
Describe minimum network requirements for voice, video, and data traffic.
Queuing Algorithms
Describe the queuing algorithms used by networking devices.
QoS Models
Describe the different QoS models.
QoS Implementation Techniques
Explain how QoS uses mechanisms to ensure transmission quality.
Hierarchical Networks
Explain how data, voice, and video are converged in a switched network.
Scalable Networks
Explain considerations for designing a scalable network.
Network Documentation
Explain how network documentation is developed and used to troubleshoot network issues.
Troubleshooting Process
Compare troubleshooting methods that use a systematic, layered approach.
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Network Transmission Quality Prioritizing Traffic
• When traffic volume is greater than what can be transported across the network, devices queue (hold) the packets in memory until resources become available to transmit them.
• Queuing packets causes delay because new packets cannot be transmitted until previous packets have been processed.
• If the number of packets to be queued continues to increase, the memory within the device fills up and packets are dropped.
• One QoS technique that can help with this problem is to classify data into multiple queues, as shown in the figure.
Note: A device implements QoS only when it is experiencing some type of congestion.
Network Transmission Quality
Bandwidth, Congestion, Delay, and Jitter
• Network bandwidth is measured in the number of bits that can be transmitted in a single second, or bits per second (bps).
• Network congestion causes delay. An interface experiences congestion when it is presented with more traffic than it can handle. Network congestion points are ideal candidates for QoS mechanisms.
• The typical congestion points are aggregation, speed mismatch, and LAN to WAN.
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Network Transmission Quality
Bandwidth, Congestion, Delay, and Jitter (Cont.)
Delay or latency refers to the time it takes for a packet to travel from the source to the
destination.
• Fixed delay is the amount of time a specific process takes, such as how long it takes to place a bit on the transmission media.
• Variable delay takes an unspecified amount of time and is affected by factors such as how much traffic is being processed.
• Jitter is the variation of delay of received packets.
Description
Code delay
The fixed amount of time it takes to compress data at the source before transmitting to the first internetworking device, usually a switch.
Packetization delay
The fixed time it takes to encapsulate a packet with all the necessary header information.
Queuing delay
The variable amount of time a frame or packet waits to be transmitted on the link.
Serialization delay
The fixed amount of time it takes to transmit a frame onto the wire.
Propagation delay
The variable amount of time it takes for the frame to travel between the source and destination.
De-jitter delay
The fixed amount of time it takes to buffer a flow of packets and then send them out in evenly spaced intervals.
Network Transmission Quality Packet Loss
Without QoS mechanisms, time-sensitive packets, such as real-time video and voice, are dropped with the same frequency as data that is not time-sensitive.
• When a router receives a Real-Time Protocol (RTP) digital audio stream for Voice over IP (VoIP), it compensates for the jitter that is encountered using a playout delay buffer.
• The playout delay buffer buffers these packets and then plays them out in a steady stream.
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Network Transmission Quality Packet Loss (Cont.)
If the jitter is so large that it causes packets to be received out of the range of the play out buffer, the out-of-range packets are discarded and dropouts are heard in the audio.
• For losses as small as one packet, the digital signal processor (DSP) interpolates what it thinks the audio should be and no problem is audible to the user.
• When jitter exceeds what the DSP can do to make up for the missing packets, audio problems are heard.
Traffic Characteristics Network Traffic Trends
Note: In a properly designed network, packet loss should be near zero.
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In the early 2000s, the predominant types of IP traffic were voice and data.
• Voice traffic has a predictable bandwidth need and known packet arrival times.
• Data traffic is not real-time and has unpredictable bandwidth need.
• Data traffic can temporarily burst, as when a large file is being downloaded. This bursting can consume the entire bandwidth of a link.
More recently, video traffic has become the increasingly important to business communications and operations.
• According to the Cisco Visual Networking Index (VNI), video traffic represented 70% of all traffic in 2017.
• By 2022, video will represent 82% of all traffic.
• Mobile video traffic will reach 60.9 exabytes per month by 2022.
The type of demands that voice, video, and data traffic place on the network are very different.
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Queuing Algorithms Queuing Overview
The QoS policy implemented by the network administrator becomes active when congestion occurs on the link. Queuing is a congestion management tool that can buffer, prioritize, and, if required, reorder packets before being transmitted to the destination.
A number of queuing algorithms are available:
• First-In, First-Out (FIFO)
• Weighted Fair Queuing (WFQ)
• Class-Based Weighted Fair Queuing (CBWFQ)
• Low Latency Queuing (LLQ)
Queuing Algorithms First in First Out
First In First Out (FIFO) queuing buffers and forwards packets in the order of their arrival.
FIFO has no concept of priority or classes of traffic and consequently, makes no decision about packet priority.
There is only one queue, and all packets are treated equally. Packets are sent out an interface in the order in which they arrive.
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Queuing Algorithms
Weighted Fair Queuing (WFQ)
Weighted Fair Queuing (WFQ) is an automated scheduling method that provides fair bandwidth allocation to all network traffic.
• WFQ applies priority, or weights, to identified traffic, classifies it into conversations or flows, and then determines how much bandwidth each flow is allowed relative to other flows.
• WFQ classifies traffic into different flows based on source and destination IP addresses, MAC addresses, port numbers, protocol, and Type of Service (ToS) value.
• WFQ is not supported with tunneling and encryption because these features modify the packet content information required by WFQ for classification.
Queuing Algorithms
Class-Based Weighted Fair Queuing (CBWFQ)
Class-Based Weighted Fair Queuing (CBWFQ) extends the standard WFQ functionality to provide support for user-defined traffic classes.
Traffic classes are defined based on match criteria including protocols, access control lists (ACLs), and input interfaces.
Packets satisfying the match criteria for a class constitute the traffic for that class.
A FIFO queue is reserved for each class, and traffic belonging to a class is directed to the queue for that class.
A class can be assigned characteristics, such as bandwidth, weight, and maximum packet limit. The bandwidth assigned to a class is the guaranteed bandwidth delivered during congestion.
Packets belonging to a class are subject to the bandwidth and queue limits, which is the maximum number of packets allowed to accumulate in the queue, that characterize the class.
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Queuing Algorithms
Class-Based Weighted Fair Queuing (CBWFQ) (Cont.)
After a queue has reached its configured queue limit, adding more packets to the class causes tail drop or packet drop to take effect, depending on how class policy is configured.
• Tail drop discards any packet that arrives at the tail end of a queue that has completely used up its packet-holding resources.
• This is the default queuing response to congestion. Tail drop treats all traffic equally and does not differentiate between classes of service.
Queuing Algorithms
Low Latency Queuing (LLQ)
The Low Latency Queuing (LLQ) feature brings strict priority queuing (PQ) to CBWFQ.
• Strict PQ allows delay-sensitive packets such as voice to be sent before packets in other queues.
• LLQ allows delay-sensitive packets such as voice to be sent first (before packets in other queues), giving delay-sensitive packets preferential treatment over other traffic.
• Cisco recommends that only voice traffic be directed to the priority queue.
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QoS Models
Selecting an Appropriate QoS Policy Model
There are three models for implementing QoS. QoS is implemented in a network using either IntServ or DiffServ.
• IntServ provides the highest guarantee of QoS, it is very resource-intensive, and therefore, not easily scalable.
• DiffServ is less resource-intensive and more scalable.
• IntServ and DiffServ are sometimes co-deployed in network QoS implementations.
Best-effort model
Integrated services (IntServ)
Differentiated services (DiffServ)
Description
• Not an implementation as QoS is not explicitly configured.
• Use when QoS is not required.
• Provides very high QoS to IP packets with guaranteed delivery.
• Defines a signaling process for applications to signal to the network that they require special
QoS for a period and that bandwidth should be reserved.
• IntServ can severely limit the scalability of a network.
• Provides high scalability and flexibility in implementing QoS.
• Network devices recognize traffic classes and provide different levels of QoS to different
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QoS Models
The basic design of the internet is best-effort packet delivery and provides no guarantees.
• The best-effort model treats all network packets in the same way, so an emergency voice message is treated the same way that a digital photograph attached to an email is treated.
• Benefits and drawbacks of the best effort model:
The model is the most scalable.
There are no guarantees of delivery.
Scalability is only limited by available bandwidth, in which case all traffic is equally affected.
Packets will arrive whenever they can and in any order possible, if they arrive at all.
No special QoS mechanisms are required.
No packets have preferential treatment.
It is the easiest and quickest model to deploy.
Critical data is treated the same as casual email is treated.
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QoS Models
Integrated Services
IntServ delivers the end-to-end QoS that real- time applications require.
• Explicitly manages network resources to provide QoS to individual flows or streams, sometimes called microflows.
• Uses resource reservation and admission- control mechanisms as building blocks to establish and maintain QoS.
• Uses a connection-oriented approach. Each individual communication must explicitly specify its traffic descriptor and requested resources to the network.
• The edge router performs admission control to ensure that available resources are sufficient in the network.
QoS Models
Integrated Services (Cont.)
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In the IntServ model, the application requests a specific kind of service from the network before sending data.
• The application informs the network of its traffic profile and requests a particular kind of service that can encompass its bandwidth and delay requirements.
• IntServ uses the Resource Reservation Protocol (RSVP) to signal the QoS needs of an application’s traffic along devices in the end-to-end path through the network.
• If network devices along the path can reserve the necessary bandwidth, the originating application can begin transmitting. If the requested reservation fails along the path, the originating application does not send any data.
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• Explicit end-to-end resource admission control
• Per-request policy admission control
• Signaling of dynamic port numbers
• Resource intensive due to the stateful architecture requirement for continuous signaling.
• Flow-based approach not scalable to large implementations such as the internet.
QoS Models
Differentiated Services
The differentiated services (DiffServ) QoS model specifies a simple and scalable mechanism for classifying and managing network traffic.
• Is not an end-to-end QoS strategy because it cannot enforce end-to-end guarantees.
• Hosts forward traffic to a router which classifies the flows into aggregates (classes) and provides the appropriate QoS policy for the classes.
• Enforces and applies QoS mechanisms on a hop-by-hop basis, uniformly applying global meaning to each traffic class to provide both flexibility and scalability.
QoS Models
Differentiated Services (Cont.)
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• DiffServ divides network traffic into classes based on business requirements. Each of the classes can then be assigned a different level of service.
• As the packets traverse a network, each of the network devices identifies the packet class and services the packet according to that class.
• It is possible to choose many levels of service with DiffServ.
• Highly scalable
• Provides many different levels of quality
• No absolute guarantee of service quality
• Requires a set of complex mechanisms to work in
concert throughout the network
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QoS Implementation Techniques Avoiding Packet Loss
Packet loss is usually the result of congestion on an interface. Most applications that use TCP experience slowdown because TCP automatically adjusts to network congestion. Dropped TCP segments cause TCP sessions to reduce their window sizes. Some applications do not use TCP and cannot handle drops (fragile flows).
The following approaches can prevent drops in sensitive applications:
• Increase link capacity to ease or prevent congestion.
• Guarantee enough bandwidth and increase buffer space to accommodate bursts of traffic from fragile flows. WFQ, CBWFQ, and LLQ can guarantee bandwidth and provide prioritized forwarding to drop-sensitive applications.
• Drop lower-priority packets before congestion occurs. Cisco IOS QoS provides queuing mechanisms, such as weighted random early detection (WRED), that start dropping lower-priority packets before congestion occurs.
QoS Implementation Techniques QoS Tools
There are three categories of QoS tool, as described in the table.
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Description
Classification and marking tools
• Sessions, or flows, are analyzed to determine what traffic class they belong to.
• Whenthetrafficclassisdetermined,thepacketsaremarked.
Congestion avoidance tools
• Trafficclassesareallottedportionsofnetworkresources,as defined by the QoS policy.
• TheQoSpolicyalsoidentifieshowsometrafficmaybeselectively dropped, delayed, or re-marked to avoid congestion.
• TheprimarycongestionavoidancetoolisWREDandisusedto regulate TCP data traffic in a bandwidth-efficient manner before tail drops caused by queue overflows occur.
Congestion management tools
• Whentrafficexceedsavailablenetworkresources,trafficisqueued to await availability of resources.
• CommonCiscoIOS-basedcongestionmanagementtoolsinclude CBWFQ and LLQ algorithms.
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QoS Implementation Techniques QoS Tools (Cont.)
The figure shows the sequence of QoS tools used when applied to packet flows.
• Ingress packets are classified and their respective IP header is marked.
• To avoid congestion, packets are then allocated resources based on defined policies.
• Packets are then queued and forwarded out the egress interface based on their defined QoS shaping and policing policy.
Note: Classification and marking can be done on ingress or egress, whereas other QoS actions such queuing and shaping are usually done on egress.
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QoS Implementation Techniques Classification and Marking
Before a packet can have a QoS policy applied to it, the packet has to be classified.
Classification determines the class of traffic to which packets or frames belong. Only after traffic is marked can policies be applied to it.
How a packet is classified depends on the QoS implementation.
• Methods of classifying traffic flows at Layer 2 and 3 include using interfaces, ACLs, and class maps.
• Traffic can also be classified at Layers 4 to 7 using Network Based Application Recognition (NBAR).
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QoS Implementation Techniques Classification and Marking (Cont.)
How traffic is marked usually depends on the technology. The decision of whether to mark traffic at Layers 2 or 3 (or both) is not trivial and should be made after consideration of the following points:
• Layer 2 marking of frames can be performed for non-IP traffic.
• Layer 2 marking of frames is the only QoS option available for switches that are not “IP aware”.
• Layer 3 marking will carry the QoS information end-to-end.
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QoS Implementation Techniques Trust Boundaries
Traffic should be classified and marked as close to its source as technically and
administratively feasible. This defines the trust boundary.
1. Trusted endpoints have the capabilities and intelligence to mark application traffic to the appropriate Layer 2 CoS and/or Layer 3 DSCP values.
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Marking Field
Width in Bits
Ethernet (802.1q, 802.1p)
Class of Service (CoS)
802.11 (Wi-Fi)
Wi-Fi Traffic Identifier (TID)
Experimental (EXP)
IPv4 and IPv6
IP Precedence (IPP)
IPv4 and IPv6
Differentiated Services Code Point (DSCP)
2. Secure endpoints can have traffic marked at the Layer 2 switch.
3. Traffic can also be marked at Layer 3 switches / routers.
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QoS Implementation Techniques Congestion Avoidance
Congestion avoidance tools monitor network traffic loads in an effort to anticipate and avoid congestion at common network and internetwork bottlenecks before congestion becomes a problem.
• They monitor network traffic loads in an effort to anticipate and avoid congestion at common
network and internetwork bottlenecks before congestion becomes a problem.
• They monitor
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