Ch 14: QoS Flashcards
Which of the following are the leading causes of quality of service issues? (Choose all that apply.)
- Bad hardware
- Lack of bandwidth
- Latency and jitter
- Copper cables
- Packet loss
2, 3 and 5.. The leading causes of quality of service issues are lack of bandwidth, latency and jitter, and packet loss.
Network latency can be broken down into which of the following types? (Choose all that apply.)
- Propagation delay (fixed)
- Time delay (variable)
- Serialization delay (fixed)
- Processing delay (fixed)
- Packet delay (fixed)
- Delay variation (variable)
1, 3, 4, and 6.
Network latency can be broken down into propagation delay, serialization delay, processing delay, and delay variation.
Which of the following is not a QoS implementation model?
- IntServ
- Expedited forwarding
- Best effort
- DiffServ
B.
Best effort, IntServ, and DiffServ are the three QoS implementation models.
Which of the following is the QoS implementation model that requires a signaling protocol?
- IntServ
- Best Effort
- DiffServ
- RSVP
A.
IntServ uses Resource Reservation Protocol (RSVP) to reserve resources throughout a network for a specific application and to provide Call Admission Control (CAC) to guarantee that no other IP traffic can use the reserved bandwidth.
Which of the following is the most popular QoS implementation model?
- IntServ
- Best effort
- DiffServ
- RSVP
C.
DiffServ is the most popular and most widely deployed QoS model. It was designed to address the limitations of the best effort and IntServ.
T/F: Traffic classification should always be performed in the core of the network.
Packet classification should take place at the network edge, as close to the source of the traffic as possible, in an effort to provide an end-to-end QoS experience.
The 16-bit TCI field is composed of which fields? (Choose three.)
- Priority Code Point (PCP)
- Canonical Format Identifier (CFI)
- User Priority (PRI)
- Drop Eligible Indicator (DEI)
- VLAN Identifier (VLAN ID)
1, 4, and 5.
The TCI (Tag Control Information) field is a two byte field composed of the 3-bit Priority Code Point (PCP) field (formerly PRI), the 1-bit Drop Eligible Indicator (DEI) field (formerly CFI), and the 12-bit VLAN Identifier (VLAN ID) field. This field is part of 802.1Q.
IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual LANs (VLANs) on an IEEE 802.3 Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames. The standard also contains provisions for a quality-of-service prioritization scheme commonly known as IEEE 802.1p and defines the Generic Attribute Registration Protocol.
Portions of the network which are VLAN-aware (i.e., IEEE 802.1Q conformant) can include VLAN tags. When a frame enters the VLAN-aware portion of the network, a tag is added to represent the VLAN membership.[a] Each frame must be distinguishable as being within exactly one VLAN. A frame in the VLAN-aware portion of the network that does not contain a VLAN tag is assumed to be flowing on the native VLAN.
T/F: The one byte DiffServ (DS or Differentiated Services) field contains a 6-bit Differentiated Services Code Point (DSCP) field that allows for classification of up to 64 values (0 to 63).
True.
The DS field replaces the outdated IPv4 ToS field and the IPV6 traffic class field. They were redefined as an 8-bit Differentiated Services (DiffServ) field.
The DiffServ field is composed of a 6-bit Differentiated Services Code Point (DSCP) field that allows for classification of up to 64 values (0 to 63) and a 2-bit Explicit Congestion Notification (ECN) field.
Which of the following is not a QoS PHB?
- Best Effort (BE)
- Class Selector (CS)
- Default Forwarding (DF)
- Assured Forwarding (AF)
- Expedited Forwarding (EF)
1.
Four PHBs (Per-Hop Behaviors) have been defined and characterized for general use:
- Class Selector (CS) PHB: The first 3 bits of the DSCP field are used as CS bits; the class selector bits make DSCP backward compatible with IP Precedence because IP Precedence uses the same 3 bits to determine class.
- Default Forwarding (DF) PHB: Used for best-effort service.
- Assured Forwarding (AF) PHB: Used for guaranteed bandwidth service.
- Expedited Forwarding (EF) PHB: Used for low-delay service.
Which traffic conditioning tool can be used to drop or mark down traffic that goes beyond a desired traffic rate?
- Policers
- Shapers
- WRR
- None of the above
1.
Policers drop or re-mark incoming or outgoing traffic that goes beyond a desired traffic rate.
What does Tc stand for? (Choose two.)
- Committed time interval
- Token credits
- Bc bucket token count
- Traffic control
1 and 3.
The Committed Time Interval (Tc) is the time interval in milliseconds (ms) over which the Committed Burst (Bc) is sent. Tc can be calculated with the formula Tc = (Bc [bits] / CIR [bps]) × 1000. For single-rate three-color markers/policers (srTCMs) and two-rate three-color markers/policers (trTCMs),
Tc can also refer to the Bc Bucket Token Count (Tc), which is the number of tokens in the Bc bucket.
Which of the following are the recommended congestion management mechanisms for modern rich-media networks? (Choose two.)
- Class-based weighted fair queuing (CBWFQ)
- Priority queuing (PQ) 14
- Weighted RED (WRED)
- Low-latency queuing (LLQ)
1 and 4.
CBWFQ and LLQ provide real-time, delay-sensitive traffic bandwidth and delay guarantees while not starving other types of traffic.
Which of the following is a recommended congestion-avoidance mechanism for modern rich-media networks?
- Weighted RED (WRED)
- Tail drop
- FIFO
- RED
1.
WRED provides congestion avoidance by selectively dropping packets before the queue buffers are full. Packet drops can be manipulated by traffic weights denoted
What are the top three leading causes of quality issues?
- Lack of bandwidth: The available bandwidth on the data path from a source to a destination equals the capacity of the lowest-bandwidth link.
-
Latency and jitter:
- One-way end-to-end delay, also referred to as network latency, is the time it takes for packets to travel across a network from a source to a destination.
- Delay variation, also referred to as jitter, is the difference in the latency between packets in a single flow.
-
Packet loss: Packet loss is usually a result of congestion on an interface. Packet loss can be prevented by implementing one of the following approaches:
- Increase link speed.
- Implement QoS congestion-avoidance and congestion-management mechanism.
- Implement traffic policing to drop low-priority packets and allow high-priority traffic through.
- Implement traffic shaping to delay packets instead of dropping them since traffic may burst and exceed the capacity of an interface buffer. Traffic shaping is not recommended for real-time traffic because it relies on queuing that can cause jitter.
Network latency can be broken down into fixed and variable latency. Give a brief definition of each of the following:
- Propagation delay (fixed)
- Serialization delay (fixed)
- Processing delay (fixed)
- Delay variation (variable)
Propagation delay is the time it takes for a packet to travel from the source to a destination at the speed of light over a medium such as fiber-optic cables or copper wires.
Serialization delay is the time it takes to place all the bits of a packet onto a link. It is a fixed value that depends on the link speed; the higher the link speed, the lower the delay.
Processing delay is the fixed amount of time it takes for a networking device to take the packet from an input interface and place the packet onto the output queue of the output interface.
Delay variation aka jitter, is the difference in the latency between packets in a single flow.
What does Cisco think is an acceptable latency for real time traffic?
200ms.
ITU Recommendation G.114 recommends that, regardless of the application type, a network latency of 400 ms should not be exceeded, and for real-time traffic, network latency should be less than 150 ms.
However, ITU and Cisco have demonstrated that real-time traffic quality does not begin to significantly degrade until network latency exceeds 200 ms.
What is the average refractive index of fiber optic cable?
The speed of light is 299,792,458 meters per second in a vacuum. The lack of vacuum conditions in a fiber-optic cable or a copper wire slows down the speed of light by a ratio known as the refractive index; the larger the refractive index value, the slower light travels.
The average refractive index value of an optical fiber is about 1.5. The speed of light through a medium v is equal to the speed of light in a vacuum c divided by the refractive index n, or v = c / n. This means the speed of light through a fiber-optic cable with a refractive index of 1.5 is approximately 200,000,000 meters per second (that is, 300,000,000 / 1.5).
What is the serialization delay for a 1500-byte packet over a 1Gbps interface?
Serialization delay is the time it takes to place all the bits of a packet onto a link. It is a fixed value that depends on the link speed; the higher the link speed, the lower the delay.
The serialization delay, s, is equal to the packet size in bits divided by the line speed in bits per second.
For example, the serialization delay for a 1500-byte packet over a 1 Gbps interface is 12 μs and can be calculated as follows:
s = packet size in bits / line speed in bps s = (1500 bytes × 8) / 1 Gbps s = 12,000 bits / 1,000,000,000 bps = 0.000012 s × 1000 = .012 ms × 1000 = **12 μs**
The processing delay depends on all of the following factors, except one. Which is incorrect?
- CPU speed (for software-based platforms)
- CPU utilization (load)
- IP packet switching mode (process switching, software CEF, or hardware CEF)
- Bandwidth of the circuit connecting the endpoints
- Router architecture (centralized or distributed)
- Configured features on both input and output interfaces
- Bandwidth is not a factor in processing delay.
Processing delay is the fixed amount of time it takes for a networking device to take the packet from an input interface and place the packet onto the output queue of the output interface. The processing delay depends on factors such as the following:
- CPU speed (for software-based platforms)
- CPU utilization (load)
- IP packet switching mode (process switching, software CEF, or hardware CEF)
- Router architecture (centralized or distributed)
- Configured features on both input and output interfaces
What causes jitter?
Jitter is experienced due to the queueing delay experienced by packets during periods of network congestion. Queuing delay depends on the number and sizes of packets already in the queue, the link speed, and the queuing mechanism. Queuing introduces unequal delays for packets of the same flow, thus producing jitter.
What is a de-jitter buffer?
Voice and video endpoints typically come equipped with de-jitter buffers that can help smooth out changes in packet arrival times due to jitter. A de-jitter buffer is often dynamic and can adjust for approximately 30 ms changes in arrival times of packets. If a packet is not received within the 30 ms window allowed for by the de-jitter buffer, the packet is dropped, and this affects the overall voice or video quality.
What is LLQ and what is it useful for?
To prevent jitter for high-priority real-time traffic, it is recommended to use queuing mechanisms such as low-latency queueing (LLQ) that allow matching packets to be forwarded prior to any other low priority traffic during periods of network congestion.
What is the usual cause of packet loss? What are some solutions to packet loss?
Packet loss is usually a result of congestion on an interface. Packet loss can be prevented by implementing one of the following approaches:
- Increase link speed.
- Implement QoS congestion-avoidance and congestion-management mechanism.
- Implement traffic policing to drop low-priority packets and allow high-priority traffic through.
- Implement traffic shaping to delay packets instead of dropping them since traffic may burst and exceed the capacity of an interface buffer. Traffic shaping is not recommended for real-time traffic because it relies on queuing that can cause jitter.
Give a brief definition of the three different QoS implementation models.
- Best effort
- Integrated Services (IntServ)
- Differentiated Services (DiffServ)
There are three different QoS implementation models:
Best effort: QoS is not enabled for this model. It is used for traffic that does not require any special treatment.
Integrated Services (IntServ): Applications signal the network to make a bandwidth reservation and to indicate that they require special QoS treatment.
Differentiated Services (DiffServ): The network identifies classes that require special QoS treatment.
In this model, applications signal their requirements to the network to reserve the end-to-end resources (such as bandwidth) they require to provide an acceptable user experience.
Which QoS model is this?
The IntServ model was created for real-time applications such as voice and video that require bandwidth, delay, and packet-loss guarantees to ensure both predictable and guaranteed service levels.
What is RSVP in QoS?
IntServ uses Resource Reservation Protocol (RSVP) to reserve resources throughout a network for a specific application and to provide call admission control (CAC) to guarantee that no other IP traffic can use the reserved bandwidth. The bandwidth reserved by an application that is not being used is wasted.
What is the biggest drawback/limitation with the IntServ model of QoS?
The biggest drawback of IntServ is that it cannot scale well on large networks that might have thousands or millions of flows due to the large number of RSVP flows that would need to be maintained.
To be able to provide end-to-end QoS, all nodes, including the endpoints running the applications, need to support, build, and maintain RSVP path state for every single flow.
With this model, there is no need for a signaling protocol, and there is no RSVP flow state to maintain on every single node, which makes it highly scalable; QoS characteristics (such as bandwidth and delay) are managed on a hop-by-hop basis with QoS policies that are defined independently at each device in the network.
Which QoS model is this?
DiffServ is not considered an end-to-end QoS solution because end-to-end QoS guarantees cannot be enforced.
DiffServ divides IP traffic into classes and marks it based on business requirements so that each of the classes can be assigned a different level of service. As IP traffic traverses a network, each of the network devices identifies the packet class by its marking and services the packets according to this class.
Many levels of service can be chosen with DiffServ. For example, IP phone voice traffic is very sensitive to latency and jitter, so it should always be given preferential treatment over all other application traffic. Email, on the other hand, can withstand a great deal of delay and could be given best-effort service, and non-business, non-critical scavenger traffic (such as from YouTube) can either be heavily rate limited or blocked entirely. The DiffServ model is the most popular and most widely deployed QoS model and is covered in detail in this chapter.
What is Packet Classification in QoS?
Packet classification is a QoS mechanism responsible for distinguishing between different traffic streams. It uses traffic descriptors to categorize an IP packet within a specific class.
T/F: Packet classification should take place at the network edge, as close to the destination of the traffic as possible.
False.
Packet classification should take place at the network edge, as close to the source of the traffic as possible.
Once an IP packet is classified, packets can then be marked/re-marked, queued, policed, shaped, or any combination of these and other actions.
What is NBAR2?
This is considered Layer 7 classification.
NBAR2 is a deep packet inspection engine that can classify and identify a wide variety of protocols and applications using Layer 3 to Layer 7 data, including difficult-to-classify applications that dynamically assign Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) port numbers.
NBAR2 can recognize more than 1000 applications, and monthly protocol packs are provided for recognition of new and emerging applications, without requiring an IOS upgrade or router reload.
NBAR2 has two modes of operation. Give a brief description of them.
- Protocol Discovery
- Modular QoS CLI (MQC)
NBAR2 has two modes of operation:
- Protocol Discovery: Protocol Discovery enables NBAR2 to discover and get real-time statistics on applications currently running in the network. These statistics from the Protocol Discovery mode can be used to define QoS classes and policies using MQC configuration.
- Modular QoS CLI (MQC): Using MQC, network traffic matching a specific network protocol such as Cisco Webex can be placed into one traffic class, while traffic that matches a different network protocol such as YouTube can be placed into another traffic class. After traffic has been classified in this way, different QoS policies can be applied to the different classes of traffic.
Diagram a L2 ethernet frame with 802.1Q marking.
The 802.1Q standard is an IEEE specification for implementing VLANs in Layer 2 switched networks.
The 802.1Q specification defines two 2-byte fields, which are inserted within an Ethernet frame following the Source Address field, as illustrated in Figure 14-2.
- Tag Protocol Identifier (TPID)
- Tag Control Information (TCI)
What is the PCP field in an ethernet header?
It is part of the TCI, Tag Control Information.
The specifications of the 3-bit PCP field are defined by the IEEE 802.1p specification. This field is used to mark packets as belonging to a specific CoS. The CoS marking allows a Layer 2 Ethernet frame to be marked with eight different levels of priority values, 0 to 7, where 0 is the lowest priority and 7 is the highest. Table 14-2 includes the IEEE 802.1p specification standard definition for each CoS.
What is a ToS field in an IPv4 header? How many bits is it? How many are used? and for what?
The ToS field is an 8-bit field where only the first 3 bits of the ToS field, referred to as IP Precedence (IPP), are used for marking, and the rest of the bits are unused.
IPP values, which range from 0 to 7, allow the traffic to be partitioned in up to six usable classes of service; IPP 6 and 7 are reserved for internal network use.
Figure 14-3 illustrates the ToS/DiffServ field within an IPv4 header.
T/F: DiffServ made the ToS field obsolete.
True.
RFC 2474 made the ToS field obsolete by introducing the DiffServ field, and the Class Selector (CS) PHB was defined to provide backward compatibility for DSCP with IP Precedence. Fun facts:
- Packets with higher IP Precedence should be forwarded in less time than packets with lower IP Precedence.
- The last 3 bits of the DSCP (bits 2 to 4), when set to 0, identify a Class Selector PHB, but the Class Selector bits 5 to 7 are the ones where IP Precedence is set. Bits 2 to 4 are ignored by non-DiffServ-compliant devices performing classification based on IP Precedence.
- There are eight CS classes, ranging from CS0 to CS7, that correspond directly with the eight IP Precedence values.
Figure 14-4 illustrates the CS PHB.
What is the DF PHB?
Default Forwarding (DF) and Class Selector 0 (CS0) provide best-effort behavior and use the DS value 000000.
Default best-effort forwarding is also applied to packets that cannot be classified by a QoS mechanism such as queueing, shaping, or policing. This usually happens when a QoS policy on the node is incomplete or when DSCP values are outside the ones that have been defined for the CS, AF, and EF PHBs.
Figure 14-5 illustrates the DF PHB (Per-hop Behavior).
What is the role of the AF PHB?
The AF PHB (Assured Forwarding Per-Hop Behavior) guarantees a minimum amount of bandwidth to an AF class and allows access to extra bandwidth, if available.
Packets requiring AF PHB should be marked with DSCP value aaadd0, where aaa is the binary value of the AF class (bits 5 to 7), and dd (bits 2 to 4) is the drop probability where bit 2 is unused and always set to 0.
Note: AF uses WRED to drop packets preemptively.
Figure 14-6 illustrates the AF PHB.
T/F: When you create a new WLAN, its QoS policy defaults to Silver, or best-effort handling.
True.
What is the difference between a traffic policer and a shaper?
Both are traffic-conditioning QoS mechanisms used to classify traffic and enforce other QoS mechanisms such as rate limiting. They classify traffic in an identical manner but differ in their implementation:
- Policers: Drop or re-mark incoming or outgoing traffic that goes beyond a desired traffic rate.
- Shapers: Buffer and delay egress traffic rates that momentarily peak above the desired rate until the egress traffic rate drops below the defined traffic rate. If the egress traffic rate is below the desired rate, the traffic is sent immediately.
Figure 14-9 illustrates the difference between traffic policing and shaping. Policers drop or re-mark excess traffic, while shapers buffer and delay excess traffic.
Figure 14-12 illustrates different actions that the ______________ policer can take.
Figure 14-12 illustrates different actions that the single-rate two-color policer can take.
The section above the dotted line on the left side of the figure represents traffic that exceeded the CIR and was marked down. The section above the dotted line on the right side of the figure represents traffic that exceeded the CIR and was dropped.
The first policers implemented use a single-rate, two-color model based on the single token bucket algorithm. For this type of policer, traffic can be either conforming to or exceeding the CIR. Marking down or dropping actions can be performed for each of the two states.
Figure 14-13 illustrates different actions that a ________________ policer can take.
Single-rate three-color policer algorithms are based on RFC 2697. This type of policer uses two token buckets, and the traffic can be classified as either conforming to, exceeding, or violating the CIR. Marking down or dropping actions are performed for each of the three states of traffic.
The exceeding and violating traffic rates vary because they rely on random tokens spilling over from the Bc bucket into the Be. The section right above the straight dotted line on the right side of the figure represents traffic that exceeded the CIR and was marked down and the top section represents traffic that violated the CIR and was dropped.
The first token bucket operates very similarly to the single-rate two-color system; the difference is that if there are any tokens left over in the bucket after each time period due to low or no activity, instead of discarding the excess tokens (overflow), the algorithm places them in a second bucket to be used later for temporary bursts that might exceed the CIR. Tokens placed in this second bucket are referred to as the excess burst (Be), and Be is the maximum number of bits that can exceed the Bc burst size.
The ___________________ marker/policer is based on RFC 2698 and is similar to the single-rate three-color policer.
The difference is that ____________________ policers rely on excess tokens from the Bc bucket, which introduces a certain level of variability and unpredictability in traffic flows; the _____________________ marker/policers address this issue by using two distinct rates, the CIR and the Peak Information Rate (PIR).
The two-rate three-color marker/policer is based on RFC 2698 and is similar to the single-rate three-color policer.
The difference is that single-rate three-color policers rely on excess tokens from the Bc bucket, which introduces a certain level of variability and unpredictability in traffic flows; the two-rate three-color marker/policers address this issue by using two distinct rates, the CIR and the Peak Information Rate (PIR).
T/F: LLQ allows for two different traffic classes to be assigned to it so that different policing rates can be applied to different types of high-priority traffic.
True.
For example, voice traffic could be policed during times of congestion to 10 Mbps, while video could be policed to 100 Mbps. This would not be possible with only one traffic class and a single policer.
