Friday, October 8, 2021 11:48 AM
Quality of Experience (QoE) -users’ perception of their use of the application on the
network.
Voice and Video Applications
Aflowisall the data moving from one application to another over the network, with one
flow for each direction
For example, if you open a website and connect to a web server, the web page content that moves from the server to the client is one flow.
From a voice perspective, a phone call between two IP phones would create a flow for each direction
Step 1. The phone user makes a phone call and begins speaking.
Step 2. A chip called a codec processes (digitizes) the sound to create a binary code (160
bytes with the G.711 codec, for example) for a certain time period (usually 20 ms).
Step 3. The phone places the data into an IP packet.
Step 4. The phone sends the packet to the destination IP phone.
guidelines for interactive voice:
Delay (one-way): 150 ms or less
Jitter: 30 ms or less
Loss: 1% or less
requirements for video
Bandwidth: 384 Kbps to 20+ Mbps
Delay (one-way): 200–400 ms
Jitter: 30–50 ms
Loss: 0.1%–1%
QoS as Mentioned in This Book
“Classification and Marking” is about the marking of packets and the definition of trust
boundaries.
“Queuing” describes the scheduling of packets to give one type of packet priority over another.
“Shaping and Policing” explains these two tools together because they are often used on opposite ends of a link.
“devices get too busy.Congestion Avoidance” addresses how to manage the packet loss that occurs when network
Classification and Marking
a type of QoS tool that by changing some bits in specific header fieldsclassifies packets based on their header contents then marks the message
Classification Basics
QoS tools sit in the path that packets take when being forwarded through a router or switch,
much like the positioning of ACLs. Like ACLs, ACLs, QoS tools are enabled for a direction: packets entering the interface (before the QoS tools are enabled on an interface. Also like
forwarding decision) or for messages exiting the interface (after the forwarding decision).
classification refers to the process of matching the fields in a message to make a choice to
take some QoS action
QoS tools perform classification (matching of header fields) to decide which packets to take certain QoS actions against.Those actions include the other types of QoS tools discussed in
this chapter, such as queuing, shaping, policing, and so on.
Matching (Classification) Basics
Cisco and by RFCs, suggests doing complex matching early in the life of a packet and then
marking the packet. Marking means that the QoS tool changes one or more header fields, setting a value in the header.
Differentiated Services Code Point (DSCP)
a 6-bit field in the IP header meant for QoS marking
Classification on Routers with ACLs and NBAR
many QoS tools support the ability to simply refer to an IP ACL,with this kind of logic:
For any packet matched by the ACL with a permit action, consider that packet a match for
QoS, so do a particular QoS action.
All these fields are matchable for QoS classification.
not every classification can be easily made by matching with an ACL. In more challenging cases, Cisco Network Based Application Recognition (NBAR)can be used. NBAR is basically in
its second major version, called NBAR2, or nextpackets for classification in a large variety of ways that are very useful for QoS.-generation NBAR. In short,NBAR2 matches
NBAR2 looks far beyond what an ACL can examine in a message
Cisco also organizes what NBAR can match in ways that make it easy to separate the traffic into different classes.
you might classify WebEx traffic and give it a unique DSCP marking. NBAR provides
easy builtof applications.-in matching ability for WebEx, plus more than 1000 different subcategories
NBAR refers to this idea of defining the characteristics of different applications as application signatures.)
application signatures.)
Marking IP DSCP and Ethernet CoS
example plan
Classify all voice payload traffic that is used for business purposes as IP DSCP EF and
CoS 5.
Classify all video conferencing and other interactive video for business purposes as IP DSCP AF41 and CoS 4.
Classify all business-critical data application traffic as IP DSCP AF21 and CoS 2
Marking the IP Header
Marking a QoS field in the IP header works well with QoS because the IP header exists for the entire trip from the source host to the destination host.
IPv4 defines a Type of Service (ToS) byte in the IPv4 headerThe original RFC defined a 3-bit IP Precedence (IPP) field for QoS marking. That field , as shown in Figure 11-6.
gave us eight separate valueswhen converted to decimal are decimals 0 through 7.—binary 000, 001, 010, and so on, through 111—which
Those last 5 bits of the ToS byte per RFC 791 were mostly defined for some purpose but were not used in practice to any significant extent.
DSCP increased the number of marking bits to 6 bits, allowing for64 unique values
that can be marked. The DiffServ RFCs, which became RFCs back in the late 1990s, have become accepted as the most common method to use when doing QoS, and
using the DSCP field for marking has become quite common.
IPv6 has a similar field to mark as well.The 6-bit field also goes by the name DSCP,
with the byte in the IPv6 header being called the IPv6 Traffic Class byte.
Marking the Ethernet 802.1Q Header
marking field exists in the 802.1Q header
It goes bytwo different names:Class of Service, orCoS, and Priority Code Point, or
PCP.
sits in the third byte of the 4possible valuesto mark (see Figure 11-byte 802.1Q header, as a 3-7). It goes by two different names: Class of -bit field,supplying eight
Service, or CoS, and Priority Code Point, or PCP.
802.1Q header is not included in all Ethernet frames
only exists when 802.1Q trunking is used on a link
only for QoS features enabled on interfaces that use trunking, as shown
Other Marking Fields
Defining Trust Boundaries
most voice traffic is marked with a DSCP called Expediteddecimal value of 46 Forwarding (EF), which has a
trust boundary
the point in the path of a packet flowing through the network at which the networking devices can trust the current QoS markings
when the access layer includes an IP Phone, the phone is typically the trust boundary
IP Phones can set the CoS and DSCP fields of the messages created by the phone, as
well as those forwarded from the PC through the phone. are actually configured on the attached access switch The specific marking values
DiffServ Suggested Marking Values
DiffServ architecture as defined originally by RFC 2475
three sets of DSCP values as used in DiffServ.
Expedited Forwarding (EF)
Expedited Forwarding (EF) DSCP value—a single value
use for packets that need low latency (delay), low jitter, and low loss.
decimal 46
Most often QoS plans use EF to mark voice payloadpackets. With voice calls, some
packets carry voice payload, and other packets carry call signaling messages.signaling messages set up (create) the voice call between two devices, and they do not Call
require low delay, jitter, and loss.
By default, Cisco IP Phones mark voice pay-load with EF,and mark voice signaling
packets sent by the phone with another value called CS3.
Assured Forwarding (AF)
Thmeant to be used in concert with each othere Assured Forwarding (AF) DiffServ RFC (2597) defines a set of 12 DSCP values
defines the defines three levels of drop priority within each queue for use with congestion concept of four separate queues in a queuing system. Additionally, it
avoidance tools.need 12 different DSCP markings, one for each combination of queue and drop With four queues, and three drop priority classes per queue, you
priority. (Queuing and congestion avoidance mechanisms are discussed later in this
chapter.)
and Y referring to the drop priority (1 through 3).The text names follow a format of AFXY, with X referring to the queue (1 through 4)
Class Selector (CS)
eight DSCP values for backward compatibility with IPP values.
The DSCP values have the last 3 bits, as shown on the left side of the figure. CSx represents the text names, same first 3 bits as the IPP field, and with binary 0s for the
where x is the matching IPP value (0 through 7).
Guidelines for DSCP Marking Values
how all devices should mark data
DSCP EF: Voice payload
AF4x: Interactive video (for example, videoconferencing)
AF3x: Streaming video
AF2x: High priority (low latency) data
CS0: Standard data
Queuing
interfacethe QoS toolset for managing the queues that hold packets while they wait their turn to exit an
To use multiple queues, the queuing system needs a classifierfunction to choose which packets
are placed into which queue
The queuing system needs a interface becomes availablescheduleras well, to decide which message to take next when the
Prioritizationrefers to the concept of giving priority to one queue over another
Round-Robin Scheduling (Prioritization) (Two Types)
weighting
the more preference to one queue over another.scheduler takes a different number of packets (or bytes) from each queue, giving
Class-Based Weighted Fair Queuing (CBWFQ)
Each class receives at least the amount of bandwidth configured during times of congestion, but maybe more
Low Latency Queuing
tells the scheduler to treat one or more queues as special priority queues
if thethe voice queue? The scheduler never services the other queues (calledspeed of the interface is X bits per second, but more than X bits per second come into queue starvation).
limit the amount of traffic placed into the priority queue, using a feature called policing
a cap on the bandwidth used by the priority queue
Call Admission Control (CAC) tools
Find a way to this link, so that the policer never discards any of the trafficlimit the amount of voice and video that the network routes out
CAC tools did not get a mention in the exam topics, so this chapter leaves those
tools at a brief mention
A Prioritization Strategy for Data, Voice, and Video
approach queuing in their QoS plans:
Key Topic.
Use a round-robin queuing method like CBWFQ for data classes and for noninteractive voice
and video.
If faced with too little bandwidth compared to the typical amount of traffic, give data classes that support business-critical applications much more guaranteed bandwidth than is given
to less important data classes.
Use a priority queue with LLQ scheduling for interactive voice and video, to achieve low
delay, jitter, and loss.
Put voice in a separate queue from video so that the policing function applies separately to each.
Define enough bandwidth for each priority queue so that the builtdiscard any messages from the priority queues. -in policer should not
Use Call Admission Control (CAC) tools to avoid adding too much voice or video to the
network, which would trigger the policer function.
Shaping and Policing #
not found in as many locations in a typical enterprise
often used at the WAN edge in an enterprise network design.
monitor the bit rate of the combined messages that flow through a device.
notes each packet that passes and measures the number of bits per second over time
attempt to keep the bit rate at or below the configured speed
policersdiscard packets,and shapershold packets in queues to delay the packets.
Does this next packet push the measured rate past the configured shaping rate or policing rate?
If no:
Let the packet keep moving through the normal path and do nothing extra to the packet.
If yes:
If shaping, delay the message by queuing it.
If policing, either discard the message or mark it differently.
policing, which discards or rewhich slows down messages that exceed the shaping rate.-marks messages that exceed the policing rate, followed by shaping,
Policing #
Policers activity.allow for a burst beyond the policing rate for a short time, after a period of low
Where to Use Policing
Policing makes sense only in certain cases, and as a general tool, it can be best used at
the edge between two networks.
a 200-Mbps committed information rate (CIR).
the Mbps of traffic in each directionSP providing the WAN service agrees to allow the enterprise to send 200. However, remember that the enterprise
routers transmit the data at the speed of the access link, or 1 Gbps in this case.
Thethat the customer chooses for that link.SP can police incoming packets, setting the policing rate to match the CIR
Policers can also re-mark packets.
SP could mark the messages with a new marking value, with this strategy:
Renetwork.-mark packets that exceed the policing rate, but let them into the SP’s
If other SP network devices are experiencing congestion when they process the packet, the different marking means that device can discard the packet.
However...
...if no other SP network devices are experiencing congestion when forwarding that re-marked packet, it gets through the SP network anyway.
that re-marked packet, it gets through the SP network anyway.
SP canstill protecting the SP’s network during times of stress.treat their customers a little better by discarding less traffic, while
key features of policing:
Key Topic.
It rate.measures the traffic rate over time for comparison to the configured policing
It allows for a burst of data after a period of inactivity.
It is enabled on an interface, in either direction, but typically at ingress.
It cancandidate for more aggressive discard later in its journey.discard excess messages but can also re-mark the message so that it is a
Use a shaper to slow down the traffic
shaping before sending data to an SP that is policing—is one of the typical uses of a shaper
slows messages down by queuing the messages.
then services the shaping queues, but not based on when the physical interface is available
schedules messages from the shaping queues based on the shaping rate
the shaper queues packets so that the sending rate through the shaper does not exceed the shaping rate; and then output queuing works as normal, if needed.
shaping queuesapply the round-robin and priority queuing features of CBWFQ and LLQ, respectively, to the
Setting a Good Shaping Time Interval for Voice and Video #
you can (and should) configure a shaper’s setting that changes the internal operation of the shaper, which then reduces the delay and jitter caused to voice and video
traffic.
Shaping #
traffic.
A shaper’s time interval refers to its internal logic and how a shaper averages, over time, sending at a particular rate
Because the interface transmits bits at 1 Gbps, it takes just .2 seconds, or 200 ms, to send all 200 million bits. Then the shaper must wait for the rest of the
time interval, another 800 ms, before beginning the next time interval.
configure a short time interval so there is less wait time
Tc = 1 second (1000 ms): Send at 1 Gbps for 200 ms, rest for 800 ms
Tc = .1 second (100 ms): Send at 1 Gbps for 20 ms, rest for 80 ms
Tc = .01 second (10 ms): Send at 1 Gbps for 2 ms, rest for 8 ms
When shaping, use a short time interval. By recommendation, use a interval to support voice and video. 10 - ms time
key features of shapers:
Key Topic.
Shapers measure the traffic rate over time for comparison to the configured shaping rate.
Shapers allow for bursting after a period of inactivity.
Shapers areenabled on an interface for egress (outgoing packets).
Shapers slow down packets by queuing them and over time releasing them from the queue at the shaping rate.
Shapers use queuing tools to create and schedule the shaping queues, which is very
important for the same reasons discussed for output queuing.
Congestion Avoidance #
attempts to reduce overall packet loss by preemptively discarding some packets used in TCP connections
connections
TCP Windowing Basics
tail drop
one queue being totally full. Any new packets that arrive for that queue right now will
be dropped because there is no room at the tail of the queue (tail drop).
Congestion Avoidance Tools
discard some TCP segments before the queues fill, hoping that enough TCP connections will
slow down, reducing congestion, and avoiding tail drop
monitor the average queue depth over time, triggering more severe actions the deeper the
queue
the queue depth passes the maximum thresholdcalled full drop. ,the tool drops all packets, in an action
congestion avoidance tools can classify messages to treat some packets better than others
3.0 IP Connectivity
3.5 Describe the purpose of First Hop Redundancy Protocol
4.0 Infrastructure Services
4.4 Explain the function of SNMP in network operations
4.9 Describe the capabilities and function of TFTP/FTP in the network
First Hop Redundancy Protocol #
11 QoS #
Friday, October 8, 2021 11:48 AM
Quality of Experience (QoE) -users’ perception of their use of the application on the
network.
Voice and Video Applications
Aflowisall the data moving from one application to another over the network, with one
flow for each direction
For example, if you open a website and connect to a web server, the web page content that moves from the server to the client is one flow.
From a voice perspective, a phone call between two IP phones would create a flow for each direction
Step 1. The phone user makes a phone call and begins speaking.
Step 2. A chip called a codec processes (digitizes) the sound to create a binary code (160
bytes with the G.711 codec, for example) for a certain time period (usually 20 ms).
Step 3. The phone places the data into an IP packet.
Step 4. The phone sends the packet to the destination IP phone.
guidelines for interactive voice:
Delay (one-way): 150 ms or less
Jitter: 30 ms or less
Loss: 1% or less
requirements for video
Bandwidth: 384 Kbps to 20+ Mbps
Delay (one-way): 200–400 ms
Jitter: 30–50 ms
Loss: 0.1%–1%
QoS as Mentioned in This Book
“Classification and Marking” is about the marking of packets and the definition of trust
boundaries.
“Queuing” describes the scheduling of packets to give one type of packet priority over another.
“Shaping and Policing” explains these two tools together because they are often used on opposite ends of a link.
“devices get too busy.Congestion Avoidance” addresses how to manage the packet loss that occurs when network
Classification and Marking
a type of QoS tool that by changing some bits in specific header fieldsclassifies packets based on their header contents then marks the message
Classification Basics
QoS tools sit in the path that packets take when being forwarded through a router or switch,
much like the positioning of ACLs. Like ACLs, ACLs, QoS tools are enabled for a direction: packets entering the interface (before the QoS tools are enabled on an interface. Also like
forwarding decision) or for messages exiting the interface (after the forwarding decision).
classification refers to the process of matching the fields in a message to make a choice to
take some QoS action
QoS tools perform classification (matching of header fields) to decide which packets to take certain QoS actions against.Those actions include the other types of QoS tools discussed in
this chapter, such as queuing, shaping, policing, and so on.
Matching (Classification) Basics
Cisco and by RFCs, suggests doing complex matching early in the life of a packet and then
marking the packet. Marking means that the QoS tool changes one or more header fields, setting a value in the header.
Differentiated Services Code Point (DSCP)
a 6-bit field in the IP header meant for QoS marking
Classification on Routers with ACLs and NBAR
many QoS tools support the ability to simply refer to an IP ACL,with this kind of logic:
For any packet matched by the ACL with a permit action, consider that packet a match for
QoS, so do a particular QoS action.
All these fields are matchable for QoS classification.
not every classification can be easily made by matching with an ACL. In more challenging cases, Cisco Network Based Application Recognition (NBAR)can be used. NBAR is basically in
its second major version, called NBAR2, or nextpackets for classification in a large variety of ways that are very useful for QoS.-generation NBAR. In short,NBAR2 matches
NBAR2 looks far beyond what an ACL can examine in a message
Cisco also organizes what NBAR can match in ways that make it easy to separate the traffic into different classes.
you might classify WebEx traffic and give it a unique DSCP marking. NBAR provides
easy builtof applications.-in matching ability for WebEx, plus more than 1000 different subcategories
NBAR refers to this idea of defining the characteristics of different applications as application signatures.)
application signatures.)
Marking IP DSCP and Ethernet CoS
example plan
Classify all voice payload traffic that is used for business purposes as IP DSCP EF and
CoS 5.
Classify all video conferencing and other interactive video for business purposes as IP DSCP AF41 and CoS 4.
Classify all business-critical data application traffic as IP DSCP AF21 and CoS 2
Marking the IP Header
Marking a QoS field in the IP header works well with QoS because the IP header exists for the entire trip from the source host to the destination host.
IPv4 defines a Type of Service (ToS) byte in the IPv4 headerThe original RFC defined a 3-bit IP Precedence (IPP) field for QoS marking. That field , as shown in Figure 11-6.
gave us eight separate valueswhen converted to decimal are decimals 0 through 7.—binary 000, 001, 010, and so on, through 111—which
Those last 5 bits of the ToS byte per RFC 791 were mostly defined for some purpose but were not used in practice to any significant extent.
DSCP increased the number of marking bits to 6 bits, allowing for64 unique values
that can be marked. The DiffServ RFCs, which became RFCs back in the late 1990s, have become accepted as the most common method to use when doing QoS, and
using the DSCP field for marking has become quite common.
IPv6 has a similar field to mark as well.The 6-bit field also goes by the name DSCP,
with the byte in the IPv6 header being called the IPv6 Traffic Class byte.
Marking the Ethernet 802.1Q Header
marking field exists in the 802.1Q header
It goes bytwo different names:Class of Service, orCoS, and Priority Code Point, or
PCP.
sits in the third byte of the 4possible valuesto mark (see Figure 11-byte 802.1Q header, as a 3-7). It goes by two different names: Class of -bit field,supplying eight
Service, or CoS, and Priority Code Point, or PCP.
802.1Q header is not included in all Ethernet frames
only exists when 802.1Q trunking is used on a link
only for QoS features enabled on interfaces that use trunking, as shown
Other Marking Fields
Defining Trust Boundaries
most voice traffic is marked with a DSCP called Expediteddecimal value of 46 Forwarding (EF), which has a
trust boundary
the point in the path of a packet flowing through the network at which the networking devices can trust the current QoS markings
when the access layer includes an IP Phone, the phone is typically the trust boundary
IP Phones can set the CoS and DSCP fields of the messages created by the phone, as
well as those forwarded from the PC through the phone. are actually configured on the attached access switch The specific marking values
DiffServ Suggested Marking Values
DiffServ architecture as defined originally by RFC 2475
three sets of DSCP values as used in DiffServ.
Expedited Forwarding (EF)
Expedited Forwarding (EF) DSCP value—a single value
use for packets that need low latency (delay), low jitter, and low loss.
decimal 46
Most often QoS plans use EF to mark voice payloadpackets. With voice calls, some
packets carry voice payload, and other packets carry call signaling messages.signaling messages set up (create) the voice call between two devices, and they do not Call
require low delay, jitter, and loss.
By default, Cisco IP Phones mark voice pay-load with EF,and mark voice signaling
packets sent by the phone with another value called CS3.
Assured Forwarding (AF)
Thmeant to be used in concert with each othere Assured Forwarding (AF) DiffServ RFC (2597) defines a set of 12 DSCP values
defines the defines three levels of drop priority within each queue for use with congestion concept of four separate queues in a queuing system. Additionally, it
avoidance tools.need 12 different DSCP markings, one for each combination of queue and drop With four queues, and three drop priority classes per queue, you
priority. (Queuing and congestion avoidance mechanisms are discussed later in this
chapter.)
and Y referring to the drop priority (1 through 3).The text names follow a format of AFXY, with X referring to the queue (1 through 4)
Class Selector (CS)
eight DSCP values for backward compatibility with IPP values.
The DSCP values have the last 3 bits, as shown on the left side of the figure. CSx represents the text names, same first 3 bits as the IPP field, and with binary 0s for the
where x is the matching IPP value (0 through 7).
Guidelines for DSCP Marking Values
how all devices should mark data
DSCP EF: Voice payload
AF4x: Interactive video (for example, videoconferencing)
AF3x: Streaming video
AF2x: High priority (low latency) data
CS0: Standard data
Queuing
interfacethe QoS toolset for managing the queues that hold packets while they wait their turn to exit an
To use multiple queues, the queuing system needs a classifierfunction to choose which packets
are placed into which queue
The queuing system needs a interface becomes availablescheduleras well, to decide which message to take next when the
Prioritizationrefers to the concept of giving priority to one queue over another
Round-Robin Scheduling (Prioritization) (Two Types)
weighting
the more preference to one queue over another.scheduler takes a different number of packets (or bytes) from each queue, giving
Class-Based Weighted Fair Queuing (CBWFQ)
Each class receives at least the amount of bandwidth configured during times of congestion, but maybe more
Low Latency Queuing
tells the scheduler to treat one or more queues as special priority queues
if thethe voice queue? The scheduler never services the other queues (calledspeed of the interface is X bits per second, but more than X bits per second come into queue starvation).
limit the amount of traffic placed into the priority queue, using a feature called policing
a cap on the bandwidth used by the priority queue
Call Admission Control (CAC) tools
Find a way to this link, so that the policer never discards any of the trafficlimit the amount of voice and video that the network routes out
CAC tools did not get a mention in the exam topics, so this chapter leaves those
tools at a brief mention
A Prioritization Strategy for Data, Voice, and Video
approach queuing in their QoS plans:
Key Topic.
Use a round-robin queuing method like CBWFQ for data classes and for noninteractive voice
and video.
If faced with too little bandwidth compared to the typical amount of traffic, give data classes that support business-critical applications much more guaranteed bandwidth than is given
to less important data classes.
Use a priority queue with LLQ scheduling for interactive voice and video, to achieve low
delay, jitter, and loss.
Put voice in a separate queue from video so that the policing function applies separately to each.
Define enough bandwidth for each priority queue so that the builtdiscard any messages from the priority queues. -in policer should not
Use Call Admission Control (CAC) tools to avoid adding too much voice or video to the
network, which would trigger the policer function.
Shaping and Policing #
not found in as many locations in a typical enterprise
often used at the WAN edge in an enterprise network design.
monitor the bit rate of the combined messages that flow through a device.
notes each packet that passes and measures the number of bits per second over time
attempt to keep the bit rate at or below the configured speed
policersdiscard packets,and shapershold packets in queues to delay the packets.
Does this next packet push the measured rate past the configured shaping rate or policing rate?
If no:
Let the packet keep moving through the normal path and do nothing extra to the packet.
If yes:
If shaping, delay the message by queuing it.
If policing, either discard the message or mark it differently.
policing, which discards or rewhich slows down messages that exceed the shaping rate.-marks messages that exceed the policing rate, followed by shaping,
Policing #
Policers activity.allow for a burst beyond the policing rate for a short time, after a period of low
Where to Use Policing
Policing makes sense only in certain cases, and as a general tool, it can be best used at
the edge between two networks.
a 200-Mbps committed information rate (CIR).
the Mbps of traffic in each directionSP providing the WAN service agrees to allow the enterprise to send 200. However, remember that the enterprise
routers transmit the data at the speed of the access link, or 1 Gbps in this case.
Thethat the customer chooses for that link.SP can police incoming packets, setting the policing rate to match the CIR
Policers can also re-mark packets.
SP could mark the messages with a new marking value, with this strategy:
Renetwork.-mark packets that exceed the policing rate, but let them into the SP’s
If other SP network devices are experiencing congestion when they process the packet, the different marking means that device can discard the packet.
However...
...if no other SP network devices are experiencing congestion when forwarding that re-marked packet, it gets through the SP network anyway.
that re-marked packet, it gets through the SP network anyway.
SP canstill protecting the SP’s network during times of stress.treat their customers a little better by discarding less traffic, while
key features of policing:
Key Topic.
It rate.measures the traffic rate over time for comparison to the configured policing
It allows for a burst of data after a period of inactivity.
It is enabled on an interface, in either direction, but typically at ingress.
It cancandidate for more aggressive discard later in its journey.discard excess messages but can also re-mark the message so that it is a
Use a shaper to slow down the traffic
shaping before sending data to an SP that is policing—is one of the typical uses of a shaper
slows messages down by queuing the messages.
then services the shaping queues, but not based on when the physical interface is available
schedules messages from the shaping queues based on the shaping rate
the shaper queues packets so that the sending rate through the shaper does not exceed the shaping rate; and then output queuing works as normal, if needed.
shaping queuesapply the round-robin and priority queuing features of CBWFQ and LLQ, respectively, to the
Setting a Good Shaping Time Interval for Voice and Video #
you can (and should) configure a shaper’s setting that changes the internal operation of the shaper, which then reduces the delay and jitter caused to voice and video
traffic.
Shaping #
traffic.
A shaper’s time interval refers to its internal logic and how a shaper averages, over time, sending at a particular rate
Because the interface transmits bits at 1 Gbps, it takes just .2 seconds, or 200 ms, to send all 200 million bits. Then the shaper must wait for the rest of the
time interval, another 800 ms, before beginning the next time interval.
configure a short time interval so there is less wait time
Tc = 1 second (1000 ms): Send at 1 Gbps for 200 ms, rest for 800 ms
Tc = .1 second (100 ms): Send at 1 Gbps for 20 ms, rest for 80 ms
Tc = .01 second (10 ms): Send at 1 Gbps for 2 ms, rest for 8 ms
When shaping, use a short time interval. By recommendation, use a interval to support voice and video. 10 - ms time
key features of shapers:
Key Topic.
Shapers measure the traffic rate over time for comparison to the configured shaping rate.
Shapers allow for bursting after a period of inactivity.
Shapers areenabled on an interface for egress (outgoing packets).
Shapers slow down packets by queuing them and over time releasing them from the queue at the shaping rate.
Shapers use queuing tools to create and schedule the shaping queues, which is very
important for the same reasons discussed for output queuing.
Congestion Avoidance #
attempts to reduce overall packet loss by preemptively discarding some packets used in TCP connections
connections
TCP Windowing Basics
tail drop
one queue being totally full. Any new packets that arrive for that queue right now will
be dropped because there is no room at the tail of the queue (tail drop).
Congestion Avoidance Tools
discard some TCP segments before the queues fill, hoping that enough TCP connections will
slow down, reducing congestion, and avoiding tail drop
monitor the average queue depth over time, triggering more severe actions the deeper the
queue
the queue depth passes the maximum thresholdcalled full drop. ,the tool drops all packets, in an action
congestion avoidance tools can classify messages to treat some packets better than others
3.0 IP Connectivity
3.5 Describe the purpose of First Hop Redundancy Protocol
4.0 Infrastructure Services
4.4 Explain the function of SNMP in network operations
4.9 Describe the capabilities and function of TFTP/FTP in the network