Table Of Contents

Quality of Service

Creating a QoS Policy

Create a QoS Policy Reference

Create QoS Policy

QoS Wizard

Interface Selection  

Queuing for Outbound Traffic

Add a New Traffic Class

Policing for Outbound Traffic

QoS Policy Generation

QoS Configuration Summary

Editing QoS Policies

Edit QoS Policy Reference

Edit QoS Policy

Add Class for the New Policy

Add Service Policy to Class

Associate or Disassociate the QoS Policy

Add or Edit a QoS Class

Edit Match DSCP Values

Edit Match Protocol Values

Add Custom Protocols  

Edit Match ACL

Configure Policing

Configure Shaping

Configure Queuing

Quality of Service

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The Quality of Service (QoS) Wizard allows a network administrator to enable Quality of Service (QoS) on the router's WAN interfaces. QoS can also be enabled on IPSec VPN interfaces and tunnels. The QoS edit windows enables the administrator to edit policies created using the wizard.

Creating a QoS Policy

Complete these steps to create a QoS policy:

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Step 1 If you want to review the Cisco IOS CLI commands that you send to the router when you complete the configuration, go to the Cisco SDM toolbar, and click Edit > Preferences > Preview commands before delivering to router. The preview screen allows you to cancel the configuration if you want to.

Step 2 In the Cisco SDM toolbar, click Configure.

Step 3 In the Cisco SDM taskbar, click QoS.

Step 4 In the Create QoS Policy tab, click Launch QoS Wizard.

Step 5 Make configuration settings in the wizard screens. Click Next to go from the current screen to the next screen. Click Back to return to a screen you have previously visited.

Step 6 Cisco SDM displays the Summary screen when you have completed the configuration. Review the configuration. If you need to make changes, click Back to return to the screen in which you need to make changes, then return to the Summary screen.

Step 7 To send the configuration to the router, click Finish.

Step 8 If you checked Preview commands before delivering to router in the Edit Preferences screen, the Cisco IOS CLI commands that you are sending are displayed. Click Deliver to send the configuration to the router, or click Cancel to discard it. If you did not make this setting, clicking Finish sends the configuration to the router.

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Create a QoS Policy Reference describes the configuration screens.

Create a QoS Policy Reference

The topics in this section describe the configuration screens.

•Create QoS Policy

•QoS Wizard

•Interface Selection

•Queuing for Outbound Traffic

•Add a New Traffic Class

•Policing for Outbound Traffic

•QoS Policy Generation

•QoS Configuration Summary

Create QoS Policy

The QoS Wizard allows a network administrator to enable Quality of Service (QoS) on the router's WAN interfaces. QoS can also be enabled on IPSec VPN interfaces and tunnels.

The policy is applied to outgoing traffic on the interface.

To create a QoS policy, click Launch QoS Wizard.

QoS Wizard

This window summarizes the information that you will be providing as you complete the QoS Policy wizard.

Click the Next button to begin configuring a QoS policy.

Interface Selection  

Choose the interface on which you want to configure the QoS policy in this window. This window lists WAN interfaces, and interfaces which do not have a configured outbound QoS policy. VPN interfaces are included in the list, but interfaces used for Easy VPN clients, and interfaces with an existing QoS policy are not included.

If the router Cisco IOS image release is 12.4(11)T or later, virtual template tunnel interfaces may appear in this list. If you choose a VTI interface, you will be able to configure shaping and queuing parameters.

Field Reference

Table 29-1 Interface Selection 

Element	Description
Details	To view configuration details about the chosen interface, click Details. The window displays the interface's IP address and subnet mask, names of access rules and policies applied to the interface, and connections the interface is used for.
DSCP marking (trusted)	To use Differentiated Services Code Point (DSCP) markings to classify traffic, click DSCP marking (trusted). Cisco network devices such as IP phones and switches add DSCP markings to packets. Configuring DSCP on the router allows these markings to be used to classify traffic. If the Cisco IOS image on the router does not support DSCP marking, this option will not appear.
NBAR protocol discovery (untrusted)	To use Networked Based Application Recognition (NBAR) protocol discovery to classify traffic, click NBAR protocol discovery (untrusted). When an application is recognized and classified by NBAR, a network can invoke services for that specific application. NBAR ensures that network bandwidth is used efficiently by classifying packets and then applying Quality of Service (QoS) to the classified traffic. If the Cisco IOS image on the router does not support NBAR protocol discovery, this option will not appear.

Queuing for Outbound Traffic

In this screen, configure queuing for outbound traffic.

Field Reference

Table 29-2 describes the fields in this screen.

Table 29-2 Queuing for Outbound Traffic 

Element	Description
Configure Shaping
To configure shaping for outbound traffic, click Configure Shaping.
Committed Information Rate	The Committed Information Rate (CIR) is the rate at which the interface is to transfer data. Enter the CIR in kilobits per second.
Bandwidth Allocation
Traffic Class	A traffic class is a specific type of traffic, such as voice traffic and routing traffic. The Cisco SDM default traffic classes and user-created traffic classes are listed in this column.
Bandwidth Percentage	To specify the bandwidth percentage for a traffic class, enter the percentage value for that class. Traffic types that depend on high transmission rates, such as voice traffic, should be given a higher percentage than traffic classes that do not need high transmission rates, such as routing traffic.
	The Cisco SDM default traffic classes are displayed with suggested values. When you change the percentage value of any traffic class, the best effort class adjusts to a higher or lower value. The total bandwidth of all classes other than best effort cannot exceed 75%.
Allocated Bandwidth	Cisco SDM displays the Allotted Bandwidth column when you configure a QoS policy for a non-VTI interface. It displays the kilobits per second allotted to the traffic class based on the CIR and the bandwidth percentage entered.
Add Class	To add a traffic class to this policy, click Add Class and enter the class information in the displayed dialog.
Remove	To remove a traffic class from this list that you have created, select the list and click Remove. Cisco SDM default classes cannot be removed.

Add a New Traffic Class

Add a new QoS traffic class in this screen.

Field Reference

Table 29-3 describes the fields in this screen.

Table 29-3 Add New Traffic Class Fields 

Element	Description
Class Name	Enter a name for the traffic class.
Classification
Match	Specify whether the QoS class is to look for matches to Any or to All of the selected criteria. If you choose Any, traffic must meet only one of the match criteria. If you choose All, traffic must meet all of the match criteria. The DSCP values chosen are displayed in the DSCP column. Any—Click Any to specify that traffic must meet only one of the criteria specified in the classification list that you create. All—Click All to specify that traffic must meet all the criteria specified in the classification list that you create.
Item Name	This column displays the types of criteria that you can include in this traffic class. If the QoS policy uses NBAR protocol discovery, you can specify protocol and ACL values. If the QoS policy uses DSCP marking, you can specify DSCP values as well as protocol and ACL values.
Item Value	This column displays the values configured for the particular type, separated by commas. For example, the Protocol row might show the following values: http, edonkey, dhcp
Edit	To add or edit the values for a particular type of entry, select the type, and click Edit. Then, add or modify entries for type in the displayed dialog.
Bandwidth Percentage	Enter the bandwidth percentage that you want to give to the class. Cisco SDM displays a message if you enter a value that causes the total percentage value of all traffic types other than best effort to exceed 75%. If that occurs, lower the percentage value.
Use LLQ (Low Latency Queuing)	To have LLQ be used for this traffic class, click Use LLQ (Low Latency Queuing).

Policing for Outbound Traffic

Configure policing for outbound traffic in this screen.

Field Reference

Table 29-4 describes the fields in this screen.

Table 29-4 Policing for Outbound Traffic Fields 

Element	Description
Configure policing for outbound traffic	If you want the QoS policy to include policing for outbound traffic, check this option and enter values in the configuration fields. Otherwise, click Next to proceed to the next screen. Policing causes packets that exceed the Committed Information Rate (CIR) to be dropped.
Traffic Class	This column lists the traffic classes included in this QoS policy.
Committed Information Rate (CIR)	Enter the CIR for each traffic class. The bandwidth of the link is listed at the bottom of the screen. Cisco SDM displays a message if any entered value causes the total to exceed the link bandwidth.

QoS Policy Generation

Use this window to allocate the bandwidth to the different types of traffic carried on the selected interface. The percentage value that you enter represents 1000 Kbps. For example, if you enter 5%, a bandwidth of 5000 Kbps is allocated. The total percentage value for all types of traffic excluding Best Effort cannot exceed 75%.

Field Reference

Table 29-5 QoS Policy Generation

Element	Description
Voice	Voice traffic. The default value is 33 percent of the bandwidth.
Call Signalling	Signalling needed to control voice traffic. The default value is 5 percent of the bandwidth
Routing	Traffic generated by this and other routers to manage the routing of packets. The default value is 5 percent of the bandwidth.
Management	Telnet, SSH and other traffic generated to manage the router. The default value is 5 percent of the bandwidth.
Transactional	Examples would be traffic generated for retail applications, or database updates. The default value is 5 percent of the bandwidth.
Best Effort	Remaining bandwidth for other traffic, such as e-mail traffic. The default value is 47 percent of the bandwidth. The value of Best Effort is dynamically updated based on the total percentage for the other types of traffic.

QoS Configuration Summary

The QoS Wizard Summary window displays a summary of the QoS policy created based on your choices in the wizard. This policy map will be attached to the selected interface. Each class that the SDM QoS wizard configures is summarized in this screen. A partial display follows. showing the interface that the policy is bound to, the classification type (NBAR or DSCP), the policy name, and several of the QoS classes created.

Interface:  FastEthernet0/0

Classification:  DSCP

Policy Name: SDM-QoS-Policy-1

Policy Details

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Class Name: SDM-Voice-1

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Enabled: Yes

Match DSCP: ef

Queuing: LLQ

Bandwidth Percentage: 33

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Class Name: SDM-Signalling-1

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Enabled: Yes

Match DSCP: cs3,af31

Queuing: CBWFQ

Bandwidth Percentage: 5

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Class Name: SDM-Routing-1

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Enabled: Yes

Match DSCP: cs6

Queuing: CBWFQ

Bandwidth Percentage: 5

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Class Name: class-default

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Enabled: Yes

Match Protocols:

Queuing: Fair Queue

Random Detect: Yes

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Class Name: SDM-Streaming-Video-1

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Enabled: No

Match DSCP: cs4

Editing QoS Policies

Complete these steps to edit a QoS policy:

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Step 1 If you want to review the Cisco IOS CLI commands that you send to the router when you complete the configuration, go to the Cisco SDM toolbar, and click Edit > Preferences > Preview commands before delivering to router. The preview screen allows you to cancel the configuration if you want to.

Step 2 In the Cisco SDM toolbar, click Configure.

Step 3 In the Cisco SDM taskbar, click QoS.

Step 4 Click Edit QoS Policy.

Step 5 Choose the QoS policy that you want to edit.

Step 6 Click Edit. Then, make changes to the settings in the displayed dialogs.

Step 7 Click OK to close the dialog and send the changes to the router.

Step 8 If you checked Preview commands before delivering to router in the Edit Preferences screen, the Cisco IOS CLI commands that you are sending are displayed. Click Deliver to send the configuration to the router, or click Cancel to discard it. If you did not make this setting, clicking Finish sends the configuration to the router.

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Edit QoS Policy Reference describes the configuration screens.

Edit QoS Policy Reference

The topics in this section describe the configuration screens.

•Edit QoS Policy

•Associate or Disassociate the QoS Policy

•Add or Edit a QoS Class

•Edit Match DSCP Values

•Edit Match Protocol Values

•Add Custom Protocols

•Edit Match ACL

Edit QoS Policy

The Edit QoS Policy window allows you to view and change configured QoS policies, and associate policies with router interfaces. This help topic contains separate sections for different parts of the screen. To view the information for a section, click on the section heading.

Policy Selection Reference

Table 29-6 Policy Selection Area 

Element	Description
View Policy on interface	Choose the interface whose QoS policies you want to view.
In Direction	Choose the traffic direction on which the policy that you want to view is applied.
Go	To view the policy for the interface and traffic direction that you chose, click Go.
Associate	To change the association of a QoS policy with an interface, click Associate. If the policy is currently associated with an interface, you can disassociate the policy, or change the traffic direction the policy is applied to. The Associate button is disabled when a frame-relay serial interface is displayed in the View Policy on Interface field.
Policy Name	This field displays the name of the policy associated with the interface and traffic direction that you chose.

QoS Class Button Reference

Table 29-7 QoS Buttons 

Element	Description
Add	To add a QoS class to the policy, click Add.
Edit	To edit a QoS class in this screen, choose the class and click Edit. The Edit button is disabled when a read-only Qos class is selected.
Delete	To remove a QoS class from this policy, select a class and click Delete. The Delete button is disabled when a read-only Qos class is selected.
Cut	To remove a class from its current position in the list, select the class and click Cut. Use the Paste button to place the class in the position that you want. The Cut button is disabled when a read-only Qos class is selected.
Copy	To copy class information, select the class and click Copy. The Copy button is disabled when a read-only Qos class is selected.
Paste	To edit copied class information and provide a new name for the class, click Paste. If you choose Add this class to the policy, the class will be placed with the enabled polices in the class. The Paste button is disabled when a read-only Qos class is selected.
Move Up	To move a class up the class list, choose a class and click Move Up. This button can only be used to move enabled classes. The Move Up button is disabled when a read-only Qos class is selected.
Move Down	To move a class down the class list, choose a class and click Move Down. This button can only be used to move enabled classes. The Move Down button is disabled when a read-only Qos class is selected.
Add Service Policy	To add a service policy to an existing QoS policy, select an existing class name from the policy, click Add Service Policy, and choose whether to add a new service policy or use an existing policy.
Remove Service Policy	To remove a service policy, choose the top-level class-default entry, and click Remove Service Policy.
Apply Changes	Changes that you make in this window are not immediately delivered to the router. To deliver changes that you make, click Apply Changes.
Discard Changes	If you do not want the changes that you have made in this window to be sent to the router, click Discard Changes.

Class List Display Reference

Table 29-8 Class List Display Area 

Element	Description
	If this icon appears next to the QoS class, it is read-only, and it cannot be edited, deleted, or moved to another position in the class list.
Class Name	The name of the QoS class. Cisco SDM predefines names for QoS classes.
Match	Whether the QoS class looks for matches to Any or to All of the selected DSCP values. If you choose Any, traffic must meet only one of the match criteria. If you choose All, traffic must meet all of the match criteria. The DSCP values chosen are displayed in the DSCP column.
Classification	This portion of the display contains the following columns: •DSCP—The DSCP values that are chosen for possible match. •Protocols—The protocols included in this QoS class. A video traffic QoS class might have protocols such as cuseeme, netshow, and vdolive. A routing traffic QoS class might have protocols such as BGP, EIGRP, and OSPF. •ACL—The name or number of an ACL that specifies the traffic that this QoS class applies to.
Action	This portion of the display contains the following columns: •Queuing—This column lists the queuing type, Class Based Weighted Fair Queuing (CBWFQ), Low Latency Queuing (LLQ), or Fair Queuing, and displays the bandwidth allocated to the class. •Shaping—This column displays Yes if shaping is configured for this policy, or No if shaping is not configured. •Policing—This column displays Yes if policing is configured for this policy, or No if policing is not configured. •Set DSCP—The DSCP value that is given to this type of traffic by the QoS class. •Drop—The column displays Yes if this type of traffic is to be dropped, or No if it is not to be dropped.

Add Class for the New Policy

Add a traffic class for a new QoS policy in this screen.

Field Reference

Table 29-9 describes the fields in this screen.

Table 29-9 Add Class for New Policy 

Element	Description
Policy Name	Enter a name for the QoS Policy.
Class Name	Enter a name for the traffic class.
Classification
Match	Specify whether the QoS class is to look for matches to Any or to All of the selected criteria. If you choose Any, traffic must meet only one of the match criteria. If you choose All, traffic must meet all of the match criteria. The DSCP values chosen are displayed in the DSCP column. •Any—Click Any to specify that traffic must meet only one of the criteria specified in the classification list that you create. •All—Click All to specify that traffic must meet all the criteria specified in the classification list that you create.
Item Name	This column displays the types of criteria that you can include in this traffic class. If the QoS policy uses NBAR protocol discovery, you can specify protocol and ACL values. If the QoS policy uses DSCP marking, you can specify DSCP values as well as protocol and ACL values.
Item Value	This column displays the values configured for the particular type, separated by commas. For example, the Protocol row might show the following values: http, edonkey, dhcp
Edit	To add or edit the values for a particular type of entry, select the type, and click Edit. Then, add or modify entries for type in the displayed dialog.

Add Service Policy to Class

In this screen, add an existing service policy to a QoS class.

Field Reference

Table 29-10 describes the fields in this screen.

Table 29-10 Add Service Policy to Class 

Element	Description
Existing service policy	Select an existing service policy from the list.

Associate or Disassociate the QoS Policy

Use this window to change the associations that a QoS policy has to router interfaces and traffic directions.

Field Reference

Table 29-11

Element	Description
Interface	This column lists the router interfaces. To choose an interface to which you want to associate the QoS policy, check the box next to the interface name. Note If you select the interface Cisco SDM uses to communicate with the router, you cause the connection between SDM and the router to be dropped.
Inbound	To associate the QoS policy to inbound traffic on the chosen interface, check the box in this column.
Outbound	To associate the QoS policy to outbound traffic on the chosen interface, check the box in this column.

Add or Edit a QoS Class

You can create and edit QoS traffic classes, and specify whether the class is to be added to the QoS policy.

Field Reference

Table 29-12 Add or Edit a QoS Class 

Element	Description
Add this class to the policy	To include this QoS class in QoS policy, check Add this class to the policy. If this option is not checked, then the selected QoS class is marked as Disabled in the Edit QoS Policy window.
Class Name	The QoS class name is displayed in this field if you are editing an existing class. You must enter a classname if you are adding a new class to a policy, or pasting information from a QoS class that you have copied.
Class Default	This option appears when there is no class-default in the QoS policy. To add class-default—the default class—instead of creating a new class, click Class Default. There are several configuration parameters that you cannot set for class-default: •Classification box—You cannot specify classification criteria. •Action box—You cannot specify that traffic be dropped. Additionally, you can only specify that Fair Queuing be used.
Classification
Choose the types of items and values that you want the router to examine traffic for.
All	To indicate that traffic must meet all criteria, click All.
Any	To indicate that traffic need only meet one criteria, click Any.
DSCP	To specify that the traffic must contain specific DSCP markings, select DSCP, and click Edit. Then choose the DSCP markings in the displayed dialog.
Protocol	To specify that the traffic must contain specific protocols, select Protocol, and click Edit. Then choose the protocols in the displayed dialog.
Access Rule	To specify that the class must match traffic defined in an ACL, select Access Rule, and then click Edit. In the dialog that appears, choose an existing ACL, create a new one, or clear existing associations if you are editing a QoS class.
Action
Choose the action that the router is to take when it finds traffic that matches the specified DSCP values.
Drop	To have the router drop the traffic, check Drop. If you check Drop, other options in the Action area are disabled.
Set DSCP	To have the router reset DSCP value for the traffic, check Set DSCP and choose the value that you want the traffic to be reset to.
Queuing	To configure queuing for this traffic class, check Queuing and then click Configure Queuing. Then configure traffic queuing in the displayed dialog. LLQ is available if the traffic uses the RTP protocol or has a DSCP value of EF. If the traffic does not have these attributes, the LLQ option is not available. If you are adding or editing the default class—class-default—only Fair Queuing is available.
Shaping	To configure shaping for this traffic, check Shaping and then click Configure Shaping to display the shaping dialog and make settings.
Policing	To configure policing for this traffic, check Policing and then click Configure Policing to display the policing dialog and make settings.

Edit Match DSCP Values

To add a DSCP value to the match list, choose a value from the Available DSCP Values column on the left, and click the top double-arrowhead button to add it to the Selected DSCP Values column. To remove a value from the Selected DSCP Values column, choose the value and click the bottom double-arrowhead button.

Edit Match Protocol Values

To add a protocol to a class, choose a protocol from the Available Protocol Values column on the left, and click the top double-arrowhead button to add it to the Selected Protocol Values column. To remove a protocol from the Selected Protocol Values column, choose the protocol and click the bottom double-arrowhead button.

Add Custom Protocols  

This window allows you to add custom protocols that are not available in the Edit Match Protocol Values window. Do the following to define a custom protocol:

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Step 1 Select the name of the custom protocol from the Name list.

Step 2 Select whether it will be used as a TCP or a UDP protocol.

Step 3 Define the port numbers that this protocol will use. Enter a port number in the New Port Number field, and click Add to add it to the Port Numbers list. To remove a port number from the list, choose the number and click Remove.

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Edit Match ACL

Choose either Select an existing rule (ACL), or Create a new rule (ACL) and select. Additional dialogs are displayed to enable you to create or select an existing rule. If you want to clear existing rule associations, you can choose None (clear associations).

Configure Policing

In this screen, configure policing for a QoS policy.

Field Reference

Table 29-13 describes the fields in this screen.

Table 29-13 Configure Policing 

Element	Description
Specify the access rate parameters for policing
Committed Information Rate (CIR)	Enter the CIR to be used for the policy in kilobits per second. When the traffic rate reaches the CIR, excess traffic is dropped or remarked.
Normal Burst Size (BC)	Optional. Enter the normal burst size in kilobits per second. The normal burst size determines how large traffic bursts can be before some traffic exceeds the CIR.
Excess Burst Size (BE)	Optional. Enter the excess burst size in kilobits per second. The excess burst size determines how large traffic bursts can be before all traffic exceeds the rate limit. Traffic that falls between the normal burst size and the excess burst size exceeds the rate limit with a probability that increases as the burst size increases.
Action Type	This column lists the names of the actions that you can choose for traffic that conforms to, exceeds, or violates the configured CIR, BC, and BE parameters.
Action	Choose what you want the router to do when traffic conditions conform, exceed or violate configured policing parameters. The conform and the exceed actions are mandatory and have default values. The violate action is optional. The available actions are the following: •Drop—(Default for exceed action) Discard the packet. •None—(Available for violate action) •Set DSCP Transmit—Set the DSCP and transmit. •Transmit—(Default for conform action) Send the packet. •Unsupported—Cisco SDM adds and selects this option in the following cases: –If Cisco SDM detects that actions other than transmit, drop, or set DSCP transmit have been configured. Actions other than those are not supported. –If Cisco SDM detects that more than one action has been configured for the same action type. When Cisco SDM encounters either of these configurations, Unsupported is the only available action, and Cisco SDM displays a tooltip popup indicating that an unsupported policing action has been configured.
DSCP Values	Options in this column are enabled when you choose the Set DSCP Transmit action. The options displayed are the available DSCP markings.

Configure Shaping

In this screen, configure shaping for a QoS policy.

Field Reference

Table 29-14 describes the fields in this screen.

Table 29-14 Configure Shaping 

Element	Description
Committed Information Rate (CIR)	Enter the CIR to be used for the policy in kilobits per second. When the traffic rate reaches the CIR, excess traffic is dropped or remarked.
Normal Burst Size (BC)	Optional. Enter the normal burst size in kilobits per second. The normal burst size determines how large traffic bursts can be before some traffic exceeds the CIR.
Excess Burst Size (BE)	Optional. Enter the excess burst size in kilobits per second. The excess burst size determines how large traffic bursts can be before all traffic exceeds the rate limit. Traffic that falls between the normal burst size and the excess burst size exceeds the rate limit with a probability that increases as the burst size increases.

Configure Queuing

In this screen, configure queuing for a QoS policy. The fields displayed change based on the queuing method chosen. You can choose the following queuing methods:

•LLQ— Low Latency Queuing

•CBWFQ—Class-Based Weighted Fair Queuing

•Fair Queue—Weighted Fair Queuing (WFQ)

Field Reference

Table 29-15 describes the fields in this screen.

Table 29-15 Configure Queuing Fields 

Element	Description
LLQ Chosen
Priority Percentage	Bandwidth is allocated as an absolute percentage of the total bandwidth of the interface or tunnel. Enter a percentage value from 1 to 100 to specify the amount of bandwidth that you want to use.
CBWFQ Chosen
Bandwidth	Enter a percentage value from 1 to 100 to specify the amount of bandwidth that you want to use. Bandwidth is allocated as an absolute percentage of the total bandwidth of the interface or tunnel.
Bandwidth Remaining	Enter a percentage value from 1 to 100 to specify the amount of available bandwidth that you want to use for this traffic class. Bandwidth is allocated as a relative percentage of the total bandwidth available on the interface. You can specify that 30 percent of the available bandwidth be allocated to one class, and 60 percent of the bandwidth be allocated to another QoS class. To use this option, all other classes must use this option.
Random Detect	To enable Weighted Random Early Detection (WRED) and Distributed WRED (DWRED), click Random Detect. WRED drops packets during periods of high congestion, thus telling the source host to decrease the transmission rate.
Fair Queue Chosen
Random Detect	To enable WRED and DWRED, click Random Detect. WRED drops packets during periods of high congestion, thus telling the source host to decrease the transmission rate.