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Cisco IOS XE Quality of Service Solutions Configuration Guide, Release 2
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Quality of Service Overview
- Part 1: Classification
- Part 2: Congestion Management
- Part 3: Congestion Avoidance
- Part 4: Policing and Shaping
- Part 5: Link Efficiency Mechanisms
- Part 6: Quality of Service Solutions
- Part 7: Modular Quality of Service Command-Line Interface
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Table Of Contents
WRED—Explicit Congestion Notification
ECN Extends WRED Functionality
How Packets Are Treated When ECN Is Enabled
Benefits of WRED—Explicit Congestion Notification
Supported Standards, MIBs, and RFCs
Configuring Explicit Congestion Notification
Verifying the Explicit Congestion Notification Configuration
Verifying the ECN Configuration: Example
WRED—Explicit Congestion Notification
Feature History
Finding Feature Information
Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE Software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Supported Standards, MIBs, and RFCs
Feature Overview
Currently, the congestion control and avoidance algorithms for Transmission Control Protocol (TCP) are based on the idea that packet loss is an appropriate indication of congestion on networks that are transmitting data using the best-effort service model. When a network uses the best-effort service model, the network delivers data if it can, without any assurance of reliability, delay bounds, or throughput. However, these algorithms and the best-effort service model are not suited to applications that are sensitive to delay or packet loss (for instance, interactive traffic including Telnet, web-browsing, and transfer of audio and video data). Weighted Random Early Detection (WRED) and, by extension, Explicit Congestion Notification (ECN) helps to solve this problem.
RFC 3168, The Addition of Explicit Congestion Notification (ECN) to IP, states that with the addition of active queue management (for example, WRED) to the Internet infrastructure, routers are no longer limited to packet loss as an indication of congestion.
How WRED Works
WRED makes early detection of congestion possible and provides a means for handling multiple classes of traffic. WRED can selectively discard lower priority traffic when the router begins to experience congestion and provide differentiated performance characteristics for different classes of service. It also protects against global synchronization. Global synchronization occurs as waves of congestion crest, only to be followed by periods of time during which the transmission link is not used to capacity. For these reasons, WRED is useful on any output interface or router where congestion is expected to occur.
WRED is implemented at the core routers of a network. Edge routers assign IP precedences to packets as the packets enter the network. With WRED, core routers then use these precedences to determine how to treat different types of traffic. WRED provides separate thresholds and weights for different IP precedences, enabling the network to provide different qualities of service, in regard to packet dropping, for different types of traffic. Standard traffic may be dropped more frequently than premium traffic during periods of congestion.
ECN Extends WRED Functionality
WRED drops packets, based on the average queue length exceeding a specific threshold value, to indicate congestion. ECN is an extension to WRED in that ECN marks packets instead of dropping them when the average queue length exceeds a specific threshold value. When configured with the WRED — Explicit Congestion Notification feature, routers and end hosts would use this marking as a signal that the network is congested and slow down sending packets.
As stated in RFC 3168, The Addition of Explicit Congestion Notification (ECN) to IP, implementing ECN requires an ECN-specific field that has two bits—the ECN-capable Transport (ECT) bit and the CE (Congestion Experienced) bit—in the IP header. The ECT bit and the CE bit can be used to make four ECN field combinations of 00 to 11. The first number is the ECT bit and the second number is the CE bit. Table 1 lists each of the ECT and CE bit combination settings in the ECN field and what the combinations indicate.
The ECN field combination 00 indicates that a packet is not using ECN.
The ECN field combinations 01 and 10—called ECT(1) and ECT(0), respectively—are set by the data sender to indicate that the endpoints of the transport protocol are ECN-capable. Routers treat these two field combinations identically. Data senders can use either one or both of these two combinations. For more information about these two field combinations, and the implications of using one over the other, refer to RFC 3168, The Addition of Explicit Congestion Notification (ECN) to IP.
The ECN field combination 11 indicates congestion to the endpoints. Packets arriving a full queue of a router will be dropped.
How Packets Are Treated When ECN Is Enabled
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For More Information
For more information about implementing ECN and about the changes required at the routers and end hosts, refer to the following RFCs:
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Benefits of WRED—Explicit Congestion Notification
Improved Method for Congestion Avoidance
This feature provides an improved method for congestion avoidance by allowing the network to mark packets for transmission later, rather than dropping them from the queue. Marking the packets for transmission later accommodates applications that are sensitive to delay or packet loss and provides improved throughput and application performance.
Enhanced Queue Management
Currently, dropped packets indicate that a queue is full and that the network is experiencing congestion. When a network experiences congestion, this feature allows networks to mark the IP header of a packet with a CE bit. This marking, in turn, triggers the appropriate congestion avoidance mechanism and allows the network to better manage the data queues. With this feature, ECN-capable routers and end hosts can respond to congestion before a queue overflows and packets are dropped, providing enhanced queue management.
Related Documents
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Supported Standards, MIBs, and RFCs
Standards
No new or modified standards are supported by this feature.
MIBs
No new or modified MIBs are supported by this feature.
To locate and download MIBs for selected platforms, Cisco IOS XE Software releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
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Prerequisites
ECN must be configured through the Modular Quality of Service Command-Line Interface (MQC). For more information about the MQC, see the "Applying QoS Features Using the MQC" module.
Configuration Tasks
See the following sections for configuration tasks for the WRED—Explicit Congestion Notification feature. Each task in the list is identified as either required or optional.
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Configuring Explicit Congestion Notification
To configure ECN, use the following commands beginning in global configuration mode:
Verifying the Explicit Congestion Notification Configuration
To verify the ECN configuration, use the following commands in EXEC or privileged EXEC mode, as needed:
Configuration Examples
This section provides the following configuration examples:
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Enabling ECN: Example
The following example enables ECN in the policy map called pol1:
Router(config)# policy-map pol1Router(config-pmap)# class class-defaultRouter(config-pmap-c)# bandwidth per 70Router(config-pmap-c)# random-detectRouter(config-pmap-c)# random-detect ecnVerifying the ECN Configuration: Example
The following is sample output from the show policy-map command. The words "explicit congestion notification" (along with the ECN marking information) in the output indicate that ECN has been enabled.
Router# show policy-mapPolicy Map pol1Class class-defaultWeighted Fair QueueingBandwidth 70 (%)exponential weight 9explicit congestion notificationclass min-threshold max-threshold mark-probability--------------------------------------------------------------------------------------------------------------------0 - - 1/101 - - 1/102 - - 1/103 - - 1/104 - - 1/105 - - 1/106 - - 1/107 - - 1/10rsvp - - 1/10The following is sample output from the show policy-map interface command. The words "explicit congestion notification" in the output indicate that ECN has been enabled.
Router# show policy-map interface Serial4/1/0Serial4/1/0Service-policy output:policy_ecnClass-map:prec1 (match-all)1000 packets, 125000 bytes30 second offered rate 14000 bps, drop rate 5000 bpsMatch:ip precedence 1Weighted Fair QueueingOutput Queue:Conversation 42Bandwidth 20 (%)Bandwidth 100 (kbps)(pkts matched/bytes matched) 989/123625(depth/total drops/no-buffer drops) 0/455/0exponential weight:9explicit congestion notificationmean queue depth:0class Transmitted Random drop Tail drop Minimum Maximum Markpkts/bytes pkts/bytes pkts/bytes threshold threshold probability0 0/0 0/0 0/0 20 40 1/101 545/68125 0/0 0/0 22 40 1/102 0/0 0/0 0/0 24 40 1/103 0/0 0/0 0/0 26 40 1/104 0/0 0/0 0/0 28 40 1/105 0/0 0/0 0/0 30 40 1/106 0/0 0/0 0/0 32 40 1/107 0/0 0/0 0/0 34 40 1/10rsvp 0/0 0/0 0/0 36 40 1/10class ECN Markpkts/bytes0 0/01 43/53752 0/03 0/04 0/05 0/06 0/07 0/0rsvp 0/0CCDE, CCENT, Cisco Eos, Cisco HealthPresence, the Cisco logo, Cisco Lumin, Cisco Nexus, Cisco StadiumVision, Cisco TelePresence, Cisco WebEx, DCE, and Welcome to the Human Network are trademarks; Changing the Way We Work, Live, Play, and Learn and Cisco Store are service marks; and Access Registrar, Aironet, AsyncOS, Bringing the Meeting To You, Catalyst, CCDA, CCDP, CCIE, CCIP, CCNA, CCNP, CCSP, CCVP, Cisco, the Cisco Certified Internetwork Expert logo, Cisco IOS, Cisco Press, Cisco Systems, Cisco Systems Capital, the Cisco Systems logo, Cisco Unity, Collaboration Without Limitation, EtherFast, EtherSwitch, Event Center, Fast Step, Follow Me Browsing, FormShare, GigaDrive, HomeLink, Internet Quotient, IOS, iPhone, iQuick Study, IronPort, the IronPort logo, LightStream, Linksys, MediaTone, MeetingPlace, MeetingPlace Chime Sound, MGX, Networkers, Networking Academy, Network Registrar, PCNow, PIX, PowerPanels, ProConnect, ScriptShare, SenderBase, SMARTnet, Spectrum Expert, StackWise, The Fastest Way to Increase Your Internet Quotient, TransPath, WebEx, and the WebEx logo are registered trademarks of Cisco Systems, Inc. and/or its affiliates in the United States and certain other countries.
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