Cisco IOS XR Modular Quality of Service Configuration Guide for the Cisco XR 12000 Series Router, Release 4.3.x
Configuring Modular QoS Congestion Avoidance
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Configuring Modular QoS Congestion Avoidance

Configuring Modular QoS Congestion Avoidance

Congestion avoidance techniques monitor traffic flow in an effort to anticipate and avoid congestion at common network bottlenecks. Avoidance techniques are implemented before congestion occurs as compared with congestion management techniques that control congestion after it has occurred.

Congestion avoidance is achieved through packet dropping. Cisco IOS XR software supports these quality of service (QoS) congestion avoidance techniques that drop packets:

  • Random early detection (RED)
  • Weighted random early detection (WRED)
  • Tail drop

The module describes the concepts and tasks related to these congestion avoidance techniques.

Feature History for Configuring Modular QoS Congestion Avoidance on Cisco IOS XR Software

Release

Modification

Release 3.2

The Congestion Avoidance feature was introduced.

Release 3.6.0

Weighted random early detection (WRED) support for IPv4 multicast egress QoS traffic was introduced on 2.5 Gbps IP Services Engine (Engine 3) linecards.

Release 4.0.0

In calculations for the average queue size, indicated that the exponential weight factor is not configurable (CSCeg75763).

Prerequisites for Configuring Modular QoS Congestion Avoidance

This prerequisite is required for configuring QoS congestion avoidance on your network:

You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance.

Information About Configuring Modular QoS Congestion Avoidance

Random Early Detection and TCP

The Random Early Detection (RED) congestion avoidance technique takes advantage of the congestion control mechanism of TCP. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. Assuming the packet source is using TCP, it decreases its transmission rate until all packets reach their destination, indicating that the congestion is cleared. You can use RED as a way to cause TCP to slow transmission of packets. TCP not only pauses, but it also restarts quickly and adapts its transmission rate to the rate that the network can support.

RED distributes losses in time and maintains normally low queue depth while absorbing traffic bursts. When enabled on an interface, RED begins dropping packets when congestion occurs at a rate you select during configuration.

Average Queue Size for WRED

The router automatically determines the parameters to use in the WRED calculations. The average queue size is based on the previous average and current size of the queue. The formula is:

average = (old_average * (1-2 -x)) + (current_queue_size * 2 -x)

where x is the exponential weight factor.

For high values of x, the previous average becomes more important. A large factor smooths out the peaks and lows in queue length. The average queue size is unlikely to change very quickly, avoiding a drastic change in size. The WRED process is slow to start dropping packets, but it may continue dropping packets for a time after the actual queue size has fallen below the minimum threshold. The slow-moving average accommodates temporary bursts in traffic.


Note


The exponential weight factor, x, is fixed and is not user configurable.



Note


If the value of x gets too high, WRED does not react to congestion. Packets are sent or dropped as if WRED were not in effect.


For low values of x, the average queue size closely tracks the current queue size. The resulting average may fluctuate with changes in the traffic levels. In this case, the WRED process responds quickly to long queues. Once the queue falls below the minimum threshold, the process stops dropping packets.

If the value of x gets too low, WRED overreacts to temporary traffic bursts and drops traffic unnecessarily.

WRED Support for IPv4 Multicast Egress QoS Traffic

Weighted random early detection (WRED) is supported for IPv4 multicast egress QoS traffic. The following features are supported:

  • Classification based on precedence and differentiated services code point (DSCP)
  • Queue selection
  • Maximum and minimum guaranteed bandwidth
  • Detection of remaining bandwidth
  • Priority queuing
  • Queue limit
  • Random early detection (RED)
  • Traffic shaping
  • Layer 2 set on ATM and Ethernet

Policy map configurations that use Random Early Detection (RED) must comply with the following requirements. These requirements must be met across all interfaces in the line card for precedence and DSCP-based WRED to be supported for multicast traffic. The policy map configuration requirements are specific to a line card and not across the line cards:

  • Multicast must be enabled.
  • The same class maps must be used in all the policy maps.
  • The class maps must be configured in the same order in all of the policy maps.
  • The same RED statements must be configured in a class map across policy maps. The minimum and maximum threshold values can be different for each policy map.
  • If a RED profile is used by more than one DSCP or precedence in a class, the precedence or DSCP values share the same RED profile in all the different policy maps.

If the policy map requirements are not met for all interfaces in the line card and are not met for at least one policy map, RED configuration is not applied to the multicast traffic and all multicast traffic in the line card is tail-dropped. See the “Tail Drop and the FIFO Queue” section on page 16.

Interfaces with no policy maps and interfaces with policy maps that do not use RED function normally.

For hierarchal policy maps that use WRED, only the child policies should conform to the policy map configuration requirements.

Supported Platforms

WRED is supported for IPv4 multicast egress QoS traffic on the following Cisco XR 12000 Series Router platforms:

  • POS line cards
    • Cisco XR 12000 Series 4xOC12c/STM4c POS Rev B
    • Cisco XR 12000 Series 16xOC3c/STM1c POS Rev B
    • Cisco XR 12000 Series 8xOC3c/STM1c POS
    • Cisco XR 12000 Series 4xOC3c/STM1c POS
    • Cisco XR 12000 Series 1xOC48c/STM16c POS
  • Ethernet line card
    • Cisco XR 12000 Series 4xGE

Tail Drop and the FIFO Queue

Tail drop is a congestion avoidance technique that drops packets when an output queue is full until congestion is eliminated. Tail drop treats all traffic flow equally and does not differentiate between classes of service. It manages the packets that are unclassified, placed into a first-in, first-out (FIFO) queue, and forwarded at a rate determined by the available underlying link bandwidth.

See the “Default Traffic Class” section of the “Configuring Modular Quality of Service Packet Classification on Cisco IOS XR Software” module.

Configuring Random Early Detection

This configuration task is similar to that used for WRED except that the random-detect precedence command is not configured and the random-detect command with the default keyword must be used to enable RED.

Restrictions

For the random-detect command to take effect, you must configure either the shape average, bandwidth/bandwidth remaining percent command in the user defined policy map class. This dependency is not applicable to the policy map class class-default.

SUMMARY STEPS

    1.    configure

    2.    policy-map policy-map-name

    3.    class class-name

    4.    random-detect {cos value | default | discard-class value | dscp value | exp value | precedence value | min-threshold [units] max-threshold [units] }

    5.    bandwidth {bandwidth [units] | percent value} or bandwidth remaining [percent value | ratio ratio-value

    6.    shape average {percent percentage | value [units]}

    7.    exit

    8.    exit

    9.    interface type interface-path-id

    10.    service-policy {input | output} policy-map

    11.    Use the commit or end command.


DETAILED STEPS
     Command or ActionPurpose
    Step 1 configure


    Example:
    RP/0/0/CPU0:router# configure
     

    Enters global configuration mode.

     
    Step 2policy-map policy-map-name


    Example:
    
    RP/0/0/CPU0:router(config)# policy-map policy1
    
     

    Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy and enters the policy map configuration mode.

     
    Step 3class class-name


    Example:
    
    RP/0/0/CPU0:router(config-pmap)# class class1
    
     

    Specifies the name of the class whose policy you want to create or change and enters the policy map class configuration mode.

     
    Step 4random-detect {cos value | default | discard-class value | dscp value | exp value | precedence value | min-threshold [units] max-threshold [units] }


    Example:
    
    RP/0/0/CPU0:router(config-pmap-c)# random-detect default
    
     

    Enables RED with default minimum and maximum thresholds.

     
    Step 5bandwidth {bandwidth [units] | percent value} or bandwidth remaining [percent value | ratio ratio-value

    Example:
    
    RP/0/0/CPU0:router(config-pmap-c)# bandwidth percent 30
    

    or

    
    RP/0/0/CPU0:router(config-pmap-c)# bandwidth remaining percent 20
    
     

    (Optional) Specifies the bandwidth allocated for a class belonging to a policy map.

    or

    (Optional) Specifies how to allocate leftover bandwidth to various classes.

     
    Step 6shape average {percent percentage | value [units]}


    Example:
    
    RP/0/0/CPU0:router(config-pmap-c)# shape average percent 50
    
     

    (Optional) Shapes traffic to the specified bit rate or a percentage of the available bandwidth.

     
    Step 7exit


    Example:
    
    RP/0/0/CPU0:router(config-pmap-c)# exit
    
     

    Returns the router to policy map configuration mode.

     
    Step 8exit


    Example:
    
    RP/0/0/CPU0:router(config-pmap)# exit
    
     

    Returns the router to global configuration mode.

     
    Step 9interface type interface-path-id


    Example:
    
    RP/0/0/CPU0:router(config)# interface TenGigE 0/2/0/0
    
     

    Enters the configuration mode and configures an interface.

     
    Step 10service-policy {input | output} policy-map


    Example:
    
    RP/0/0/CPU0:router(config-if)# service-policy output policy1
    
     

    Attaches a policy map to an input or output interface to be used as the service policy for that interface. In this example, the traffic policy evaluates all traffic leaving that interface.

     
    Step 11 Use the commit or end command.  

    commit—Saves the configuration changes, and remains within the configuration session.

    end—Prompts user to take one of these actions:
    • Yes— Saves configuration changes and exits the configuration session.
    • No—Exits the configuration session without committing the configuration changes.
    • Cancel—Remains in the configuration mode, without committing the configuration changes.
     

    Configuring Weighted Random Early Detection

    WRED drops packets selectively based on IP precedence. Edge routers assign IP precedences to packets as they enter the network. WRED uses these precedences to determine how to treat different types of traffic.

    When a packet arrives, the following actions occur:

    • The average queue size is calculated.
    • If the average queue size is less than the minimum queue threshold, the arriving packet is queued.
    • If the average queue size is between the minimum queue threshold for that type of traffic and the maximum threshold for the interface, the packet is either dropped or queued, depending on the packet drop probability for that type of traffic.
    • If the average queue size is greater than the maximum threshold, the packet is dropped.

    Restrictions

    You cannot configure WRED in a class that has been set for priority queueing (PQ).

    You cannot use the random-detect command in a class configured with the priority command.

    For the random-detect command to take effect, you must configure either the shape average or bandwidth/bandwidth remaining percent command in the user defined policy map class. This dependency is not applicable to the policy map class class-default.

    SUMMARY STEPS

      1.    configure

      2.    policy-map policy-name

      3.    class class-name

      4.    random-detect dscp dscp-value min-threshold [units] max-threshold [units]

      5.    bandwidth {bandwidth [units] | percent value} or bandwidth remaining [percent value | ratio ratio-value]

      6.    bandwidth {bandwidth [units] | percent value}

      7.    bandwidth remaining percent value

      8.    shape average {percent percentage | value [units]}

      9.    queue-limit value [units]

      10.    exit

      11.    interface type interface-path-id

      12.    service-policy {input | output} policy-map

      13.    Use the commit or end command.


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 configure


      Example:
      RP/0/0/CPU0:router# configure
       

      Enters global configuration mode.

       
      Step 2policy-map policy-name


      Example:
      
      RP/0/0/CPU0:router(config)# policy-map policy1
      
       

      Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy and enters the policy map configuration mode.

       
      Step 3class class-name


      Example:
      
      RP/0/0/CPU0:router(config-pmap)# class class1
      
       

      Specifies the name of the class whose policy you want to create or change and enters the policy map class configuration mode.

       
      Step 4random-detect dscp dscp-value min-threshold [units] max-threshold [units]


      Example:
      
      RP/0/0/CPU0:router(config-pmap-c)# random-detect dscp af11 1000000 bytes 2000000 bytes
      
       

      Modifies the minimum and maximum packet thresholds for the DSCP value.

      • Enables RED.
      • dscp-value—Number from 0 to 63 that sets the DSCP value. Reserved keywords can be specified instead of numeric values.
      • min-threshold—Minimum threshold in the specified units. The value range of this argument is from 512 to 1073741823. When the average queue length reaches the minimum threshold, WRED randomly drops some packets with the specified DSCP value.
      • max-threshold—Maximum threshold in the specified units. The value range of this argument is from the value of the min-threshold argument to 1073741823. When the average queue length exceeds the maximum threshold, WRED drops all packets with the specified DSCP value.
      • units—Units of the threshold value. This can be bytes, gbytes, kbytes, mbytes, ms (milliseconds), packets, or us (microseconds). The default is packets.
      • This example shows that for packets with DSCP AF11, the WRED minimum threshold is 1,000,000 bytes and maximum threshold is 2,000,000 bytes.
       
      Step 5bandwidth {bandwidth [units] | percent value} or bandwidth remaining [percent value | ratio ratio-value]

      Example:
      
      RP/0/0/CPU0:router(config-pmap-c)# bandwidth percent 30
      

      or

      
      RP/0/0/CPU0:router(config-pmap-c)# bandwidth remaining percent 20
      
       

      (Optional) Specifies the bandwidth allocated for a class belonging to a policy map.

      or

      (Optional) Specifies how to allocate leftover bandwidth to various classes.

       
      Step 6bandwidth {bandwidth [units] | percent value}


      Example:
      
      RP/0/0/CPU0:router(config-pmap-c)# bandwidth percent 30
      
       

      (Optional) Specifies the bandwidth allocated for a class belonging to a policy map. This example guarantees 30 percent of the interface bandwidth to class class1.

       
      Step 7bandwidth remaining percent value


      Example:
      
      RP/0/0/CPU0:router(config-pmap-c)# bandwidth remaining percent 20
      
       

      (Optional) Specifies how to allocate leftover bandwidth to various classes.

      • The remaining bandwidth of 70 percent is shared by all configured classes.
      • In this example, class class1 receives 20 percent of the 70 percent.
       
      Step 8shape average {percent percentage | value [units]}


      Example:
      
      RP/0/0/CPU0:router(config-pmap-c)# shape average percent 50
      
       

      (Optional) Shapes traffic to the specified bit rate or a percentage of the available bandwidth.

       
      Step 9queue-limit value [units]


      Example:
      RP/0/0/CPU0:router(config-pmap-c)# queue-limit 50 ms
      
       

      (Optional) Changes queue-limit to fine-tune the amount of buffers available for each queue. The default queue-limit is 100 ms of the service rate for a non-priority class and 10ms of the service rate for a priority class.

       
      Step 10exit


      Example:
      
      RP/0/0/CPU0:router(config-pmap)# exit
      
       

      Returns the router to global configuration mode.

       
      Step 11interface type interface-path-id


      Example:
      
      RP/0/0/CPU0:router(config)# interface pos 0/2/0/0
      
       

      Enters the configuration mode and configures an interface.

       
      Step 12service-policy {input | output} policy-map


      Example:
      
      RP/0/0/CPU0:router(config-if)# service-policy output policy1
      
       

      Attaches a policy map to an input or output interface to be used as the service policy for that interface.

      • In this example, the traffic policy evaluates all traffic leaving that interface.
      • Ingress policies are not valid; the bandwidth and bandwidth remaining commands cannot be applied to ingress policies.
       
      Step 13 Use the commit or end command.  

      commit—Saves the configuration changes and remains within the configuration session.

      end—Prompts user to take one of these actions:
      • Yes— Saves configuration changes and exits the configuration session.
      • No—Exits the configuration session without committing the configuration changes.
      • Cancel—Remains in the configuration mode, without committing the configuration changes.
       

      Configuring Tail Drop

      Packets satisfying the match criteria for a class accumulate in the queue reserved for the class until they are serviced. The queue-limit command is used to define the maximum threshold for a class. When the maximum threshold is reached, enqueued packets to the class queue result in tail drop (packet drop).

      The queue-limit value uses the guaranteed service rate (GSR) of the queue as the reference value for the queue_bandwidth. If the class has bandwidth percent associated with it, the queue-limit is set to a proportion of the bandwidth reserved for that class.

      When a class has no guaranteed service rate, the default queue limit depends on whether shaping is applied. If shaping is not applied, the default queue limit is 16384 packets. If shaping is applied, the default queue limit is:

      default queue limit (in packets) = (200 ms * (queue bandwidth or shaper rate) / 8) / average packet size, which is 250 bytes

      Restrictions

      • When configuring the queue-limit command in a class, you must configure one of the following commands: priority, shape average, bandwidth, or bandwidth remaining, except for the default class.
      SUMMARY STEPS

        1.    configure

        2.    policy-map policy-name

        3.    class class-name

        4.    queue-limit value [units]

        5.    priority[level priority-level ]

        6.    police rate percent percentage

        7.    class class-name

        8.    bandwidth {bandwidth [units] | percent value}

        9.    bandwidth remaining percent value

        10.    exit

        11.    exit

        12.    interface type interface-path-id

        13.    service-policy {input | output} policy-map

        14.    Use the commit or end command.


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 configure


        Example:
        RP/0/0/CPU0:router# configure
         

        Enters global configuration mode.

         
        Step 2policy-map policy-name


        Example:
        
        RP/0/0/CPU0:router(config)# policy-map policy1
        
         

        Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy and also enters the policy map configuration mode.

         
        Step 3class class-name


        Example:
        
        RP/0/0/CPU0:router(config-pmap)# class class1
        
         

        Specifies the name of the class whose policy you want to create or change and enters the policy map class configuration mode.

         
        Step 4queue-limit value [units]


        Example:
        
        RP/0/0/CPU0:router(config-pmap-c)# queue-limit 1000000 bytes
        
         

        Specifies or modifies the maximum the queue can hold for a class policy configured in a policy map. The default value of the units argument is packets. In this example, when the queue limit reaches 1,000,000 bytes, enqueued packets to the class queue are dropped.

         
        Step 5priority[level priority-level ]


        Example:
        
        RP/0/0/CPU0:router(config-pmap-c)# priority level 1
        
         

        Specifies priority to a class of traffic belonging to a policy map.

         
        Step 6police rate percent percentage


        Example:
        
        RP/0/0/CPU0:router(config-pmap-c)# police rate percent 30
        
         

        Configures traffic policing.

         
        Step 7class class-name


        Example:
        
        RP/0/0/CPU0:router(config-pmap)# class class2
        
         

        Specifies the name of the class whose policy you want to create or change. In this example, class2 is configured.

         
        Step 8bandwidth {bandwidth [units] | percent value}


        Example:
        
        RP/0/0/CPU0:router(config-pmap-c)# bandwidth percent 30
        
         

        (Optional) Specifies the bandwidth allocated for a class belonging to a policy map. This example guarantees 30 percent of the interface bandwidth to class class2.

         
        Step 9bandwidth remaining percent value


        Example:
        
        RP/0/0/CPU0:router(config-pmap-c)# bandwidth remaining percent 20
        
         

        (Optional) Specifies how to allocate leftover bandwidth to various classes. This example allocates 20 percent of the leftover interface bandwidth to class class2.

         
        Step 10exit


        Example:
        
        RP/0/0/CPU0:router(config-pmap-c)# exit
        
         

        Returns the router to policy map configuration mode.

         
        Step 11exit


        Example:
        
        RP/0/0/CPU0:router(config-pmap)# exit
        
         

        Returns the router to global configuration mode.

         
        Step 12interface type interface-path-id


        Example:
        
        RP/0/0/CPU0:router(config)# interface POS 0/2/0/0
        
         

        Enters the configuration mode and configures an interface.

         
        Step 13service-policy {input | output} policy-map


        Example:
        
        RP/0/0/CPU0:router(config-if)# service-policy output policy1
        
         

        Attaches a policy map to an input or output interface to be used as the service policy for that interface. In this example, the traffic policy evaluates all traffic leaving that interface.

         
        Step 14 Use the commit or end command.  

        commit—Saves the configuration changes and remains within the configuration session.

        end—Prompts user to take one of these actions:
        • Yes— Saves configuration changes and exits the configuration session.
        • No—Exits the configuration session without committing the configuration changes.
        • Cancel—Remains in the configuration mode, without committing the configuration changes.
         

        Configuring Multicast Egress QoS

        This task configures multicast egress QoS and disables multicast egress traffic on the priority queue.

        Prerequisites

        Multicast egress QoS can be configured only on an interface that is already configured for multicast routing. See Cisco IOS XR Multicast Configuration Guide for Cisco XR 12000 Series Router for information on configuring multicast routing.

        SUMMARY STEPS

          1.    configure

          2.    hw-module qos multicast location node-id

          3.    hw-module qos multicast priorityq disable location node-id

          4.    Use the commit or end command.


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 configure


          Example:
          RP/0/0/CPU0:router# configure
           

          Enters global configuration mode.

           
          Step 2hw-module qos multicast location node-id


          Example:
          
          RP/0/0/CPU0:router(config)# hw-module qos multicast location 0/2/0
          
           

          Enables multicast egress QoS for a specified location.

          This command is needed only for multicast QoS on 2.5 Gbps IP Services Engine (Engine 3) linecards.

           
          Step 3hw-module qos multicast priorityq disable location node-id


          Example:
          
          RP/0/0/CPU0:router(config)# hw-module qos multicast priorityq disable location 0/2/0
          
           

          Diverts multicast traffic slated for the priority queue to the default queue QoS for a specified location.

           
          Step 4 Use the commit or end command.  

          commit—Saves the configuration changes and remains within the configuration session.

          end—Prompts user to take one of these actions:
          • Yes— Saves configuration changes and exits the configuration session.
          • No—Exits the configuration session without committing the configuration changes.
          • Cancel—Remains in the configuration mode, without committing the configuration changes.
           

          Configuration Examples for Configuring Policy Maps

          This section provides these configuration examples:

          Enabling WRED for IPv4 Egress Multicast Traffic

          The following example shows a policy map configuration that meets the requirements to enable WRED for IPv4 egress multicast traffic:

          
          policy-map policy_A
           class prec0123
             bandwidth percent 20
             random-detect precedence routine 1000 packets 2000 packets
             random-detect precedence priority 1000 packets 4000 packets
             random-detect precedence immediate 2000 packets 5000 packets
           class prec45
             bandwidth percent 40
             random-detect precedence flash-override 4000 packets 8000 packets
          
          policy-map policy_B
           class prec0123
             bandwidth percent 10
             random-detect precedence routine 1000 packets 4000 packets
             random-detect precedence priority 2000 packets 5000 packets
             random-detect precedence immediate 3000 packets 6000 packets
           class prec45
             bandwidth percent 30
             random-detect precedence flash-override 4000 packets 8000 packets
          
          policy-map policy_C
           class prec012
             bandwidth percent 20
           class prec34
             bandwidth percent 40
          
          interface pos0/1/0/0
           service-policy output policy_A
          interface pos 0/1/0/1
           service-policy output policy_B
          interface pos 0/1/0/3
           service-policy output policy_C
          

          The interfaces mapped to policy_A and policy_B are configured with the same classes and RED statements. The precedences for policy_A and policy_B are mapped to the same RED profile although the minimum and maximum RED thresholds are different. The interface mapped to policy_C is configured differently than the interfaces mapped to policy_A and policy_B.

          Additional References

          These sections provide references related to implementing QoS congestion avoidance.

          Related Documents

          Related Topic

          Document Title

          Initial system bootup and configuration

          Cisco IOS XR Getting Started Guide for the Cisco XR 12000 Series Router

          Master command reference

          Cisco XR 12000 Series Router Master Command Listing

          QoS commands

          Cisco IOS XR Modular Quality of Service Command Reference for the Cisco XR 12000 Series Router

          User groups and task IDs

          “Configuring AAA Services on Cisco IOS XR Software” module of Cisco IOS XR System Security Configuration Guide

          Standards

          Standards

          Title

          No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

          MIBs

          MIBs

          MIBs Link

          To locate and download MIBs using Cisco IOS XR software, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http:/​/​cisco.com/​public/​sw-center/​netmgmt/​cmtk/​mibs.shtml

          RFCs

          RFCs

          Title

          No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

          Technical Assistance

          Description

          Link

          The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

          http:/​/​www.cisco.com/​cisco/​web/​support/​index.html