NAME

       congestion.config - Traffic Server congestion control configuration file

       The  congestion.config file (by default, located in /usr/local/etc/trafficserver/) enables
       you to configure Traffic Server to stop forwarding HTTP requests to  origin  servers  when
       they  become  congested,  and then send the client a message to retry the congested origin
       server later. After you modify the congestion.config file, navigate to the Traffic  Server
       bin  directory;  then run the traffic_ctl config reload command to apply changes. When you
       apply the changes to a node in a cluster, Traffic Server automatically applies the changes
       to  all other nodes in the cluster. Traffic Server uses the congestion.config file only if
       you enable the proxy.config.http.congestion_control.enabled option.

       You can create rules in the congestion.config file to specify:

       • Which origin servers Traffic Server tracks for congestion.

       • The timeouts Traffic Server uses, depending on whether a server is congested.

       • The page Traffic Server sends to the client when a server becomes congested.

       • If Traffic Server tracks the origin servers per IP address or per hostname.

FORMAT

       Each line in congestion.config must follow the format below. Traffic  Server  applies  the
       rules  in  the  order  listed,  starting at the top of the file. Traffic Server recognizes
       three space-delimited tags:

          primary_destination=value secondary_specifier=value action=value

       The following list shows possible primary destinations with allowed values.

       dest_domain
              A requested domain name.

       dest_host
              A requested hostname.

       dest_ip
              A requested IP address.

       url_regex
              A regular expression (regex) to be found in a URL.

       The secondary specifiers are optional in the congestion.config file.  The  following  list
       shows  possible  secondary  specifiers  with  allowed  values.   You can use more than one
       secondary specifier in a rule; however, you cannot repeat a secondary specifier.

       port   A requested URL port or range of ports.

       prefix A prefix in the path part of a URL.

       The following list shows the possible tags and their allowed values.

       max_connection_failures
              Default: 5 The maximum number of connection failures allowed within the fail window
              described below before Traffic Server marks the origin server as congested.

       fail_window
              Default:  120  seconds.   The  time  period  during  which  the  maximum  number of
              connection failures can occur before Traffic Server  marks  the  origin  server  as
              congested.

       proxy_retry_interval
              Default:  10  seconds.   The  number  of  seconds  that Traffic Server waits before
              contacting a congested origin server again.

       client_wait_interval
              Default: 300 seconds.  The number of seconds that the client  is  advised  to  wait
              before retrying the congested origin server.

       wait_interval_alpha
              Default:  30  seconds The upper limit for a random number that is added to the wait
              interval.

       live_os_conn_timeout
              Default: 60 seconds.  The connection  timeout  to  the  live  (uncongested)  origin
              server.  If a client stops a request before the timeout occurs, then Traffic Server
              does not record a connection failure.

       live_os_conn_retries
              Default: 2 The maximum number of retries allowed to the live  (uncongested)  origin
              server.

       dead_os_conn_timeout
              Default: 15 seconds.  The connection timeout to the congested origin server.

       dead_os_conn_retries
              Default: 1 The maximum number of retries allowed to the congested origin server.

       max_connection
              Default:  -1  The  maximum number of connections allowed from Traffic Server to the
              origin server.

       error_page
              Default: "congestion#retryAfter" The error page sent to the client when a server is
              congested. You must enclose the value in quotes;

       congestion_scheme
              Default:  "per_ip"  Specifies  if  Traffic  Server  applies  the rule on a per-host
              ("per_host") or per-IP basis ("per_ip"). You must enclose the value in quotes.

              For example: if the server www.host1.com has two IP addresses and you use  the  tag
              value  "per_ip", then each IP address has its own number of connection failures and
              is marked as congested independently. If you use the tag value "per_host"  and  the
              server  www.host1.com  is marked as congested, then both IP addresses are marked as
              congested.

EXAMPLES

       The following congestion.config rule configures Traffic Server to stop forwarding requests
       to the server www.host.com on port 80 (HTTP traffic) if the server is congested, according
       to the timeouts specified. Traffic Server uses the default tag values because no  tag  has
       been specified.

          dest_host=www.host.com port=80

       You  can  use  one  or  more tags in a rule, but each tag must have one value only. If you
       specify no tags in the rule, then Traffic Server uses the default values.

       You can override any of the default tag values by adding configuration  variables  at  the
       end of records.config as follows:

          CONFIG proxy.config.http.congestion_control.default.tag INT|STRING value

       where  tag  is  one of the tags described in the list under congestion.config and value is
       the value you want to use.

       For example:

          CONFIG proxy.config.http.congestion_control.default.congestion_scheme STRING per_host

       IMPORTANT:
          Rules  in  the  congestion.config  file  override  the  following  variables   in   the
          records.config file:

          proxy.config.http.connect_attempts_max_retries
          proxy.config.http.connect_attempts_max_retries_dead_server
          proxy.config.http.connect_attempts_rr_retries
          proxy.config.http.connect_attempts_timeout
          proxy.config.http.down_server.cache_time
          proxy.config.http.down_server.abort_threshold

COPYRIGHT

       2018, dev@trafficserver.apache.org