Network Congestion and Congestion Control

Network congestion typically occurs in the form of router buffer overflows, when nodes send more packets than the network can accommodate. Various algorithms prevent traffic congestion through establishing controls on the sending systems. These algorithms are supported in Oracle Solaris and can be easily added or directly plugged in to the operating system.

The following table lists and describes the supported algorithms.

In Oracle Solaris, congestion control is enabled by setting the following control-related TCP properties. Although these properties are listed for TCP, the control mechanism that is enabled by these properties also applies to SCTP traffic.

• cong_enabled – contains a list of algorithms, separated by commas, that are currently operational in the system. You can add or remove algorithms to enable only those algorithms you want to use.

• cong_default – the algorithm that is used by default when applications do not specify the algorithms explicitly in socket options. Currently, the value of the cong_default property applies to both global and non-global zones.

To set these properties, you use the ipadm set-prop command. You use either the += modifier to add an algorithm or the -= modifier to remove an algorithm.

How to Implement TCP and SCTP Network Congestion Control

1. Become an administrator.

   For more information, see How to Obtain Administrative Rights in Oracle Solaris Administration: Security Services.

2. Display the current values of the TCP protocol's congestion-control properties.

   # ipadm show-prop -p cong_enabled,cong_default tcp

   If you do not specify the properties, then all of the properties are displayed.

   The command displays both the current values as well as the possible algorithms that you can assign to the properties.

3. Set the congestion control properties of the TCP protocol.

   # ipadm set-prop -p cong-ctrl-property+=algorithm tcp

   where

   cong-ctrl-property

   Refers to either the cong_enabled property or the cong_default property.

   algorithm

   Specifies the algorithm that you are setting for the property. You can specify any algorithm that is listed under the POSSIBLE field heading in the output of the ipadm show-prop command.

4. (Optional) Remove an algorithm that is currently enabled.

   # ipadm set-prop -p cong-ctrl-property-=algorithm tcp

   ---

   Note - No sequence rules are followed when you add or remove algorithms. You can remove an algorithm before adding other algorithms to a property. However, the cong_default property must always have a defined algorithm.

   ---

5. (Optional) Display the new values of the congestion control properties.

   # ipadm show-prop -p cong_enabled,cong_default tcp

Example 26-1 Setting Algorithms for Congestion Control

This example changes the default algorithm for the TCP protocol from newreno to cubic. It also removes vegas from the list of enabled algorithms.

# ipadm show-prop -p cong_default,cong_enabled tcp
PROTO  PROPERTY      PERM  CURRENT          PERSISTENT   DEFAULT   POSSIBLE
tcp    cong_default  rw    newreno          --           newreno   -
tcp    cong_enabled  rw    newreno,cubic,   --           newreno   newreno,cubic,
                           highspeed,                              highspeed,vegas
                           vegas

# ipadm set-prop -p cong_enabled-=vegas tcp
# ipadm set-prop -p cong_default=cubic tcp

# ipadm show-prop -p cong_default,confg_enabled tcp
PROTO  PROPERTY      PERM  CURRENT          PERSISTENT   DEFAULT   POSSIBLE
tcp    cong_default  rw    cubic            --           newreno   -
tcp    cong_enabled  rw    newreno,cubic,   --           newreno   newreno,cubic,
                           highspeed                               highspeed,vegas