# Defining Protocol parameters

Many Protocols provide options for you to set in the Service that uses them.

Some of the parameters are only used by a specific type of component.

To set options for a Protocol, you must attach it to a custom Service element: either open the properties of a custom Service you have created previously or create a Service.

Link selection options in the Protocol Parameters specify how important different quality metrics are for traffic that is associated with the Protocol. Traffic uses the VPN link that best matches the link selection options.

The default link selection values are predefined and cannot be changed even if you duplicate the default elements. You can optionally use QoS Class elements in Access rules to override the default link selection values.