1. A method of implementing a service model in a datacenter comprising a plurality of host computers executing a plurality of source compute nodes (SCNs), the method comprising: providing a particular host computer with parameters for establishing first and second tunnels between the particular host computer and first and second service nodes of first and second service providers external to the datacenter; and providing the particular host computer with a service-action set that includes first and second service actions to be respectively performed by the first and second service nodes on at least one data message flow of one SCN executing on the particular host computer, each service action in the service-action set defined by referencing an identifier that identifies one service provider for performing the service action, each tunnel for use in relaying each data message of said data message flow to a service node of a service provider to perform a service action on the data message, wherein after performing the first service action on each data message of said data message flow, the first service node of the first service provider sends a response data message back to the particular host computer along the first tunnel and the data message is sent to the second service node of the second service provider along the second tunnel for the second service node to perform the second service action on the data message.