Network Filter Chain
As discussed in the listener section, network level (L3/L4) filters form the core of Envoy connection handling.
The network filters are chained in a ordered list known as filter chain. Each listener has multiple filter chains and an optional default filter chain. associated with each filter chain. If the best match filter chain cannot be found, the default filter chain will be chosen to serve the request. If the default filter chain is not supplied, the connection will be closed.
Filter chain only update
Filter chains can be updated independently. Upon listener config update, if the listener manager determines that the listener update is a filter chain only update, the listener update will be executed by adding, updating and removing filter chains. The connections owned by these destroying filter chains will be drained as described in listener drain.
Not all the listener config updates can be executed by filter chain update. For example, if the listener metadata is updated within the new listener config, the new metadata must be picked up by the new filter chains. In this case, the entire listener is drained and updated.