WebSocket and HTTP upgrades

Envoy Upgrade support is intended mainly for WebSocket but may be used for non-WebSocket upgrades as well. Upgrades pass both the HTTP headers and the upgrade payload through an HTTP filter chain. One may configure the upgrade_configs with or without custom filter chains. If only the upgrade_type is specified, both the upgrade headers, any request and response body, and WebSocket payload will pass through the default HTTP filter chain. To avoid the use of HTTP-only filters for upgrade payload, one can set up custom filters for the given upgrade type, up to and including only using the router filter to send the WebSocket data upstream.

Upgrades can be enabled or disabled on a per-route basis. Any per-route enabling/disabling automatically overrides HttpConnectionManager configuration as laid out below, but custom filter chains can only be configured on a per-HttpConnectionManager basis.

Note that the statistics for upgrades are all bundled together so websocket statistics are tracked by stats such as downstream_cx_upgrades_total and downstream_cx_upgrades_active

Handling H2 hops

Envoy currently has an alpha implementation of tunneling websockets over H2 streams for deployments that prefer a uniform H2 mesh throughout, for example, for a deployment of the form:

[Client] —- HTTP/1.1 —- [Front Envoy] —- HTTP/2 —- [Sidecar Envoy —- H1 —- App]

In this case, if a client is for example using WebSocket, we want the Websocket to arive at the upstream server functionally intact, which means it needs to traverse the HTTP/2 hop.

This is accomplished via extended CONNECT support. The WebSocket request will be transformed into an HTTP/2 CONNECT stream, with :protocol header indicating the original upgrade, traverse the HTTP/2 hop, and be downgraded back into an HTTP/1 WebSocket Upgrade. This same Upgrade-CONNECT-Upgrade transformation will be performed on any HTTP/2 hop, with the documented flaw that the HTTP/1.1 method is always assumed to be GET. Non-WebSocket upgrades are allowed to use any valid HTTP method (i.e. POST) and the current upgrade/downgrade mechanism will drop the original method and transform the Upgrade request to a GET method on the final Envoy-Upstream hop.

Note that the H2 upgrade path has very strict HTTP/1.1 compliance, so will not proxy WebSocket upgrade requests or responses with bodies.