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http3_upstream.md

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Overview

Support for Upstream HTTP/3 connections is slightly more complex than for HTTP/1 or HTTP/2 and requires specific configurations to enable it. If HTTP/3 is configured in auto_config, HTTP/3 connections will only be attempted to servers which advertise HTTP/3 support either via HTTP Alternative Services or the HTTPS DNS resource record. If no such advertisement exists, then HTTP/2 or HTTP/1 will be used instead. Further, HTTP/3 runs over QUIC (which uses UDP) and not over TCP (which HTTP/1 and HTTP/2 use). It is not uncommon for network devices to block UDP traffic, and hence block HTTP/3. This means that upstream HTTP/3 connection attempts might be blocked by the network and should fall back to using HTTP/2. On such networks, the upstream connection code needs to track that HTTP/3 connects attempts are not succeeding and avoid making connections which are doomed to fail. On networks where HTTP/3 is working correctly, however, the upstream connection code should avoid attempting HTTP/2 unnecessarily.

Components

Broken HTTP/3 tracking

The BrokenHttp3Tracker is a per-grid object which tracks the state of HTTP/3 for that grid. When HTTP/3 is marked as broken, it remains broken for 5 minutes after which point it is no longer broken. If it is marked as broken again, it remains broken for twice the previous period, up to a maximum of 1 day. If it is instead marked as confirmed, then the period is reset to the default and the next time HTTP/3 is marked as broken it will be for the initial 5 minutes.

Alternate Protocols Cache

The AlternateProtocolsCache is responsible for tracking servers which advertise HTTP/3. These advertisements may arrive via HTTP Alternative Service (alt-svc) or soon via the HTTPS DNS RR. Currently only advertisements which specify the same hostname and port are stored.

Connectivity Grid

The ConnectivityGrid is a ConnectionPool which wraps a Http3ConnPoolImpl connection pool (QUIC) and a HttpConnPoolImplMixed connection pool (TCP). If HTTP/3 is currently broken, stream requets will be sent directly to the mixed connection pool. If HTTP/3 is not advertised in the alternate protocols cache, stream requets will be sent directly to the mixed connection pool. If, however, HTTP/3 is advertised and working then the request will be sent to the HTTP/3 connection pool. After 300ms, if this request has not succeeded, then the request will also be sent to the mixed pool, and whichever succeeds first will be passed back up to the caller.

Required configuration

Specific Envoy configuration is required in order to enable the previously described components.

Alternate Protocols Cache Filter

The Alternate Protocols Cache Filter must be enabled in order for alt-svc headers to be parsed and stored in the cache, and the alternate_protocols_cache_options field must be specified in the filter config.

Auto Cluster Pool

The "Auto" must be enabled via the Upstream HttpProtocolOptions message. The upstream_protocol_options field needs to specify auto_config. This config muse specify all three protocols: HTTP/1, HTTP/2 and HTTP/3.

Alternate Protocols Cache Options

In addition, the alternate_protocols_cache_options field must be specified in upstream_protocol_options and the value must match that in the filter config.