Alternative services client support

[vietj]

Implementation of client HTTP Alternative Services (RFC 7838)

Implementation is based on the HTTP client address resolver which provides already client service discovery. Alternative services can be considered as a discovery mechanism that is populated by the interactions the client has with servers.

Origin resolver

The OriginResolver resolves Origin address to an OriginEndpoint.

An origin is the triplet (scheme,host,port) described by The Web Origin Concept.

An OriginEndpoint maintains

• a primary OriginServer resulting from DNS resolution and for which we cannot really make an assumption on the HTTP protocol it can handle (although it is TCP based).
• a list of OriginServer alternatives for which the HTTP protocol is determined by the alternative services discovery

The initial resolution uses the classic A/AAAA DNS resolution and thus initially an origin endpoint is the list of DNS resolved socket addresses.

When the client considers a connection could send alt-svc events, it sets an alt-svc event handler on the connection and relays those events to the origin resolver: an origin endpoint is looked up and when the endpoint is found, it is updated with the alternative services and potentially considered stale.

When an origin endpoint is stale, the resolver updates it on the next resolver lookup and we use the DNS resolver to obtain the alternative services adresses and keep only the ones we can DNS resolve.

Address resolver modifications

Several modifications have been made to the address resolver.

An endpoint is now capable of maintaining a filtered view of the server, which allows to create views for a given protocol and perform load balancing on this view.

HTTP modifications

The HTTP client relies on the origin resolver to obtain an origin endpoint. When the request

• does not mandate a specific HTTP version, the endpoint is used as is.
• mandates a specific HTTP version, a view of the endpoint for a given protocol is obtained and used for load balancing

The HttpConnectOptions now has a protocolVersion field that allows to set a desired HTTP version of the protocol instead of the version defined by HttpClientOptions.

A new HttpProtocol enum has been added to describe more precisely the HTTP protocol used as defined by https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids .

TLS changes

The SslContextManager and SslContextProvider have been updated to cache ssl contexts with the application protocols it supports. When a request is made to resolve an ssl context, the list of application protocols is passed instead of using an initial list provided at build time of the resolver.

Parser

A bunch of parsers have been added to parse alt-svc header strings.

DNS load balancing

The internal resolver for load balancing with DNS A/AAAA records is merged into the new origin resolver since this is pretty much the same thing.

Further work: SVCB and HTTPS Resource Records

The implementation of SVCB and HTTPS Resource Records will piggy back on this PR when the DNS resolver will support it. The origin resolver will obtain HTTPS resource records and directly populate the alternative services for a given origin.