Alt-Svc

curl features EXPERIMENTAL support for the Alt-Svc: HTTP header.

Experimental

Experimental support in curl means:

1. Experimental features are provided to allow users to try them out and provide feedback on functionality and API etc before they ship and get "carved in stone".
2. You must enable the feature when invoking configure as otherwise curl will not be built with the feature present.
3. We strongly advice against using this feature in production.
4. We reserve the right to change behavior of the feature without sticking to our API/ABI rules as we do for regular features, as long as it is marked experimental.
5. Experimental features are clearly marked so in documentation. Beware.

Enable Alt-Svc in build

./configure --enable-alt-svc

Standard

RFC 7838

What works

• read alt-svc file from disk
• write alt-svc file from disk
• parse Alt-Svc: response headers, including ma, clear and persist.
• replaces old entries when new alternatives are received
• unit tests to verify most of this functionality (test 1654)
• act on Alt-Svc: response headers
• build conditionally on configure --enable-alt-svc only, feature marked as EXPERIMENTAL
• implement CURLOPT_ALTSVC_CTRL
• implement CURLOPT_ALTSVC
• document CURLOPT_ALTSVC_CTRL
• document CURLOPT_ALTSVC
• document --alt-svc
• add CURL_VERSION_ALTSVC
• make curl -V show 'alt-svc' as a feature if built-in
• support curl --alt-svc [file] to enable caching, using that file
• make tests/runtests.pl able to filter tests on the feature alt-svc
• actually use the existing in-memory alt-svc cache for outgoing connections
• alt-svc cache expiry
• test 355 and 356 verify curl acting on Alt-Svc, received from header and loaded from cache. The latter needs a debug build since it enables Alt-Svc for plain HTTP.

What is left

• handle multiple response headers, when one of them says clear (should override them all)
• using Age: value for caching age as per spec
• CURLALTSVC_IMMEDIATELY support
• CURLALTSVC_ALTUSED support