RFC 7838: HTTP Alternative Services.
This allows HTTP responses to indicate alternative servers that a client can use ( but it isn’t required to do so ). One example, from the RFC:
Alt-Svc: h2=":8000"This indicates the “h2” protocol ([RFC7540]) on the same host using
the indicated port 8000.
Google already uses this to announce QUIC as an alternative, with this header included in a response from https://www.google.com/:
Alt-Svc: quic=":443"; ma=2592000; v="43,42,41,39,35"
That breaks down as:
– QUIC is available on port 443, of the same origin
– Max age ( ma ) for this information is 2,592,000 seconds ( default is 24 hours )
– Version negotiation hints ( v ) for QUIC
That gives sites a direct option for announcing other means of making requests. Right now QUIC is the obvious alternative, but by being an open format, others could use it too.
Mark Nottingham has a good write up on “Alternative Services”.