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Re: #492: Alt-Svc header host restriction

From: Julian Reschke <julian.reschke@gmx.de>
Date: Thu, 12 Jun 2014 16:27:40 +0200
Message-ID: <5399B8DC.9060100@gmx.de>
To: HTTP Working Group <ietf-http-wg@w3.org>
On 2014-06-12 15:33, Julian Reschke wrote:
> Hi there,
>
> turns out that this is a bit messy:
>
> 1) People usually think of "host:port", and using that syntax requires
> putting the value into double quotes, as ":" is a token character --
> unless we choose a different delimiter.
>
> 2) I started with the allowing either a simple port (as used before), or
> host:port, but here the problem is that a naked port number can be
> parsed as host name as well. Thus the colon becomes required even is the
> host part is absent.
>
> With that, I currently have:
>
>
> 3.  The Alt-Svc HTTP Header Field
>
>     An HTTP(S) origin server can advertise the availability of
>     alternative services to clients by adding an Alt-Svc header field to
>     responses.
>
>     Alt-Svc       = 1#( alternative *( OWS ";" OWS parameter ) )
>     alternative   = protocol-id "=" alt-authority
>     protocol-id   = token ; percent-encoded ALPN protocol identifier
>     alt-authority = token / quoted-string
>                     ; containing [ uri-host ] ":" port
>
>     ALPN protocol names are octet sequences with no additional
>     constraints on format.  Octets not allowed in tokens ([RFC7230],
>     Section 3.2.6) MUST be percent-encoded as per Section 2.1 of
>     [RFC3986].  Consequently, the octet representing the percent
>     character "%" (hex 25) MUST be percent-encoded as well.
>
>     In order to have precisely one way to represent any ALPN protocol
>     name, the following additional constraints apply:
>
>     1.  Octets in the ALPN protocol MUST NOT be percent-encoded if they
>         are valid token characters except "%", and
>
>     2.  When using percent-encoding, uppercase hex digits MUST be used.
>
>     With these constraints, recipients can apply simple string comparison
>     to match protocol identifiers.
>
>     The "alt-authority" component consists of an OPTIONAL uri-host
>     ("host" in Section 3.2.2 of [RFC3986]), a colon (":"), and a port
>     number.
>
>     For example:
>
>     Alt-Svc: http2=":8000"
>
>     This indicates the "http2" protocol on the same host using the
>     indicated port 8000.
>
>     An example involving a change of host:
>
>     Alt-Svc: http2="new.example.org:80"
>
>     This indicates the "http2" protocol on the host "new.example.org",
>     running on port 80.  Note that the "quoted-string" syntax needs to be
>     used when a host is specified in addition to a port (":" is not an
>     allowed character in "token").
>
>     Examples for protocol name escaping:
>
>     +--------------------+-------------+---------------------+
>     | ALPN protocol name | protocol-id | Note                |
>     +--------------------+-------------+---------------------+
>     | http2              | http2       | No escaping needed  |
>     +--------------------+-------------+---------------------+
>     | w=x:y#z            | w%3Dx%3Ay#z | "=" and ":" escaped |
>     +--------------------+-------------+---------------------+
>     | x%y                | x%25y       | "%" needs escaping  |
>     +--------------------+-------------+---------------------+
>
>     Alt-Svc MAY occur in any HTTP response message, regardless of the
>     status code.
>
>     Alt-Svc does not allow advertisement of alternative services on other
>     hosts, to protect against various header-based attacks.
>
>     It can, however, have multiple values:
>
>     Alt-Svc: h2c=":8000", h2=":443"
>
>     The value(s) advertised by Alt-Svc can be used by clients to open a
>     new connection to one or more alternative services immediately, or
>     simultaneously with subsequent requests on the same connection.
>
>     To reduce the ability of servers to track individual clients over
>     time (see Section 9.4), an alternative service indication sent by a
>     client SHOULD NOT include any alternative service information other
>     than the protocol, host and port.
>
>     When using HTTP/2 ([HTTP2]), clients SHOULD instead send an ALTSVC
>     frame.  A single ALTSVC frame can be sent for a connection; a new
>     frame is not needed for every request.
>
>     Note that all field elements that allow "quoted-string" syntax MUST
>     be processed as per Section 3.2.6 of [RFC7230].
>
> Can people live with that or should we try to come up with something
> more elegant?
> ...

For instance, something like

   Alt-Svc: h2; port=443, h2c; host=www.examplecom; port=80

?

Best regards, Julian
Received on Thursday, 12 June 2014 14:28:15 UTC

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