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Re: #446: alt-svc header field syntax

From: Mark Nottingham <mnot@mnot.net>
Date: Wed, 2 Apr 2014 13:59:42 +1100
Cc: HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <F5EC3050-7F5B-4C9E-9CDE-458C6C589CAA@mnot.net>
To: "Julian F. Reschke" <julian.reschke@gmx.de>
Not hearing any pushback, closing the issue.


On 2 Apr 2014, at 11:27 am, Mark Nottingham <mnot@mnot.net> wrote:

> +1, looks good.
> 
> On 1 Apr 2014, at 7:04 pm, Julian Reschke <julian.reschke@gmx.de> wrote:
> 
>> On 2014-03-05 20:59, Julian Reschke wrote:
>>> So,
>>> 
>>>> Alt-Svc     = 1#( alternate *( OWS ";" OWS parameter ) )
>>>>  alternate   = <"> protocol-id <"> "=" port
>>> 
>>> <"> can be written better as DQUOTE.
>>> 
>>> That being said, HTTPbis P2 has this advice:
>>> 
>>> "Note that double-quote delimiters almost always are used with the
>>> quoted-string production; using a different syntax inside double-quotes
>>> will likely cause unnecessary confusion."
>>> 
>>> So I'd propose to either make it a full blown quoted-string, or to use a
>>> different quote character ("<" and ">"?).
>>> 
>>>>  Finally, note that while it may be technically possible to put
>>>>  content other than printable ASCII in a HTTP header, some
>>>>  implementations only support ASCII (or a superset of it) in header
>>>>  field values.  Therefore, this field SHOULD NOT be used to convey
>>>>  protocol identifiers that are not printable ASCII, or those that
>>>>  contain quote characters.
>>> 
>>> The note wrt to quote characters is either a statement of fact (can't),
>>> or should be a MUST.
>>> 
>>> A simpler way out of this might be to say:
>>> 
>>>  alternate   = alt-token "=" port
>>>  alt-token   = token ; alpn protocol identifier where non-token octets
>>> are uri-percent-escaped
>>> 
>>> That (1) avoids quoting, (2) makes it possible to use all syntactically
>>> valid protocol identifiers, and last but not least (3) makes the common
>>> case simpler.
>>> 
>>> Best regards, Julian
>> 
>> So I have made this change in the editor's copy of the spec. The change makes the description of the field slightly more complex, but I claim actual implementations will simpler.
>> 
>> The field description now reads:
>> 
>> 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 "=" port
>>  protocol-id = token ; percent-encoded ALPN protocol identifier
>> 
>>  ALPN protocol names are octet sequences with no additional
>>  constraints on format.  Octets not allowed in tokens ([HTTP-p1],
>>  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.
>> 
>>  For example:
>> 
>>  Alt-Svc: http2=8000
>> 
>>  This indicates that the "http2" protocol on the same host using the
>>  indicated port (in this case, 8000).
>> 
>>  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.
>> 
>>  Intermediaries MUST NOT change or append Alt-Svc field values.
>> 
>> 
>> Best regards, Julian
>> 
>> 
>> 
> 
> --
> Mark Nottingham   http://www.mnot.net/
> 
> 
> 
> 

--
Mark Nottingham   http://www.mnot.net/
Received on Wednesday, 2 April 2014 03:00:03 UTC

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