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Re: Submitted new I-D: Cache Digests for HTTP/2

From: Kazuho Oku <kazuhooku@gmail.com>
Date: Thu, 21 Jan 2016 10:26:39 +0900
Message-ID: <CANatvzxrYpMpzSTi8KdoGjF0mQHcuvFDmpVPozXX40MgXBR-kw@mail.gmail.com>
To: Stefan Eissing <stefan.eissing@greenbytes.de>
Cc: Julian Reschke <julian.reschke@gmx.de>, Martin Thomson <martin.thomson@gmail.com>, Ilya Grigorik <ilya@igvita.com>, Amos Jeffries <squid3@treenet.co.nz>, HTTP Working Group <ietf-http-wg@w3.org>
2016-01-20 21:57 GMT+09:00 Stefan Eissing <stefan.eissing@greenbytes.de>:
> I make another attempt, based on the feedback (Kazuho, please stop me if you prefer to work on this):

Sorry.  I thought I had responded to your email.  It has always been
my pleasure (and fun) to discuss how we should design and implement
cache digests.

I think we mostly agree on how the header should be designed.

Below, I will cover the semantic differences between yours and mine
(laid out in https://lists.w3.org/Archives/Public/ietf-http-wg/2016JanMar/0120.html),
and my thoughts.

> Cache-Digest      = "Cache-Digest" ":" #digest-value
> digest-value    = base64url-encoded-digest *( ";" digest-param )

I would like to have the encoded digest stored as a name-value pair,
thereby allowing the client to store several types of digests within a
single digest-value.

At the moment, the draft only covers how the digests of fresh
resources in cache should be encoded.  However if all goes well, I
anticipate that we would extend the spec., to support stale resources
as well.  For such purpose, being able to store more than one type of
digests in a single digest-value would make the header less redundant.

As an example, the header conveying a digest for fresh resources and a
digest for stale resources with if-modified-since headers can look

  cache-digest: fresh=xxxxx; if-modified-since=yyyyy; authority=*.example.com

only in case if we use name-value pairs to encode the digest.

> A digest replaces any existing digest if its domain parameter
> matches the domain of an existing digest (with '*' matching any
> domain) and the path parameter is a prefix of or equal to the
> existing path.

I believe that a server should be able to determine the domain(s) the
client considers the server to be authoritative for.

Therefore, I think that a) the default scope should be the value
passed in by the `:authority` pseudo header, and b) otherwise the
client should explicitly specify the scope using the language defined
in RFC 6125 6.4.3.

Unless the scope is transferred over HTTP, HTTP servers need to have
the knowledge of the common name in the TLS certificate being used.
However that might not always be easy considering the fact that in
some cases TLS terminators (run by a different entity) might be placed
in front of the HTTP server, or in case we want to rolling-update
certificates of a TLS terminator cluster from a host-level certificate
to a wildcard certificate.

Requiring the client to always specify the scope of the digest aligns
with the fact that in HTTP/1.1 and HTTP/2 specifications the Host
header (or the :authority header) is transferred even if TLS
certificate is used (and the fact that not the common name stored in
the certificate but the value of the header IS used as the name of the

> If the codec of a digest is unknown, the digest MUST be assumed
> to be empty

If the codec of a digest is unknown, a server must behave as if it did
not receive that digest.

Assuming it to be empty would mean that a server will consider the
cache of the client to be empty.  It should mean to the server that
the client's cache state is unknown (for the category of the resources
(e.g. fresh)).

And this definition is important for extending the spec., to cover
stale resources.  For example, if a client sends a digest of fresh
resources and a digest of stale resources, a server knowing only how
to handle the former should respect the former and ignore the latter.

> A candidate of a server push is matched against a cache digest
> by comparing its :authority against the digest domain (port number?)
> and its :path against the digest path (equal or prefix). Only
> then is the candidate matched against the digest content.


>> Am 18.01.2016 um 15:46 schrieb Kazuho Oku <kazuhooku@gmail.com>:
>> 2016-01-18 19:32 GMT+09:00 Julian Reschke <julian.reschke@gmx.de>:
>>> On 2016-01-18 11:19, Stefan Eissing wrote:
>>>> +1 generic header parameter form.
>>>> If I may make a proposal:
>>>> Cache-Digest      = "Cache-Digest" ":" #digest-value
>>>>   digest-value    = "<" base64url encoded digest ">" *( ";" digest-param
>>>> )
>>>>   digest-param    = ( ( "domain" "=" domain-value )
>>>>                   | ( "path" "=" path-value )
>>>>                   | ( "codec" "=" codec-value )
>>>>                   | ( "update" ( "=" update-param ) )
>>>>                   | ( digest-extension ) )
>>>>  digest-extension = ( parmname [ "=" ( ptoken | quoted-string ) ] )
>>>>  domain-value     =
>>>>        authority              # defined in RFC3986
>>>>        | wildcard-identifier  # defined in RFC 6125 6.4.3
>>>>  path-value       = path-absolute    # defined in RFC3986 3.3
>>>>  codec            = ( "GCS-SHA256" | ( ptoken | quoted-string ) )
>>>> ...
>>> I wouldn't use angle brackets here; let's leave them for use in places where
>>> the value is a URI (Link header field and several WebDAV header fields).
>>> Just us token/quoted-sting here.
>>> Also, do not wire parameter names into the ABNF; this mixes syntax with
>>> semantics.
>> Thank you for the advice.
>> Obviously I have used an old-fashioned RFC as a reference when writing
>> the ABNF (that Stefan probably modified).  I should have looked into a
>> newer one, such as the Alt-Svc draft.
>>> Best regards, Julian
>> --
>> Kazuho Oku

Kazuho Oku
Received on Thursday, 21 January 2016 01:27:09 UTC

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