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Re: draft-bryan-metalinkhttp-18.txt

From: Anthony Bryan <anthonybryan@gmail.com>
Date: Wed, 19 Jan 2011 14:58:39 -0500
Message-ID: <AANLkTi=1kbtpHqW6td2huNwwu8JJCNrVrk24zikbUPG+@mail.gmail.com>
To: Julian Reschke <julian.reschke@gmx.de>
Cc: ietf-http-wg@w3.org, draft-bryan-metalinkhttp@tools.ietf.org
On Wed, Jan 19, 2011 at 5:27 AM, Julian Reschke <julian.reschke@gmx.de> wrote:
> On 19.01.2011 10:33, Anthony Bryan wrote:
>> ...
>>>   ...
>>>   [[ Discussion of this draft should take place on IETF HTTP WG mailing
>>>   list at ietf-http-wg@w3.org or the Metalink discussion mailing list
>>>   located at metalink-discussion@googlegroups.com.  To join the list,
>>>   visit http://groups.google.com/group/metalink-discussion . ]]
>>> This should go on the front page as "Editorial Note".
>> in<abstract>?
>> I've moved it to there where it says:
>> [[ Discussion of this draft should take place on IETF HTTP WG mailing
>> list at ietf-http-wg@w3.org although this draft is not a WG item. ]]
>> ...
> No, add a <note> element after abstract. (Example:
> <http://tools.ietf.org/id/draft-ietf-httpbis-p2-semantics-12.xml>)

nifty, didn't know how to do that. thanks!

>>>   ...
>>> 1.2.  Examples
>>>   A brief Metalink server response with ETag, mirrors, .metalink,
>>>   OpenPGP signature, and a cryptographic hash of the whole file:
>>>   Etag: "thvDyvhfIqlvFe+A9MYgxAfm1q5="
>>>   Link:<http://www2.example.com/example.ext>; rel="duplicate"
>>>   Link:<ftp://ftp.example.com/example.ext>; rel="duplicate"
>>>   Link:<http://example.com/example.ext.torrent>; rel="describedby";
>>>   type="application/x-bittorrent"
>>>   Link:<http://example.com/example.ext.metalink>; rel="describedby";
>>>   type="application/metalink4+xml"
>>>   Link:<http://example.com/example.ext.asc>; rel="describedby";
>>>   type="application/pgp-signature"
>>>   DYzYjRjYzFhOTlmNTQwYzI2M2QwM2U2MQ==
>>> Note: there's no need quote the relation type here.
>> here or throughout? they are quoted in the RFC 5988 examples.
> Not for single relation names that just use token characters.

I removed them from all the duplicate and describedby ones.

>>    Link:<http://example.com/TheBook/chapter2>; rel="previous";
>>          title="previous chapter"
>> or the MIME type in the type parameter doesn't need to be quoted?
> RFC 5988 allows it to be non-quoted, but I would advise against it as it
> violates that 2616 grammar for tokens...


>>> Note: it's unfortunate that there doesn't seem to be a registered media
>>> type
>>> for torrent files.
>> true. I'd fix that if I could. :)
> Go ahead. You can :-)

any pointers?

>>>   ...
>>>   Metalink resources include a Link header [RFC5988] to present a list
>>>   of mirrors in the response to a client request for the resource.
>>>   Metalink servers MUST include the cryptographic hash of a resource
>>>   via Instance Digests in HTTP [RFC3230].  Valid algorithms are found
>>>   in the IANA registry named "Hypertext Transfer Protocol (HTTP) Digest
>>>   Algorithm Values" at
>>>   http://www.iana.org/assignments/http-dig-alg/http-dig-alg.xhtml .
>>> Surplus whitespace. Maybe put the URI into angle brackets.
>> I've also seen registries cited in the references. is that better?
> I think in-lining has a better chance to survive the RFC Editor changes (I
> believe they don't want to have the URIs in the document at all).

I guess you can always search by the registry name but the URI seems useful?

>> ...
>>> As the term "Etag policy" is important, it might make sense to introduce
>>> it
>>> more formally.
>> "Metalink servers and their associated mirror servers SHOULD all share
>> the same ETag policy, meaning ETags are synchronized across servers,
>> i.e. byte-for-byte identical files will have the same ETag on all
>> servers. ETags could be based on the file contents (cryptographic
>> hash) and not server-unique filesystem metadata."
>> ...
> Maybe "To have the same ETag policy means..."...?


> Also, it appears that you require more than what's needed.
> For instance, why would it be a problem when byte-for-byte identical files
> have different ETags on the same server? I think what's relevant is the
> requirement for the mirror files, not any additional requirements for other
> files^h^h^h^h^hresources on the server.

is this better?

"To have the same ETag policy means that ETags are synchronized across
servers for duplicate resources, i.e. byte-for-byte identical files
will have the same ETag on mirrors that they have on the Metalink

>>>   ...
>>>   There are two types of mirror servers: preferred and normal.
>>>   Preferred mirror servers are HTTP mirror servers that MUST share the
>>>   same ETag policy as the originating Metalink server.  Preferred
>>>   mirrors make it possible to detect early on, before data is
>>>   transferred, if the file requested matches the desired file.
>>> Note: that also could be achieved by introducing a new conditional header
>>> for the digest, or by using the extension points in the WebDAV "If"
>>> header.
>> I don't know much about WebDAV but I read section 10.4 of RFC 4918. I
>> still don't understand.
>> ...
> The "If" header can use arbitrary state tokens, and you could make the
> instance digests one of those. Theoretically.

your other mail clears this up, thanks :)

>> we use If-Match in the examples. is that wrong?
> No, this was just a suggestion that you *could* define to make the request
> conditional based on the digest, in which case the dance about ETag policies
> wouldn't be needed.
>>>   ...
>>> Wow. This appears to ignore the overhead of Range requests on the
>>> *server*.
>>> Note that sometimes, content is not served directly from the filesystem,
>>> and
>>> implementing Range may not be possible using seeks. Now one could argue
>>> that
>>> servers suffering from the problem should not support this in the first
>>> place, but still...
>>> Given the file sizes for which parallel downloads make any sense today,
>>> is
>>> it *really* a good idea to recommend 10K segments?
>> nope. how about removing the last 3 sentences&  adding
>> "Note that Range requests impose an overhead on servers and clients
>> need to be aware of that and not abuse them. "
> +1

done :)

I'll upload these changes shortly.

(( Anthony Bryan ... Metalink [ http://www.metalinker.org ]
  )) Easier, More Reliable, Self Healing Downloads
Received on Wednesday, 19 January 2011 20:00:26 UTC

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