Re: Alt-Svc alternative cache invalidation (ext#16)

Mark Nottingham <mnot@mnot.net> Tue, 17 March 2015 05:47 UTC

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From: Mark Nottingham <mnot@mnot.net>
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Date: Tue, 17 Mar 2015 16:42:41 +1100
Cc: Martin Thomson <martin.thomson@gmail.com>, "Julian F. Reschke" <julian.reschke@gmx.de>, HTTP Working Group <ietf-http-wg@w3.org>
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To: Erik Nygren <erik@nygren.org>
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Subject: Re: Alt-Svc alternative cache invalidation (ext#16)
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I’m not hearing any pushback on this, so I’ve marked #16 as editor-ready, with this note:

“””
Discussed on-list. Cache invalidation is to be scoped to a specific discovery mechanism; e.g., the alternatives you discover via the response header will be invalidated when you see a new response header, while those that were discovered via the frame will be invalidated only when a new frame is received.

This means each mechanism needs to define its own exact invalidation semantics, and probably needs to be capable of carrying multiple alternatives.
“””

Cheers,


> On 20 Feb 2015, at 3:24 pm, Mark Nottingham <mnot@mnot.net> wrote:
> 
> Reading the thread again -- AIUI the intent is for invalidation to be scoped to a single discovery mechanism (the frame, a header, whatever).
> 
> If that's the case, the use cases below will work, because they both use different mechanisms.
> 
> So, I'm OK with this. We will need to be *very* careful to scope the invalidations, however.
> 
> Cheers,
> 
> 
>> On 25 Aug 2014, at 10:30 am, Mark Nottingham <mnot@mnot.net> wrote:
>> 
>> So, to be clear, you're suggesting that both the Alt-Svc header field and the ALTSVC frame type have the side effect of cache invalidation?
>> 
>> Personally -- I'm not sure that's a good idea. 
>> 
>> For example, imagine a http:// service that a) wants to use Opp-Sec and b) the alternate wants to do some load balancing, etc.
>> 
>> The http:// service sets an Alt-Svc header field with a very long lifetime, so that Opp-Sec is as sticky as possible.
>> 
>> The alternate, OTOH, uses a fairly short lifetime for load balancing.
>> 
>> With cache invalidation, the alternate doing load balancing is going to clear the cache of the Opp-Sec hint, thereby forcing the client to go back to the http:// origin once the (short lifetime) load balancing policy expires.
>> 
>> Without invalidation, it'd fall back to the original Opp-Sec alternative.
>> 
>> Likewise for the SNI segmentation use case. 
>> 
>> Regards,
>> 
>> 
>> On 24 Aug 2014, at 11:30 am, Erik Nygren <erik@nygren.org> wrote:
>> 
>>> On Fri, Aug 22, 2014 at 7:50 PM, Martin Thomson <martin.thomson@gmail.com> wrote:
>>> On 22 August 2014 14:53, Erik Nygren <erik@nygren.org> wrote:
>>>> but does not define anything similar for the ALTSVC frame.  Aligning the
>>>> frame and the
>>>> header would allow this to apply to both.
>>> 
>>> I think that we would want to move the Origin field up to the header
>>> with Max-Age.  Logically, you store alternatives for different origins
>>> separately, so requiring different frames makes sense there.  It also
>>> removes any potential for duplication.
>>> 
>>> Also 8 bits of length is not sufficient for an HTTP origin if the name
>>> is maximum size.  I'd assume that the same applies to authority.
>>> 
>>> 
>>> Agreed on both counts.  What about this, then:
>>> 
>>> 0                   1                   2                   3
>>> 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
>>> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>> |                          Max-Age (32)                         |
>>> +---------------+---------------+-------------------------------+
>>> | Origin-Len (16)               |         Origin? (*)         ...
>>> +---------------------------------------------------------------+
>>> |Num-Alt-Auth(8)|
>>> +---------------+---------------+-------------------------------+
>>> | Proto-Len(8)  |        Protocol-ID (*)                        |
>>> +---------------+-----------------------------------------------+
>>> | Alt-Auth-Len (16)             |        Alt-Auth (*)         ...
>>> +---------------+-----------------------------------------------+
>>> |                        Ext-Param? (*)                       ...
>>> +---------------------------------------------------------------+
>>> 
>>> where Origin-Len=0 would be used in the case where this was part of a Stream != 0
>>> and Num-Alt-Auth>=1.  The {Proto-Len, Protocol-ID, Alt-Auth-Len, Alt-Auth} would be
>>> repeated Num-Alt-Auth times.  Alt-Auth is a string such as "server.example.com:443"
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>> 
>> --
>> Mark Nottingham   https://www.mnot.net/
>> 
>> 
> 
> --
> Mark Nottingham   https://www.mnot.net/
> 
> 

--
Mark Nottingham   https://www.mnot.net/