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

Mark Nottingham <mnot@mnot.net> Mon, 25 August 2014 00:34 UTC

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From: Mark Nottingham <mnot@mnot.net>
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Date: Mon, 25 Aug 2014 10:30:40 +1000
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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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/