Re: [TLS] New Cached info draft

Stefan Santesson <stefan@aaa-sec.com> Tue, 30 March 2010 18:10 UTC

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Date: Tue, 30 Mar 2010 20:08:46 +0100
From: Stefan Santesson <stefan@aaa-sec.com>
To: <mrex@sap.com>
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Thread-Topic: [TLS] New Cached info draft
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Cc: tls@ietf.org
Subject: Re: [TLS] New Cached info draft
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Martin,


On 10-03-30 6:23 PM, "Martin Rex" <mrex@sap.com>; wrote:

> Stefan Santesson wrote:
>> 
>> The most important reason being that the server need to reply with the same
>> extension that the client sends, everything else is a violation of TLS.
> 
> To me it sounded like Brian disliked the ServerHelloExtension
> to be allowed empty on the initial discovery, rather than telling
> the client for which handshake data the server supports caching.
> 
> I do not think that he suggested to not return the extension _and_
> replace cached data.
> 

I interpreted the ServerCachedInformation structure as a separate extension
sent only by the server.

> 
>> On 10-03-30 5:34 PM, "Brian Smith" <brian@briansmith.org>; wrote:
>> 
>>> * The draft says that CachedInformation.cached_info can be up to (2^16-1)*9
>>> = 590KB in size. extension_data can't be larger than 64KB, so the max bound
>>> for the CachedInformation.cached_info array must be 7281 or less. But,
>>> really, sending more than a few hashes per type of cached info is likely to
>>> run into DoS countermeasures. It would be better to have the specification
>>> require and/or at least recommend that there not be more than one (or at
>>> most a few) hashes per information type in the client hello.
> 
> To me, allowing the client to cache distinct values for the same
> server leads to cache management problems.  How should a client expire
> outdated content from his cache?  If the client only caches one item
> per "server:port" pair, then expiring of outdated cached information
> is a non-issue.
> 
> 

It's a non-issue in any case. A timer for example works well. Nothing
prevents the client to refuse caching more than one object per type and
server, but that restriction doesn't strike me as necessary.

>> 
>>> * The draft says "A present non-empty digest_value indicates that the server
>>> will honor caching of objects of the specified type that matches the present
>>> digest value." I don't see why this is necessary. The server should always
>>> be supporting the digests of the values that it most recently returned, for
>>> the information items it claims to support, so the semantics for empty
>>> digest_values in the server extension are good enough.
> 
> I would also appreciate semantics as suggested here.
> Allow the server to return a ServerHelloExtension that explicitly list
> the types of information for which the server supports caching, but
> _without_ a digest_value, both on discovery and on actual use of
> the caching extension by the client, so that the server does not
> have to pre-calculate this data of future handshake message
> while it is composing ServerHello.
> 

The server doesn't have to send digest values in current draft.

> 
> -Martin