Re: [TLS] New Cached info draft

Stefan Santesson <stefan@aaa-sec.com> Tue, 30 March 2010 19:23 UTC

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Date: Tue, 30 Mar 2010 21:21:22 +0100
From: Stefan Santesson <stefan@aaa-sec.com>
To: Brian Smith <brian@briansmith.org>, <tls@ietf.org>
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Thread-Topic: [TLS] New Cached info draft
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Subject: Re: [TLS] New Cached info draft
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Brian,

I see that I misunderstood you a bit.

I agree that it could be wise to limit the max size of this extension. It
does not have to be that large.

I'll get back to the rest later.

/Stefan


On 10-03-30 7:30 PM, "Brian Smith" <brian@briansmith.org>; wrote:

> Stefan Santesson wrote:
>> On 10-03-30 6:23 PM, "Martin Rex" <mrex@sap.com>; wrote:
>>> 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.
> 
> No, I meant for the client and the server to use the same extension ID, but
> with different syntax for the extension_data. That is allowed. In fact the
> current draft already has slightly different syntax for the client and
> server extension data; the client digest_value is fixed at 8 bytes and the
> server digest value can be either 0 or 8 bytes.
> 
>>>> On 10-03-30 5:34 PM, "Brian Smith" <brian@briansmith.org>; wrote:
>>>>> * The draft says that CachedInformation.cached_info can be up to
>>>>> 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 outdated content from his cache?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 per "server:port" pair, then expiring of outdated cached information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.
> 
> It is good to keep the maximum size of extensions small so that the server
> can allocate and reuse fixed-size buffers that are as small as possible. I
> don't see the use for allowing multiple values per information type, but at
> least I think a small cap on the total size of the extension_data (say, 1KB)
> would be useful. There's no need for a server to waste resources to support
> clients that send dozens, hundreds, or thousands of digests.
> 
>>>>> * 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.
> 
> AFAICT, there's nothing in the draft that says that the client should use
> this information in any way. As long as the client is free to ignore the
> server-sent digest_values when present, it doesn't hurt. But, I don't see
> how it really helps either. It's better to keep the syntax as simple as
> possible.
> 
> Again, it is best to require that the server explicitly list the information
> types for which it supports caching. It costs the server basically nothing
> to provide the few extra bytes, and it is very useful information for the
> client to have.
> 
> Regards,
> Brian
>