Re: [TLS] New Cached info draft

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

Return-Path: <stefan@aaa-sec.com>
X-Original-To: tls@core3.amsl.com
Delivered-To: tls@core3.amsl.com
Received: from localhost (localhost [127.0.0.1]) by core3.amsl.com (Postfix) with ESMTP id 385E13A6877 for <tls@core3.amsl.com>; Tue, 30 Mar 2010 12:23:50 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.618
X-Spam-Level:
X-Spam-Status: No, score=-1.618 tagged_above=-999 required=5 tests=[AWL=0.501, BAYES_00=-2.599, DNS_FROM_OPENWHOIS=1.13, HELO_EQ_SE=0.35, RCVD_IN_DNSWL_LOW=-1]
Received: from mail.ietf.org ([64.170.98.32]) by localhost (core3.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 5jLAy1PLP77d for <tls@core3.amsl.com>; Tue, 30 Mar 2010 12:23:48 -0700 (PDT)
Received: from s87.loopia.se (s87.loopia.se [194.9.95.115]) by core3.amsl.com (Postfix) with ESMTP id 70B8C3A6AB6 for <tls@ietf.org>; Tue, 30 Mar 2010 12:23:36 -0700 (PDT)
Received: from s128.loopia.se (s34.loopia.se [194.9.94.70]) by s87.loopia.se (Postfix) with ESMTP id A49FE2BCA83 for <tls@ietf.org>; Tue, 30 Mar 2010 21:21:30 +0200 (CEST)
Received: (qmail 64153 invoked from network); 30 Mar 2010 19:21:22 -0000
Received: from 213-64-142-247-no153.business.telia.com (HELO [192.168.1.16]) (stefan@fiddler.nu@[213.64.142.247]) (envelope-sender <stefan@aaa-sec.com>) by s128.loopia.se (qmail-ldap-1.03) with DES-CBC3-SHA encrypted SMTP for <brian@briansmith.org>; 30 Mar 2010 19:21:22 -0000
User-Agent: Microsoft-Entourage/12.24.0.100205
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
Message-ID: <C7D81BD2.9BEF%stefan@aaa-sec.com>
Thread-Topic: [TLS] New Cached info draft
Thread-Index: AQH2zpkx6SXE20QoIjYvnNXk//OcBgFxK2uqAU89Q72RnNvl4w==
In-Reply-To: <003501cad037$1b37c960$51a75c20$@briansmith.org>
Mime-version: 1.0
Content-type: text/plain; charset="US-ASCII"
Content-transfer-encoding: 7bit
Subject: Re: [TLS] New Cached info draft
X-BeenThere: tls@ietf.org
X-Mailman-Version: 2.1.9
Precedence: list
List-Id: "This is the mailing list for the Transport Layer Security working group of the IETF." <tls.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/listinfo/tls>, <mailto:tls-request@ietf.org?subject=unsubscribe>
List-Archive: <http://www.ietf.org/mail-archive/web/tls>
List-Post: <mailto:tls@ietf.org>
List-Help: <mailto:tls-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/tls>, <mailto:tls-request@ietf.org?subject=subscribe>
X-List-Received-Date: Tue, 30 Mar 2010 19:23:50 -0000

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
>