Re: [imapext] AD review of draft-ietf-imapapnd-appendlimit-extension-06

["Adrien de Croy" <[e-mail]>]

Understood

However I think you'll find some implementations are already 
non-compliant in that they handle larger messages already.  We already 
use 64bits.

I understand at the time that IMAP was written the concept of a 4GB 
message was almost inconceivable, and probably Mark didn't want people 
to use 16bits, but for a protocol that sends numbers as texts with no 
limit on the number of decimal digits being transmitted, it makes no 
sense to set a max size, only a minimum upper limit.  I'm not aware of 
any other protocol (leastwise not in the email space) that uses text to 
send numbers that places a limit like this on the underlying data type 
or width.  In fact the whole benefit of using text is that it frees you 
from that, implementers can choose whatever underlying type they see 
fit, bearing in mind the environment they work in.

I'd suggest if someone wants to advertise no limit, it should use a 
non-numeric keyword, rather than overloading 0, or -1 or whatever which 
flavour of maxint is at the moment.

Adrien

------ Original Message ------
From: "Stu Brandt" <[e-mail]>
To: "Adrien de Croy" <[e-mail]>; "[e-mail]" 
<[e-mail]>
Sent: 11/12/2015 1:20:58 p.m.
Subject: Re: [imapext] AD review of 
draft-ietf-imapapnd-appendlimit-extension-06

>Adrien -
>
>I'm not looking to further entrench it, rather I'm pointing out that as 
>long as the BNF itself defines a limit on the size of 'number', then a 
>server's claim of having no limit is irrelevant. Such a server could 
>express its limitless APPENDLIMIT by referring to the BNF defined 
>limit.  I would hope that if/when 'number' as MAX_UINT32 is extended, 
>the next logical step would be MAX_UINT64 which itself is yet another 
>limit.
>
>- Stuart
>
>
>On 12/10/15 6:51 PM, Adrien de Croy wrote:
>>please don't further entrench the hard MAX_INT32 limit on message 
>>size, it will have to be overturned eventually.  4GB gets smaller 
>>every day.
>>
>>------ Original Message ------
>>From: "Stu Brandt" <[e-mail]>
>>To: mailto:[e-mail] <[e-mail]>
>>Sent: 11/12/2015 12:14:11 p.m.
>>Subject: Re: [imapext] AD review of 
>>draft-ietf-imapapnd-appendlimit-extension-06
>>
>>>Barry -
>>>
>>>On the issue of "there's no limit on this mailbox", doesn't the IMAP 
>>>protocol itself establish an append size limit through rfc3501 BNF 
>>>alone?
>>>
>>>literal = "{" number "}" CRLF *CHAR8
>>>number = 1*DIGIT ; Unsigned 32-bit integer ; (0 <= n < 
>>>4,294,967,296)This would support the notion of advertising 
>>>APPENDLIMIT=4294967295 for those who *claim* to have no limit since 
>>>ultimately there *is* a limit.
>>>
>>>- Stuart
>>>
>>>On 12/10/15 5:39 PM, Jayantheesh S B wrote:
>>>>Hi Barry, Please find our response inline. Regards, Jay 
>>>>-----Original Message----- From: imapext 
>>>>[mailto:[e-mail]] On Behalf Of Barry Leiba Sent: 
>>>>Thursday, December 10, 2015 2:10 PM To: Narendra Bisht; Adrien de 
>>>>Croy Cc: [e-mail]; 
>>>>[e-mail] Subject: Re: [imapext] AD review of 
>>>>draft-ietf-imapapnd-appendlimit-extension-06 Hi, Narendra, and 
>>>>thanks for the responses. I'm eliminating all the things we're in 
>>>>agreement on, so we can discuss the others a bit.
>>>>>The substantive issue is this: Suppose my server supports 
>>>>>APPENDLIMIT, and wants to advertise that to the client, but there's 
>>>>>no global limit. So it says "APPENDLIMIT" in response to 
>>>>>CAPABILITY, without giving a number. Good. Now you're requiring 
>>>>>that there be a limit on every mailbox -- the protocol provides no 
>>>>>way to tell the client that there's no limit on this mailbox. Why 
>>>>>is that not a problem? [Naren] This draft is to publish the 
>>>>>APPENDLIMIT, if there is any. If you accept any size then there is 
>>>>>no need to publish this policy.
>>>>I think you missed my point. The situation I'm talking about is 
>>>>this: - The server has no global limit, so no number is advertised 
>>>>on the CAPABILITY response. The spec says, therefore, that 
>>>>per-mailbox limits have to be advertised on, say, STATUS commands. 
>>>>Fine. - There's a limit of, say, 50,000 for INBOX. So when I do 
>>>>STATUS INBOX (APPENDLIMIT), the server responds with "* STATUS INBOX 
>>>>(APPENDLIMIT 50000)". Fine. - But there's no limit on the 
>>>>"Stuff-to-save" mailbox. No limit at all. How does the server 
>>>>respond when I do "STATUS Stuff-to-save (APPENDLIMIT)" ? Why is it 
>>>>not a problem that the spec doesn't answer this question? [Jay] We 
>>>>understood the situation. In this case we can have a big value to 
>>>>the APPENDLIMIT, like 2^32 - 1. We can have it documented. We can 
>>>>have a line like: "If server decides not to have any upload limit 
>>>>for a mailbox, then it can have a huge number set for APPENDLIMIT"
>>>>>-- Section 2 -- "An IMAP server that supports APPENDLIMIT extension 
>>>>>advertises this by including the name APPENDLIMIT in its capability 
>>>>>list. IMAP server MAY advertise this capability after user has 
>>>>>logged in." This says nothing about whether the server is allowed 
>>>>>to advertise the capability before the user has logged in. I also 
>>>>>don't think this is an appropriate use of 2119 "MAY". If you mean, 
>>>>>"The IMAP server MUST NOT advertise this capability until after the 
>>>>>user has logged in," then you should say it that way. (And if 
>>>>>that's not what you mean, please discuss it with me and clarify.)
>>>>>[Naren] We purposefully marked it as a MAY, because it alerts 
>>>>>implementations to the possibility, but doesn't have any effect in 
>>>>>interoperability. What we mean here is client should be ready to 
>>>>>handle this no matter when the capability is sent.
>>>>Great, and I saw the discussion in the other thread. My sense here 
>>>>is that 2119 key words aren't appropriate for this at all: there's 
>>>>no choice here that needs a "MAY" -- you're just talking about how 
>>>>the protocol works when this option is in effect: NEW An IMAP server 
>>>>that supports APPENDLIMIT extension advertises this by including the 
>>>>name APPENDLIMIT in its capability list in Authenticated state. The 
>>>>server may also advertise this extension before the user has logged 
>>>>in. END (With this formulation, if you really want to say "MAY" in 
>>>>the last sentence, I won't object further. [Jay] OK
>>>>>-- Section 4 -- "Client can avoid use of LITERAL+ [RFC2088], when 
>>>>>maximum upload size supported by the IMAP server is unknown." What? 
>>>>>Don't you mean "The client SHOULD avoid"? I'd even use this as an 
>>>>>opportunity to make it firmer, and say "The client MUST avoid". No? 
>>>>>If not, why not? [Naren] We will change it to a MUST
>>>>Hold off on this, because there's still discussion based on Adrian's 
>>>>message in another thread... which I'll bring back here: On Tue, Dec 
>>>>8, 2015 at 4:31 PM, Adrien de Croy mailto:[e-mail] wrote:
>>>>>The proposal that a client MUST avoid LITERAL+/NSLs presumes there 
>>>>>is a limit when in fact there may actually not be one. Of course 
>>>>>there is always a finite limit, but there may be no policy limit. 
>>>>>In fact we don't plan to implement the limit as we've never had a 
>>>>>request for it and don't see a need to deny authenticated users 
>>>>>from appending a mail (and see some dangers in that). I think MAY 
>>>>>works in that it proposes a strategy, and doesn't confuse issues 
>>>>>with servers that already implement LITERAL+ but not a limit. 
>>>>>Otherwise you may be placing a new requirement on old software to 
>>>>>police the new MUST, or implementing the limit places addition 
>>>>>requirements to alter behaviour of LITERAL+ support to enforce this 
>>>>>which IMO over-complicates it.
>>>>But the point of the use of LITERAL+ with APPEND isn't just about 
>>>>this spec and overall limits -- it's about whether we should use 
>>>>LITERAL+ with APPEND *at all*. There are other reasons that any 
>>>>particular APPEND might fail, and one point of using literals (and 
>>>>*never* allowing quoted strings, for example) is exactly to give the 
>>>>server a chance to say "NO" to the APPEND *before* the message data 
>>>>is shipped over. Using LITERAL+ for APPEND data violates that. It 
>>>>was always the intent of LITERAL+ that it be used as a way to 
>>>>eliminate the extra round trip on short strings, where the OK from 
>>>>the server isn't necessary -- such as for username and password at 
>>>>login, or for mailbox names in various places (including the mailbox 
>>>>name in an APPEND command. My point here is that we now have an 
>>>>opportunity to stress this: that it's not a generally good idea to 
>>>>use LITERAL+ for the message data in an APPEND command, because it 
>>>>doesn't give the server the opportunity to say, "OK, yes, go ahead 
>>>>and send me the message." I'm absolutely willing to accept that 
>>>>"MUST NOT" use LITERAL+ for that is too strong. But I'm still going 
>>>>to hold out for "SHOULD NOT", and would like to continue the 
>>>>discussion of why you disapprove. [Jay] "SHOULD NOT" looks 
>>>>reasonable to me, as per the above explanation.
>>>>>"STATUS APPENDLIMIT is considered to be fast" What does that mean? 
>>>>>Is this meant to be advice to the server, to make sure that it *is* 
>>>>>fast? If so, then say it more that way. If not, then please tell me 
>>>>>what it's supposed to mean. [Naren] It's not an advice to the 
>>>>>server, we are trying to convey that this approach is considered to 
>>>>>be faster than evaluating the remaining quota for a mailbox.
>>>>"Is considered to be" is not useful specification language. If you 
>>>>want this to affect what clients or servers do, you should word this 
>>>>in terms that more specifically tell them what you want them to do. 
>>>>Please come up with a better way to say what you really mean here. 
>>>>Let me take a stab, and you can see if this has the right sense 
>>>>(edit as appropriate, of course): OLD STATUS APPENDLIMIT is 
>>>>considered to be fast and there is no need to evaluate remaining 
>>>>quotas (if any) when returning APPENDLIMIT values. APPEND can still 
>>>>fail due to ACL and quota related issues, even if the message being 
>>>>appended is smaller than the APPENDLIMIT. NEW Computing the 
>>>>APPENDLIMIT should be fast, and need not take ACLs, quotas, and 
>>>>other such information into account. The APPENDLIMIT specifies one 
>>>>part of the policy, but an APPEND command can still fail due to 
>>>>issues related to ACLs and quotas issues, even if the message being 
>>>>appended is smaller than the APPENDLIMIT. END [Jay] Fine Also, why 
>>>>is this in Section 4, rather than in, say, a Section 3.3? It has 
>>>>nothing to do with the APPEND response, which is the subject of 
>>>>Section 4. [Jay] Alright
>>>>>-- Section 6 -- "The IMAP APPENDLIMIT extension described in this 
>>>>>document can conceivably be used to facilitate Denial-of-Service 
>>>>>attacks. Specifically, the information contained in the APPENDLIMIT 
>>>>>capability and use of the APPEND command make it somewhat quicker 
>>>>>and easier to devise an efficacious Denial-of-Service attack." 
>>>>>What? How? If it's true (it might be, but you haven't said), then 
>>>>>you should explain it. [Naren] What we meant here is that using 
>>>>>this extension it will easy and quicker to get the upload limit and 
>>>>>using that limit against the server.
>>>>Why? I don't understand why that helps anyone mount an attack. And 
>>>>in any case, it needs to be explained here in the document. And you 
>>>>didn't respond to these questions in my review: [Jay] If the 
>>>>APPENDLIMIT is known beforehand, it's easy to overwhelm server with 
>>>>huge data which is beyond the APPENDLIMIT. This might facilitate 
>>>>Denial-of-Service attacks. Makes sense?
>>>>>"In addition, no issues are addressed involving trusted systems and 
>>>>>possible release of information via the mechanisms described in 
>>>>>this document." What does this mean, and why is it here?
>>>>[Jay] We will remove this
>>>>>"IMAP APPENDLIMIT extension doesn't add any new security 
>>>>>considerations that are not already present in the base IMAP 
>>>>>protocol [RFC3501]." Doesn't the presence of the first paragraph 
>>>>>belie that?
>>>>[Jay] Yes, we will modify accordingly Barry 
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>>>
>