Subject: RE: header and data digest issue

From: "Julian Satran" <[email-masked]>
Date: Thu, 28 Feb 2002 08:58:34 +0200

David,

Usually (not always) middleboxes terminate TCP connections. That is the 
reason some of us wanted a separate CRC on the data - even with IPsec. The 
scenario suggested is not the only way in which middleboxes may mess up 
things.  An incrementaly recomputed  CRC may solve this type of failure 
but who says that the middle box will play by the rules and not mess-up 
with the data in unexpected ways?  I think that we have a decent solution 
for SCSI data integrity that is both simple and robust with respect to 
accidental failures.

Julo

[email-masked]
Sent by: [email-masked]
27-02-02 21:47

        To:     [email-masked], [email-masked]
        cc: 
        Subject:        RE: header and data digest issue

> I think there is a problem with the current design of the
> header and data digest.
> There is a non-zero chance that an error could cause
> the target to process a single pdu that has a header from
> one pdu and data from a second pdu.
> 
> This type of error is undetectable by the target with the CRC32C
> digests turned on.
> 
> For example:
>         Initiator sends pdu 1 which contains header H1 and data D1
>         Initiator sends pdu 2 which contains header H2 and data D2
>         Target receives pdu 1 which contains header H1 and data D1
>         Target receives pdu 2 which contains header H2 and data D1 (bad)
> In this example the header and data digests will checksum correctly.
> However the wrong data will be written to addresses indicated in H2.
> 
> There are three obvious places this error can occur:
>         1. the initiator
>         2. an intelligent router/switch
>         3. the target
> 
> Not much can be done about initiator problems.
> But I would like to detect problems introduced by the other areas.

The middlebox problem is inherent in allowing middlebox functionality
which was the rationale for using separate CRCs.  One can stop a middlebox
from making this mistake by running IPsec ESP's keyed cryptographic
integrity
check end-to-end - if some middlebox tries to get clever, the ESP 
integrity
check fails and the packets that make up the corrupted PDU are dropped; if
the middlebox error is persistent, the TCP connection will probably close.
Changing the CRC check won't stop middleboxes - if some CRC gets in the 
way,
the middlebox will strip the offending CRC and regenerate it - the result
is *still* vulnerable to implementation bugs in the middlebox (and 
cleverer
things like putting the data digest in the header or vice versa are 
vulnerable to similarly cleverer implementation bugs).

> I think it is actually likely that my target software will run
> in an environment that produces this type of error.
> Imagining how an intelligent router/switch or off-board iSCSI target
> hardware might operate, it is probably only a single bit logic error
> that could cause this type of error.

If "intelligent" means the router/switch is iSCSI-aware, ESP cryptographic
integrity is the right means to protect against modifications - anything
less (especially omitting the keying) is an invitation to the "strip the
CRC and regenerate" scenario akin to the one in the middlebox paragraph
above.

If an iSCSI target is off-board, it has stripped, parsed, and discarded
the iSCSI header - no CRC of any form will protect against a bug in that
target that got confused about which data went with which header. Protocol
implementations remove headers from data and have to keep track of what
they're doing - there's no free lunch here (e.g., if a CRC covering header
and data is added, an implementation will likely to check that and strip 
it,
after which it can still get confused.
 
> Maybe it is not appropriate to address this problem in the iSCSI spec
> because I have described a problem that would not occur on the wire.
> However iSCSI is the mechanism that allows more hardware and software
> to be interposed between the SCSI initiator and the SCSI target.
> iSCSI should not allow a single bit logic error in this additional
> hardware and software to produce an undetectable error.

The only problem scenario I see involves middleboxes, and iSCSI
provides a tool (IPsec ESP cryptographic integrity) that addresses
that problem.  Did I miss anything?

Thanks,
--David

---------------------------------------------------
David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
+1 (508) 249-6449 *NEW*      FAX: +1 (508) 497-8500
[email-masked]         Cell: +1 (978) 394-7754
---------------------------------------------------

---

From: Paul Koning <[email-masked]>
Date: Thu, 28 Feb 2002 11:17:59 -0500

>>>>> "Julian" == Julian Satran <[email-masked]> writes:

 Julian> David, Usually (not always) middleboxes terminate TCP
 Julian> connections. That is the reason some of us wanted a separate
 Julian> CRC on the data - even with IPsec. The scenario suggested is
 Julian> not the only way in which middleboxes may mess up things.  An
 Julian> incrementaly recomputed CRC may solve this type of failure
 Julian> but who says that the middle box will play by the rules and
 Julian> not mess-up with the data in unexpected ways?  I think that
 Julian> we have a decent solution for SCSI data integrity that is
 Julian> both simple and robust with respect to accidental failures.

Agreed.

For any integrity mechanism, you can postulate an implementation
sufficiently broken that it defeats the mechanism.  That isn't the
point of the CRC, or the TCP checksum, or other mechanisms.  All those
mechanisms are there to deal with bit errors caused by noise, and
things of that sort.

       paul