RE: MPA CRC normative text (was DRAFT San Diego minutes)

The original intent of using CRC32C with MPA was to ensure that DDP/RDMAP had the same level of data integrity from TCP as it had from SCTP.  
 
I don't object to Jim's proposed wording and agree that MPA should have CRC enabled by default. 
 
--jc
 
 
 -----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of Jim Pinkerton
Sent: Thursday, August 12, 2004 2:53 PM
To: Somesh Gupta; Julian Satran; [email protected]
Cc: [email protected]
Subject: MPA CRC normative text (was [rddp] DRAFT San Diego minutes)



Renaming the subject line.

 

To me Somesh's point devolves into the classic argument of who owns this issue - the application or the administrator? Personally, I don't see why we should decide here. Saying the administrator has the unique ability to dictate levels of error detection robustness when we have a diverse set of protocols running on the system (SCTP, TCP, UDP, RDMAP, etc) seems odd.

 

Another way of thinking about it is some applications are so paranoid about error detection that they embed a data check in their data - and take a perf hit because of it. Some applications that sit on top of storage embed this all the way to the disk. They are perfectly fine with trading off performance for the additional level of error detection. To me this is a perfectly reasonable level of flexibility, which the administrator doesn't necessarily want to get in the middle of. 

 

Back to Jim William's points. I think Jim was primarily making two points:

 

1) How to state the requirement? In David's original email, he stated "at least 

CRC32-class integrity" be used. And in Jim Williams email, he stated "CRC32 is not a class of integrity, it is a mechanism for providing integrity."  

 

I'd prefer that we don't resurrect the exact CRC-32C technical requirements - seems like it is over specifying the interface (i.e. XX sequential bit errors 100% detectable, up to YY non-sequential bit errors 100% detected, etc, etc). What value does it add? And we aren't the experts in any case. We know what MPA/SCTP gives us. Let's keep it simple. Possibly something like "at least the same level of data integrity verification as CRC32C"?

 

2) Why are we stating this as a requirement? I suspect there is some truth in his argument. Options are bad, basic assumptions are good. Look how UDP checksums evolved. In the early days, the checksum was optional. That was changed to mandatory to implement, on by default, because everyone wanted to know they had a data integrity check. I think the same argument can be applied here. In the world of high-speed networking and large data movement, checksum silent escapes are much more probable. Thus the need for CRC32C level of integrity check verses checksum. It's a lot tougher argument to make if telnet is the primary application. But I don't see telnet as being the application that is motivating the creation of a completely new transport protocol (RDMAP/DDP). For similar reasons that UDP apps motivated UDP implementations to turn checksums on by default (and it was an ugly migration, if memory serves me correctly, between 1980's RFC768 and 1989's RFC1122), let's skip the ugly migration path and turn on CRC32C or equivalent data checking by default.

 

 

Jim

 

 

 

 


  _____  


From: [email protected] [mailto:[email protected]] On Behalf Of Somesh Gupta
Sent: Thursday, August 12, 2004 12:28 PM
To: Julian Satran; [email protected]
Cc: [email protected]
Subject: RE: [rddp] DRAFT San Diego minutes

 

Julian,

 

I think there is some validity to Jim's point. Maybe the application is

satisfied if there is end-to-end IPSec instead. And although there is

a point of how does the application know, it is really the administrative

policy that determines what level of reliability is good enough or not.

 

Somesh

-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of Julian Satran
Sent: Thursday, August 12, 2004 6:58 AM
To: [email protected]
Cc: [email protected]
Subject: RE: [rddp] DRAFT San Diego minutes


The reasoning behind this requirement was  that an I_T pair may  want to negotiate CRC (or equivalent) as an e2e guarantee but have no way of doing that if they use iSER. 

Does it still look (or smell) as a red herring? 

Julo 




[email protected] 
Sent by: [email protected] 

11/08/04 17:48 


To

<[email protected]> 


cc

 


Subject

RE: [rddp] DRAFT San Diego minutes

 


 

 




> iSER is a protocol that uses MPA (RDDP mapping to TCP) for iSCSI.
> The iSER draft authors have found MPA's current CRC requirements text
> to be insufficient for iSER usage of MPA, and hence currently impose
> stronger requirements.  iSER requires that RDDP provide at least
> CRC32-class integrity, but the current MPA draft text allows CRC32C
> to be disabled in situations where that would not be the result.
> 
> The RDDP WG's rough consensus has been that ULPs can assume at least
> CRC-class integrity from RDDP.  Hence the sense of the room is that the
> MPA draft text needs to be tightened to do this and remove the need
> for iSER to add requirements to MPA.

This smells like a red herring.  CRC32 is not a class of integrity,
it is a mechanism for providing integrity.  Quantifying integrity 
should be done with metrics like mean time between undetected
failure or mean GB of data transferred between undetected failure,
not bits of CRC.  RDDP should provide the ULP with a guaranteed 
level of integrity, not guarantee a mechanism for providing it.

The case that iSER requires a higher level of reliability that
iSCSI or NFS seems like a difficult case to make (but since I
wasn't at the meeting, perhaps it was made effectively).  iSCSI
and NFS do not require mandatory CRC.

I suspect that the real reason is that all planned implementations
provide CRC for free, and options are a pain.  However this it
not a bad argument, so I am not troubled by the conclusion.

> --> Important Security Issue #1: IPsec - required or optional?
> 
> A lengthy discussion ensued on this topic.  Important points:
> - For optional: IPsec can involve significant hardware complexity.
> - For mandatory: Some attacks can only be mitigated by IPsec (WG has
>                  little interest in designing its own security mechanism), and
>                  they will be of concern in some deployment environments.

OK, I'll be politically incorrect.  Running RDMA over IPsec is
sufficiently stupid that NOBODY will implement IPsec regardless of
how mandatory it is.  Just make it mandatory and quit worrying.

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