Re: iSCSI: IP fragmentation

[Glen Turner <glen.turner@aarnet.edu.au>]

Joshua Tseng wrote:
> 
> I was wondering if it would make sense to require that the
> DO NOT FRAGMENT bit to be set on IPv4 packets carrying iSCSI

Hosts already SHOULD do path MTU discovery, so it is not
the sender who's behaviour you are specifing, but the
behaviour of the receiver.

I think you are asking that the reciever of iSCSI IP
packets with DF=0 send a ICMP error, or drop the
packets quietly.

If the receiver sends an ICMP, how do you prevent
malicious unauthenticated people from using iSCSI
receivers as denial of service multiplers?

If the receiver doesn't send an ICMP, how do users
discover the configuration error?

It is also not clear to me how you would implement
this proposal on a general purpose operating system
(that is, on many iSCSI initiators) where the Path
MTU feature is globally set for all protocols.

The mere act of checking the DF bit may prove
difficult on some platforms.  For example a
mainframe that does TCP offload will have no
idea what the DF bit of an individual IP packet
is, and to find out will cost many more channel
accesses than allowing fragment reassembly
on the offload processor.

Finally, and most significantly, I would oppose your
suggestion simply because it is a long-held IETF design
philosophy that receivers should be robust.

The best IETF standards also avoid prescribing implementation
optimisations, leaving these for less normative informational
RFCs.  Until we have more experience with the protocol
in practice, it would seem to me unwise to seek to optimise
unbenchmarked performance.

I suggest that your proposal only considers one
of the two partners in a iSCSI session, the
target.  Be careful in that optimising the target
performance you don't reduce the initiator
performance and suboptimize the system.

Glen

-- 
 Glen Turner                                 Network Engineer
 (08) 8303 3936      Australian Academic and Research Network
 glen.turner@aarnet.edu.au          http://www.aarnet.edu.au/
--
 The revolution will not be televised, it will be digitised