Re: [Int-area] FW: Last Call: 'Fragmentation Considered Very Harmful' to Informational RFC (draft-heffner-frag-harmful)

On Fri, 13 Oct 2006, Joe Touch wrote:

> > (*mostly, because the other route is to strictly enforce the IP ID wrap time
> > and fragment lifetimes.)
>
> I'd prefer that approach to deliberately encourage breaking fragmentation.

let me get this straight: you would like to declare all IPv4 TCP
connections running at full speed over fast Ethernet and faster to be
protocol violations?  Some how I don't think others would agree.

> > (Ok one caveat: tunnels can also work if they greatly strengthen the IP ID
> > and/or do their own fragmentation).
>
> This is the bigger issue. Tunnels supposed to honor the DF bit, but
> basically cannot. There are two alternatives:
>
> - clear DF in the outer header and take your chances

You missed my point (and part of why the problem space starts looking
fractal).  If you think of the tunnel as a separate protocol that uses
additional methods to protect itself from corruption (say by use of IPSEC, or
an enhanced fragmentation mechanism, etc.) then it can be designed to support
safe fragmentation.  The fact that the payload is also IP packets which
happen to be tagged DF, becomes irrelevant because they are not participating in
the fragmentation itself.  They are just opaque payload data.  (Note that the
tunnel should use it's own IPID space, not copied from the payload.)

There is not an easy way to detect if any particular combination of tunnel and
endpoint features are safe, except to test if it fails with IPID=0.
Unfortunately there are too many false fails to ship products in this
configuration.

One of the things I have musing about is writing a super jumbo tunnel protocol
that would use its own fragmentation to put 64k IP jumbograms into 1500 byte
IP packets with FEC (and without using IP fragmentation).   Think of the
implications....

Thanks,
--MM--
-------------------------------------------
Matt Mathis      http://www.psc.edu/~mathis
Work:412.268.3319    Home/Cell:412.654.7529
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Evil is defined by mortals who think they know
"The Truth" and use force to apply it to others.

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