Subject: Re: draft-ietf-nat-protocol-complications

From: Vernon Schryver <[email-masked]>
Date: Fri, 21 Apr 2000 10:01:29 -0600 (MDT)

> From: Pyda Srisuresh <[email-masked]>
> To: Vernon Schryver <[email-masked]>, [email-masked],
>         [email-masked]
> Cc: [email-masked], [email-masked]

> ...
> But, IP fragmentation problem is listed as one of known NAT limitations
> in section 6.3 of <draft-ietf-nat-traditional-03.txt> as follows. 

draft-ietf-nat-traditional-03.txt is at neither ftp.isi.edu nor
http://www.ietf.org/internet-drafts

but please don't send me a private copy.  I've intentionally not
subscribed to [email-masked].  If you think the document says enough
about IP fragmentation, then fine.

> ...
> What excatly is your concern with NAT boxes and DF bit? Please elaborate.

My guess is that many NAT boxes choose to pretend to be too-smart-by-half
bridges instead of routers and ignore the DF bit.

> > ICMP as an application whose payloads can need to be modified; consider
> > the packet headers in some ICMP payloads.)
>
> Agreed. This is part of NAT operation described in traditional-nat draft.
> I dont believe, this belongs in "NAT complications" draft. 

If I were writing a document listing complications of NAT, I would
include at least a reference to descriptions of other serious problem.

> > NAT boxes (or perhaps "ALG" in the terms of that draft) and redirection
>
> NAT boxes are not same as ALGs. NAT boxes are those that implement a 
> NAT type (typically NAPT or Basic-NAT, as described RFC 2663) and some
> number of ALGs.

I think I now understand the very positive, even pollyannaish tone
of the document.  And note again, that while I think third party
redirection proxies are somewhere between criminal and completely
unethical and immoral, I think NAT boxes have been and will continue
for at least a while to be better than any of the (un)available
alternatives, and so one might think my prejudices would cause me
to not sensitive to a overly rosey pictures.

> ...
> > Neither kind of box can even redirect incoming IP fragments to the correct
> > local host, since the UDP and TCP port numbers are only in the first
> > fragment.
> > 
> When IP fragments are directed to the NAT box, NAT will need to 
> reassemble the fragments into a single MTU prior to translation.
> And, NAT devices do just that. 

They should, but do most?

Public experience with somewhat related boxes is relevant.  It was always
completely obvious that an FDDI-Ethernet bridge must act like a router
for large IP/FDDI packets, and generate IP fragments if allowed by the DF
bit and send ICMP errors if not.  However, for years more than one vendor
of such bridges insisted in public simply droppping large FDDI frames was
perfectly fine, and that FDDI hosts should just know enough to not use
the FDDI MTU.  The best FDDI-Ethernet bridges would IP fragment (and at
wire speed), but none sent both honored the DF bit and sent ICMP errors.

Vernon Schryver    [email-masked]