Re: Maybe I'm misreading this but...

[Dean Anderson <dean () av8 com>]

Sorry. I stand corrected.

                --Dean

At 02:00 PM 10/16/98 -0700, I Am Not An Isp wrote:
> At 04:37 PM 10/16/98 -0400, Dean Anderson wrote:
>
> > But Path MTU discovery is a bit more complicated.  I'm not looking at any
> > docs at the moment, so I hope I'm not completely wrong about this, but as I
> > recall the discovery process tries to send packets to each hop. First
> > discovering the route path, and then trying to determine the mtu of each
> > hop.   While the intermediate RFC1918 addresses can reply to things they
> > happen to get, you can't directly send a packet to them to see if they will
> > want to fragment it.  
>
> This is incorrect.  From RFC1191, which can be found at
> http://www.pmg.lcs.mit.edu/cgi-bin/rfc/view-plain?number=1191, page 2,
> section "2. Protocol overview":
>
> <quote>
>   The basic idea is that a source host initially assumes that the PMTU
>   of a path is the (known) MTU of its first hop, and sends all
>   datagrams on that path with the DF bit set.  If any of the datagrams
>   are too large to be forwarded without fragmentation by some router
>   along the path, that router will discard them and return ICMP
>   Destination Unreachable messages with a code meaning "fragmentation
>   needed and DF set" [7].  Upon receipt of such a message (henceforth
>   called a "Datagram Too Big" message), the source host reduces its
>   assumed PMTU for the path.
> </quote>
>
> So, as I stated in my last post, it will work unless you filter RFC1918
> space.  I've received lots & lots of replies saying "I filter it", or "I
> RFC1918 in my own LAN, so the firewall drops the packets assuming they are
> spoofed" or stuff like that.  This is fine, and possibly even desirable.
> However, there is nothing to distinguish a packet with RFC1918 space as the
> source address from any other "legal" packet on the 'Net other than your
> own administrative policies - which can break *anything* on the 'Net, not
> just PMTU with RFC1918 space.  Sorry, but I have no control over your
> policy.  So, if someone asks "does this break...", the answer is no.
>
> >              --Dean
>
> TTFN,
> patrick
>
> I Am Not An Isp
> www.ianai.net
> "Think of it as evolution in action." - Niven & Pournelle
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           Plain Aviation, Inc                  dean () av8 com
           LAN/WAN/UNIX/NT/TCPIP          http://www.av8.com
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