On Mon, 26 Feb 2001, Kane, Christopher A. wrote:
> As part of this thread, several people have mentioned that one of the
> problems created is "breaking MTU path discovery." Could someone explain
> what this means?
The smallest MTU in the path of a link is the Path MTU. How do routers
know what size MTU to use, when the link may consist of 10 hops and a
varying degree of routers and media types? This can be done via path mtu
discovery.
Two systems establish a connection, they let eachther know their MTU/MSS
sizes. The lessor of the two is used. Packets are sent using this size,
and with the DF bit set, so that they won't be fragmented. If a transited
router receives the packet with the DF bit set, and its too big, it will
send back a "ICMP Can't Fragment" to the source. This tells the source to
re-attempt at a lower size. Once a packet can make it all the way thru
with the DF bit set, then the Path MTU has been discovered.
Brian
>
> Thanks
>
> -----Original Message-----
> From: Howard C. Berkowitz [mailto:[EMAIL PROTECTED]]
> Sent: Sunday, February 25, 2001 11:21 PM
> To: [EMAIL PROTECTED]
> Subject: Re: Private Internet Addressing
>
>
> >
> >and the reason an ISP would be considered "clueless" for using RFC1918 on
> >internal point to points is..........?
> >
> >Brian
> >
>
> Let's see...
>
> It confuses troubleshooting because valid routes may appear to be
> looping, with the same address traversed more than once.
>
> The addresses can't be resolved with reverse DNS.
>
> It breaks MTU path discovery.
>
> It violates the spirit of RFC 2827 and reverse path verification.
>
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