On Mon, 2 Feb 1998, Jukka Marin wrote:

> On Sun, Feb 01, 1998 at 01:46:35PM -0700, Marc Slemko wrote:
> > > 13:44:04.878280 mailhost.orion.fi.34036 > kyyhky.jmp.fi.smtp: P 138:1598(1460) ack 373 win 8760 (DF)
> > > 13:44:04.878446 muikku.jmp.fi > mailhost.orion.fi: icmp: kyyhky.jmp.fi unreachable - need to frag (DF)
> > 
> > The problem isn't the system, but rather that some dumb admin has decided
> > to filter all ICMP.
> 
> They allowed ICMP through their firewall and now the mail message got
> through.
> 
> I still don't understand why they try to send packets of 1460 bytes with DF
> set..
> 
> BTW, what's the basic idea of DF, anyway?  The fragmented packets will be
> reassembled at the receiving end, so why not allow fragmenting and let the
> routers do what they have to? :-)  (A stupid question probably, but it had
> to be asked.. ;)  Sure, it may affect performance with high packet losses,
> but..

Read RFC-1191.

Fragmentation is quite inefficient, high overhead, requires too much
buffering, and can cause significant performance problems with
packet-based losses (as opposed to # of byte based losses).