Date: Sun, 24 Mar 1996 19:30:34 -0500 From: Andrew Herdman <andrew@whine.com> To: "Louis A. Mamakos" <louie@TransSys.COM> Cc: Brian Tao <taob@io.org>, FREEBSD-HACKERS-L <freebsd-hackers@FreeBSD.org> Subject: Re: Changing Ethernet frame size to 576 bytes? Message-ID: <Pine.BSF.3.91.960324192723.850A-100000@why> In-Reply-To: <199603240511.AAA20790@wa3ymh.transsys.com>
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Actually Brian is having a fragment problem, this is from the IRC server
machine which I run the IRC server on:
ip:
44034396 total packets received
0 bad header checksums
0 with size smaller than minimum
0 with data size < data length
0 with header length < data size
0 with data length < header length
0 with bad options
0 with incorrect version number
245769 fragments received
0 fragments dropped (dup or out of space)
1886 fragments dropped after timeout
39671 packets reassembled ok
43571756 packets for this host
256542 packets for unknown/unsupported protocol
0 packets forwarded
0 packets not forwardable
0 redirects sent
40712754 packets sent from this host
816 packets sent with fabricated ip header
0 output packets dropped due to no bufs, etc.
13 output packets discarded due to no route
20314 output datagrams fragmented
91863 fragments created
0 datagrams that can't be fragmented
Quite a bit really, perhaps Brian should drop it down to 1006. On my
workstation at work I get the following, although not much traffic has
occured since the last boot:
ip:
281116 total packets received
0 bad header checksums
0 with size smaller than minimum
79 with data size < data length
0 with header length < data size
0 with data length < header length
0 with bad options
0 with incorrect version number
0 fragments received
0 fragments dropped (dup or out of space)
0 fragments dropped after timeout
0 packets reassembled ok
276621 packets for this host
335 packets for unknown/unsupported protocol
0 packets forwarded
4081 packets not forwardable
0 redirects sent
702701 packets sent from this host
0 packets sent with fabricated ip header
0 output packets dropped due to no bufs, etc.
0 output packets discarded due to no route
0 output datagrams fragmented
0 fragments created
0 datagrams that can't be fragmented
Andrew
-----------------------------------------------------------------------
On Sun, 24 Mar 1996, Louis A. Mamakos wrote:
>
> There is much confusion evident in the email message.
>
> First, the 576 byte packet size mentioned is the *minimum* MTU which
> is "required" to be supported. Or something like that. In practice,
> most every link level encapsulation in use supports a 1500 byte MTU.
> The big exception to this these days is SLIP/PPP where a much smaller
> MTU is used to prevent interactive traffic from being "stuck" behind
> really large packets and screwing the echo response.
>
> Second, any fragmentation which would occur would happen in a IP
> router, which has an IP datagram it needs to transmit. If the
> interface which the packet needs to be forwarded over has an MTU
> smaller than the packet, then the fragmentation function occurs at
> that point. That is, unless the "don't fragment" bit is set. It's
> somewhat likely that it may be set, as the TCP in FreeBSD does MTU
> path discovery to "feel" the MTU along the path from the sender to the
> recpient's TCP. This is done by setting "Don't Fragment", and looking
> for ICMP messages coming back. When the path's MTU is discovered, the
> TCP can arrange to not send TCP segments larger than the MTU.
>
> Third, most long-haul links on the Internet today likely have 4K MTUs,
> which is usually used on FDDI and HSSI (to connect to DS3 links)
> interfaces. So, it's pretty unlikely that ethernet MTU sizes packets
> would be a problem.
>
> Fourth, you can verify that there's actually dropped fragments
> happening by looking on the receiving machine and doing a 'netstat -s'
> to look for the number of fragments received, as well as the number
> dropped due to reassembly timeouts. I suspect that the number will be
> 0.
>
> I don't think that you actually have a problem, or anything that you
> need to change..
>
> louie
>
>
> > Anyone know what this guy is saying? I figured fragmentation and
> > reassembly would happen between the FTP server's Ethernet interface
> > and that of the router to the Internet. Is there any validity to this
> > guy's suggestion?
>
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