Subject: [pmtud] The numbers 1280 and 1380

From: Matthew J Zekauskas <[email-masked]>
Date: Wed, 27 Aug 2003 01:05:14 -0400

I asked Dave Thaler where these numbers came from, and here's
his reply.  Forwarded with permission.

--Matt

---------- Forwarded Message ----------
Date: Tuesday, August 26, 2003 4:17 PM -0700
From: Dave Thaler <[email-masked]>
To: Matthew J Zekauskas <[email-masked]>
Cc: Erik Nordmark <[email-masked]>
Subject: RE: pmtud comments...

Matthew writes:
> I'm curious (and someone actually asked on the pmtud list) where
> the 1280/1380 numbers come from.

RFC 2460, section 5:
>  IPv6 requires that every link in the internet have an MTU of 1280
>  octets or greater.

1380 comes from the fact that a widely-deployed L2TP tunnel MTU is 1400.
So you end up with 1400 - IPv4 header size = 1380 bytes available for
the IPv4 payload.  (Aside: the number 1400 I believe was originally
obtained from Ethernet MTU - space for IPv4+AH+ESP+UDP+L2TP+PPP.)
Hence, when IPv6 is tunneled inside this tunnel (e.g. using ISATAP or
whatever other IPv6-in-IPv4 encapsulation method), the link MTU of that
tunnel could be anything 1280 or higher.  However, once you go over
1380, you'll cause fragmentation when going over an underlying link with
an MTU of 1400.

In a separate scenario, you have Mobile IPv6 in use where you're
tunneling
over IPv6.  If the MTU of the underlying IPv6 link is small (like 1280)
then you'll force Mobile IPv6 to be doing fragmentation when it
encapsulates, since the MTU of its tunnel has to be at least 1280, which
doesn't leave room for the encapsulation overhead.

Hence, when discussing a "default" tunnel MTU, anything over 1380 causes
gratuitous fragmentation in one scenario we expect to be common.
Anything much below 1380 will cause gratuitous fragmentation in a
different
scenario (Mobile IPv6).  And of course lower MTUs result in more
overhead to get the same throughput.

This is why I recommend the value 1380 as the best default tunnel MTU.
Those who argue to just say 1280 are essentially saying they don't care
about gratuitous fragmentation caused by Mobile IPv6 over the tunnel.

-Dave

P.S. I think I'm not yet on the PMTUD list, so feel free to forward this
if you want.

---------- End Forwarded Message ----------

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