Subject: (ngtrans) Shipworm path MTU

From: Erik Nordmark <[email-masked]>
Date: Thu, 31 Jan 2002 14:28:08 +0100 (CET)

4.1.3	Maximum Transmission Unit

Since Shipworm uses UDP as an underlying transport, a Shipworm
Maximum Transfer Unit (MTU) could potentially be as large as the 
payload of the largest valid UDP datagram (65527 bytes).  However, 
since Shipworm packets can travel unto unpredictable path over the 
Internet, it is best to contain this MTU to a small size, in order 
to minimize the effect of packet fragmentation and reassembly. The 
default link MTU assumed by a host, and the link MTU supplied by a 
Shipworm server during Router Advertisement SHOULD normally be set 
to the minimum IPv6 MTU size of 1280 bytes [RFC2460].

---

The above recommendation is presumably a sound one but I'm not sure
about the preceeding argument.
Do NAT boxes handle fragmented UDP packets?
(Since there are no port numbers in but the first fragment that would require 
that the NAT box create temporary state for each fragment chain in order
to know what IP address translation to apply.)

   Erik

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From: "Christian Huitema" <[email-masked]>
Date: Thu, 31 Jan 2002 16:09:00 -0800

> to know what IP address translation to apply.)

YMMV. Some NAT do reassemble packets before forwarding; the most stupid
ones just send the first fragment and drop the next ones. This is not
much of an issue in broadband deployments, as the link MTU for DSL or
Cable is typically 1500 bytes, minus some overhead when PPPoE is used.
This will be an issue if you stick a "stupid" NAT behind a dial-up line;
this is hopefully a very rare configuration.

-- Christian Huitema