On Jul 28, 2022, at 12:24, Andrew McConachie <and...@depht.com> wrote:
> PMTUD doesn’t work through NAT
That's a very definitive statement considering that there's no useful standard
for NAT.
If there's actual research on this to demonstrate that, pragmatically speaking,
no implementations use the payload of a type 3 code 4 ICMP message to identify
a translated target for the packet I would like to read it, because that sounds
interesting.
>> Currently, DNS is known to be the largest
>> user of IP fragmentation.
>
> Compared to what? I would just drop this sentence because it doesn’t add
> anything to the document and it’s trying to make a point that doesn’t need to
> be made.
I'd also like to see a citation for this one if there has been a study. I agree
that it's probably the most familiar example of fragmentation for an audience
mainly preoccupied with the DNS, but that's probably not a helpful observation
:-)
> Before I was interested in the DNS I worked for an ethernet switch vendor for
> 8 years, and I often find the way MTU gets talked about in IETF documents
> simply weird.
RFC 791 introduces the term "maximum transmission unit" to be the maximum size
of a datagram, not the maximum size of a frame whose payload is a datagram.
The maximum sized datagram that can be transmitted through the
next network is called the maximum transmission unit (MTU).
> MTU is a measurement of maximum frame size for a network segment starting at
> Layer 2.
I have also heard MTU used in that way. I have always assumed it was just
sloppy writing.
There may be prior use of the phrase that I'm not aware of (prior to 1981) but
even if that's the case I think it's reasonable to use the IETF definition of
the phrase in the IETF.
I think Ethernet was not standardised until the publication of IEEE 802.3 in
1983. I also think the original specification did not anticipate switches but
described a multi-access network with a broader collision domain.
So perhaps it's reasonable to say that the IETF use of MTU pre-dates Ethernet
switch vendors' usage, since it pre-dates Ethernet switches, since it pre-dates
Ethernet.
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