[sclug] Bizarre IMAP problem

[Roland Turner SCLUG]

Alex Butcher wrote:


> On Thu, 25 Aug 2005, Roland Turner (SCLUG) wrote:
...
>> without attribution. The google search "site:ietf.org mtu 576" yields
>> _no_
>> hits which suggests that, if it is a standard, it's certainly not a
>> formal
>> one.
>
> It is as formal as any other Internet standard:
>
> "All hosts must be prepared to accept datagrams of up to 576 octets
> (whether
>   they arrive whole or in fragments).  It is recommended that hosts only
>   send datagrams larger than 576 octets if they have assurance that the
>   destination is prepared to accept the larger datagrams."
>   - RFC791, Internet Protocol


I've just noticed that I mistyped "site:ietf.org" as "sit:ietf.org". Memo
to Google - error messages are sometimes helpful! (0 hits really did seem
a little low.)


Thanks for the quote. I'd forgotten about this, but do note that it is
about the minimum datagram size that hosts/routers are required to be able
to deal with, not the minimum fragment size that links are required to
carry. A complying link could still exist with, say, a 100 byte MTU, so
long as the devices on each end of that link were still willing to receive
576 byte datagrams (and chop them into seven or eight fragments;
reassembly can still be someone else's problem).


The bearing that this has on Darren's problem is minimal though. If it's a
PMTU discovery problem, then there's a need to get


   MSS <= PMTU - headers

where, as per RFC791, 576 bytes is _not_ a lower bound for PMTU. We do
know, however, that his WiFi link works, which suggests that if this is a
PMTU problem, then it is (fortunately) well above the 1000 byte mark.


- Raz