On 7 Jun 2001, at 18:13, Zachary Uram wrote:
> what is an MTU?
>
> Minimum Time Up?
Maximum Transmission Unit. A parameter to the outbound portion of
the protocol stack, it dictates the largest packets that can be sent.
[In *implementation*, it's a parameter to the encapsulate-and-send
code. In *theory*, it's a characteristic of the virtual circuit;
odds of errors requiring retransmission rise with packet size, so
there is some optimal trade-off between error rate and per-packet
overhead.]
Typical issue is that some intermediate box -- in this case, the
one that had had AOL on it -- is receiving packets that are much
larger than its MTU setting. So every received packet that it wants
to forward needs to be broken into smaller packets, and while it's
sending the first, it has to put the others somewhere. When it has
nowhere to put them, things start to break.
It can ask the sender to send smaller packets, but that request may
not be honoured. The large packets may have the DF (Don't Fragment)
flag set, requesting that it not break them up this way. The code to
break them up could be broken, or the code to put them back together
could be -- as I recall, part of the issue with the Ping of Death was
that many implementors never expected their code to have to
reassemble a fragmented ICMP packet.
David Gillett
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