Re: MTU and 2.4.x kernel

From: Alan Cox (
Date: Thu Feb 15 2001 - 16:01:21 EST

> > I ran DNS reliably over AX.25 networks. They have an MTU of 216. They work.
> 512 is maximal message size, which is transmitted without troubles,
> hardwired to almost all the datagram protocols.

Message size != MTU. DNS doesnt use DF. In fact DNS can even fall back to

> > > B. Accoutning, classification, resource reervation does not work on
> > > fragmented packets.
> > Thats a bug in accounting classification and resource reservation.
> Sorry? It is bug in client mtu selection. Functions above are impossible
> on fragmented packet even in theory. And because of A, if client uses mtu
> 296, it cannot use 100% of emerging and existing IP functions.

Tragic. You are required to accept existing realities and degrade nicely.

> > Over a 9600 mobile phone link mtu 296 makes measurable differences to the
> > latency when mixing a mail fetch with typing.
> It is myth. Changing mtu until ~4K does not affect latency, it stays on 4K/bw.

Please tell that to my phone.

> > Over a radio link where
> > error rate causes exponential increases in probability of packet loss as
> Another myth. All they do error correction and have so high latency,
> that _increasing_ mtu only helps. And helps a lot.

No. There is large amounts of real world hardware that this is not true for.
You cannot do good FEC on a narrow band link.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

This archive was generated by hypermail 2b29 : Thu Feb 15 2001 - 21:00:27 EST