MTU = 1280 everywhere? / QUIC

Jeroen Massar jeroen at
Tue Nov 11 11:05:40 CET 2014

On 2014-11-11 10:55, Vincent Bernat wrote:
>  ❦ 11 novembre 2014 10:42 +0100, Jeroen Massar <jeroen at> :
>> From:
>> "UDP PACKET FRAGMENTATION" but IPv6 dos not fragment...
> IPv6 routers don't fragment but IPv6 hosts still do.

Correct. But that means if you are sending 1350 bytes on a 1280 link you
are sending two packets, not one.

As they do cool stuff like FEC in QUIC, they assume lossy networks (good
thing they think that way), but that also means that you will be sending
more data (due to FEC) and also assume you are losing packets.

Hence, if your FEC protocol assumes that 1 packet is lost while actually
only half the packet was, you got more loss than you are anticipating.

Knowing what the MTU is on the link, thus is a smart thing.

Hence, why PMTUD is important.

Also, fragments are evil and there is no real reason to have any
fragments at all.


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