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0 pointspclmulqdq2y ago0 comments

This is probably more accurately called IP fragmentation (since that is the layer where the fragmentation happens), and a lot of companies make it optional to support in networking gear. I'm surprised that you are using it or seeing it, because it is essentially obsolete today.

It has a legitimate purpose in old-timey systems which have bespoke MTUs on each link, but now the usual thing is to use 1500 bytes for WAN traffic, which is the generic Ethernet MTU, and reserve larger sizes for intra-datacenter communications.

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colmmacc2y ago

There's a number of UDP protocols that have large enough payloads to fragment. DNSSEC and EDNS0 in particular made it much more common, though the EDNS0 flag day in 2020 partially undid some of the damage by getting folks to ratchet down their EDNS0 buffer sizes.

1500 is absolutely not a pervasively usable WAN MTU, you're going to need pMTUd if you're sending 1500 byte packets broadly. Plenty of WAN links won't tolerate it. If you don't want to deal with fragmentation at all ... 500 is the minimum guaranteed MTU, but in practice it's exceptionally rare to see anything below about 1200 require fragmentation. But you can always only control what you send, not what others are sending you.

tptacek2y ago

One thing I've learned since joining Fly.io in 2020 is to laugh when people point to the 1500 MTU. You absolutely can't count on that: IPv6 cuts into it, and so does every additional layer of encapsulation on your path.

pclmulqdqOP2y ago

Yeah, you have to account for the headers in the 1500 byte MTU, which I suppose can be substantial if you have several VLAN tags, IPSec, IPv6, and a bunch of IP options. Presumably most of that encapsulation happens inside a datacenter, though, where you can use jumbo frames.

pantalaimon2y ago

With IPv6 only the endpoint can fragment, not any hop in between.

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