MTU

Posted Jun 25, 2018 9:16 UTC (Mon) by farnz (subscriber, #17727)
In reply to: MTU by meuh
Parent article: TCP small queues and WiFi aggregation — a war story

Nope; the answer is IEE 802.3 Ethernet. WiFi (IEEE 802.11) is designed to transparently interoperate with 802.3 Ethernets. The IEEE has declared that the Ethernet MTU is fixed at 1500 bytes[1]; this implies that WiFi per-frame MTUs are also fixed at 1500 bytes. Given that it is a hard requirement for WiFi that the frame MTU is no more than 1500 bytes, you need things like aggregation to get a decent speed.

If larger frames were permitted on 802.11, then you would not be able to bridge 802.11 with IEEE standard 802.3; while it's common to support jumbo frames on Ethernet, this is technically a non-standard extension, and IEEE standard 802.11 can't assume that any Ethernet it is connected to will permit jumbo frames.

[1] While the IEEE 802.3 MTU is 1500 bytes, they also now require all equipment to handle frames of up to 2000 bytes in total size, to allow for headers, checksums, VLAN tags etc. WiFi is similar - 2304 byte maximum MSDU frame, of which 1500 bytes maximum is user MTU, and the other 804 bytes are reserved for VLAN tags etc.

MTU

Posted Jun 25, 2018 16:01 UTC (Mon) by raven667 (subscriber, #5198) [Link] (3 responses)

That's what I first though about this but the issue is not that you can't have jumbo 9000 byte MTU on the local layer2 link, either Ethernet or WiFi (although I think WiFi standard only supports 1500), which would be negotiated between endpoints as part of the TCP MSS, it's that intervening links over the Internet at some point are likely to only permit 1500 byte frames, so somewhere along the line you would need to fragment the layer3 packets, which is not allowed for IPv6 which relies on PMTU discovery to find the correct MSS/MTU that can cleanly make it through the whole path, leading to needing aggregation on the first WiFi hop for efficiency, to make up for not requiring upstream routers to fragment/reassemble. You could probably sidestep this if you had a jumbo-clean path, and you weren't concerned about stations monopolizing airtime with large frames, but that's unlikely unless you control the whole infrastructure between endpoints.

MTU

Posted Jun 25, 2018 16:20 UTC (Mon) by farnz (subscriber, #17727) [Link] (2 responses)

Nope. The issue is that you cannot have an MTU above 1500 on Ethernet without breaking the IEEE specs for Ethernet and for WiFi. You are simply not allowed a jumbo MTU on the Layer 2 link, and the IEEE won't accept changes to 802 series standards that increase the user MTU beyond 1500.

IPv6 is not relevant here - it's an IEEE decision because even in IPv4, with router fragmentation allowed, the IEEE doesn't like it.

MTU

Posted Jun 25, 2018 17:28 UTC (Mon) by raven667 (subscriber, #5198) [Link] (1 responses)

That seems bogus, I'm pretty sure I have many pieces of equipment using jumbo frames, whatever IEEE has in their written specs or whatever they "like".

MTU

Posted Jun 25, 2018 17:40 UTC (Mon) by farnz (subscriber, #17727) [Link]

You do, but they're not using IEEE standard Ethernet (jumbo frames implies not IEEE standard) - and WiFi standards (including frame aggregation) are written to get high performance when using IEEE standard Ethernet.

Hence frame aggregation rather than high MTUs - a high MTU for performance means being outside the IEEE standard, while a 1500 MTU allows you to be inside the standard.