So you think you understand IP fragmentation?

Posted Feb 9, 2024 16:05 UTC (Fri) by paulj (subscriber, #341)
In reply to: So you think you understand IP fragmentation? by intelfx
Parent article: So you think you understand IP fragmentation?

Nearly all equipment has supported at least 9k MTU for a long while. The problem now is the network protocols we use have no reliable way to figure out how to use larger MTUs.

See my blog link in another comment in this article. It has quotes from an early paper on TCPIP from Kahn and Cerf explaining why it is important to have a reliable network mechanism to allow different MTU networks to inter-op. Unfortunately, we - collectively - failed to heed their wise words.

A reliable mechanism needs to be in-band. E.g., data-plane fragmentation. Side-band end-host solutions - i.e., relying on ICMP messages - have proven to be fragile. Pure end-host probing (i.e. Path-MTU Discovery, in protocol or out) is also inefficient, temporally unreliable, and fragile.

So you think you understand IP fragmentation?

Posted Feb 9, 2024 17:00 UTC (Fri) by Cyberax (✭ supporter ✭, #52523) [Link] (1 responses)

> Nearly all equipment has supported at least 9k MTU for a long while

Except for WiFi. Its PHY MTU is just 2300 bytes.

So you think you understand IP fragmentation?

Posted Feb 9, 2024 17:40 UTC (Fri) by farnz (subscriber, #17727) [Link]

That's the MAC MTU; the PHY MTU can be as large as 2,097,148 bytes in 802.11ac networks (noting that the PHY MTU depends both on static parameters like channel width, but also dynamic parameters like time it will take to transmit the frame). For 802.11ax, the PHY MTU is permitted to go as high as 6,500,631 bytes. Even as early as 802.11n (in 2009), the PHY MTU was allowed to go as large as 65,536 bytes under good conditions.

This is made useful with a much smaller MAC MTU by having aggregation options, so that a single PHY frame contains many MAC frames; the downside is that there is overhead for each and every MAC frame in the PHY frame, which would go down if the MAC frames were larger. There would still be overhead mapping MAC frames into PHY frames, so you wouldn't have as large a MAC MTU as the PHY MTUs, but there would be large MTUs involved.