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FS#1999 - Support MPDU/A-MSDU (jumbo frames) on supported WiFi hardware #6885

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openwrt-bot opened this issue Dec 13, 2018 · 4 comments
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openwrt-bot commented Dec 13, 2018

n8v8R:

  • Device problem occurs on
  • Turris Omnia ARMv7 Processor rev 1 (v7l)
  • Software versions of OpenWrt/LEDE release, packages, etc.
  • Chaos Calmer r47055 | target:mvebu/generic | description:OpenWrt omnia 15.05
  • Steps to reproduce
/etc/config/network config device option name wlan option mtu 2305

/etc/config/wireless
config wifi-iface
option device 'radio0'
option ifname 'wlan'

Start the network and the mtu remains at 1500. Lowering the setting to 2304 and it is accepted.

According to my understanding from reading various publications, and I take this one as sample reference https://www.oreilly.com/library/view/80211ac-a-survival/9781449357702/ch03.html, 802.11n and 802.11ac support jumbo frames through MPDU (MAC frame) size and A-MSDU (aggregate MAC payload) respectively.

Thus I would trust it would benefit WLAN performance, particularly when also deploying the kernel's MTU probing feature (net.ipv4.tcp_mtu_probing = 1).

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openwrt-bot commented Dec 14, 2018

jow-:

This is a kernel limitation:

  • mac80211/iface.c: ''ndev->max_mtu = IEEE80211_MAX_DATA_LEN;''
  • linux/ieee80211.h: ''#define IEEE80211_MAX_DATA_LEN 2304''

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openwrt-bot commented Jan 5, 2019

n8v8R:

That has been extended for freq > 40Ghz

https://github.com/torvalds/linux/blob/master/include/linux/ieee80211.h

/* 802.11ad extends maximum MSDU size for DMG (freq > 40Ghz) networks

  • to 7920 bytes, see 8.2.3 General frame format
    /
    #define IEEE80211_MAX_DATA_LEN_DMG 7920
    /
    30 byte 4 addr hdr, 2 byte QoS, 2304 byte MSDU, 12 byte crypt, 4 byte FCS */

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openwrt-bot commented Jan 13, 2019

n8v8R:

Despite the available support in the kernel still won't fix considering the cited reason not being valid?

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openwrt-bot commented Jan 14, 2019

jogo:

The extended MSDU size is only available on 802.11ad devices, it doesn't affect any 2.4/5 GHz devices. Those are still limited to 2304 bytes.

And if you use a 802.11ad device (or more specifically a wil6210 device), the kernel will allow the higher MTU.

Furthermore neither A-MPDU nor A-MSDU allow you sending larger frames, these are just means of sending multiple frames in one transmission. This is already done by the kernel if supported by the hardware.

So the correct resolution would be "not a bug", not "won't fix".

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