#18590: MTU issue involving bridge or VLAN ?
-------------------------+----------------------------------
Reporter: ambrosa | Owner: developers
Type: defect | Status: new
Priority: high | Milestone: Chaos Calmer (trunk)
Component: base system | Version: Trunk
Keywords: |
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Chaos Calmer trunk@43620
Device TP-LINK W8970 (arch. LANTIQ -> XRX200)
This issue is reported by many people (see this italian forum
http://www.hwupgrade.it/forum/showthread.php?t=2643260&page=70 )
This issue is not present with original TP-LINK firmware, is not present
with Barrier Braker, is not present using another modem-router in our
setup (i.e. Asus DSL55)
Preface:
About 2 weeks ago I've bought a TP-LINK W8970 and I've installed OpenWrt
without any problem. I've compiled myself some Chaos Calmer trunk
versions. Latest installed in my W8970 is trunk@43620 (kernel 3.14)
I have very strange issue with interfaces MTU and it looks to be
introduced with Chaos Calmer.
My network (like other people affected with the issue):
1) my data provider use ADSL with PPPoE protocol (wan has MTU=1492)
2) all other W8970 network interfaces have MTU=1500
3) in my internal LAN I have some servers: a small ARM http server
(http://cubieboard.org) with Debian Wheezy, some Set-Top-Box (DVB-S2
satellite receiver) Linux based and others. All servers are connected
"wired"
4) in my internal LAN I have some clients : some PC (wired), some notebook
(wifi), some smartphone (wifi)
5) all internal servers and clients have default MTU=1500
This is my W8970 ifconfig output:
{{{
br-lan Link encap:Ethernet HWaddr E8:DE:xx:xx:xx:14
inet addr:192.168.1.1 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:627265 errors:0 dropped:0 overruns:0 frame:0
TX packets:920637 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:86104574 (82.1 MiB) TX bytes:981600054 (936.1 MiB)
eth0 Link encap:Ethernet HWaddr E8:DE:xx:xx:xx:14
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:522645 errors:0 dropped:1 overruns:0 frame:0
TX packets:695165 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:91408915 (87.1 MiB) TX bytes:738973973 (704.7 MiB)
eth0.1 Link encap:Ethernet HWaddr E8:DE:xx:xx:xx:14
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:522614 errors:0 dropped:0 overruns:0 frame:0
TX packets:695158 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:77818404 (74.2 MiB) TX bytes:733262952 (699.2 MiB)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:3251 errors:0 dropped:0 overruns:0 frame:0
TX packets:3251 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:264440 (258.2 KiB) TX bytes:264440 (258.2 KiB)
nas0 Link encap:Ethernet HWaddr E8:DE:xx:xx:xx:15
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:904195 errors:0 dropped:0 overruns:0 frame:0
TX packets:662954 errors:2082 dropped:0 overruns:2082 carrier:0
collisions:0 txqueuelen:1000
RX bytes:933036803 (889.8 MiB) TX bytes:94763776 (90.3 MiB)
pppoe-wan Link encap:Point-to-Point Protocol
inet addr:151.xx.xx.175 P-t-P:151.xx.xx.72
Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1492 Metric:1
RX packets:810196 errors:0 dropped:0 overruns:0 frame:0
TX packets:568938 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:922230793 (879.5 MiB) TX bytes:72797166 (69.4 MiB)
wlan0 Link encap:Ethernet HWaddr E8:DE:xx:xx:xx:14
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:110950 errors:0 dropped:0 overruns:0 frame:0
TX packets:205437 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:11165927 (10.6 MiB) TX bytes:232462453 (221.6 MiB)
}}}
'''Problem:'''
a) all communications between LAN (wifi or wired) <==> WAN are perfect
b) all communications between LAN wired <==> LAN wired are perfect
'''c) all communications between LAN wifi <==> LAN wired *FAIL* (tested
with notebook-wifi and smartphone-wifi)'''
There are many TCP packets loss and after some debug we have discovered
that internal LAN servers (wired) need to have MTU=1496 (or less) or Wifi
devices need to have MTU=1496 (or less)
If I use default MTU=1500 in servers (wired) and/or wifi devices, many TCP
packets 'disappear' during communication. As TCP fragmentation is not
handled correctly.
WiFi interface dislike MTU=1500 and this is a nonsense: W8970 interfaces
(wifi too) have MTU=1500 so all should be fine.
We have no idea why this happen and it's sound very strange for me.
4 bytes (1500-1496) are the overhead due to 802.1Q (VLAN): is the
VLAN/bridge the problem origin ?
Thanks
--
Ticket URL: <https://dev.openwrt.org/ticket/18590>
OpenWrt <http://openwrt.org>
Opensource Wireless Router Technology
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