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Topic: Chaos Calmer trunk@43620 : bug involving Wifi and MTU ?

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Post #1
ambrosa
14 Dec 2014, 17:49

Hi guys.
About 2 weeks ago I've bought a TP-LINK W8970 (Lantiq -> XRX200 architecture) 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)
A friend of mine follows my steps and also he has bought a W8970 and installed my OpenWrt image.

I and my friend have very strange issue with interfaces MTU. We suspect a bug in kernel 3.14 or Chaos Calmer or our misconfiguration.
Before opening a support ticket, we want open a discussion here.

Preface:
1) our data provider use ADSL with PPPoE protocol (wan/nas0 has MTU=1492)
2) all other network interfaces have MTU=1500
3) in our internal LAN we 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 our internal LAN we have some clients : some PC (wired), some notebook (wifi), some smartphone (wifi)
5) all internal servers and clients have 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 many TCP packet loss and after some debug we have discovered that our internal LAN servers (wired) need to have MTU=1492 or Wifi devices need to have MTU=1492
If we leave default MTU=1500 in servers (wired) or wifi devices, many TCP packet '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 us.
And sound strange that MTU=1492 is the same as PPPOE wan connection. Is it a chance or there is some very strange issue involving wan connection , mtu and wifi ?

Any idea ?

Thanks

(Last edited by ambrosa on 14 Dec 2014, 17:52)

Post #2
ambrosa
14 Dec 2014, 18:15

Obviously with original TP-LINK firmware and with another router (Asus DSL55) this issue is NOT present.
So it's OpenWrt related.

(Last edited by ambrosa on 14 Dec 2014, 19:16)

Post #3
ambrosa
15 Dec 2014, 18:59

Many people are confirming thi bug.

I've raised one by one the MTU : I've discovered that with MTU <= 1496 it works.
With MTU >= 1497   WIFI-LAN connection fail.

We also have checked that with Barrier Braker this issue is NOT present.

The discussion might have continued from here.