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Author Topic: MTU issue  (Read 2633 times)

lfourquaux

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MTU issue
« on: June 04, 2010, 12:48:49 PM »

Since the recent tunnel server upgrade, I'm seeing dropped packets on my IPv6 tunnel.  I have a tentative explanation (below) and would appreciate if some people with the same kind of network setup could check if it also happen for them.

The problem happens when the MTU of the IPv4 connection is < 1500, e.g. if your ISP uses PPPoE.  If I execute
Code: [Select]
(Unix) ping6 -s 1432 <insert here an IPv6 address on your network>
(Windows) ping -6 -l 1432 <insert here an IPv6 address on your network>
on a remote computer (with IPv6 MTU>=1480), the packets never get to my network.  On the other hand, pinging with small packets does work.

It looks like the tunnel broker server has MTU=1480, which makes it possible to send packets with up to 1500 bytes, and that it sets the DF flag on all IPv4 packets.  If the IPv4 path has MTU < 1500, the larger packets get dropped.
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Ninho

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Re: MTU issue
« Reply #1 on: June 06, 2010, 04:38:18 AM »

IPv6 ping from http://www.subnetonline.com/pages/ipv6-network-tools/online-ipv6-ping.php
to my end point of the IPv6 tunnel using exactly 1480 bytes (the max they support) works :)

I'm on pppoA though, not pppoE (if your ISP allows you to choose, pppoA is /much/ better! Indeed pppoE may be slightly advantageous to the provider, but sure it is disadvantageous to the subscriber)

Just my 2 cents... Cheers !

___________________________________________________________________
IPv6 Ping Output:

PING 2001:470:xxxx:xxxx::2(2001:470:xxxx:xxxx::2) 1480 data bytes
1488 bytes from 2001:470:xxxx:xxxx::2: icmp_seq=0 ttl=123 time=135 ms
1488 bytes from 2001:470:xxxx:xxxx::2: icmp_seq=1 ttl=123 time=141 ms
(... + success lines suppressed)
1488 bytes from 2001:470:xxxx:xxxx::2: icmp_seq=7 ttl=123 time=135 ms

--- 2001:470:xxxx:xxxx::2 ping statistics ---
8 packets transmitted, 8 received, 0% packet loss, time 6997ms
rtt min/avg/max/mdev = 134.046/135.641/141.004/2.105 ms, pipe 2


« Last Edit: June 06, 2010, 04:39:53 AM by Ninho »
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lfourquaux

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Re: MTU issue
« Reply #2 on: June 06, 2010, 06:51:24 AM »

Thanks for testing!

I'm on pppoA though, not pppoE

If you are on PPPoA, it will work, since your IPv4 MTU is 1500.

(if your ISP allows you to choose, pppoA is /much/ better! Indeed pppoE may be slightly advantageous to the provider, but sure it is disadvantageous to the subscriber)

My ISP (which is Orange, the same as yours IIRC) is (slowly) moving from an ATM infrastructure to an Ethernet one.  On the new DSLAM/BAS, PPPoA is not really supported.  Sometimes it works but is re-encapsulated into PPPoE at the DSLAM, and the are reports that it does not work at all in some places.  Moreover, PPPoA won't be available for FTTH (coming "soon" in some areas).

Thank you for the workaround, though.  I had not thought of this one.

Regards,

L.
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Ninho

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Re: MTU issue
« Reply #3 on: June 06, 2010, 09:57:10 AM »

If you are on PPPoA, it will work, since your IPv4 MTU is 1500.

Indeed, this is why I love PPPoA over PPPoE. One level less of useless encapsulation for a start! and no problems with services failing unexplainably.

My ISP (which is Orange, the same as yours IIRC) is (slowly) moving from an ATM infrastructure to an Ethernet one.  ... Moreover, PPPoA won't be available for FTTH (coming "soon" in some areas).

Hopefully they're not "upgrading" the ADSL infrastructure in my remote whereabouts soon ;=) And fiber-to-the-farm is not going to happen in my lifetime I guess.

Quote
Thank you for the workaround, though.  I had not thought of this one.

You're very welcome...


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