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 Post subject: traffic halt on tunnel when any traffic sent by client
PostPosted: Sat May 12, 2012 3:01 am 

Joined: Wed May 09, 2012 10:34 pm
Posts: 2
Hi, this is a strange situation and I need your help to proceed.

We're porting openl2tpd (v1.6) to an embedded system running kernel 2.6.30, with PPPd version 2.4.5. The router has only one wan port which is uplink to ISP via DSL/PPPoE (always assigned interface "ppp2"). Remote openl2tp client connects to server and is assigned "ppp0" for the connected tunnel on the server. Once connected, the default route for the client is the server's ppp0 IP and ALL traffic is directed to the server.

The strange thing is: when client ping an Internet host, e.g. http://www.google.com, then router "intermittently" drops packet and in most cases, it eventually halts (not rx or tx) all traffic from client after random time from 5 to 20 minutes. I could not find a clue about what is triggering this to happen.

We found at least two unusual things during debugging:
(1) On server, tcpdump "occasionally" captures "truncated" packet like this:
"19:23:04.067133 IP 75.36.153.47.1701 > 68.65.79.106.38306: l2tp:[] 6252/30753) {IP truncated-ip - 38 bytes missing! 192.168.1.3.54627 > 10.7.1.2.60577: Flags [.], seq 3306127340:3306128780, ack 319099951, win 91, options [nop,nop,TS val 1945320155 ecr 1590286527], length 1440}

After decoding the packet, it seems the encapsulating headers (IP+UDP+L2TP+PPP, total 38 bytes) are trimmed when passing for tcpdump's l2tp print. Note this happens only "occasionally" and we don't know if this is related to the packet drop issue.

(2) In very few cases that ping can get through successfully for 5 minutes, then we run ftp to download large sized files from Internet. usually it is halted again; but while it runs fine, we noticed that the xfer rate from the l2tp client is only half of the router's locally connected host downloading from the same site (the local host does not go through l2tp tunnel).

We have PPTP server on the system so we then try that in the same network topology as for l2tp, for comparison reason, and it went perfectly fine. No issue as mentioned above. Here's the topology.

l2tp client ------- INTERNET ----- (DSL/ppp2) Router ----- local host
ppp0:10.7.1.2 <====================> ppp0:10.7.1.1
default route (l2tp tunnel)
=10.7.1.1

The traffic flow from client to Internet is like this:
client ping -> 10.7.1.2 --(l2tp tunnel)-> 10.7.1.1 -> Router ppp2/pppoe -> google.com -> google reply -> Router ppp2/pppoe -> 10.7.1.1 -> 10.7.1.2 -> client

Since pptp server/client works fine in the same topology, that makes me think maybe pppol2tp kernel driver has something to do with this?

Could someone help me how to proceed from here?

Thanks in advance!

Peter


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 Post subject: Re: traffic halt on tunnel when any traffic sent by client
PostPosted: Fri Aug 03, 2012 12:41 pm 
Site Admin

Joined: Sun Jul 27, 2008 1:39 pm
Posts: 122
Hi, sorry for my late reply.
mwrtest10 wrote:
(1) On server, tcpdump "occasionally" captures "truncated" packet like this:
"19:23:04.067133 IP 75.36.153.47.1701 > 68.65.79.106.38306: l2tp:[] 6252/30753) {IP truncated-ip - 38 bytes missing! 192.168.1.3.54627 > 10.7.1.2.60577: Flags [.], seq 3306127340:3306128780, ack 319099951, win 91, options [nop,nop,TS val 1945320155 ecr 1590286527], length 1440}

After decoding the packet, it seems the encapsulating headers (IP+UDP+L2TP+PPP, total 38 bytes) are trimmed when passing for tcpdump's l2tp print. Note this happens only "occasionally" and we don't know if this is related to the packet drop issue.

How are you running tcpdump? What interface is it capturing?

Make sure that the ppp mtu is 38 bytes less than the MTU of the tunnel interface.


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