Messages in this thread | | | From | "DuBuisson, Thomas" <> | Subject | RE: IPsec packet dropping due to MTU problem (was RE: XFRM / DF F lag / Fragmentation Needed) | Date | Wed, 17 Nov 2004 16:16:53 -0500 |
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>> After A establishes an SSH connection with C and tries to transfer the >> patches the size of a packet from A destined for C quickly reaches 1500 >> while the MTU >> to A is ~1400. At this point A sends an ICMP 'Fragmentation Needed' >> packet to its self (see xfrm_output.c xfrm4_tunnel_check_size(...)). It >> seems this packet is never acted on - it just disappears into the >> loopback interface. The proper mtu trial/error process never takes >> place. >There is a known problem in xfrm4_tunnel_check_size if your underlying >path MTU is a multiple of 8. So if your path MTU is 1480, you'll need >to lower it to 1476 before it will work. The problem occurs even when the PMTU is any value, 1476 would be one example. While I don't quite understand the xfrm4_tunnel_check_size bug by looking at the code but I have instrumented the function and don't see any behavior I disagree with in the tested cases.
It seems to me that this problem is more with Linux ignoring the received ICMP error. ICMP messages from a host to itself, imh observation, aren't properly handled and that is where the problem I have encountered is coming from.
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