netdev - ipsec and snat: mtu question

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Date:	Thu, 14 Jun 2012 15:48:02 +0200
From:	Marco Berizzi <pupilla@...mail.com>
To:	<netdev@...r.kernel.org>
Subject: ipsec and snat: mtu question

Hello everybody.

Kindly, I would like to ask for explanations
about a linux ipsec gateway snatting packets.

Here is the network schema.

customer private network 10.16.0.0/16
|
|
+ipsec customer gateway (checkpoint)
||
||---ipsec tunnel 10.16.0.0/16<->172.16.128.0/28 (des3/md5)
||   mtu=1446
||
++ linux_gw_snat ipsec gateway (SNAT all packets from 172.22.1.0/24 to 172.16.128.1)
|| 
||---ipsec tunnel 10.16.0.0/16<->172.22.1.0/24 (aes/sha1/ipcomp)
||   mtu=1430
||
+linux_final ipsec gateway
|
|
client 172.22.1.50

SYN packet start behind the linux_final (172.22.1.50)
for 10.16.237.66 customer network. MSS is 1460 byte.
DF flag is set on outgoing packets.
Packet travel inside the ipsec tunnel: tunnel mtu is
1430
At the linux_gw_snat, the packet get decryped, snatted
(ip src change from 172.22.1.50 to 172.16.128.1) and
encryped again.
Packets are delivered to the checkpoint: tunnel mtu is
1446
Checkpoint deliver the decryped packet to 10.16.237.66
So far, so good.

At some point, 10.16.237.66 will send a 1500 byte
packet for 172.16.128.1: checkpoint will reply with
an icmp packet too large need to frag: mtu is 1446

10.16.237.66 will send back a 1446 byte packet to
the checkpoint which will encrypt and deliver to the
linux_gw_snat which will decrypt and deSNAT. Now
linux_gw_snat must send this 1446 byte packet to
172.22.1.50 but mtu is only 1430: packet will be
dropped (DF is set).

Now, IMHO, linux_gw_snat should send an imcp message
to 10.16.237.66 telling that max mtu is 1430, but I
don't see any icmp packet.
Is this the expected behaviour?

TIA

PS: linux_gw_snat is 3.3.5

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