[Openswan dev] MTU: tcpdump vs pmtu 1446

[Vincent Tamet]

Hi,
I'm trying to understand why the MTU in my test tunnel is 1446.
Ruben in the irc canal #openswan, tell me to try here.

The 2 linux-box are in the same ethernet lan.

Mode Tunnel: 3des/md5-96

My calculs:
   MTU     IP SPI SN  IV  Data   Pad PL  NH  AUTH
  1500    -20 -4 -4 -( 8    x )  -0  -1  -1  -12  = 1450
The PMTU from a ping -M do give me 1446.

Can't understand where the problem is, I must miss something, but what ?

Best regards

Vincent Tamet.
OSG[PCQ]

PS: The dump is from a lan to internet configuration, but it's the same results.
-----------------------------------------------------------------------------
* ping 192.168.3.1 -c 1 -s 2
17:25:56.555463 00:06:5b:8a:a4:2b > 00:24:14:d9:f1:90, ethertype IPv4 (0x0800), length 44: (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto ICMP (1), length 30)
    192.168.2.5 > 192.168.3.1: ICMP echo request, id 46448, seq 1, length 10
        0x0000:  4500 001e 0000 4000 4001 b488 c0a8 0205  E..... at .@.......
        0x0010:  c0a8 0301 0800 428d b570 0001 0001       ......B..p....
-----------------------------------------------------------------------------
16:25:59.221603 08:1f:f3:e7:0e:65 > 00:23:7d:fd:bb:04, ethertype IPv4 (0x0800), length 94: (tos 0x0, ttl 253, id 1992, offset 0, flags [DF], proto ESP (50), length 80) 80.94.1.136 > 10.0.0.2: ESP(spi=0xdb14b228,seq=0x8), length 60
        0x0000:  4500 0050 07c8 4000 fd32 19cc 505e 0188  E..P.. at ..2..P^..
        0x0010:  0a00 0002 db14 b228 0000 0008 5957 445a  .......(....YWDZ
        0x0020:  5dcd 42b4 4500 001e 0000 4000 3f01 b588  ].B.E..... at .?...
        0x0030:  c0a8 0205 c0a8 0301 0800 428d b570 0001  ..........B..p..
        0x0040:  0001 0004 58c2 f376 69fa ede5 2584 f199  ....X..vi...%...
-----------------------------------------------------------------------------