IP bad-len 0

Did someone knows what's that mean?

root_at_cassini:~# tcpdump -n host xx.162.141.1 and port 80                                                                                                                                                                             
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on em0, link-type EN10MB (Ethernet), capture size 96 bytes
17:04:40.734189 IP xx.162.141.1.47484 > xxx.53.51.235.80: S 
3567180455:3567180455(0) win 5840 <mss 1452,sackOK,timestamp 8804899 
0,nop,wscale 6>
17:04:40.734236 IP xx.53.51.235.80 > xx.162.141.1.47484: S 
3608546905:3608546905(0) ack 3567180456 win 8192 <mss 1452,nop,wscale 
5,sackOK,timestamp 709854070 8804899>
17:04:40.925032 IP xx.162.141.1.47484 > xxx.53.51.235.80: . ack 1 win 92 
<nop,nop,timestamp 8804947 709854070>
17:04:40.933559 IP xx.162.141.1.47484 > xxx.53.51.235.80: P 1:935(934) ack 
1 win 92 <nop,nop,timestamp 8804947 709854070>
17:04:40.995133 IP bad-len 0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I see quite a lot of this messages on my web server xxx.53.51.235 and the 
connection is a bit sluggish sometimes.
It's a em card on 7.0-RC1 GENERIC amd64 kernel:

em0_at_pci0:6:0:0: class=0x020000 card=0x109a15d9 chip=0x109a8086 rev=0x00 
hdr=0x00
    vendor     = 'Intel Corporation'
    device     = '82573L Intel PRO/1000 PL Network Adaptor'
    class      = network
    subclass   = ethernet

dev.em.0.%desc: Intel(R) PRO/1000 Network Connection Version - 6.7.3
dev.em.0.%driver: em
dev.em.0.%location: slot=0 function=0 handle=\_SB_.PCI0.EXP5.PXS5
dev.em.0.%pnpinfo: vendor=0x8086 device=0x109a subvendor=0x15d9 
subdevice=0x109a class=0x020000
dev.em.0.%parent: pci6
dev.em.0.debug: -1
dev.em.0.stats: -1
dev.em.0.rx_int_delay: 0
dev.em.0.tx_int_delay: 66
dev.em.0.rx_abs_int_delay: 66
dev.em.0.tx_abs_int_delay: 66
dev.em.0.rx_processing_limit: 100
dev.sc.0.%desc: System console



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- Best regards, Nikolay Pavlov. <<<-----------------------------------    
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