Sir / Madam

I am doing my research on fragmentation avoidance technique for mip6. I am using FreeBSD4.4 with kame snap and ethereal to capture packets

I have a query regarding the packet length

If i am correct ,
packet length = IPHdr +extHdr+TCPHdr+TCPOpt+Data

=IPHdr+Routing Header + Frag Header + TCP Header+ Tcpopt+ Data

40+24+8+20+12+1176=1280 (Which is the MTU of the gif0 interface)

But my ethereal capture says the packet length to be 1284 btyes. Can anyone please let me know what that 4 bytes is accounted for?

With this mail, i am pasting a capture packet of my experiment

Frame 973 (1284 on wire 1284 captured)
Arrival Time: Dec 6 2002 39:54.8
Time delta from previous packet: 0.000052 seconds
Time relative to first packet: 97.69262 seconds
Frame Number: 973
Packet Length: 1284 bytes
Capture Length: 1284 bytes
Family: IPv6 (0x0000001c)
Internet Protocol Version 6
Version: 6
Traffic class: 0x00
Flowlabel: 0x6017f
Payload length: 1240
Next header: IPv6 routing (0x2b)
Hop limit: 64
Source address: 2001:618:400::8949:b94 (2001:618:400::8949:b94)
Destination address: 3ffe:400b:6004:3:200:39ff:fe11:f32 (3ffe:400b:6004:3:200:39ff:fe11:f32)
Routing Header Type 0
Next header: IPv6 fragment (0x2c)
Length: 2 (24 bytes)
Type: 0
Segments left: 1
address 0:00 3ffe:327e:2:1:200:39ff:fe11:f32 (3ffe:327e:2:1:200:39ff:fe11:f32)
Fragmention Header
Next header: TCP (0x06)
Offset: 0
More fragments: Yes
Identification: 0xef9a0000
Transmission Control Protocol Src Port: 49165 -49165 Dst Port: iad2 -1031 Seq: 2.68E+09 Ack: 1.12E+09
Source port: 49165 -49165
Destination port: iad2 -1031
Sequence number: 2.68E+09
Next sequence number: 2.68E+09
Acknowledgement number: 1.12E+09
Header length: 32 bytes
Flags: 0x0010 (ACK)
0... .... = Congestion Window Reduced (CWR): Not set
.0.. .... = ECN-Echo: Not set
..0. .... = Urgent: Not set
...1 .... = Acknowledgment: Set
.... 0... = Push: Not set
.... .0.. = Reset: Not set
.... ..0. = Syn: Not set
.... ...0 = Fin: Not set
Window size: 16912
Checksum: 0x7783
Options: (12 bytes)
Time stamp: tsval 1931258 tsecr 159233
Data (1176 bytes)

Here the packet length is 1284 instead of 1280. Can anyone please explain me the situation . Is it because of any header or something else.

Best Regards

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