packets with capture length in Wireshark larger than configured MTU

[Andrej van der Zee <andrejvanderzee () gmail com>]

Hi,

I am using Ubuntu Maverick (kernel 2.6.35-25) with the following
config for eth0:

eth0      Link encap:Ethernet  HWaddr b8:ac:6f:99:6d:be
          inet addr:85.17.148.22  Bcast:85.17.148.255  Mask:255.255.255.0
          inet6 addr: fe80::baac:6fff:fe99:6dbe/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:126634506 errors:0 dropped:0 overruns:0 frame:0
          TX packets:100914149 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:88690663907 (88.6 GB)  TX bytes:66274092073 (66.2 GB)
          Interrupt:16 Memory:da000000-da012800

It sais MTU of 1500. When I capture in Wireshark I see packets which
are much larger than the 1500 (see below). I am wondering how this is
possible.

Thank you,
Andrej


No.     Time            Source                Destination
Protocol Info
     61 09:19:15.599088 85.17.148.22          175.105.93.20
HTTP     HTTP/1.1 200 OK  (application/x-amf)

Frame 61 (8478 bytes on wire, 8478 bytes captured)
    Arrival Time: Apr 14, 2011 09:19:15.599088000
    [Time delta from previous captured frame: 0.030731000 seconds]
    [Time delta from previous displayed frame: 0.030731000 seconds]
    [Time since reference or first frame: 14.020010000 seconds]
    Frame Number: 61
    Frame Length: 8478 bytes
    Capture Length: 8478 bytes
    [Frame is marked: False]
    [Protocols in frame: eth:ip:tcp:http:media]
    [Coloring Rule Name: Checksum Errors]
    [Coloring Rule String: cdp.checksum_bad==1 || edp.checksum_bad==1
|| ip.checksum_bad==1 || tcp.checksum_bad==1 || udp.checksum_bad==1 ||
mstp.checksum_bad==1]
Ethernet II, Src: Dell_99:6d:be (b8:ac:6f:99:6d:be), Dst:
All-HSRP-routers_12 (00:00:0c:07:ac:12)
    Destination: All-HSRP-routers_12 (00:00:0c:07:ac:12)
        Address: All-HSRP-routers_12 (00:00:0c:07:ac:12)
        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
        .... ..0. .... .... .... .... = LG bit: Globally unique
address (factory default)
    Source: Dell_99:6d:be (b8:ac:6f:99:6d:be)
        Address: Dell_99:6d:be (b8:ac:6f:99:6d:be)
        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
        .... ..0. .... .... .... .... = LG bit: Globally unique
address (factory default)
    Type: IP (0x0800)
Internet Protocol, Src: 85.17.148.22 (85.17.148.22), Dst:
175.105.93.20 (175.105.93.20)
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
        0000 00.. = Differentiated Services Codepoint: Default (0x00)
        .... ..0. = ECN-Capable Transport (ECT): 0
        .... ...0 = ECN-CE: 0
    Total Length: 8464
    Identification: 0xd304 (54020)
    Flags: 0x02 (Don't Fragment)
        0.. = Reserved bit: Not Set
        .1. = Don't fragment: Set
        ..0 = More fragments: Not Set
    Fragment offset: 0
    Time to live: 64
    Protocol: TCP (0x06)
    Header checksum: 0x513e [correct]
        [Good: True]
        [Bad : False]
    Source: 85.17.148.22 (85.17.148.22)
    Destination: 175.105.93.20 (175.105.93.20)
Transmission Control Protocol, Src Port: http (80), Dst Port: 52787
(52787), Seq: 2671789300, Ack: 2723574492, Len: 8412
    Source port: http (80)
    Destination port: 52787 (52787)
    [Stream index: 3]
    Sequence number: 2671789300
    [Next sequence number: 2671797712]
    Acknowledgement number: 2723574492
    Header length: 32 bytes
    Flags: 0x10 (ACK)
        0... .... = Congestion Window Reduced (CWR): Not set
        .0.. .... = ECN-Echo: Not set
        ..0. .... = Urgent: Not set
        ...1 .... = Acknowledgement: Set
        .... 0... = Push: Not set
        .... .0.. = Reset: Not set
        .... ..0. = Syn: Not set
        .... ...0 = Fin: Not set
    Window size: 116
    Checksum: 0x16a8 [incorrect, should be 0xbea6 (maybe caused by
"TCP checksum offload"?)]
        [Good Checksum: False]
        [Bad Checksum: True]
            [Expert Info (Error/Checksum): Bad checksum]
                [Message: Bad checksum]
                [Severity level: Error]
                [Group: Checksum]
    Options: (12 bytes)
        NOP
        NOP
        Timestamps: TSval 474870612, TSecr 789164
    [SEQ/ACK analysis]
        [Number of bytes in flight: 8412]
Hypertext Transfer Protocol
    HTTP/1.1 200 OK\r\n
        [Expert Info (Chat/Sequence): HTTP/1.1 200 OK\r\n]
            [Message: HTTP/1.1 200 OK\r\n]
            [Severity level: Chat]
            [Group: Sequence]
        Request Version: HTTP/1.1
        Response Code: 200
    Date: Thu, 14 Apr 2011 00:19:15 GMT\r\n
    Server: Apache/2.2.16 (Ubuntu)\r\n
    Cache-Control: no-cache\r\n
    Expires: Sat, 25 Dec 1999 00:00:00 GMT\r\n
    Pragma: no-cache\r\n
    Content-Length: 13087\r\n
        [Content length: 13087]
    Keep-Alive: timeout=15, max=97\r\n
    Connection: Keep-Alive\r\n
    Content-Type: application/x-amf\r\n
    \r\n
Media Type
    Media Type: application/x-amf (8129 bytes)
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