[Eloy Paris <peloy@xxxxxxxxxx>]

Hello,

For each layer (protocol) in a packet I need to obtain the offset into
the packet. For example, for "eth:ip:icmp:data", the offsets would be:

    eth:   0
    ip:   14 (IP with no options)
    icmp: 34 (ICMP echo request)
    data: 42

I have been using the value of the "start" field of "struct field_info"
(epan/proto.h). However, I just found out that in some cases "start" can
be zero. For example, a packet with the following structure is giving me
a zero "start" for protocol "tcp", and offsets for the following layers
(telnet in this example), start counting from 0:

    eth:     0
    vlan:   14
    ipv6:   18
    tcp:     0
    telnet: 20

What is causing field_info's "start" field to be zero is the presence of
an IPv6 fragmentation header (see below for packet details.)

The question is: is this normal behavior, and if so, is there some other
reliable way to obtain offsets into a packet from dissection results?

Thanks in advance for any suggestions.

Cheers,

Eloy Paris.-
netexpect.org

----------------------------------------------------------------------
No.     Time        Source                Destination           Protocol Info
    532 124.395809  2001:db8:1:208::1047  2001:db8:1:208::20    TCP      1029 > 23 [SYN] Seq=192 Win=65535 Len=0

Frame 532 (90 bytes on wire, 90 bytes captured)
    Arrival Time: May 27, 2008 15:27:19.862097000
    [Time delta from previous captured frame: 0.002587000 seconds]
    [Time delta from previous displayed frame: 124.395809000 seconds]
    [Time since reference or first frame: 124.395809000 seconds]
    Frame Number: 532
    Frame Length: 90 bytes
    Capture Length: 90 bytes
    [Frame is marked: False]
    [Protocols in frame: eth:vlan:ipv6:tcp]
    [Coloring Rule Name: TCP SYN/FIN]
    [Coloring Rule String: tcp.flags & 0x02 || tcp.flags.fin == 1]
Ethernet II, Src: 0a:12:bc:34:ef:61 (0a:12:bc:34:ef:61), Dst: Cisco_b5:a4:1b (00:18:74:b5:a4:1b)
    Destination: Cisco_b5:a4:1b (00:18:74:b5:a4:1b)
        Address: Cisco_b5:a4:1b (00:18:74:b5:a4:1b)
        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
    Source: 0a:12:bc:34:ef:61 (0a:12:bc:34:ef:61)
        Address: 0a:12:bc:34:ef:61 (0a:12:bc:34:ef:61)
        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
        .... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
    Type: 802.1Q Virtual LAN (0x8100)
802.1Q Virtual LAN, PRI: 0, CFI: 0, ID: 909
    000. .... .... .... = Priority: 0
    ...0 .... .... .... = CFI: 0
    .... 0011 1000 1101 = ID: 909
    Type: IPv6 (0x86dd)
    Trailer: B1508EC3
Internet Protocol Version 6
    0110 .... = Version: 6
        [0110 .... = This field makes the filter "ip.version == 6" possible: 6]
    .... 0000 0000 .... .... .... .... .... = Traffic class: 0x00000000
    .... .... .... 0000 0000 0000 0000 0000 = Flowlabel: 0x00000000
    Payload length: 28
    Next header: IPv6 fragment (0x2c)
    Hop limit: 255
    Source: 2001:db8:1:208::1047 (2001:db8:1:208::1047)
    Destination: 2001:db8:1:208::20 (2001:db8:1:208::20)
    Fragmentation Header
        Next header: TCP (0x06)
        0000 0000 0000 0... = Offset: 0 (0x0000)
        0 = More Fragment: No
        Identification: 0x00000007
    [IPv6 Fragments (20 bytes): #532(20)]
        [Frame: 532, payload: 0-19 (20 bytes)]
Transmission Control Protocol, Src Port: 1029 (1029), Dst Port: 23 (23), Seq: 192, Len: 0
    Source port: 1029 (1029)
    Destination port: 23 (23)
    Sequence number: 192
    Header length: 20 bytes
    Flags: 0x02 (SYN)
        0... .... = Congestion Window Reduced (CWR): Not set
        .0.. .... = ECN-Echo: Not set
        ..0. .... = Urgent: Not set
        ...0 .... = Acknowledgment: Not set
        .... 0... = Push: Not set
        .... .0.. = Reset: Not set
        .... ..1. = Syn: Set
        .... ...0 = Fin: Not set
    Window size: 65535
    Checksum: 0x3b1c [correct]
        [Good Checksum: True]
        [Bad Checksum: False]