Inconsistencies with Linux tc filter u32 matching on packet data payload - can someone explain?

I found this may solve the problem.

http://linux-tc-notes.sourceforge.net/tc/doc/cls_u32.txt

Header Offsets

The IP header (and other headers) are variable length. This creates a problem if you are trying to use "match" to look at a value in a header that follows - you don't know where it is. It is not an impossible problem because every header in an IP packet contains a length field. The "header offsets" feature of u32 allows you to extract that length from the packet, and then add it to the offset specified in the "match" option.

Here is how it works. Recall that the match option looks like this:

match u32 VALUE MASK at OFFSET

I said earlier that OFFSET tells the kernel which word in the packet to compare to VALUE. That statement was a simplification. Two other values can be added to OFFSET to determine which word to use. Both those values start off as 0, but they can be modified when a "link" option calls another filter list. Any modification made only applies while called filter list is being executed as the old values are restored if the called filter list fails to classify the packet. Here are the two values and the names I call them:

permoff     This value is unconditionally added to every OFFSET
        that is done in the destination link, ie that one
    that is called.  This includes calculations of new
    permoff's and tempoff's.  Permoff's are cumulative
    in that if the destination link calls another link
    and calculates a new permoff, the result is added to
    this one.

tempoff A "match" option in the destination link can
        optionally add this value its OFFSET.  Tempoff's are
    temporary, in that it does not apply to any links the
    destination link calls.  It also does not effect the
    calculation of OFFSET's for new permoff's and
    tempoff's.

Time for an example. Consider this command:

# tc filter add dev eth0 parent 999:0 protocol ip prio 99 u32 \
    link 1: offset at 0 mask 0f00 shift 6 plus 0 eat \
match ip protocol 6 ff

The match expression selects tcp packets (which is IP protocol 6). If we have protocol 6 we execute filter 1:0. Now for the rest of it:

offset  This signals that we want to modify permoff or tempoff
        if the link is executed.  If this is not present,
    neither permoff nor tempoff are effected - in other
    words the target of the link inherits the current
    permoff and tempoff.

at 0    This says the 16 bit word that contains the value we
        are going to use to calculate permoff or tempoff lives
    offset 0 the IP packet - ie at the start of the packet.
    This offset must be even.  If not specified 0 is used.

mask 0f00   This mask (which is in hex) is bit-wise anded with the
        16 bit word extracted from the packet header.  It
    isolates the header length from the rest of the
    information in the word.  If not specified 0 is used
    for the extracted value.

shift 6 This says the word extracted is to be divided by 64
        after being masked.  If not present the value is not
    shifted.

plus 0  After extracting the word, masking it and dividing it by
        64, this value is now added to it.  If not present is
    assumed to be 0.

eat     If this is present we are calculating permoff, and the
        result of the calculation above is added to it.  Tempoff
    is set to 0 in this case.  If this is not present we are
    calculating tempoff, and the result of the calculation
    becomes tempoff's new value.  Permoff is not altered in
    this case.