Understanding the U32 filter.

Linux Advanced Routing and Traffic Control

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Hi,
  I am trying to understand filters.
1) Under the U32 section of the lartc howto there is
an example (to match ACKs on packets smaller than 64
bytes):
# tc filter add dev ppp14 parent 1:0 protocol ip prio
10 u32 \
   match ip protocol 6 0xff \
   match u8 0x05 0x0f at 0 \
   match u16 0x0000 0xffc0 at 2 \
   match u8 0x10 0xff at 33 \
   flowid 1:3
The howto says 'the filter above does only work on 32
bit systems', can someone tell me why this is so?
2) Doesn't U32 only mean 32 bits are read at one time?
3) Also a 'match tcp dport 53 0xffff' also needs a
'match ip protocol 0x6 0xff' for differentiating
between tcp and udp. Why is this so? The first match
phrase uses the words "match tcp dport ..." so in the
kernel memory shouldn't a equivalent entry be there;
then why do I need a second protocol match rule?

Thank you.

Regards,
suraj.

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