Networking and communication interface

Hi guys , Im trying to build an interface that takes packets from streaming packets and analyze this packet if it's valid or not - I main with valid ..its length is ok or not ok - it sounds really simple but Im finding it hard to program a function that does this thing, to capture packet from streaming packets -must capture every packet in the stream .... and verify if it's ok or not ok-

could anyone please help me how could I start doing this? thanks alot

for simplicity and understand the concept, lets say that from bit 17 to 25 the length of the data, and if its big than 32 then its not ok otherwise this packet is ok - this data is ok-

attaching down the packet format
Attachment 16014