"""
Transmission Control Protocol (TCP/IP protocol stack)
"""
from construct import *
from binascii import unhexlify
import six
tcp_header = Struct("tcp_header",
UBInt16("source"),
UBInt16("destination"),
UBInt32("seq"),
UBInt32("ack"),
EmbeddedBitStruct(
ExprAdapter(Nibble("header_length"),
encoder = lambda obj, ctx: obj / 4,
decoder = lambda obj, ctx: obj * 4,
),
Padding(3),
Struct("flags",
Flag("ns"),
Flag("cwr"),
Flag("ece"),
Flag("urg"),
Flag("ack"),
Flag("psh"),
Flag("rst"),
Flag("syn"),
Flag("fin"),
),
),
UBInt16("window"),
UBInt16("checksum"),
UBInt16("urgent"),
Field("options", lambda ctx: ctx.header_length - 20),
)
if __name__ == "__main__":
cap = unhexlify(six.b("0db5005062303fb21836e9e650184470c9bc0000"))
obj = tcp_header.parse(cap)
print (obj)
built = tcp_header.build(obj)
print (built)
assert cap == built
