The following countermeasure against SYN flood attacks

is widely adopted, and is known as a $$SYN cache$.$ For simplicity, we assume that the server only

responds on port $80,$ and only needs to remember the client$$s sequence number C and its own sequence

number S between the second and third message in the TCP handshake. The server maintains a table

with exactly B cells $($e$.$g$.,$ B $=216),$ numbered $1,...,$ B$,$ and each cell in the table can store an entry

with form

$($ClientIP$,$ ClientPort, S$,$ C$).$