Acknowledgment Number: 32 bits
If the ACK control bit is set this field contains the value of the
next sequence number the sender of the segment is expecting to
receive. Once a connection is established this is always sent.
#include <stdio.h>
int main()
{
char string[] = "abcdef";
asm volatile(
"incb (%0)\n"
:: "r"(string) : "memory");
printf("%s\n", string);
return 0;
}