// allocate space for ip packet
        mov %esp, %ebp
        sub $0x2038, %esp

        // create our ip pointer
        lea 0xffffdfc8(%ebp), %eax
        add  $0x10, %eax
        sub $0x2, %eax
        mov %eax, 0xfffffff4(%ebp)

        // create our tcp pointer
        lea 0xffffdfc8(%ebp), %eax
        add $0x24, %eax
        //sub $0x2, %eax
        mov %eax, 0xfffffff8(%ebp)


        ... farther down the code path


        // get our ip pointer
        mov 0xfffffff4(%ebp), %ebx

        // get protocol into ebx
        movzbl 0x9(%ebx), %ebx

        // make sure its a tcp packet
        cmp $0x6, %bl
        jne read_loop

        // get our tcp pointer
        mov 0xffffff8(%ebp), %ebx

        // get source port into ebx
        movzwl (%ebx), %ebx                 <-- this bit crashes, even though it is addressing the right offset.