IPv4 datagram structure


Note: 
1. Total Length

This 16-bit field defines the entire packet size, including header and data, in bytes. The minimum-length packet is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 bytes — the maximum value of a 16-bit word. All hosts are required to be able to reassemble datagrams of size up to 576 bytes, but most modern hosts handle much larger packets. Sometimes subnetworks impose further restrictions on the packet size, in which case datagrams must be fragmented. Fragmentation is handled in either the host or router in IPv4. 

2. Identification

This field is an identification field and is primarily used for uniquely identifying the group of fragments of a single IP datagram. Some experimental work has suggested using the ID field for other purposes, such as for adding packet-tracing information to help trace datagrams with spoofed source addresses,[11] but RFC 6864 now prohibits any such use. 

3. Flags

A three-bit field follows and is used to control or identify fragments. They are (in order, from high order to low order):
bit 0: Reserved; must be zero.[note 1]
bit 1: Don't Fragment (DF)
bit 2: More Fragments (MF)


Refer:


OSI model

Enterprise Architect - skill & experience

1. User (Role-Based) Security
User security is included in the Corporate, Business and Software Engineering, Systems Engineering and Ultimate editions. Trial users can enable user security on a project, by entering the following key: {C27378C8-6CD8-4efa-92F8-4E0E4F1A8B65}

Refer: http://www.sparxsystems.com/resources/corporate/

2. Create a project in Mysql DB
http://www.sparxsystems.com/enterprise_architect_user_guide/12/projects_and_teams/upsizingtomysql.html


Các lệnh quản lý Mysql - Mysql manager command

1. show big tables in a database

2. show slave status \G
3. Master
   show master status;
   reset master;    -- use to reset master

4. show table status;
5. show create database db_name;

6. Grant privileges:
Ex:
GRANT SELECT, INSERT, DELETE, UPDATE, INDEX, LOCK TABLES, CREATE, EXECUTE  ON db_name.* TO 'db_user'@'%' IDENTIFIED BY 'password';

7. Create random data base on a list values
update your_table_name set method = (SELECT ELT(0.5 + RAND() * 4, 'VALUE_1', 'VALUE_2', 'VALUE_3', 'VALUE_4') )

8. 

Một số lệnh thông dụng trong linux

1. Xóa toàn bộ các file .svn
  find . -name .svn -print0 | xargs -0 rm -rf
2. Xử lý với các file và thư mục trong thư mục hiện hành.
Ví dụ: chmod riêng folder hoặc file

find . -type d -print0 |xargs -0 chmod 755
find . -type f -print0 |xargs -0 chmod 644
hoặc
find . -type d | xargs chmod 644

3. Forward port thông qua ssh
sudo ssh -L 3308:IPserver:3306   user@IPserver

4. Grep: get 10 lines After ,  get 10 lines Before
cat file.txt | grep -A10 "test" | wc -l
cat file.txt | grep -B10 "test" | wc -l

Còn nữa..

How to extract text on font color from a cell in Excel?

You have a data list with some red text in each cell in Excel as shown as below screenshot, and do you know how to extract the red text only? Now I will introduce a quick way to solve the puzzle that extracts text based on the font color from a cell in Excel.

Create Custom Functions in Excel 2007, 2010, 2013

1. Press Alt+F11 to open the Visual Basic Editor, and then click Insert, Module.




2. Write your function here
Such as here is the function to get Red text in a Cell. It's very useful :)

Function GetRedText(pRange As Range) As String
   
    Dim xOut As String
    Dim xValue As String
    Dim i As Long
    xValue = pRange.Text

    For i = 1 To VBA.Len(xValue)

        If pRange.Characters(i, 1).Font.Color = vbRed Then
            xOut = xOut & VBA.Mid(xValue, i, 1)
        End If

    Next

    GetRedText = xOut
End Function

symfony 1.4 log rotation

Execute following command on your project root folder:

php symfony log:rotate frontend prod --period=1 --history=5

In above command,

period 1 means:

frontend_prod.log contains 1 day log data and very next day backup of same file will be saved under log/history folder and also empty it.

history 5 means:

Under log/history folder, backup of 5 days will be stored only.