TCP中的Flag options

使用Network monitor來分析網路通訊包的時候, 經常會使用到TCP Flag這樣的知識點.

 

 

那麼這些Flag分別都代表什麼意義呢?

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Urgent flag is used to identify incoming data as 'urgent'. Such incoming segments do not have to wait until the previous segments are consumed by the receiving end but are sent directly and processed immediately.

 

ACKnowledgement flag is used to acknowledge the successful receipt of packets.

 

Reset flag is used when a segment arrives that is not intended for the current connection. In other words, if you were to send a packet to a host in order to establish a connection, and there was no such service waiting to answer at the remote host, then the host would automatically reject your request and then send you a reply with the RST flag set. This indicates that the remote host has reset the connection.

 

SYN flag is initialy sent when establishing the classical 3-way handshake between two hosts.

 

FIN flag, standing for the word FINished. This flag is used to tear down the virtual connections created using the previous flag (SYN), so because of this reason, the FIN flag always appears when the last packets are exchanged between a connection.

 

TCP三向交握, 建立串連

 

TCP中斷串連, 過程

 

資料來源:

TCP Analysis - Section 4:TCP Flag Options

http://www.firewall.cx/tcp-analysis-section-4.php