Introduction

• Transport layer
  Allows traffic to be directed to specific network applications.
• Application layer
  Allows these applications to communicate in a way they understand.

The Transport layer

• port
  160bit number that's used to direct traffic to specific services running on a networked computer.

• TCP segment
  Made up of a TCP header and a data section.

• 6 TCP control flags
  1. URG(Urgent) : indicates that the segment is considered urgent and that the urgent pointer field has more data about this.

  2. ACK(Acknowledge) : means that the acknowledgement number field should be examined.
  3. PSH(Push) : means that the transmitting device wants the receiving device to push currently-buffered data to the application on the receiving end as soon as possible.
  4. RST(Reset) : means one of the sides in a TCP connection hasn't been able to properly recover from a series of a missing or malformed segments.
  5. SYN(Synchronize) : used when first establishing a TCP connection and make sure that receiving end knows to examine the sequence number field.
  6. FIN(Finish) : when this flag is set to one, it means that the transmitting computer doesn't have any mroe data to send and the connectiobn can be closed.

• Handshake
  A way for two devices to ensure that they're speaking the same protocol and will be able to understand each other.

• Three way handshake.
  Once the three way handshake is complete, the TCP connection is established.
  

• Four way handshake.
  Once on of the devices involved with the TCP connection is ready to close the connection, four way handshake happens.
  

• Socket
  The instantiation of an end-point in a potential TCP connection.

• Instantiation
  actual implementation of something defined elsewhere.

• TCP sockets states
  1. LISTEN : means that a TCP socket is readt and listening for incoming connections.

  2. SYN_SENT: means that a synchronization request has been sent, but the connection hasn't been established yet.

  3. SYN_RECEIVED : means that a socket previously in a listener state, has received a synchronization request and sent a SYN_ACK back.
     But is hasn't received the final ACK from the client yet.

  4. ESTABLISHED : means that the TCP connection is in working order, and both sides are free to send each other data.

  5. FIN_WAIT : measns that a FIN has bben send, but the corresponding ACK from the other end hasn't been received yet.

  6. CLOSE_WAIT : This means that the connection has been closed at the TCP layer, but that the application that opened the socket hasn't released its hold on the socket yet.

  7. CLOSED : means that the connection has been fully terminated, and that no further communication is possible.

     Socket states and their names can vary from operating system and operating system.

  • Firewall
    A device that blocks traffic that meets certain criteria.