TCP

Transmission Control Protocol (TCP/IP)

• Designed in conjunction with IP
• Interface to services
  • Port
  • A server could be running different number of services, you can port scan it, for example use an utility called nmap
• Reliable delivery
  • The goal of TCP/IP
• Error checking
• Congestion avoidance
  • How much data are you sending at once?
  • When you ride a car you do not instantly go to 100 mph you start slow then accelerate then slow down
  • Speed of transfer data may vary

TCP header

- The sequence numbers change over the lifetime of the connection
- They changed based on how much data is sent.

Three-way Handshake

• Synchronization before transportation
  • The sequence number is so that if packets arrive out of order we can put them back in order
  • Data comes and is assembled even if specific data segments are lost, TCP will detect it and will retrieve that data again.
  • SYN
    • Synchronization request
    • Let's say we put it in port 22
  • SYN, ACK
    • I acknowledge the SYN you just sent me
    • Here is my SYN number (SYN-1024)
  • ACK
    • Is that ok to follow?

RST and FIN flags

In TCP (Transmission Control Protocol), the FIN and RST flags are used for managing the state of a connection:

1. FIN (Finish):
   • Indicates that the sender has finished sending data and wants to gracefully close the connection.
   • When a device sends a FIN flag, it is signaling the end of data transmission, and the other device should acknowledge it by sending an ACK (Acknowledgment) back.
   • This is part of the four-way handshake for closing a TCP connection.
2. RST (Reset):
   • Indicates that the connection should be immediately reset or aborted.
   • Typically used when there is an error, such as a non-existent port, or when a device wants to abruptly terminate the connection for some reason.
   • Unlike FIN, which is part of the graceful termination process, RST abruptly ends the connection without the normal handshakes.