What are TCP Flags?

Hi All.

What are TCP Flags?

Any easy explanation?

Admin note: this post was updated with image below.
What are TCP Flags

TCP Flag Options available to make life either more difficult, or easy, depending on how you look at the picture :slight_smile:

TCP Flags are used to influence the flow of Data across a TCP connection.

  1. PUSH (PSH): It pushes the buffered data to the receiver’s application. If data is to be send
    on the immediate basic, we will push it.
  2. Reset (RST): It Reset the connection.
  3. Finish (FIN): It Finishes the session. It means No More Data form the sender.
  4. Urgent (URG): It is used to set the priority to tell the receiver that this data is important
    for you.
  5. Acknowlegement (ACK): All packets after SYS packet sent by the client should have this
    flag set.ACK=10 means host has received 0 through 9 and is expecting byte 10 next.
  6. Synchronize (SYN): It initiated a connection. It Synchronize sequence number.
    28.What is the importance of Sequence Number and Acknowledgement Number?
  • Sequence Number is a 32-bit field which indicates the amount of data that is send during a TCP session. By sequence number, the sender can be assured that the receiver received the data because the receiver uses this sequence number as the acknowledgement number in the next segment it sends to acknowledge the received data. When the TCP session starts, the initial sequences number can be any number in the range of 0-4294967295.

  • Acknowledgement number is used to Acknowledge the received data and is equal to the received sequence number plus 1.

  • Which is the importance of the identification field in the IP Packets?

    • This is used to identify each fragmented packet so that destination device can rearrange the whole communication in order.
  • What is the MTU (Maximum Transmission Unit)?

    • A maximum transmission unit (MTU) is the largest packet or frame size, specified in octets (eight-bit bytes) that can be sent in a packet- or frame-based network such as the internet. The internet’s transmission control protocol (TCP) uses the MTU to determine the maximum size of each packet in any transmission.
  • What is the Fragmentation?

    • Fragmentation is a process of breaking the IP packet into smaller pieces (fragment). Fragmentation is required when the datagram is large r than the MTU. Each fragment then becomes a datagram and transmitted independently from source. These datagrams are reassembled by the destination.
  • How is the packet reassembled?

    • IP fragmentation is an Internet Protocol (IP) process that breaks packets into smaller pieces (fragments), so that the resulting pieces can pass through a link with a smaller maximum transmission unit (MTU) than the original packet size. The fragments are reassembled by the receiving host.

See more details here:

What are TCP Flags