TCP & UDP

TCP

• TCP is a connection-oriented protocol, which means that it first establishes the connection between the sender and receiver in the form of a handshake.

• After both the connections are verified, it begins transmitting packets. It makes the transmission process error-free and ensures the delivery of data.

• TCP is one of the most common protocols that ensure end-to-end delivery. It guarantees the security and integrity of the data being transmitted. It always establishes a secure connection between the sender and receiver.

• TCP is used in most of the high-level protocols, such as FTP, HTTP, and SMTP.

• TCP connection and termination are full-duplex, which means that the data can travel in both the directions simultaneously.

TCP Message Types

Message	Description
Syn	Used to initiate and establish a connection. It also helps you to synchronize sequence numbers between devices.
ACK	Helps to confirm to the other side that it has received the SYN.
SYN-ACK	SYN message from local device and ACK of the earlier packet.
FIN	Used to terminate a connection.

TCP Connection (3-Way Handshake)

Handshake refers to the process to establish connection between the client and server. To transmit a packet, TCP needs a three-way handshake before it starts sending data. The reliable communication in TCP is termed as PAR (Positive Acknowledgement Re-transmission). When a sender sends the data to the receiver, it requires a positive acknowledgement from the receiver confirming the arrival of data. If the acknowledgement has not reached the sender, it needs to resend that data. The positive acknowledgement from the receiver establishes a successful connection.

Three-Way Handshake Process (SYN, SYN-ACK,ACK)

• Step 1: In the first step, the client establishes a connection with a server. It sends a segment with SYN and informs the server about the client should start communication, and with what should be its sequence number.

• Step 2: In this step server responds to the client request with SYN-ACK signal set. ACK helps you to signify the response of segment that is received and SYN signifies what sequence number it should able to start with the segments.

• Step 3: In this final step, the client acknowledges the response of the Server, and they both create a stable connection will begin the actual data transfer process.

Both X and Y are randomly chosen. Once the numbers are exchanged both sides simply increment their sequence numbers in the following packets for the same session.

In detail

• The client sends the SYN to the server:

  When the client wants to connect to the server. It sets the 'SYN' flag as 1 and sends the message to the server.

  The message has also some additional information like the sequence number, the ACK (is set here to 0), the window size, and the maximum segment size.

  For Example, if the window size is 2000 bits, and the maximum segment size is 200 bits then a maximum of 10 data segments (2000/200 = 10) can be transmitted in the connection.

• The server replies with the SYN and the ACK to the client:

  After receiving the client's synchronization request, the server sends an acknowledgement to the client by setting the ACK flag to '1'. The acknowledgement number of the ACK is one more than the received sequence number.

  For Example, if the client has sent the SYN with sequence number = 1000, then the server will send the ACK with acknowledgement number = 10001.

  Also, the server sets the SYN flag to '1' and sends it to the client, if the server also wants to establish the connection. The sequence number used here for the SYN will be different from the client's SYN.

  The server also advertises its window size and maximum segment size to the client.

• The client sends the ACK to the server:

  After receiving the SYN from the server, the client sets the ACK flag to '1' and sends it with an acknowledgement number 1 greater than the server's SYN sequence number to the client.

  Here, the SYN flag is kept '0'. After completion of this step, the connection is now established from the server to the client-side also. After the connection is being established, the minimum of the sender's and receiver's maximum segment size is taken under consideration for data transmission.

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TCP Termination (4-way handshake)

• To terminate the data transmission, it requires a 4-way handshake. The segments required for TCP termination are similar to the segments to build a TCP connection (ACK and SYN) except the FIN segment. The FIN segment specifies a termination request sent by one device to the other.

TCP Four-Way Handshake Process (FIN, ACK, FIN, ACK)

• Step 1: When a TCP client/server close the connection first (active close), then TCP sends a FIN segment.
• Step 2: On the other hand who receives the FIN performs the passive close and acknowledged by TCP.
• Step 3: Later of time the application that received the end-of-file will close its socket and thus TCP to send a FIN.
• Step 4: Finally, TCP which receives this final FIN (the end which initiated the active close) acknowledges the FIN.

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TCP Header

• The size of the TCP header in 32-bit words.

• The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header.

• Source port:

  • This is a 16 bit field that specifies the port number of the sender.

• Destination port:

  • This is a 16 bit field that specifies the port number of the receiver.

• Sequence Number:

  • Sequence number is a 32 bits (in the range of 0 to (2^32 -1)) number which is assigned to the first bit of the data.

• Acknowledgement Number:

  • This value will be the received sequence number incremented by 1.

  • This is a 32 bit field

• Window

  • The 16 bit window field specifies how many bytes the device is willing to receive (Window / buffer size).

• Reserved:

  • These are 6 bits for the reserved field. They are unused and are always set to 0.

• Hlen

  • This is the 4 bit, known as the header length.

  • It indicates the length of the TCP header so that we know where the actual data begins.

  • Header length = Header length field value x 4 bytes where the minimum value for this field is 5 and the maximum is 15.

  • Examples:

    • If header length field contains decimal value 5 (represented as 0101), then Header length = 5 x 4 = 20 bytes

    • If header length field contains decimal value 10 (represented as 1010), then Header length = 10 x 4 = 40 bytes

    • If header length field contains decimal value 15 (represented as 1111), then Header length = 15 x 4 = 60 bytes

• Flags:

  • There are 6 bits for flags, we also call them control bits. We use them to establish connections, send data and terminate connections:

    • FIN:

      • FIN stands for Finished. After the data transmission has been completed, devices have to terminate the connection using the FIN flag.

      • If FIN is 1, the device wants to terminate the connection, else not.

      • This is the normal method how we end an connection.

    • SYN:

      • SYN stands for synchronization. It can be described as a request for establishing a connection.

      • If SYN is 1, it means that the device wants to establish a secure connection, else not.

    • RST:

      • This resets the connection.

      • When you receive this you have to terminate the connection right away. This is only used when there are unrecoverable errors and it’s not a normal way to finish the TCP connection.

    • PSH:

      • This is Push Function.

      • This tells an application that the data should be transmitted immediately and that we don’t want to wait to fill the entire TCP segment.

    • ACK:

      • ACK stands for acknowledgement. It can be described as the response of SYN.

      • If ACK is 1, the device has received the SYN message and acknowledges it, else not.

    • URG:

      • Stands for Urgent Pointer.

      • When this bit is set, the data should be treated as priority over other data.

    Suppose the receiving buffer has already some data to be processed by the application. A segment with the PSH flag set to 1 is sent now. The sending buffer will not wait to be filled, instead, it will immediately push the data into the wire. Now, this data will queue up behind the already non-processed data in the receiving buffer. After the previous data is processed then only the data with PSH=1 can be forwarded to the application. When URG is set, the data can be immediately forwarded to the application ignoring the already existent data in the receiving buffer. In other words, URG=1 violates the FIFO structure.

• Urgent Pointer:

  • These 16 bits are used when the URG bit has been set, the urgent pointer is used to indicate how much data in the current segment is urgent.

  • When the receiving TCP receives a segment with the URG bit set, it extracts the urgent data from the segment, using the value of this urgent pointer, and delivers them, out of order, to the receiving application program.

• Checksum:

  • 16 bits are used for a checksum to check if the TCP header is OK or not.

• Options:

  • This field is optional and can be anywhere between 0 and 320 bits (40 bytes).

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UDP

• It is an unreliable and connectionless protocol.

• Since UDP avoids the overhead associated with connections, error checks and the retransmission of missing data, it’s suitable for real-time or high performance applications that don’t require data verification or correction. If verification is needed, it can be performed at the application layer.

  

UDP Header

• UDP header is an 8-bytes fixed and simple header.

• Source Port:

  • This is a 16 bit field, used to identify the port number of the source.

• Destination Port:

  • This is a 16 bit field, used to identify the port of the destined packet.

• Length:

  • The length of UDP including the header and the data. It is a 16-bits field.

• Checksum:

  • This is a 16 bit field.

  • It is an optional field, which means that it depends upon the application, whether it wants to write the checksum or not. If it does not want to write the checksum, then all the 16 bits are zero; otherwise, it writes the checksum.

  • There is no sequence ordering and retransmission mechanism in UDP. If the checksum does not match packet is simply discarded.

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TCP vs UDP