Stateless connection in UDP explained with examples

User Datagram Protocol (UDP):

The User Datagram Protocol (UDP) is a lightweight layer 4 (or transport layer) protocol that sends data in the form of datagrams to other communicating hosts in a network that utilizes the Internet Protocol (IP). In short, UDP must run on top of IP in order to work. Therefore, a network utilizing UDP is sometimes regarded to as a UDP/IP network.

The transfer of datagrams via UDP is done in a connectionless or stateless manner; but that isn't necessary a bad thing. In fact, some type of data are best transferred in this manner. A typical example of such data is Voice over Internet Protocol (VoIP).

 

When data is transferred in a stateless manner, it only mean that both connecting devices did not partake in an initial three way handshake (or connection setup) before the communication began. In this regard, you can think of it as both computers connecting without exchanging some form of tracking information about each other. As a result, there is no sequence number, acknowledgement number and flow control associated with this connection; hence the name 'connectionless' or 'stateless'. For this reason, UDP is sometimes referred to an 'Unreliable' Data Protocol.

Please read an introduction to the transport layer here.

 

A stateless protocol is also referred to as a connectionless protocol; and that's exactly how UDP functions.

In a stateless connection, there is no guarantee of datagram delivery to communicating devices. Also the datagrams can arrive out of other or may even be duplicated when sent. In an IPv4 network, it is important to state that UDP may perform checksum on datagrams in an attempt to ascertain their integrity. However, a checksum is always performed when IPv6 is utilized at the network layer.

A stateless communication ensures that one of the communicating device send data or datagrams to the other computers without any prior notice to the latter. If something happens to the sent data along its path, (probably traffic was heavy along its route) and it was unable to reach a destination computer, then such data will be lost (not delivered to the destination computer or device); and there won't be any form of notification from the receiving device, hence the retransmission of the lost data is impossible.

The image below shows a connection and connectionless (or stateful and stateless) communication between two devices:

Please read on the functions of the transport layer here.

 

Voice communication is a function of the connectionless User Datagram Protocol (UDP). In fact, this is the reason why we sometimes hear distorted sounds in voice communications. This is so because some specific voice data could not reach its destination device; and there isn't retransmission of the lost data. For this reason, voice conversations happen in real-time and that's good enough.

 

Apart from VoIP, applications that utilizes UDP at its transport layer includes:

• Instant messaging apps like facebook messenger, whatsapp, telegram, michat and wechat

• Video calls

• Online multiplayer games

• Robotic telepresence

• Live streaming media

• Teleconferencing

 

Application layer protocols that are transmitted via UDP, alongside their port numbers include:

• Trivial File Transfer Protocol (TFTP): Port 59

• Dynamic Host Configuration Protocol (DHCP): Port 67

• Network Time Protocol (NTP): Port 123

• Domain Name Service (DNS): Port 53

• Routing Information Protocol (RIP): Port 520

• Simple Network Management Protocol (SNMP): Port 161

• Network File System (NFS): Port 111

• Bootstrap Protocol (BOOTP): Port 68

• Quote of the day protocol: Port 17

 

From our explanations so far, we can safely say that the stateless connection in UDP is achieved via port numbering and an optional checksum computation. Port numbering functions to distinguish communicating users request while a checksum might be calculated to confirm datagram integrity.

 

UDP Header

Below is a diagram showing UDP's header.

The User Datagram Protocol header has a data field of eight bytes (or 64 bits) in total. In other words, four fields are present in the header, with each taking 2 bytes (16 bits) respectively. The fields include:

1. Source port number: It's a 2 byte field containing the source port of the sending device.

2. Destination port number: A 2 byte field containing the destination port to which datagrams are to be sent.

3. Length: The length of UDP datagram specifies the number of bytes present in the UDP header and the UDP payload (that is, the data it contains). It is a 2 byte field.

4. Checksum: It computes error checks on datagrams in order to maintain data integrity. It is required in IPv6 but optional in IPv4.

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