Reliable and Unreliable Connections Explained

The transport layer facilitates data transfer between devices in a network using either reliable or unreliable connections. Reliable connections guarantee data delivery, although they typically incur greater latency. In contrast, unreliable connections offer faster transmission but do not ensure data delivery.

Reliable connections

Reliable connections use the Transmission Control Protocol (TCP), which employs mechanisms such as the three-way handshake, windowing, and sequence numbers to ensure guaranteed data delivery. Let us take an example demonstrates how TCP implements these techniques to achieve guaranteed data delivery.

A user on PC-1 wants to transfer some files to a user on PC-2. Since these files require guaranteed data delivery, the transport layer on PC-1 implements the TCP protocol and delegates this responsibility.

The Three-Way Handshake process

Before data transmission, TCP confirms that PC-2 is operational and ready to receive data by initiating the three-way handshake. This process consists of the following steps.

PC-1 (source device) sends a SYN (synchronization) segment to PC2 (destination device) to request the establishment of a reliable session for data transfer. PC-2 responds with a segment containing both acknowledgment and synchronization information. The acknowledgment confirms receipt of the SYN segment, and the SYN flag indicates that PC-2 is prepared to establish the connection.

Upon receiving the SYN/ACK segment, PC-1 replies with an ACK segment, notifying PC-2 that its SYN was received and that the session is fully established. Once the three-way handshake is complete, both devices can transfer data within the established session. TCP executes this process for each data transfer and negotiates data transmission parameters as needed.

Windowing

Windowing defines the number of segments the source can transmit before receiving an acknowledgment from the destination. For example, with a window size of 4, PC-1 may send 4 segments before awaiting an acknowledgment from PC-2. Additional segments are transmitted only after the previous set is acknowledged.

Sequencing

TCP assigns an incremental sequence number to each segment to facilitate tracking. The destination device uses these numbers to notify the source device of received segments and to identify any segments lost during transmission. The following example illustrates this process.

Suppose the window size is four. PC-1 transmits four segments with sequence numbers 40, 41, 42, and 43. PC-2 receives all segments and sends an acknowledgment confirming receipt of segments 40, 41, 42, and 43, indicating readiness for the next four segments.

PC-1 then transmits the next four segments, numbered 44, 45, 46, and 47. Suppose the segment 46 is lost during transmission. PC-2 sends an acknowledgment indicating receipt of segments 44, 45, and 47, and signals readiness for the next four segments. Since PC-1 does not receive acknowledgment for segment 46, it infers that PC-2 did not receive it. PC-1 resends segment 46 in the subsequent batch, along with segments 48, 49, and 50.

By utilizing sequence numbers and acknowledgment signals, TCP ensures guaranteed data delivery.

Unreliable connections

A primary limitation of reliable connections is that TCP must complete a handshake process before data transfer can begin and must subsequently acknowledge receipt of all transmitted data. In scenarios requiring only a single message and response, the handshake process introduces unnecessary overhead and delays. For example, when a device resolves a fully qualified domain name to an IP address, it sends a single query to a DNS server and waits for a response.

In such cases, where only two messages are exchanged (the client’s query and the server’s response), establishing a reliable connection before sending the query is worthless. Instead, the device transmits its query and waits for a response. If no response is received, the device retransmits the query. Transmitting data without first establishing a connection is called connectionless or unreliable data delivery.

Conclusion

Reliable connections, such as those provided by TCP, ensure accurate and ordered data delivery through handshakes, acknowledgments, and sequencing. These features are essential for applications that require guaranteed delivery. However, they add additional overhead and slower transmission speeds. In contrast, unreliable or connectionless connections, as used in DNS queries, prioritize speed over guaranteed delivery, making them suitable for scenarios where rapid, lightweight communication is preferred. Understanding the distinctions between these approaches lets you select the most appropriate method for specific application requirements.