Question

Suppose a host wants to establish the reliability of a link by sending packets and measuring the percentage that are received; routers, for example, do this. Explain the difficulty of doing this over a TCP connection.

Short answer

TCP masks packet loss through retransmissions and acknowledgments, hiding true link reliability. Using simpler protocols like UDP can better measure packet loss.

Step-by-step solution

Understanding TCP

Transmission Control Protocol (TCP) is a reliable, connection-oriented protocol that ensures all packets sent are received and acknowledged. Due to its built-in error-checking and retransmission mechanisms, TCP inherently guarantees that all packets will eventually be received, making it difficult to measure lost packets directly with TCP.

Packet Loss Concealment

TCP masks packet loss by retransmitting lost packets until they are successfully received. As a result, while TCP provides overall reliable transmission, it does not reveal how many packets were originally lost and retransmitted, hiding the actual link reliability.

Acknowledgment and Timing

TCP relies on acknowledgments (ACKs) to confirm packet receipt. Timers and sequence numbers are used to detect lost packets and trigger retransmissions. This means the sender does not differentiate between originally received packets and those that were retransmitted, making direct measurement of packet loss difficult.

Impact of Retransmissions

Frequent retransmissions due to packet loss can significantly affect overall transmission time and efficiency. However, TCP's goal is to deliver all packets accurately, not to provide insight into the transmission quality of each original packet, obfuscating the true reliability of the link.

Alternative Methods

To measure the link reliability accurately, it would be more effective to use a simpler protocol like UDP (User Datagram Protocol) that does not guarantee delivery and, therefore, allows direct observation of packet loss.

Key concepts

Packet Loss Measurement

Measuring packet loss over a TCP connection can be tricky. TCP has built-in mechanisms to ensure all packets arrive reliably, potentially leading to misleading results about the actual packet loss.
When packets are lost, TCP automatically retransmits them, so the receiving end gets the complete data without ever knowing packets were lost initially.
This makes it hard to measure the real packet loss rate as TCP hides the loss through its retransmission processes.

Retransmission Mechanics

TCP achieves reliability through retransmission mechanics. Whenever TCP detects a lost packet, it retransmits it to ensure the data arrives correctly at the destination.
Here’s a breakdown of how this works:

• TCP uses sequence numbers: Every packet is assigned a sequence number. This helps in tracking which packet has been lost or requires retransmission.
• Acknowledgments (ACKs): The receiving end sends back ACKs for packets received. If the sender doesn’t receive an ACK in time, it assumes the packet was lost.
• Timers: TCP uses timers to detect lost packets. If an ACK isn’t received within a certain period, the sender retransmits the packet.

This robust system ensures data reliability but masks the actual packet loss, hiding the real picture of network reliability.

TCP versus UDP

When it comes to understanding link reliability and packet loss, it’s vital to compare TCP with UDP (User Datagram Protocol). TCP and UDP serve different purposes in network communication:

• TCP (Transmission Control Protocol): Ensures complete and reliable data transfer. It uses mechanisms like acknowledgment, retransmissions, and sequence numbers to achieve this, but it hides packet loss because it’s focused on ensuring all data arrives successfully.
• UDP (User Datagram Protocol): Does not guarantee delivery, order, or error-checking. It sends packets without waiting for acknowledgments or retransmitting lost packets. This makes it simpler and faster, but also unreliable for data integrity. However, it allows for direct measurement of packet loss since lost packets are not retransmitted.

If accurately measuring packet loss is the goal, UDP is the better choice, as it does not hide the loss like TCP does through its reliability mechanisms.