Two-way delay, often synonymous with Round Trip Delay or Round Trip Time (RTT), is the duration it takes for a data packet to travel from the source to the destination and then back from the destination to the source. While RTT is the more commonly used term, two-way delay explicitly emphasizes the bidirectional nature of the delay being measured.
Components of Two-way Delay:
Propagation Delay:
- The time required for a signal or packet to traverse the medium (like fiber, copper, or air) from the source to the destination and then back.
Transmission Delay:
- The time taken to push the entire packet’s bits onto the network link at the source and subsequently pull them off at the destination. It depends on the packet’s size and the bandwidth of the link.
Processing Delay:
- The time routers or other intermediary devices take to process the packet. It includes operations like error checking, determining the next hop for the packet, and handling the packet at the transport layer at the destination.
Queuing Delay:
- The duration a packet spends waiting in queues at routers or other devices, typically due to congestion.
Acknowledgment Delay:
- For protocols requiring acknowledgments (like TCP), this is the duration taken for the acknowledgment packet to be generated and sent back to the original sender.
How Two-way Delay is Measured:
The most straightforward method to measure two-way delay is using the “ping” tool. When you execute a ping to a target address, your device dispatches an ICMP (Internet Control Message Protocol) echo request to the intended target. Upon receipt, the target sends back an ICMP echo reply. The duration between sending the request and getting the reply represents the two-way delay or RTT.
Significance of Two-way Delay:
A low two-way delay is crucial for real-time applications such as VoIP calls, video conferencing, and online gaming. High delays can lead to inferior user experiences, with disruptions in voice clarity, video lag, or gaming responsiveness. Additionally, for some protocols, like TCP, the two-way delay is essential for determining parameters like the suitable window size for efficient data transfers.