How does a message travel across the internet?

A photograph can appear on a friend's screen on another continent even though no single cable connects your two devices. Its data travels through a network of networks. Along the way, the information may be represented by radio signals, electrical signals or pulses of light. The physical signals change from one connection to another while the devices preserve the information they carry.

The sending device prepares the data in manageable pieces. Each travels with information that helps networks deliver it, including an internet address for the destination. A router receives a packet, examines its addressing information and chooses an onward connection using its routing information. The next router makes its own forwarding decision. A router does not need to understand the photograph or store the whole conversation to do that job.

Many users share these connections. Their packets take turns being transmitted instead of each conversation reserving an entire end-to-end wire. A busy router may have a queue of packets waiting for an outgoing link. If it runs out of room, a packet can be discarded. Different delays and changing routes also mean that packets need not reach the far end in the order they were sent.

Reliable delivery therefore needs more than forwarding. One widely used transport protocol, TCP, gives the endpoints responsibility for delivering an ordered stream of bytes. Sequence numbers describe where data belongs in that stream. The receiving endpoint can hold later data while waiting for an earlier missing section. Acknowledgments tell the sender about progress, while a timeout or other evidence of loss can lead the sender to transmit missing data again.

Consider three successive pieces labeled 1, 2 and 3. Pieces 1 and 3 arrive, but piece 2 is missing. The receiver cannot turn 1 followed by 3 into the complete original message. When a retransmission brings piece 2, the endpoint can assemble 1–2–3. These short labels stand for sections of data; actual TCP sequence numbers identify byte positions rather than simply numbering packets one, two and three.

If the connection keeps failing, retransmission cannot promise that delivery will eventually succeed. Other applications also choose different transport arrangements. The remarkable achievement is that shared forwarding rules, combined with suitable endpoint handling, let very different machines exchange coherent information across many independently operated networks. No one router needs to carry a map of the conversation's meaning.