IP addresses allow for the routing of packets between networks. However, IP makes no guarantees about delivery. The transport layer is responsible for the reliable transport of and regulation of data flow from source to destination. This is accomplished using sliding windows and sequencing numbers along with a synchronization process that ensures each host is ready and willing to communicate.

To understand reliability and flow control, think of a student who studies a foreign language for one year. Now imagine the student visits a country where the language is used. In conversation, the student must ask people to repeat their words (for reliability) and to speak slowly, so the student can understand the words (flow control). The transport layer, Layer 4 of the OSI model, provides these services to Layer 5 by way of TCP.