Traditionally the Internet has provided best-effort delivery of datagram traffic from senders to receivers. No guarantees are made regarding when or if a datagram will be delivered to a receiver. However datagrams are normally only dropped when a router exceeds a queue size limit due to congestion. The best-effort Internet service model does not assume first-in-first-out (FIFO, also known as first-come-first-served) queueing, although many routers have implemented this. The effect is to provide rather unfair distribution of resources.
With best-effort service, if a link is not congested, queues will not build at routers, datagrams will not be discarded in routers, and delays will consist of serialisation delays at each hop plus propagation delays. With sufficiently fast link speeds, serialisation delays are insignificant compared to propagation delays. However, if a link is congested, with best-effort service queueing delays will start to influence end-to-end delays, and packets will start to be lost as queue size limits are exceeded.