Editing Network service (section)

==Application layer==
{{IP stack}}
{{OSI model}}
In [[computer network programming]], the [[application layer]] is an [[abstraction layer]] reserved for [[communications protocol]]s and methods designed for process-to-process communications across an [[IP network]]. Application layer protocols use the underlying [[transport layer]] protocols to establish host-to-host connections for network services.

===TCP-IP network services===

====Port numbers====
Many IP-based services are associated with a particular [[well-known port number]] which is standardized by the Internet technical governance.

For example, World-Wide-Web servers operate on port 80, and email relay servers usually listen on port 25.

===TCP versus UDP===

Different services use different packet transmission techniques.

In general, packets that must get through in the correct order, without loss, use TCP, whereas real time services where later packets are more important than older packets use UDP.

For example, file transfer requires complete accuracy and so is normally done using TCP, and audio conferencing is frequently done via UDP, where momentary glitches may not be noticed.

UDP lacks built-in [[network congestion]] avoidance and the protocols that use it must be extremely carefully designed to prevent network collapse.