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In computer networking, a network service is an application running at the network layer and above, that provides data storage, manipulation, presentation, communication or other capability which is often implemented using a client–server or peer-to-peer architecture based on application layer network protocols.
Each service is usually provided by a server component running on one or more computers (often a dedicated server computer offering multiple services) and accessed via a network by client components running on other devices. However, the client and server components can both be run on the same machine.
Clients and servers will often have a user interface, and sometimes other hardware associated with it.
Email, printing and distributed (network) file system services are common services on local area networks. They require users to have permissions to access the shared resources.
Other network services include:
For example, World-Wide-Web servers operate on port 80, and email relay servers usually listen on port 25.
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.
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