Application Services Layer
The uppermost layer of the Internet protocol suite (TCP/IP) is the Application Services Layer. Here we find the definition of some of the basic network services, tools, and utilities that are used by many user applications. Unlike the OSI Reference Model (OSI-RM), this layer does not merely define protocols, but in some cases also defines the applications themselves. The TCP/IP community is a little less clear in defining the boundary between the application and the protocol than the ISO was.
Some of the more important Applications Services Layer protocols include:
- File Transfer Protocol (FTP): Defines a mechanism for transferring files between hosts. FTP operates over the Transmission Control Protocol (TCP). There is a secure version of FTP called Secure File Transfer Protocol (SFTP) that can run over any underlying reliable Transport Layer but is intended to run over one that provides authentication and encryption, such as the Secure Socket Layer (SSL).
- Telnet: A remote login service that allows users to access applications and run programs on a remote system. Telnet also operates over TCP.
- Simple Mail Transfer Protocol (SMTP): Provides an electronic messaging service. SMTP also operates over TCP.
- Post Office Protocol (POP): Enables a mechanism for users to access and manage their email, particularly as they are receiving messages. POP is currently in version 3, so the acronym is usually POP3.
- Hypertext Transfer Protocol (HTTP): Allows for the exchange of World Wide Web (WWW) text, graphical, audio, and video documents over the Internet. HTTP operates over the Transport Control Protocol (TCP). A secure version of HTTP, known as Hypertext Transfer Protocol over SSL (HTTPS) also exists and is widely used for ecommerce applications.
- Domain Name System (DNS): A distributed database that allows users to refer to programs, devices, and services by easy-to-remember names rather than numeric addresses. Information exchange between DNS servers (called zone transfers) operate over TCP; DNS query applications, such as NSLOOKUP, operate over the User Datagram Protocol (UDP).
- Simple Network Management Protocol (SNMP): Defines a protocol for network management and the structure of network management information. SNMP operates over UDP.
- Session Initiation Protocol (SIP): Responsible for providing signaling within a voice over IP (VoIP) or IP telephony (IPT) system, or establishing any multimedia session (e.g., IPTV).
Other Application Services protocols are listed below.
- PING: Allows users to learn status information about other hosts. PING operates over the Internet Control Message Protocol (ICMP).
- Traceroute: Provides a list of the routers between the tracing system and a specified destination.
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q3/application_services_layer.mp3%7Cdownload</mp3> | Application Services Layer|