Session Layer
The session layer is level five of the seven level OSI model. It responds to service requests from the presentation layer and issues service requests to the transport layer.
The Session layer provides the mechanism for managing the dialogue between end-user application processes. It provides for either full duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures.
The Session layer is typically completely unused, but it does have a few places where it is useful. The idea is to allow information on different streams, perhaps originating from different sources, to be properly combined. In particular, it deals with synchronization issues, and ensuring nobody ever sees inconsistent versions of data, and similar things.
One application which is fairly intuitively clear is web conferencing. Here, we want to make sure that the streams of audio and video match up - or in other words, that we do not have lipsync problems. We may also want to do "floor control" - ensuring that the person displayed on screen and whose words are relayed is the one selected by the speaker, or by some other criteria.
Another big application is in live TV programs, where streams of audio and video need to be seamlessly merged from one to the other so that we do not have half a second of blank airtime, or half a second when we transmit two pictures simultaneously.
In brief: the session layer establishes, manages and terminates connections (sessions) among cooperating applications. It also adds traffic flow information.
Examples
- ADSP, AppleTalk Data Stream Protocol
- ASP, AppleTalk Session Protocol
- H.245, Call Control Protocol for Multimedia Communication
- iSNS, Internet Storage Name Service
- L2F, Layer 2 Forwarding Protocol
- L2TP, Layer 2 Tunneling Protocol
- NetBIOS, Network Basic Input Output System
- PAP, Printer Access Protocol
- PPTP, Point-to-Point Tunneling Protocol
- RPC, Remote Procedure Call Protocol
- RTP, Real-time Transport Protocol
- RTCP, Real-time Transport Control Protocol
- SMPP, Short Message Peer-to-Peer
- SCP, Secure Copy Protocol
- SSH, Secure Shell
- ZIP, Zone Information Protocol
| Physical Layer | |||
| The physical layer is level one in the seven level OSI model of computer networking as well as in the five... | |||
| Data Link Layer | |||
| The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference... | |||
| Network Layer | |||
| The network layer is level three of the seven level OSI model as well as of the five layer TCP/IP model... | |||
| Transport Layer | |||
In computing and telecommunications, the transport layer is the second highest layer in the four... |
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| Session Layer | |||
| The session layer is level five of the seven level OSI model. It responds to service requests from the presentation... | |||
| Presentation Layer | |||
| The presentation layer is the sixth level of the seven layer OSI model. It responds to service requests... | |||
| Application Layer | |||
| The application layer is the seventh level of the seven-layer OSI model. It interfaces directly to and performs... | |||
