- Session Layer
The Session Layer is level five of the seven level
OSI model . It responds to service requests from thePresentation Layer and issues service requests to theTransport Layer .The Session Layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session Layers are commonly used in application environments that make use of
remote procedure call s (RPCs).An example of a Session Layer protocol is the
OSI protocol suite Session Layer Protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the Session Layer protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and providessynchronization point s in the stream of exchanged messages. [ [http://www.itu.int/rec/T-REC-X.225/en/ ITU-T Recommendation X.225] ]Other examples of session-layer implementations include
Zone Information Protocol (ZIP) – theAppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP) – theDECnet Phase IV Session Layer protocol.In brief: the Session Layer establishes, manages and terminates connections (sessions) among cooperating applications. It also adds traffic flow information.
ession Layer services
*
Authentication
*Permission s
*Session restoration (checkpointing and recovery)ession Layer support in the Internet
The Session Layer does not exist as a separate protocol layer or process in the
TCP/IP model , but its functionality is partly provided by the TCP/IP modelTransport Layer , and partly by some TCP/IP modelApplication Layer protocols.The OSI model made the Session Layer responsible for "graceful close" of sessions, which is a property of the TCP protocol, provided by the four-way SYN handshake process.
The OSI model also made the Session Layer responsible for session
checkpointing and recovery, which is not usually used in the Internet protocols suite. However, there are a few applications layer protocols where the concept 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 intuitively fairly 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 havelip synch problems. We may also want to dofloor 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 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.Session Layer Protocols
*ADSP, AppleTalk Data Stream Protocol
*ASP, AppleTalk Session Protocol
*H.245, Call Control Protocol for Multimedia Communication
*ISO-SP, OSI Session Layer Protocol (X.225, ISO 8327)
*iSNS, Internet Storage Name Service
*L2F, Layer 2 Forwarding Protocol
*L2TP, Layer 2 Tunneling Protocol
*NetBIOS, Network Basic Input Output System
*PAP,Password Authentication Protocol
*PPTP, Point-to-Point Tunneling Protocol
*RPC, Remote Procedure Call Protocol
*RTCP, Real-time Transport Control Protocol
*SMPP, Short Message Peer-to-Peer
*SCP,Secure Copy Protocol
*SIP,Session Initiation Protocol
*SSH,Secure Shell
*ZIP,Zone Information Protocol
*SDP,Sockets Direct Protocol ee also
*
Session (computer science) References
Wikimedia Foundation. 2010.
