Chapter 13 Session Layer Functions of Session Layer

Chapter 13 Session Layer

Functions of Session Layer OSI Application Presentation Session Transport Network Data Link Physical Interhost Communication Establishes, Manages, and terminates sessions between applications

Why Session Layer is needed? To avoid communication problems between 2 hosts: Problem • Messages may cross during conversation (sent at the same time) Solution Two-way Alternate Communication Taking turns to send messages to avoid interrupting each other Another Solution Two-way Simultaneous Communication Each person types whenever he wishes, regardless of who is transmitting, and assumes that more information is always on the way

Why Session Layer is needed ? To avoid communication problems between 2 hosts: Problem • Need to pause (to save your current conversation as a file) or to check each other’s previous conversation, or to re-synchronize your communication after an interruption Solution Synchronization Send a checkpoint to each other, which means that each person should save the conversation as a file. Then, each person should re-read the last part of his conversation and check the time on the clock. Two very important checkpoints are how the conversation starts and how it ends. This is referred to as orderly initiation and termination of the conversation

Dialogue Control • Communication between two computers involves many mini-conversations, thus ensuring that the two computers can communicate effectively • One requirement of these mini-conversations is that each host plays dual roles: Client (requesting service) & Server (providing service) • Determining which role a host is playing at a given time (client or server) is called dialogue control

Dialogue Control • Session layer decides whether to use two-way simultaneous or two-way alternate communication. This decision is referred to as Dialogue Control. • Session layer does little in managing the two-way simultaneous conversation. Usually, other layers of the communicating computers manage the conversation • With two-way simultaneous communication it is possible to have session layer collisions. • Collisions can only occur as two messages pass each other, and cause confusion in either, or both, communicating hosts.

Dialogue Separation • To avoid Session layer collisions, use two-way alternate communication. • Two-way alternate communication involves a session layer data token that allows each host to take turns. • Dialogue separation is the orderly initiation, termination, and managing of communication. It is done through minor and major synchronizations.

Dialogue Separation Data Host A Dialogue 1 T=Checkpoint Data Host B Dialogue 1 Minor Sync T=Checkpoint • Backup the particular files • Save the network settings • Save the clock settings • Make note of the end point in the conversation

Dialogue Separation Data Host A Dialogue 1 T=Checkpoint Data Dialogue 1 Minor Sync Data Dialogue 2 Host B Data T=Checkpoint Dialogue 2

Session Layer Protocols • SQL : Structured Query Language Standardized query language for requesting information from a database • RPC : Remote Procedure Call An application issues an RPC when it wants to use a function running on another system in the same network • X Window System A graphic system primarily used on Unix and Linux systems that provides a client/server base for displaying windows graphics

Session Layer Protocols • ASP : Apple Talk Session Protocol A session layer protocol used by Apple systems to enable sequences of communication to occur • SCP : DNA Session Control Porotocol Digital Network Architecture SCP is a digital DECnet protocol