Chapter 13 Session Layer

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

Why Session Layer is needed? To avoid communication problems between 2 hosts: Messages may cross during conversation (sent at the same time) Problem Solution Two-way Alternate Communication Taking turns to send messages to avoid interrupting each other Two-way Simultaneous Communication Another Solution 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: 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 Problem 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 Host AHost B Dialogue 1 Data Minor T=Checkpoint Sync Backup the particular files Save the network settings Save the clock settings Make note of the end point in the conversation

Dialogue Separation Host AHost B Dialogue 1 Data Minor T=Checkpoint Sync Data 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