Chapter Nine

The Session Layer

Objectives• We’ll see how a new session is created,

maintained, and dismantled.

• The process of logon authentication will be revisited.

• Dialogue control will be examined.

Why we need session layer?• Before the network starts to transfer data, it needs to make

sure that the user has all the necessary rights and privileges.– User credentials are verified.

• The processes or applications at the source and destination need to figure out how to talk to one another, such as service, protocol, speed, duration, etc.– Establish a connection ID number– Agree upon which services are required and for what duration– Determine which device begins the conversation– Coordinate the Acknowledgement numbering and retransmission

rules.

Session Layer Responsibilities

• The responsibilities of session layer are:– Establish a connection– Provide dialogue management and

synchronization– Release the connection

Establishing a Connection

• In order for two devices to communicate, they must first establish a session.– A session can also defined as a virtual connection between

two devices for the purpose of transferring data.– Many times, data is transferred over a series of session in

order to keep network congestion at a minimum.– Any time multiple sessions are used by devices, it is

considered to be a dialogue.

Initiating a Link• When a client application or process decides that it requires a

service that exists on a remote device, it is up to Session layer to establish a link with that remote device.

• The Session layer will rely heavily upon the service provided by the lower lays in the OSI model– To create a new session, the Session layer entity issues a T-connect

request to the Transport Layer.– Transport layer will identify the services for the particular session and

the protocol that will be used. The Transport layer issue a N-connect request to the Network Layer and wait for confirmation.

– The confirmation will be handed back to the Session layer and used for all subsequent processes.

Initiating a Link

Logon Authentication• In any connection-oriented protocol, logon

authentication is required before any session can be built– At a minimum, the logon authentication requires User ID

and password.

• The credentials are sent to the target device and compared against the security database.– If the user is authenticated, the session is created.– If not, the session is immediately aborted.

Logon Authentication• Once users log on to the network, their credentials

stay in cache.• The system makes use of this credential every time

they attempt to access any network resource– Once the client host has located the resource, if the

resource has password restriction, the device housing that resource will forward the logon authentication to the server.

– The security database will be checked, and if all information is correct, the user will be allowed to access the resource.

Logon Authentication Example

The Connection ID• Everything has to have a name.

• On any given host, it is high likely that there are multiple session open simultaneously– The connection ID keeps multiple sessions from getting confused.

• For each session, the network will create a source connection ID and destination connection ID. This number will be given to the lower layer in order to make sure this information is added to the appropriate fields.

• TCP/IP makes very little use of connection IDs. TCP/IP uses sequence number to keep packets from different session.

Duration of the Connection

• An estimated duration is negotiated between entities.

• In the event that one of the device drops the connection for any reason, the other device will not just leave the virtual link hanging open.

• When the time to live expires, the still-connected device can either reestablish the connection or close it.

Service Required• The device also negotiates what services are being

requested and those that will be required.

• The services provided by the Session Layer include:

The Final Steps of Initializing a Session

• The last two steps of initializing a session are:– Determine which entity begins the conversation– Coordinate the acknowledgement numbering and

retransmission procedure.

• Deciding who talks first is simply a matter of what type of service is being provided. – The client may be requesting access to the resource on the

server– The server may be synchronizing data on the client.

The Final Steps of Initializing a Session

• Connection-oriented protocol require that acknowledgements be sent for each packet received. It is the Session layer to negotiate the method of acknowledgement– Sliding window– Stop and wait– etc

Dialogue Management• Duplex or half-duplex operation is selected.

– If the full duplex is selected, either device can send data any time it is required

– If half-duplex (Simplex) is selected, the process of communication is controlled by a token.• Data Token – permit the transmission of data• Synchronize minor token – A fine-tuning of the session’s flow is required,

or a checkpoint is being issued.• Major activity token – The entire is out of synch and needs to be

realigned and reset• Release token – All finished. Let’s go home• Disconnect token – The data transfer is complete and the session can be

broken down.• Abort token – Some catastrophic failure has occurred and the session is

to be ended immediately.

Dialogue Management

• During the transmission, the Session layer periodically places Checkpoints in the data steam.

• In the event of a session failure, certain protocols can

reestablish a connection and pick up where they left off.

• A synchronization minor token is used so that the receiving device will not confuse the checkpoint with actual data

Dialogue Synchronization

• Broken down into four types– Initial synchronization

• Starts the conversation

– Major synchronization• Starts the conversation over again, if need be

– Minor synchronization• Fine-tunes data exchange as needed

– Resynchronization• Either restarts or completely aborts the session. It can take

advantage of checkpoint to resume the transmission. A set command can be used to return to a specific checkpoint.

Releasing the Connection• The final responsibility of the Session layer is to

disconnect a communication link and remove the temporary files that have been created during the session.

• Disconnection fall under two categories, the expected and the unexpected– Expected disconnections occur because either the

duration of the session has expired or the transfer of data is complete.

– Unexpected disconnection occurs because of an unrecoverable error

Releasing the Connection

– An expected disconnect results in a finish Protocol Data Unit being issued.• The session is broken down and all TEMP files cleaned up.

– An unexpected disconnection results in a not-finished PDU.• TEMP files remain intact. Once the reconnection has been made,

the checkpoint procedure will be used to resume the transfer where it left off.

Summary

• The Session layer (Layer 5) of the OSI model performs the following tasks:– Starts and ends sessions on the Presentation

layer’s request and passes connection information to the Transport layer

– Maintain the dialogue.– Performs authentication, verifies permissions, and

session recovery and checkpoint