The fifth layer of the OSI model is the session layer. It organises the data transfer, syn­chron­ises systems. and is re­spons­ible for the con­nec­tion between two end systems. In the event that a con­nec­tion is ter­min­ated, the session layer allows the systems to continue com­mu­nic­at­ing where they left off once the con­nec­tion is restored.

What is the session layer?

The session layer is also known as the com­mu­nic­a­tion man­age­ment layer. It is the fifth of seven layers in the OSI model. This con­cep­tu­al model was developed in the 1970s to create a standard for com­mu­nic­a­tion between different devices and systems within a network.

All seven layers have their own tasks and are built upon each other. The session layer is re­spons­ible for the or­gan­isa­tion of the con­nec­tion between two end systems. To do this, the layer creates sessions, which offer different services. The session layer works closely with the layers around it in the OSI model. The seven layers are as follows:

  1. Physical layer
  2. Data link layer
  3. Network layer
  4. Transport layer
  5. Session layer
  6. Present­a­tion layer
  7. Ap­plic­a­tion layer

How does the session layer manage com­mu­nic­a­tion between systems?

The main task of the session layer is to create a logical con­nec­tion between two systems. This con­nec­tion is known as a session, which is then monitored by the session layer. To enable sessions, this layer tem­por­ar­ily allows access to another system and then manages the com­mu­nic­a­tion between the two systems.

Sessions can occur at the same time, parallel to each other or in one direction. You might hear the term dialog control, which is the re­spons­ib­il­ity of the session layer. In the event of one-sided com­mu­nic­a­tion, the session layer can send out a token to control the direction and allow for un­af­fected dialog.

These tokens are separated into four cat­egor­ies:

  • Data token: these show which side may send data for half-duplex op­er­a­tions.
  • Activity major token: activity major tokens separate a con­nec­tion into different activ­it­ies. If an activity is ter­min­ated or cancelled, it can be restored in the same session or a later one.
  • Syn­chron­ise minor token: these tokens are numbered from 0 to 999,999 and are used to separate com­mu­nic­a­tions. You can find out more about syn­chron­isa­tion in the next section of this article.
  • Release token: these mark the end of the session.

Syn­chron­isa­tion on the session layer

Alongside or­gan­ising and managing com­mu­nic­a­tion, the session layer carries out the syn­chron­isa­tion of the data transfer. This primarily comes into effect when a data transfer is un­ex­pec­tedly and in­vol­un­tar­ily ter­min­ated on the fourth layer or below.

In such cases, the session layer creates special syn­chron­isa­tion points. If the com­mu­nic­a­tion is ter­min­ated, the transfer can be restored at the same point. This is of par­tic­u­lar use if the com­mu­nic­a­tion occurs across a slow or unstable con­nec­tion, or if there is a lot of data being trans­ferred.

The syn­chron­isa­tion points created by the session layer can be broken down into two cat­egor­ies.

  • Major syn­chron­isa­tion points separate data which can be trans­ferred in single units. These syn­chron­isa­tion points must be spe­cific­ally confirmed.
  • Minor syn­chron­isa­tion points ensure logical and practical struc­tures within the units. They do not need to be confirmed.

Which services does the session layer carry out?

The services offered by the session layer are made available to ap­plic­a­tion processes and aim to better organise and syn­chron­ise com­mu­nic­a­tion. They are used when the session layer asks the transport layer (layer 4) to connect to another system. The various services are grouped into func­tion­al units. The In­ter­na­tion­al Or­gan­iz­a­tion for Stand­ard­iz­a­tion (ISO), which also took part in de­vel­op­ing the OSI model, has suggested the following clas­si­fic­a­tion for func­tion­al units. The ap­pro­pri­ate com­bin­a­tion is de­term­ined by both parties prior to the beginning of the session.

  • Basic Combined Subset (BCS): com­pat­ible with kernel, half-duplex and duplex
  • Basic Syn­chron­ised Subset (BSS): com­pat­ible with kernel, half-duplex, ne­go­ti­ated release, minor and major syn­chron­isa­tion points as well as re­syn­chron­isa­tion
  • Basic Combined Subset (BAS): com­pat­ible with kernel, half-duplex, minor syn­chron­isa­tion points, ex­cep­tions, activ­it­ies man­age­ment

Session layer protocols

There are numerous protocols that use the OSI session layer. For this purpose, the session layer makes its protocols and services available to the higher layers via pro­gram­ming in­ter­faces. The para­met­ers and char­ac­ter­ist­ics of the lower layers are not relevant for these com­mu­nic­a­tion con­nec­tions. Among the protocols that use the session layer are:

  • ADSP
  • ASP
  • FTP
  • H.245
  • HTTP
  • ISO-SP
  • iSNS
  • L2F
  • L2TP
  • NetBIOS
  • NNTP
  • PAP
  • PPTP
  • RPC
  • RTCP
  • SCP
  • SDP
  • SMPP
  • SMTP
  • SOCKS
  • Telnet
  • TFTP
  • ZIP
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