Page 209 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes For example, if you are performing a one-hour file transfer between two machines, and a
network crash occurs approximately every 30 minutes, you might never be able to complete the
file transfer. After each transfer aborts, you have to start all over again. To avoid this problem,
you can treat the entire file transfer as a single activity with checkpoints inserted into the data
stream. That way, if a crash occurs, the session layer can synchronize to a previous checkpoint.
These checkpoints are called synchronization points.
There are two types of synchronization points: major and minor synchronization points. A
major synchronized point inserted by any communicating side must be acknowledged by the
other communicating side, whereas a minor synchronization point is not acknowledged. That
portion of the session that is between two major synchronization points is called a dialog unit.
The operation of managing an entire activity is called activity management. An activity can
consist of one or more dialog units.
Synchronization is accomplished with the use of sequence numbers. The session layer protocols
provide both major and minor synchronization points. Major synchronization points divide a
message into a series of dialogs. Each major synchronization points are acknowledged before
the session is continued. When an error happens, data is recovered only up to the last major
point. A session layer activity is divided into a single dialog or several dialogs separated by
major synchronization points. If resynchronizing, one only goes back as far as the previous
major synchronization point. Besides, major synchronization points are acknowledged with the
help of the explicit messages thus making their use expensive.
Minor synchronization points are just markers and inserted into the middle of dialogs. They,
depending on the applications, may or may not require acknowledgement. They are security
zones to recover the data from one or more minor synchronization points within a dialog when
an error happens.

Notes It is to be noted carefully that major synchronization points are required to be
acknowledged so that the control could begin retransmission of data from the point of last
major synchronization points just before the point of occurrence of the errors. The minor
synchronization points are used as security blankets and therefore do not need to
acknowledge. When an error happens, the control goes back to one or more minor
synchronization points to retransmit the data.
Figure 13.1 explains the concept of synchronization point lucidly.

Figure 13.1: Synchronization Points
An activity made of only one dialog
Dialog

Major Sync Minor Minor Sync Minor Sync Major Sync
Sync

An activity made of more than one dialog
Dialog Dialog Dialog

Major Minor Minor Major Minor Minor Major Minor Minor Major
Sync Sync Sync Sync Sync Sync Sync Sync Sync Sync

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