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PDU (Protocol Data Unit)

OSI Layers and Name

PDU names for each OSI layers are as follow.

Layer 1 - Physical → Bits
Layer 2 - Data Link → Frame
Layer 3 - Network→ Packet
Layer 4 - Transport → Segment or datagram
Layer 5 - Session → Data
Layer 6 - Presentation → Data
Layer 7 - Application → Data


Since layer 7, 6 and 5 are represent actual data including following basic elements which are component, collaboration, interface, function, interaction, process, event, service and objects. In short it defines how application should talk to another application, defines encryption and decryption of data from application layer, including the data types. Lastly layer 5 controls the connection between computer, in general starting-stopping and managing the connection between applications. If anybody has worked with apache httpd, keep alive variable in configuration closely control this behavior.

In layer 4 which is responsible for delivering data to appropriate application process, addes following values to the header. Source and destination ports, sequence identifier, acknowledgement status, checksum, urgency information, padding, window size and status bits. Layer 4 and 5 are shared for general use of ssl (well-known encryption, for data security widely vary based on how it is implemented.)

In layer 3, PDU named as packet, it added header information such as version, header length, TOS, total length, identification, flags and fragments, TTL, protocol, Header checksum, source and destination.

In Layer 2, PDU named as Frame, it adds header information such as destination, source and type. Header indicates the source and destination layer 2 end points, pay load is IP Packet. Which is also considered to use by directly connected nodes, it also manages the flow control from layer 1. This layer highly uses the MAC layer and logical link layer.

In Layer 1 where the data is steamed in digital or analog signal, which identify the media and transfer varies widely.

pdu.txt · Last modified: 2019/01/10 20:50 by k2patel