Network Protocol Data Unit (NPDU)

Protocol Identifier
This is set to a fixed value to distinguish this header from other Network Layer protocols.
Length Indicator
Length of the header
PDU lifetime
The remaining lifetime of the PDU in units of 500ms. This is set to an initial value by the sender. Each node that the data unit passes through decrements the lifetime value by the amount corresponding to the time that it has taken to transit the node. When the lifetime value is decremented to zero, the data unit is discarded.
Segmentation permitted flag. This should be set for OSI applications
More segments flag. This indicates the NPDU has been segmented when it is set to 1. When set to 0, the segment either contains the last part of the NPDU or it had not been segmented.
Error report flag. If it is set by the sender, an error report is returned to the sender if the NPDU is discarded.
PDU Type
This is either Data (11100) or Error Report (00001).
Segment length
Length of this segment of the NPDU.
Header error check bits. A NPDU received with an invalid checksum is discarded.
Variable length fields are used to convey NSAP addresses.
Data Unit Identifier
This field is given a unique value by the originating network entity to identify the initial PDU and all derived segments
Segment Offset
Relative position of this segment with respect to the original.
Total length
Length of the original NPDU.
Options include padding, security, source routing, recording of route, QOS and priority.