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
- 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.