ISO - CLNP 

Reference of the protocol: Standard ISO 8473 - Connection Less Network Protocol 

PDU structure: fields

PDU length field
9 octets Fixed part
(n-10) octets Address part
6 octets (first n) Segmentation part (optional)
(p-n-5) octets (last p) Option part (optional)
(z-p) octets Data part (optional)

PDU structure: fixed part

bits 8 _ _ _ _ _ _ 1 fixed part
octet 1 Network Layer Protocol Identifier
The value of this field is set to binary 1000 0001 = 81 H to identify this Network Layer Protocol as ISO8473.
octet 2 Length indicator
The Length is indicated by a binary number with a maximum value of 254.
The length indicated is the length in octets of the header.
octet 3 Version / Protocol Identifier Extension
The value of this field is set to binary 1000 0001 = 81 H which identifies the standard version 1 of ISO8473.
octet 4 Lifetime
The PDU Lifetime field is encoded as a binary number representing the remaining lifetime of the PDU in units of 500 milliseconds.
octet 5, bits 8-6 Flags: SP, MS, E/R
octet 5, bits 5-1 Type
octets 6-7 Segment length
The Segment Length field specifies the entire length in octets of the PDU, including both header and data.
octets 8-9 Checksum
The checksum is computed on the entire PDU header.
For the Data PDU this includes the segmentation and option part.
For the Error report PDU this includes the reason for discard field as well.

PDU structure: Flags

- Octet 5: Flags
bit 8, SP Segmentation Permitted
  • 1 = Permitted
  • 0 = Not Permitted
bit 7, MS More Segments
  • 1 = the segmentation has occurred and the last octet of NSDU is not contained in this PDU
  • 0 = the last octet of the PDU is the last octet of NSDU
bit 6, ER Error Report
  • 1 = used to detemine whether to generate an Error Report PDU if it is necessary to discard this PDU
  • 0 = discard of the Data PDU will not cause the generation of an Error Report PDU


PDU structure: Type

bits 5-4-3-2-1 Type designation
1 1 1 0 0 DT Data TPDU
1 0 0 0 0 ER Error

PDU structure: option part

The number of parameters that may be contained in the options part is constrained by the length of the option part which is determined by the following formula:
Option part Length= PDU Header Length - (length fixed part + length address part + length segmentation part)
bits 8 _ _ _ _ _ _ 1 option part
octet n+6 Parameter Code
octet n+7 Parameter Length
octets (n+8)-p Parameter Value

PDU structure: segmentation part

If the SP Flag =1 the segmentation part of the Header shall be present.

bits 8 _ _ _ _ _ _ 1 segmentation part
octets n-(n+1) Data Unit Identifier
The Data Unit Identifier identifies an initial PDU (and hence its derived PDU)
so that a segmented data unit may be correctly reassembled.
octets (n+2)-(n+3) Segment offset
For each derived PDU, the Segment offset specifies the relative position
of the segment contained in the Data Part of the derived PDU with respect to the start
of the Data Part of the initial PDU.
octets (n+4)-(n+5) Total Length
The Total Length field specifies the entire length of the initial PDU in octets,
 including the header and data.

PDU structure: address part

bits 8 _ _ _ _ _ _ 1 address part
octet 10 Destination Address Length Indicator
octets 11-(m-1) Destination Address
octet m Source Address Length Indicator
octets (m+1)-(n-1) Source Address

PDU structure: data part

bits 8 _ _ _ _ _ _ 1 data part
octets (p+1)-z data field

 

 

 

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