Tlv Encoding Of Oids - Magtek DynaFlex II Go Programmer's Manual

8 - Configuration
A.3

TLV Encoding of OIDs

The primary method of encoding OIDs, as specified in X.660 section 8 Basic encoding rules, is BER
Tag-Length-Value (TLV) format. A TLV-encoded OID is a set of nested TLV data objects, where:
•
The tag of each TLV data object equals the primary value of the node it represents in the OID
sequence, encoded according to the BER TLV tag number rules; the assigned primary values in this
standard are all private (11nnnnnn), the bottom level value / leaf of any OID arc is encoded as
primitive (nn0nnnnn), and the levels above it are encoded as composed (nn1nnnnn). This
information makes it possible to use the bit definitions for Tag in section 3.2.1 Tag-Length-Value
(TLV) Encoding as a reference to encode each level of the OID as a tag. Generally the primary
values in this standard are selected to be less than 0x1F, and so do not require multi-byte tags.
•
The length of each TLV data object, like all TLV data objects, is equal to the length of its value.
•
The value of each TLV data object is equal to a nested TLV data object containing all subsequent
nodes in the OID sequence, so TLV-encoded OIDs must be built from the bottom up. The primitive
value in the bottom node of a TLV-encoded OID is the set of actual values contained by that OID
(e.g., string, byte[s], and so on), or in the case of requests where the value is (by definition) not
known to the requester, the length is 0x00.
For example, the process of TLV encoding the OID in a request to retrieve the contents of OID 3.5.7.9
with 9 being a leaf node looks like this (all values are hexadecimal unless otherwise noted):
1) First, realize OID 3.5.7.9 is to be interpreted as a nested set of containers. Node 3 contains node 5,
which contains node 7, which contains node 9. Although node 9 presumably contains one or more
values, the entity making the request for its contents, by definition, does not know the contents when
composing the request, so in this example, node 9 is treated like an "empty bucket." In the
corresponding response, node 9 would include its actual contents.
2) Because TLV is built from the inside out (bottom up), start the encoding process with the node at the
end which has a primary value equal to 9. Like all OIDs in this standard it is private, and because it
is a leaf node at the bottom / end of the OID tree and therefore does not contain any further nested
TLV data objects, it is primitive: 0x09 primary value OR 0b11000000 private OR 0b00000000
primitive = C9. As explained above, in this case it is an empty bucket, so its length is 00. Its TLV
encoding is C9 00.
3) Up one level to the node with primary value equal to 7, it is also private, but unlike the previous level
it contains a nested TLV data object (C9 00 from above) so it is constructed instead of primitive:
0x07 primary value OR 0b11000000 private OR 0b00100000 constructed = E7. It has contents C9
00 so length is 02. Its TLV encoding is E7 02 C9 00.
4) Up one level to the node with primary value 5, it is private and contains TLV data object E7 02 C9
00 from above, so it is constructed: 0x05 primary value OR 0b11000000 private OR 0b00100000
constructed = E5. It has contents E7 02 C9 00 so length is 04. Its TLV encoding is E5 04 E7 02 C9
00.
5) Up to the root with primary value 3, it is private and constructed so tag is E3, value is the TLV data
object from above, so length is equal to that object's length 06. Its encoding is E3 06 E5 04 E7 02
C9 00, which is the final encoding of whole "empty bucket" OID 3.5.7.9.
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