1720   Type and size of Length field value (TLV) encoding in SV

Created: 18 Sep 2020

Status: Ballot Period

Part: Part 9-2 (2020; Edition 2.1)

Links:

Page: 16

Clause: 5.4

Paragraph: 3

Issue

Encoding of SV message shall be according to ASN.1 and BER rules. BER rules allow for three length field encoding formats: short definite (1 byte), long definite (minimum 2 bytes) and indefinite format.

SV messages needs to have determinable size, based on (SCL) settings, to enable efficient parsing.

In order for SV message to have deterministic size, encoding of Length field must be well defined. As it is now, any Length field encoding is allowed, leading to indeterministic message size. For example, length of 4 can be coded as 04 (short definite) or 81 04 (long definite).

Length of some fields depends on settings (e.g. svID or ASDU depending on number of U/I element in it) and some are fixed (e.g. smpCnt always 2 bytes), but still Length field encoding of all of them needs to be minimum size.

Proposal

The size for encoding the Length field (TLV) of the variable size elements shall always use minimum length encoding.

This will make SV encoding to be in line with fix-length GOOSE encoding, part 8-1 annex A-3.

Discussion Created Status
Starting ballot period 15 Feb 24 Ballot Period
Test case sSvp7 already states the encoding shall use the minimum length TLV encoding.

No change needed.
25 Jul 23 Conformance Test Verification
Next state: Conformance Test Preparation.
The test procedure for SV publisher shall verify that all BER TLV Length fields of published SV messages are definite form, and encoded using the minimum size BER encoding possible.
09 Dec 22 Conformance Test Preparation
Go to status "Analysis of compatibility":
I'm not aware of any implementation of SV publishers not already complying with the proposal.
As well, I don't expect any SV subscribers not accepting SV complying with the proposal.
07 Nov 22 Analysis Of Compatibility
Proposed change:
adding a sentence to the beginning of the "8.5.2 Presentation Layer Functionality" clause:

[... the sampled value buffer is encoded as specified in Table 14.] All BER TLV Length fields shall be definite form, and encoded using the minimum size BER encoding possible.
07 Oct 22 Verify Draft Implementation

 

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