| Internet-Draft | SCHC AC | February 2023 |
| Minaburo & Toutain | Expires 27 August 2023 | [Page] |
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RFC9363 defines a YANG Data Model for SCHC rules. [I-D.ietf-core-sid] specifies the process for SID allocation and management. This document discuss of the SID allocation for RFC9363.¶
The version @2023-01-18 of the SCHC YANG Data Model published in the RFC 9363 contains 136 SIDs (92 for identities, 2 for features and 42 for data). [I-D.ietf-core-sid] indicates that the SID range for the YANG Data Model specified in RFC is between 1000 and 59 000 and that the maximum request pool SHOULD NOT exceed 1000. The draft also gives some pre allocated values.¶
Since SIDs will be used either to represent unique identity contained in the data model and also leaves (data) forming this data model, it could be wise to distinguish between identifiers and data.¶
Data structures are delta encoded and included as a CBOR element, the size depends on the value. Deltas between -24 and +23 are encoded on a single byte. Deltas between -256 and +255 use 2 bytes and larger values corresponding to the RFC SID range will be encoded into 3 bytes. To optimize the CORECONF representation delta should be smaller as possible for the more frequent leaves.¶
On the other hand identities are included in the CORECONF representation and for the RFC SID range the size is constant and equal to 3 bytes.¶
CORECONF
{5095: {1: [{4:
[{1: 5015,
5: 5018,
6: 5068,
7: 4,
8: 1,
9: 5083,
13: [{1: 0, 2: h'06'}]},
{1: 5015,
5: 5018,
6: 2000003,
7: 8,
8: 1,
9: 5083,
13: [{1: 0, 2: h'00'}]}]
}]}
}
RESTCONF
{"ietf-schc:schc": {"rule": [{"entry":
[{"comp-decomp-action": "ietf-schc:cda-not-sent",
"direction-indicator": "ietf-schc:di-bidirectional",
"field-id": "ietf-schc:fid-ipv6-version",
"field-length": 4,
"field-position": 1,
"matching-operator": "ietf-schc:mo-equal",
"target-value": [{"index": 0, "value": "Bg=="}]},
{"comp-decomp-action": "ietf-schc:cda-not-sent",
"direction-indicator": "ietf-schc:di-bidirectional",
"field-id": "ietf-schc-oam:fid-icmpv6-type",
"field-length": 8,
"field-position": 1,
"matching-operator": "ietf-schc:mo-equal",
"target-value": [{"index": 0, "value": "gA=="}]} ]
}]}
}
The example in Figure 1 gives a CORECONF structure transposed the CBOR diagnostic notation and its equivalent in RESTCONF with JSON. For readability and compactness, this example is edited and do not encode a full rule as defined in RFC9363.¶
The default SID numbering produced by pyang is used, starting from 5000 for SCHC Data Model defined in RFC9363 and 2000000 for an experimental module for OAM.¶
We can see the delta encoding. The first SID 5095 represents "ietf-schc:schc". "/ietf-schc:schc/rule" which is coded with a +1 since SID 5096 has been assigned. "/ietf-schc:schc/rule/entry" is coded with a delta of 4. Then a list of Field Description follows. +1 represents the leaf "ietf-schc:schc/rule/entry/comp-decomp-action" and the value assigned to that key contains the SID of "ietf-schc:cda-not-sent" identity.¶
Note that the second element contains a "field-id" belonging to the "ietf-schc-oam" module and the associate SID is 2000003.¶
The SCHC YANG Data Model defined in RFC 9363 will probably be augmented, to include for instance access control data. To keep a compact representation, delta values must be kept as small as possible. The LPWAN working group should not use the automatic SID numbering and provide a more optimal allocation scheme for augmentation of the SCHC YANG Data Model.¶
A first recommendation is to avoid merging data and identity in order to limit the delta encoding. The distance between these two sections can be 255 SID to allow deltas on 2 bytes.¶
The second recommendation is to leave some unused SID around SCHC rules to allow augmentation.¶
We propose to use a range of 300 values for the YANG Data Model defined in RFC9263, which introduce room for future augmentation of the Data Model, such as [I-D.toutain-lpwan-access-control] or [I-D.ietf-lpwan-schc-compound-ack]. This will break the automatic allocation process done by pyang and based on the nature of the SID and the alphabetical order.¶
It is also worth noting that in the current SID allocation based on alphabetical order places rule-id-value and rule-id-length, rule-nature from the 33 to 35 position. CBOR encoding will be on two bytes for each of the values. Since these three values are present in all the rules, a smaller value will optimize the CORECONF representation.¶
The allocation algorithm is the following:¶
The LPWAN group will receive an range of SID values (we suppose starting at 5000). The SIDs will be allocated following the previous algorithm.¶
Other RFCs modifying the SCHC YANG Data Model will include a YANG module. The lpwan WG will decide of the SID allocation and produce a SID file with the mapping.¶
We propose the following allocation scheme for RFC9363:¶
5000 - 5022 : RESERVED FOR /ietf-schc:schc 5023 module ietf-schc 5024 data /ietf-schc:schc 5025 - 5046 : RESERVED FOR /ietf-schc:schc AND /ietf-schc:schc/rule 5047 data /ietf-schc:schc/rule 5048 data /ietf-schc:schc/rule/rule-id-length 5049 data /ietf-schc:schc/rule/rule-id-value 5050 data /ietf-schc:schc/rule/rule-nature 5051 - 5069 : RESERVED FOR /ietf-schc:schc/rule AND /ietf-schc:schc/rule/entry 5070 data /ietf-schc:schc/rule/entry 5071 data /ietf-schc:schc/rule/entry/comp-decomp-action 5072 data /ietf-schc:schc/rule/entry/comp-decomp-action-value 5073 data /ietf-schc:schc/rule/entry/comp-decomp-action-value/index 5074 data /ietf-schc:schc/rule/entry/comp-decomp-action-value/value 5075 data /ietf-schc:schc/rule/entry/direction-indicator 5076 data /ietf-schc:schc/rule/entry/field-id 5077 data /ietf-schc:schc/rule/entry/field-length 5078 data /ietf-schc:schc/rule/entry/field-position 5079 data /ietf-schc:schc/rule/entry/matching-operator 5080 data /ietf-schc:schc/rule/entry/matching-operator-value 5081 data /ietf-schc:schc/rule/entry/matching-operator-value/index 5082 data /ietf-schc:schc/rule/entry/matching-operator-value/value 5083 data /ietf-schc:schc/rule/entry/target-value 5084 data /ietf-schc:schc/rule/entry/target-value/index 5085 data /ietf-schc:schc/rule/entry/target-value/value 5086 - 5094 : RESERVED 5094 data /ietf-schc:schc/rule/ack-behavior 5095 data /ietf-schc:schc/rule/direction 5096 data /ietf-schc:schc/rule/dtag-size 5097 data /ietf-schc:schc/rule/fcn-size 5098 data /ietf-schc:schc/rule/fragmentation-mode 5099 data /ietf-schc:schc/rule/inactivity-timer 5100 data /ietf-schc:schc/rule/inactivity-timer/ticks-duration 5101 data /ietf-schc:schc/rule/inactivity-timer/ticks-numbers 5102 data /ietf-schc:schc/rule/l2-word-size 5103 data /ietf-schc:schc/rule/max-ack-requests 5104 data /ietf-schc:schc/rule/max-interleaved-frames 5105 data /ietf-schc:schc/rule/maximum-packet-size 5106 data /ietf-schc:schc/rule/rcs-algorithm 5107 data /ietf-schc:schc/rule/retransmission-timer 5108 data /ietf-schc:schc/rule/retransmission-timer/ticks-duration 5109 data /ietf-schc:schc/rule/retransmission-timer/ticks-numbers 5110 - 5115 : RESERVED FOR TIMER 5116 data /ietf-schc:schc/rule/tile-in-all-1 5117 data /ietf-schc:schc/rule/tile-size 5118 data /ietf-schc:schc/rule/w-size 5119 data /ietf-schc:schc/rule/window-size 5120 - 5299 : RESERVED FOR 2 BYTES DELTAS 5300 identity ack-behavior-after-all-0 5301 identity ack-behavior-after-all-1 5302 identity ack-behavior-base-type 5303 identity ack-behavior-by-layer2 5304 identity all-1-data-base-type 5305 identity all-1-data-no 5306 identity all-1-data-sender-choice 5307 identity all-1-data-yes 5308 identity cda-appiid 5309 identity cda-base-type 5310 identity cda-compute 5311 identity cda-deviid 5312 identity cda-lsb 5313 identity cda-mapping-sent 5314 identity cda-not-sent 5315 identity cda-value-sent 5316 identity di-base-type 5317 identity di-bidirectional 5318 identity di-down 5319 identity di-up 5320 identity fid-base-type 5321 identity fid-coap-base-type 5322 identity fid-coap-code 5323 identity fid-coap-code-class 5324 identity fid-coap-code-detail 5325 identity fid-coap-mid 5326 identity fid-coap-option 5327 identity fid-coap-option-accept 5328 identity fid-coap-option-block1 5329 identity fid-coap-option-block2 5330 identity fid-coap-option-content-format 5331 identity fid-coap-option-etag 5332 identity fid-coap-option-if-match 5333 identity fid-coap-option-if-none-match 5334 identity fid-coap-option-location-path 5335 identity fid-coap-option-location-query 5336 identity fid-coap-option-max-age 5337 identity fid-coap-option-no-response 5338 identity fid-coap-option-observe 5339 identity fid-coap-option-oscore-flags 5340 identity fid-coap-option-oscore-kid 5341 identity fid-coap-option-oscore-kidctx 5342 identity fid-coap-option-oscore-piv 5343 identity fid-coap-option-proxy-scheme 5344 identity fid-coap-option-proxy-uri 5345 identity fid-coap-option-size1 5346 identity fid-coap-option-size2 5347 identity fid-coap-option-uri-host 5348 identity fid-coap-option-uri-path 5349 identity fid-coap-option-uri-port 5350 identity fid-coap-option-uri-query 5351 identity fid-coap-tkl 5352 identity fid-coap-token 5353 identity fid-coap-type 5354 identity fid-coap-version 5355 identity fid-ipv6-appiid 5356 identity fid-ipv6-appprefix 5357 identity fid-ipv6-base-type 5358 identity fid-ipv6-deviid 5359 identity fid-ipv6-devprefix 5360 identity fid-ipv6-flowlabel 5361 identity fid-ipv6-hoplimit 5362 identity fid-ipv6-nextheader 5363 identity fid-ipv6-payload-length 5364 identity fid-ipv6-trafficclass 5365 identity fid-ipv6-trafficclass-ds 5366 identity fid-ipv6-trafficclass-ecn 5367 identity fid-ipv6-version 5368 identity fid-oscore-base-type 5369 identity fid-udp-app-port 5370 identity fid-udp-base-type 5371 identity fid-udp-checksum 5372 identity fid-udp-dev-port 5373 identity fid-udp-length 5374 identity fl-base-type 5375 identity fl-token-length 5376 identity fl-variable 5377 identity fragmentation-mode-ack-always 5378 identity fragmentation-mode-ack-on-error 5379 identity fragmentation-mode-base-type 5380 identity fragmentation-mode-no-ack 5381 identity mo-base-type 5382 identity mo-equal 5383 identity mo-ignore 5384 identity mo-match-mapping 5385 identity mo-msb 5386 identity nature-base-type 5387 identity nature-compression 5388 identity nature-fragmentation 5389 identity nature-no-compression 5390 identity rcs-algorithm-base-type 5391 identity rcs-crc32 5392 feature compression 5393 feature fragmentation 5394 - 5500 : RESERVED FOR IDENTITY¶
For instance [I-D.toutain-lpwan-access-control] augments the model with "ac-modify-set-of-rules" at the top level, "ac-modify-compression-rule" for each compression rule, "ac-modify-field" in each Field Description of a compression rule and finally "ac-modify-timers" in fragmentation rules. Delta representation will be on 1 byte.¶
The following SIDs could be assigned:¶
[I-D.ietf-lpwan-schc-compound-ack] augments the model for fragmentation, with 3 identity and two leaves. identities can get a SID 5394 to 5396 and the two SIDs for the leaves can be 5120 and 5122. There delta representations will be coded on 2 bytes.¶