Data Model for Static Context Header Compression (SCHC)
draft-ietf-lpwan-schc-yang-data-model-06
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9363.
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|
|---|---|---|---|
| Authors | Ana Minaburo , Laurent Toutain | ||
| Last updated | 2021-11-24 (Latest revision 2021-09-09) | ||
| Replaces | draft-toutain-lpwan-schc-yang-data-model | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-lpwan-schc-yang-data-model-06
lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Standards Track L. Toutain
Expires: 28 May 2022 Institut MINES TELECOM; IMT Atlantique
24 November 2021
Data Model for Static Context Header Compression (SCHC)
draft-ietf-lpwan-schc-yang-data-model-06
Abstract
This document describes a YANG data model for the SCHC (Static
Context Header Compression) compression and fragmentation rules.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 28 May 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SCHC rules . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Compression Rules . . . . . . . . . . . . . . . . . . . . 3
2.2. Identifier generation . . . . . . . . . . . . . . . . . . 4
2.3. Field Identifier . . . . . . . . . . . . . . . . . . . . 5
2.4. Field length . . . . . . . . . . . . . . . . . . . . . . 7
2.5. Field position . . . . . . . . . . . . . . . . . . . . . 8
2.6. Direction Indicator . . . . . . . . . . . . . . . . . . . 8
2.7. Target Value . . . . . . . . . . . . . . . . . . . . . . 9
2.8. Matching Operator . . . . . . . . . . . . . . . . . . . . 10
2.8.1. Matching Operator arguments . . . . . . . . . . . . . 11
2.9. Compression Decompression Actions . . . . . . . . . . . . 11
2.9.1. Compression Decompression Action arguments . . . . . 13
2.10. Fragmentation rule . . . . . . . . . . . . . . . . . . . 13
2.10.1. Fragmentation mode . . . . . . . . . . . . . . . . . 13
2.10.2. Fragmentation Header . . . . . . . . . . . . . . . . 14
2.10.3. Last fragment format . . . . . . . . . . . . . . . . 15
2.10.4. Acknowledgment behavior . . . . . . . . . . . . . . 17
2.10.5. Fragmentation Parameters . . . . . . . . . . . . . . 18
2.10.6. Layer 2 parameters . . . . . . . . . . . . . . . . . 19
3. Rule definition . . . . . . . . . . . . . . . . . . . . . . . 19
3.1. Compression rule . . . . . . . . . . . . . . . . . . . . 21
3.2. Fragmentation rule . . . . . . . . . . . . . . . . . . . 24
3.3. YANG Tree . . . . . . . . . . . . . . . . . . . . . . . . 26
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
5. Security considerations . . . . . . . . . . . . . . . . . . . 28
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
7. YANG Module . . . . . . . . . . . . . . . . . . . . . . . . . 28
8. Normative References . . . . . . . . . . . . . . . . . . . . 49
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction
2. SCHC rules
SCHC is a compression and fragmentation mechanism for constrained
networks defined in [RFC8724]. It is based on a static context
shared by two entities at the boundary of the constrained network.
[RFC8724] provides a non formal representation of the rules used
either for compression/decompression (or C/D) or fragmentation/
reassembly (or F/R). The goal of this document is to formalize the
description of the rules to offer:
* the same definition on both ends, even if the internal
representation is different.
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* an update of the other end to set up some specific values (e.g.
IPv6 prefix, Destination address,...)
* ...
This document defines a YANG module to represent both compression and
fragmentation rules, which leads to common representation for values
for all the rules elements.
SCHC compression is generic, the main mechanism does not refer to a
specific protocol. Any header field is abstracted through an ID, a
position, a direction, and a value that can be a numerical value or a
string. [RFC8724] and [RFC8824] specify fields for IPv6, UDP, CoAP
and OSCORE.
SCHC fragmentation requires a set of common parameters that are
included in a rule. These parameters are defined in [RFC8724].
2.1. Compression Rules
[RFC8724] proposes a non formal representation of the compression
rule. A compression context for a device is composed of a set of
rules. Each rule contains information to describe a specific field
in the header to be compressed.
+-----------------------------------------------------------------+
| Rule N |
+-----------------------------------------------------------------+|
| Rule i ||
+-----------------------------------------------------------------+||
| (FID) Rule 1 |||
|+-------+--+--+--+------------+-----------------+---------------+|||
||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+--+------------+-----------------+---------------+|||
||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+--+------------+-----------------+---------------+|||
||... |..|..|..| ... | ... | ... ||||
|+-------+--+--+--+------------+-----------------+---------------+||/
||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
|+-------+--+--+--+------------+-----------------+---------------+|/
| |
\-----------------------------------------------------------------/
Figure 1: Compression Decompression Context
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2.2. Identifier generation
Identifier used in the SCHC YANG Data Model are from the identityref
statement to ensure to be globally unique and be easily augmented if
needed. The principle to define a new type based on a group of
identityref is the following:
* define a main identity ending with the keyword base-type.
* derive all the identity used in the Data Model from this base
type.
* create a typedef from this base type.
The example (Figure 2) shows how an identityref is created for RCS
algorithms used during SCHC fragmentation.
// -- RCS algorithm types
identity rcs-algorithm-base-type {
description
"Identify which algorithm is used to compute RSC.
The algorithm also defines the size if the RSC field.";
}
identity rcs-RFC8724 {
base rcs-algorithm-base-type;
description
"CRC 32 defined as default RCS in RFC8724.";
}
typedef rcs-algorithm-type {
type identityref {
base rcs-algorithm-base-type;
}
description
"type used in rules";
}
Figure 2: Principle to define a type based on identityref.
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2.3. Field Identifier
In the process of compression, the headers of the original packet are
first parsed to create a list of fields. This list of fields is
matched against the rules to find the appropriate rule and apply
compression. [RFC8724] do not state how the field ID value can be
constructed. In examples, identification is done through a string
indexed by the protocol name (e.g. IPv6.version, CoAP.version,...).
The current YANG Data Model includes fields definitions found in
[RFC8724], [RFC8824].
Using the YANG model, each field MUST be identified through a global
YANG identityref. A YANG field ID for the protocol always derives
from the fid-base-type. Then an identity for each protocol is
specified using the naming convention fid-<<protocol name>>-base-
type. All possible fields for this protocol MUST derive from the
protocol identity. The naming convention is "fid" followed by the
protocol name and the field name. If a field has to be divided into
sub-fields, the field identity serves as a base.
The full field-id definition is found in Section 7. The example
Figure 3 gives the first field ID definitions. A type is defined for
IPv6 protocol, and each field is based on it. Note that the DiffServ
bits derives from the Traffic Class identity.
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identity fid-base-type {
description
"Field ID base type for all fields";
}
identity fid-ipv6-base-type {
base fid-base-type;
description
"Field IP base type for IPv6 headers described in RFC 8200";
}
identity fid-ipv6-version {
base fid-ipv6-base-type;
description
"IPv6 version field from RFC8200";
}
identity fid-ipv6-trafficclass {
base fid-ipv6-base-type;
description
"IPv6 Traffic Class field from RFC8200";
}
identity fid-ipv6-trafficclass-ds {
base fid-ipv6-trafficclass;
description
"IPv6 Traffic Class field from RFC8200,
DiffServ field from RFC3168";
}
...
Figure 3: Definition of identityref for field IDs
The type associated to this identity is fid-type (cf. Figure 4)
typedef fid-type {
type identityref {
base fid-base-type;
}
description
"Field ID generic type.";
}
Figure 4: Type definition for field IDs
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2.4. Field length
Field length is either an integer giving the size of a field in bits
or a specific function. [RFC8724] defines the "var" function which
allows variable length fields in byte and [RFC8824] defines the "tkl"
function for managing the CoAP Token length field.
The naming convention is "fl" followed by the function name.
identity fl-base-type {
description
"Used to extend field length functions.";
}
identity fl-variable {
base fl-base-type;
description
"Residue length in Byte is sent as defined in
for CoAP in RFC 8824 (cf. 5.3).";
}
identity fl-token-length {
base fl-base-type;
description
"Residue length in Byte is sent as defined in
for CoAP in RFC 8824 (cf. 4.5).";
}
Figure 5: Definition of identityref for Field Length
Field ID, field length function can be defined as an identityref as
shown in Figure 5.
Therefore, the type for field length is a union between an integer
giving in bits the size of the length and the identityref (cf.
Figure 6).
typedef fl-type {
type union {
type int64; /* positive length in bits */
type identityref { /* function */
base fl-base-type;
}
}
description
"Field length either a positive integer giving the size in bits
or a function defined through an identityref.";
}
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Figure 6: Type definition for field Length
2.5. Field position
Field position is a positive integer which gives the position of a
field, the default value is 1, and incremented at each repetition.
value 0 indicates that the position is not important and is not
considered during the rule selection process.
Field position is a positive integer. The type is an uint8.
2.6. Direction Indicator
The Direction Indicator (di) is used to tell if a field appears in
both direction (Bi) or only uplink (Up) or Downlink (Dw).
identity di-base-type {
description
"Used to extend direction indicators.";
}
identity di-bidirectional {
base di-base-type;
description
"Direction Indication of bi directionality in
RFC 8724 (cf. 7.1).";
}
identity di-up {
base di-base-type;
description
"Direction Indication of upstream defined in
RFC 8724 (cf. 7.1).";
}
identity di-down {
base di-base-type;
description
"Direction Indication of downstream defined in
RFC 8724 (cf. 7.1).";
}
Figure 7: Definition of identityref for direction indicators
Figure 7 gives the identityref for Direction Indicators. The naming
convention is "di" followed by the Direction Indicator name.
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The type is "di-type" (cf. Figure 8).
typedef di-type {
type identityref {
base di-base-type;
}
description
"Direction in LPWAN network, up when emitted by the device,
down when received by the device, bi when emitted or
received by the device.";
}
Figure 8: Type definition for direction indicators
2.7. Target Value
The Target Value is a list of binary sequences of any length, aligned
on the left. Figure 9 gives the definition of a single element of a
Target Value. In the rule, this will be used as a list, with
position as a key. The highest position value is used to compute the
size of the index sent in residue for LSB CDA. The position allows
to specify several values:
* For Equal and LSB, a single value is used, such as for the equal
or LSB CDA, the position is set to 0.
* For match-mapping, several of these values can be contained in a
Target Value field. Position values must start from 0 and be
contiguous.
grouping tv-struct {
description
"Define the target value element. Always a binary type, strings
must be converted to binary. field-id allows the conversion
to the appropriate type.";
leaf value {
type binary;
description
"Target Value";
}
leaf position {
type uint16;
description
"If only one element position is 0, otherwise position is the
the position in the matching list.";
}
}
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Figure 9: Definition of target value
2.8. Matching Operator
Matching Operator (MO) is a function applied between a field value
provided by the parsed header and the target value. [RFC8724]
defines 4 MO as listed in Figure 10.
identity mo-base-type {
description
"Used to extend Matching Operators with SID values";
}
identity mo-equal {
base mo-base-type;
description
"Equal MO as defined RFC 8724 (cf. 7.3)";
}
identity mo-ignore {
base mo-base-type;
description
"Ignore MO as defined RFC 8724 (cf. 7.3)";
}
identity mo-msb {
base mo-base-type;
description
"MSB MO as defined RFC 8724 (cf. 7.3)";
}
identity mo-match-mapping {
base mo-base-type;
description
"match-mapping MO as defined RFC 8724 (cf. 7.3)";
}
Figure 10: Definition of identityref for Matching Operator
The naming convention is "mo" followed by the MO name.
The type is "mo-type" (cf. Figure 11)
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typedef mo-type {
type identityref {
base mo-base-type;
}
description
"Matching Operator (MO) to compare fields values with
target values";
}
Figure 11: Type definition for Matching Operator
2.8.1. Matching Operator arguments
They are viewed as a list of tv-struct.
2.9. Compression Decompression Actions
Compression Decompression Action (CDA) identified the function to use
either for compression or decompression. [RFC8724] defines 6 CDA.
Figure 13 gives some CDA definition, the full definition is in
Section 7.
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identity cda-base-type {
description
"Compression Decompression Actions.";
}
identity cda-not-sent {
base cda-base-type;
description
"not-sent CDA as defines in RFC 8724 (cf. 7.4).";
}
identity cda-value-sent {
base cda-base-type;
description
"value-sent CDA as defines in RFC 8724 (cf. 7.4).";
}
identity cda-lsb {
base cda-base-type;
description
"LSB CDA as defines in RFC 8724 (cf. 7.4).";
}
identity cda-mapping-sent {
base cda-base-type;
description
"mapping-sent CDA as defines in RFC 8724 (cf. 7.4).";
}
....
Figure 12: Definition of identityref for Compresion Decompression
Action
The naming convention is "cda" followed by the CDA name.
typedef cda-type {
type identityref {
base cda-base-type;
}
description
"Compression Decompression Action to compression or
decompress a field.";
}
Figure 13: Type definition for Compresion Decompression Action
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2.9.1. Compression Decompression Action arguments
Currently no CDA requires arguments, but the future some CDA may
require several arguments. They are viewed as a list of target-
values-type.
2.10. Fragmentation rule
Fragmentation is optional in the data model and depends on the
presence of the "fragmentation" feature.
Most of parameters for fragmentation are defined in Annex D of
[RFC8724].
Since fragmentation rules work for a specific direction, they contain
a mandatory direction. The type is the same as the one used in
compression entries, but the use of bidirectional is forbidden.
2.10.1. Fragmentation mode
[RFC8724] defines 3 fragmentation modes:
* No Ack: this mode is unidirectionnal, no acknowledgment is sent
back.
* Ack Always: each fragmentation window must be explicitly
acknowledged before going to the next.
* Ack on Error: A window is acknowledged only when the receiver
detects some missing fragments.
Figure 14 give the definition for identifiers from these three modes.
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identity fragmentation-mode-base-type {
description
"fragmentation mode.";
}
identity fragmentation-mode-no-ack {
base fragmentation-mode-base-type;
description
"No Ack of RFC 8724.";
}
identity fragmentation-mode-ack-always {
base fragmentation-mode-base-type;
description
"Ack Always of RFC8724.";
}
identity fragmentation-mode-ack-on-error {
base fragmentation-mode-base-type;
description
"Ack on Error of RFC8724.";
}
typedef fragmentation-mode-type {
type identityref {
base fragmentation-mode-base-type;
}
description
"type used in rules";
}
Figure 14: Definition of fragmentation mode identifer
The naming convention is "fragmentation-mode" followed by the
fragmentation mode name.
2.10.2. Fragmentation Header
A data fragment header, directly following the rule ID can be sent on
the fragmentation direction. The direction is mandatory and must be
up or down. bidirectional is forbidden. The SCHC header may be
composed of (cf. Figure 15):
* a Datagram Tag (Dtag) identifying the datagram being fragmented if
the fragmentation applies concurrently on several datagrams. This
field in optional and its length is defined by the rule.
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* a Window (W) used in Ack-Always and Ack-on-Error modes. In Ack-
Always, its size is 1 and depends on the rule in Ack-on-Error.
This field is not need in No-Ack mode.
* a Fragment Compressed Number (FCN) indicating the fragment/tile
position on the window. This field is mandatory on all modes
defined in [RFC8724], its size is defined by the rule.
|-- SCHC Fragment Header ----|
|-- T --|-M-|-- N --|
+-- ... -+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~
| RuleID | DTag | W | FCN | Fragment Payload | padding (as needed)
+-- ... -+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~
Figure 15: Data fragment header from RFC8724
2.10.3. Last fragment format
The last fragment of a datagram is sent with an RCS (Reassembly Check
Sequence) field to detect residual transmission error and possible
losses in the last window. [RFC8724] defines a single algorithm
based on Ethernet CRC computation. The identity of the RCS algorithm
is shown in Figure 16.
// -- RCS algorithm types
identity rcs-algorithm-base-type {
description
"Identify which algorithm is used to compute RSC.
The algorithm also defines the size if the RSC field.";
}
identity rcs-RFC8724 {
base rcs-algorithm-base-type;
description
"CRC 32 defined as default RCS in RFC8724.";
}
typedef rcs-algorithm-type {
type identityref {
base rcs-algorithm-base-type;
}
description
"type used in rules.";
}
Figure 16: type definition for RCS
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The naming convention is "rcs" followed by the algorithm name.
For Ack-on-Error mode, the All-1 fragment may just contain the RCS or
can include a tile. The parameters defined in Figure 17 allows to
define the behavior:
* all1-data-no: the last fragment contains no data, just the RCS
* all1-data-yes: the last fragment includes a single tile and the
RCS
* all1-data-sender-choice: the last fragment may or may not contain
a single tile. The receiver can detect if a tile is present.
// -- All1 with data types
identity all1-data-base-type {
description
"Type to define when to send an Acknowledgment message.";
}
identity all1-data-no {
base all1-data-base-type;
description
"All1 contains no tiles.";
}
identity all1-data-yes {
base all1-data-base-type;
description
"All1 MUST contain a tile.";
}
identity all1-data-sender-choice {
base all1-data-base-type;
description
"Fragmentation process choose to send tiles or not in all1.";
}
typedef all1-data-type {
type identityref {
base all1-data-base-type;
}
description
"Type used in rules.";
}
Figure 17: type definition for RCS
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The naming convention is "all1-data" followed by the behavior
identifier.
2.10.4. Acknowledgment behavior
A cknowledgment fragment header goes in the opposite direction of
data. The header is composed of (see Figure 18):
* a Dtag (if present).
* a mandatory window as in the data fragment.
* a C bit giving the status of RCS validation. In case of failure,
a bitmap follows, indicating received fragment/tile. The size of
the bitmap is given by the FCN value.
NOTE: IN THE DATA MODEL THERE IS A max-window-size FIELD TO LIMIT THE
BITMAP SIZE, BUT IS NO MORE IN RFC8724! DO WE KEEP IT?
|--- SCHC ACK Header ----|
|-- T --|-M-| 1 |
+-- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
| RuleID | DTag | W |C=1| padding as needed (success)
+-- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
+-- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
| RuleID | DTag | W |C=0|Compressed Bitmap| pad. as needed (failure)
+-- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
Figure 18: Acknowledgment fragment header for RFC8724
For Ack-on-Error, SCHC defined when acknowledgment can be sent. This
can be at any time defined by the layer 2, at the end of a window
(FCN All-0) or at the end of the fragment (FCN All-1). The following
identifiers (cf. Figure 19) define the acknowledgment behavior.
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// -- Ack behavior
identity ack-behavior-base-type {
description
"Define when to send an Acknowledgment .";
}
identity ack-behavior-after-All0 {
base ack-behavior-base-type;
description
"Fragmentation expects Ack after sending All0 fragment.";
}
identity ack-behavior-after-All1 {
base ack-behavior-base-type;
description
"Fragmentation expects Ack after sending All1 fragment.";
}
identity ack-behavior-always {
base ack-behavior-base-type;
description
"Fragmentation expects Ack after sending every fragment.";
}
typedef ack-behavior-type {
type identityref {
base ack-behavior-base-type;
}
description
"Type used in rules.";
}
Figure 19: bitmap generation behavior
The naming convention is "ack-behavior" followed by the algorithm
name.
2.10.5. Fragmentation Parameters
The state machine requires some common values to handle
fragmentation:
* retransmission-timer gives in seconds the duration before sending
an ack request (cf. section 8.2.2.4. of [RFC8724]). If specified,
value must be higher or equal to 1.
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* inactivity-timer gives in seconds the duration before aborting
(cf. section 8.2.2.4. of [RFC8724]), value of 0 explicitly
indicates that this timer is disabled.
* max-ack-requests gives the number of attempts before aborting (cf.
section 8.2.2.4. of [RFC8724]).
* maximum-packet-size gives in bytes the larger packet size that can
be reassembled.
The are defined as unsigned integer, see Section 7.
2.10.6. Layer 2 parameters
The data model includes two parameters needed for fragmentation:
* l2-word-size: [RFC8724] base fragmentation on a layer 2 word which
can be of any length. The default value is 8 and correspond to
the default value for byte aligned layer 2. A value of 1 will
indicate that there is no alignment and no need for padding.
* maximum-packet-size: defines the maximum size of a uncompressed
datagram. By default, the value is set to 1280 bytes.
They are defined as unsigned integer, see Section 7.
3. Rule definition
A rule is either a C/D or an F/R rule. A rule is identified by the
rule ID value and its associated length. The YANG grouping rule-id-
type defines the structure used to represent a rule ID. Length of 0
is allowed to represent an implicit rule.
Three types of rules are defined in [RFC8724]:
* Compression: a compression rule is associated to the rule ID.
* No compression: nothing is associated to the rule ID.
* Fragmentation: fragmentation parameters are associated to the rule
ID. Fragmentation is optional and feature "fragmentation" should
be set.
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grouping rule-id-type {
leaf rule-id-value {
type uint32;
description
"Rule ID value, this value must be unique combined with
the length.";
}
leaf rule-id-length {
type uint8 {
range "0..32";
}
description
"Rule ID length in bits, value 0 is for implicit rules.";
}
description
"A rule ID is composed of a value and a length in bit.";
}
// SCHC table for a specific device.
container schc {
list rule {
key "rule-id-value rule-id-length";
uses rule-id-type;
choice nature {
case fragmentation {
//if-feature "fragmentation";
uses fragmentation-content;
}
case compression {
uses compression-content;
}
case no-compression {
description
"RFC8724 allows a rule for uncompressed headers.";
}
description
"A rule is either for compression, no compression or
fragmentation.";
}
description
"Set of rules compression, no compression or fragmentation
rules identified by their rule-id.";
}
description
"a SCHC set of rules is composed of a list of rule which are
either compression or fragmentation.";
}
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}
Figure 20: Definition of a SCHC Context
To access to a specific rule, rule-id and its specific length is used
as a key. The rule is either a compression or a fragmentation rule.
Each context can be identified though a version id.
3.1. Compression rule
A compression rule is composed of entries describing its processing
(cf. Figure 21). An entry contains all the information defined in
Figure 1 with the types defined above.
The compression rule described Figure 1 is defined by compression-
content. It defines a list of compression-rule-entry, indexed by
their field id, position and direction. The compression-rule-entry
element represent a line of the table Figure 1. Their type reflects
the identifier types defined in Section 2.1
Some controls are made on the values:
* target value must be present for MO different from ignore.
* when MSB MO is specified, the matching-operator-value must be
present
grouping compression-rule-entry {
description
"These entries defines a compression entry (i.e. a line)
as defined in RFC 8724 and fragmentation parameters.
+-------+--+--+--+------------+-----------------+---------------+
|Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|
+-------+--+--+--+------------+-----------------+---------------+
An entry in a compression rule is composed of 7 elements:
- Field ID: The header field to be compressed. The content is a
YANG identifer.
- Field Length : either a positive integer of a function
defined as a YANF id.
- Field Position: a positive (and possibly equal to 0) integer.
- Direction Indicator: a YANG identifier giving the direction.
- Target value: a value against which the header Field is
compared.
- Matching Operator: a YANG id giving the operation, paramters
may be associated to that operator.
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- Comp./Decomp. Action: A YANG id giving the compression or
decompression action, paramters may be associated to that
action.
";
leaf field-id {
type schc:fid-type;
mandatory true;
description
"Field ID, identify a field in the header with a YANG
referenceid.";
}
leaf field-length {
type schc:fl-type;
mandatory true;
description
"Field Length in bit or through a function defined as a
YANG referenceid.";
}
leaf field-position {
type uint8;
mandatory true;
description
"Field position in the header is a integer. If the field is not
repeated in the header the value is 1, and incremented for each
repetition of the field. Position 0 means that the position is
not important and order may change when decompressed";
}
leaf direction-indicator {
type schc:di-type;
mandatory true;
description
"Direction Indicator, a YANG referenceid to say if the packet
is bidirectional, up or down";
}
list target-value {
key "position";
uses tv-struct;
description
"A list of value to compare with the header field value.
If target value is a singleton, position must be 0.
For matching-list, should be consecutive position
values starting from 1.";
}
leaf matching-operator {
type schc:mo-type;
must "../target-value or derived-from-or-self(., 'mo-ignore')" {
error-message
"mo-equal, mo-msb and mo-match-mapping need target-value";
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description
"target-value is not required for mo-ignore";
}
must "not (derived-from-or-self(., 'mo-msb')) or
../matching-operator-value" {
error-message "mo-msb requires length value";
}
mandatory true;
description
"MO: Matching Operator";
}
list matching-operator-value {
key "position";
uses tv-struct;
description
"Matching Operator Arguments, based on TV structure to allow
several arguments.
In RFC 8724, only MSB define a single argument: length in
bits";
}
leaf comp-decomp-action {
type schc:cda-type;
mandatory true;
description
"CDA: Compression Decompression Action.";
}
list comp-decomp-action-value {
key "position";
uses tv-struct;
description
"CDA Arguments, based on TV structure to allow several
arguments. In RFC 8724, no argument is defined for CDA.";
}
}
grouping compression-content {
list entry {
key "field-id field-position direction-indicator";
uses compression-rule-entry;
description
"A compression rule is a list of rule entry describing
each header field. An entry is identifed through a field-id,
its position in the packet and its direction.";
}
description
"Define a compression rule composed of a list of entries.";
}
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Figure 21: Definition of a compression entry
3.2. Fragmentation rule
A Fragmentation rule is composed of entries describing the protocol
behavior. Some on them are numerical entries, others are identifiers
defined in Section 2.10.
The data model defines some relations between the entries:
* direction must be either up or down (not bidirectional).
* W size is only needed for Ack Always and Ack on Error modes.
grouping fragmentation-content {
description
"This grouping defines the fragmentation parameters for
all the modes (No Ack, Ack Always and Ack on Error) specified in
RFC 8724.";
leaf l2-word-size {
type uint8;
default "8";
description
"Size in bit of the layer 2 word";
}
leaf direction {
type schc:di-type;
must "derived-from-or-self(., 'di-up') or
derived-from-or-self(., 'di-down')" {
error-message
"direction for fragmentation rules are up or down.";
}
mandatory true;
description
"Should be up or down, bi directionnal is forbiden.";
}
leaf dtag-size {
type uint8;
default "0";
description
"Size in bit of the DTag field (T variable from RFC8724).";
}
leaf w-size {
when "not(derived-from(../fragmentation-mode,
'fragmentation-mode-no-ack'))";
type uint8;
description
"Size in bit of the window field (M variable from RFC8724).";
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}
leaf fcn-size {
type uint8;
mandatory true;
description
"Size in bit of the FCN field (M variable from RFC8724).";
}
leaf rcs-algorithm {
type rcs-algorithm-type;
default "schc:rcs-RFC8724";
description
"Algoritm used for RCS";
}
leaf maximum-window-size {
type uint16;
description
"By default 2^wsize - 1";
}
leaf retransmission-timer {
type uint64 {
range "1..max";
}
description
"Duration in seconds of the retransmission timer.";
}
leaf inactivity-timer {
type uint64;
description
"Duration is seconds of the inactivity timer,
0 indicates the timer is disabled.";
}
leaf max-ack-requests {
type uint8 {
range "1..max";
}
description
"The maximum number of retries for a specific SCHC ACK.";
}
leaf maximum-packet-size {
type uint16;
default "1280";
description
"When decompression is done, packet size must not
strictly exceed this limit in Bytes.";
}
leaf fragmentation-mode {
type schc:fragmentation-mode-type;
mandatory true;
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description
"which fragmentation mode is used (noAck, AckAlways,
AckonError)";
}
choice mode {
case no-ack;
case ack-always;
case ack-on-error {
leaf tile-size {
when "derived-from(../fragmentation-mode,
'fragmentation-mode-ack-on-error')";
type uint8;
description
"Size in bit of tiles, if not specified or set to 0,
tile fills the fragment.";
}
leaf tile-in-All1 {
when "derived-from(../fragmentation-mode,
'fragmentation-mode-ack-on-error')";
type schc:all1-data-type;
description
"When true, sender and receiver except a tile in
All-1 frag.";
}
leaf ack-behavior {
when "derived-from(../fragmentation-mode,
'fragmentation-mode-ack-on-error')";
type schc:ack-behavior-type;
description
"Sender behavior to acknowledge, after All-0, All-1 or
when the LPWAN allows it (Always).";
}
}
description
"RFC 8724 defines 3 fragmentation modes.";
}
}
3.3. YANG Tree
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module: ietf-schc
+--rw schc
+--rw rule* [rule-id-value rule-id-length]
+--rw rule-id-value uint32
+--rw rule-id-length uint8
+--rw (nature)?
+--:(fragmentation)
| +--rw l2-word-size? uint8
| +--rw direction schc:di-type
| +--rw dtag-size? uint8
| +--rw w-size? uint8
| +--rw fcn-size uint8
| +--rw rcs-algorithm? rcs-algorithm-type
| +--rw maximum-window-size? uint16
| +--rw retransmission-timer? uint64
| +--rw inactivity-timer? uint64
| +--rw max-ack-requests? uint8
| +--rw maximum-packet-size? uint16
| +--rw fragmentation-mode schc:fragmentation-mode-type
| +--rw (mode)?
| +--:(no-ack)
| +--:(ack-always)
| +--:(ack-on-error)
| +--rw tile-size? uint8
| +--rw tile-in-All1? schc:all1-data-type
| +--rw ack-behavior? schc:ack-behavior-type
+--:(compression)
| +--rw entry* [field-id field-position direction-indicator]
| +--rw field-id schc:fid-type
| +--rw field-length schc:fl-type
| +--rw field-position uint8
| +--rw direction-indicator schc:di-type
| +--rw target-value* [position]
| | +--rw value? binary
| | +--rw position uint16
| +--rw matching-operator schc:mo-type
| +--rw matching-operator-value* [position]
| | +--rw value? binary
| | +--rw position uint16
| +--rw comp-decomp-action schc:cda-type
| +--rw comp-decomp-action-value* [position]
| +--rw value? binary
| +--rw position uint16
+--:(no-compression)
Figure 22
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4. IANA Considerations
This document has no request to IANA.
5. Security considerations
This document does not have any more Security consideration than the
ones already raised on [RFC8724]
6. Acknowledgements
The authors would like to thank Dominique Barthel, Carsten Bormann,
Alexander Pelov.
7. YANG Module
<code begins> file ietf-schc@2021-11-10.yang
module ietf-schc {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-schc";
prefix schc;
organization
"IETF IPv6 over Low Power Wide-Area Networks (lpwan) working group";
contact
"WG Web: <https://datatracker.ietf.org/wg/lpwan/about/>
WG List: <mailto:p-wan@ietf.org>
Editor: Laurent Toutain
<mailto:laurent.toutain@imt-atlantique.fr>
Editor: Ana Minaburo
<mailto:ana@ackl.io>";
description
"
Copyright (c) 2021 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject to
the license terms contained in, the Simplified BSD License set
forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC XXXX
(https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
for full legal notices.
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
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NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
'MAY', and 'OPTIONAL' in this document are to be interpreted as
described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
they appear in all capitals, as shown here.
*****************************************************************
Generic Data model for Static Context Header Compression Rule for
SCHC, based on RFC 8724 and RFC8824. Include compression, no
compression and fragmentation rules.
This module is a YANG model for SCHC rules (RFc 8724).
RFC 8724 describes a rule in a abstract way through a table.
|-----------------------------------------------------------------|
| (FID) Rule 1 |
|+-------+--+--+--+------------+-----------------+---------------+|
||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||
|+-------+--+--+--+------------+-----------------+---------------+|
||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||
|+-------+--+--+--+------------+-----------------+---------------+|
||... |..|..|..| ... | ... | ... ||
|+-------+--+--+--+------------+-----------------+---------------+|
||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||
+-------+--+--+--+------------+-----------------+---------------+||
|-----------------------------------------------------------------|
This module proposes a global data model that can be used for rule
exchanges or modification. It proposes both the data model format
and the global identifiers used to describes some operations in
fields.
This data model applies both to compression and fragmentation.";
revision 2021-11-10 {
description
"Initial version from RFC XXXX ";
reference
"RFC XXX: Data Model for Static Context Header Compression
(SCHC)";
}
feature fragmentation {
description
"Fragmentation is usually required only at the transportation
level.";
}
// -------------------------
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// Field ID type definition
//--------------------------
// generic value TV definition
identity fid-base-type {
description
"Field ID base type for all fields";
}
identity fid-ipv6-base-type {
base fid-base-type;
description
"Field IP base type for IPv6 headers described in RFC 8200";
}
identity fid-ipv6-version {
base fid-ipv6-base-type;
description
"IPv6 version field from RFC8200";
}
identity fid-ipv6-trafficclass {
base fid-ipv6-base-type;
description
"IPv6 Traffic Class field from RFC8200";
}
identity fid-ipv6-trafficclass-ds {
base fid-ipv6-trafficclass;
description
"IPv6 Traffic Class field from RFC8200,
DiffServ field from RFC3168";
}
identity fid-ipv6-trafficclass-ecn {
base fid-ipv6-trafficclass;
description
"IPv6 Traffic Class field from RFC8200,
ECN field from RFC3168";
}
identity fid-ipv6-flowlabel {
base fid-ipv6-base-type;
description
"IPv6 Flow Label field from RFC8200";
}
identity fid-ipv6-payloadlength {
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base fid-ipv6-base-type;
description
"IPv6 Payload Length field from RFC8200";
}
identity fid-ipv6-nextheader {
base fid-ipv6-base-type;
description
"IPv6 Next Header field from RFC8200";
}
identity fid-ipv6-hoplimit {
base fid-ipv6-base-type;
description
"IPv6 Next Header field from RFC8200";
}
identity fid-ipv6-devprefix {
base fid-ipv6-base-type;
description
"correspond either to the source address or the desdination
address prefix of RFC 8200. Depending if it is
respectively a uplink or an downklink message.";
}
identity fid-ipv6-deviid {
base fid-ipv6-base-type;
description
"correspond either to the source address or the desdination
address prefix of RFC 8200. Depending if it is respectively
a uplink or an downklink message.";
}
identity fid-ipv6-appprefix {
base fid-ipv6-base-type;
description
"correspond either to the source address or the desdination
address prefix of RFC 768. Depending if it is respectively
a downlink or an uplink message.";
}
identity fid-ipv6-appiid {
base fid-ipv6-base-type;
description
"correspond either to the source address or the desdination
address prefix of RFC 768. Depending if it is respectively
a downlink or an uplink message.";
}
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identity fid-udp-base-type {
base fid-base-type;
description
"Field IP base type for UDP headers described in RFC 768";
}
identity fid-udp-dev-port {
base fid-udp-base-type;
description
"UDP length from RFC 768";
}
identity fid-udp-app-port {
base fid-udp-base-type;
description
"UDP length from RFC 768";
}
identity fid-udp-length {
base fid-udp-base-type;
description
"UDP length from RFC 768";
}
identity fid-udp-checksum {
base fid-udp-base-type;
description
"UDP length from RFC 768";
}
identity fid-coap-base-type {
base fid-base-type;
description
"Field IP base type for UDP headers described in RFC 768";
}
identity fid-coap-version {
base fid-coap-base-type;
description
"CoAP version from RFC 7252";
}
identity fid-coap-type {
base fid-coap-base-type;
description
"CoAP type from RFC 7252";
}
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identity fid-coap-tkl {
base fid-coap-base-type;
description
"CoAP token length from RFC 7252";
}
identity fid-coap-code {
base fid-coap-base-type;
description
"CoAP code from RFC 7252";
}
identity fid-coap-code-class {
base fid-coap-code;
description
"CoAP code class from RFC 7252";
}
identity fid-coap-code-detail {
base fid-coap-code;
description
"CoAP code detail from RFC 7252";
}
identity fid-coap-mid {
base fid-coap-base-type;
description
"CoAP message ID from RFC 7252";
}
identity fid-coap-token {
base fid-coap-base-type;
description
"CoAP token from RFC 7252";
}
identity fid-coap-option-if-match {
base fid-coap-base-type;
description
"CoAP option If-Match from RFC 7252";
}
identity fid-coap-option-uri-host {
base fid-coap-base-type;
description
"CoAP option URI-Host from RFC 7252";
}
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identity fid-coap-option-etag {
base fid-coap-base-type;
description
"CoAP option Etag from RFC 7252";
}
identity fid-coap-option-if-none-match {
base fid-coap-base-type;
description
"CoAP option if-none-match from RFC 7252";
}
identity fid-coap-option-observe {
base fid-coap-base-type;
description
"CoAP option Observe from RFC 7641";
}
identity fid-coap-option-uri-port {
base fid-coap-base-type;
description
"CoAP option Uri-Port from RFC 7252";
}
identity fid-coap-option-location-path {
base fid-coap-base-type;
description
"CoAP option Location-Path from RFC 7252";
}
identity fid-coap-option-uri-path {
base fid-coap-base-type;
description
"CoAP option Uri-Path from RFC 7252";
}
identity fid-coap-option-content-format {
base fid-coap-base-type;
description
"CoAP option Content Format from RFC 7252";
}
identity fid-coap-option-max-age {
base fid-coap-base-type;
description
"CoAP option Max-Age from RFC 7252";
}
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identity fid-coap-option-uri-query {
base fid-coap-base-type;
description
"CoAP option Uri-Query from RFC 7252";
}
identity fid-coap-option-accept {
base fid-coap-base-type;
description
"CoAP option Max-Age from RFC 7252";
}
identity fid-coap-option-location-query {
base fid-coap-base-type;
description
"CoAP option Location-Query from RFC 7252";
}
identity fid-coap-option-block2 {
base fid-coap-base-type;
description
"CoAP option Block2 from RFC 7959";
}
identity fid-coap-option-block1 {
base fid-coap-base-type;
description
"CoAP option Block1 from RFC 7959";
}
identity fid-coap-option-size2 {
base fid-coap-base-type;
description
"CoAP option size2 from RFC 7959";
}
identity fid-coap-option-proxy-uri {
base fid-coap-base-type;
description
"CoAP option Proxy-Uri from RFC 7252";
}
identity fid-coap-option-proxy-scheme {
base fid-coap-base-type;
description
"CoAP option Proxy-scheme from RFC 7252";
}
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identity fid-coap-option-size1 {
base fid-coap-base-type;
description
"CoAP option Size1 from RFC 7252";
}
identity fid-coap-option-no-response {
base fid-coap-base-type;
description
"CoAP option No response from RFC 7967";
}
identity fid-coap-option-oscore-flags {
base fid-coap-base-type;
description
"CoAP option oscore flags (see draft schc coap, section 6.4)";
}
identity fid-coap-option-oscore-piv {
base fid-coap-base-type;
description
"CoAP option oscore flags (see draft schc coap, section 6.4)";
}
identity fid-coap-option-oscore-kid {
base fid-coap-base-type;
description
"CoAP option oscore flags (see draft schc coap, section 6.4)";
}
identity fid-coap-option-oscore-kidctx {
base fid-coap-base-type;
description
"CoAP option oscore flags (see draft schc coap, section 6.4)";
}
//----------------------------------
// Field Length type definition
//----------------------------------
identity fl-base-type {
description
"Used to extend field length functions.";
}
identity fl-variable {
base fl-base-type;
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description
"Residue length in Byte is sent as defined in
for CoAP in RFC 8824 (cf. 5.3).";
}
identity fl-token-length {
base fl-base-type;
description
"Residue length in Byte is sent as defined in
for CoAP in RFC 8824 (cf. 4.5).";
}
//---------------------------------
// Direction Indicator type
//---------------------------------
identity di-base-type {
description
"Used to extend direction indicators.";
}
identity di-bidirectional {
base di-base-type;
description
"Direction Indication of bi directionality in
RFC 8724 (cf. 7.1).";
}
identity di-up {
base di-base-type;
description
"Direction Indication of upstream defined in
RFC 8724 (cf. 7.1).";
}
identity di-down {
base di-base-type;
description
"Direction Indication of downstream defined in
RFC 8724 (cf. 7.1).";
}
//----------------------------------
// Matching Operator type definition
//----------------------------------
identity mo-base-type {
description
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"Used to extend Matching Operators with SID values";
}
identity mo-equal {
base mo-base-type;
description
"Equal MO as defined RFC 8724 (cf. 7.3)";
}
identity mo-ignore {
base mo-base-type;
description
"Ignore MO as defined RFC 8724 (cf. 7.3)";
}
identity mo-msb {
base mo-base-type;
description
"MSB MO as defined RFC 8724 (cf. 7.3)";
}
identity mo-match-mapping {
base mo-base-type;
description
"match-mapping MO as defined RFC 8724 (cf. 7.3)";
}
//------------------------------
// CDA type definition
//------------------------------
identity cda-base-type {
description
"Compression Decompression Actions.";
}
identity cda-not-sent {
base cda-base-type;
description
"not-sent CDA as defines in RFC 8724 (cf. 7.4).";
}
identity cda-value-sent {
base cda-base-type;
description
"value-sent CDA as defines in RFC 8724 (cf. 7.4).";
}
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identity cda-lsb {
base cda-base-type;
description
"LSB CDA as defines in RFC 8724 (cf. 7.4).";
}
identity cda-mapping-sent {
base cda-base-type;
description
"mapping-sent CDA as defines in RFC 8724 (cf. 7.4).";
}
identity cda-compute-length {
base cda-base-type;
description
"compute-length CDA as defines in RFC 8724 (cf. 7.4)";
}
identity cda-compute-checksum {
base cda-base-type;
description
"compute-checksum CDA as defines in RFC 8724 (cf. 7.4)";
}
identity cda-deviid {
base cda-base-type;
description
"deviid CDA as defines in RFC 8724 (cf. 7.4)";
}
identity cda-appiid {
base cda-base-type;
description
"appiid CDA as defines in RFC 8724 (cf. 7.4)";
}
// -- type definition
typedef fid-type {
type identityref {
base fid-base-type;
}
description
"Field ID generic type.";
}
typedef fl-type {
type union {
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type int64; /* positive length in bits */
type identityref { /* function */
base fl-base-type;
}
}
description
"Field length either a positive integer giving the size in bits
or a function defined through an identityref.";
}
typedef di-type {
type identityref {
base di-base-type;
}
description
"Direction in LPWAN network, up when emitted by the device,
down when received by the device, bi when emitted or
received by the device.";
}
typedef mo-type {
type identityref {
base mo-base-type;
}
description
"Matching Operator (MO) to compare fields values with
target values";
}
typedef cda-type {
type identityref {
base cda-base-type;
}
description
"Compression Decompression Action to compression or
decompress a field.";
}
// -- FRAGMENTATION TYPE
// -- fragmentation modes
identity fragmentation-mode-base-type {
description
"fragmentation mode.";
}
identity fragmentation-mode-no-ack {
base fragmentation-mode-base-type;
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description
"No Ack of RFC 8724.";
}
identity fragmentation-mode-ack-always {
base fragmentation-mode-base-type;
description
"Ack Always of RFC8724.";
}
identity fragmentation-mode-ack-on-error {
base fragmentation-mode-base-type;
description
"Ack on Error of RFC8724.";
}
typedef fragmentation-mode-type {
type identityref {
base fragmentation-mode-base-type;
}
description
"type used in rules";
}
// -- Ack behavior
identity ack-behavior-base-type {
description
"Define when to send an Acknowledgment .";
}
identity ack-behavior-after-All0 {
base ack-behavior-base-type;
description
"Fragmentation expects Ack after sending All0 fragment.";
}
identity ack-behavior-after-All1 {
base ack-behavior-base-type;
description
"Fragmentation expects Ack after sending All1 fragment.";
}
identity ack-behavior-always {
base ack-behavior-base-type;
description
"Fragmentation expects Ack after sending every fragment.";
}
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typedef ack-behavior-type {
type identityref {
base ack-behavior-base-type;
}
description
"Type used in rules.";
}
// -- All1 with data types
identity all1-data-base-type {
description
"Type to define when to send an Acknowledgment message.";
}
identity all1-data-no {
base all1-data-base-type;
description
"All1 contains no tiles.";
}
identity all1-data-yes {
base all1-data-base-type;
description
"All1 MUST contain a tile.";
}
identity all1-data-sender-choice {
base all1-data-base-type;
description
"Fragmentation process choose to send tiles or not in all1.";
}
typedef all1-data-type {
type identityref {
base all1-data-base-type;
}
description
"Type used in rules.";
}
// -- RCS algorithm types
identity rcs-algorithm-base-type {
description
"Identify which algorithm is used to compute RSC.
The algorithm also defines the size if the RSC field.";
}
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identity rcs-RFC8724 {
base rcs-algorithm-base-type;
description
"CRC 32 defined as default RCS in RFC8724.";
}
typedef rcs-algorithm-type {
type identityref {
base rcs-algorithm-base-type;
}
description
"type used in rules.";
}
// -------- RULE ENTRY DEFINITION ------------
grouping tv-struct {
description
"Define the target value element. Always a binary type, strings
must be converted to binary. field-id allows the conversion
to the appropriate type.";
leaf value {
type binary;
description
"Target Value";
}
leaf position {
type uint16;
description
"If only one element position is 0, otherwise position is the
the position in the matching list.";
}
}
grouping compression-rule-entry {
description
"These entries defines a compression entry (i.e. a line)
as defined in RFC 8724 and fragmentation parameters.
+-------+--+--+--+------------+-----------------+---------------+
|Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|
+-------+--+--+--+------------+-----------------+---------------+
An entry in a compression rule is composed of 7 elements:
- Field ID: The header field to be compressed. The content is a
YANG identifer.
- Field Length : either a positive integer of a function defined
as a YANF id.
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- Field Position: a positive (and possibly equal to 0) integer.
- Direction Indicator: a YANG identifier giving the direction.
- Target value: a value against which the header Field is
compared.
- Matching Operator: a YANG id giving the operation, paramters
may be associated to that operator.
- Comp./Decomp. Action: A YANG id giving the compression or
decompression action, paramters may be associated to that
action.
";
leaf field-id {
type schc:fid-type;
mandatory true;
description
"Field ID, identify a field in the header with a YANG
referenceid.";
}
leaf field-length {
type schc:fl-type;
mandatory true;
description
"Field Length in bit or through a function defined as a
YANG referenceid.";
}
leaf field-position {
type uint8;
mandatory true;
description
"Field position in the header is a integer. If the field is not
repeated in the header the value is 1, and incremented for each
repetition of the field. Position 0 means that the position is
not important and order may change when decompressed";
}
leaf direction-indicator {
type schc:di-type;
mandatory true;
description
"Direction Indicator, a YANG referenceid to say if the packet
is bidirectional, up or down";
}
list target-value {
key "position";
uses tv-struct;
description
"A list of value to compare with the header field value.
If target value is a singleton, position must be 0.
For matching-list, should be consecutive position
values starting from 1.";
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}
leaf matching-operator {
type schc:mo-type;
must "../target-value or derived-from-or-self(., 'mo-ignore')" {
error-message
"mo-equal, mo-msb and mo-match-mapping need target-value";
description
"target-value is not required for mo-ignore";
}
must "not (derived-from-or-self(., 'mo-msb')) or
../matching-operator-value" {
error-message "mo-msb requires length value";
}
mandatory true;
description
"MO: Matching Operator";
}
list matching-operator-value {
key "position";
uses tv-struct;
description
"Matching Operator Arguments, based on TV structure to allow
several arguments.
In RFC 8724, only MSB define a single argument: length in
bits";
}
leaf comp-decomp-action {
type schc:cda-type;
mandatory true;
description
"CDA: Compression Decompression Action.";
}
list comp-decomp-action-value {
key "position";
uses tv-struct;
description
"CDA Arguments, based on TV structure to allow several
arguments. In RFC 8724, no argument is defined for CDA.";
}
}
grouping compression-content {
list entry {
key "field-id field-position direction-indicator";
uses compression-rule-entry;
description
"A compression rule is a list of rule entry describing
each header field. An entry is identifed through a field-id,
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its position in the packet and its direction.";
}
description
"Define a compression rule composed of a list of entries.";
}
grouping fragmentation-content {
description
"This grouping defines the fragmentation parameters for
all the modes (No Ack, Ack Always and Ack on Error) specified in
RFC 8724.";
leaf l2-word-size {
type uint8;
default "8";
description
"Size in bit of the layer 2 word";
}
leaf direction {
type schc:di-type;
must "derived-from-or-self(., 'di-up') or
derived-from-or-self(., 'di-down')" {
error-message
"direction for fragmentation rules are up or down.";
}
mandatory true;
description
"Should be up or down, bi directionnal is forbiden.";
}
leaf dtag-size {
type uint8;
default "0";
description
"Size in bit of the DTag field (T variable from RFC8724).";
}
leaf w-size {
when "not(derived-from(../fragmentation-mode,
'fragmentation-mode-no-ack'))";
type uint8;
description
"Size in bit of the window field (M variable from RFC8724).";
}
leaf fcn-size {
type uint8;
mandatory true;
description
"Size in bit of the FCN field (M variable from RFC8724).";
}
leaf rcs-algorithm {
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type rcs-algorithm-type;
default "schc:rcs-RFC8724";
description
"Algoritm used for RCS";
}
leaf maximum-window-size {
type uint16;
description
"By default 2^wsize - 1";
}
leaf retransmission-timer {
type uint64 {
range "1..max";
}
description
"Duration in seconds of the retransmission timer.";
}
leaf inactivity-timer {
type uint64;
description
"Duration is seconds of the inactivity timer,
0 indicates the timer is disabled.";
}
leaf max-ack-requests {
type uint8 {
range "1..max";
}
description
"The maximum number of retries for a specific SCHC ACK.";
}
leaf maximum-packet-size {
type uint16;
default "1280";
description
"When decompression is done, packet size must not
strictly exceed this limit in Bytes.";
}
leaf fragmentation-mode {
type schc:fragmentation-mode-type;
mandatory true;
description
"which fragmentation mode is used (noAck, AckAlways,
AckonError)";
}
choice mode {
case no-ack;
case ack-always;
case ack-on-error {
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leaf tile-size {
when "derived-from(../fragmentation-mode,
'fragmentation-mode-ack-on-error')";
type uint8;
description
"Size in bit of tiles, if not specified or set to 0,
tile fills the fragment.";
}
leaf tile-in-All1 {
when "derived-from(../fragmentation-mode,
'fragmentation-mode-ack-on-error')";
type schc:all1-data-type;
description
"When true, sender and receiver except a tile in
All-1 frag.";
}
leaf ack-behavior {
when "derived-from(../fragmentation-mode,
'fragmentation-mode-ack-on-error')";
type schc:ack-behavior-type;
description
"Sender behavior to acknowledge, after All-0, All-1 or
when the LPWAN allows it (Always).";
}
}
description
"RFC 8724 defines 3 fragmentation modes.";
}
}
// Define rule ID. Rule ID is composed of a RuleID value and a
// Rule ID Length
grouping rule-id-type {
leaf rule-id-value {
type uint32;
description
"Rule ID value, this value must be unique combined with
the length.";
}
leaf rule-id-length {
type uint8 {
range "0..32";
}
description
"Rule ID length in bits, value 0 is for implicit rules.";
}
description
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"A rule ID is composed of a value and a length in bit.";
}
// SCHC table for a specific device.
container schc {
list rule {
key "rule-id-value rule-id-length";
uses rule-id-type;
choice nature {
case fragmentation {
//if-feature "fragmentation";
uses fragmentation-content;
}
case compression {
uses compression-content;
}
case no-compression {
description
"RFC8724 allows a rule for uncompressed headers.";
}
description
"A rule is either for compression, no compression or
fragmentation.";
}
description
"Set of rules compression, no compression or fragmentation
rules identified by their rule-id.";
}
description
"a SCHC set of rules is composed of a list of rule which are
either compression or fragmentation.";
}
}
<code ends>
Figure 23
8. Normative References
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
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[RFC8824] Minaburo, A., Toutain, L., and R. Andreasen, "Static
Context Header Compression (SCHC) for the Constrained
Application Protocol (CoAP)", RFC 8824,
DOI 10.17487/RFC8824, June 2021,
<https://www.rfc-editor.org/info/rfc8824>.
Authors' Addresses
Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Laurent Toutain
Institut MINES TELECOM; IMT Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
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