| lpwan Working Group | A. Minaburo |
| Internet-Draft | Acklio |
| Intended status: Standards Track | L. Toutain |
| Expires: July 26, 2020 | Institut MINES TELECOM; IMT Atlantique |
| January 23, 2020 |
Data Model for Static Context Header Compression (SCHC)
draft-ietf-lpwan-schc-yang-data-model-01
This document describes a YANG data model for the SCHC (Static Context Header Compression) compression and fragmentation rules.
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SCHC is a compression and fragmentation mechanism for constrained networks defined in [I-D.ietf-lpwan-ipv6-static-context-hc] it is based on a static context shared by two entities at the boundary this contrained network. Draft [I-D.ietf-lpwan-ipv6-static-context-hc] provides an abstract 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:
This document defines a YANG module to represent both compression and fragmentation rules, which leads to common representation and values for the elements of the rules. SCHC compression is generic, the main mechanism do no refers to a specific fields. A field is abstractedh through an ID, a position, a direction and a value that can be a numerical value or a string.
[I-D.ietf-lpwan-ipv6-static-context-hc] and [I-D.ietf-lpwan-coap-static-context-hc] specifies fields for IPv6, UDP, CoAP and OSCORE.
Fragmentation requires a set of common parameters that are included in a rule.
[I-D.ietf-lpwan-ipv6-static-context-hc] proposes an abstract 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
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 again the rules to find the appropriate one and apply compression. The link between the list given by the parsed fields and the rules is doen through a field ID. [I-D.ietf-lpwan-ipv6-static-context-hc] do not state how the field ID value can be constructed. In the given example, it was given through a string indexed by the protocol name (e.g. IPv6.version, CoAP.version,…).
Using the YANG model, each field can be identified through a global YANG identityref. A YANG field ID derives from the field-id-base-type. Figure 2 gives some field ID definitions. Note that some field IDs can be splitted is smaller pieces. This is the case for “fid-ipv6-trafficclass-ds” and “fid-ipv6-trafficclass-ecn” which are a subset of “fid-ipv6-trafficclass-ds”.
identity field-id-base-type {
description "Field ID with SID";
}
identity fid-ipv6-version {
base field-id-base-type;
description "IPv6 version field from RFC8200";
}
identity fid-ipv6-trafficclass {
base field-id-base-type;
description "IPv6 Traffic Class field from RFC8200";
}
identity fid-ipv6-trafficclass-ds {
base field-id-base-type;
description "IPv6 Traffic Class field from RFC8200,
DiffServ field from RFC3168";
}
identity fid-ipv6-trafficclass-ecn {
base field-id-base-type;
description "IPv6 Traffic Class field from RFC8200,
ECN field from RFC3168";
}
...
identity fid-coap-option-if-match {
base field-id-base-type;
description "CoAP option If-Match from RFC 7252";
}
identity fid-coap-option-uri-host {
base field-id-base-type;
description "CoAP option URI-Host from RFC 7252";
}
...
Figure 2: Definition of indentityref for field IDs
Figure 2 gives an example of field ID identityref definitions. The base identity is field-id-base-type, and field id are derived for it. The naming convention is “fid” followed by the protocol name and the field name.
The yang model in annex gives the full definition of the field ID for [I-D.ietf-lpwan-ipv6-static-context-hc] and [I-D.ietf-lpwan-coap-static-context-hc].
The type associated to this identity is field-id-type (cf. {{Fig-field-id-type}})
typedef field-id-type {
description "Field ID generic type.";
type identityref {
base field-id-base-type;
}
}
Figure 3: Definition of indentityref for field IDs
Field length is either an integer giving the size of a field in bits or a function. [I-D.ietf-lpwan-ipv6-static-context-hc] defines the “var” function which allows variable length fields in byte and [I-D.ietf-lpwan-coap-static-context-hc] defines the “tkl” function for managing the CoAP Token length field.
identity field-length-base-type {
description "used to extend field length functions";
}
identity fl-variable {
base field-length-base-type;
description "residue length in Byte is sent";
}
identity fl-token-length {
base field-length-base-type;
description "residue length in Byte is sent";
}
Figure 4: Definition of indetntyref for field IDs
As for field ID, field length function can be defined as a identityref as shown in Figure 4.
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 5).
typedef field-length-type {
type union {
type int64; /* positive length */
type identityref { /* function */
base field-length-base-type;
}
}
}
Figure 5: Definition of indetntyref for field IDs
The naming convention is fl followed by the function name as defined in SCHC specifications.
Field position is a positive integer which gives the position of a field, the default value is 1, but if the field is repeated several times, the value is higher. value 0 indicates that the position is not important and is not taken into account during the rule selection process.
Field position is a positive integer. The type is an uint8.
The Direction Indicator (DI) is used to tell if a field appears in both direction (Bi) or only uplink (Up) or Downlink (Dw).
identity direction-indicator-base-type {
description "used to extend field length functions";
}
identity di-bidirectional {
base direction-indicator-base-type;
description "Direction Indication of bi directionality";
}
identity di-up {
base direction-indicator-base-type;
description "Direction Indication of upstream";
}
identity di-down {
base direction-indicator-base-type;
description "Direction Indication of downstream";
}
Figure 6: Definition of identityref for direction indicators
Figure 6 gives the identityref for Direction Indicators.
The type is “direction-indicator-type” (cf. Figure 7).
typedef direction-indicator-type {
type identityref {
base direction-indicator-base-type;
}
}
Figure 7: Definition of identityref for direction indicators
Target Value may be either a string or binary sequence. For match-mapping, several of these values can be contained in a Target Value field. In the data model, this is generalized by adding a position, which orders the list of values. By default the position is set to 0.
The leaf “value” is not mandatory to represent a non existing value in a TV.
grouping target-values-struct {
leaf value {
type union {
type binary;
type string;
}
}
leaf position {
type uint16;
}
}
Figure 8: Definition of target value
Figure 8 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.
Matching Operator (MO) is a function applied between a field value provided by the parsed header and the target value. [I-D.ietf-lpwan-ipv6-static-context-hc] defines 4 MO.
identity matching-operator-base-type {
description "used to extend Matching Operators with SID values";
}
identity mo-equal {
base matching-operator-base-type;
description "SCHC draft";
}
identity mo-ignore {
base matching-operator-base-type;
description "SCHC draft";
}
identity mo-msb {
base matching-operator-base-type;
description "SCHC draft";
}
identity mo-matching {
base matching-operator-base-type;
description "SCHC draft";
}
Figure 9: Definition of Matching Operator identity
the type is “matching-operator-type” (cf. Figure 10)
typedef matching-operator-type {
type identityref {
base matching-operator-base-type;
}
}
Figure 10: Definition of Matching Operator type
Some Matching Operator such as MSB can take some values. Even if currently LSB is the only MO takes only one argument, in the future some MO may require several arguments. They are viewed as a list of target-values-type.
Compresion Decompression Action (CDA) idenfied the function to use either for compression or decompression. [I-D.ietf-lpwan-ipv6-static-context-hc] defines 6 CDA.
identity compression-decompression-action-base-type;
identity cda-not-sent {
base compression-decompression-action-base-type;
description "from SCHC draft";
}
identity cda-value-sent {
base compression-decompression-action-base-type;
description "from SCHC draft";
}
identity cda-lsb {
base compression-decompression-action-base-type;
description "from SCHC draft";
}
identity cda-mapping-sent {
base compression-decompression-action-base-type;
description "from SCHC draft";
}
identity cda-compute-length {
base compression-decompression-action-base-type;
description "from SCHC draft";
}
identity cda-compute-checksum {
base compression-decompression-action-base-type;
description "from SCHC draft";
}
identity cda-deviid {
base compression-decompression-action-base-type;
description "from SCHC draft";
}
identity cda-appiid {
base compression-decompression-action-base-type;
description "from SCHC draft";
}
Figure 11: Definition of Compresion Decompression Action identity
The type is “comp-decomp-action-type” (cf. Figure 12)
typedef comp-decomp-action-type {
type identityref {
base compression-decompression-action-base-type;
}
}
Figure 12: Definition of Compresion Decompression Action type
Currently no CDA requires argumetns, but the future some CDA may require several arguments. They are viewed as a list of target-values-type.
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.
// Define rule ID. Rule ID is composed of a RuleID value and a Rule ID Length
grouping rule-id-type {
leaf rule-id {
type uint32;
description "rule ID value, this value must be unique combined with the length";
}
leaf rule-length {
type uint8 {
range 0..32;
}
description "rule ID length in bits, value 0 is for implicit rules";
}
}
// SCHC table for a specific device.
container schc {
leaf version{
type uint64;
mandatory false;
description "used as an indication for versioning";
}
list rule {
key "rule-id rule-length";
uses rule-id-type;
choice nature {
case fragmentation {
uses fragmentation-content;
}
case compression {
uses compression-content;
}
}
}
}
Figure 13: Definition of a SCHC Context
To access to a specfic 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 identify though a version id.
A compression rule is composed of entries describing its processing (cf. Figure 14). An entry contains all the information defined in Figure 1 with the types defined above.
The compression rule described Figure 1 is associated to a rule ID. The compression rule entry is defined in Figure 14. Each column in the table is either represented by a leaf or a list. Note that Matching Operators and Compression Decompression actions can have arguments. They are viewed a ordered list of strings and numbers as in target values.
grouping compression-rule-entry {
leaf field-id {
mandatory true;
type schc-id:field-id-type;
}
leaf field-length {
mandatory true;
type schc-id:field-length-type;
}
leaf field-position {
mandatory true;
type uint8;
}
leaf direction-indicator {
mandatory true;
type schc-id:direction-indicator-type;
}
list target-values {
key position;
uses target-values-struct;
}
leaf mo {
mandatory true;
type schc-id:matching-operator-type;
}
// /!\ Not always good, it allows to give several arguments to a MO, but
// theses arguments are only int or strings, cannot be arrays. Is it necessary?
list mo-value {
key position;
uses target-values-struct;
}
leaf cda {
mandatory true;
type schc-id:cda-type;
}
list cda-value {
key position;
uses target-values-struct;
}
}
Figure 14: Definition of a compression entry
A compression rule is a list of entries.
grouping compression-content {
list entry {
key "field-id field-position direction-indicator"; // field-position direction-indicator";
uses compression-rule-entry;
}
}
Figure 15: Definition of a compression rule
To identify a specific entry Field ID, position and direction is needed.
TBD
grouping fragmentation-content {
leaf dtagsize {
type uint8;
}
leaf wsize {
type uint8;
}
leaf fcnsize {
type uint8;
}
choice mode {
case no-ack;
case ack-always;
case ack-on-error {
leaf ack-method {
type enumeration {
enum afterAll0;
enum afterAll1;
enum always;
}
}
}
}
}
Figure 16: Definition of a fragmentation rule
module: schc
+--rw schc
+--rw version? uint64
+--rw rule* [rule-id rule-length]
+--rw rule-id uint32
+--rw rule-length uint8
+--rw (nature)?
+--:(fragmentation)
| +--rw dtagsize? uint8
| +--rw wsize? uint8
| +--rw fcnsize? uint8
| +--rw (mode)?
| +--:(no-ack)
| +--:(ack-always)
| +--:(ack-on-error)
| +--rw ack-method? enumeration
+--:(compression)
+--rw entry* [field-id field-position direction-indicator]
+--rw field-id schc-id:field-id-type
+--rw field-length schc-id:field-length-type
+--rw field-position uint8
+--rw direction-indicator schc-id:direction-indicator-type
+--rw target-values* [position]
| +--rw value? union
| +--rw position uint16
+--rw mo schc-id:matching-operator-type
+--rw mo-value* [position]
| +--rw value? union
| +--rw position uint16
+--rw cda schc-id:comp-decomp-action-type
+--rw cda-value* [position]
+--rw value? union
+--rw position uint16
Figure 17
This document has no request to IANA.
This document does not have any more Security consideration than the ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc]
The authors would like to thank Dominique Barthel, Carsten Bormann, Alexander Pelov.
| [I-D.ietf-lpwan-coap-static-context-hc] | Minaburo, A., Toutain, L. and R. Andreasen, "LPWAN Static Context Header Compression (SCHC) for CoAP", Internet-Draft draft-ietf-lpwan-coap-static-context-hc-12, December 2019. |
| [I-D.ietf-lpwan-ipv6-static-context-hc] | Minaburo, A., Toutain, L., Gomez, C., Barthel, D. and J. Zuniga, "Static Context Header Compression (SCHC) and fragmentation for LPWAN, application to UDP/IPv6", Internet-Draft draft-ietf-lpwan-ipv6-static-context-hc-24, December 2019. |
| [RFC7252] | Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014. |