RFC 9363 | LPWAN SCHC YANG Data Model | March 2023 |
Minaburo & Toutain | Standards Track | [Page] |
- Stream:
- Internet Engineering Task Force (IETF)
- RFC:
- 9363
- Category:
- Standards Track
- Published:
- ISSN:
- 2070-1721
RFC 9363
A YANG Data Model for Static Context Header Compression (SCHC)
Abstract
This document describes a YANG data model for the Static Context Header Compression (SCHC) compression and fragmentation Rules.¶
This document formalizes the description of the Rules for better interoperability between SCHC instances either to exchange a set of Rules or to modify the parameters of some Rules.¶
Status of This Memo
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9363.¶
Copyright Notice
Copyright (c) 2023 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/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
1. Introduction
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 an informal representation of the Rules used either for compression/decompression (C/D) or fragmentation/reassembly (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, and¶
- an update of the other end to set up some specific values (e.g., IPv6 prefix, destination address, etc.).¶
[LPWAN-ARCH] illustrates the exchange of Rules using the YANG data model.¶
This document defines a YANG data model [RFC7950] to represent both compression and fragmentation Rules, which leads to common representation for values for all the Rules' elements.¶
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
3. Terminology
This section defines the terminology and acronyms used in this document. It extends the terminology of [RFC8376].¶
- App:
- Low-Power WAN (LPWAN) Application, as defined by [RFC8376]. An application sending/receiving packets to/from the Dev.¶
- Bi:
- Bidirectional. Characterizes a Field Descriptor that applies to headers of packets traveling in either direction (Up and Dw; see this glossary).¶
- CDA:
- Compression/Decompression Action. Describes the pair of actions that are performed at the compressor to compress a header field and at the decompressor to recover the original value of the header field.¶
- Context:
- A set of Rules used to compress/decompress headers.¶
- Dev:
- Device, as defined by [RFC8376].¶
- DevIID:
- Device Interface Identifier. The IID that identifies the Dev interface.¶
- DI:
- Direction Indicator. This field tells which direction of packet travel (Up, Dw, or Bi) a Field Descriptor applies to. This allows for asymmetric processing, using the same Rule.¶
- Dw:
- Downlink direction for compression/decompression, from SCHC C/D in the network to SCHC C/D in the Dev.¶
- FID:
- Field Identifier or Field ID. This identifies the protocol and field a Field Descriptor applies to.¶
- FL:
- Field Length. This is the length of the original packet header field. It is expressed as a number of bits for header fields of fixed lengths or as a type (e.g., variable, token length, ...) for Field Lengths that are unknown at the time of Rule creation. The length of a header field is defined in the corresponding protocol specification (such as IPv6 or UDP).¶
- FP:
- Field Position. When a field is expected to appear multiple times in a header, the Field Position specifies the occurrence this Field Descriptor applies to (for example, first Uri-Path option, second Uri-Path, etc. in a Constrained Application Protocol (CoAP) header), counting from 1. The value 0 is special and means "don't care" (see Section 7.2 of [RFC8724]).¶
- IID:
- Interface Identifier. See the IPv6 addressing architecture [RFC7136].¶
- L2 Word:
- This is the minimum subdivision of payload data that the Layer 2 (L2) will carry. In most L2 technologies, the L2 Word is an octet. In bit-oriented radio technologies, the L2 Word might be a single bit. The L2 Word size is assumed to be constant over time for each device.¶
- MO:
- Matching Operator. An operator used to match a value contained in a header field with a value contained in a Rule.¶
- RuleID:
- Rule Identifier. An identifier for a Rule. SCHC C/D on both sides share the same RuleID for a given packet. A set of RuleIDs are used to support SCHC F/R functionality.¶
- TV:
- Target Value. A value contained in a Rule that will be matched with the value of a header field.¶
- Up:
- Uplink direction for compression/decompression, from the Dev SCHC C/D to the network SCHC C/D.¶
4. SCHC Rules
SCHC compression is generic; the main mechanism does not refer to a specific protocol. Any header field is abstracted through a Field Identifier (FID), a position (FP), a direction (DI), and a value that can be a numerical value or a string. [RFC8724] and [RFC8824] specify fields for IPv6 [RFC8200], UDP [RFC0768], and CoAP [RFC7252], including options defined for no server response [RFC7967] and Object Security for Constrained RESTful Environments (OSCORE) [RFC8613]. For the latter, [RFC8824] splits this field into subfields.¶
SCHC fragmentation requires a set of common parameters that are included in a Rule. These parameters are defined in [RFC8724].¶
The YANG data model enables the compression and the fragmentation selection using the feature statement.¶
4.1. Compression Rules
[RFC8724] proposes an informal 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.¶
4.2. Identifier Generation
Identifiers used in the SCHC YANG data model are from the identityref statement to ensure global uniqueness and easy augmentation 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 identities used in the data model from this base type.¶
- Create a typedef from this base type.¶
The example below (Figure 2) shows how an identityref is created for Reassembly Check Sequence (RCS) algorithms used during SCHC fragmentation.¶
4.3. Convention for 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] does not state how the Field ID value is constructed. In examples, identification is done through a string indexed by the protocol name (e.g., IPv6.version, CoAP.version, etc.).¶
The current YANG data model includes field definitions found in [RFC8724] and [RFC8824].¶
Using the YANG data model, each field MUST be identified through a global YANG identityref.¶
A YANG Field ID for the protocol is always derived 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 subfields, the field identity serves as a base.¶
The full field-id definition is found in Section 6. A type is defined for the IPv6 protocol, and each field is based on it. Note that the Diffserv bits derive from the Traffic Class identity.¶
4.4. Convention for Field Length
The 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 (whose length is expressed in bytes), and [RFC8824] defines the "tkl" function for managing the CoAP Token Length field.¶
The naming convention is "fl-" followed by the function name.¶
The Field Length function can be defined as an identityref, as described in Section 6. Therefore, the type for the Field Length is a union between an integer giving the size of the length in bits and the identityref.¶
4.5. Convention for Field Position
The Field Position is a positive integer that gives the occurrence times of a specific field from the header start. The default value is 1 and is incremented at each repetition. Value 0 indicates that the position is not important and is not considered during the Rule selection process.¶
The Field Position is a positive integer. The type is uint8.¶
4.6. Convention for Direction Indicator
The Direction Indicator is used to tell if a field appears in both directions (Bi) or only uplink (Up) or Downlink (Dw). The naming convention is "di" followed by the Direction Indicator name.¶
The type is "di-type".¶
4.7. Convention for Target Value
The Target Value is a list of binary sequences of any length, aligned to the left. In the Rule, the structure will be used as a list, with the index as a key. The highest index value is used to compute the size of the index sent in residue for the match-mapping Compression Decompression Action (CDA). The index can specify several values:¶
- For equal and most significant bits (MSBs), the Target Value contains a single element. Therefore, the index is set to 0.¶
- For match-mapping, the Target Value can contain several elements. Index values MUST start from 0 and MUST be contiguous.¶
If the header field contains text, the binary sequence uses the same encoding.¶
4.8. Convention for Matching Operator
The Matching Operator (MO) is a function applied between a field value provided by the parsed header and the Target Value. [RFC8724] defines 4 MOs.¶
The naming convention is "mo-" followed by the MO name.¶
The type is "mo-type".¶
4.8.1. Matching Operator Arguments
They are viewed as a list, built with a tv-struct (see Section 4.7).¶
4.9. Convention for Compression Decompression Actions
The Compression Decompression Action (CDA) identifies the function to use for compression or decompression. [RFC8724] defines 7 CDAs.¶
The naming convention is "cda-" followed by the CDA name.¶
4.9.1. Compression Decompression Action Arguments
Currently no CDA requires arguments, but some CDAs may require one or several arguments in the future. They are viewed as a list of target-value type.¶
4.10. Fragmentation Rule
Fragmentation is optional in the data model and depends on the presence of the "fragmentation" feature.¶
Most of the fragmentation parameters are listed in Appendix D of [RFC8724].¶
Since fragmentation Rules work for a specific direction, they MUST contain a mandatory Direction Indicator. The type is the same as the one used in compression entries, but bidirectional MUST NOT be used.¶
4.10.1. Fragmentation Mode
[RFC8724] defines 3 fragmentation modes:¶
- No ACK: This mode is unidirectional; 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.¶
The type is "fragmentation-mode-type". The naming convention is "fragmentation-mode-" followed by the fragmentation mode name.¶
4.10.2. Fragmentation Header
A data fragment header, starting with the RuleID, can be sent in the fragmentation direction. [RFC8724] indicates that the SCHC header may be composed of the following (cf. Figure 3):¶
- a Datagram Tag (DTag) identifying the datagram being fragmented if the fragmentation applies concurrently on several datagrams. This field is optional, and its length is defined by the Rule.¶
- a Window (W) used in ACK-Always and ACK-on-Error modes. In ACK-Always, its size is 1. In ACK-on-Error, it depends on the Rule. This field is not needed in No-ACK mode.¶
- a Fragment Compressed Number (FCN) indicating the fragment/tile position within the window. This field is mandatory on all modes defined in [RFC8724], and its size is defined by the Rule.¶
4.10.3. Last Fragment Format
The last fragment of a datagram is sent with a Reassembly Check Sequence (RCS) field to detect residual transmission errors and possible losses in the last window. [RFC8724] defines a single algorithm based on Ethernet CRC computation.¶
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 following parameters define the behavior:¶
- all-1-data-no: The last fragment contains no data, just the RCS.¶
- all-1-data-yes: The last fragment includes a single tile and the RCS.¶
- all-1-data-sender-choice: The last fragment may or may not contain a single tile. The receiver can detect if a tile is present.¶
The naming convention is "all-1-data-" followed by the behavior identifier.¶
4.10.4. Acknowledgment Behavior
The acknowledgment fragment header goes in the opposite direction of data. [RFC8724] defines the header, which is composed of the following (see Figure 4):¶
- 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 the received tile.¶
For ACK-on-Error, SCHC defines when an 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 as a response to receiving the last fragment (FCN all-1). The naming convention is "ack-behavior" followed by the algorithm name.¶
4.10.5. Timer Values
The state machine requires some common values to handle fragmentation correctly.¶
- The Retransmission Timer gives the duration before sending an ACK request (cf. Section 8.2.2.4 of [RFC8724]). If specified, the value MUST be strictly positive.¶
- The Inactivity Timer gives the duration before aborting a fragmentation session (cf. Section 8.2.2.4 of [RFC8724]). The value 0 explicitly indicates that this timer is disabled.¶
[RFC8724] does not specify any range for these timers. [RFC9011] recommends a duration of 12 hours. In fact, the value range should be between milliseconds for real-time systems to several days for worse-than-best-effort systems. To allow a large range of applications, two parameters must be specified:¶
- the duration of a tick. It is computed by this formula: 2tick-duration/106. When tick-duration is set to 0, the unit is the microsecond. The default value of 20 leads to a unit of 1.048575 seconds. A value of 32 leads to a tick-duration of about 1 hour 11 minutes.¶
- the number of ticks in the predefined unit. With the default tick-duration value of 20, the timers can cover a range between 1.0 second and 19 hours, as recommended in [RFC9011].¶
4.10.6. Fragmentation Parameter
The SCHC fragmentation protocol specifies the number of attempts before aborting through the parameter:¶
- max-ack-requests (cf. Section 8.2.2.4 of [RFC8724])¶
4.10.7. Layer 2 Parameters
The data model includes two parameters needed for fragmentation:¶
- l2-word-size: [RFC8724] base fragmentation, in bits, on a Layer 2 Word that can be of any length. The default value is 8 and corresponds to the default value for the 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 an uncompressed datagram. By default, the value is set to 1280 bytes.¶
5. Rule Definition
A Rule is identified by a unique Rule Identifier (RuleID) comprising both a RuleID value and a RuleID length. The YANG grouping rule-id-type defines the structure used to represent a RuleID. A length of 0 is allowed to represent an implicit Rule.¶
Three natures of Rules are defined in [RFC8724]:¶
- Compression: A compression Rule is associated with the RuleID.¶
- No-compression: This identifies the default Rule used to send a packet integrally when no-compression Rule was found (see Section 6 of [RFC8724]).¶
- Fragmentation: Fragmentation parameters are associated with the RuleID. Fragmentation is optional, and the feature "fragmentation" should be set.¶
The YANG data model respectively introduces these three identities :¶
The naming convention is "nature-" followed by the nature identifier.¶
To access a specific Rule, the RuleID length and value are used as a key. The Rule is either a compression or a fragmentation Rule.¶
5.1. Compression Rule
A compression Rule is composed of entries describing its processing. 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 represents a line in Figure 1. Their type reflects the identifier types defined in Section 4.1.¶
Some checks are performed on the values:¶
5.2. 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 4.10.¶
7. IANA Considerations
This document registers one URI and one YANG data model.¶
7.1. URI Registration
IANA registered the following URI in the "IETF XML Registry" [RFC3688]:¶
8. Security Considerations
The YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS [RFC8446].¶
The Network Configuration Access Control Model (NACM) [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.¶
There are a number of data nodes defined in this YANG module that are writable/creatable/deletable (i.e., config true, which is the default). These data nodes may be considered sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) to these data nodes without proper protection can have a negative effect on network operations. These are the subtrees and data nodes and their sensitivity/vulnerability:¶
- /schc:
- All the data nodes may be modified. The Rule contains sensitive information, such as the application IPv6 address where the device's data will be sent after decompression. An attacker may try to modify other devices' Rules by changing the application address and may block communication or allows traffic eavesdropping. Therefore, a device must be allowed to modify only its own rules on the remote SCHC instance. The identity of the requester must be validated. This can be done through certificates or access lists. Modification may be allowed regarding the Field Descriptor (i.e., IPv6 addresses field descriptors should not be modified, but UDP dev port could be changed).¶
Some of the readable data nodes in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access (e.g., via get, get-config, or notification) to these data nodes. These are the subtrees and data nodes and their sensitivity/vulnerability:¶
- /schc:
- By reading a module, an attacker may learn the traffic generated by a device and can also learn about application addresses or REST API.¶
9. References
9.1. Normative References
- [RFC0768]
- Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, , <https://www.rfc-editor.org/info/rfc768>.
- [RFC2119]
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
- [RFC3688]
- Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, , <https://www.rfc-editor.org/info/rfc3688>.
- [RFC6020]
- Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, , <https://www.rfc-editor.org/info/rfc6020>.
- [RFC6241]
- Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, , <https://www.rfc-editor.org/info/rfc6241>.
- [RFC6242]
- Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, , <https://www.rfc-editor.org/info/rfc6242>.
- [RFC7136]
- Carpenter, B. and S. Jiang, "Significance of IPv6 Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, , <https://www.rfc-editor.org/info/rfc7136>.
- [RFC7252]
- Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <https://www.rfc-editor.org/info/rfc7252>.
- [RFC8040]
- Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, , <https://www.rfc-editor.org/info/rfc8040>.
- [RFC8174]
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
- [RFC8200]
- Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <https://www.rfc-editor.org/info/rfc8200>.
- [RFC8341]
- Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, , <https://www.rfc-editor.org/info/rfc8341>.
- [RFC8342]
- Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K., and R. Wilton, "Network Management Datastore Architecture (NMDA)", RFC 8342, DOI 10.17487/RFC8342, , <https://www.rfc-editor.org/info/rfc8342>.
- [RFC8446]
- Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <https://www.rfc-editor.org/info/rfc8446>.
- [RFC8613]
- Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, , <https://www.rfc-editor.org/info/rfc8613>.
- [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, , <https://www.rfc-editor.org/info/rfc8724>.
- [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, , <https://www.rfc-editor.org/info/rfc8824>.
9.2. Informative References
- [LPWAN-ARCH]
- Pelov, A., Thubert, P., and A. Minaburo, "LPWAN Static Context Header Compression (SCHC) Architecture", Work in Progress, Internet-Draft, draft-ietf-lpwan-architecture-02, , <https://datatracker.ietf.org/doc/html/draft-ietf-lpwan-architecture-02>.
- [RFC7950]
- Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, , <https://www.rfc-editor.org/info/rfc7950>.
- [RFC7967]
- Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T. Bose, "Constrained Application Protocol (CoAP) Option for No Server Response", RFC 7967, DOI 10.17487/RFC7967, , <https://www.rfc-editor.org/info/rfc7967>.
- [RFC8376]
- Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) Overview", RFC 8376, DOI 10.17487/RFC8376, , <https://www.rfc-editor.org/info/rfc8376>.
- [RFC9011]
- Gimenez, O., Ed. and I. Petrov, Ed., "Static Context Header Compression and Fragmentation (SCHC) over LoRaWAN", RFC 9011, DOI 10.17487/RFC9011, , <https://www.rfc-editor.org/info/rfc9011>.
Appendix A. Example
The informal Rules given Figure 7 are represented in XML, as shown in Figure 8.¶
Acknowledgments
The authors would like to thank Dominique Barthel, Carsten Bormann, Ivan Martinez, and Alexander Pelov for their careful reading and valuable inputs. A special thanks for Joe Clarke, Carl Moberg, Tom Petch, Martin Thomson, and Éric Vyncke for their explanations and wise advice when building the model.¶