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Internet SCHC Working Group Internet-Draft The framework for SCHC defines an abstract view of the rules, formalized through a YANG Data Model. In its original description, rules are static and shared by two endpoints. The use of YANG authorizes rules to be uploaded or modified in a SCHC instance and leads to some possible attacks if the changes are not controlled. This document defines a threat model, summarizes some possible attacks, and defines augmentation to the existing Data Model in order to restrict the changes in the rule and, therefore, the impact of possible attacks.

Introduction SCHC is a compression and fragmentation mechanism defined in while provides a YANG Data Model for formal representation of SCHC Rules used either for compression/decompression (C/D) or fragmentation/reassembly (F/R). illustrates the use of several protocols for rule management using the YANG Data Model, such as CORECONF , NETCONF, RESTCONF . The inappropriate use of any of these protocols leads to some possible attacks. The goal of this document is to define a threat model, summarize some possible attacks, and define augmentation to the existing Data Model in order to restrict the changes in the rules and, therefore, the impact of possible attacks.

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here.

Terminology It is expected that the reader will be familiar with the terms and concepts associated with the SCHC framework , , and managmente request processing , NETCONF, RESTCONF . ToDo * Access Control. * Management request processing: The NETCONF, RESTCONF or CORECONF request is processed and passed to the end-point Rule Manager. * Rule Manager (RM). * Context. SCHC Rules

SCHC Management Architecture Figure presents the management part of the SCHC architecture.

| RM |<-->|Mgmt.|<==|Access |<==|Other end|<=== (-------) update +====+ |req. | |Control| |auth. | Management delete |proc.| +=======+ +=========+ Request +=====+ NETCONF, RESTCONF or CORECONF ]]>

When a management request arrives on a SCHC endpoint, several processes should be passed before effectively creating or updating a Rule: Other end authentication: the identity of the requester must be verified: This can be implicit, for example, an LPWAN device that receives it from the SCHC core. Hence, authentication is done at Layer 2. This can be an L2 address. In a LoRaWAN network, for example, the DevEUI allows the SCHC core to identify the device. IP addresses may also be used, as well as cryptographic keys. Access control: Once authenticated, the associated Set of Rules of the instance is retrieved. These rules are enriched with access control information that will be defined in this document. If the Set of Rules does not contain any access control information, the end-point is not allowed to modify the Rules' content. Management request processing: The NETCONF, RESTCONF or CORECONF request is processed and passed to the end-point Rule Manager. The Rule Manager (RM) applies the changes (create, read, update, or delete) to the Rule database.

Threat Model The RM is in charge of applying changes to the context when a management request arrives at a SCHC end-point. It is assumed that these changes should only be effectively applied when it is sure that all end-points of an instance have made the change. This means that in all cases, a peer of peers in an instance always shares the same Set of Rules. The selection of a rule to be applied in an accurate endpoint when a packet arrives is made by selecting the rule offering the best-performance SCHC packet after compression. The attack scenarios considered below are limited to the rule management layer and only involve that a single endpoint in a given instance has been compromised. This means that the authentication is bypassed. Therefore, the compromised endpoint is able to effectively deliver management requests using NETCONF, RESTCONF, or CORECONF to the other endpoint.

SCHC TV/MO/CDA possible combinations SCHC compression behavior uses the TV, MO, and CDA to generate the correct residue. But not all the combinations of this fields descriptors are possible, and then an attack can be detected or avoided. shows all the combinations and those that are enabled. SCHC defines two TV values: set and not set. SCHC MO can be Equal, Ignore, MSB, or Match-mapping. And SCHC CDA can be not-sent, value-sent, mapping-sent, LSB, compute-*, DevIID, or AppIID.

Attack Scenarios ## Scenario 1: Compromised Device A Device RM, under the control of an attacker, sends some management messages to modify the SCHC rules in the core in order to direct the traffic to another application. The impact of this attack is different depending on the original rule: Rules containing exclusively the pair MO -- CDA : (ignore -- not-sent) or rules such as no-compress or no-fragmentation: There is no risk of information lost. There is a risk of a DoS-type attack as it can flood empty packets that pass at the core level. For example ... TBD The attack is limited to a single end-point (the device) since it does not have the right to change core-level rules. Management messages aiming at changing rules where the length of the residue changes: There can be a risk of desynchronizing rules between the core and the compromised device. The attack is limited to a single end-point (the device) since it does not have the right to change core-level rules. As SCHC rules are defined for specific traffic. An example of this can be an attacker changing an element of the rule (the dev UDP port number, for instance), and therefore no rule matches the traffic. Therefore, the core may be saturated by no-compressed messages.

Scenario 2: Compromised Core A Core RM, under the control of an attacker, sends some management messages to modify the SCHC rules in the device in order to delete the device's data. In such a scenario, the attacker will try to inject destructive rules. The main characteristic of these rules is that the combination of MA -- CA reduces the size of the residue, which has, in turn, made it more attractive since it increases the rate of compression. The impact of this attack could be: * Lost of devices' information if nothing is done to preempt a compromised core to change such a rule. An example of this attack could be ... TBD

YANG Access Control YANG language allows to specify read-only or read write nodes. NACM extends this by allowing users or groups of users to perform specific actions. This granularity does not fit this rule model. For instance, the goal is not to allow all the field-id leaves to be modified. The objective is to allow a specific rule entry to be changed and, therefore, some of the leaves to be modified. For instance, an entry with FID containing Uri-path may have its target value modified, as in the same rule, the entry regarding the application prefix should not be changed. The SCHC access control augments the YANG module defined in to allow a remote entity to manipulate the rules. Several levels are defined. in the set of rules, it authorizes or not a new rule to be added. in a compression rule, it allows adding or removing field descriptions. in a compression rule, it allows modifying some elements of the rule, such as the TV, MO, or/and CDA, and associated values. in a fragmentation rule, it allows modifying some parameters.

YANG Data Model The YANG DM proposed in extends the SCHC YANG Data Model introduced in . It adds read-only leaves containing access rights. If these leaves are not present, the information cannot be modified.

leaf ac-modify-set-of-rules This leaf controls modifications applied to a set of rules. They are specified with the rule-access-right enumeration: no-change (0): rules cannot be modified in the Set of Rules. This is the equivalent of having no access control elements in the set of rules. modify-existing-element (1): an existing rule may be modified. add-remove-element (2): a rule can be added or deleted from the Set of Rules, or an existing rule can be modified.

leaf ac-modify-compression-rule This leaf allows to modify a compression element. To be active, leaf ac-modify-set-of-rules MUST be set to modify-existing-element or add-remove-element. This leaf uses the same enumeration as add-remove-element: no-change (0): The rule cannot be modified. modify-existing-element (1): an existing Field Description may be modified. add-remove-element (2): a Field Description can be added or deleted from the Rule or an existing rule can be modified.

leaf ac-modify-field This leaf allows to modify a Field Description in a compression rule. To be active, leaves ac-modify-set-of-rules and ac-modify-compression-rule MUST be set to modify-existing-element or add-remove-element and ac-modify-compression-rule and leaf

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs). This document obsoletes RFC 4741. [STANDARDS-TRACK] This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in the Network Configuration Protocol (NETCONF). RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document defines the Static Context Header Compression and fragmentation (SCHC) framework, which provides both a header compression mechanism and an optional fragmentation mechanism. SCHC has been designed with Low-Power Wide Area Networks (LPWANs) in mind. SCHC compression is based on a common static context stored both in the LPWAN device and in the network infrastructure side. This document defines a generic header compression mechanism and its application to compress IPv6/UDP headers. This document also specifies an optional fragmentation and reassembly mechanism. It can be used to support the IPv6 MTU requirement over the LPWAN technologies. Fragmentation is needed for IPv6 datagrams that, after SCHC compression or when such compression was not possible, still exceed the Layer 2 maximum payload size. The SCHC header compression and fragmentation mechanisms are independent of the specific LPWAN technology over which they are used. This document defines generic functionalities and offers flexibility with regard to parameter settings and mechanism choices. This document standardizes the exchange over the LPWAN between two SCHC entities. Settings and choices specific to a technology or a product are expected to be grouped into profiles, which are specified in other documents. Data models for the context and profiles are out of scope. This document defines how to compress Constrained Application Protocol (CoAP) headers using the Static Context Header Compression and fragmentation (SCHC) framework. SCHC defines a header compression mechanism adapted for Constrained Devices. SCHC uses a static description of the header to reduce the header's redundancy and size. While RFC 8724 describes the SCHC compression and fragmentation framework, and its application for IPv6/UDP headers, this document applies SCHC to CoAP headers. The CoAP header structure differs from IPv6 and UDP, since CoAP uses a flexible header with a variable number of options, themselves of variable length. The CoAP message format is asymmetric: the request messages have a header format different from the format in the response messages. This specification gives guidance on applying SCHC to flexible headers and how to leverage the asymmetry for more efficient compression Rules. 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. The standardization of network configuration interfaces for use with the Network Configuration Protocol (NETCONF) or the RESTCONF protocol requires a structured and secure operating environment that promotes human usability and multi-vendor interoperability. There is a need for standard mechanisms to restrict NETCONF or RESTCONF protocol access for particular users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. This document defines such an access control model. This document obsoletes RFC 6536. Trilliant Networks Inc. consultant Acklio YumaWorks Universität Bremen TZI This document describes a network management interface for constrained devices and networks, called CoAP Management Interface (CORECONF). The Constrained Application Protocol (CoAP) is used to access datastore and data node resources specified in YANG, or SMIv2 converted to YANG. CORECONF uses the YANG to CBOR mapping and converts YANG identifier strings to numeric identifiers for payload size reduction. CORECONF extends the set of YANG based protocols, NETCONF and RESTCONF, with the capability to manage constrained devices and networks. Acklio Cisco Systems Acklio This document defines the LPWAN SCHC architecture.

YANG Data Model

file "ietf-schc-access-control@2023-02-14.yang" module ietf-schc-access-control { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-schc-access-control"; prefix schc-ac; import ietf-schc { prefix schc; } organization "IETF Static Context Header Compression (schc) working group"; contact "WG Web: WG List: Editor: Ana Minaburo "; 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 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. ************************************************************************* This module extends the ietf-schc module to include the rule access control behaviour in RFC YYYY."; revision 2023-02-14 { description "Initial version for RFC YYYY "; reference "RFC YYYY: SCHC AC"; } typedef rule-access-right { type enumeration { enum no-changes { value 0; description "No change are allowed."; } enum modify-existing-element { value 1; description "can modify content inside an element."; } enum add-remove-element { value 2; description "Allows to add or remove or modify an element."; } } } typedef field-access-right { type enumeration { enum no-change { value 0; description "Reserved slot number."; } enum change-tv { value 1; description "Reserved slot number."; } enum change-mo-cda-tv { value 2; description "Reserved slot number."; } } } augment "/schc:schc/schc:rule" { leaf ac-modify-set-of-rules { config false; type rule-access-right; } } augment "/schc:schc/schc:rule/schc:nature/schc:compression" { leaf ac-modify-compression-rule { config false; type rule-access-right; } } augment "/schc:schc/schc:rule/schc:nature/schc:compression/schc:entry" { leaf ac-modify-field { config false; type field-access-right; } } augment "/schc:schc/schc:rule/schc:nature/schc:fragmentation" { leaf ac-modify-timers { config false; type boolean; } } } ]]>

Security Considerations TBD

IANA Considerations TBD