LPWAN Static Context Header Compression (SCHC) Architecture

The IETF LPWAN WG defined the necessary operations to enable IPv6 over selected Low-Power Wide Area Networking (LPWAN) radio technologies. presents an overview of those technologies. The core product of the working group is the Static Context Header Compression (SCHC) technology. SCHC provides an extreme compression capability based on a state that must match on the compressor and decompressor side. This state if formed of an ordered set of Compression/Decompression (C/D) rules. The first rule that matches is used to compress, and is indicated with the compression residue. Based on the rule identifier (RuleID) the decompressor can rebuild the original bitstream based on the residue. also provides a Fragmentation/Reassembly (F/R) capability to cope with a constrained Maximum Transmit Unit (MTU) below the IPv6 minimum link MTU of 1280 bytes (see section 5 of ), which is typically the case on an LPWAN network. was defined to compress IPv6 and UDP; but SCHC really is a generic compression and fragmentation technology. As such, SCHC is agnostic to which protocol it compresses and at which layer it is operated. The C/D peers may be hosted by different entities for different layers, and the F/R operation may also be performed between different parties, or different sub-layers in the same stack. If a protocol or a layer requires additional capabilities, it is always possible to document more specifically how to use SCHC in that context, or to specify additional behaviours. For instance, extends the compression to CoAP and OSCORE . SCHC is also designed to be profiled to adapt to the specific necessities of the various LPWAN technologies to which it is applied. Appendix D. “SCHC Parameters” of lists the information that an LPWAN technology-specific document must provide to profile SCHC for that technology. As an example, provides the profile for LoRaWAN networks. In order to deploy SCHC, it is mandatory that the C/D and F/R peers are provisionned with the exact same set of rules. To be able to provision end-points from different vendors, a common data model is needed that expresses the SCHC rules in an interoperable fashion. To that effect, defines a rule representation using the YANG formalism. Finally, section 3 of depicts a typical network architecture for an LPWAN network, simplified from that shown in and reproduced in .

| |Application| () () () () / \==========| v |=============| Server | () () () / \ +---------+ +-----------+ Dev RGWs NGW App ]]> Typically, an LPWAN network topology is star-oriented, which means that all packets between the same source-destination pair follow the same path from/to a central point. In that model, highly constrained Devices (Dev) exchange information with LPWAN Application Servers (Apps) through a central Network Gateway (NGW), which can be powered and is typically a lot less constrained than the Devices. Because devices embed built-in applications, the traffic flows to be compressed are known in advance and the location of the C/D and F/R functions (e.g., at the Dev and NGW), and the associated rules, can be pre provisionned in the network . Then again, SCHC is very generic and its applicability is not limited to star-oriented deployments and/or to use cases where applications are very static and the state can provisionned in advance. describes an alternate deployment where the C/D and/or F/R operations are performed between peers of equal capabilities over a PPP connection. SCHC over PPP illustrates that with SCHC, the protocols that are compressed can be discovered dynamically and the rules can be fetched on-demand by both parties from the same Uniform Resource Name (URN) , ensuring that the peers use the exact same set of rules.

over PPP ]]> This document provides a general architecture for a SCHC deployment, positioning the required specifications, describing the possible deployment types, and indicating models whereby the rules can be distributed and installed to enable reliable and scalable operations.

As states, the SCHC compression and fragmentation mechanism can be implemented at different levels and/or managed by different organizations. For instance, as represented figure , IP compression and fragmentation may be managed by the network SCHC instance and end-to-end compression between the device and the application. The former can itself be split in two instances since encryption hides the field structure.

This document defines a generic architecture for SCHC that can be used at any of these levels. The goal of the architectural document is to orchestrate the different protocols and data model defined by the LPWAN woeking group to design an operational and interoperable framework for allowing IP application over contrained networks.

As described in a SCHC service is composed of a Parser, analyzing packets and creating a list of fields what will be used to match against the compression rules. If a packet matches rules, compression can be applied following rules instructions. If SCHC compressed packet is too large to be send in a single L2 frame, fragmentation will apply. The process is similar, device rules are checked to find the most appropriate fragmentation rule, regarding the SCHC packet size, the link error rate, the reliability required by the application, … On the other direction, when a packet SCHC arrives, the ruleID is used to find the rule. Its nature allows to select if it is a compression or fragmentation rule. The rule database contains a set of rules specific to a single device. The indicates that the SCHC instance reads the rules to process C/D and F/R, rules are not modified during these actions. A SCHC instance, summarized in the , implies C/D and F/R present in both end. The device connected to a constrained network is in one end and the other end is either located in the core network or at the application. In any cases, the rules must be the same in both ends to perform C/D and F/R.

| C&F |=>..............................=>| R&D |===> +-----+ +-----+ ]]> To enable rule synchronization between both ends, a common rule representation must be defined.

defines an YANG data model to represent the rules. This enables the use of several protocols for rule management, such as NETCONF, RESTCONF and CORECONF. NETCONF uses SSH, RESTCONF uses HTTPS, and CORECONF uses CoAP(s) as their respective transport layer protocols. The data is represented in XML under NETCONF, in JSON under RESTCONF and in CBOR under CORECONF.

|Rule |<--|--------> (-------) update |Manager| NETCONF, RESTCONF or CORECONF . read delete +=======+ request . +-------+ <===| R & D |<=== ===>| C & F |===> +-------+ ]]> Rule Manager (RM) is in charge of handling data derived from the YANG Data Model and apply changes to the rules database . The RM is a application using the Internet to exchange information, therefore: for the network-level SCHC, the communication does not require routing. Each of the end-points having an RM and both RMs can be viewed on the same link, therefore wellknown Link Local addresses can be used to identify the device and the core RM. L2 security MAY be deemed as sufficient, if it provides the necessary level of protection. for application-level SCHC, routing is involved and global IP addresses SHOULD be used. End-to-end encryption is recommended. Management messages can also be carried in the negotiation protocol as proposed in The RM traffic may be itself compressed by SCHC, especially if CORECONF is used, can be used.

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