]>

Austria christian@amsuess.com

RISE AB

Isafjordsgatan 22 Kista 16440 Sweden marco.tiloca@ri.se

RISE AB

Isafjordsgatan 22 Kista 16440 Sweden rikard.hoglund@ri.se

t2trg tor onion coap oscore edhoc hidden The CoAP protocol was designed with direct connections and proxies in mind. This document defines mechanisms by which chains of proxies can be set up. In combination, they enable the operation of hidden services and of clients similar to how Tor (onion routing) enables it for TCP-based protocols. Discussion Venues Discussion of this document takes place on the Thing-to-Thing Research Group mailing list (t2trg@irtf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction The Constrained Application Protocol (CoAP) protocol was designed with direct connections and proxies in mind. This document defines mechanisms by which chains of proxies can be set up. In combination, they enable the operation of hidden services and of clients similar to how Tor (onion routing) enables it for TCP-based protocols. Onion CoAP is a framework for building a multi-party anonymization network with the goal of enhancing endpoints’ privacy through anonymous communication, while providing protection against traffic analysis. The key goal of its design is to provide confidential and anonymous communication. Achieving this goal critically depends on multiple non-colluding parties providing particular network services. Specifically, Onion CoAP takes advantage of layered encryption of messages and relaying of traffic through a chain of proxies. Each layer of encryption is removed at successive proxies in the chain, ensuring that no single proxy or network monitor can determine both the source and destination of a given communication flow. Onion CoAP uses Object Security for Constrained RESTful Environments (OSCORE) to protect communications, and Ephemeral Diffie-Hellman Over COSE (EDHOC) for establishing the required OSCORE associations.

Terminology 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.

Uses cases Using Onion CoAP allows for minimizing information disclosure, by hiding the network location of the communicating endpoints as well as their traffic pattern. This also prevents linking the physical position of individuals that use communication endpoints to their organizational or personal affiliations, or inferring their activities. For example, Onion CoAP is relevant in the following use cases.

Transport Applications Some industrial applications can have particularly high privacy and confidentiality requirements, by deeming physical positions and communication patterns of the different parties to be sensitive information and a critical asset to protect. For instance, this can be the case for businesses operating in logistics and for their customers, as interested in a highly discrete tracking of transported goods that exposes as little information as possible. The use of Onion CoAP ensures that the physical location of goods remains undisclosed to unintended and unauthorized parties, thus reducing risks such as tampering, theft, or industrial espionage, e.g., by competitors or saboteurs. Furthermore, anonymizing the communication endpoints and their traffic patterns protects cargo details and logistics chains from being linked to specific operators, routes, or customers. A concrete example is a transport container that includes tracking sensors belonging to a manufacturer company. The container contains products made by such manufacturer and is physically anonymized (i.e., the container does not provide any visual hint pointing to the manufacturer). As the container is moved from one harbor to another, the sensor therein is intended to send a status update to the manufacturer server. When this happens, the manufacturer does not want to reveal itself and its position as something understandable by observing the communication started by the sensor. For instance, the manufacturer wants to keep the details of its logistics strategy confidential, including the routes and schedule of its delivery and distribution process. The reliability and commercial success of the manufacturer may well rely on such an optimized combination or routes and schedule, which is a key asset to not expose to competitors.

Hidden Deployments In some scenarios, not only is the physical location of communicating endpoints a critical asset to protect, but even their deployment and topology should be hidden and impractical to infer. That is, the fact that communication devices are deployed and the details of their deployment should themselves be hidden. For example, one may want to build a network of (mobile) sensors and actuators that are confidentially and secretly deployed in locations which must not be revealed, and from where they act as clients, servers, or both. Concrete use cases can include the supporting of military actions such as intelligence collection and assistance for in-the-field operations, or circumventing repressive actions such as censorship and (pervasive) traffic monitoring.

High-level Overview

Design considerations The network described in this document is designed to allow participation of Class 1 devices as defined in as servers and clients. It should reuse building blocks these devices will already implement if they use EDHOC for authenticated key establishment and OSCORE for encryption. Operations that are costly for constrained devices, such as creating and verifying signatures, should not be part of regular operation.

Components overview This document introduces separate mechanisms that in combination enable setups similar to how Tor is used for anonymous web access and anonymous hosting of web sites. Some of the mechanisms need no new protocol components, but merely describe which existing steps are used to obtain the desired results.

• A client can use EDHOC to establish a unilaterally authenticated OSCORE context with proxies (see ).
• A server can use EDHOC to establish a unilaterally authenticated OSCORE context to establish a reverse proxy address (see ).
• A discovery mechanism for proxies (see ).
• A naming and discovery mechanism for hidden services (see ).

Note that these mechanisms should be largely independent: A server that does not intend to hide its position can still advertise a cryptographic name at its real network coordinates, and thus be available both to clients that do hide their location (even if their proxies do not work as “exit nodes” in Tor terminology) and to clients on a local network. illustrates an example topology, and illustrates a cross-section of the OSCORE layers along one path.

Example topology of an onion style CoAP network showing two routes in separate graphs. Note that the hop P4→P2 is present in both chains, and can be pooled into one TLS connection P3 | v P4 / | P5 ---> Server 1 \ | (published as | ^ \ | server addr.) | / v | | / P6 v / P7 / Client 2 v P1 +------> P2 -----+ / | | v | v P3 P4 P5 ---> Server \ <--------+ \ | v | P6 -------------------> P7 (published as server addr.) ]]>

Cross section of the Client 1 → Server 1 connection of [ TBD reference from label ]. Numbers indicate the sequence in which EDHOC is performed (precisely: message 1 is transmitted over the wire through whichever encapsulation), and are placed on the side of the initiator. Not depicted at step 8: Server 1 and/or P4 (TBD) publishes P4 as the public address of the hidden service; 9: client obtains the list of available proxies. 15: Client 1 looks up the introduction point of Server 1 through the proxy chain up to P1 to discover P4 (may also be implicit).

Mechanisms

Client proxy chain A client can pick one or more proxies to hide its position in the network. Without OSCORE proxies, only one proxy hop can be chosen, because the CoAP requests contains at most two addresses: The address in the IP header, and the address in the Uri-Host option. With the mechanisms introduced in , CoAP request can contain a Uri-Host option in each layer of OSCORE, effectively building a source routing chain. To build the chain, the client first chooses its first proxy hop, and runs EDHOC to establish an OSCORE context. In this process, the proxy authenticates with its long-term credentials, whereas the client uses an ephemeral key (a plain CWT Cliams Set, ). The process can take as little as one round-trip per proxy; when message 3 of EDHOC is sent along with the OSCORE message (see ) that contains the next hop’s message 1, Once one proxy context is established, EDHOC can be run through that proxy with the next proxy, until a chain of sufficient length has been established. Care has to be taken to never use one of the later proxies with any chain other than the chain through which the connection was established, for otherwise the client can be deanonymized mor easily. When forwarding messages, every forward proxy strips off a layer of OSCORE from the request, and adds one to the response. Possible optimizations:

• Can EDHOC be run without transmitting two public keys (G_X and G_I) for the client? (I.e., Can G_X be re-used as G_I without harm to EDHOC (likely not), and how would that be communicated?)
• For hops that are only ever used with a single next-hop, as is typical with all but the first proxy (see guidance below): Can default values for Proxy-Scheme and Uri-Host be communicated during EDHOC, values that would later be elided? Otherwise, every request would contain explicit addresses of the full chain. If taken to the extreme, this might be setting up a SCHC context that also compresses parts of the OSCORE option, where the client tells each proxy what the KID used with the next proxy is, and uses the same sender sequence number for the hops. (This has own security considerations; might be necessary to apply offsets, at which point it gets overly complex). Effectively, setting a default value for Proxy-Scheme and Uri-Host makes that (originally forward) proxy a reverse proxy.

Guidance for setting up proxy chains TBD: This section should contain guidance distilled from Tor operations. In particular, it might recommend that a client pick one proxy hop as a long-term first hop, while building the remaining chain individually for each new origin server. Following common tor practice, it is expected that typical chain lengths are around 3 hops. Note that the amount of processing on the peer side is independent of the length of the chain chosen by a host. If a client deems a one-hop setup sufficient and only has resources for maintaining one extra OSCORE context, it can still use a server that is hidden behind a 3 long proxy chain.

Server proxy chain A server can pick one or more proxies to hide its position in the network. Unlike forward proxies, which are configured per request, this requires a dedicated mechanism. TBD: This document does not yet specify such a mechanism, but may draw upon the reverse proxy request of . When forwarding messages, every reverse proxy adds a layer of OSCORE to the request, and removes one from the response. Possible optimizations:

• Rather than explicitly advertise the name for which the proxies should be set up, the advertised name could be derived from the CRED_x used in EDHOC.

Naming and name resolution The mechanisms discussed in can be used by hidden services to come up with names for their services. (That document will need to be updated to use mechanisms from ). Along with the service’s public key (that is announced as part of the name), the published record may also include the public key of the introduction point, as that will allow the client to establish an extra layer with the introduction point. As the published record is not trusted, the client can use the EAD option described in to verify the proxy’s public key as part of the end-to-end session. If client and server support this, they can rule out that an attacker might advertise itself as the introduction address and could thus monitor large portions of the traffic toward a hidden service (even though that attacker would still not learn the location of the server, the location of hidden clients, or the content of the communication). As an alternative (TBD: when would which be chosen), the client’s last chosen proxy, when seeing the cryptographic address of the hidden service, may not just establish an EDHOC session with the introduction proxy, but also with the hidden service, therein performing the same verification. The server should therefore allow for at least one level of nesting within incoming EDHOC sessions.

Proxy discovery A mechanism for discovering forward proxies is already described in ; discovery of reverse proxies suitable for servers will depend on the precise mechanism used.

Discovering the introduction proxy for services Services with cryptographic identifiers outlined in {#naming} can register these names in a distributed Resource Directory following the same style setup. Unlike described there, they would not enter their network address into the distributed directory, but the address of their most remote reverse proxy (the introduction point). This directory propagates changes relatively fast, limited by the performance of the underlying Distributed Hash Table (DHT). Clients looking for services may not need to use the discovery service directly: Instead, they can send requests to a proxy of their chosing, and rely on the proxy to utilize the directory to look up a next hop. (They do need to perform discovery of the introductory node if they want to hide the ciphertext of their conversation from their last proxy and establish a secure connection to the introduction proxy chosen by the server, verifying it using the EAD option described in instead of relying on their own last proxy).

Discovery for eligible forward and reverse proxies In order to hide their location, clients as well as servers need to discovery lists of eligible proxies, along with metadata that indicates whether the proxy is willing to proxy to arbitrary locations on the Internet, or merely to hidden peers. That distinction in forwrad proxies would be similar to how Tor distinguishes relay and exit nodes. In reverse proxies, there is an analogous distinction that is not so much based on policy but rather on the structure of the authority component used by that reverse proxy: If the proxy can offer names that are resolvable on regular CoAP stacks (i.e., DNS can resolve it to a global IP address), then regular CoAP clients can use the introduction address as an entry point. The hidden service trusts the user to establish an end-to-end connection: If the client is unauthenticated (i.e., using a plain CCS as its credential), the hidden server can not tell whether the incoming EDHOC session is end-to-end or merely set up by a proxy, let alone whether the client is using a chain of proxies or not. Many proxies may not offer such names, and services may not want to rely on such names anyway -- in that case, clients are required to use (most probably by proxy) the DHT in which services are announced. Maintenance of this list is out of scope of this document, but the produced list will have some properties required for the constrained devices: * For each proxy that is available to form a hiding circuit, the list includes: * the proxy’s cryptographic identity (eg. in a CCS): to authenticate the proxy, * affiliation information (operator and location): this enables hiding nodes to find paths of probably non-colluding proxies * optionally a public IP address: this enables nodes to use the proxy as a first hop * The list is updated regularly, with an update rate measured in hours or a few days. * The list needs to be signed by independent entities. (This is the only place in the whole setup where signatures are required: it appears unrealistic that the maintainers of the network will be online to perform non-signing challenges for the document all the time. Devices that can not even perform that verification might have a trusted source, possibly their firmware update source, that performs the verification for them). * The list’s size will excede the memory capacity of individual devices, so it needs to be split up, possibly in a way similar to a Merkle tree. (At a bare minimum, a Tor sized network of 10k nodes with 32 bytes of key material for each node would already exceed the RAM available to Class 2 devices ). It may be beneficial for long-term stability if the list is structured such that there is always a fragment with long-term stable addresses that nodes can store. TBD: Describe operations of this service in a separate document. The three tasks of proxying, participation in the distributed Resource Directory and participation in the dissemination of the proxy list are conceptually separate. None the less, it is expected that proxies eligible for the list will perform all those roles. Nodes partipating in this network will always keep at least some verified fragments of the list across restarts, and should be provisioned with a current state of the list at setup time. As the proxies also provide the list, devices can obtain the latest version through the first EDHOC connection they establish with a proxy they know from the most recent version the have. For the unlikely event that all stored proxies have become unavailable, nodes may accept recent signed versions of the list through other means.

Establishing TLS connections between proxies Proxy-to-proxy requests, which are the majority of transmitted request, are transmitted between unconstrained devices across the Internet. As such, protecting those connections with an extra layer of TLS (as specified in ) is desirable, because

• it profits from TCP’s flow control,
• it hides more request metadata (even though none of the metadata visible at this point should be linkable to the server or the client, with the exception of message length), and
• it allows requests to be mixed across MTU and thus to hide their length and number (provided there is sufficient traffic on the link to ensure that multiple messages are processed within one Nagle interval).

[ TBD: Explore whether coercing traffic through specific pairs of nodes instead of random node pairings would make sense. If it is dangerous, maybe servers might pair up on their own to ensure that it is hard to monitor their ingress and egress traffic for correlation. ] A challenge in establishing TLS connections on that link is that proxies are advertised with EDHOC credentials in the network’s discovery area. The tools of may help bridging that gap. If that work does not progress, proxies might establish an EDHOC session inside an intially unauthenticated / self-signed TLS session, tying the sessions together by the use of a data item exported from the TLS key material exporter.

Other tricks TBD. Current ideas:

• For increased privacy, it may make sense to spool requests and responses in proxies for “as long as practical”. Those setting up the proxy may indicate that in the security context. While it increases the proxy’s memory requirements a lot to keep several seconds of traffic around, it is not expected that these proxies will be operating at network capacity limits.
• Add chatter between proxies. With the stark contrast between constrained device bandwidths and Internet bandwidths, this can be tolerable.
• Access point assistance: While all of this is aimed at constrained devices as defined in , devices of Class 1 may not be capable of using the full proxy discovery service or setting up security contexts with all the hops in the chain. Instead, they may only provide end-to-end encryption, and use a service provided by a local node (e.g. the border router in a 6LoWPAN network ) to set up the hops. Such a setup can also be used to defer policy decisions to the network, which may decide to advertise its own address as an introduction point, or use a suitable length of reverse proxies.
• The introduction point would be the only suitable location to place a caching proxy when is used. As the server is be aware that cacheable OSCORE is used, it can select a reverse proxy that was advertised with caching capabilities (with that metadatum still TBD).

Overhead and optimizations TBD. Main points:

• Establishing a default Uri-Host likely gives most savings.
• For intermediate hops, using a shorter AEAD tag might be an option.

Security Considerations

• When using proxy chains, only contact a proxy through the one chain it is set up with, and only accept messages into a context if they were transported in the hop they are expected to be received from. It is of utmost importance to not have observably different behavior between messages with an unknown context and messages whose context is known but not expected at this point. For example, if an attacker controls a server’s introduction point and intends to deanonymize clients, it may attempt to send responses directly to the suspected address of the client. In implementations, this can be mitigated by first looking up the list of contexts depending on the outer layer, and then looking up inside that list whether the security context is known and the message expected.
• What are the effects of sequence numbers on correlation? Is it good or bad to use the same sequence number for all hops in a chain?

IANA Considerations TBD.

References Normative References The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. The Constrained Application Protocol (CoAP), although inspired by HTTP, was designed to use UDP instead of TCP. The message layer of CoAP over UDP includes support for reliable delivery, simple congestion control, and flow control. Some environments benefit from the availability of CoAP carried over reliable transports such as TCP or Transport Layer Security (TLS). This document outlines the changes required to use CoAP over TCP, TLS, and WebSockets transports. It also formally updates RFC 7641 for use with these transports and RFC 7959 to enable the use of larger messages over a reliable transport. CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios, and a main use case is to establish an Object Security for Constrained RESTful Environments (OSCORE) security context. By reusing CBOR Object Signing and Encryption (COSE) for cryptography, Concise Binary Object Representation (CBOR) for encoding, and Constrained Application Protocol (CoAP) for transport, the additional code size can be kept very low. Ericsson RISE AB RISE AB Fraunhofer AISEC Ericsson The lightweight authenticated key exchange protocol Ephemeral Diffie- Hellman Over COSE (EDHOC) can be run over the Constrained Application Protocol (CoAP) and used by two peers to establish a Security Context for the security protocol Object Security for Constrained RESTful Environments (OSCORE). This document details this use of the EDHOC protocol, by specifying a number of additional and optional mechanisms. These especially include an optimization approach for combining the execution of EDHOC with the first OSCORE transaction. This combination reduces the number of round trips required to set up an OSCORE Security Context and to complete an OSCORE transaction using that Security Context. RISE AB RISE AB Object Security for Constrained RESTful Environments (OSCORE) can be used to protect CoAP messages end-to-end between two endpoints at the application layer, also in the presence of intermediaries such as proxies. This document defines how to use OSCORE for protecting CoAP messages also between an origin application endpoint and an intermediary, or between two intermediaries. Also, it defines rules to escalate the protection of a CoAP option, in order to encrypt and integrity-protect it whenever possible. Finally, it defines how to secure a CoAP message by applying multiple, nested OSCORE protections, e.g., both end-to-end between origin application endpoints, and between an application endpoint and an intermediary or between two intermediaries. Therefore, this document updates RFC 8613. Furthermore, this document updates RFC 8768, by explicitly defining the processing with OSCORE for the CoAP option Hop-Limit. The approach defined in this document can be seamlessly used with Group OSCORE, for protecting CoAP messages when group communication is used in the presence of intermediaries. TUD Dresden University of Technology The Constrained Application Protocol (CoAP, [RFC7252]) is available over different transports (UDP, DTLS, TCP, TLS, WebSockets), but lacks a way to unify these addresses. This document provides terminology and provisions based on Web Linking [RFC8288] and Service Bindings (SVCB, [RFC9460]) to express alternative transports available to a device, and to optimize exchanges using these. 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. 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. Informative References The Internet Protocol Suite is increasingly used on small devices with severe constraints on power, memory, and processing resources, creating constrained-node networks. This document provides a number of basic terms that have been useful in the standardization work for constrained-node networks. This specification introduces a new IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) dispatch type for use in 6LoWPAN route-over topologies, which initially covers the needs of Routing Protocol for Low-Power and Lossy Networks (RPL) data packet compression (RFC 6550). Using this dispatch type, this specification defines a method to compress the RPL Option (RFC 6553) information and Routing Header type 3 (RFC 6554), an efficient IP-in-IP technique, and is extensible for more applications. RISE AB Group communication with the Constrained Application Protocol (CoAP) can be secured end-to-end using Group Object Security for Constrained RESTful Environments (Group OSCORE), also across untrusted intermediary proxies. However, this sidesteps the proxies' abilities to cache responses from the origin server(s). This specification restores cacheability of protected responses at proxies, by introducing consensus requests which any client in a group can send to one server or multiple servers in the same group. RISE AB RISE AB Object Security for Constrained RESTful Environments (OSCORE) can be used to protect CoAP messages end-to-end between two endpoints at the application layer, also in the presence of intermediaries such as proxies. This document defines how to use OSCORE for protecting CoAP messages also between an origin application endpoint and an intermediary, or between two intermediaries. Also, it defines how to secure a CoAP message by applying multiple, nested OSCORE protections, e.g., both end-to-end between origin application endpoints, as well as between an application endpoint and an intermediary or between two intermediaries. Thus, this document updates RFC 8613. The same approach can be seamlessly used with Group OSCORE, for protecting CoAP messages when group communication with intermediaries is used. A collection of extensions to the Resource Directory [rfc9176] that can stand on their own, and have no clear future in specification yet. Discussion Venues This note is to be removed before publishing as an RFC. Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (core@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/core/. Source for this draft and an issue tracker can be found at https://gitlab.com/chrysn/resource-directory-extensions. Arm Limited Arm Limited The TLS protocol supports different credentials, including pre-shared keys, raw public keys, and X.509 certificates. For use with public key cryptography developers have to decide between raw public keys, which require out-of-band agreement and full-fletched X.509 certificates. For devices where the reduction of code size is important it is desirable to minimize the use of X.509-related libraries. With the CBOR Web Token (CWT) a structure has been defined that allows CBOR-encoded claims to be protected with CBOR Object Signing and Encryption (COSE). This document registers a new value to the "TLS Certificate Types" sub-registry to allow TLS and DTLS to use CWTs. Conceptually, CWTs can be seen as a certificate format (when with public key cryptography) or a Kerberos ticket (when used with symmetric key cryptography). Thing to thing communication in Constrained RESTful Environments (CoRE) relies on URIs to link to servers. Next to hierarchical configuration and short-lived IP addresses, this document introduces a naming scheme for devices based on cryptographic identifiers. A special purpose domain is reserved for expressing those identifiers, and mechanisms for constrained devices to announce their names and to look them up are described.

High-Level Comparison with Tor TBD.

Examples TBD.

Change log Since -03:

• Refresh to keep document active.
• Minimal editorial fixes.

Since -02:

• Fixes in the markdown.
• Revised table of contents.
• Updated and fixed references.
• Editorial fixes.
• Acknowledgments.
• Include use cases.
• Add introduction.

Since -01:

• Elaborate on bootstrapping: Describe list of proxies.
• Add design considerations section.
• Suggest exported material from TLS to bind EDHOC sessions if TLS-CWT does not fly.
• Various clarifications.

Since -00:

• Shaped into separate sections on the bits and pieces involved.
• Added illustrations.
• Moved all points from the previous notes in with the new text.

Since :

• The main body of the text was moved here and will be absent from the -07 version of that document.
• An abstract was added.

Acknowledgments The authors sincerely thank and for their comments and feedback. The work on this document has been partly supported by the Sweden’s Innovation Agency VINNOVA and the Celtic-Next project CYPRESS.