| P2PSIP | J. Jimenez |
| Internet-Draft | Ericsson |
| Intended status: Standards Track | J. Lopez-Vega |
| Expires: January 24, 2016 | University of Granada |
| J. Maenpaa | |
| G. Camarillo | |
| Ericsson | |
| July 23, 2015 |
A Constrained Application Protocol (CoAP) Usage for REsource LOcation And Discovery (RELOAD)
draft-jimenez-p2psip-coap-reload-10
This document defines a Constrained Application Protocol (CoAP) Usage for REsource LOcation And Discovery (RELOAD). The CoAP Usage provides the functionality to federate Wireless Sensor Networks (WSN) in a peer-to-peer fashion. The CoAP Usage for RELOAD allows CoAP nodes to store resources in a RELOAD peer-to-peer overlay, provides a lookup service, and enables the use of RELOAD overlay as a cache for sensor data. This functionality is implemented in the RELOAD overlay itself, without the use of centralized servers. The RELOAD AppAttach method is used to establish a direct connection between nodes through which CoAP messages are exchanged.
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The CoAP Usage for RELOAD allows CoAP nodes to store resources in a RELOAD peer-to-peer overlay, provides a lookup service, and enables the use of RELOAD overlay as a cache for sensor data. This functionality is implemented in the RELOAD overlay itself, without the use of centralized servers.
This usage is intended for interconnected devices over a wide-area geographical coverage, such as in cases where multiple Wireless Sensor Networks (WSN) need to be federated over some wider-area network. These WSNs would interconnect by means of nodes that are equipped with long range modules (e.g., 2G, 3G, 4G) as well as short range ones (e.g., XBee, ZigBee, BLE).
Constrained devices are likely to be heterogeneous when it comes to their radio layer, however we expect them to use a common application layer protocol, CoAP. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol [RFC7252]. It realizes the Representational State Transfer (REST) architecture for the most constrained nodes, such as sensors and actuators. CoAP can be used not only between nodes on the same constrained network but also between constrained nodes and nodes on the Internet. The latter is possible since CoAP can be translated to Hypertext Transfer Protocol (HTTP) for integration with the web. Application areas of CoAP include different forms of M2M communication, such as home automation, construction, health care or transportation. Areas with heavy use of sensor and actuator devices that monitor and interact with the surrounding environment.
As specified in [RFC6940] RELOAD is fundamentally an overlay network. Providing a layered architecture with pluggable application layers than can use the underlaying forwarding, storage and lookup functionalities. Figure 1 illustrates where the CoAP Usage is placed within the RELOAD architecture.
Application
+-------+
| CoAP | ...
| Usage |
+-------+
------------------------------------ Messaging Service
+------------------+ +---------+
| Message |<--->| Storage |
| Transport | +---------+
+------------------+ ^
^ ^ |
| v v
| +-------------------+
| | Topology |
| | Plug-in |
| +-------------------+
| ^
v v
+------------------+
| Forwarding & |
| Link Management |
+------------------+
------------------------------------ Overlay Link Service
+-------+ +-------+
|TLS | |DTLS | ...
|Overlay| |Overlay|
|Link | |Link |
+-------+ +-------+
Figure 1: Architecture
The CoAP Usage involves three basic functions:
Registration: CoAP nodes that can use the RELOAD data storage functionality, can store a mapping from their CoAP URI to their Node-ID in the overlay. They can also retrieve the Node-IDs of other nodes. Nodes that are not RELOAD aware can use other mechanisms, for example [I-D.ietf-core-resource-directory] in their local network.
Lookup: Once a CoAP node has identified the Node-ID for an URI it wishes to retrieve, it can use the RELOAD message routing system to set up a connection which can be used to exchange CoAP messages. Similarly as with the registration, nodes that are not RELOAD aware can use CoAP messages with an Reload Node (RN) that will in turn perform the lookup in the overlay.
Caching: Nodes can use the RELOAD overlay as a caching mechanism for information about what CoAP resources are available on the node. This is specially useful for battery constrained nodes that can make their data available in the cache provided by the overlay while in sleep mode.
For instance, a CoAP proxy (See Section 3) could register its Node-ID (e.g. "9996172") and a list of sensors (e.g. "/sensors/temp-1; /sensors/temp-2; /sensors/light, /sensors/humidity") under its URI (e.g. "coap://overlay-1.com/proxy-1/").
When a node wants to discover the values associated with that URI, it queries the overlay for "coap://overlay-1.com/proxy-1/" and gets back the Node-ID of the proxy and the list of its associated sensors. The requesting node can then use the RELOAD overlay to establish a direct connection with the proxy and to read sensor values.
Moreover, the CoAP proxy can store the sensor information in the overlay. In this way information can be retrieved directly from the overlay without performing a direct connection to the storing proxy.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
We use the terminology and definitions from the RELOAD Base Protocol [RFC6940] extensively in this document. Some of those concepts are further described in the Concepts and Terminology for Peer to Peer SIP [I-D.ietf-p2psip-concepts] document.
In our architecture we extend the different nodes present in RELOAD (Peer, Client) and add support for sensor devices or other constrained devices. Figure 2 illustrates the overlay topology. The different nodes, according to their functionality are :
Physical devices can have one or several of the previous functional roles. According to the functionalities that are present in each of the nodes, they can be:
+------+ | | +--------+ RN +---------+ | | | | +---+--+ +------+ +--+---+ | | | | | RN | | RN | | | | | +------------+ +---+--+ +--+---+ | WSN | | RELOAD | | +----+ | | OVERLAY | | +---+ CN | | +---+--+ +--+---+ | | +----+ | | | | +-----+ | | RN | | PN | | | | | | +-----+ | +---+--+ +------+ +--+---+ | | +----+ | | | | | | +---+ CN | | +--------+ PN +---------+ | +----+ | | | +------------+ +-+--+-+ | | +--------|--|--------+ | +--+ +--+ | | | | | | +--+-+ +-+--+ | | | CN | | CN | | | +----+ +----+ | | WSN | +--------------------+
Figure 2: Overlay Topology
CoAP URIs are typically resolved using a DNS. When CoAP is needed in a RELOAD environment, URI resolution is provided by the overlay as a whole. Instead of registering a URI, a peer stores a CoAPRegistration structure under a hash of its own URI. This uses the CoAP REGISTRATION Kind-ID, which is formally defined in Section 6, and that uses a DICTIONARY data model.
As an example, if a CoAP proxy that is located in an overlay overlay-1.com using a Node-ID "9996172" wants to register four different sensors to the URI "coap://overlay-1.com/proxy-1/.well-known/". We will be using the link format specified in [RFC6690] to store the following mapping in the overlay:
Resource-ID = h(coap://overlay-1.com/proxy-1/.well-known/)
KEY = 9996172,
VALUE = [
</sensors/temp-1>;rt="temperature-c";if="sensor",
</sensors/temp-2>;rt="temperature-c";if="sensor",
</sensors/light>;rt="light-lux";if="sensor",
</sensors/humidity>;rt="humidity-p";if="sensor"
]
Note that the Resource-ID stored in the overlay is calculated as hash over the URI (i.e. h(URI)), which in RELOAD usually is SHA-1.
This would inform any other node performing a lookup for the previous URI "coap://overlay-1.com/proxy-1/.well-known" that the Node-ID value for proxy-1 is "9996172". In addition, this mapping provides relevant information as to the number of sensors (CNs) and the URI path to connect to them using CoAP.
The RELOAD overlay supports a rendezvous system that can be used for the lookup of other CoAP nodes. This is done by fetching mapping information between CoAP URIs and Node-IDs.
As an example, if a node RN located in the overlay overlay-1.com wishes to read which resources are served at a RN with URI coap://overlay-1.com/proxy-1/, it performs a fetch in the overlay. The Resource-ID used in this fetch is a SHA-1 hash over the URI "coap://overlay-1.com/proxy-1/.well-known/".
After this fetch request, the overlay will return the following result:
Resource-ID = h(coap://overlay-1.com/proxy-1/.well-known/) KEY = 9996172, VALUE = [ </sensors/temp-1>;rt="temperature-c";if="sensor", </sensors/temp-2>;rt="temperature-c";if="sensor", </sensors/light>;rt="light-lux";if="sensor", </sensors/humidity>;rt="humidity-p";if="sensor" ]
The obtained KEY is the Node-ID of the RN responsible of this KEY/VALUE pair. The VALUE is the set of URIs necessary to read data from the CNs associated with the RN.
Using the RELOAD DICTIONARY model allows for multiple nodes to perform a store to the same Resource-ID. This can be used, for example, to perform a store of resources of the same type or with similar characteristics. After performing a lookup, this feature allows to fetch those multiple RNs that host CNs of the same class.
As an example, providing that the previous peer (9996172) and another peer (9996173) have stored the links to their respective temperature resources in this same Resource-ID (temperature), a RN (e.g., node-A) can do a fetch to the URI "coap://overlay-1.com/temperature/.well-known/", obtaining the following results:
Resource-ID = h(coap://overlay-1.com/temperature/.well-known/)
KEY = 9996172,
VALUE = [
</sensors/temp-1>;rt="temperature-c";if="sensor",
</sensors/temp-2>;rt="temperature-c";if="sensor",
]
KEY = 9996173,
VALUE = [
</sensors/temp-a>;rt="temperature-c";if="sensor",
</sensors/temp-b>;rt="temperature-c";if="sensor"
]
Once a RN (e.g., node-A) has obtained the lookup information for a node in the overlay (e.g., proxy-1), it can directly connect to that node. This is performed by sending an AppAttach request to the Node-ID obtained during the lookup process.
After the AppAttach negotiation, node-A can access to the values of the CNs at proxy-1 using the information obtained during the lookup. Following the example in Section 5, and according to [RFC6690], the requests for accessing to the CNs at proxy-1 would be:
REQ: GET /sensors/temp-1
REQ: GET /sensors/temp-2
Figure 3 shows a sample of a node reading temperature data.
+-----+ +---------+ +-----+ +---+ | PNA | | OVERLAY | | PNB | |CNB| +-----+ +---------+ +-----+ +---+ | | | | | | | | | 1.RELOAD | | | | FetchReq | | | |+----------->| | | | | | | | 2.RELOAD | | | | FetchAns | | | |<-----------+| | | | | | | | 3.RELOAD | | | | AppAttach | | | |+----------->| | | | | 4.RELOAD | | | | AppAttach | | | |+---------->| | | | | | | | 5.RELOAD | | | 6.RELOAD |AppAttachAns| | |AppAttachAns |<----------+| | |<-----------+| | | | | | | | | | | --------------------- | | | / 7.ICE \| | | \ connectivity checks /| | | --------------------- | | | | | | 8.CoAP CON | | | GET /sensors/temp-1 | | |+------------------------>| | | | 9.CoAP GET | | |/sensors/temp-1 | | |+-------------->| | | 10.CoAP | | 11.CoAP | ACK 200 | | ACK 200 |<--------------+| |<------------------------+| | | | |
Figure 3: An Example of a Message Sequence
The CoAP protocol itself supports the caching of sensor information in order to reduce the response time and network bandwidth consumption of future, equivalent requests. CoAP caching is specified in the Section 5 of the CoAP RFC [RFC7252], it consists on reusing stored responses when new requests arrives. This type of storage is done in CoAP proxies.
This CoAP usage for RELOAD proposes an additional caching mechanism for storing sensor information directly in the overlay. In order to do so, it is necessary to define how the data should be stored. Such caching mechanism is primarily intended for CNs with sensor capabilities, not for RN sensors. This is due to the battery constrains of CNs, forcing them to stay in sleep mode for long periods of time.
Whenever a CN wakes up, it sends the most recent data from its sensors to its proxy (PN), which stores the data in the overlay using a RELOAD StoredData structure defined in Section 6 of the RELOAD RFC [RFC6940]. We use the StoredDataValue structure defined in Section 6.2 of the RELOAD RFC, in particular we use the SingleValue format type to store the cached values in the overlay. From that structure length, storage_time, lifetime and Signature are used in the same way. The only difference is DataValue which in our case can be either a ProxyCache or a SensorCache:
enum { reserved (0), proxy_cache(1), sensor_cache(2), (255) }
CoAPCachingType;
struct {
CoAPCachingType coap_caching_type;
select(coap_caching_type) {
case proxy_cache: ProxyCache proxy_cache_entry;
case sensor_cache: SensorCache sensor_cache_entry;
/* extensions */
}
} CoAPCaching;
ProxyCache is meant to store values and sensor information (e.g. inactivity time) for all the sensors associated with a certain proxy, as well as their CoAP URIs. On the other hand, SensorCache is used for storing the information and cached value of only one sensor (CoAP URI is not necessary, as is the same as the one used for generating the Resource-ID associated to that SensorCache entry).
struct {
Node-ID Node_ID;
uint32 length;
SensorEntry sensors[count];
} ProxyCache;
ProxyCache contains the fields Node-ID and series of SensorEntry types.
struct {
opaque coap_uri;
SensorInfo sensor_info;
uint32 length;
SensorValue sensor_value[count];
} SensorEntry;
SensorEntry contains the coap_uri, sensor_info and a series of SensorValue types.
struct {
Node-ID Node_ID;
SensorInfo sensor_info;
uint32 length;
SensorValue sensor_value[count];
} SensorCache;
SensorCache: contains the information related to one sensor.
struct {
opaque dev_info;
/* extensions */
} SensorInfo;
SensorInfo contains relevant sensor information that is dependent on the use case. As an example we use the sensor manufacturer as relevant information.
struct {
uint32 measurement_time;
uint32 lifetime;
opaque value;
/* extensions */
} SensorValue;
SensorValue contains the measurement_time, lifetime and value of the measurement.
This section defines the CoAP-REGISTRATION and CoAP-CACHING kinds.
struct {
Node-ID Node_ID;
uint16 coap_uris_length;
opaque coap_uris (0..2^16-1);
} CoAPRegistration;
Data stored under the COAP-REGISTRATION kind is of type CoAPRegistration, defined below.
Data stored under the CoAP-CACHING kind is of type CoAPCaching, defined in Section 7.
As specified in RELOAD base [RFC6940], every kind which is storable in an overlay must be associated with an access control policy. This policy defines whether a request from a given node to operate on a given value should succeed or fail. Usages can define any access control rules they choose, including publicly writable values.
CoAP Usage for RELOAD requires an access control policy that allows multiple nodes in the overlay read and write access. This access is for registering and caching information using CoAP URIs as identifiers. Therefore, none of the access control policies specified in RELOAD base are sufficient.
This document defines two access control policies, called URI-MATCH and URI-NODE-MATCH. In URI-MATCH policy, a given value MUST be written and overwritten if and only if the signer's certificate contains an uniformResourceIdentifier entry in the subjectAltName Extension [RFC5280] that in canonicalized form hashes to the Resource-ID for the resource. As explained on Section 6.3 of the COAP RFC [RFC7252] the "coap" and "coaps" schemes conform to the generic URI, thus they are normalized in the generic form as explained on Section 6 of [RFC3986]. The hash function used is specified in Section 10.2 of [RFC6940]. The certificate can be generated as specified on Section 9 of the COAP RFC [RFC7252], Certificate Option.
In URI-NODE-MATCH policy, a given value MUST be written and overwritten if and only if the condition for URI-MATCH is met and, in addition, the dictionary key is equal to the Node-ID in the certificate and that Node-ID is the one indicated in the SignerIdentity value cert_hash.
These Access Control Policies are specified for IANA in Section Section 11.3.
The security considerations of RELOAD [RFC6940] and CoAP [RFC7252] apply to this specification. RELOAD's security model is based on public key certificates, which are used for signing messages and stored objects. At the connection level RELOAD can use either TLS or DTLS. In the case of CoAP, several security modes have been defined. Implementations of this specification MUST follow all the security-related rules specified in the RELOAD [RFC6940] and CoAP [RFC7252] specifications.
Additionally, in RELOAD every kind which is storable in an overlay must be associated with an access control policy. This document specifies two new access control policies, which are specified in Section 9. These policies cover the most typical deployment scenarios.
During the phase of registration and lookup, security considerations relevant to RELOAD apply. A CoAP node that advertises it's existence via this mechanism, is more likely to be attacked, compared to a node (especially a sleepy node) that does not advertise it's existence. Section 11 of [RFC7252] and Section 13 of [RFC6940] have more information on the kinds of attack and mitigation possible.
The caching mechanism specified in this draft is additional to the caching already done in CoAP. Access control is handled by the RELOAD overlay, where the peer storing the data is responsible of validating the signature on the data being stored.
This document introduces one additional data Kind-ID to the "RELOAD Data Kind-ID" Registry:
+-------------------+------------+----------+ | Kind | Kind-ID | RFC | +-------------------+------------+----------+ | CoAP-REGISTRATION | 105 | RFC-AAAA | +-------------------+------------+----------+
This Kind-ID was defined in Section 4.
This document introduces one additional data Kind-ID to the "RELOAD Data Kind-ID" Registry:
+--------------+------------+----------+ | Kind | Kind-ID | RFC | +--------------+------------+----------+ | CoAP-CACHING | 106 | RFC-AAAA | +--------------+------------+----------+
This Kind-ID was defined in Section 4.
IANA is asked to create a "CoAP Usage for RELOAD Access Control Policy" Registry. This registry is to be added to the existing RELOAD registry. Entries in this registry are strings denoting access control policies, as described in Section 8.1. New entries in this registry are to be registered per the Specification Required policy in [RFC5226]. The initial contents of this registry are:
+-----------------+----------+ | Access Policy | RFC | +-----------------+----------+ | URI-NODE-MATCH | RFC-AAAA | | URI-MATCH | RFC-AAAA | +-----------------+----------+
This access control policy was described in Section 9.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC3986] | Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005. |
| [RFC5280] | Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008. |
| [RFC6690] | Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012. |
| [RFC6940] | Jennings, C., Lowekamp, B., Rescorla, E., Baset, S. and H. Schulzrinne, "REsource LOcation And Discovery (RELOAD) Base Protocol", RFC 6940, DOI 10.17487/RFC6940, January 2014. |
| [RFC7252] | Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014. |
| [I-D.ietf-core-resource-directory] | Shelby, Z., Koster, M., Bormann, C. and P. Stok, "CoRE Resource Directory", Internet-Draft draft-ietf-core-resource-directory-04, July 2015. |
| [I-D.ietf-p2psip-concepts] | Bryan, D., Matthews, P., Shim, E., Willis, D. and S. Dawkins, "Concepts and Terminology for Peer to Peer SIP", Internet-Draft draft-ietf-p2psip-concepts-07, May 2015. |
| [RFC5226] | Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008. |
| [RFC7228] | Bormann, C., Ersue, M. and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014. |