CoRE Working Group B. Silverajan Internet-Draft Tampere University of Technology Intended status: Informational T. Savolainen Expires: December 22, 2015 Nokia June 20, 2015 CoAP Communication with Alternative Transports draft-silverajan-core-coap-alternative-transports-08 Abstract CoAP has been standardised as an application level REST-based protocol. A single CoAP message is typically encapsulated and transmitted using UDP or DTLS as transports. These transports are optimal solutions for CoAP use in IP-based constrained environments and nodes. However compelling motivation exists for allowing CoAP to operate with other transports and protocols. Examples are M2M communication in cellular networks using SMS, more suitable transport protocols for firewall/NAT traversal, end-to-end reliability and security such as TCP and TLS, or employing proxying and tunneling gateway techniques such as the WebSocket protocol. This draft examines the requirements for conveying CoAP messages to end points over such alternative transports. It also provides a new URI format for representing CoAP resources over alternative transports. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on December 22, 2015. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. Silverajan & Savolainen Expires December 22, 2015 [Page 1] Internet-Draft CoAP Alternative Transports June 2015 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Usage Cases . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Use of SMS . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Use of WebSockets . . . . . . . . . . . . . . . . . . . . 4 2.3. Use of P2P Overlays . . . . . . . . . . . . . . . . . . . 4 2.4. Use of TCP and TLS . . . . . . . . . . . . . . . . . . . 5 3. Node Types based on Transport Availability . . . . . . . . . 5 4. CoAP Alternative Transport URI . . . . . . . . . . . . . . . 6 4.1. Design Considerations . . . . . . . . . . . . . . . . . . 7 4.2. URI format . . . . . . . . . . . . . . . . . . . . . . . 8 5. Alternative Transport Analysis and Properties . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . . 12 9.2. Informative References . . . . . . . . . . . . . . . . . 12 Appendix A. Expressing transport in the URI in other ways . . . 14 A.1. Transport information as part of the URI authority . . . 14 A.1.1. Usage of DNS records . . . . . . . . . . . . . . . . 15 A.2. Making CoAP Resources Available over Multiple Transports 15 A.3. Transport as part of a 'service:' URL scheme . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 1. Introduction The Constrained Application Protocol (CoAP) [RFC7252] has been standardised by the CoRE WG as a lightweight, HTTP-like protocol providing a request/response model that constrained nodes can use to communicate with other nodes, be those servers, proxies, gateways, less constrained nodes, or other constrained nodes. CoAP has been definied to utilise UDP and DTLS as transports. As the Internet evolves by integrating new kinds of networks, services and devices, the need for a consistent, lightweight method for resource representation, retrieval and manipulation becomes Silverajan & Savolainen Expires December 22, 2015 [Page 2] Internet-Draft CoAP Alternative Transports June 2015 evident. Owing to its simplicity and low overhead, CoAP is a highly suitable protocol for this purpose. However, communicating CoAP endpoints can reside in networks where end-to-end UDP-based communication can be challenging. These include networks separated by NATs and firewalls, cellular networks in which the Short Messaging Service (SMS) can be utilised as between nodes, or simply situations where an endpoint has no possibility to communicate over UDP. Consequently in addition to UDP and DTLS, alternative transport channels for conveying CoAP messages should be considered. Extending CoAP over alternative transports allows CoAP implementations to have a significantly larger relevance in constrained as well as non-constrained networked environments: it leads to better code optimisation in constrained nodes and broader implementation reuse across new transport channels. As opposed to implementing new resource retrieval mechanisms, an application in an end-node can continue relying on using CoAP's REST-based resource retrieval and manipulation for this purpose, while changes in end point identification and the transport protocol can be addressed by a transport-specific messaging sublayer. This simplifies development and memory requirements. Resource representations are also visible in an end-to-end manner for any CoAP client. In certain conditions, the processing and computational overhead for conveying CoAP Requests and Responses from one underlying transport to another, would be less than that of an application-level gateway performing protocol translation of individual messages between CoAP and another resource retrieval protocol such as HTTP. This document first provides scenarios where usage of CoAP over alternative transports is either currently underway, or may prove advantageous in the future. A simple transport type classification for CoAP-capable nodes is provided next. Then a new URI format is described through which a CoAP resource representation can be formulated that expresses transport identification in addition to endpoint information and resource paths. Following that, a discussion of the various transport properties which influence how CoAP Request and Response messages are mapped to transport level payloads, is presented. This document however, does not touch on application QoS requirements, user policies or network adaptation, nor does it advocate replacing the current practice of UDP-based CoAP communication. Silverajan & Savolainen Expires December 22, 2015 [Page 3] Internet-Draft CoAP Alternative Transports June 2015 2. Usage Cases Apart from UDP and DTLS, CoAP usage is being specified for the following environments as of this writing: 2.1. Use of SMS CoAP messages can be sent via SMS between CoAP end-points in a cellular network [I-D.becker-core-coap-sms-gprs]. A CoAP Request message can also be sent via SMS from a CoAP client to a sleeping CoAP Server as a wake-up mechanism and trigger communication via IP. For this reason, the Open Mobile Alliance (OMA) specifies both UDP and SMS as transports for M2M communication in cellular networks. The OMA Lightweight M2M (LWM2M) protocol being drafted uses CoAP, and as transports, specifies both UDP as well as Short Message Service (SMS) bindings [OMALWM2M]. DTLS is being proposed for securing CoAP messages over SMS between Mobile Stations [I-D.fossati-dtls-over-gsm-sms]. 2.2. Use of WebSockets The WebSocket protocol has been proposed as a transport channel between WebSocket enabled CoAP end-points on the Internet [I-D.savolainen-core-coap-websockets]. This is particularly useful to enable CoAP communication within HTML5 apps and web browsers, especially in smart devices, that do not have any means to use low- level socket interfaces. Embedded client side scripts create new WebSocket connections to various WebSocket-enabled servers, through which CoAP messages can be exchanged. This also allows a browser containing an embedded CoAP server to open a connection to a WebSocket enabled CoAP Mirror Server [I-D.vial-core-mirror-server] to register and update its resources. 2.3. Use of P2P Overlays [I-D.jimenez-p2psip-coap-reload] specifices how CoAP nodes can use a peer-to-peer overlay network called RELOAD, as a resource caching facility for storing wireless sensor data. When a CoAP node registers its resources with a RELOAD Proxy Node (PN), the node computes a hash value from the CoAP URI and stores it as a structure together with the PN's Node ID as well as the resources. Resource retrieval by CoAP nodes is accomplished by computing the hash key over the Request URI, opening a connection to the overlay and using its message routing system to contact the CoAP server via its PN. Silverajan & Savolainen Expires December 22, 2015 [Page 4] Internet-Draft CoAP Alternative Transports June 2015 2.4. Use of TCP and TLS Using TCP [I-D.tschofenig-core-coap-tcp-tls], allows easier communication between CoAP clients and servers separated by firewalls and NATs. This also allows CoAP messages to be transported over push notification services from a notification server to a client app on a smartphone, that may previously have subscribed to receive change notifications of CoAP resource representations, possibly by using CoAP Observe [I-D.ietf-core-observe]. [I-D.tschofenig-core-coap-tcp-tls] also discusses using TLS as a transport to securely convey CoAP messages over TCP. 3. Node Types based on Transport Availability The term "alternative transport" in this document thus far has been used to refer to any non-UDP and non-DTLS transport that can convey CoAP messages in its payload. A node however, may in fact possess the capability to utilise CoAP over multiple transport channels at its disposal, simultaneously or otherwise, at any point in time to communicate with a CoAP end-point. Such communication can obviously take place over UDP and DTLS as well. Inevitably, if two CoAP endpoints reside in distinctly separate networks with orthogonal transports, a CoAP proxy node is needed between the two networks so that CoAP Requests and Responses can be exchanged properly. In [RFC7228], Tables 1, 3 and 4 introduced classification schemes for devices, in terms of their resource constraints, energy limitations and communication power. For this document, in addition to these capabilities, it seems useful to additionally identify devices based on their transport capabilities. +-------+----------------------------+ | Name | Transport Availability | +-------+----------------------------+ | T0 | Single transport | | | | | T1 | Multiple transports, with | | | one or more active at any | | | point in time | | | | | T2 | Multiple active transports| | | at all times | +-------+----------------------------+ Table 1: Classes of Available Transports Silverajan & Savolainen Expires December 22, 2015 [Page 5] Internet-Draft CoAP Alternative Transports June 2015 Type T0 nodespossess the capability of exactly 1 type of transport channel for CoAP, at all times. These include both active and sleepy nodes, which may choose to perform duty cycling for power saving. Type T1 nodes possess multiple different transports, and can retrieve or expose CoAP resources over any or all of these transports. However, not all transports are constantly active and certain transport channels and interfaces could be kept in a mostly-off state for energy-efficiency, such as when using CoAP over SMS (refer to section 2.1) Type T2 nodes possess more than 1 transport, and multiple transports are simultaneously active at all times. CoAP proxy nodes which allow CoAP endpoints from disparate transports to communicate with each other, are a good example of this. 4. CoAP Alternative Transport URI Based on the usage scenarios as well as the transport classes presented in the preceding sections, this section discusses the formulation of a new URI format for representing CoAP resources over alternative transports. CoAP is logically divided into 2 sublayers, whereby the upper layer is responsible for the protocol functionality of exchanging request and response messages, while the messaging layer is bound to UDP. These 2 sublayers are tightly coupled, both being responsible for properly encoding the header and body of the CoAP message. The CoAP URI is used by both logical sublayers. For a URI that is expressed generically as URI = scheme ":" "//" authority path-abempty ["?" query ] a simple example CoAP URI, "coap://server.example.com/sensors/ temperature" is interpreted as follows: coap :// server.example.com /sensors/temperature \___/ \______ ________/ \______ _________/ | \/ \/ protocol endpoint parameterised identifier identifier resource identifier Figure 1: The CoAP URI format Silverajan & Savolainen Expires December 22, 2015 [Page 6] Internet-Draft CoAP Alternative Transports June 2015 The resource path is explicitly expressed, and the endpoint identifier, which contains the host address at the network-level is also directly bound to the scheme name containing the application- level protocol identifier. The choice of a specific transport for a scheme, however, cannot be embedded with a URI, but is defined by convention or standardisation of the protocol using the scheme. As examples, [RFC5092] defines the 'imap' scheme for the IMAP protocol over TCP, while [RFC2818] requires that the 'https' protocol identifier be used to differentiate using HTTP over TLS instead of TCP. 4.1. Design Considerations Several ways of formulating a URI which express an alternative transport binding to CoAP, can be envisioned. When such a URI is provided from an application to its CoAP implementation, the URI component containing transport-specific information can be checked to allow CoAP to use the appropriate transport for a target endpoint identifier. The following design considerations influence the formulation of a new URI expressing CoAP resources over alternative transports: 1. The CoAP Transport URI must conform to the generic syntax for a URI described in [RFC3986]. By ensuring conformance to RFC3986, the need for custom URI parsers as well as resolution algorithms can be obviated. In particular, a URI format needs to be described in which each URI component clearly meets the syntax and percent-encoding rules described. 2. A CoAP Transport URI can be supplied as a Proxy-Uri option by a CoAP end-point to a CoAP forward proxy. This allows communication with a CoAP end-point residing in a network using a different transport. Section 6.4 of [RFC7252] provides an algorithm for parsing a received URI to obtain the request's options. Conformance to [RFC3986] is also necessary in order for the parsing algorithm to be successful. 3. Request messages sent to a CoAP endpoint using a CoAP Transport URI may be responded to with a relative URI reference, for example, of the form "../../path/to/resource". In such cases, the requesting endpoint needs to resolve the relative reference against the original CoAP Transport URI to then obtain a new target URI to which a request can be sent to, to obtain a resource representation. [RFC3986] provides an algorithm to establish how relative references can be resolved against a base URI to obtain a target URI. Given this algorithm, a URI format needs to be described in which relative reference resolution does Silverajan & Savolainen Expires December 22, 2015 [Page 7] Internet-Draft CoAP Alternative Transports June 2015 not result in a target URI that loses its transport-specific information 4. The host component of current CoAP URIs can either be an IPv4 address, an IPv6 address or a resolvable hostname. While the usage of DNS can sometimes be useful for distinguishing transport information (see section 4.3.1), accessing DNS over some alternative transport environments may be challenging. Therefore, a URI format needs to be described which is able to represent a resource without heavy reliance on a naming infrastructure, such as DNS. 4.2. URI format To meet the design considerations previously discussed, the transport information is expressed as part of the URI scheme component. This is performed by minting new schemes for alternative transports using the form "coap+" and/or "coaps+", where the name of the transport is clearly and unambiguously described. Each scheme name formed in this manner is used to differentiate the use of CoAP, or CoAP using DTLS, over an alternative transport respectively. The endpoint identifier, path and query components together with each scheme name would be used to uniquely identify each resource. Examples of such URIs are: o coap+tcp://[2001:db8::1]:5683/sensors/temperature for using CoAP over TCP o coap+tls://[2001:db8::1]:5683/sensors/temperature for using CoAP over TLS o coaps+sctp://[2001:db8::1]:5683/sensors/temperature for using CoAP over DTLS over SCTP o coap+sms://0015105550101/sensors/temperature for using CoAP over SMS with the endpoint identifier being a telephone subscriber number o coaps+sms://0015105550101/sensors/temperature for using CoAP over DTLS over SMS with the endpoint identifier being a telephone subscriber number o coap+ws://www.example.com/sensors/temperature for using CoAP over WebSockets Silverajan & Savolainen Expires December 22, 2015 [Page 8] Internet-Draft CoAP Alternative Transports June 2015 o coap+wss://www.example.com/sensors/temperature for using CoAP over secure WebSockets (WebSockets using TLS) A URI of this format to distinguish transport types is simple to understand and not dissimilar to the CoAP URI format. As the usage of each alternative transport results in an entirely new scheme, IANA intervention is required for the registration of each scheme name. The registration process follows the guidelines stipulated in [I-D.ietf-appsawg-uri-scheme-reg], particularly where permanent URI scheme registration is concerned. CoAP resources transported over UDP or DTLS must conform to Section 6 of [RFC7252] and utilise "coap" or "coaps" for the URI scheme, instead of "coap+udp" or "coap+dtls". It is also entirely possible for each new scheme to specify its own rules for how resource and transport endpoint information can be presented. However, the URIs and resource representations arising from their usage should meet the URI design considerations and guidelines mentioned in Section 4.1. In addition, each new transport being defined should take into consideration the various transport- level properties that can have an impact on how CoAP messages are conveyed as payload. This is elaborated on in the next section. 5. Alternative Transport Analysis and Properties In this section the various characteristics of alternative transports for successfully supporting various kinds of functionality for CoAP are considered. CoAP factors lossiness, unreliability, small packet sizes and connection statelessness into its protocol logic. General transport differences and their impact on carrying CoAP messages here are discussed. Property 1: 1:N communication support. This refers to the ability of the transport protocol to support broadcast and multicast communication. For example, group communication for CoAP is based on multicasting Request messages and receiving Response messages via unicast [RFC7390]. A protocol such as TCP would be ill-suited for group communications using multicast. Anycast support, where a message is sent to a well defined destination address to which several nodes belong, on the other hand, is supported by TCP. Property 2: Transport-level reliability. This refers to the ability of the transport protocol to support properties such as guaranteeing reliability against packet loss, ensuring ordered packet delivery and having error control. When CoAP Request and Response messages are delivered over such transports, the Silverajan & Savolainen Expires December 22, 2015 [Page 9] Internet-Draft CoAP Alternative Transports June 2015 CoAP implementations elide certain fields in the packet header. As an example, if the usage of a connection-oriented transport renders it unnecessary to specify the various CoAP message types, the Type field can be elided. For some connection-oriented transports, such as WebSockets, the version of CoAP being used can be negotiated during the opening transfer. Consequently, the Version field in CoAP packets can also be elided. Property 3: Message encoding. While parts of the CoAP payload are human readable or are transmitted in XML, JSON or SenML format, CoAP is essentially a low overhead binary protocol. Efficient transmission of such packets would therefore be met with a transport offering binary encoding support. Techniques exist in allowing binary payloads to be transferred over text-based transport protocols such as base-64 encoding. When using SMS as a transport, for example, although binary encoding is supported, Appendix A.5 of [I-D.bormann-coap-misc] indicates binary encoding for SMS may not always be viable. A fuller discussion about performing CoAP message encoding for SMS can be found in Appendix A.5 of [I-D.bormann-coap-misc] Property 4: Network byte order. CoAP, as well as transports based on the IP stack use a Big Endian byte order for transmitting packets over the air or wire, while transports based on Bluetooth and Zigbee prefer Little Endian byte ordering for packet fields and transmission. Any CoAP implementation that potentially uses multiple transports has to ensure correct byte ordering for the transport used. Property 5: MTU correlation with CoAP PDU size. Section 4.6 of [RFC7252] discusses the avoidance of IP fragmentation by ensuring CoAP message fit into a single UDP datagram. End-points on constrained networks using 6LoWPAN may use blockwise transfers to accommodate even smaller packet sizes to avoid fragmentation. The MTU sizes for Bluetooth Low Energy as well as Classic Bluetooth are provided in Section 2.4 of [I-D.ietf-6lo-btle]. Transport MTU correlation with CoAP messages helps ensure minimal to no fragmentation at the transport layer. On the other hand, allowing a CoAP message to be delivered using a delay-tolerant transport service such as the Bundle Protocol [RFC5050] would imply that the CoAP message may be fragmented (or reconstituted) along various nodes in the DTN as various sized bundles and bundle fragments. Property 6: Framing Silverajan & Savolainen Expires December 22, 2015 [Page 10] Internet-Draft CoAP Alternative Transports June 2015 When using CoAP over a streaming transport protocol such as TCP, as opposed to datagram based protocols, care must be observed in preserving message boundaries. Commonly applied techniques at the transport level include the use of delimiting characters for this purpose as well as message framing and length prefixing. Property 7: Transport latency. A confirmable CoAP request would be retransmitted by a CoAP end-point if a response is not obtained within a certain time. A CoAP end- point registering to a Resource Directory uses a POST message that could include a lifetime value. A sleepy end-point similarly uses a lifetime value to indicate the freshness of the data to a CoAP Mirror Server. Care needs to be exercised to ensure the latency of the transport being used to carry CoAP messages is small enough not to interfere with these values for the proper operation of these functionalities. Property 8: Connection Management. A CoAP endpoint using a connection-oriented transport should be responsible for proper connection establishment prior to sending a CoAP Request message. Both communicating endpoints may monitor the connection health during the Data Transfer phase. Finally, once data transfer is complete, at least one end point should perform connection teardown gracefully. 6. IANA Considerations This memo includes no request to IANA. 7. Security Considerations New security risks are not envisaged to arise from the guidelines given in this document, for describing a new URI format containing transport identification within the URI scheme component. However, when specific alternative transports are selected for implementing support for carrying CoAP messages, risk factors or vulnerabilities can be present. Examples include privacy trade-offs when MAC addresses or phone numbers are supplied as URI authority components, or if specific URI path components employed for security-specific interpretations are accidentally encountered as false positives. While this document does not make it mandatory to introduce a security mode with each transport, it recommends ascribing meaning to the use of "coap+" and "coaps+" prefixes in the scheme component, with the "coaps+" prefix used for DTLS-based CoAP messages over the alternative transport. Silverajan & Savolainen Expires December 22, 2015 [Page 11] Internet-Draft CoAP Alternative Transports June 2015 8. Acknowledgements The draft has benefited greatly from reviews, comments and ideas from Thomas Fossati, Akbar Rahman, Klaus Hartke, Martin Thomson, Mark Nottingham, Dave Thaler, Graham Klyne, Carsten Bormann and Markus Becker. 9. References 9.1. Normative References [I-D.ietf-appsawg-uri-scheme-reg] Thaler, D., Hansen, T., Hardie, T., and L. Masinter, "Guidelines and Registration Procedures for URI Schemes", draft-ietf-appsawg-uri-scheme-reg-06 (work in progress), April 2015. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, May 2014. [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, June 2014. 9.2. Informative References [BTCorev4.1] BLUETOOTH Special Interest Group, "BLUETOOTH Specification Version 4.1", December 2013. [I-D.becker-core-coap-sms-gprs] Becker, M., Li, K., Kuladinithi, K., and T. Poetsch, "Transport of CoAP over SMS", draft-becker-core-coap-sms- gprs-05 (work in progress), August 2014. [I-D.bormann-coap-misc] Bormann, C. and K. Hartke, "Miscellaneous additions to CoAP", draft-bormann-coap-misc-27 (work in progress), November 2014. Silverajan & Savolainen Expires December 22, 2015 [Page 12] Internet-Draft CoAP Alternative Transports June 2015 [I-D.fossati-dtls-over-gsm-sms] Fossati, T. and H. Tschofenig, "Datagram Transport Layer Security (DTLS) over Global System for Mobile Communications (GSM) Short Message Service (SMS)", draft- fossati-dtls-over-gsm-sms-01 (work in progress), October 2014. [I-D.ietf-6lo-btle] Nieminen, J., Savolainen, T., Isomaki, M., Patil, B., Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low Energy", draft-ietf-6lo-btle-13 (work in progress), May 2015. [I-D.ietf-core-observe] Hartke, K., "Observing Resources in CoAP", draft-ietf- core-observe-16 (work in progress), December 2014. [I-D.jimenez-p2psip-coap-reload] Jimenez, J., Lopez-Vega, J., Maenpaa, J., and G. Camarillo, "A Constrained Application Protocol (CoAP) Usage for REsource LOcation And Discovery (RELOAD)", draft-jimenez-p2psip-coap-reload-09 (work in progress), June 2015. [I-D.savolainen-core-coap-websockets] Savolainen, T., Hartke, K., and B. Silverajan, "CoAP over WebSockets", draft-savolainen-core-coap-websockets-04 (work in progress), March 2015. [I-D.tschofenig-core-coap-tcp-tls] Bormann, C., Lemay, S., Technologies, Z., and H. Tschofenig, "A TCP and TLS Transport for the Constrained Application Protocol (CoAP)", draft-tschofenig-core-coap- tcp-tls-04 (work in progress), June 2015. [I-D.vial-core-mirror-server] Vial, M., "CoRE Mirror Server", draft-vial-core-mirror- server-01 (work in progress), April 2013. [OMALWM2M] Open Mobile Alliance (OMA), "Lightweight Machine to Machine Technical Specification Version 1.0", 2015. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2609] Guttman, E., Perkins, C., and J. Kempf, "Service Templates and Service: Schemes", RFC 2609, June 1999. Silverajan & Savolainen Expires December 22, 2015 [Page 13] Internet-Draft CoAP Alternative Transports June 2015 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant Networking Architecture", RFC 4838, April 2007. [RFC5050] Scott, K. and S. Burleigh, "Bundle Protocol Specification", RFC 5050, November 2007. [RFC5092] Melnikov, A. and C. Newman, "IMAP URL Scheme", RFC 5092, November 2007. [RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, December 2011. [RFC6568] Kim, E., Kaspar, D., and JP. Vasseur, "Design and Application Spaces for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 6568, April 2012. [RFC6733] Fajardo, V., Arkko, J., Loughney, J., and G. Zorn, "Diameter Base Protocol", RFC 6733, October 2012. [RFC7390] Rahman, A. and E. Dijk, "Group Communication for the Constrained Application Protocol (CoAP)", RFC 7390, October 2014. [WWWArchv1] http://www.w3.org/TR/webarch/#uri-aliases, "Architecture of the World Wide Web, Volume One", December 2004. Appendix A. Expressing transport in the URI in other ways Other means of indicating the transport as a distinguishable component within the CoAP URI are possible, but have been deemed unsuitable by not meeting the design considerations listed, or are incompatible with existing practices outlined in [RFC7252]. They are however, retained in this section for historical documentation and completeness. A.1. Transport information as part of the URI authority A single URI scheme, "coap-at" can be introduced, as part of an absolute URI which expresses the transport information within the authority component. One approach is to structure the component with a transport prefix to the endpoint identifier and a delimiter, such as "-endpoint_identifier". Examples of resulting URIs are: Silverajan & Savolainen Expires December 22, 2015 [Page 14] Internet-Draft CoAP Alternative Transports June 2015 o coap-at://tcp-server.example.com/sensors/temperature o coap-at://sms-0015105550101/sensors/temperature An implementation note here is that some generic URI parsers will fail when encountering a URI such as "coap-at://tcp- [2001:db8::1]/sensors/temperature". Consequently, an equivalent, but parseable URI from the ip6.arpa domain needs to be formulated instead. For [2001:db8::1] using TCP, this would result in the following URL: coap-at://tcp-1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0 .1.0.0.2.ip6.arpa:5683/sensors/temperature Usage of an IPv4-mapped IPv6 address such as [::ffff.192.100.0.1] can similarly be expressed with a URI from the ip6.arpa domain. This URI format allows the usage of a single scheme to represent multiple types of transport end-points. Consequently, it requires consistency in ensuring how various transport-specific endpoints are identified, as a single URI format is used. Attention must be paid towards the syntax rules and encoding for the URI host component. Additionally, against a base URI of the form "coap-at://tcp- server.example.com/sensors/temperature", resolving a relative reference, such as "//example.net/sensors/temperature" would result in the target URI "coap-at://example.net/sensors/temperature", in which transport information is lost. A.1.1. Usage of DNS records DNS names can be used instead of IPv6 address literals to mitigate lengthy URLs referring to the ip6.arpa domain, if usage of DNS is possible. DNS SRV records can also be employed to formulate a URL such as: coap-at://srv-_coap._tcp.example.com/sensors/temperature in which the "srv" prefix is used to indicate that a DNS SRV lookup should be used for _coap._tcp.example.com, where usage of CoAP over TCP is specified for example.com, and is eventually resolved to a numerical IPv4 or IPv6 address. A.2. Making CoAP Resources Available over Multiple Transports The CoAP URI used thus far is as follows: Silverajan & Savolainen Expires December 22, 2015 [Page 15] Internet-Draft CoAP Alternative Transports June 2015 URI = scheme ":" hier-part [ "?" query ] hier-part = "//" authority path-abempty A new URI format could be introduced, that does not possess an "authority" component, and instead defining "hier-part" to instead use another component, "path-rootless", as specified by RFC3986 [RFC3986]. The partial ABNF format of this URI would then be: URI = scheme ":" hier-part [ "?" query ] hier-part = path-rootless path-rootless = segment-nz *( "/" segment ) The full syntax of "path-rootless" is described in [RFC3986]. A generic URI defined this way would conform to the syntax of [RFC3986], while the path component can be treated as an opaque string to indicate transport types, endpoints as well as paths to CoAP resources. A single scheme can similarly be used. A constrained node that is capable of communicating over several types of transports (such as UDP, TCP and SMS) would be able to convey a single CoAP resource over multiple transports. This is also beneficial for nodes performing caching and proxying from one type of transport to another. Requesting and retrieving the same CoAP resource representation over multiple transports could be rendered possible by prefixing the transport type and endpoint identifier information to the CoAP URI. This would result in the following example representation: coap-at:tcp://example.com?coap://example.com/sensors/temperature \_______ ______/ \________________ __________________/ \/ \/ Transport-specific CoAP Resource Prefix Figure 2: Prefixing a CoAP URI with TCP transport Such a representation would result in the URI being decomposed into its constituent components, with the CoAP resource residing within the query component as follows: Silverajan & Savolainen Expires December 22, 2015 [Page 16] Internet-Draft CoAP Alternative Transports June 2015 Scheme: coap-at Path: tcp://example.com Query: coap://example.com/sensors/temperature The same CoAP resource, if requested over a WebSocket transport, would result the following URI: coap-at:ws://example.com/endpoint?coap://example.com/sensors/temperature \___________ __________/ \_______________ ___________________/ \/ \/ Transport-specific CoAP Resource Prefix Figure 3: Prefixing a CoAP URI with WebSocket transport While the transport prefix changes, the CoAP resource representation remains the same in the query component: Scheme: coap-at Path: ws://example.com/endpoint Query: coap://example.com/sensors/temperature The URI format described here overcomes URI aliasing [WWWArchv1] when multiple transports are used, by ensuring each CoAP resource representation remains the same, but is prefixed with different transports. However, against a base URI of this format, resolving relative references of the form "//example.net/sensors/temperature" and "/sensor2/temperature" would again result in target URIs which lose transport-specific information. Implementation note: While square brackets are disallowed within the path component, the '[' and ']' characters needed to enclose a literal IPv6 address can be percent-encoded into their respective equivalents. The ':' character does not need to be percent-encoded. This results in a significantly simpler URI string compared to section 2.2, particularly for compressed IPv6 addresses. Additionally, the URI format can be used to specify other similar address families and formats, such as Bluetooth addresses [BTCorev4.1]. Silverajan & Savolainen Expires December 22, 2015 [Page 17] Internet-Draft CoAP Alternative Transports June 2015 A.3. Transport as part of a 'service:' URL scheme The "service:" URL scheme name was introduced in [RFC2609] and forms the basis of service description used primarily by the Service Location Protocol. An abstract service type URI would have the form "service::" where refers to a service type name that can be associated with a variety of protocols, while the then providing the specific details of the protocol used, authority and other URI components. Adopting the "service:" URL scheme to describe CoAP usage over alternative transports would be rather trivial. To use a previous example, a CoAP service to discover a Resource Directory and its base RD resource using TCP would take the form service:coap:tcp://host.example.com/.well-known/core?rt=core-rd The syntax of the "service:" URL scheme differs from the generic URI syntax and therefore such a representation should be treated as an opaque URI as Section 2.1 of [RFC2609] recommends. Authors' Addresses Bilhanan Silverajan Tampere University of Technology Korkeakoulunkatu 10 FI-33720 Tampere Finland Email: bilhanan.silverajan@tut.fi Teemu Savolainen Nokia Hermiankatu 12 D FI-33720 Tampere Finland Email: teemu.savolainen@nokia.com Silverajan & Savolainen Expires December 22, 2015 [Page 18]