Network Working Group A. Keränen Internet-Draft Ericsson Intended status: Standards Track C. Bormann Expires: 9 January 2022 Universität Bremen TZI 8 July 2021 SenML Data Value Content-Format Indication draft-ietf-core-senml-data-ct-04 Abstract The Sensor Measurement Lists (SenML) media type supports multiple types of values, from numbers to text strings and arbitrary binary data values. In order to simplify processing of the data values, this document proposes to specify a new SenML field for indicating the Content-Format of the data. 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 https://datatracker.ietf.org/drafts/current/. 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Keränen & Bormann Expires 9 January 2022 [Page 1] Internet-Draft SenML Data Content-Format Indication July 2021 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. SenML Content-Format ("ct") Field . . . . . . . . . . . . . . 4 4. SenML Base Content-Format ("bct") Field . . . . . . . . . . . 4 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. Security Considerations . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . 8 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction The Sensor Measurement Lists (SenML) media types [RFC8428] can be used to send various kinds of data. In the example given in Figure 1, a temperature value, an indication whether a lock is open, and a data value (with SenML field "vd") read from an NFC reader is sent in a single SenML pack. [ {"bn":"urn:dev:ow:10e2073a01080063:","n":"temp","u":"Cel","v":7.1}, {"n":"open","vb":false}, {"n":"nfc-reader","vd":"aGkgCg"} ] Figure 1: SenML pack with unidentified binary data The receiver is expected to know how to interpret the data in the "vd" field based on the context, e.g., name of the data source and out-of-band knowledge of the application. However, this context may not always be easily available to entities processing the SenML pack. To facilitate automatic interpretation it is useful to be able to indicate an Internet media type and content-coding right in the SenML Record. The CoAP Content-Format (Section 12.3 of [RFC7252]) provides just this information; enclosing a Content-Format number (in this case number 60 as defined for content-type application/cbor in [RFC8949]) in the Record is illustrated in Figure 2. All registered CoAP Content-Formats are listed in the Content-Formats subregistry of the CoRE Parameters registry [IANA.core-parameters]. {"n":"nfc-reader", "vd":"gmNmb28YKg", "ct":"60"} Figure 2: SenML Record with binary data identified as CBOR Keränen & Bormann Expires 9 January 2022 [Page 2] Internet-Draft SenML Data Content-Format Indication July 2021 In this example SenML Record, the data value contains a string "foo" and a number 42 encoded in a CBOR [RFC8949] array. Since the example above uses the JSON format of SenML, the data value containing the binary CBOR value is base64-encoded (Section 5 of [RFC4648]). The data value after base64 decoding is shown with CBOR diagnostic notation in Figure 3. 82 # array(2) 63 # text(3) 666F6F # "foo" 18 2A # unsigned(42) Figure 3: Example Data Value in CBOR diagnostic notation 2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Media Type: A registered label for representations (byte strings) prepared for interchange, identified by a Media-Type-Name [RFC1590], [RFC6838]. Media-Type-Name: A combination of a type-name and a subtype-name registered in [IANA.media-types] as per [RFC6838], conventionally identified by the two names separated by a slash. Content-Type: A Media-Type-Name, optionally associated with parameters (separated from the media type name and from each other by a semicolon). In HTTP and many other protocols, used in a "Content-Type" header field. Content-Coding: a registered name for an encoding transformation that has been or can be applied to a representation. Confusingly, in HTTP the Content-Coding is then given in a header field called "Content-Encoding"; we *never* use this term (except when we are in error). Content-Format: the combination of a Content-Type and a Content- Coding, identified by (1) a numeric identifier defined by the "CoAP Content-Formats" subregistry of [IANA.core-parameters] as per [RFC7252] or (2) a Content-Format-String. Content-Format-String: the string representation of the combination of a Content-Type and a Content-Coding. Keränen & Bormann Expires 9 January 2022 [Page 3] Internet-Draft SenML Data Content-Format Indication July 2021 Content-Format-Spec: the string representation of a Content-Format; either a Content-Format-String or the (decimal) string representation of a Content-Format number. Readers should also be familiar with the terms and concepts discussed in [RFC8428]. 3. SenML Content-Format ("ct") Field When a SenML Record contains a Data Value field ("vd"), the Record MAY also include a Content-Format indication field, using label "ct". The value of this field is a Content-Format-Spec, i.e., one of: * a CoAP Content-Format identifier in decimal form with no leading zeros (except for the value "0" itself). This value represents an unsigned integer in the range of 0-65535, similar to the CoRE Link Format [RFC6690] "ct" attribute). * or a Content-Format-String containing a Content-Type and optionally a Content-Coding (see below). The syntax of this field is formally defined in Section 6. The CoAP Content-Format number provides a simple and efficient way to indicate the type of the data. Since some Internet media types and their content coding and parameter alternatives do not have assigned CoAP Content-Format numbers, using Content-Type and Content-Coding is also allowed. Both methods use a string value in the "ct" field to keep its data type consistent across uses. When the "ct" field contains only digits, it is interpreted as a CoAP Content-Format identifier. To indicate that a Content-Coding is used with a Content-Type, the Content-Coding value (e.g., "deflate" [RFC7230]) is appended to the Content-Type value (media type and parameters, if any), separated by a "@" sign. For example: "text/plain; charset=utf-8@deflate". If no "@" sign is present outside the media type parameters, the Content- Coding is not specified and the "identity" Content-Coding is used -- no encoding transformation is employed. 4. SenML Base Content-Format ("bct") Field The Base Content-Format Field, label "bct", provides a default value for the Content-Format Field (label "ct") within its range. The range of the base field includes the Record containing it, up to (but not including) the next Record containing a "bct" field, if any, or up to the end of the pack otherwise. Resolution (Section 4.6 of [RFC8428]) of this base field is performed by adding its value with Keränen & Bormann Expires 9 January 2022 [Page 4] Internet-Draft SenML Data Content-Format Indication July 2021 the label "ct" to all Records in this range that carry a "vd" field but do not already contain a Content-Format ("ct") field. 5. Examples The following examples are valid values for the "ct" and "bct" fields (explanation/comments in parenthesis): * "60" (CoAP Content-Format for "application/cbor") * "0" (CoAP Content-Format for "text/plain" with parameter "charset=utf-8") * "application/json" (JSON Content-Type -- equivalent to "50" CoAP Content-Format identifier) * "application/json@deflate" (JSON Content-Type with "deflate" as Content-Coding -- equivalent to "11050" CoAP Content-Format identifier) * "text/csv" (Comma-Separated Values (CSV) [RFC4180] Content-Type) * "text/csv@gzip" (CSV with "gzip" as Content-Coding) 6. ABNF This specification provides a formal definition of the syntax of Content-Format-Spec strings using ABNF notation [RFC5234], which contains three new rules and a number of rules collected and adapted from various RFCs [RFC7231] [RFC6838] [RFC5234] [RFC8866]. ; New in this document Content-Format-Spec = Content-Format-Number / Content-Format-String Content-Format-Number = "0" / (POS-DIGIT *DIGIT) Content-Format-String = Content-Type ["@" Content-Coding] ; Cleaned up from RFC 7231: Content-Type = Media-Type-Name *( *SP ";" *SP parameter ) parameter = token "=" ( token / quoted-string ) token = 1*tchar tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA quoted-string = %x22 *qdtext %x22 Keränen & Bormann Expires 9 January 2022 [Page 5] Internet-Draft SenML Data Content-Format Indication July 2021 qdtext = SP / %x21 / %x23-5B / %x5D-7E ; Adapted from section 3.1.2.1 of RFC 7231 Content-Coding = token ; Adapted from various specs Media-Type-Name = type-name "/" subtype-name ; RFC 6838 type-name = restricted-name subtype-name = restricted-name restricted-name = restricted-name-first *126restricted-name-chars restricted-name-first = ALPHA / DIGIT restricted-name-chars = ALPHA / DIGIT / "!" / "#" / "$" / "&" / "-" / "^" / "_" restricted-name-chars =/ "." ; Characters before first dot always ; specify a facet name restricted-name-chars =/ "+" ; Characters after last plus always ; specify a structured syntax suffix ; Boilerplate from RFC 5234 and RFC 8866 DIGIT = %x30-39 ; 0 – 9 POS-DIGIT = %x31-39 ; 1 – 9 ALPHA = %x41-5A / %x61-7A ; A – Z / a – z SP = %x20 Figure 4: ABNF syntax of Content-Format-Spec 7. Security Considerations The indication of a media type in the data does not exempt a consuming application from properly checking its inputs. Also, the ability for an attacker to supply crafted SenML data that specify media types chosen by the attacker may expose vulnerabilities of handlers for these media types to the attacker. This includes "decompression bombs", compressed data that is crafted to decompress to extremely large data items. 8. IANA Considerations (Note to RFC Editor: Please replace all occurrences of "RFC-AAAA" with the RFC number of this specification and remove this note.) Keränen & Bormann Expires 9 January 2022 [Page 6] Internet-Draft SenML Data Content-Format Indication July 2021 IANA is requested to assign new labels in the "SenML Labels" subregistry of the SenML registry [IANA.senml] (as defined in [RFC8428]) for the Content-Format indication as per Table 1: +=====================+=======+===========+==========+===========+ | Name | Label | JSON Type | XML Type | Reference | +=====================+=======+===========+==========+===========+ | Base Content-Format | bct | String | string | RFC-AAAA | +---------------------+-------+-----------+----------+-----------+ | Content-Format | ct | String | string | RFC-AAAA | +---------------------+-------+-----------+----------+-----------+ Table 1: IANA Registration for new SenML Labels 9. References 9.1. Normative References [IANA.core-parameters] IANA, "Constrained RESTful Environments (CoRE) Parameters", . [IANA.media-types] IANA, "Media Types", . [IANA.senml] IANA, "Sensor Measurement Lists (SenML)", . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . Keränen & Bormann Expires 9 January 2022 [Page 7] Internet-Draft SenML Data Content-Format Indication July 2021 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C. Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, DOI 10.17487/RFC8428, August 2018, . 9.2. Informative References [RFC1590] Postel, J., "Media Type Registration Procedure", RFC 1590, DOI 10.17487/RFC1590, March 1994, . [RFC4180] Shafranovich, Y., "Common Format and MIME Type for Comma- Separated Values (CSV) Files", RFC 4180, DOI 10.17487/RFC4180, October 2005, . [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, . [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, . [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, . [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, . [RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP: Session Description Protocol", RFC 8866, DOI 10.17487/RFC8866, January 2021, . Keränen & Bormann Expires 9 January 2022 [Page 8] Internet-Draft SenML Data Content-Format Indication July 2021 [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, December 2020, . Acknowledgements The authors would like to thank Sérgio Abreu for the discussions leading to the design of this extension and Isaac Rivera for reviews and feedback. Klaus Hartke suggested not burdening this draft with a separate mandatory-to-implement version of the fields. Alexey Melnikov, Jim Schaad, and Thomas Fossati provided helpful comments at Working-Group last call. Marco Tiloca asked for clarifying and using the term Content-Format-Spec. Authors' Addresses Ari Keränen Ericsson FI-02420 Jorvas Finland Email: ari.keranen@ericsson.com Carsten Bormann Universität Bremen TZI Postfach 330440 D-28359 Bremen Germany Phone: +49-421-218-63921 Email: cabo@tzi.org Keränen & Bormann Expires 9 January 2022 [Page 9]