CoRE Working Group K. Hartke Internet-Draft Ericsson Updates: 7252, 8323 (if approved) September 10, 2018 Intended status: Standards Track Expires: March 14, 2019 Extended Tokens and Stateless Clients in the Constrained Application Protocol (CoAP) draft-hartke-core-stateless-00 Abstract This document provides considerations for alleviating CoAP clients and intermediaries of maintaining per-request state. Additionally, it introduces a new, optional CoAP protocol extension for extended tokens. [[The present draft presents two alternative variants for extending CoAP with extended tokens. Variant A makes use of the unused values in the TKL field of the CoAP header. Variant B duplicates the functionality of the token field of the CoAP header in a new CoAP option, "Extended-Token". The considerations for stateless clients making use of either variant are the same.]] This document updates RFCs 7252 and 8323 (if Variant A). 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/. 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 March 14, 2019. Hartke Expires March 14, 2019 [Page 1] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://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 . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4 2. (Variant A:) Extended Tokens . . . . . . . . . . . . . . . . 4 2.1. Extended Token Length (TKL) Field . . . . . . . . . . . . 4 2.2. Discovering Support . . . . . . . . . . . . . . . . . . . 5 2.2.1. Extended-Token-Lengths Capability Option . . . . . . 5 2.2.2. Trial and Error . . . . . . . . . . . . . . . . . . . 6 3. (Variant B:) Extended Tokens . . . . . . . . . . . . . . . . 6 3.1. Extended-Token Option . . . . . . . . . . . . . . . . . . 6 3.2. Discovering Support . . . . . . . . . . . . . . . . . . . 7 3.2.1. Extended-Token-Option Capability Option . . . . . . . 7 3.2.2. Trial and Error . . . . . . . . . . . . . . . . . . . 8 4. Stateless Clients . . . . . . . . . . . . . . . . . . . . . . 9 4.1. Intermediaries . . . . . . . . . . . . . . . . . . . . . 9 4.2. Extended Tokens . . . . . . . . . . . . . . . . . . . . . 9 4.3. Message Transmission . . . . . . . . . . . . . . . . . . 10 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 6. (Variant A:) IANA Considerations . . . . . . . . . . . . . . 11 6.1. CoAP Signaling Option Number . . . . . . . . . . . . . . 11 7. (Variant B:) IANA Considerations . . . . . . . . . . . . . . 11 7.1. CoAP Option Number . . . . . . . . . . . . . . . . . . . 11 7.2. CoAP Signaling Option Number . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8.1. Normative References . . . . . . . . . . . . . . . . . . 12 8.2. Informative References . . . . . . . . . . . . . . . . . 13 Appendix A. (Variant A:) Updated Message Formats . . . . . . . . 13 A.1. CoAP over UDP . . . . . . . . . . . . . . . . . . . . . . 14 A.2. CoAP over TCP . . . . . . . . . . . . . . . . . . . . . . 15 A.3. CoAP over WebSockets . . . . . . . . . . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17 Hartke Expires March 14, 2019 [Page 2] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 1. Introduction The Constrained Application Protocol (CoAP) [RFC7252] is a RESTful application-layer protocol for constrained environments [RFC7228]. In CoAP, clients (or proxies in the client role) make requests to servers (or proxies in the server role), which are serving these requests by returning responses. While a request is outstanding, a client typically maintains some state that it requires for processing the response when it arrives. Identification of this state is done by means of a _token_ in CoAP, an opaque sequence of bytes chosen by the client and in the CoAP request. The server returns it verbatim in any resulting CoAP response (Figure 1). +-----------------+ request with +------------+ | | | state identifier | | | | | as token | | | .-<-+->------|--------------------->|------. | | _|_ | | | | | / \ stored | | | | | \___/ state | | | | | | | | | | | '->-+-<------|<---------------------|------' | | | | response with | | | v | token echoed back | | +-----------------+ +------------+ Client Server Figure 1: Token as an Identifier for Request State In some scenarios, it can be beneficial to reduce the amount of state stored at the client at the cost of increased message sizes. Client implementations can accomplish this by serializing their state into the token and recovering the state from the token in the response when it comes back (Figure 2). Hartke Expires March 14, 2019 [Page 3] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 +-----------------+ request with +------------+ | | | serialized state | | | | | as token | | | +--------|=====================>|------. | | | | | | | look, no | | | | | state, ma! | | | | | | | | | | +--------|<=====================|------' | | | | response with | | | v | token echoed back | | +-----------------+ +------------+ Client Server Figure 2: Token as Serialization of Request State Section 4 of this document provides considerations for making clients "stateless" in this way, i.e., avoiding per-request state. (They'll still need to maintain per-server state and other kinds of state, though, so they're not entirely stateless.) Serializing state into tokens is complicated by the fact that both CoAP over UDP [RFC7252] and CoAP over reliable transports [RFC8323] limit the maximum token length to 8 bytes. To overcome this limitation, Section 2 (Variant A) / Section 3 (Variant B) of this document first introduces a CoAP protocol extension for extended tokens. 1.1. Requirements Notation 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. 2. (Variant A:) Extended Tokens 2.1. Extended Token Length (TKL) Field The message formats defined for CoAP over UDP [RFC7252] and CoAP over TCP, TLS, and WebSockets [RFC8323] are updated with the following new definition of the TKL field: Token Length (TKL): 4-bit unsigned integer. A value between 0 and 12 inclusive indicates the length of the variable-length Token field in bytes. Three values are reserved for special constructs: Hartke Expires March 14, 2019 [Page 4] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 13: An 8-bit unsigned integer precedes the Token field and indicates the length of the Token field minus 13. 14: A 16-bit unsigned integer in network byte order precedes the Token field and indicates the length of the Token field minus 269. 15: Reserved. This value MUST NOT be sent and MUST be processed as a message format error. All other fields retain their definitions. The updated message formats are illustrated in Appendix A. 2.2. Discovering Support Extended token lengths require support from the server or, if there are intermediaries between the client and the origin server, the intermediary in the server role that the client is interacting with. Support can be discovered by a client (or intermediary in the client role) in one of two ways: If Capabilities and Settings Messages (CSMs) are available, such as in CoAP over TCP, then support can be discovered using the new Extended-Token-Lengths Capability Option defined in Section 2.2.1. Otherwise, such as in CoAP over UDP, support can only be discovered by trial and error, which is described in Section 2.2.2. 2.2.1. Extended-Token-Lengths Capability Option A sender can use the elective Extended-Token-Lengths Capability Option to indicate its support for the updated definition of the TKL field as specified above. +----+---+---+-------+--------------------+-------+--------+--------+ | # | C | R | Appli | Name | Forma | Length | Base | | | | | es to | | t | | Value | +----+---+---+-------+--------------------+-------+--------+--------+ | TB | | | CSM | Extended-Token- | empty | 0 | (none) | | D | | | | Lengths | | | | +----+---+---+-------+--------------------+-------+--------+--------+ C=Critical, R=Repeatable Table 1: The Extended-Token-Lengths Capability Option Hartke Expires March 14, 2019 [Page 5] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 2.2.2. Trial and Error A request with a TKL field value outside the range from 0 to 8 will be considered a message format error (Section 3 of RFC 7252) and be rejected by the next hop if that does not support extended token lengths. A client can therefore determine support by sending a request with an extended token length and checking if the next hop rejects the message. For CoAP over UDP, the recipient rejects a malformed confirmable message by sending an Reset message and otherwise ignoring the message (Section 4.2 of RFC 7252). In case of a non-confirmable message, the Reset message is optional (Section 4.3 of RFC 7252), so it is RECOMMENDED that the client uses a confirmable message. As per RFC 7252, a Reset message does not contain a token and only echoes back the Message ID of the confirmable or non-confirmable message (Figure 3). +-----------------+ request message +------------+ | | | with extended | | | | | token length | | | .-<-+->------|--------------------->|------. | | _|_ | | | | | / \ stored | | | | | \___/ state | | | | | | | | | | | '->-+-<------|<---------------------|------' | | | | reset message | | | v | with only message | | +-----------------+ ID echoed back +------------+ Client Server Figure 3: A Confirmable Request With an Extended Token is Rejected With a Reset Message if the Next Hop Does Not Support It For CoAP over TCP, TLS, and WebSockets, the recipient rejects a malformed message by sending an Abort message and shutting down the connection (Section 5.6 of RFC 8323). There is no specific Abort message option indicating the reason defined for this case. 3. (Variant B:) Extended Tokens 3.1. Extended-Token Option The Extended-Token Option contains an extension of the token that can additionally be used for matching a response with a request. Hartke Expires March 14, 2019 [Page 6] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 +-----+---+---+---+---+----------------+--------+---------+---------+ | # | C | U | N | R | Name | Format | Length | Default | +-----+---+---+---+---+----------------+--------+---------+---------+ | TBD | x | x | | | Extended-Token | opaque | 0-65535 | (none) | +-----+---+---+---+---+----------------+--------+---------+---------+ C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable Table 2: The Extended-Token Option A client (or intermediary in the role of a client) can include the Extended-Token Option in a request with any client-generated value. A server (or intermediary in the role of a server) MUST echo the value without modification in an Extended-Token Option in any resulting response. It is entirely up to the client how it splits the token between the Token field in the message header and the Extended-Token Option. 3.2. Discovering Support The Extended-Token Option requires support from the server or, if there are intermediaries between the client and the origin server, the intermediary in the server role that the client is interacting with. Support can be discovered by a client (or intermediary in the client role) in one of two ways: If Capabilities and Settings Messages (CSMs) are available, such as in CoAP over TCP, then support can be discovered using the new Extended-Token-Option Capability Option defined in Section 3.2.1. Otherwise, such as in CoAP over UDP, support can only be discovered by trial and error, which is described in Section 3.2.2. 3.2.1. Extended-Token-Option Capability Option A sender can use the elective Extended-Token-Option Capability Option to indicate its support for the Extended-Token Option specified above. Hartke Expires March 14, 2019 [Page 7] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 +----+---+---+--------+-------------------+-------+--------+--------+ | # | C | R | Applie | Name | Forma | Length | Base | | | | | s to | | t | | Value | +----+---+---+--------+-------------------+-------+--------+--------+ | TB | | | CSM | Extended-Token- | empty | 0 | (none) | | D | | | | Option | | | | +----+---+---+--------+-------------------+-------+--------+--------+ C=Critical, R=Repeatable Table 3: The Extended-Token-Option Capability Option 3.2.2. Trial and Error A request with a critical, unsafe-to-forward option such as the Extended-Token Option will cause the return of a 4.02 (Bad Option) response from the next hop if that does not support the option. A client can therefore determine support by sending a request with an Extended-Token Option to next hop and checking if that returns such a response. As per RFC 7252, the response will not contain an Extended-Token Option, so only the regular token can be used for matching the response to the request. Note that a 4.02 (Bad Option) response does not necessarily mean that the Extended-Token Option is unsupported if there is another unsupported critical option in the request. +-----------------+ request message +------------+ | | | with Extended- | | | | | Token option | | | .-<-+->------|--------------------->|------. | | _|_ | | | | | / \ stored | | | | | \___/ state | | | | | | | | | | | '->-+-<------|<---------------------|------' | | | | 4.02 (Bad Option) | | | v | response without | | +-----------------+ Extended-Token +------------+ Client Option Server Figure 4: A Request With an Extended-Token Option Results in a 4.02 (Bad Option) Response if the Next Hop Does Not Support It Hartke Expires March 14, 2019 [Page 8] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 4. Stateless Clients A client can be alleviated of keeping request state by serializing the state into a sequence of bytes and sending this as the token in a request. The server will return the token to the client verbatim in the response, so that the client can recover the state and process the response as if it had kept the state locally. The format of the serialized state is client implementation specific and opaque to any server implementation. For security, the serialized state MUST be integrity protected, MUST include a freshness indicator (e.g., a sequence number or a timestamp), and MAY additionally be encrypted. These requirement can be satisfied, for example, by using COSE [RFC8152] with a key only known to the client. 4.1. Intermediaries Tokens are a hop-by-hop feature: If a client makes a request to an intermediary, that intermediary needs to store the client's token (along with the client's transport address) while it makes a request to the next hop towards the origin server and waits for the response. An intermediary can be alleviated of storing the client's token and transport address by serializing this information along the request state into its own token to the next hop. When the next hop returns the response, the intermediary can recover the information from the token and satisfy the client's outstanding request. The downside of this approach is that the intermediary, without keeping request state, is unable to use a cacheable response to satisfy multiple outstanding requests, which degrades efficiency. It is also unable to aggregate requests when it acts on behalf of multiple clients observing [RFC7641] the same resource (which is REQUIRED by RFC 7641). When using blockwise transfers [RFC7959], servers might not be able to distinguish blocks originating from different clients once they have been forwarded by an intermediary. To ensure that this does not lead inconsistent resource state, a stateless intermediary MUST include the Request-Tag Option [I-D.ietf-core-echo-request-tag] with a value that uniquely identifies the originating endpoint in the intermediary namespace. 4.2. Extended Tokens A client or intermediary in the client role that depends on support for extended tokens from the next hop to avoid keeping request state MUST perform a discovery of support (Section 2.2/Section 3.2) before Hartke Expires March 14, 2019 [Page 9] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 it can be stateless. This discovery MUST be performed in a stateful way, i.e., keeping state for the request (Figure 5): If the client was stateless from the start and the next hop doesn't support extended tokens, then any error message couldn't be processed as the state would neither be present at the client nor echoed back in the error message. +-----------------+ dummy request +------------+ | | | with extended | | | | | token | | | .-<-+->------|=====================>|------. | | _|_ | | | | | / \ stored | | | | | \___/ state | | | | | | | | | | | '->-+-<------|<=====================|------' | | | | response with | | | | | extended token | | | | | echoed back | | | | | | | | | | | | | | | request with | | | | | serialized state | | | | | as token | | | +--------|=====================>|------. | | | | | | | look, no | | | | | state, ma! | | | | | | | | | | +--------|<=====================|------' | | | | response with | | | v | token echoed back | | +-----------------+ +------------+ Client Server Figure 5: Depending on Extended Tokens for Being Stateless First Requires a Successful Stateful Discovery of Support 4.3. Message Transmission In the case of CoAP over UDP [RFC7252], a client can use confirmable or non-confirmable messages for requests. When using confirmable messages, it needs to keep message exchange state for performing retransmissions and for handling Acknowledgement and Reset messages. When using non-confirmable messages, it can keep no message exchange state. However, in either case the client needs to keep congestion control state. That is, it needs to maintain state for each node it communicates with and, e.g., enforce NSTART. Hartke Expires March 14, 2019 [Page 10] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 In accordance with RFC 7252, a client must always be prepared to receive a separate response or non-confirmable response in reply to a request, regardless of the message type it used for that request. If a separate response contains a valid authentication tag and freshness indicator in the token, the client MUST process the message as specified in Section 4.2 of RFC 7252; otherwise, it SHOULD reject the response as specified in Section 4.2 of RFC 7252. 5. Security Considerations TODO. 6. (Variant A:) IANA Considerations 6.1. CoAP Signaling Option Number The following entries are added to the "CoAP Signaling Option Numbers" registry within the "CoRE Parameters" registry. +------------+--------+------------------------+-------------------+ | Applies to | Number | Name | Reference | +------------+--------+------------------------+-------------------+ | 7.01 | TBD | Extended-Token-Lengths | [[this document]] | +------------+--------+------------------------+-------------------+ [[The three lowest available option numbers with the properties specified in Table 1 are 6, 8, and 10 at the time of writing.]] 7. (Variant B:) IANA Considerations 7.1. CoAP Option Number The following entries are added to the "CoAP Option Numbers" registry within the "CoRE Parameters" registry. +--------+----------------+-------------------+ | Number | Name | Reference | +--------+----------------+-------------------+ | TBD | Extended-Token | [[this document]] | +--------+----------------+-------------------+ [[The three lowest available option numbers with the properties specified in Table 2 are 19, 31, and 43 at the time of writing.]] Hartke Expires March 14, 2019 [Page 11] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 7.2. CoAP Signaling Option Number The following entries are added to the "CoAP Signaling Option Numbers" registry within the "CoRE Parameters" registry. +------------+--------+-----------------------+-------------------+ | Applies to | Number | Name | Reference | +------------+--------+-----------------------+-------------------+ | 7.01 | TBD | Extended-Token-Option | [[this document]] | +------------+--------+-----------------------+-------------------+ [[The three lowest available option numbers with the properties specified in Table 3 are 6, 8, and 10 at the time of writing.]] 8. References 8.1. Normative References [I-D.ietf-core-echo-request-tag] Amsuess, C., Mattsson, J., and G. Selander, "Echo and Request-Tag", draft-ietf-core-echo-request-tag-02 (work in progress), June 2018. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, . [RFC7641] Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015, . [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, August 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Hartke Expires March 14, 2019 [Page 12] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 [RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K., Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets", RFC 8323, DOI 10.17487/RFC8323, February 2018, . 8.2. Informative References [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014, . [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, July 2017, . Appendix A. (Variant A:) Updated Message Formats This appendix illustrates the CoAP message formats updated with the new definition of the TKL field (Section 2). Hartke Expires March 14, 2019 [Page 13] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 A.1. CoAP over UDP 0 1 2 3 4 5 6 7 +-------+-------+---------------+ | | | | | Ver | T | TKL | 1 byte | | | | +-------+-------+---------------+ | | | Code | 1 byte | | +-------------------------------+ | | | | | | +- Message ID -+ 2 bytes | | | | | | +-------------------------------+ \ \ / TKL / 0-2 bytes \ (extended) \ +-------------------------------+ \ \ / Token / 0 or more bytes \ \ +-------------------------------+ \ \ / / \ \ / Options / 0 or more bytes \ \ / / \ \ +---------------+---------------+ | | | | 15 | 15 | 1 byte (if payload) | | | +---------------+---------------+ \ \ / / \ \ / Payload / 0 or more bytes \ \ / / \ \ +-------------------------------+ Hartke Expires March 14, 2019 [Page 14] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 A.2. CoAP over TCP 0 1 2 3 4 5 6 7 +---------------+---------------+ | | | | Len | TKL | 1 byte | | | +---------------+---------------+ \ \ / Len / 0-2 bytes \ (extended) \ +-------------------------------+ | | | Code | 1 byte | | +-------------------------------+ \ \ / TKL / 0-2 bytes \ (extended) \ +-------------------------------+ \ \ / Token / 0 or more bytes \ \ +-------------------------------+ \ \ / / \ \ / Options / 0 or more bytes \ \ / / \ \ +---------------+---------------+ | | | | 15 | 15 | 1 byte (if payload) | | | +---------------+---------------+ \ \ / / \ \ / Payload / 0 or more bytes \ \ / / \ \ +-------------------------------+ Hartke Expires March 14, 2019 [Page 15] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 A.3. CoAP over WebSockets 0 1 2 3 4 5 6 7 +---------------+---------------+ | | | | 0 | TKL | 1 byte | | | +---------------+---------------+ | | | Code | 1 byte | | +-------------------------------+ \ \ / TKL / 0-2 bytes \ (extended) \ +-------------------------------+ \ \ / Token / 0 or more bytes \ \ +-------------------------------+ \ \ / / \ \ / Options / 0 or more bytes \ \ / / \ \ +---------------+---------------+ | | | | 15 | 15 | 1 byte (if payload) | | | +---------------+---------------+ \ \ / / \ \ / Payload / 0 or more bytes \ \ / / \ \ +-------------------------------+ Hartke Expires March 14, 2019 [Page 16] Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018 Author's Address Klaus Hartke Ericsson Torshamnsgatan 23 Stockholm SE-16483 Sweden Email: klaus.hartke@ericsson.com Hartke Expires March 14, 2019 [Page 17]