CORE C. Bormann, Ed. Internet-Draft Universitaet Bremen TZI Intended status: Standards Track S. Lemay Expires: May 21, 2016 V. Solorzano Barboza Zebra Technologies H. Tschofenig ARM Ltd. November 18, 2015 A TCP and TLS Transport for the Constrained Application Protocol (CoAP) draft-ietf-core-coap-tcp-tls-00 Abstract The Hypertext Transfer Protocol (HTTP) was designed with TCP as the underlying transport protocol. The Constrained Application Protocol (CoAP), while inspired by HTTP, has been defined to make use of UDP instead of TCP. Therefore, reliable delivery and a simple congestion control and flow control mechanism are provided by the message layer of the CoAP protocol. A number of environments benefit from the use of CoAP directly over a reliable byte stream such as TCP, which already provides these services. This document defines the use of CoAP over TCP as well as CoAP over TLS. 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 May 21, 2016. Bormann, et al. Expires May 21, 2016 [Page 1] Internet-Draft TCP/TLS Transport for CoAP November 2015 Copyright Notice Copyright (c) 2015 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 (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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Constrained Application Protocol . . . . . . . . . . . . . . 3 4. Message Format . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . 6 5. Message Transmission . . . . . . . . . . . . . . . . . . . . 7 6. CoAP URI . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. coap+tcp URI scheme . . . . . . . . . . . . . . . . . . . 7 6.2. coaps+tcp URI scheme . . . . . . . . . . . . . . . . . . 8 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8.1. Service Name and Port Number Registration . . . . . . . . 8 8.2. URI Schemes . . . . . . . . . . . . . . . . . . . . . . . 9 8.3. ALPN Protocol ID . . . . . . . . . . . . . . . . . . . . 10 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 10.1. Normative References . . . . . . . . . . . . . . . . . . 10 10.2. Informative References . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction The Constrained Application Protocol (CoAP) [RFC7252] was designed for Internet of Things (IoT) deployments, assuming that UDP can be used unimpeded -- UDP [RFC0768], or DTLS [RFC6347] over UDP; it is a good choice for transferring small amounts of data across networks that follow the IP architecture. Some CoAP deployments, however, may have to integrate well with existing enterprise infrastructure, where the use of UDP-based protocols may not be well-received or may even be blocked by firewalls. Middleboxes that are unaware of CoAP usage Bormann, et al. Expires May 21, 2016 [Page 2] Internet-Draft TCP/TLS Transport for CoAP November 2015 for IoT can make the use of UDP brittle, resulting in lost or malformed packets. Where NATs are still present, CoAP over TCP can also help with their traversal. NATs often calculate expiration timers based on the transport layer protocol being used by application protocols. Many NATs are built around the assumption that a transport layer protocol such as TCP gives them additional information about the session life cycle and keep TCP-based NAT bindings around for a longer period. UDP, on the other hand, does not provide such information to a NAT and timeouts tend to be much shorter, as research confirms [HomeGateway]. Some environments may also benefit from the more sophisticated congestion control capabilities provided by many TCP implementations. (Note that there is ongoing work to add more elaborate congestion control to CoAP as well, see [I-D.bormann-core-cocoa].) Finally, CoAP may be integrated into a Web environment where the front-end uses CoAP from IoT devices to a cloud infrastructure but the CoAP messages are then transported in TCP between the back-end services. A TCP-to-UDP gateway can be used at the cloud boundary to talk to the UDP-based IoT. To make IoT devices work smoothly in these demanding environments, CoAP needs to make use of a different transport protocol, namely TCP [RFC0793], in some situations secured by TLS [RFC5246]. The present document describes a shim header that conveys length information about each CoAP message. Modifications to CoAP beyond the replacement of the message layer (e.g., to introduce further optimizations) are intentionally avoided. 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 [RFC2119]. 3. Constrained Application Protocol The interaction model of CoAP over TCP is very similar to the one for CoAP over UDP, with the key difference that using TCP voids the need to provide certain transport layer protocol features, such as reliable delivery, fragmentation and reassembly, as well as congestion control, at the CoAP level. The protocol stack is illustrated in Figure 1 (derived from [RFC7252], Figure 1). Bormann, et al. Expires May 21, 2016 [Page 3] Internet-Draft TCP/TLS Transport for CoAP November 2015 +----------------------+ | Application | +----------------------+ +----------------------+ | Requests/Responses | CoAP (RFC7252) |----------------------| | Message adapter | This Document +----------------------+ +-----------+ ^ | TLS | | +-----------+ v +----------------------+ | TCP | +----------------------+ Figure 1: The CoAP over TLS/TCP Protocol Stack Since TCP offers reliable delivery, there is no need to offer a redundant acknowledgement at the CoAP messaging layer. Since there is no need to carry around acknowledgement semantics, messages do not require a message type; no message layer acknowledgement is expected or even possible. Because something needs to be put into the two bits indicating the message type (unless alternative L3 below is chosen), we put the bits for a Non- Confirmable message (NON) into the header. By the nature of TCP, messages are always transmitted reliably over TCP. Figure 2 (derived from [RFC7252], Figure 3) shows this message exchange graphically. A UDP-to-TCP gateway will therefore discard all empty messages, such as empty ACKs (after operating on them at the message layer), and re- pack the contents of all non-empty CON, NON, or ACK messages (i.e., those ACK messages that have a piggy-backed response) into untyped messages (that happen to look like NON messages). Similarly, there is no need to detect duplicate delivery of a message. In UDP CoAP, the Message ID is used for relating acknowledgements to Confirmable messages as well as for duplicate detection. Since the Message ID thus is not meaningful over TCP, it is elided (as indicated by the dashes in Figure 2). Client Server | | | (no type) [------] | +-------------------->| | | Figure 2: Untyped Message Transmission over TCP. Bormann, et al. Expires May 21, 2016 [Page 4] Internet-Draft TCP/TLS Transport for CoAP November 2015 A response is sent back as defined in [RFC7252], as illustrated in Figure 3 (derived from [RFC7252], Figure 6). Client Server | | | (no type) [------] | | GET /temperature | | (Token 0x74) | +------------------->| | | | (no type) [------] | | 2.05 Content | | (Token 0x74) | | "22.5 C" | |<-------------------+ | | Figure 3 4. Message Format The CoAP message format defined in [RFC7252], as shown in Figure 4, relies on the datagram transport (UDP, or DTLS over UDP) for keeping the individual messages separate. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Ver| T | TKL | Code | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Token (if any, TKL bytes) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1 1 1 1 1| Payload (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: RFC 7252 defined CoAP Message Format. In a stream oriented transport protocol such as TCP, a form of message delimitation is needed. For this purpose, CoAP over TCP introduces a length field. Figure 5 shows a 2-byte shim header carrying length information prepended to the CoAP message header. Bormann, et al. Expires May 21, 2016 [Page 5] Internet-Draft TCP/TLS Transport for CoAP November 2015 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Length |Ver|0 1| TKL | Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Token (TKL bytes) ... | Options (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1 1 1 1 1| Payload (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: CoAP Header with prepended Shim Header. The 'Message Length' field is a 16-bit unsigned integer in network byte order. It provides the length of the subsequent CoAP message (including the CoAP header but excluding this message length field) in bytes (so its minimum value is 2). The Message ID and message type are meaningless and thus elided (what would have been the message type field is always filled with what would be the code for NON (01)). The semantics of the other CoAP header fields are left unchanged. 4.1. Discussion One observation is that, over a reliable byte stream transport, the message size limitations defined in Section 4.6 of [RFC7252] are no longer strictly necessary. Consenting [[how: There is currently no defined way to arrive at this consent. --cabo]] implementations may want to interchange messages with payload sizes larger than 1024 bytes, potentially also obviating the need for the Block protocol [I-D.ietf-core-block]. It must be noted that entirely getting rid of the block protocol is not a generally applicable solution, as: o a UDP-to-TCP gateway may simply not have the context to convert a message with a Block option into the equivalent exchange without any use of a Block option; o large messages might also cause undesired head-of-line blocking; o the 2-byte message length field causes another, larger upper bound to the message length. The general assumption is therefore that the block protocol will continue to be used over TCP, even if TCP-based applications occasionally do exchange messages with payload sizes larger than desirable in UDP. Bormann, et al. Expires May 21, 2016 [Page 6] Internet-Draft TCP/TLS Transport for CoAP November 2015 5. Message Transmission As CoAP exchanges messages asynchronously over the TCP connection, the client can send multiple requests without waiting for responses. For this reason, and due to the nature of TCP, responses are returned during the same TCP connection as the request. In the event that the connection gets terminated, all requests that have not yet elicited a response are implicitly canceled; clients may transmit the request again once a connection is reestablished. Furthermore, since TCP is bidirectional, requests can be sent from both the connecting host and the endpoint that accepted the connection. In other words, the question who initiated the TCP connection has no bearing on the meaning of the CoAP terms client and server. 6. CoAP URI CoAP [RFC7252] defines the "coap" and "coaps" URI schemes for identifying CoAP resources and providing a means of locating the resource. RFC 7252 defines these resources for use with CoAP over UDP. The present specification introduces two new URI schemes, namely "coap+tcp" and "coaps+tcp". The rules from Section 6 of [RFC7252] apply to these two new URI schemes. [RFC7252], Section 8 (Multicast CoAP), does not apply to the URI schemes defined in the present specification. Resources made available via one of the "coap+tcp" or "coaps+tcp" schemes have no shared identity with the other scheme or with the "coap" or "coaps" scheme, even if their resource identifiers indicate the same authority (the same host listening to the same port). The schemes constitute distinct namespaces and, in combination with the authority, are considered to be distinct origin servers. 6.1. coap+tcp URI scheme coap-tcp-URI = "coap+tcp:" "//" host [ ":" port ] path-abempty [ "?" query ] The semantics defined in [RFC7252], Section 6.1, apply to this URI scheme, with the following changes: o The port subcomponent indicates the TCP port at which the CoAP server is located. (If it is empty or not given, then the default port 5683 is assumed, as with UDP.) Bormann, et al. Expires May 21, 2016 [Page 7] Internet-Draft TCP/TLS Transport for CoAP November 2015 6.2. coaps+tcp URI scheme coaps-tcp-URI = "coaps+tcp:" "//" host [ ":" port ] path-abempty [ "?" query ] The semantics defined in [RFC7252], Section 6.2, apply to this URI scheme, with the following changes: o The port subcomponent indicates the TCP port at which the TLS server for the CoAP server is located. If it is empty or not given, then the default port 443 is assumed (this is different from the default port for "coaps", i.e., CoAP over DTLS over UDP). o When CoAP is exchanged over TLS port 443 then the "TLS Application Layer Protocol Negotiation Extension" [RFC7301] MUST be used to allow demultiplexing at the server-side unless out-of-band information ensures that the client only interacts with a server that is able to demultiplex CoAP messages over port 443. This would, for example, be true for many IoT deployments where clients are pre-configured to only ever talk with specific servers. [[alwaysalpn: Shouldn't we simply always require ALPN? The protocol should not be defined in such a way that it depends on some undefined pre-configuration mechanism. --cabo]] 7. Security Considerations This document defines how to convey CoAP over TCP and TLS. It does not introduce new vulnerabilities beyond those described already in the CoAP specification. CoAP [RFC7252] makes use of DTLS 1.2 and this specification consequently uses TLS 1.2 [RFC5246]. CoAP MUST NOT be used with older versions of TLS. Guidelines for use of cipher suites and TLS extensions can be found in [I-D.ietf-dice-profile]. 8. IANA Considerations 8.1. Service Name and Port Number Registration IANA is requested to assign the port number 5683 and the service name "coap+tcp", in accordance with [RFC6335]. Service Name. coap+tcp Transport Protocol. tcp Assignee. IESG Bormann, et al. Expires May 21, 2016 [Page 8] Internet-Draft TCP/TLS Transport for CoAP November 2015 Contact. IETF Chair Description. Constrained Application Protocol (CoAP) Reference. [RFCthis] Port Number. 5683 Similarly, IANA is requested to assign the service name "coaps+tcp", in accordance with [RFC6335]. However, no separate port number is used for "coaps" over TCP; instead, the ALPN protocol ID defined in Section 8.3 is used over port 443. Service Name. coaps+tcp Transport Protocol. tcp Assignee. IESG Contact. IETF Chair Description. Constrained Application Protocol (CoAP) Reference. [RFC7301], [RFCthis] Port Number. 443 (see also Section 8.3 of [RFCthis]}) 8.2. URI Schemes This document registers two new URI schemes, namely "coap+tcp" and "coaps+tcp", for the use of CoAP over TCP and for CoAP over TLS over TCP, respectively. The "coap+tcp" and "coaps+tcp" URI schemes can thus be compared to the "http" and "https" URI schemes. The syntax of the "coap" and "coaps" URI schemes is specified in Section 6 of [RFC7252] and the present document re-uses their Bormann, et al. Expires May 21, 2016 [Page 9] Internet-Draft TCP/TLS Transport for CoAP November 2015 semantics for "coap+tcp" and "coaps+tcp", respectively, with the exception that TCP, or TLS over TCP is used as a transport protocol. IANA is requested to add these new URI schemes to the registry established with [RFC7595]. 8.3. ALPN Protocol ID IANA is requested to assign the following value in the registry "Application Layer Protocol Negotiation (ALPN) Protocol IDs" created by [RFC7301]: Protocol: CoAP Identification Sequence: 0x63 0x6f 0x61 0x70 ("coap") Reference: [RFCthis] 9. Acknowledgements We would like to thank Stephen Berard, Geoffrey Cristallo, Olivier Delaby, Michael Koster, Matthias Kovatsch, Szymon Sasin, Andrew Summers, and Zach Shelby for their feedback. 10. References 10.1. Normative References [I-D.ietf-dice-profile] Tschofenig, H. and T. Fossati, "TLS/DTLS Profiles for the Internet of Things", draft-ietf-dice-profile-17 (work in progress), October 2015. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, . Bormann, et al. Expires May 21, 2016 [Page 10] Internet-Draft TCP/TLS Transport for CoAP November 2015 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/ RFC5246, August 2008, . [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/ RFC7252, June 2014, . [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, July 2014, . [RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines and Registration Procedures for URI Schemes", BCP 35, RFC 7595, DOI 10.17487/RFC7595, June 2015, . 10.2. Informative References [HomeGateway] Eggert, L., "An experimental study of home gateway characteristics", Proceedings of the 10th annual conference on Internet measurement, 2010. [I-D.bormann-core-cocoa] Bormann, C., Betzler, A., Gomez, C., and I. Demirkol, "CoAP Simple Congestion Control/Advanced", draft-bormann- core-cocoa-03 (work in progress), October 2015. [I-D.ietf-core-block] Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP", draft-ietf-core-block-18 (work in progress), September 2015. [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, . [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011, . Bormann, et al. Expires May 21, 2016 [Page 11] Internet-Draft TCP/TLS Transport for CoAP November 2015 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, . Authors' Addresses Carsten Bormann (editor) Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63921 Email: cabo@tzi.org Simon Lemay Zebra Technologies 820 W. Jackson Blvd. Suite 700 Chicago 60607 United States of America Phone: +1-847-634-6700 Email: slemay@zebra.com Valik Solorzano Barboza Zebra Technologies 820 W. Jackson Blvd. suite 700 Chicago 60607 United States of America Phone: +1-847-634-6700 Email: vsolorzanobarboza@zebra.com Hannes Tschofenig ARM Ltd. 110 Fulbourn Rd Cambridge CB1 9NJ Great Britain Email: Hannes.tschofenig@gmx.net URI: http://www.tschofenig.priv.at Bormann, et al. Expires May 21, 2016 [Page 12]