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CORE                                                     C. Bormann, Ed.
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                                S. Lemay
Expires: May 6, 2016                                V. Solorzano Barboza
                                                      Zebra Technologies
                                                           H. Tschofenig
                                                                ARM Ltd.
                                                       November 03, 2015


A TCP and TLS Transport for the Constrained Application Protocol (CoAP)
                 draft-tschofenig-core-coap-tcp-tls-05

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 6, 2016.








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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  . . . . . . . . . . . . . . . . . . . .   6
   6.  CoAP URI  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  coap+tcp URI scheme . . . . . . . . . . . . . . . . . . .   7
     6.2.  coaps+tcp URI scheme  . . . . . . . . . . . . . . . . . .   7
   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  . . . . . . . . . . . . . . . . . . . . . . .  11

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




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   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).



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           +----------------------+
           |      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.




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   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 1-byte shim header
   carrying length information prepending the CoAP message header.








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        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 the message size limitations defined in
   Section 4.6 of [RFC7252] are no longer strictly necessary.
   Consenting [how?] 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.

   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.

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



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   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, whoever 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.)

6.2.  coaps+tcp URI scheme

   coaps-tcp-URI = "coaps+tcp:" "//" host [ ":" port ] path-abempty
                   [ "?" query ]




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   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.  [[_1:
      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 <iesg@ietf.org>

   Contact.
      IETF Chair <chair@ietf.org>

   Description.



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      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 <iesg@ietf.org>

   Contact.
      IETF Chair <chair@ietf.org>

   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
   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].



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8.3.  ALPN Protocol ID

   This document requests a value from the "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,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
              RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.




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   [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, <http://www.rfc-editor.org/info/rfc7301>.

   [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,
              <http://www.rfc-editor.org/info/rfc7595>.

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,
              <http://www.rfc-editor.org/info/rfc768>.

   [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,
              <http://www.rfc-editor.org/info/rfc6335>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

Authors' Addresses








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   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













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