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CoRE Working Group                                         B. Silverajan
Internet-Draft                                                       TUT
Intended status: Informational                                   M. Ocak
Expires: January 9, 2017                                        Ericsson
                                                            July 8, 2016


                       CoAP Protocol Negotiation
           draft-silverajan-core-coap-protocol-negotiation-03

Abstract

   CoAP has been standardised as an application-level REST-based
   protocol.  When multiple transport protocols exist for exchanging
   CoAP resource representations, this document introduces a way forward
   for CoAP endpoints as well as intermediaries to agree upon alternate
   transport and protocol configurations as well as URIs for CoAP
   messaging.

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 January 9, 2017.

Copyright Notice

   Copyright (c) 2016 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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Overcoming Middlebox Issues . . . . . . . . . . . . . . .   4
     2.2.  Better resource caching and serving in proxies  . . . . .   5
     2.3.  Interaction with Energy-constrained Servers . . . . . . .   5
   3.  Node Types based on Transport Availability  . . . . . . . . .   6
   4.  New Link Attribute and Relation types . . . . . . . . . . . .   7
   5.  Observing Transport Types and Resource Representations  . . .   8
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  13
     A.1.  From -02 to -03 . . . . . . . . . . . . . . . . . . . . .  13
     A.2.  From -01 to -02 . . . . . . . . . . . . . . . . . . . . .  13
     A.3.  From -00 to -01 . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] allows clients,
   origin servers and proxies, to exchange and manipulate resource
   representations using REST-based methods using UDP or DTLS.  CoAP
   messaging is however being extended to use other alternative
   underlying transports.  These include reliable transports such as
   TCP, WebSockets and TLS.  In addition, the use of SMS as a CoAP
   transport remains a possibility for simple communication in cellular
   networks.

   When CoAP-based endpoints and proxies possess the ability to perform
   CoAP messaging over multiple transports, significant benefits can be
   obtained if communicating client endpoints can discover that multiple
   transport bindings may exist on an origin server over which CoAP
   resources can be retrieved.  This allows a client to understand and
   possibly subsitute a different transport protocol configuration for
   the same CoAP resources on the origin server, based on the
   preferences of the communicating peers.  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.



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   A URI in CoAP, however, serves two purposes simultaneously.  It
   firstly functions as a locator, by specifying the network location of
   the endpoint hosting the resource, and the underlying transport used
   by CoAP for accessing the resource representation.  It secondly
   identifies the name of the specific resource found at that endpoint
   together with its namespace, or resource path.  A single CoAP URI
   cannot be used to express the identity of the resource independently
   of alternate underlying transports or protocol configuration.
   Multiple URIs can result for a single CoAP resource representations
   if:

   o  the authority components of the URI differ, owing to the same
      physical host exposing several network endpoints.  For example,
      "coap://example.org/sensors/temperature" and
      "coap://example.net/sensors/temperature"

   o  the scheme components of the URI differ, owing to the origin
      server exposing several underlying transport alternatives.  For
      example, "coap://example.org/sensors/temperature" and
      "coap+tcp://example.org/sensors/temperature"

   o  the path components of the URI differ, should an origin server
      also allow alternative transport endpoint such as the WebSocket
      protocol, to be expressed using the path.  For example,
      "coap://example.org/sensors/temperature" and
      "coap+ws://example.org/ws-endpoint/sensors/temperature"

   Without a priori knowledge, clients would be unable to ascertain if
   two or more URIs provided by an origin server are associated to the
   same representation or not.  Consequently, a communication mechanism
   needs to be conceived to allow an origin server to properly capture
   the relationship between these alternate representations or locations
   and then subsequently supply this information to clients.  This also
   goes some way in limiting URI aliasing [WWWArchv1].

   In order to support CoAP clients, proxies and servers wishing to use
   CoAP over multiple transports, this draft proposes the following:

   o  A means for CoAP clients to interact with an origin server's CoRE
      resource directory interface to discover alternative transports
      and links describing alternate locations of CoAP resources.

   o  An ability for servers to convey the names of supported CoAP
      transports to requesting clients, in order of preference, as well
      as any optional lifetime values associated with them.






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   o  A new link format attribute as well as a new link relation type
      that together enable an origin server to serve a resource from
      other protocol configurations or endpoints.

2.  Aim

   The following simple scenarios aim to better portray how CoAP
   protocol negotiation benefits communicating nodes

2.1.  Overcoming Middlebox Issues

   Discovering which transports are available is important for a client
   to determine the optimal alternative to perform CoAP messaging
   according to its needs, particularly when separated from a CoAP
   server via a NAT.  It is well-known that some firewalls as well as
   many NATs, particularly home gateways, hinder the proper operation of
   UDP traffic.  NAT bindings for UDP-based traffic do not have as long
   timeouts as TCP-based traffic.



        +-------------+-----+      +---+      +-----+-------------+
        |             |     |--1-->|   |--1-->|     |             |
        |             | UDP |      | N |      | UDP |             |
        |             |     |<--2--|   |<--2--|     |             |
        | CoAP Client +-----+      | A |      +-----+ CoAP Server |
        |             |     |--3-->|   |--3-->|     |             |
        |             | TCP |      | T |      | TCP |             |
        |             |     |<--4--|   |<--4--|     |             |
        +-------------+-----+      +---+      +-----+-------------+



     Figure 1: CoAP Client initially accesses CoAP Server over UDP and
                           then switching to TCP

   Figure 1 depicts such a scenario, where a CoAP client uses UDP
   initially for accessing a CoAP Server, and engages in discovering
   alternative transports offered by the server.  The client
   subsequently decides to use TCP for CoAP messaging instead of UDP to
   set up an Observe relationship for a resource at the CoAP Server, in
   order to avoid incoming packets containing resource updates being
   discarded by the NAT.








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2.2.  Better resource caching and serving in proxies

   Figure 2 outlines a more complex example of intermediate nodes such
   as CoAP-based proxies to intelligently cache and respond to CoAP or
   HTTP clients with the same resource representation requested over
   alternative transports or server endpoints.

   In this example, a CoAP over WebSockets client successfully obtains a
   response from a CoAP forward proxy to retrieve a resource
   representation from an origin server using UDP, by supplying the CoAP
   server's endpoint address and resource in a Proxy-URI option.  Arrow
   1 represents a GET request to "coap+ws://proxy.example.com" which
   subsequently retrieves the resource from the CoAP server using the
   URI "coap://example.org/sensors/temperature", shown as arrow 2.



        +---------+
        | CoAP+WS |     +--------+-------+---+     +-----+---------+
        | Client  |<-1->|  Web   |       |   |<-2->|     |         |
        +---------+     | Socket | CoAP  | U |<-4->| UDP |   CoAP  |
        +---------+     +--------+ Proxy | D |     +-----+  Server |
        |  HTTP   |<-3->|  HTTP  |       | P |     | TCP |         |
        | Client  |<-5->|        |       |   |     |     |         |
        +---------+     +--------+-------+---+     +-----+---------+



      Figure 2: Proxying and returning a resource's alternate cached
                    representations to multiple clients

   Subsequently, assume an HTTP client requests the same resource, but
   instead specifies a CoAP over TCP alternative URI instead.  Arrow 3
   represents this event, where the HTTP client performs a GET request
   to "http://proxy.example.com/coap+tcp://example.org/sensors/
   temperature".  When the proxy receives the request, instead of
   immediately retrieving the temperature resource again over TCP, it
   first verifies from the CoAP server whether the cached resource
   retrieved over UDP is a valid equivalent representation of the
   resource requested by the HTTP client over TCP (arrow 4).  Upon
   confirmation, the proxy is able to supply the same cached
   representation to the HTTP client as well (arrow 5).

2.3.  Interaction with Energy-constrained Servers

   Figure 3 illustrates discovery and communication between a CoAP
   client and an energy-constrained CoAP Server.  Such a server aims at
   conserving its energy unless a need arises otherwise.  The figure



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   depicts the server maintaining its communication in a low-power state
   by listening only for incoming SMS messages while disabling
   communication on radio interfaces requiring greater energy.  This is
   depicted as the server's initial state in the figure, showing an
   active SMS endpoint and a disabled or dormant UDP interface.



            +-------------+-----+        +-----+-------------+
            |             |     |--1---->|     |             |
            |             | SMS |        | SMS |             |
            |             |     |<---2---|     |             |
            | CoAP Client +-----+        +-----+ CoAP Server |
            |             |     |--3---->|     |             |
            |             | UDP |        |(UDP)|             |
            |             |     |<---4---|     |             |
            +-------------+-----+        +-----+-------------+



   Figure 3: CoAP client interacting over SMS to discover a server's IP-
                              based endpoint

   A CoAP client wishing to perform CoAP operations can query a CoAP
   server for available transports via SMS, as shown in arrow 1.  Upon
   reception of the message, should the server have its radio and IP
   interface up, it can send an SMS response containing the location of
   the CoAP IP endpoint and supported transports.  Alternatively, the
   incoming SMS can be also used by the server as a triggering event to
   temporarily power up its radio interface so that UDP or other
   transport-based CoAP communication can instead be employed, and
   likewise provide this information in its response.  This is depicted
   as arrow 2.  Subsequently, low latency IP-based CoAP communication
   can occur between the endpoints as shown by arrows 3 and 4.

3.  Node Types based on Transport Availability

   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.









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     +-------+----------------------------+
     | Name  |  Transport Availability    |
     +-------+----------------------------+
     |  T0   |  Single transport          |
     |       |                            |
     |  T1   |  Multiple transports, with |
     |       |  one or more active at any |
     |       |  point in time             |
     |       |                            |
     |  T2   |  Multiple active and       |
     |       |  persistent transports     |
     |       |  at all times              |
     +-------+----------------------------+

    Table 1: Classes of Available Transports


   Type T0 nodes possess 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 in a persistent manner.  CoAP
   proxy nodes which allow CoAP endpoints from disparate transports to
   communicate with each other, are a good example of this.

4.  New Link Attribute and Relation types

   A CoAP server wishing to allow interactions with resources from
   multiple locations or transports can do so by specifying the
   Transport Type "tt" link attribute, which is an opaque string.
   Multiple transport types can be included in the value of this
   parameter, each separated by a space.  In such cases, transport types
   appear in a prioritised list, with the most preferred transport type
   by the CoAP server specified first and the lowest priority transport
   type last.

   At the same time, each transport type supported by the server is also
   described with an "altloc" link relation type.  The "altloc" relation
   type specifices a URI (containing the URI scheme, authority and




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   optionally path) providing an alternate endpoint location up to but
   not including the resource path of a representation.

   Each URI specified by "altloc" link relation type can also have an
   active lifetime value described with "al" link extension, which is an
   integer showing the active lifetime in seconds.  The "al" link
   extension specifies how long the CoAP server will respond to the
   specified URI in "altloc" relation type.

   Both "tt" and "altloc" are optional CoAP features.  If supported,
   they occur at the granularity level of an origin server, ie. they
   cannot be applied selectively on some resources only.  Therefore
   "altloc" is always anchored at the root resource ("/").  The "al"
   link attribute, while also being optional, exists at the granularity
   of each transport protocol used.  When it is absent, it is assumed
   that the transport protocol is persistent.

   Additionally, the "tt" and "al" link attributes as well as the
   "altloc" relation type can be ignored by unsupported CoAP clients.

5.  Observing Transport Types and Resource Representations

   A CoAP client interested in being notified of changes in an origin
   server's transport protocols for CoAP, can choose to do so with an
   Observe relationship [RFC7641].  The client registers its interest on
   the available active transports by setting the Observe option with a
   GET to ".well-known/core" on a CoAP server, with a client-specified
   parameter value for "tt" as depicted in Figure 4.  Updates on the
   active transports will be sent to the CoAP client as CoAP
   notifications.





















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         Client                         Server
            |                              |
            |  GET /.well-known/core?tt=*  |
            |    Token: 0x7a               |   Registration
            |  Observe: 0                  |
            +----------------------------->|
            |                              |
            |    2.05 Content              |
            |    Token: 0x7a               |   Notification of
            |  Observe: 18                 |   transport type
            |  Payload: "udp sms"          |
            |<-----------------------------+
            |                              |
            |    2.05 Content              |
            |    Token: 0x7a               |   Notification of new
            |  Observe: 32                 |   transport
            |  Payload: "udp sms tcp"      |
            |<-----------------------------+
            |                              |
            |    2.05 Content              |
            |    Token: 0x7a               |   Notification of
            |  Observe: 56                 |   transport type
            |  Payload: "udp sms"          |
            |<-----------------------------+



    Figure 4: CoAP client observing .well-known/core for all transport
                                   types

   Observe relationships between a CoAP client and a CoAP server must
   conform to established norms specified in [RFC7641].  Subsequent
   notifications are considered to simply be additional responses to the
   original client GET request.  Therefore, should a client subsequently
   switch to a different transport protocol (such as from UDP to TCP),
   it is the responsibility of the client to deregister its interest
   beforehand or cancel its interest as specified in section 3.6 of
   [RFC7641].  No assumptions of session continuation should be made and
   the client should instead re-register its interest using the new
   transport, either actively, or upon the Max-Age of a stored
   representation being exceeded at the client.

   A server can also prevent notifications to be perpetually sent to a
   client on a previous transport, by using confirmable CoAP messages
   for responses.  This allows the server to remove an unresponsive
   client from its list of interested observers.





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6.  Examples

   Figure 5 shows a CoAP server returning all transport types and the
   alternate resource locations to a CoAP client performing a CoAP
   Request to ./well-known/core

   In this case, the server supplies two different locations to interact
   with resources using CoAP over TCP i.e. the resources given in the
   CoAP response are available from multiple hosts with different entry
   points and transport layer security.

   At the same time, the path to the WebSocket endpoint is provided in
   addition to the FQDN of the server, for using CoAP over WebSockets,
   exemplifying the ability to separate a CoAP resource path from a web-
   based CoAP endpoint path in a URI.



   REQ: GET /.well-known/core

   RES: 2.05 Content
   </sensors>;ct=40;title="Sensor Index", tt="tcp ws sms",
   </sensors/temp>;rt="temperature-c";if="sensor",
   </sensors/light>;rt="light-lux";if="sensor",
   <coap+tcp://server.example.com/>;rel="altloc",
   <coaps+tcp://server.example.net/>;rel="altloc",
   <coap+ws://server.example.com/ws-endpoint/>;rel="altloc",
   <coap+sms://001234567>;rel="altloc"


                   Figure 5: Example of Server response

   Figure 6 shows a CoAP client actively soliciting a CoAP server for
   all supported transport types and protocol configurations.


   REQ: GET /.well-known/core?tt=*

   RES: 2.05 Content
   </sensors>;tt="tcp sms ws"
   <coap+tcp://server.example.com/>;rel="altloc",
   <coap+tcp://server.example.net/>;rel="altloc",
   <coap+ws://server.example.com/ws-endpoint/>;rel="altloc",
   <coap+sms://001234567/>;rel="altloc"


     Figure 6: CoAP client discovering transports supported by a CoAP
                                  server.



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   Figure 7 shows a CoAP client explicitly soliciting support for a
   specific transport type using a query filter parameter.


   REQ: GET /.well-known/core?tt=sms

   RES: 2.05 Content
   </sensors>;tt="tcp sms ws"
   <coap+sms://001234567/>;rel="altloc"


   Figure 7: CoAP client looking for a specific transport to use with a
                               CoAP server.

   Figure 8 shows a CoAP client making a CoAP over SMS request to an
   energy-constrained CoAP server, explicitly soliciting support for
   UDP-based communication by using a query filter parameter.  The
   server temporarily activates its UDP interface, responds with the
   location of the UDP endpoint and also provides the expected lifetime
   of the transport, which in this case is 120 seconds.


   REQ: GET /.well-known/core?tt=udp

   RES: 2.05 Content
   </sensors>;tt="udp sms"
   <coap://server.example.com/>;rel="altloc";al=120


      Figure 8: CoAP client using CoAP over SMS to discover UDP-based
                      address and transport lifetime.

7.  IANA Considerations

   This document requests the registration of a new link relation type
   "altloc".

   Relation name:
      altloc

   Description:
      Identifies an alternate CoAP endpoint location for a resource.

   Reference:
      This document.






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8.  Security Considerations

   When multiple transports, locations and representations are used,
   some obvious risks are present both at the origin server as well as
   by requesting clients.

   An energy-constrained node exposing its resource representations
   using CoAP over SMS, but subsequently enabling its IP interface on-
   demand, can be subjected to denial-of-sleep as well as battery
   draining attacks by attackers.  The risk can be somewhat mitigated at
   the server by strict requirements on the active lifetime of IP-based
   communication as well as restricting which clients are allowed to
   request for IP-based communication and referring other incoming
   requests to a caching proxy instead.

   When a client is presented with alternate URIs for retrieving
   resources, it presents an opportunity for attackers to mount a series
   of attacks, either by hijacking communication and masquerading as an
   alternate location or by using a man-in-the-middle attack on TLS-
   based communication to a server and redirecting traffic to an
   alternate location.  A malicious or compromised server could also be
   used for reflective denial-of-service attacks on innocent third
   parties.  Moreover, clients may obtain web links to alternate URIs
   containing weaker security properties than the existing session.

9.  Acknowledgements

   Thanks to Klaus Hartke for comments and reviewing this draft, and
   Teemu Savolainen for initial discussions about protocol negotations
   and lifetime values.

10.  References

10.1.  Normative References

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <http://www.rfc-editor.org/info/rfc7228>.

   [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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   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <http://www.rfc-editor.org/info/rfc7641>.

10.2.  Informative References

   [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>.

   [WWWArchv1]
              http://www.w3.org/TR/webarch/#uri-aliases, "Architecture
              of the World Wide Web, Volume One", December 2004.

Appendix A.  Change Log

A.1.  From -02 to -03

   Added new author

   Rewrite of "Introduction" section

   Added new Aims Section

   Added new Section on Node Types

   Introduced "al" Active Lifetime link attribute

   Added new Section on Observing transports and resources

   Security and IANA considerations sections populated

A.2.  From -01 to -02

   Freshness update.

A.3.  From -00 to -01

   Reworked "Introduction" section, added "Rationale", and "Goals"
   sections.

Authors' Addresses







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   Bilhanan Silverajan
   Tampere University of Technology
   Korkeakoulunkatu 10
   FI-33720 Tampere
   Finland

   Email: bilhanan.silverajan@tut.fi


   Mert Ocak
   Ericsson
   Hirsilantie 11
   02420 Jorvase
   Finland

   Email: mert.ocak@ericsson.com



































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