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ANIMA WG                                                     M. Pritikin
Internet-Draft                                             P. Kampanakis
Intended status: Informational                             Cisco Systems
Expires: May 4, 2017                                    October 31, 2016


                            BRSKI over CoAP
                    draft-pritikin-coap-bootstrap-01

Abstract

   This document provides an initial discussion of Bootstrapping of
   Remote Secure key infrastructures (BRSKI) when the device being
   bootstrapped speaks CoAP.  The HTTPS REST methods leveraged by BRSKI
   are mapped to CoAP methods.  Fragmentation management of large
   messages during EST certificate enrollment is addressed.

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 4, 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Scope of solution . . . . . . . . . . . . . . . . . . . . . .   3
   4.  DTLS  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Message Bindings  . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  cacerts . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  enroll / reenroll . . . . . . . . . . . . . . . . . . . .   6
     5.3.  csrattr . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.4.  requestaudittoken, requestauditlog  . . . . . . . . . . .   7
   6.  Data Fragmentation  . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Fragmented response example with Block2 . . . . . . . . .   8
     6.2.  Fragmented request example with Block1  . . . . . . . . .  11
     6.3.  Fragmented request/response example with Block1, Size1
           and Block2  . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Proxying  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  CoAP Parameters . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. Update Tracking . . . . . . . . . . . . . . . . . . . . . . .  11
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Many IoT and other devices are expected to use CoAP over UDP
   extensively.  Bootstrapping these devices without requiring a full
   TCP stack is an often raised requirement for
   [I-D.ietf-anima-bootstrapping-keyinfra].  BRSKI provides REST methods
   over TLS that can be functional in a UDP setting with the folling
   necessary additions:

   DTLS:  Because the CoAP use of DTLS includes support for large
      handshake messages there is little to describe here.  BRSKI and
      EST [RFC7030] are expanded to include DTLS.

   REST:  The mapping of BRSKI and EST messages to CoAP REST calls is
      described.

   Fragmentation:  Use of block chaining to support fragmentation of
      large BRSKI and EST messages is described.

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].



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3.  Scope of solution

   The definition of BRSKI over DTLS and CoAP is not intended to expand
   the scope of BRSKI to highly constrained devices. (ref: [RFC7228]).
   Instead it is intended to ensure that bootstrapping works for less
   constained devices that choose to limit their communications stack to
   UDP/CoAP.

   The BRSKI document details extensions to EST as well as making
   section 5.7 requirements on EST flows.  This document's references to
   BRSKI are intended to include all BRSKI extensions and all existing
   EST messages.  This document could replace BRSKI -03 section 5.7.5.
   [[TODO: making this section 5.8 might make the most sense.]]

   Support for Observe CoAP options ( https://tools.ietf.org/html/
   rfc7641 ) in Blocks with BRSKI is not supported in the current BRSKI/
   EST message flows and is thus out-of-scope for this discussion.
   Observe options could be used by the server to notify clients about a
   change in the cacerts or csr attributes (resources) and might be an
   area of future work.

4.  DTLS

   COAP was designed to avoid fragmentation.  DTLS is used to secure
   COAP messages.  When using DTLS, even though it can be avoided by
   using pre-shared keys or ECC ciphersuites, sometimes fragmentation
   will be needed.  During the DTLS handshake, if fragmentation is
   necessary, "DTLS provides a mechanism for fragmenting a handshake
   message over a number of records, each of which can be transmitted
   separately, thus avoiding IP fragmentation" [RFC6347].

   Within BRSKI and EST when "TLS" is referred to, it is understood that
   CoAP security is provided using DTLS instead.  No other changes are
   necessary (all provisional modes etc are the same as for TLS).

   In a constrained CoAP environment, endpoints can't afford to
   establish a DTLS connection for every EST transaction.
   Authenticating and negotiating DTLS keys requires resources on low-
   end endpoints and consumes valuable bandwidth.  The DTLS connection
   SHOULD remain open for persistent EST connections.  For example, an
   EST cacerts request that is followed by an simpleenroll request can
   use the same authenticated DTLS connection.  Given that after a
   successfull enrollment, it is more likely that a new EST transaction
   will take place after a significant amount of time, the DTLS
   connections SHOULD only be kept alive for EST messages that are
   relatively close to each other.





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5.  Message Bindings

   This section describes BRSKI to CoAP message mappings.

   CoAP defines confirmed (CON), acknowledgements (ACK), reset (RST) and
   non-corfirmed (NON) message types.  For confirmable messages, the
   responses are CoAP ACKs or RSTs.  All /cacerts, /simpleenroll,
   /simplereenroll, /csrattrs, /fullcmc and /serverkeygen EST messages
   expect a response, so they are all COAP CON messages.

   The HTTP responses in BRSKI are translated to COAP Response Codes as
   explained in [RFC7252] section 5.3.1

   A CoAP message has the following fields ([I-D.ietf-core-block]):

       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) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Then Ver, TKL, Token, Message ID are not affected in BRSKI.  Their
   use is the same as in CoAP.

   The options that can be used in a CoAP header have the following
   format (from [I-D.ietf-core-block] Figure 8):

               0   1   2   3   4   5   6   7
      +---------------+---------------+
      |  Option Delta | Option Length |   1 byte
      +---------------+---------------+
      /         Option Delta          /   0-2 bytes
      \          (extended)           \
      +-------------------------------+
      /         Option Length         /   0-2 bytes
      \          (extended)           \
      +-------------------------------+
      \                               \
      /         Option Value          /   0 or more bytes
      \                               \
      +-------------------------------+




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   Options are used to convey Uri-Host, Uri-Path, Uri-Port, Content-
   Format and more in COAP.  For BRSKI, COAP Options are used to
   communicate the HTTP fields used in the BRSKI REST messages.

   BRSKI URLs are HTTPS based (https:// ), in CoAP these will be assumed
   to be transformed to coaps (coaps://)

   Some examples of how an BRSKI message would be translated in CoAP
   follow.  [[TODO: This section to be expanded to ensure it covers all
   BRSKI edge conditions.]]

5.1.  cacerts

   First let's see how a get cacerts message in EST would be in CoAP.
   The HTTPS cacerts message can be

     GET /.well-known/est/cacerts HTTP/1.1
        User-Agent: curl/7.22.0 (i686-pc-linux-gnu) libcurl/7.22.0
                    OpenSSL/1.0.1 zlib/1.2.3.4 libidn/1.23 librtmp/2.3
        Host: 192.0.2.1:8085
        Accept: */*

   The corresponding CoAP fields would be:

     Ver = 1
     T = 0 (CON)
     Code = 0x01 (0.01 is GET)
     Options
      Option1 (Uri-Host)
        Option Delta = 0x3
        Option Length = 0x9
        Option Value = 192.0.2.1
      Option2 (Uri-Port)
        Option Delta = 0xA
        Option Length = 0x4
        Option Value = 8085
      Option3 (Uri-Path)
        Option Delta = 0xD
        Option Length = 0xD
        Extended Option Delta = 0x08
        Extended Option Length = 0x14
        Option Value = /.well-known/est/cacerts HTTP/1.1
     Payload = [Empty]

   Now let's say we have a 200 OK response with a cert in EST:






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      HTTP/1.1 200 OK
      Status: 200 OK
      Content-Type: application/pkcs7-mime
      Content-Transfer-Encoding: base64 (TODO: Verify if we need a new
                                         option registry for Encoding?)
      Content-Length: 4246 (TODO: this example overflows and would
                            need fragmentation. Choose a better example.
                            Regardless we might need an CoAP option for
                            the content-length ie the CoAP payload?)

      MIIMOQYJKoZIhvcNAQcCoIIMKjCCDCYCAQExADALBgkqhkiG9w0BBwGgggwMMIIC
      +zCCAeOgAwIBAgIJAJpY3nUZO3qcMA0GCSqGSIb3DQEBBQUAMBsxGTAXBgNVBAMT
      ...

   The corresponding CoAP fields would be:

     Ver = 1
     T = 2 (ACK)
     Code = 0x21 (TODO: Maybe we need to create a 0x200 response code.)
     Options
       Option1 (Content-Format)
         Option Delta = 0xC
         Option Length = 0xD
         Extended Option Length = 0x09
         Option Value = <number for application/pkcs7-mime>
                   (TODO: We need a new CoAP IANA registered value
                   application/pkcs7-mime; smime-type=certs-only,
                   application/csrattrs, application/pkcs10,
                   application/pkcs8,
                   application/pkcs12 )
     Payload = MIIMOQYJKoZIhvcNAQcCoIIMKjCCDCYCAQExA \
                 DALBgkqhkiG9w0BBwGgggwMMIIC...

5.2.  enroll / reenroll

   [[TODO: username/password authentication can be described here but is
   not a primary focus for BRSKI.  It is important for generic EST
   exchanges but would an endpoint device with sufficient user interface
   to allow username/password input from an end user be required to use
   CoAP instead of a full HTTPS exchange?]]

   [[TODO: We might need a new Option for the Retry-After response
   message.  We might need a new Option for the WWW-Authenticate
   response.]]







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5.3.  csrattr

5.4.  requestaudittoken, requestauditlog

6.  Data Fragmentation

   Even though ECC certificates are small in size, they can vary greatly
   based on signature algorithms, key sizes, and OID fields used.  Even
   with ECC certs, BRSKI CoAP messages carrying such certificates can
   still exceed sizes in MTU of 1280 for IPv6 or 60-80 bytes for 6LoWPAN
   [RFC4919]) (section 2 of [I-D.ietf-core-block]).  For 256-bit curves,
   common ECDSA cert sizes are 500-1000 bytes which could fluctuate
   further based on the algorithms, OIDs, SANs and cert fields.  For
   384-bit curves, ECDSA certs increase in size and can sometimes reach
   1.5KB.  Additionally, there are times when the EST cacert response
   from the server can include multiple certs that amount large
   payloads.

   CoAP RFC section 4.6 describes the possible payload sizes: "if
   nothing is known about the size of the headers, good upper bounds are
   1152 bytes for the message size and 1024 bytes for the payload size".
   Also "If IPv4 support on unusual networks is a consideration,
   implementations may want to limit themselves to more conservative
   IPv4 datagram sizes such as 576 bytes; per [RFC0791], the absolute
   minimum value of the IP MTU for IPv4 is as low as 68 bytes, which
   would leave only 40 bytes minus security overhead for a UDP payload".
   Thus, after the DTLS connection is established, fragmentation will
   sometimes be needed for the CoAP messages which involve certificate
   enrollment and management.  A fragmentation solution for BRSKI and
   EST CoAP message is required.

   The [I-D.ietf-core-block] document describes how fragmentation can be
   done by using a pair of Block options added to the CoAP flow.  Block1
   options are used by the client PUT and POST requests.  A Block1 in a
   client request requires a Block1 option in the responses.  A Block2
   comes from a server response that will also need Block2 from the
   client to acknowledge the block and get the rest of blocks from the
   server.  So, Block1 is used when a request (POST for example) is done
   in BRSKI over CoAP with a payload that needs fragmentation.  Then the
   server responds with Block1 option to acknowledge the fragment-
   blocks.  Block2 is used when a BRSKI server response is big and needs
   fragmentation.  The Block2 acknowledgements are requests with the
   same options as the initial request and a Block2 option.  "To
   influence the block size used in a response, the requester MAY also
   use the Block2 Option on the initial request, giving the desired
   size, a block number of zero and an M bit of zero".  As explained in
   see section 2.8 of [I-D.ietf-core-block]), blockwise transfers SHOULD




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   be used in Confirmable COAP messages to avoid the exacerbation of
   lost blocks.

   In a scenario with a big BRSKI POST request we might have Block1
   options from client to server and Block2 from server to client.  In
   this case the Block1 blocks get completed and then the Block2 comes
   the other direction.

   The BLOCK draft also defines Size1 and Size2 options.  These are used
   to convey the size of the resources in the requests or responses.
   The Size1 response MAY be parsed by the client as an size indication
   of the Block2 resource in the server response or by the server as a
   request for a size estimate by the client.  Similarly, Size2 option
   defined in BLOCK should be parsed by the server as an indication of
   the size of the resource carried in Block1 options and by the client
   as a maximum size expected in the 4.13 (Request Entity Too Large)
   response to a request.

6.1.  Fragmented response example with Block2

   An example of a server cacerts response that exceeds the MTU is:

   An example of a server cacerts response that exceeds the MTU is
   HTTP/1.1 200 OK
      Status: 200 OK
      Content-Type: application/pkcs7-mime; smime-type=certs-only
      Content-Transfer-Encoding: base64
      Content-Length: 1122

      MIIDOAYJKoZIhvcNAQcCoIIDKTCCAyUCAQExADALBgkqhkiG9w0BBwGgggMLMIID
      BzCCAe+gAwIBAgIBFTANBgkqhkiG9w0BAQUFADAbMRkwFwYDVQQDExBlc3RFeGFt
      cGxlQ0EgTndOMB4XDTEzMDUwOTIzMTU1M1oXDTE0MDUwOTIzMTU1M1owHzEdMBsG
      A1UEAxMUZGVtb3N0ZXA0IDEzNjgxNDEzNTIwggEiMA0GCSqGSIb3DQEBAQUAA4IB
      DwAwggEKAoIBAQClNp+kdz+Nj8XpEp9kaumWxDZ3eFYJpQKz9ddD5e5OzUeCm103
      ZIXQIxc0eVtMCatnRr3dnZRCAxGjwbqoB3eKt29/XSQffVv+odbyw0WdkQOIbntC
      Qry8YdcBZ+8LjI/N7M2krmjmoSLmLwU2V4aNKf0YMLR5Krmah3Ik31jmYCSvwTnv
      6mx6pr2pTJ82JavhTEIIt/fAYq1RYhkM1CXoBL+yhEoDanN7TzC94skfS3VV+f53
      J9SkUxTYcy1Rw0k3VXfxWwy+cSKEPREl7I6k0YeKtDEVAgBIEYM/L1S69RXTLuji
      rwnqSRjOquzkAkD31BE961KZCxeYGrhxaR4PAgMBAAGjUjBQMA4GA1UdDwEB/wQE
      AwIEsDAdBgNVHQ4EFgQU/qDdB6ii6icQ8wGMXvy1jfE4xtUwHwYDVR0jBBgwFoAU
      scRp5lujBKfYl6OLO7+5arIyQjwwDQYJKoZIhvcNAQEFBQADggEBACmxg1hvL6+7
      a+lFTARoxainBx5gxdZ9omSb0L+qL+4PDvg/+KHzKsDnMCrcU6M4YP5n0EDKmGa6
      4lY8fbET4tt7juJg6ixb95/760Th0vuctwkGr6+D6ETTfqyHnrbhX3lAhnB+0Ja7
      o1gv4CWxh1I8aRaTXdpOHORvN0SMXdcrlCys2vrtOl+LjR2a3kajJO6eQ5leOdzF
      QlZfOPhaLWen0e2BLNJI0vsC2Fa+2LMCnfC38XfGALa5A8e7fNHXWZBjXZLBCza3
      rEs9Mlh2CjA/ocSC/WxmMvd+Eqnt/FpggRy+F8IZSRvBaRUCtGE1lgDmu6AFUxce
      R4POrT2xz8ChADEA




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   Block options in CoAP messages can contain fields, SZX, M and NUM
   which are not affected by BRSKI.

   Let's assume that the cacerts message will need to be broken up to 3
   messages.  The first Block2 will be:

     Ver = 1
     T = 2 (ACK)
     Code = 0x21 (2.01 success message.
            TODO: Do we need to create a 0x200 respond code.)
     Options
       Option1 (Content-Format)
         Option Delta = 0xC
         Option Length = 0xD
         Extended Option Length = 0x09
         Option Value = <number for application/pkcs7-mime;
                         smime-type=certs-only>
                   (TODO: We need a new CoAP IANA registered value
                   application/pkcs7-mime, application/csrattrs,
                   application/pkcs10, application/pkcs8,
                   application/pkcs12 )
       Option2 (Block2)
         Option Delta = 0xD
         Option Length = 0x1
         Extended Option Delta = 0x16
         Option Value = 0x0D
     Payload = MIIMOQYJKoZIhvcNAQcCoIIMKjCCDCYC \
               AQExADALBgkqhkiG9w0BBwGgggwMMIIC... (512 bytes)

   The second Block2:





















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     Ver = 1
     T = 2 (means ACK)
     Code = 0x21
     Options
       Option1 (Content-Format)
         Option Delta = 0xC
         Option Length = 0xD
         Extended Option Length = 0x09
         Option Value = <number for application/pkcs7-mime;
                        smime-type=certs-only>
                   (TODO: We need a new CoAP IANA registered value
                   application/pkcs7-mime, application/csrattrs,
                   application/pkcs10, application/pkcs8,
                   application/pkcs12 )
       Option2 (Block2)
         Option Delta = 0xD
         Option Length = 0x1
         Extended Option Delta = 0x16
         Option Value = 0x1D
     Payload = ... (512 bytes)

   The third and final Block2:

     Ver = 1
     T = 2 (means ACK)
     Code = 0x21
     Options
       Option1 (Content-Format)
         Option Delta = 0xC
         Option Length = 0xD
         Extended Option Length = 0x09
         Option Value = <number for application/pkcs7-mime;
                         smime-type=certs-only>
                   (TODO: We need a new CoAP IANA registered value
                   application/pkcs7-mime, application/csrattrs,
                   application/pkcs10, application/pkcs8,
                   application/pkcs12 )
       Option2 (Block2)
         Option Delta = 0xD
         Option Length = 0x1
         Extended Option Delta = 0x16
         Option Value = 0x25
     Payload = ...








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6.2.  Fragmented request example with Block1

6.3.  Fragmented request/response example with Block1, Size1 and Block2

7.  Proxying

   [[TODO: This section to be populated.  It will address how proxying
   can take place by an entity that resides at the edge of the CoAP
   network, such as the Registrar, and can reach the BRSKI server
   residing in a traditional "TCP setting".  ]]

8.  CoAP Parameters

   [[TODO: This section to be populated.  It will address transmission
   parameters for BRSKI described in sections 4.7 and 4.8 of the CoAP
   draft.  BRSKI does not impose any unique parameters that affect the
   CoAP parameters in Table 2 and 3 in the CoAP draft but the ones in
   CoAP could be affecting BRSKI.  For example the processing delay of
   CAs could be less then 2s, but in this case they should send an CoAP
   ACK every 2s while processing. ]]

9.  Security Considerations

   [[TODO: This section to be populated.  This document describes an
   existing protocol moved to CoAP and there should not be additional
   security concerns added beyond the protocol's or CoAP's specifics
   security considerations.]]

10.  Update Tracking

   -01:

      Added more binding usecases and Block examples subsections to be
      expanded on later.  Made various text improvements and
      clarifications.

      Added two clarifying sentences in the relevant sections to address
      Brian C.'s comments and explain that message fragmentation can be
      avoided to a degree in DTLS and that fragments of block transfers
      should be confirmed to prevent exacerbated fragment loss in
      constrained networks.

11.  Normative References








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   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., and S.
              Bjarnason, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-03 (work in progress), June 2016.

   [I-D.ietf-core-block]
              Bormann, D. and Z. Shelby, "Block-wise transfers in CoAP",
              draft-ietf-core-block-20 (work in progress), April 2016.

   [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>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <http://www.rfc-editor.org/info/rfc7030>.

   [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>.

Authors' Addresses

   Max Pritikin
   Cisco Systems

   Email: pritikin@cisco.com


   Panos Kampanakis
   Cisco Systems

   Email: pkampana@cisco.com









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