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Versions: 00

anima                                                           S. Kumar
Internet-Draft                                 Philips Lighting Research
Intended status: Standards Track                         P. van der Stok
Expires: May 2, 2017                                          Consultant
                                                        October 29, 2016


               EST based on DTLS secured CoAP (EST-coaps)
                   draft-vanderstok-core-coap-est-00

Abstract

   Low-resource devices in a Low-power and Lossy Network (LLN) can
   operate in a mesh network using the IPv6 over Low-power Personal Area
   Networks (6LoWPAN) and IEEE 802.15.4 link-layer standards.
   Provisioning these devices in a secure manner with keys (often called
   security bootstrapping) used to encrypt and authenticate messages is
   the subject of Bootstrapping of Remote Secure Key Infrastructures
   (BRSKI) [I-D.ietf-anima-bootstrapping-keyinfra].  Enrollment over
   Secure Transport (EST) [RFC7030], based on TLS and HTTP, is used for
   BRSKI.  This document defines how low-resource devices are expected
   to use EST over DTLS and CoAP. 6LoWPAN fragmentation management and
   minor extensions to CoAP are needed to enable EST over DTLS-secured
   CoAP (EST-coaps).

Note

   Many of the concepts in this document are taken over from [RFC7030].
   Consequently, much text is directly traceable to [RFC7030].  The same
   document structure is followed to point out the differences and
   commonalities between EST and EST-coaps.

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 2, 2017.



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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Operational Scenarios Overview  . . . . . . . . . . . . . . .   4
   3.  Protocol Design and Layering  . . . . . . . . . . . . . . . .   5
     3.1.  CoAP response codes . . . . . . . . . . . . . . . . . . .   7
     3.2.  Message fragmentation using Block . . . . . . . . . . . .   7
     3.3.  CoAP message headers  . . . . . . . . . . . . . . . . . .   8
   4.  Protocol Exchange Details . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Operational Scenario Example Messages  . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   IPv6 over Low-power Wireless Personal Area Networks (6LoWPANs)
   [RFC4944] on IEEE 802.15.4 [ieee802.15.4] wireless networks is
   becoming common in many professional application domains such as
   lighting controls.  However commissioning of such networks suffers
   from a lack of standardized secure bootstrapping mechanisms for these
   networks.

   Although IEEE 802.15.4 defines how security can be enabled between
   nodes within a single mesh network, it does not specify the
   provisioning and management of the keys.  Therefore securing a
   6LoWPAN network with devices from multiple manufacturers with



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   different provisioning techniques is often tedious and time
   consuming.

   Bootstrapping of Remote Secure Infrastructures (BRSKI)
   [I-D.ietf-anima-bootstrapping-keyinfra] addresses the issue of
   bootstrapping networked devices in the context of Autonomic
   Networking Integrated Model and Approach (ANIMA).  However, BRSKI has
   not been developed specifically for low-resource devices in
   constrained networks.  These networks use DTLS [RFC6347], CoAP
   [RFC7252], and UDP instead of TLS [RFC5246], HTTP [RFC7230] and TCP.
   BRSKI relies on Enrollment over Secure Transport (EST) [RFC7030] for
   the provisioning of the operational domain certificates.  Replacing
   the EST invocations of TLS and HTTP by DTLS and CoAP invocations
   enables applying BRSKI on CoAP-based low-resource devices.

   The Figure 1 below shows the EST-coaps architecture.

   +---------------------------------------------------------+
   |                                                         |
   |    EST request/response messages                        |
   |                                                         |
   +---------------------------------------------------------+
   |                                                         |
   |    CoAP for message transfer and signaling              |
   |                                                         |
   +---------------------------------------------------------+
   |                                                         |
   |    DTLS for transport security                          |
   |                                                         |
   +---------------------------------------------------------+
   |                                                         |
   |    UDP for transport                                    |
   |                                                         |
   +---------------------------------------------------------+


                    Figure 1: EST-coaps protocol layers

   Although EST-coaps paves the way for the utilization of BRSKI for
   constrained devices on constrained networks, some devices will not
   have enough resources to handle the large payloads that come with
   EST-coaps.  It is up to the network designer to decide which devices
   execute the BRSKI protocol and which not.

   EST-coaps is designed for use in professional control networks such
   as lighting.  The autonomic bootstrapping is interesting because it
   reduces the manual intervention during the commissioning of the




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   network.  Typing in passwords is contrary to this wish.  Therefore,
   the password authentication of EST is not supported in EST-coaps.

   In the constrained devices context it is very unlikely that full PKI
   request messages will be used.  For that reason, full PKI messages
   are not supported in EST-coaps.

   Because the relatively large messages involved in EST cannot be
   readily transported over constrained (6LoWPAN, LLN) wireless
   networks, this document defines the use of CoAP Block-Wise Transfer
   ("Block") [RFC7959] combined with DTLS to fragment EST messages at
   the application layer.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   All the terminology from EST [RFC7030] is included in this document
   by reference.

2.  Operational Scenarios Overview

   Only the differences to EST with respect to operational scenarios are
   described in this section.  EST-coaps server authentication differs
   from EST as follows:

   o  Replacement of TLS by DTLS and HTTP by CoAP, resulting in:

      *  DTLS-secured CoAP sessions between EST-coaps client and EST-
         coaps server.

   o  Only certificate-based client authentication is supported, with as
      result:

      *  The EST-coaps client does not support manual authentication (as
         described in Section 4.4.1 of [RFC7030])

      *  The EST-coaps client does not support authentication at the
         application layer.

   o  EST-coaps does not support full PKI request messages [RFC5272].

   The following EST-coaps protocol parts are supported as described for
   the equivalent EST parts:





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   1.  Request of client certificates by submitting a enrollment request
       to EST-coaps server.

   2.  Renewal of existing client certificates by submitting a re-
       enrollment request to EST-coaps server.

   3.  Request of certificate with key pair generated by EST-coaps
       server.

   4.  The EST-coaps client can request the attributes needed for
       enrollment before the enrollment request is issued"

3.  Protocol Design and Layering

   The EST-coaps protocol design follows closely the EST design,
   excluding some aspects that are not relevant for automatic
   bootstrapping of constrained devices within a professional context.
   The parts supported by EST-coaps are:

   Message types:

      *  Simple PKI messages.

      *  CA certificate retrieval.

      *  CSR Attributes Request.

      *  Server-generated key request.

   CoAP with Block-Wise Transfer:

      *  CoAP Block-Wise Transfer header Options for control of the
         transfer of larger EST messages.

   DTLS for transport security:

      *  Authentication of the EST-coaps server.

      *  Authentication of the EST-coaps client.

      *  Communication integrity and confidentiality.

      *  Channel-binding information for linking proof-of-identity with
         message-based proof-of-possession (OPTIONAL).

   Given that CoAP and DTLS can provide proof of identity for EST-coaps
   clients and server, simple PKI messages can be used conformant to
   section 3.1 of [RFC5272].  EST-coaps supports the certificate types



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   and Trust Anchors (TA) that are specified for EST in section 3 of
   [RFC7030].

   The EST-coaps server URI is identical to the EST URI (except for
   replacing the scheme https by coaps):

   coaps://www.example.com/.well-known/est
   coaps://www.example.com/.well-known/est/arbitraryLabel1

   See Figure 5 in section 3.2.2 of [RFC7030] for the path-suffixes
   (operations) that are supported by EST.

   EST-coaps uses CoAP to transfer EST messages, aided by Block-Wise
   Transfer [RFC7959] to transport CoAP messages in blocks thus avoiding
   (excessive) 6LoWPAN fragmentation of UDP datagrams.  The use of
   "Block" is specified in Section 3.2.

   The content-format (media type equivalent) of the CoAP message
   determines which EST message is transported in the CoAP payload.  The
   media types specified in the HTTP Content-Type header(see section
   3.2.2 of [RFC7030]) are in EST-coaps specified by the Content-Format
   Option (12) of CoAP.  The combination of URI path-suffix and content-
   format used MUST map to an allowed combination of path-suffix and
   media type as defined for EST.

   EST-coaps is designed for use between low-resource devices using CoAP
   and hence does not need to send base64-encoded data.  Simple binary
   coding is more efficient (30% less payload compared to base64) and
   well supported by CoAP.  Therefore, the content formats specification
   in Section 5 requires the use of binary encoding for all EST-coaps
   CoAP payloads.

   The functions of TLS specified for EST are in EST-coaps mapped to the
   equivalent DTLS functions.  However, DTLS sessions SHOULD remain open
   for persistent EST-coaps connections to reduce storage load.  For
   example, a cacerts request followed by an enrollments request SHOULD
   use the same DTLS session.

   The mandatory cipher suite for DTLS is
   TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 defined in [RFC7251] which is the
   mandatory-to-implement cipher suite in CoAP.  Additionally the curve
   secp256r1 MUST be supported [RFC4492]; this curve is equivalent to
   the NIST P-256 curve.  The hash algorithm is SHA-256.  DTLS
   implementations MUST use the Supported Elliptic Curves and Supported
   Point Formats Extensions [RFC4492]; the uncompressed point format
   MUST be supported; [RFC6090] can be used as an implementation method.





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3.1.  CoAP response codes

   Section 5.9 of [RFC7252] specifies the mapping of HTTP response codes
   to CoAP response codes.  Every time the HTTP response code 200 is
   specified in [RFC7030] in response to a GET request, in EST-coaps the
   equivalent CoAP response code 2.05 MUST be used.  Response code HTTP
   202 in EST is mapped as indicated below; while other HTTP 2xx
   response codes are not used by EST.  For the following HTTP 4xx error
   codes that may occur: 400, 401, 403, 404, 405, 406, 412, 413, 415 ;
   the equivalent CoAP response code for EST-coaps is 4.xx.  For the
   HTTP 5xx error codes: 500, 501, 502, 503, 504 the equivalent CoAP
   response code is 5.xx.

   HTTP response code 202 needs a different treatment from the one
   described for [RFC7030].  A new CoAP response code 2.06 is needed.
   When the EST over CoAP request cannot be treated immediately, a CoAP
   response code 2.06 Delayed is returned with Content-Format:
   application/link-format described in [RFC6690].  The payload of the
   response contains a link to receive the delayed response.
   ALTERNATIVE (to discuss) : a 2.06 Delayed response without payload
   and the link to receive the delayed response indicated using the
   Location-Path and Location-Query Options.

   The waiting client may send GET requests to the returned link.  When
   the response is not available, the server returns response code 2.06
   with again the link for the client to query.  When the response is
   available, the server returns the response code 2.05 Content with a
   payload containing the requested response in the appropriate content
   format.

3.2.  Message fragmentation using Block

   DTLS defines fragmentation only for the handshake part and not for
   secure data exchange (DTLS records).  [RFC6347] states "Each DTLS
   record MUST fit within a single datagram".  In order to avoid using
   IP fragmentation, which is not supported by 6LoWPAN, invokers of the
   DTLS record layer MUST size DTLS records so that they fit within any
   Path MTU estimates obtained from the record layer.  In addition,
   invokers residing on a 6LoWPAN over IEEE 802.15.4 network SHOULD
   attempt to size CoAP messages such that each DTLS record will fit
   within one or two IEEE 802.15.4 frames only by choosing the
   appropriate block sizes.

   Certificates can vary greatly in size dependent on signature
   algorithms and key sizes.  For a 256-bit curve, common ECDSA sizes
   fluctuate between 500 bytes and 1 KB.  Some EST messages may be
   several kilobytes in size.  Given non-existence of IP fragmentation
   in 6LoWPAN networks and its 1280 bytes MTU, EST-coaps needs to be



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   able to fragment EST messages into multiple DTLS datagrams with each
   DTLS datagram containing a block of CoAP payload data.  Further
   considering the small payload size available to a CoAP message, which
   can be as low as 68 bytes in case the message needs to fit into a
   single IEEE 802.15.4 frame, fine-grained fragmentation of EST
   messages is essential.

   For CoAP, [RFC7959] specifies the "Block1" option for fragmentation
   of the request payload and the "Block2" option for fragmentation of
   the return payload.  The CoAP client MAY specify the Block1 size and
   MAY also specify the Block2 size.  The CoAP server MAY specify the
   Block2 size, but not the Block1 size.

   Examples of fragmented messages are shown in Appendix A.

3.3.  CoAP message headers

   EST-coaps uses CoAP payload blocks that each fit in a single DTLS
   record i.e. UDP datagram without causing IP fragmentation.  The
   returned CoAP response codes are specified in Section 3.1.  The CoAP
   Token value is not specified by EST-coaps and may be chosen by the
   CoAP client according to [RFC7252].

   An example HTTP request message cacerts in EST will look like:

   REQ:
           GET /.well-known/est/cacerts HTTP/1.1
              Host: 192.0.2.1:8085
              Accept: */*

   RES:
           HTTP/1.1 200 OK
           Status: 200 OK
           Content-Type: application/pkcs7-mime
           Content-Transfer-Encoding : base64
           Content-Length: 4246
           payload

   The corresponding EST-coaps request looks like:

   REQ:
           GET coaps://[192.0.2.1:8085]/.well-known/est/cacerts

   RES:
           2.05 Content (Content-Format: application/pkcs7-mime)
           {payload}





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4.  Protocol Exchange Details

   The EST-coaps client MUST be configured with an implicit TA database
   or an explicit TA database.  The authentication of the EST-coaps
   server by the EST-coaps client is based on Certificate authentication
   in the DTLS handshake.

   The authentication of the EST-coaps client is based on client
   certificate in the DTLS handshake.  This can either be

   o  DTLS with a previously issued client certificate (e.g., an
      existing certificate issued by the EST CA);

   o  DTLS with a previously installed certificate (e.g., manufacturer-
      installed certificate or a certificate issued by some other
      party);

   The details on checking the validity of the certificates are
   identical to EST.

   The other protocol aspects such as simple enrollment (re-enrollment),
   certificate attributes and CA certificate request are similar to EST
   with the exception that these are performed on coaps (CoAP+DTLS) as
   the transport.  The required content-formats for these request and
   response messages are defined in Section 5.  The CoAP response codes
   are defined in Section 3.1.

   EST-coaps does not support full PKI Requests.  Consequently, the
   fullcmc request of section 4.3 of [RFC7030] and response MUST NOT be
   supported by EST-coaps.

5.  IANA Considerations

   Additions to the sub-registry "CoAP Content-Formats", within the
   "CoRE Parameters" registry are needed for the below media types.
   These can be registered either in the Expert Review range (0-255) or
   IETF Review range (256-9999).

   1.

       *  application/pkcs7-mime

       *  Type name: application

       *  Subtype name: pkcs7-mime

       *  smime-type: certs-only




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       *  ID: TBD1

       *  Required parameters: None

       *  Optional parameters: None

       *  Encoding considerations: Binary

       *  Security considerations: As defined in this specification

       *  Published specification: [RFC5751]

       *  Applications that use this media type: ANIMA Bootstrap (BRSKI)
          and EST

   2.

       *  application/pkcs8

       *  Type name: application

       *  Subtype name: pkcs8

       *  ID: TBD2

       *  Required parameters: None

       *  Optional parameters: None

       *  Encoding considerations: Binary

       *  Security considerations: As defined in this specification

       *  Published specification: [RFC5958]

       *  Applications that use this media type: ANIMA Bootstrap (BRSKI)
          and EST

   3.

       *  application/csrattrs

       *  Type name: application

       *  Subtype name: csrattrs

       *  ID: TBD3




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       *  Required parameters: None

       *  Optional parameters: None

       *  Encoding considerations: Binary

       *  Security considerations: As defined in this specification

       *  Published specification: [RFC7030]

       *  Applications that use this media type: ANIMA Bootstrap (BRSKI)
          and EST

   4.

       *  application/pkcs10

       *  Type name: application

       *  Subtype name: pkcs10

       *  ID: TBD4

       *  Required parameters: None

       *  Optional parameters: None

       *  Encoding considerations: binary

       *  Security considerations: As defined in this specification

       *  Published specification: [RFC5967]

       *  Applications that use this media type: ANIMA bootstrap (BRSKI)
          and EST

   Additions to the sub-registry "CoAP Response Code", within the "CoRE
   Parameters" registry are needed for the following response codes:

   o  Code: 2.06

   o  Description: Delayed

   o  Reference: this document







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6.  Security Considerations

   The security considerations mentioned in EST applies also to EST-
   coaps.

7.  Acknowledgements

   The authors are very grateful to Klaus Hartke for his detailed
   explanations on the use of Block with DTLS.  The authors would like
   to thank Esko Dijk and Michael Verschoor for the valuable discussions
   that helped in shaping the solution.

8.  Change Log

9.  References

9.1.  Normative References

   [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.

   [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>.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
              <http://www.rfc-editor.org/info/rfc5272>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <http://www.rfc-editor.org/info/rfc5751>.

   [RFC5958]  Turner, S., "Asymmetric Key Packages", RFC 5958,
              DOI 10.17487/RFC5958, August 2010,
              <http://www.rfc-editor.org/info/rfc5958>.




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   [RFC5967]  Turner, S., "The application/pkcs10 Media Type", RFC 5967,
              DOI 10.17487/RFC5967, August 2010,
              <http://www.rfc-editor.org/info/rfc5967>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <http://www.rfc-editor.org/info/rfc6090>.

   [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>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <http://www.rfc-editor.org/info/rfc6690>.

   [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>.

   [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
              CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
              TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
              <http://www.rfc-editor.org/info/rfc7251>.

   [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>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <http://www.rfc-editor.org/info/rfc7959>.

9.2.  Informative References

   [ieee802.15.4]
              Institute of Electrical and Electronics Engineers, , "IEEE
              Standard 802.15.4-2006", 2006.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <http://www.rfc-editor.org/info/rfc4944>.




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   [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>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

Appendix A.  Operational Scenario Example Messages

   This appendix provides detailed examples of the messages using DTLS
   and BLOCK option Block2.  The minimum PMTU is 1280 bytes, which is
   the example value assumed for the DTLS datagram size.  The example
   block length is taken as 64 which gives an SZX value of 2.

   The following is an example of a valid /cacerts exchange.

   During the initial DTLS handshake, the client can ignore the optional
   server-generated "certificate request" and can instead proceed with
   the CoAP GET request.  The content length of the cacerts response in
   appendix A.1 of [RFC7030] is 4246 bytes using base64.  This leads to
   a length of 3185 bytes in binary.  The CoAP message adds around 10
   bytes, the DTLS record 29 bytes.

   To avoid IP fragmentation, the CoAP block option is used and an MTU
   of 127 is assumed to stay within one IEEE 802.15.4 packet.  To stay
   below the MTU of 127, the payload is split in 50 packets with a
   payload of 64 bytes each.  Fifty times the client sends an IPv6
   packet containing the UDP datagram with the DTLS record that
   encapsulates the CoAP Request.  The server returns an IPv6 packet
   containing the UDP datagram with the DTLS record that encapsulates
   the CoAP response.

   The CoAP request-response exchange with block option is shown below.
   Block option is shown in a decomposed way indicating the kind of
   Block option (2 in this case because used in the response) followed
   by a colon, and then the block number (NUM), the more bit (M = 0
   means last block), and block size exponent (2**(SZX+4)) separated by
   slashes.  The Length 64 is used with SZX= 2 to avoid IP
   fragmentation.




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Internet-Draft                  EST-coaps                   October 2016


   The CoAP Request is sent with confirmable (CON) option and the
   content format of the Response is /application/cacerts.


   GET [192.0.2.1:8085]/.well-known/est/cacerts     -->
                 <--   (2:0/1/64) 2.05 Content
       GET URI (2:1/1/64)                           -->
                 <--   (2:1/1/64) 2.05 Content
                         |
                         |
                         |
        GET URI (2:49/1/64)                         -->
                 <--   (2:49/0/64) 2.05 Content


Authors' Addresses

   Sandeep S. Kumar
   Philips Lighting Research
   High Tech Campus 7
   Eindhoven  5656 AE
   NL

   Email: ietf@sandeep.de


   Peter van der Stok
   Consultant

   Email: consultancy@vanderstok.org





















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