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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 anima S. Kumar 3 Internet-Draft Philips Lighting Research 4 Intended status: Standards Track P. van der Stok 5 Expires: May 2, 2017 Consultant 6 October 29, 2016 8 EST based on DTLS secured CoAP (EST-coaps) 9 draft-vanderstok-core-coap-est-00 11 Abstract 13 Low-resource devices in a Low-power and Lossy Network (LLN) can 14 operate in a mesh network using the IPv6 over Low-power Personal Area 15 Networks (6LoWPAN) and IEEE 802.15.4 link-layer standards. 16 Provisioning these devices in a secure manner with keys (often called 17 security bootstrapping) used to encrypt and authenticate messages is 18 the subject of Bootstrapping of Remote Secure Key Infrastructures 19 (BRSKI) [I-D.ietf-anima-bootstrapping-keyinfra]. Enrollment over 20 Secure Transport (EST) [RFC7030], based on TLS and HTTP, is used for 21 BRSKI. This document defines how low-resource devices are expected 22 to use EST over DTLS and CoAP. 6LoWPAN fragmentation management and 23 minor extensions to CoAP are needed to enable EST over DTLS-secured 24 CoAP (EST-coaps). 26 Note 28 Many of the concepts in this document are taken over from [RFC7030]. 29 Consequently, much text is directly traceable to [RFC7030]. The same 30 document structure is followed to point out the differences and 31 commonalities between EST and EST-coaps. 33 Status of This Memo 35 This Internet-Draft is submitted in full conformance with the 36 provisions of BCP 78 and BCP 79. 38 Internet-Drafts are working documents of the Internet Engineering 39 Task Force (IETF). Note that other groups may also distribute 40 working documents as Internet-Drafts. The list of current Internet- 41 Drafts is at http://datatracker.ietf.org/drafts/current/. 43 Internet-Drafts are draft documents valid for a maximum of six months 44 and may be updated, replaced, or obsoleted by other documents at any 45 time. It is inappropriate to use Internet-Drafts as reference 46 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on May 2, 2017. 50 Copyright Notice 52 Copyright (c) 2016 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (http://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 68 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 69 2. Operational Scenarios Overview . . . . . . . . . . . . . . . 4 70 3. Protocol Design and Layering . . . . . . . . . . . . . . . . 5 71 3.1. CoAP response codes . . . . . . . . . . . . . . . . . . . 7 72 3.2. Message fragmentation using Block . . . . . . . . . . . . 7 73 3.3. CoAP message headers . . . . . . . . . . . . . . . . . . 8 74 4. Protocol Exchange Details . . . . . . . . . . . . . . . . . . 9 75 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 76 6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 77 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 78 8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 12 79 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 80 9.1. Normative References . . . . . . . . . . . . . . . . . . 12 81 9.2. Informative References . . . . . . . . . . . . . . . . . 13 82 Appendix A. Operational Scenario Example Messages . . . . . . . 14 83 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 85 1. Introduction 87 IPv6 over Low-power Wireless Personal Area Networks (6LoWPANs) 88 [RFC4944] on IEEE 802.15.4 [ieee802.15.4] wireless networks is 89 becoming common in many professional application domains such as 90 lighting controls. However commissioning of such networks suffers 91 from a lack of standardized secure bootstrapping mechanisms for these 92 networks. 94 Although IEEE 802.15.4 defines how security can be enabled between 95 nodes within a single mesh network, it does not specify the 96 provisioning and management of the keys. Therefore securing a 97 6LoWPAN network with devices from multiple manufacturers with 98 different provisioning techniques is often tedious and time 99 consuming. 101 Bootstrapping of Remote Secure Infrastructures (BRSKI) 102 [I-D.ietf-anima-bootstrapping-keyinfra] addresses the issue of 103 bootstrapping networked devices in the context of Autonomic 104 Networking Integrated Model and Approach (ANIMA). However, BRSKI has 105 not been developed specifically for low-resource devices in 106 constrained networks. These networks use DTLS [RFC6347], CoAP 107 [RFC7252], and UDP instead of TLS [RFC5246], HTTP [RFC7230] and TCP. 108 BRSKI relies on Enrollment over Secure Transport (EST) [RFC7030] for 109 the provisioning of the operational domain certificates. Replacing 110 the EST invocations of TLS and HTTP by DTLS and CoAP invocations 111 enables applying BRSKI on CoAP-based low-resource devices. 113 The Figure 1 below shows the EST-coaps architecture. 115 +---------------------------------------------------------+ 116 | | 117 | EST request/response messages | 118 | | 119 +---------------------------------------------------------+ 120 | | 121 | CoAP for message transfer and signaling | 122 | | 123 +---------------------------------------------------------+ 124 | | 125 | DTLS for transport security | 126 | | 127 +---------------------------------------------------------+ 128 | | 129 | UDP for transport | 130 | | 131 +---------------------------------------------------------+ 133 Figure 1: EST-coaps protocol layers 135 Although EST-coaps paves the way for the utilization of BRSKI for 136 constrained devices on constrained networks, some devices will not 137 have enough resources to handle the large payloads that come with 138 EST-coaps. It is up to the network designer to decide which devices 139 execute the BRSKI protocol and which not. 141 EST-coaps is designed for use in professional control networks such 142 as lighting. The autonomic bootstrapping is interesting because it 143 reduces the manual intervention during the commissioning of the 144 network. Typing in passwords is contrary to this wish. Therefore, 145 the password authentication of EST is not supported in EST-coaps. 147 In the constrained devices context it is very unlikely that full PKI 148 request messages will be used. For that reason, full PKI messages 149 are not supported in EST-coaps. 151 Because the relatively large messages involved in EST cannot be 152 readily transported over constrained (6LoWPAN, LLN) wireless 153 networks, this document defines the use of CoAP Block-Wise Transfer 154 ("Block") [RFC7959] combined with DTLS to fragment EST messages at 155 the application layer. 157 1.1. Terminology 159 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 160 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 161 document are to be interpreted as described in [RFC2119]. 163 All the terminology from EST [RFC7030] is included in this document 164 by reference. 166 2. Operational Scenarios Overview 168 Only the differences to EST with respect to operational scenarios are 169 described in this section. EST-coaps server authentication differs 170 from EST as follows: 172 o Replacement of TLS by DTLS and HTTP by CoAP, resulting in: 174 * DTLS-secured CoAP sessions between EST-coaps client and EST- 175 coaps server. 177 o Only certificate-based client authentication is supported, with as 178 result: 180 * The EST-coaps client does not support manual authentication (as 181 described in Section 4.4.1 of [RFC7030]) 183 * The EST-coaps client does not support authentication at the 184 application layer. 186 o EST-coaps does not support full PKI request messages [RFC5272]. 188 The following EST-coaps protocol parts are supported as described for 189 the equivalent EST parts: 191 1. Request of client certificates by submitting a enrollment request 192 to EST-coaps server. 194 2. Renewal of existing client certificates by submitting a re- 195 enrollment request to EST-coaps server. 197 3. Request of certificate with key pair generated by EST-coaps 198 server. 200 4. The EST-coaps client can request the attributes needed for 201 enrollment before the enrollment request is issued" 203 3. Protocol Design and Layering 205 The EST-coaps protocol design follows closely the EST design, 206 excluding some aspects that are not relevant for automatic 207 bootstrapping of constrained devices within a professional context. 208 The parts supported by EST-coaps are: 210 Message types: 212 * Simple PKI messages. 214 * CA certificate retrieval. 216 * CSR Attributes Request. 218 * Server-generated key request. 220 CoAP with Block-Wise Transfer: 222 * CoAP Block-Wise Transfer header Options for control of the 223 transfer of larger EST messages. 225 DTLS for transport security: 227 * Authentication of the EST-coaps server. 229 * Authentication of the EST-coaps client. 231 * Communication integrity and confidentiality. 233 * Channel-binding information for linking proof-of-identity with 234 message-based proof-of-possession (OPTIONAL). 236 Given that CoAP and DTLS can provide proof of identity for EST-coaps 237 clients and server, simple PKI messages can be used conformant to 238 section 3.1 of [RFC5272]. EST-coaps supports the certificate types 239 and Trust Anchors (TA) that are specified for EST in section 3 of 240 [RFC7030]. 242 The EST-coaps server URI is identical to the EST URI (except for 243 replacing the scheme https by coaps): 245 coaps://www.example.com/.well-known/est 246 coaps://www.example.com/.well-known/est/arbitraryLabel1 248 See Figure 5 in section 3.2.2 of [RFC7030] for the path-suffixes 249 (operations) that are supported by EST. 251 EST-coaps uses CoAP to transfer EST messages, aided by Block-Wise 252 Transfer [RFC7959] to transport CoAP messages in blocks thus avoiding 253 (excessive) 6LoWPAN fragmentation of UDP datagrams. The use of 254 "Block" is specified in Section 3.2. 256 The content-format (media type equivalent) of the CoAP message 257 determines which EST message is transported in the CoAP payload. The 258 media types specified in the HTTP Content-Type header(see section 259 3.2.2 of [RFC7030]) are in EST-coaps specified by the Content-Format 260 Option (12) of CoAP. The combination of URI path-suffix and content- 261 format used MUST map to an allowed combination of path-suffix and 262 media type as defined for EST. 264 EST-coaps is designed for use between low-resource devices using CoAP 265 and hence does not need to send base64-encoded data. Simple binary 266 coding is more efficient (30% less payload compared to base64) and 267 well supported by CoAP. Therefore, the content formats specification 268 in Section 5 requires the use of binary encoding for all EST-coaps 269 CoAP payloads. 271 The functions of TLS specified for EST are in EST-coaps mapped to the 272 equivalent DTLS functions. However, DTLS sessions SHOULD remain open 273 for persistent EST-coaps connections to reduce storage load. For 274 example, a cacerts request followed by an enrollments request SHOULD 275 use the same DTLS session. 277 The mandatory cipher suite for DTLS is 278 TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 defined in [RFC7251] which is the 279 mandatory-to-implement cipher suite in CoAP. Additionally the curve 280 secp256r1 MUST be supported [RFC4492]; this curve is equivalent to 281 the NIST P-256 curve. The hash algorithm is SHA-256. DTLS 282 implementations MUST use the Supported Elliptic Curves and Supported 283 Point Formats Extensions [RFC4492]; the uncompressed point format 284 MUST be supported; [RFC6090] can be used as an implementation method. 286 3.1. CoAP response codes 288 Section 5.9 of [RFC7252] specifies the mapping of HTTP response codes 289 to CoAP response codes. Every time the HTTP response code 200 is 290 specified in [RFC7030] in response to a GET request, in EST-coaps the 291 equivalent CoAP response code 2.05 MUST be used. Response code HTTP 292 202 in EST is mapped as indicated below; while other HTTP 2xx 293 response codes are not used by EST. For the following HTTP 4xx error 294 codes that may occur: 400, 401, 403, 404, 405, 406, 412, 413, 415 ; 295 the equivalent CoAP response code for EST-coaps is 4.xx. For the 296 HTTP 5xx error codes: 500, 501, 502, 503, 504 the equivalent CoAP 297 response code is 5.xx. 299 HTTP response code 202 needs a different treatment from the one 300 described for [RFC7030]. A new CoAP response code 2.06 is needed. 301 When the EST over CoAP request cannot be treated immediately, a CoAP 302 response code 2.06 Delayed is returned with Content-Format: 303 application/link-format described in [RFC6690]. The payload of the 304 response contains a link to receive the delayed response. 305 ALTERNATIVE (to discuss) : a 2.06 Delayed response without payload 306 and the link to receive the delayed response indicated using the 307 Location-Path and Location-Query Options. 309 The waiting client may send GET requests to the returned link. When 310 the response is not available, the server returns response code 2.06 311 with again the link for the client to query. When the response is 312 available, the server returns the response code 2.05 Content with a 313 payload containing the requested response in the appropriate content 314 format. 316 3.2. Message fragmentation using Block 318 DTLS defines fragmentation only for the handshake part and not for 319 secure data exchange (DTLS records). [RFC6347] states "Each DTLS 320 record MUST fit within a single datagram". In order to avoid using 321 IP fragmentation, which is not supported by 6LoWPAN, invokers of the 322 DTLS record layer MUST size DTLS records so that they fit within any 323 Path MTU estimates obtained from the record layer. In addition, 324 invokers residing on a 6LoWPAN over IEEE 802.15.4 network SHOULD 325 attempt to size CoAP messages such that each DTLS record will fit 326 within one or two IEEE 802.15.4 frames only by choosing the 327 appropriate block sizes. 329 Certificates can vary greatly in size dependent on signature 330 algorithms and key sizes. For a 256-bit curve, common ECDSA sizes 331 fluctuate between 500 bytes and 1 KB. Some EST messages may be 332 several kilobytes in size. Given non-existence of IP fragmentation 333 in 6LoWPAN networks and its 1280 bytes MTU, EST-coaps needs to be 334 able to fragment EST messages into multiple DTLS datagrams with each 335 DTLS datagram containing a block of CoAP payload data. Further 336 considering the small payload size available to a CoAP message, which 337 can be as low as 68 bytes in case the message needs to fit into a 338 single IEEE 802.15.4 frame, fine-grained fragmentation of EST 339 messages is essential. 341 For CoAP, [RFC7959] specifies the "Block1" option for fragmentation 342 of the request payload and the "Block2" option for fragmentation of 343 the return payload. The CoAP client MAY specify the Block1 size and 344 MAY also specify the Block2 size. The CoAP server MAY specify the 345 Block2 size, but not the Block1 size. 347 Examples of fragmented messages are shown in Appendix A. 349 3.3. CoAP message headers 351 EST-coaps uses CoAP payload blocks that each fit in a single DTLS 352 record i.e. UDP datagram without causing IP fragmentation. The 353 returned CoAP response codes are specified in Section 3.1. The CoAP 354 Token value is not specified by EST-coaps and may be chosen by the 355 CoAP client according to [RFC7252]. 357 An example HTTP request message cacerts in EST will look like: 359 REQ: 360 GET /.well-known/est/cacerts HTTP/1.1 361 Host: 192.0.2.1:8085 362 Accept: */* 364 RES: 365 HTTP/1.1 200 OK 366 Status: 200 OK 367 Content-Type: application/pkcs7-mime 368 Content-Transfer-Encoding : base64 369 Content-Length: 4246 370 payload 372 The corresponding EST-coaps request looks like: 374 REQ: 375 GET coaps://[192.0.2.1:8085]/.well-known/est/cacerts 377 RES: 378 2.05 Content (Content-Format: application/pkcs7-mime) 379 {payload} 381 4. Protocol Exchange Details 383 The EST-coaps client MUST be configured with an implicit TA database 384 or an explicit TA database. The authentication of the EST-coaps 385 server by the EST-coaps client is based on Certificate authentication 386 in the DTLS handshake. 388 The authentication of the EST-coaps client is based on client 389 certificate in the DTLS handshake. This can either be 391 o DTLS with a previously issued client certificate (e.g., an 392 existing certificate issued by the EST CA); 394 o DTLS with a previously installed certificate (e.g., manufacturer- 395 installed certificate or a certificate issued by some other 396 party); 398 The details on checking the validity of the certificates are 399 identical to EST. 401 The other protocol aspects such as simple enrollment (re-enrollment), 402 certificate attributes and CA certificate request are similar to EST 403 with the exception that these are performed on coaps (CoAP+DTLS) as 404 the transport. The required content-formats for these request and 405 response messages are defined in Section 5. The CoAP response codes 406 are defined in Section 3.1. 408 EST-coaps does not support full PKI Requests. Consequently, the 409 fullcmc request of section 4.3 of [RFC7030] and response MUST NOT be 410 supported by EST-coaps. 412 5. IANA Considerations 414 Additions to the sub-registry "CoAP Content-Formats", within the 415 "CoRE Parameters" registry are needed for the below media types. 416 These can be registered either in the Expert Review range (0-255) or 417 IETF Review range (256-9999). 419 1. 421 * application/pkcs7-mime 423 * Type name: application 425 * Subtype name: pkcs7-mime 427 * smime-type: certs-only 428 * ID: TBD1 430 * Required parameters: None 432 * Optional parameters: None 434 * Encoding considerations: Binary 436 * Security considerations: As defined in this specification 438 * Published specification: [RFC5751] 440 * Applications that use this media type: ANIMA Bootstrap (BRSKI) 441 and EST 443 2. 445 * application/pkcs8 447 * Type name: application 449 * Subtype name: pkcs8 451 * ID: TBD2 453 * Required parameters: None 455 * Optional parameters: None 457 * Encoding considerations: Binary 459 * Security considerations: As defined in this specification 461 * Published specification: [RFC5958] 463 * Applications that use this media type: ANIMA Bootstrap (BRSKI) 464 and EST 466 3. 468 * application/csrattrs 470 * Type name: application 472 * Subtype name: csrattrs 474 * ID: TBD3 475 * Required parameters: None 477 * Optional parameters: None 479 * Encoding considerations: Binary 481 * Security considerations: As defined in this specification 483 * Published specification: [RFC7030] 485 * Applications that use this media type: ANIMA Bootstrap (BRSKI) 486 and EST 488 4. 490 * application/pkcs10 492 * Type name: application 494 * Subtype name: pkcs10 496 * ID: TBD4 498 * Required parameters: None 500 * Optional parameters: None 502 * Encoding considerations: binary 504 * Security considerations: As defined in this specification 506 * Published specification: [RFC5967] 508 * Applications that use this media type: ANIMA bootstrap (BRSKI) 509 and EST 511 Additions to the sub-registry "CoAP Response Code", within the "CoRE 512 Parameters" registry are needed for the following response codes: 514 o Code: 2.06 516 o Description: Delayed 518 o Reference: this document 520 6. Security Considerations 522 The security considerations mentioned in EST applies also to EST- 523 coaps. 525 7. Acknowledgements 527 The authors are very grateful to Klaus Hartke for his detailed 528 explanations on the use of Block with DTLS. The authors would like 529 to thank Esko Dijk and Michael Verschoor for the valuable discussions 530 that helped in shaping the solution. 532 8. Change Log 534 9. References 536 9.1. Normative References 538 [I-D.ietf-anima-bootstrapping-keyinfra] 539 Pritikin, M., Richardson, M., Behringer, M., and S. 540 Bjarnason, "Bootstrapping Remote Secure Key 541 Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping- 542 keyinfra-03 (work in progress), June 2016. 544 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 545 Requirement Levels", BCP 14, RFC 2119, 546 DOI 10.17487/RFC2119, March 1997, 547 . 549 [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. 550 Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites 551 for Transport Layer Security (TLS)", RFC 4492, 552 DOI 10.17487/RFC4492, May 2006, 553 . 555 [RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS 556 (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008, 557 . 559 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 560 Mail Extensions (S/MIME) Version 3.2 Message 561 Specification", RFC 5751, DOI 10.17487/RFC5751, January 562 2010, . 564 [RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, 565 DOI 10.17487/RFC5958, August 2010, 566 . 568 [RFC5967] Turner, S., "The application/pkcs10 Media Type", RFC 5967, 569 DOI 10.17487/RFC5967, August 2010, 570 . 572 [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic 573 Curve Cryptography Algorithms", RFC 6090, 574 DOI 10.17487/RFC6090, February 2011, 575 . 577 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 578 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 579 January 2012, . 581 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 582 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 583 . 585 [RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., 586 "Enrollment over Secure Transport", RFC 7030, 587 DOI 10.17487/RFC7030, October 2013, 588 . 590 [RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES- 591 CCM Elliptic Curve Cryptography (ECC) Cipher Suites for 592 TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014, 593 . 595 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 596 Application Protocol (CoAP)", RFC 7252, 597 DOI 10.17487/RFC7252, June 2014, 598 . 600 [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in 601 the Constrained Application Protocol (CoAP)", RFC 7959, 602 DOI 10.17487/RFC7959, August 2016, 603 . 605 9.2. Informative References 607 [ieee802.15.4] 608 Institute of Electrical and Electronics Engineers, , "IEEE 609 Standard 802.15.4-2006", 2006. 611 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 612 "Transmission of IPv6 Packets over IEEE 802.15.4 613 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 614 . 616 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 617 (TLS) Protocol Version 1.2", RFC 5246, 618 DOI 10.17487/RFC5246, August 2008, 619 . 621 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 622 Specifications and Registration Procedures", BCP 13, 623 RFC 6838, DOI 10.17487/RFC6838, January 2013, 624 . 626 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 627 Protocol (HTTP/1.1): Message Syntax and Routing", 628 RFC 7230, DOI 10.17487/RFC7230, June 2014, 629 . 631 Appendix A. Operational Scenario Example Messages 633 This appendix provides detailed examples of the messages using DTLS 634 and BLOCK option Block2. The minimum PMTU is 1280 bytes, which is 635 the example value assumed for the DTLS datagram size. The example 636 block length is taken as 64 which gives an SZX value of 2. 638 The following is an example of a valid /cacerts exchange. 640 During the initial DTLS handshake, the client can ignore the optional 641 server-generated "certificate request" and can instead proceed with 642 the CoAP GET request. The content length of the cacerts response in 643 appendix A.1 of [RFC7030] is 4246 bytes using base64. This leads to 644 a length of 3185 bytes in binary. The CoAP message adds around 10 645 bytes, the DTLS record 29 bytes. 647 To avoid IP fragmentation, the CoAP block option is used and an MTU 648 of 127 is assumed to stay within one IEEE 802.15.4 packet. To stay 649 below the MTU of 127, the payload is split in 50 packets with a 650 payload of 64 bytes each. Fifty times the client sends an IPv6 651 packet containing the UDP datagram with the DTLS record that 652 encapsulates the CoAP Request. The server returns an IPv6 packet 653 containing the UDP datagram with the DTLS record that encapsulates 654 the CoAP response. 656 The CoAP request-response exchange with block option is shown below. 657 Block option is shown in a decomposed way indicating the kind of 658 Block option (2 in this case because used in the response) followed 659 by a colon, and then the block number (NUM), the more bit (M = 0 660 means last block), and block size exponent (2**(SZX+4)) separated by 661 slashes. The Length 64 is used with SZX= 2 to avoid IP 662 fragmentation. 664 The CoAP Request is sent with confirmable (CON) option and the 665 content format of the Response is /application/cacerts. 667 GET [192.0.2.1:8085]/.well-known/est/cacerts --> 668 <-- (2:0/1/64) 2.05 Content 669 GET URI (2:1/1/64) --> 670 <-- (2:1/1/64) 2.05 Content 671 | 672 | 673 | 674 GET URI (2:49/1/64) --> 675 <-- (2:49/0/64) 2.05 Content 677 Authors' Addresses 679 Sandeep S. Kumar 680 Philips Lighting Research 681 High Tech Campus 7 682 Eindhoven 5656 AE 683 NL 685 Email: ietf@sandeep.de 687 Peter van der Stok 688 Consultant 690 Email: consultancy@vanderstok.org