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Network Working Group                                        J. Mattsson
Internet-Draft                                               Ericsson AB
Intended status: Informational                            March 13, 2017
Expires: September 14, 2017


            Message Size Overhead of CoAP Security Protocols
                draft-mattsson-core-security-overhead-00

Abstract

   This document analyzes and compares per-packet message size overheads
   when using different security protocols to secure CoAP.  The analyzed
   security protocols are DTLS 1.2, DTLS 1.3, TLS 1.2, TLS 1.3, and
   OSCOAP.  DTLS and TLS are analyzed with and without compression.
   DTLS are analyzed with two different alternatives for header
   compression.

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   This Internet-Draft will expire on September 14, 2017.

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   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.  Overhead of Security Protocols  . . . . . . . . . . . . . . .   2
     2.1.  DTLS 1.2  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  DTLS 1.2 with 6LoWPAN-GHC . . . . . . . . . . . . . . . .   3
     2.3.  DTLS 1.2 with raza-6lo-compressed-dtls  . . . . . . . . .   4
     2.4.  DTLS 1.3  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.5.  DTLS 1.3 with 6LoWPAN-GHC . . . . . . . . . . . . . . . .   5
     2.6.  DTLS 1.3 with raza-6lo-compressed-dtls  . . . . . . . . .   6
     2.7.  TLS 1.2 . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.8.  TLS 1.2 with 6LoWPAN-GHC  . . . . . . . . . . . . . . . .   7
     2.9.  TLS 1.3 . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.10. TLS 1.3 with 6LoWPAN-GHC  . . . . . . . . . . . . . . . .   8
     2.11. OSCOAP  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Overhead with Different Sequence Numbers  . . . . . . . . . .   9
   4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   This document analyzes and compares per-packet message size overheads
   when using different security protocols to secure CoAP over UPD
   [RFC7252] and TCP [I-D.ietf-core-coap-tcp-tls].  The analyzed
   security protocols are DTLS 1.2 [RFC6347], DTLS 1.3
   [I-D.rescorla-tls-dtls13], TLS 1.2 [RFC5246], TLS 1.3
   [I-D.ietf-tls-tls13], and OSCOAP [I-D.ietf-core-object-security].
   The DTLS and TLS record layers are analyzed with and without
   compression.  DTLS are analyzed with two different alternatives
   ([RFC7400] and [raza-6lo-compressed-dtls]) for header compression.

2.  Overhead of Security Protocols

   To enable comparison, all the overhead calculations in this section
   use AES-CCM with a tag length of 8 bytes, a plaintext of 6 bytes, and
   the sequence number '05'.  This follows the example in [RFC7400],
   Figure 16.








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2.1.  DTLS 1.2

   This example is taken directly from [RFC7400], Figure 16.  The nonce
   follow the strict profiling given in [RFC7925].

   DTLS 1.2 Record Layer (35 bytes, 29 bytes overhead):
   17 fe fd 00 01 00 00 00 00 00 05 00 16 00 01 00
   00 00 00 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4
   cb 35 b9

   Content type:
   17
   Version:
   fe fd
   Epoch:
   00 01
   Sequence number:
   00 00 00 00 00 05
   Length:
   00 16
   Nonce:
   00 01 00 00 00 00 00 05
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   DTLS 1.2 gives 29 bytes overhead.

2.2.  DTLS 1.2 with 6LoWPAN-GHC

   Note that the compressed overhead is dependent on the parameters
   epoch, sequence number, and length.  The following is only an
   example.

   Note that the sequence number '01' used in [RFC7400], Figure 15 gives
   an exceptionally small overhead that is not representative at all.

   Note that this header compression is not available when DTLS is
   exchanged over transports that do not use 6LoWPAN together with
   6LoWPAN-GHC.










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   Compressed DTLS 1.2 Record Layer (22 bytes, 16 bytes overhead):
   b0 c3 03 05 00 16 f2 0e ae a0 15 56 67 92 4d ff
   8a 24 e4 cb 35 b9

   Compressed DTLS 1.2 Record Layer Header and Nonce:
   b0 c3 03 05 00 16 f2 0e
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   When compressed with 6LoWPAN-GHC, DTLS 1.2 with the above parameters
   (epoch, sequence number, length) gives 16 bytes overhead.

2.3.  DTLS 1.2 with raza-6lo-compressed-dtls

   Note that the compressed overhead is dependent on the parameters
   epoch and sequence number.  The following is only an example.

   Note that this header compression is not available when DTLS is
   exchanged over transports that do not use 6LoWPAN together with raza-
   6lo-compressed-dtls.

   Compressed DTLS 1.2 Record Layer (19 bytes, 13 bytes overhead):
   90 17 01 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4
   cb 35 b9

   NHC
   90
   Compressed DTLS 1.2 Record Layer Header and Nonce:
   17 01 00 05
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   When compressed with raza-6lo-compressed-dtls, DTLS 1.2 with the
   above parameters (epoch, sequence number) gives 13 bytes overhead.

2.4.  DTLS 1.3

   The only change compared to DTLS 1.2 is that the DTLS 1.3 record
   layer does not have an explicit nonce.








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   DTLS 1.3 Record Layer (27 bytes, 21 bytes overhead):
   17 fe fd 00 01 00 00 00 00 00 05 00 0e ae a0 15
   56 67 92 4d ff 8a 24 e4 cb 35 b9

   Content type:
   17
   Version:
   fe fd
   Epoch:
   00 01
   Sequence number:
   00 00 00 00 00 05
   Length:
   00 0e
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   DTLS 1.3 gives 21 bytes overhead.

2.5.  DTLS 1.3 with 6LoWPAN-GHC

   Note that the overhead is dependent on the parameters epoch, sequence
   number, and length.  The following is only an example.

   Note that this header compression is not available when DTLS is
   exchanged over transports that do not use 6LoWPAN together with
   6LoWPAN-GHC.

   Compressed DTLS 1.3 Record Layer (20 bytes, 14 bytes overhead):
   b0 c3 11 05 00 0e ae a0 15 56 67 92 4d ff 8a 24
   e4 cb 35 b9

   Compressed DTLS 1.3 Record Layer Header and Nonce:
   b0 c3 11 05 00 0e
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   When compressed with 6LoWPAN-GHC, DTLS 1.3 with the above parameters
   (epoch, sequence number, length) gives 14 bytes overhead.








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2.6.  DTLS 1.3 with raza-6lo-compressed-dtls

   Note that the compressed overhead is dependent on the parameters
   epoch and sequence number.  The following is only an example.

   Note that this header compression is not available when DTLS is
   exchanged over transports that do not use 6LoWPAN together with raza-
   6lo-compressed-dtls.

   Note that this header compression is not available when DTLS is
   exchanged over transports that do not use 6LoWPAN together with raza-
   6lo-compressed-dtls.

   Compressed DTLS 1.3 Record Layer (19 bytes, 13 bytes overhead):
   90 17 01 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4
   cb 35 b9

   NHC
   90
   Compressed DTLS 1.3 Record Layer Header and Nonce:
   17 01 00 05
   c3 03 05 00 16 f2 0e
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   When compressed with raza-6lo-compressed-dtls, DTLS 1.3 with the
   above parameters (epoch, sequence number) gives 13 bytes overhead.

2.7.  TLS 1.2

   The changes compared to DTLS 1.2 is that the TLS 1.2 record layer
   does not have epoch and sequence number, and that the version is
   different.
















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   TLS 1.2 Record Layer (27 bytes, 21 byte overhead):
   17 03 03 00 16 00 00 00 00 00 00 00 05 ae a0 15
   56 67 92 4d ff 8a 24 e4 cb 35 b9

   Content type:
   17
   Version:
   03 03
   Length:
   00 16
   Nonce:
   00 00 00 00 00 00 00 05
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   TLS 1.2 gives 21 bytes overhead.

2.8.  TLS 1.2 with 6LoWPAN-GHC

   Note that the overhead is dependent on the parameters epoch, sequence
   number, and length.  The following is only an example.

   Note that this header compression is not available when TLS is
   exchanged over transports that do not use 6LoWPAN together with
   6LoWPAN-GHC.

   Compressed TLS 1.2 Record Layer (23 bytes, 17 bytes overhead):
   05 17 03 03 00 16 85 0f 05 ae a0 15 56 67 92 4d
   ff 8a 24 e4 cb 35 b9

   Compressed TLS 1.2 Record Layer Header and Nonce:
   05 17 03 03 00 16 85 0f 05
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   When compressed with 6LoWPAN-GHC, TLS 1.2 with the above parameters
   (epoch, sequence number, length) gives 17 bytes overhead.

2.9.  TLS 1.3

   The change compared to TLS 1.2 is that the TLS 1.3 record layer uses
   a different version.





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   TLS 1.3 Record Layer (27 bytes, 21 byte overhead):
   17 03 01 00 16 00 00 00 00 00 00 00 05 ae a0 15
   56 67 92 4d ff 8a 24 e4 cb 35 b9

   Content type:
   17
   Version:
   03 01
   Length:
   00 16
   Nonce:
   00 00 00 00 00 00 00 05
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   TLS 1.3 gives 21 bytes overhead.

2.10.  TLS 1.3 with 6LoWPAN-GHC

   Note that the overhead is dependent on the parameters epoch, sequence
   number, and length.  The following is only an example.

   Note that this header compression is not available when TLS is
   exchanged over transports that do not use 6LoWPAN together with
   6LoWPAN-GHC.

   Compressed TLS 1.3 Record Layer (23 bytes, 17 bytes overhead):
   02 17 03 c3 01 16 85 0f 05 ae a0 15 56 67 92 4d
   ff 8a 24 e4 cb 35 b9

   Compressed TLS 1.3 Record Layer Header and Nonce:
   02 17 03 c3 01 16 85 0f 05
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   When compressed with 6LoWPAN-GHC, TLS 1.3 with the above parameters
   (epoch, sequence number, length) gives 17 bytes overhead.

2.11.  OSCOAP

   Note that the overhead is dependent on the included CoAP Option
   numbers, if the CoAP method allows payload, as well as the length of
   the OSCOAP parameters Sender ID and sequence number.  The below




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   calculation uses Method = POST, Option Delta = '9', and Sender ID =
   '25', and is only an example.

   OSCOAP Request (19 bytes, 13 bytes overhead):
   90 19 05 41 25 ae a0 15 56 67 92 4d ff 8a 24 e4
   cb 35 b9

   CoAP Delta and Option Length:
   90
   Compressed COSE Header:
   19 05 41 25
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   OSCOAP Response (15 bytes, 9 bytes overhead):
   90 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9

   CoAP Delta and Option Length:
   90
   Ciphertext:
   ae a0 15 56 67 92
   ICV:
   4d ff 8a 24 e4 cb 35 b9

   OSCOAP with the above parameters gives 13 bytes overhead for requests
   and 9 bytes overhead for responses.

   Unlike DTLS and TLS, OSCOAP has much smaller overhead for responses
   than requests.

3.  Overhead with Different Sequence Numbers

   The compression overhead (GHC) is dependent on the parameters epoch,
   sequence number, and length.  The following overheads should be
   representative for sequence numbers with the same length.

   The compression overhead (raza-6lo-compressed-dtls) is dependent on
   the length of the parameters epoch and sequence number.  The
   following overheads apply for all sequence numbers with the same
   length.

   The OSCOAP overhead is dependent on the included CoAP Option numbers,
   if the CoAP method allows payload, as well as the length of the
   OSCOAP parameters Sender ID and sequence number.





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        Sequence Number             '05'       '1005'     '100005'
        ----------------------------------------------------------
        DTLS 1.2                     29          29          29
        DTLS 1.3                     21          21          21
        TLS  1.2                     21          21          21
        TLS  1.3                     21          21          21
        ----------------------------------------------------------
        DTLS 1.2 (GHC)               16          16          17
        DTLS 1.2 (Raza)              13          13          14
        DTLS 1.3 (GHC)               14          14          15
        DTLS 1.3 (Raza)              13          13          14
        TLS  1.2 (GHC)               17          18          19
        TLS  1.3 (GHC)               17          18          19
        ----------------------------------------------------------
        OSCOAP Request               13          14          15
        OSCOAP Response               9           9           9

            Figure 1: Overhead as a function of sequence number

4.  Summary

   DTLS 1.2 has quite a large overhead as it uses an explicit sequence
   number and an explicit nonce.  DTLS 1.3, TLS 1.2, and TLS 1.3 have
   significantly less overhead.

   Both DTLS compression methods provides very good compression. raza-
   6lo-compressed-dtls achieves slightly better compression but requires
   state.  GHC is stateless but provides slightly worse compression.  As
   DTLS 1.3 uses the same version number as DTLS 1.2, both GHC and raza-
   6lo-compressed-dtls works well also for DTLS 1.3.

   The Generic Header Compression (6LoWPAN-GHC) is not very generic (the
   static dictionary is more or less a DTLS record layer) and the
   compression of TLS is significantly worse than the compression of
   DTLS.  Similar compression levels as for DTLS could be achieved also
   for TLS, but this would require different static dictionaries for
   each version of TLS (as TLS 1.2 and TLS 1.3 uses different version
   numbers).

   The header compression is not available when (D)TLS is exchanged over
   transports that do not use 6LoWPAN together with 6LoWPAN-GHC or raza-
   6lo-compressed-dtls.

   OSCOAP has much lower overhead than DTLS and TLS.  The overhead of
   OSCOAP is smaller than DTLS over 6LoWPAN with compression, and this
   small overhead is achieved even on deployments without 6LoWPAN or
   6LoWPAN without DTLS compression.  OSCOAP is lightweight because it
   makes use of some excellent features in CoAP, CBOR, and COSE.



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5.  Security Considerations

   This document is purely informational.

6.  Acknowledgments

   The authors want to thank Ari Keraenen for reviewing previous
   versions of the draft.

7.  Informative References

   [I-D.ietf-core-coap-tcp-tls]
              Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              draft-ietf-core-coap-tcp-tls-07 (work in progress), March
              2017.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-ietf-core-
              object-security-01 (work in progress), December 2016.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-19 (work in progress),
              March 2017.

   [I-D.rescorla-tls-dtls13]
              Rescorla, E. and H. Tschofenig, "The Datagram Transport
              Layer Security (DTLS) Protocol Version 1.3", draft-
              rescorla-tls-dtls13-00 (work in progress), October 2016.

   [raza-6lo-compressed-dtls]
              Raza, S., Shafagh, H., and O. Dupont, "Compression of
              Record and Handshake Headers for Constrained
              Environments", March 2017,
              <http://shahidraza.info/draft-raza-6lo-compressed.txt>.

   [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>.

   [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>.




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   [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>.

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
              2014, <http://www.rfc-editor.org/info/rfc7400>.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,
              <http://www.rfc-editor.org/info/rfc7925>.

Author's Address

   John Mattsson
   Ericsson AB
   Faeroegatan 6
   Kista  SE-164 80 Stockholm
   Sweden

   Email: john.mattsson@ericsson.com


























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