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ICNRG                                                        C. Gundogan
Internet-Draft                                               TC. Schmidt
Intended status: Experimental                                HAW Hamburg
Expires: May 7, 2020                                             D. Oran
                                     Network Systems Research and Design
                                                            M. Waehlisch
                                                    link-lab & FU Berlin
                                                        November 4, 2019


  An Alternative Delta Time encoding for CCNx using Interval Time from
                                RFC5497
                draft-gundogan-icnrg-ccnx-timetlv-00

Abstract

   CCNx utilizes Delta Time for a number of functions.  When using CCNx
   in environments with constrained nodes and/or bandwidth constrained
   networks, it is valuable to have a compressed representation of delta
   time.  In order to do so, either accuracy or dynamic range has to be
   sacrificed.  Since the current uses of delta time do not require both
   simultaneously, one can consider a logarithmic encoding such as that
   specified in RFC5497.  This document updates _CCNx messages in TLV
   Format_ (RFC8609) to specify this alternative encoding.

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
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   This Internet-Draft will expire on May 7, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   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
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Usage of Time values CCNx . . . . . . . . . . . . . . . . . .   3
     3.1.  Relative Time usage in CCNx . . . . . . . . . . . . . . .   3
     3.2.  Absolute Time usage in CCNx . . . . . . . . . . . . . . .   3
       3.2.1.  Signature Time  . . . . . . . . . . . . . . . . . . .   3
       3.2.2.  Expiry Time . . . . . . . . . . . . . . . . . . . . .   4
       3.2.3.  Recommended Cache Time  . . . . . . . . . . . . . . .   4
   4.  A Compact Time Representation with Logarithmic Range  . . . .   5
   5.  Alternatives for a compact time representation in CCNx
       messages  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   CCNx utilizes Time values for a number of functions.  Some of these
   are expressed as absolute time, others as delta time.  When using
   CCNx in environments with constrained nodes and/or bandwidth
   constrained networks, it is valuable to have a compact representation
   of time values.  For example [I-D.irtf-icnrg-icnlowpan] specifies a
   compression scheme useful over IEEE 804.15.4 networks.  However, any
   compact time representation has to sacrifice either accuracy or
   dynamic range or both.  For some time uses this is relatively
   straightforward to achieve, for other uses, it is not.  This document
   discusses the various cases, and proposes a compact encoding that is
   easily accommodated for delta times, and a more complicated method
   that can be considered for some uses of absolute time.







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2.  Requirements Language

   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 RFC 2119 [RFC2119].

3.  Usage of Time values CCNx

3.1.  Relative Time usage in CCNx

   CCNx, as currently specified in [RFC8569], utilizes delta time for
   only the lifetime of an Interest message (sections 2.1, 2.2, 2.4.2,
   10.3).  It is a hop-by-hop header value, and is currently encoded via
   the T_INTLIFE TLV as a 64-bit integer ([RFC8609] section 3.4.1).
   While formally an optional TLV, in all but some corner cases every
   Interest Message is expected to carry the Interest Lifetime TLV, and
   hence having compact encoding is particularly valuable for keeping
   Interest messages short.

   Since the current uses of delta time do not require both accuracy and
   dynamic range simultaneously, one can consider a logarithmic encoding
   such as that specified in [RFC5497] and outlined in Section 4.  This
   document updates CCNx messages in TLV Format ([RFC8609]) to permit
   this alternative encoding for selected Time values.  See Section 6
   for the specific actions needed to register this alternative compact
   representation of Interest Lifetime.

3.2.  Absolute Time usage in CCNx

   CCNx, as currently specified in [RFC8569], utilizes absolute time for
   various important functions.  Each of these absolute time usages
   poses a different challenge for a compact representation.  These are
   discussed in the following subsections.

3.2.1.  Signature Time

   _Signature Time_ is the time the signature of a content object was
   generated (sections 8.2-8.4 [RFC8569]).  This is a content message
   TLV stating the time the signature on the content object was
   generated, in milliseconds since the UTC epoch (i.e.  An NTP
   timestamp).  It is currently encoded via the T_SIGTIME TLV as a
   64-bit unsigned integer (see section 3.6.4.1.4.5 [RFC8609]).

   Given that a signature time could be essentially at any time in the
   past, and is included in the hash securing the content object, it
   seems there is no practical way to define an alternative compact
   encoding that preserves its semantics and security properties; hence
   we don't consider it further as a candidate.



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3.2.2.  Expiry Time

   _Expiry Time_ indicates the expiry time of a content object (section
   4 [RFC8569]).  This is a content message TLV (covered by the hash and
   signature of the content object) stating the expiration time of the
   content object in milliseconds since the UTC epoch (i.e.  An NTP
   timestamp).  It is currently encoded via the T_EXPIRY TLV as a 64-bit
   unsigned integer (see section 3.6.2.2.2 [RFC8609]).

   Expiry time could be in the past, or in the future, potentially by a
   large delta, and is included in the hash securing the content object.
   Therefore, it seems there is no practical way to define an
   alternative compact encoding that preserves its semantics and
   security properties; hence we don't consider it further as a
   candidate.

3.2.3.  Recommended Cache Time

   _Recommended Cache Time_ (RCT) for a content object (see section 4
   [RFC8569]) is a hop-by-hop header stating the expiration time for a
   cached content object in milliseconds since the UTC epoch (i.e.  An
   NTP timestamp).  It is currently encoded via the T_CACHETIME TLV as a
   64-bit unsigned integer (see section 3.4.2 [RFC8609]).

   Recommended cache time could be far in the future, but cannot be in
   the past and is likely to be a reasonably short offset from the
   current time.  Therefore, there are a couple of alternatives for
   defining a compact representation.  These are:

   o  allow the compact representation to be interpreted as a delta time
      rather than an absolute time, since the semantics associated with
      an absolute time value do not seem to be critical to the utility
      of this value.

   o  define an alternative absolute time base against which the RCT
      delta can be measured.  One potential candidate would be a
      _message generation time_ included as a message TLV (not a hop-by-
      hop TLV).  While doing this would not save any space if used for
      just RCT (it would actually make the message bigger) if its cost
      could be amortitized across multiple other time TLVs in the same
      message, it might be a win.

   This possibility is for further analysis and discussion as the
   document progresses.







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4.  A Compact Time Representation with Logarithmic Range

   This document defines a compact time representation with logarithmic
   range support that is inspired by [RFC5497].  Compact time-codes are
   one or two octets wide and represent time-values that range from
   milliseconds to days.  Figure 1 depicts the logarithmic nature of
   this time representation.

     ||||| | | | | |  |  |  |  |  |   |   |   |   |   |      |      |
     +--------------------------------------------------------------+
     milliseconds                                                days

      Figure 1: A logarithmic range representation allows for higher
    precision in the smaller time ranges and still supports large time
                                  deltas.

   Time-codes encode an exponent and a mantissa as illustrated in
   Figure 2.  Appropriate sizes for the exponent (b) and mantissa (a)
   are yet to be discussed.

                     <--   one or two octets wide  -->
                     +---+---+---+---+---+---+---+---+
                     |  exponent (b) |  mantissa (a) |
                     +---+---+---+---+---+---+---+---+

       Figure 2: A time-code with exponent and mantissa to encode a
                  logarithmic range time representation.

5.  Alternatives for a compact time representation in CCNx messages

   A straightforward way to accommodate the TimeTLV approach as an
   alternative encoding is to simply allow a 1 or 2-byte length as an
   alternative to the 8-byte length while retaining the existing TLV
   Registry entries.  While this has backward compatibility problems,
   the authors recommend this approach for the following reasons:

   o  Both CCNx RFCs are experimental and not Standards Track, hence
      expectations for forward and backward compatibility are not as
      stringent.  "Flag day" upgrades of deployed CCNx networks, while
      inconvenient, are still feasible.

   o  The major use case for these compressed encodings are smaller-
      scale IoT and/or sensor networks where the population of
      consumers, producers, and forwarders is reasonably small.

   o  Since the current TLVs have hop-by-hop semantics, they are not
      covered by any signed hash and hence may be freely re-encoded by




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      any forwarder.  That means a forwarder supporting the new encoding
      can translate freely between the two encodings.

   o  The alternative of assigning new TLV registry values does not
      substantially mitigate the interoperability problems anyway.

6.  IANA Considerations

   Please change the registry for the T_INTLIFE to permit a length of 1
   or 2 in addition to a length of 8, as follows:.

   .

                        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
   +---------------+---------------+---------------+---------------+
   |             T_INTLIFE         |               1               |
   +---------------+---------------+---------------+---------------+
   | INTERVAL_TIME |
   +---------------+

                        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
   +---------------+---------------+---------------+---------------+
   |             T_INTLIFE         |               2               |
   +---------------+---------------+---------------+---------------+
   |           INTERVAL_TIME       |
   +-------------------------------+


         Figure 3: Alternate Delta Time encoding based on RFC4574

7.  Security Considerations

   This document makes no semantic changes to RFC8965, nor to any of the
   security properties of the message encodings of RFC8609, and hence
   has the same security considerations as those two existing documents.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.





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   [RFC5497]  Clausen, T. and C. Dearlove, "Representing Multi-Value
              Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497,
              DOI 10.17487/RFC5497, March 2009, <https://www.rfc-
              editor.org/info/rfc5497>.

   [RFC8569]  Mosko, M., Solis, I., and C. Wood, "Content-Centric
              Networking (CCNx) Semantics", RFC 8569,
              DOI 10.17487/RFC8569, July 2019, <https://www.rfc-
              editor.org/info/rfc8569>.

   [RFC8609]  Mosko, M., Solis, I., and C. Wood, "Content-Centric
              Networking (CCNx) Messages in TLV Format", RFC 8609,
              DOI 10.17487/RFC8609, July 2019, <https://www.rfc-
              editor.org/info/rfc8609>.

8.2.  Informative References

   [I-D.irtf-icnrg-icnlowpan]
              Gundogan, C., Schmidt, T., Waehlisch, M., Scherb, C.,
              Marxer, C., and C. Tschudin, "ICN Adaptation to LowPAN
              Networks (ICN LoWPAN)", draft-irtf-icnrg-icnlowpan-05
              (work in progress), September 2019.

Authors' Addresses

   Cenk Gundogan
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099
   Germany

   Phone: +4940428758067
   Email: cenk.guendogan@haw-hamburg.de
   URI:   http://inet.haw-hamburg.de/members/cenk-gundogan


   Thomas C. Schmidt
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099
   Germany

   Email: t.schmidt@haw-hamburg.de
   URI:   http://inet.haw-hamburg.de/members/schmidt







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   Dave Oran
   Network Systems Research and Design
   4 Shady Hill Square
   Cambridge, MA  02138
   USA

   Email: daveoran@orandom.net


   Matthias Waehlisch
   link-lab & FU Berlin
   Hoenower Str. 35
   Berlin  D-10318
   Germany

   Email: mw@link-lab.net
   URI:   http://www.inf.fu-berlin.de/~waehl


































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