ICNRG C. Gundogan Internet-Draft TC. Schmidt Intended status: Experimental HAW Hamburg Expires: January 28, 2021 D. Oran Network Systems Research and Design M. Waehlisch link-lab & FU Berlin July 27, 2020 Alternative Delta Time Encoding for CCNx Using Compact Floating-Point Arithmetic draft-gundogan-icnrg-ccnx-timetlv-02 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 [IEEE.754.2019]. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://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 January 28, 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. C. Gundogan, et al. Expires January 28, 2021 [Page 1] Internet-Draft TimeTLV for CCNx July 2020 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://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 in CCNx . . . . . . . . . . . . . . . . 3 3.1. Relative Time in CCNx . . . . . . . . . . . . . . . . . . 3 3.2. Absolute Time in CCNx . . . . . . . . . . . . . . . . . . 3 3.2.1. Signature Time and Expiry Time . . . . . . . . . . . 3 3.2.2. Recommended Cache Time . . . . . . . . . . . . . . . 4 4. A Compact Time Representation with Logarithmic Range . . . . 4 5. Protocol Integration of the Compact Time Representation in CCNx Messages . . . . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 8.1. Normative References . . . . . . . . . . . . . . . . . . 7 8.2. Informative References . . . . . . . . . . . . . . . . . 8 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 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 802.15.4 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. C. Gundogan, et al. Expires January 28, 2021 [Page 2] Internet-Draft TimeTLV for CCNx July 2020 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 in CCNx 3.1. Relative Time in CCNx CCNx, as currently specified in [RFC8569], utilizes delta time for only the lifetime of an Interest message (see sections 2.1, 2.2, 2.4.2, 10.3 of [RFC8569]). 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 [IEEE.754.2019] 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 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 and Expiry Time _Signature Time_ is the time the signature of a content object was generated (sections 8.2-8.4 [RFC8569]). _Expiry Time_ indicates the expiry time of a content object (section 4 [RFC8569]). Both values are content message TLVs and represent absolute timestamps in milliseconds since the UTC epoch (i.e., an NTP timestamp). They are currently encoded via the T_SIGTIME and T_EXPIRY TLVs as 64-bit unsigned integers (see section 3.6.4.1.4.5 [RFC8609] and section 3.6.2.2.2 [RFC8609]). Both time values could be in the past, or in the future, potentially by a large delta. They are also included in the security envelope of the message. Therefore, it seems there is no practical way to define C. Gundogan, et al. Expires January 28, 2021 [Page 3] Internet-Draft TimeTLV for CCNx July 2020 an alternative compact encoding that preserves its semantics and security properties; hence we don't consider it further as a candidate. 3.2.2. 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]). A 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, this document allows the recommended cache time to be interpreted as a relative time value 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. This document therefore updates the recommended cache time with the following rule set: o Use absolute time as per [RFC8609] o Use relative time, if the compact time representation is used (see Section 4 and Section 5) 4. A Compact Time Representation with Logarithmic Range This document defines a compact time representation with logarithmic range support that is inspired by [RFC5497] and [IEEE.754.2019]. Compact time codes are one octet wide and represent time values that range from milliseconds to years. Figure 1 depicts the logarithmic nature of this time representation. || | | | | | | | | | | +-----------------------------------------------------------------+ milliseconds years 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. In contrast to the representation in [IEEE.754.2019], time codes only encode positive numbers and hence do not include an extra sign bit. C. Gundogan, et al. Expires January 28, 2021 [Page 4] Internet-Draft TimeTLV for CCNx July 2020 <--- one octet wide ---> +---+---+---+---+---+---+---+---+ | exponent (e) | mantissa (m) | +---+---+---+---+---+---+---+---+ Figure 2: A time code with exponent and mantissa to encode a logarithmic range time representation. The base unit for time values are seconds. A time value is calculated using the following formula, where (e) represents the exponent, (m) the mantissa, (m_max) the maximum mantissa value, and (b) the bias. Subnormal (e == 0): (0 + m/m_max) * 2^(1+b) Normalized (e > 0): (1 + m/m_max) * 2^(e+b) The subnormal form is only applied when the exponent is zero. This form provides a gradual underflow for the gap between the smallest normalized number and zero. The normalized form is applied for all values with an exponent greater than zero. This document allocates "5" bits for the exponent and "3" bits for the mantissa. Therefore, exponents have the range [0,31] and mantissae have the range [0,7]. This document further applies a bias of "-5". The following properties result from this configuration: o Minimum subnormal number: 0 seconds o Maximum subnormal number: ~0.054688 seconds o Minimum normalized number: ~0.062500 seconds o Maximum normalized number: ~3.99 years Eight time values exist in the subnormal range [0,~0.054688s] with a step size of ~0.007813s between each time value. In addition, this configuration also encodes the following convenient numbers in seconds: [1, 2, 4, 8, 16, 32, 64, ...]. An example algorithm to encode a time value into the corresponding exponent and mantissa is given as pseudo code in Figure 3. Not all time values can be represented by a time code. For these instances, the closest time code is chosen that is smaller than the value to encode. C. Gundogan, et al. Expires January 28, 2021 [Page 5] Internet-Draft TimeTLV for CCNx July 2020 input: float v // time value output: int e, m // exponent, mantissa of time code (e, m) encode (v): if (v == 0) return (0, 0) if (v < 0.0625) m = floor ((v * 2^7)) // round down return (0, m) else e = floor (log2(v)) // round down m = floor (((v / 2^e - 1) * 8)) // round down return (e + 5, m) // add bias Figure 3: Algorithm in pseudo code. As an example: No specific time code for "0.063" exists, but this algorithm maps to the closest valid time code that is smaller, i.e., exponent "1" and mantissa "0" (the same as for time value "0.0625"). 5. Protocol Integration of the Compact Time Representation in CCNx Messages A straightforward way to accommodate the compact time approach as an alternative encoding is to simply allow a 1-byte length as an alternative to the 8-byte length while retaining the existing TLV registry entries. This approach has backward compatibility problems, but may still be considered 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 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. C. Gundogan, et al. Expires January 28, 2021 [Page 6] Internet-Draft TimeTLV for CCNx July 2020 The following lists alternative approaches of integrating the compact time representation for time offsets in CCNx messages. A further analysis, discussion, and decision on the best suited approach will be added as the document progresses. 1. Relative time TLVs (e.g., T_INTLIFETIME) include nested TLVs to hint at the used encoding. This approach is the least intrusive integration, but adds a TLV overhead that negates the benefits of the compact time representation. 2. A new TLV type for T_INTLIFETIME with a compact time representation (T_INTLIFETIME_COMPACT) is defined. The packet header grammar from [RFC8609] is updated to allow for T_INTLIFETIME_COMPACT at the same level of the currently defined T_INTLIFETIME with an exclusive or. 6. IANA Considerations Based on the approach of integration, certain TLV registries from [RFC8609] need to be updated. 7. Security Considerations This document makes no semantic changes to [RFC8569], 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 [IEEE.754.2019] Institute of Electrical and Electronics Engineers, C/MSC - Microprocessor Standards Committee, "Standard for Floating-Point Arithmetic", June 2019, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5497] Clausen, T. and C. Dearlove, "Representing Multi-Value Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497, DOI 10.17487/RFC5497, March 2009, . C. Gundogan, et al. Expires January 28, 2021 [Page 7] Internet-Draft TimeTLV for CCNx July 2020 [RFC8569] Mosko, M., Solis, I., and C. Wood, "Content-Centric Networking (CCNx) Semantics", RFC 8569, DOI 10.17487/RFC8569, July 2019, . [RFC8609] Mosko, M., Solis, I., and C. Wood, "Content-Centric Networking (CCNx) Messages in TLV Format", RFC 8609, DOI 10.17487/RFC8609, July 2019, . 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-08 (work in progress), May 2020. Appendix A. Test Vectors The test vectors in Table 1 show sample time codes and their corresponding time values according to the algorithm outlined in Section 4. +-----------+----------------------+ | Time Code | Time Value (seconds) | +-----------+----------------------+ | 0x00 | 0.000000 | | 0x04 | 0.031250 | | 0x08 | 0.062500 | | 0x15 | 0.203125 | | 0x28 | 1.000000 | | 0x30 | 2.000000 | | 0xF8 | 67108864.000000 | | 0xFF | 125829120.000000 | +-----------+----------------------+ Table 1: Test Vectors Authors' Addresses C. Gundogan, et al. Expires January 28, 2021 [Page 8] Internet-Draft TimeTLV for CCNx July 2020 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 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 C. Gundogan, et al. Expires January 28, 2021 [Page 9]