Alternative Delta Time Encoding for CCNx Using Compact Floating-Point Arithmetic

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 29 October 2022.¶

Copyright Notice

Copyright (c) 2022 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 (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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

CCNx utilizes time values for a number of functions. Some of these are expressed as absolute time, others as delta time. CCNx is well suited for Internet of Things (IoT) applications [RFC7927]. When using CCNx in environments with constrained nodes or bandwidth constrained networks, it is valuable to have a compact representation of time values. For example [RFC9139] and [ICNLOWPAN] specify a compression scheme useful over IEEE 802.15.4 networks. However, any compact time representation has to sacrifice accuracy or dynamic range. 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.¶

4. A Compact Time Representation with Logarithmic Range

This document uses the compact time representation of ICNLoWPAN (see section 7 of [RFC9139]) that is inspired by [RFC5497] and [IEEE.754.2019]. Its logarithmic encoding supports a representation ranging from milliseconds to years. Figure 1 depicts the logarithmic nature of this time representation.¶

|| |  |   |    |     |      |       |        |         |          |
+-----------------------------------------------------------------+
milliseconds                                                  years

Time codes encode exponent and mantissa values in a single byte, but in contrast to the representation in [IEEE.754.2019], time codes only encode positive numbers and hence do not include an extra sign bit. Figure 2 shows the configuration of a time code: an exponent width of 5 bits, and a mantissa width of 3 bits.¶

<---          one byte wide          --->
+----+----+----+----+----+----+----+----+
|      exponent (b)      | mantissa (a) |
+----+----+----+----+----+----+----+----+
  0    1    2    3    4    5    6    7

The base unit for time values are seconds. A time value is calculated using the following formula (adopted from [RFC5497] and [RFC9139]), where (a) represents the mantissa, (b) the exponent, and (C) a constant factor set to C := 1/32.¶

Subnormal (b == 0):

(0 + a/8) * 2 * C¶

Normalized (b > 0):

(1 + a/8) * 2^b * C¶

The subnormal form provides a gradual underflow between zero and the smallest normalized number. Eight time values exist in the subnormal range [0s,~0.054688s] with a step size of ~0.007812s between each time value. This configuration also encodes the following convenient numbers in seconds: [1, 2, 4, 8, 16, 32, 64, ...]. Appendix A further includes test vectors to illustrate the logarithmic range.¶

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.¶

 input: float v    // time value
output:   int a, b // mantissa, exponent of time code

(a, b) encode (v):

    if (v == 0)
        return (0, 0)

    if (v < 2 * C)                              // subnormal
        a = floor (v * 4 / C)                   // round down
        return (a, 0)
    else                                        // normalized
        if (v > (1 + 7/8) * 2^31 * C)           // check bounds
            return (7, 31)                      // return maximum
        else
            b = floor (log2(v / C))             // round down
            a = floor ((v / (2^b * C) - 1) * 8) // round down
            return (a, b)

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

A straightforward way to accommodate the compact time approach is to use a 1-byte length field to indicate this alternative encoding while retaining the existing TLV registry entries. This approach has backward compatibility problems, but may still be considered for the following reasons:¶

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

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 allows for versatility in defining new time encodings, but adds a TLV overhead that negates the benefits of the compact time representation.¶
2. A new TLV type for the compact time representation is defined (T_INTLIFETIME_COMPACT). The packet header grammar from [RFC8609] is updated to allow for T_INTLIFETIME_COMPACT at the same level of the currently defined T_INTLIFETIME.¶

                     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           |           Length = 1          |
+---------------+---------------+---------------+---------------+
| COMPACT_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           |           Length > 1          |
+---------------+---------------+---------------+---------------+
/                                                               /
/                      Lifetime (Length octets)                 /
/                                                               /
+---------------+---------------+---------------+---------------+

                     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_CACHETIME          |           Length = 1          |
+---------------+---------------+---------------+---------------+
| COMPACT_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_CACHETIME          |           Length = 8          |
+---------------+---------------+---------------+---------------+
/                                                               /
/                    Recommended Cache Time                     /
/                                                               /
+---------------+---------------+---------------+---------------+

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.¶

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.¶

Appendix B. Efficient Time Value Approximation

A forwarder frequently converts compact time into milliseconds to compare Interest lifetimes and the duration of cache entries. On many architectures, multiplication and division perform slower than addition, subtraction, and bit shifts. The following equations approximate the formulas in Section 4, and scale from seconds into the milliseconds range by applying a factor of 2^10 instead of 10^3. This results in an error of 2.4%.¶

b == 0:

2^10 * a * 2^-3 * 2^1 * 2^-5
= a << 3¶

b > 0:

(2^10 + a * 2^-3 * 2^10) * 2^b * 2^-5
= (1 << 5 + a << 2) << b¶

Authors' Addresses