Alternative Delta Time Encoding for CCNx Using Compact Floating-Point Arithmetic
draft-irtf-icnrg-ccnx-timetlv-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9510.
|
|
|---|---|---|---|
| Authors | Cenk Gündoğan , Thomas C. Schmidt , David R. Oran , Matthias Wählisch | ||
| Last updated | 2022-10-02 (Latest revision 2022-04-27) | ||
| Replaces | draft-gundogan-icnrg-ccnx-timetlv | ||
| RFC stream | Internet Research Task Force (IRTF) | ||
| Formats | |||
| IETF conflict review | conflict-review-irtf-icnrg-ccnx-timetlv, conflict-review-irtf-icnrg-ccnx-timetlv, conflict-review-irtf-icnrg-ccnx-timetlv, conflict-review-irtf-icnrg-ccnx-timetlv, conflict-review-irtf-icnrg-ccnx-timetlv, conflict-review-irtf-icnrg-ccnx-timetlv | ||
| Additional resources | Mailing list discussion | ||
| Stream | IRTF state | (None) | |
| Consensus boilerplate | Unknown | ||
| Document shepherd | Dirk Kutscher | ||
| IESG | IESG state | Became RFC 9510 (Experimental) | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | ietf@dkutscher.net |
draft-irtf-icnrg-ccnx-timetlv-00
ICNRG C. Gündoğan
Internet-Draft TC. Schmidt
Intended status: Experimental HAW Hamburg
Expires: 29 October 2022 D. Oran
Network Systems Research and Design
M. Waehlisch
link-lab & FU Berlin
27 April 2022
Alternative Delta Time Encoding for CCNx Using Compact Floating-Point
Arithmetic
draft-irtf-icnrg-ccnx-timetlv-00
Abstract
CCNx utilizes delta time for a number of functions. When using CCNx
in environments with constrained nodes 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] and [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
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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.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Usage of Time Values in CCNx . . . . . . . . . . . . . . . . 3
3.1. Relative Time in CCNx . . . . . . . . . . . . . . . . . . 3
3.2. Absolute Time in CCNx . . . . . . . . . . . . . . . . . . 3
4. A Compact Time Representation with Logarithmic Range . . . . 4
5. Protocol Integration of the Compact Time Representation . . . 6
5.1. Interest Lifetime . . . . . . . . . . . . . . . . . . . . 7
5.2. Recommended Cache Time . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Efficient Time Value Approximation . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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.
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2. Terminology
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].
This document uses the terminology of [RFC8569] and [RFC8609] for
CCNx entities.
The following terms are used in the document and defined as follows:
byte: synonym for octet
time value: a time offset measured in seconds
time code: an 8-bit encoded time value
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.
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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 and 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
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:
* Use absolute time as per [RFC8609]
* 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 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.
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|| | | | | | | | | | |
+-----------------------------------------------------------------+
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 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
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 (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.
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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)
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
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.
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* 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.
5.1. Interest Lifetime
The Interest Lifetime definition in [RFC8609] allows for a variable-
length lifetime representation, where a length of 1 encodes the
linear range [0,255] in milliseconds. This document changes the
definition to always encode 1-byte Interest lifetime values in the
compact time value representation (Figure 4).
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) /
/ /
+---------------+---------------+---------------+---------------+
Figure 4: Changes to the definition of the Interest Lifetime TLV.
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A note on legacy forwarders: A forwarder that does not support this
compact time representation will interpret the time value as an
Interest lifetime between 0 and 255 milliseconds. This may lead to a
degradation of network performance, since Pending Interest
Table (PIT) entries timeout quicker than expected.
5.2. Recommended Cache Time
The Recommended Cache Time definition in [RFC8609] specifies an
absolute time representation that is of a length fixed to 8 bytes.
This document changes the definition to always encode 1-byte
Recommended Cache Time values in the compact relative time value
representation (Figure 5).
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 /
/ /
+---------------+---------------+---------------+---------------+
Figure 5: Changes to the definition of the Recommended Cache Time
TLV.
The packet processing is adapted to calculate an absolute time from
the relative time code based on the absolute reception time. On
transmission, a new relative time code is calculated based on the
current system time.
A note on legacy forwarders: A forwarder that does not support this
compact time representation is expected to consider a Recommended
Cache Time with length 1 as a structural or syntactical error and
discard the packet. Otherwise, a forwarder interprets the compact
1-byte time value as an absolute time far in the past, which impacts
cache utilization.
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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
[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>.
8.2. Informative References
[ICNLOWPAN]
Gündoğan, C., Kietzmann, P., Schmidt, T., and M. Wählisch,
"Designing a LoWPAN convergence layer for the Information
Centric Internet of Things", Computer Communications, Vol.
164, No. 1, p. 114–123, Elsevier, December 2020,
<https://doi.org/10.1016/j.comcom.2020.10.002>.
[IEEE.754.2019]
Institute of Electrical and Electronics Engineers, C/MSC -
Microprocessor Standards Committee, "Standard for
Floating-Point Arithmetic", June 2019,
<https://standards.ieee.org/content/ieee-standards/en/
standard/754-2019.html>.
[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>.
[RFC7927] Kutscher, D., Ed., Eum, S., Pentikousis, K., Psaras, I.,
Corujo, D., Saucez, D., Schmidt, T., and M. Waehlisch,
"Information-Centric Networking (ICN) Research
Challenges", RFC 7927, DOI 10.17487/RFC7927, July 2016,
<https://www.rfc-editor.org/info/rfc7927>.
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[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>.
[RFC9139] Gündoğan, C., Schmidt, T., Wählisch, M., Scherb, C.,
Marxer, C., and C. Tschudin, "Information-Centric
Networking (ICN) Adaptation to Low-Power Wireless Personal
Area Networks (LoWPANs)", RFC 9139, DOI 10.17487/RFC9139,
November 2021, <https://www.rfc-editor.org/info/rfc9139>.
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 |
+-----------+----------------------+
| 0x01 | 0.007812 |
+-----------+----------------------+
| 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
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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
Acknowledgments
We would like to thank the active members of the ICNRG research group
for constructive thoughts. In particular, we thank Marc Mosko and
Ken Calvert for their valuable feedback on the encoding scheme and
the protocol integration.
Authors' Addresses
Cenk Gündoğan
HAW Hamburg
Berliner Tor 7
D-20099 Hamburg
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
D-20099 Hamburg
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
United States of America
Email: daveoran@orandom.net
Matthias Waehlisch
link-lab & FU Berlin
Hoenower Str. 35
D-10318 Berlin
Germany
Email: mw@link-lab.net
URI: http://www.inf.fu-berlin.de/~waehl
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