Internet DRAFT - draft-irtf-icnrg-ccnx-timetlv
draft-irtf-icnrg-ccnx-timetlv
ICNRG C. Gündoğan
Internet-Draft Huawei
Updates: 8609 (if approved) TC. Schmidt
Intended status: Experimental HAW Hamburg
Expires: 6 April 2024 D. Oran
Network Systems Research and Design
M. Wählisch
TU Dresden
4 October 2023
Alternative Delta Time Encoding for CCNx Using Compact Floating-Point
Arithmetic
draft-irtf-icnrg-ccnx-timetlv-05
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. This
document updates RFC 8609 ("CCNx messages in TLV Format") to specify
this alternative encoding.
This document is a product of the IRTF Information-Centric Networking
Research Group (ICNRG).
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 6 April 2024.
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Copyright Notice
Copyright (c) 2023 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
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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 . . . . . . . . . . . . . . . . . . 4
4. A Compact Time Representation with Logarithmic Range . . . . 5
5. Protocol Integration of the Compact Time Representation . . . 7
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 . . . . . . . . . . . . . . . . . 10
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Efficient Time Value Approximation . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
CCNx is well suited for Internet of Things (IoT) applications
[RFC7927]. LoWPAN adaptation layers (e.g., [RFC9139]) confirm the
advantages of a space-efficient packet encoding for low-power and
lossy networks. CCNx utilizes absolute and delta time values for a
number of functions. When using CCNx in resource-constrained
environments, it is valuable to have a compact representation of time
values. 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. As a result
of experiments in bandwidth-constrained IEEE 802.15.4 deployments
[ICNLOWPAN], this document discusses the various cases of time
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values, proposes a compact encoding for delta times, and updates
[RFC8609] to utilize this encoding format in CCNx messages.
This document has received fruitful reviews by the members of the
research group (see the Acknowledgments section). It is the
consensus of ICNRG that this document should be published in the IRTF
Stream of the RFC series. This document does not constitute an IETF
standard.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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 as defined in [RFC5497]
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.
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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 POSIX epoch. 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 POSIX epoch. 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)
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If relative time is used, the time offset recorded in a message will
typically not account for residence times on lower layers (e.g., for
processing, queuing) and link delays for every hop. The recommended
cache time can thus not be expressed as accurate as with absolute
time. This document targets low-power networks, where delay bounds
are rather loose, or do not exist. An accumulated error due to
transmission delays in the range of milliseconds and seconds for the
recommended cache time is still tolerable in these networks, and does
not impact the protocol performance.
Networks with tight latency bounds use dedicated hardware, optimized
software routines, and traffic engineering to reduce latency
variations. Time offsets can then be corrected on every hop to yield
exact cache 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
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 non-negative numbers and hence do not include a separate 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.
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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,~0.0546875s] with a step size of ~0.0078125s 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)
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).
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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 is still 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.
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).
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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.
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.
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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.
6. IANA Considerations
This document has no IANA actions.
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. Additional considerations need to be made in networks
that deploy forwarders with support (updated forwarder) and without
support (legacy forwarder) for this compact time representation:
Interest Lifetime: A legacy forwarder interprets a time code 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. An updated
forwarder interprets short lifetimes set by a legacy forwarder as
time codes, thus configuring timeouts of up to 4 years. This
leads to an inefficient occupation of state space.
Recommended Cache Time: A legacy forwarder considers a Recommended
Cache Time with length 1 as a structural or syntactical error and
it SHOULD discard the packet (section 10.3.9 [RFC8569]).
Otherwise, a legacy forwarder interprets time codes as absolute
time values far in the past, which impacts cache utilization.
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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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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>.
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>.
[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.
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+===========+======================+
| Time Code | Time Value (seconds) |
+===========+======================+
| 0x00 | 0.0000000 |
+-----------+----------------------+
| 0x01 | 0.0078125 |
+-----------+----------------------+
| 0x04 | 0.0312500 |
+-----------+----------------------+
| 0x08 | 0.0625000 |
+-----------+----------------------+
| 0x15 | 0.2031250 |
+-----------+----------------------+
| 0x28 | 1.0000000 |
+-----------+----------------------+
| 0x30 | 2.0000000 |
+-----------+----------------------+
| 0xF8 | 67108864.0000000 |
+-----------+----------------------+
| 0xFF | 125829120.0000000 |
+-----------+----------------------+
Table 1: Test Vectors
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. Special thanks also to Carsten Bormann for
his constructive in-depth comments during the IRSG review.
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Authors' Addresses
Cenk Gündoğan
Huawei Technologies Duesseldorf GmbH
Riesstrasse 25
D-80992 Munich
Germany
Email: cenk.gundogan@huawei.com
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
Dave Oran
Network Systems Research and Design
4 Shady Hill Square
Cambridge, MA 02138
United States of America
Email: daveoran@orandom.net
Matthias Wählisch
TUD Dresden University of Technology
Helmholtzstr. 10
D-01069 Dresden
Germany
Email: m.waehlisch@tu-dresden.de
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