Internet Research Task Force (IRTF) C. Gündoğan

Internet Research Task Force (IRTF) C. Gündoğan
Request for Comments: 9510 Huawei
Updates: 8609 TC. Schmidt
Category: Experimental HAW Hamburg
ISSN: 2070-1721 D. Oran
Network Systems Research and Design
M. Wählisch
TU Dresden
February 2024

Alternative Delta Time Encoding for Content-Centric Networking (CCNx)
Using Compact Floating-Point Arithmetic

Abstract

Content-Centric Networking (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

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Research Task
Force (IRTF). The IRTF publishes the results of Internet-related
research and development activities. These results might not be
suitable for deployment. This RFC represents the consensus of the
Information-Centric Networking Research Group of the Internet
Research Task Force (IRTF). Documents approved for publication by
the IRSG are not candidates for any level of Internet Standard; see
Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9510.

Copyright Notice

Copyright (c) 2024 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
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Table of Contents

1. Introduction
2. Terminology
3. Usage of Time Values in CCNx
3.1. Relative Time in CCNx
3.2. Absolute Time in CCNx
4. A Compact Time Representation with Logarithmic Range
5. Protocol Integration of the Compact Time Representation
5.1. Interest Lifetime
5.2. Recommended Cache Time
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Test Vectors
Appendix B. Efficient Time Value Approximation
Acknowledgments
Authors' Addresses

1. Introduction

CCNx is well suited for Internet of Things (IoT) applications
[RFC7927]. Low-Power Wireless Personal Area Network (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
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, and 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
(Section 3.4.1 of [RFC8609]). While formally an optional TLV, every
Interest message is expected to carry the Interest Lifetime TLV in
all but some corner cases; hence, having compact encoding is
particularly valuable to keep 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 as outlined in
Section 4. This document updates CCNx messages in TLV format
[RFC8609] to permit this alternative encoding for selected time
values.

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 (see Sections 8.2-8.4 of [RFC8569]). Expiry Time indicates
the time after which a Content Object is considered expired
(Section 4 of [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 Sections 3.6.4.1.4.5 and 3.6.2.2.2 of
[RFC8609], respectively).

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; therefore, this approach is not considered
further.

3.2.2. Recommended Cache Time

Recommended Cache Time (RCT) for a Content Object (Section 4 of
[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 of [RFC8609]).

A Recommended Cache Time could be far in the future, but it 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, because 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
Sections 4 and 5)

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. Thus, the
Recommended Cache Time cannot be as accurate as when absolute time is
used. 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 described in
"Information-Centric Networking (ICN) Adaptation to Low-Power
Wireless Personal Area Networks (LoWPANs)" (see Section 7 of
[RFC9139]) that was 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. In
contrast to the representation in [IEEE.754.2019], time codes only
encode non-negative numbers and hence do not include a separate bit
to indicate integer signedness. 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 is 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
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 pseudocode in Figure 3. Not all
time values can be represented by a time code. For these instances,
a time code is produced that represents a time value closest to and
smaller than the initial time value input.

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 pseudocode.

For example, no specific time code for 0.063 exists. However, this
algorithm maps to the closest valid time code that is smaller than
0.063, 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 it is still considered for the
following reasons:

* Both CCNx RFCs ([RFC8569] and [RFC8609]) 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 (see Figure 4). For any other
length, Interest Lifetimes are encoded as described in Section 3.4.1
of [RFC8609].

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 |
+---------------+

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 (see Figure 5). For any other length, Recommended
Cache Times are encoded as described in Section 3.4.2 of [RFC8609].

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 |
+---------------+

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.

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 described in
[RFC8609]; hence, it has the same security considerations as those
two documents. Careful consideration is needed for 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 of [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>.

[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]
IEEE, "Standard for Floating-Point Arithmetic", IEEE
Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229, 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.

+===========+======================+
| 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 ICNRG for their
constructive thoughts. In particular, we thank Marc Mosko and Ken
Calvert for their valuable feedback on the encoding scheme and
protocol integration. Special thanks also go to Carsten Bormann for
his constructive in-depth comments during the IRSG review.

Authors' Addresses

Cenk Gündoğan
Huawei Technologies Duesseldorf GmbH
Riesstrasse 25
D-80992 Munich
Germany
Email: [email protected]

Thomas C. Schmidt
HAW Hamburg
Berliner Tor 7
D-20099 Hamburg
Germany
Email: [email protected]
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: [email protected]

Matthias Wählisch
TUD Dresden University of Technology
Helmholtzstr. 10
D-01069 Dresden
Germany
Email: [email protected]