MANET time TLVs: Representing Multi-Value Time in Mobile Ad Hoc Networks (MANETs)
A compact way for ad hoc network protocols to say how often to expect a message and how long its contents stay valid.
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>.
@misc{rfc5497,
series = {Request for Comments},
number = 5497,
howpublished = {RFC 5497},
publisher = {RFC Editor},
doi = {10.17487/RFC5497},
url = {https://www.rfc-editor.org/info/rfc5497},
author = {T. Clausen and C. Dearlove},
title = {{Representing Multi-Value Time in Mobile Ad Hoc Networks (MANETs)}},
pagetotal = 14,
year = 2009,
month = mar,
} RFC text
Contents
Network Working Group T. Clausen
Request for Comments: 5497 LIX, Ecole Polytechnique
Category: Standards Track C. Dearlove
BAE Systems ATC
March 2009
Representing Multi-Value Time in Mobile Ad Hoc Networks (MANETs)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
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Abstract
This document describes a general and flexible TLV (type-length-value
structure) for representing time-values, such as an interval or a
duration, using the generalized Mobile Ad hoc NETwork (MANET) packet/
message format. It defines two Message TLVs and two Address Block
TLVs for representing validity and interval times for MANET routing
protocols.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation and Rationale . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 6
4. Protocol Overview and Functioning . . . . . . . . . . . . . . 6
5. Representing Time . . . . . . . . . . . . . . . . . . . . . . 6
6. General Time TLV Structure . . . . . . . . . . . . . . . . . . 7
6.1. Single-Value Time TLVs . . . . . . . . . . . . . . . . . . 8
6.2. Multi-Value Time TLVs . . . . . . . . . . . . . . . . . . 9
7. Message TLVs . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. INTERVAL_TIME TLV . . . . . . . . . . . . . . . . . . . . 10
7.2. VALIDITY_TIME TLV . . . . . . . . . . . . . . . . . . . . 10
8. Address Block TLVs . . . . . . . . . . . . . . . . . . . . . . 10
8.1. INTERVAL_TIME TLV . . . . . . . . . . . . . . . . . . . . 10
8.2. VALIDITY_TIME TLV . . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9.1. Expert Review: Evaluation Guidelines . . . . . . . . . . . 11
9.2. Message TLV Types . . . . . . . . . . . . . . . . . . . . 12
9.3. Address Block TLV Types . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
1. Introduction
The generalized packet/message format [RFC5444] specifies a signaling
format that MANET routing protocols can employ for exchanging
protocol information. This format presents the ability to express
and associate attributes to packets, messages, or addresses, by way
of a general TLV (type-length-value) mechanism.
This document specifies a general Time TLV structure, which can be
used by any MANET routing protocol that needs to express either
single time-values or a set of time-values with each time-value
associated with a range of hop counts, as provided by the Message
Header of [RFC5444]. This allows a receiving node to determine a
single time-value if either it knows its hop count from the
originator node or the Time TLV specifies a single time-value.
A time-value is, in this context, not an "absolute point in time",
but rather an interval or a duration. An instance of a Time TLV can,
therefore, express an interval or a duration such as "10 seconds".
This document also specifies two Message TLV Types, which use the TLV
structure proposed. These TLV Types are INTERVAL_TIME and
VALIDITY_TIME, specifying, respectively, the maximum time before
another message of the same type as this message from the same
originator should be received, and the duration for which the
information in this message is valid after receipt. Note that, if
both are present, then the latter will usually be greater than the
former in order to allow for possible message loss.
This document also specifies two Address Block TLV Types, which use
the TLV structure proposed. These TLV Types are INTERVAL_TIME and
VALIDITY_TIME, defined equivalently to the two Message TLVs with the
same names.
1.1. Motivation and Rationale
The Time TLV structure, specified in this document, is intended to be
used as a component in a MANET routing protocol, e.g., to indicate
the expected spacing between successive transmissions of a given
Message Type, by including a Time TLV in transmitted messages.
Some MANET routing protocols may employ very short spacing for some
messages and very long spacing for others, or may change the message
transmission rate according to observed behavior. For example, if a
network is observed at some point in time to exhibit a highly dynamic
topology, a very short (sub-second) message spacing could be
appropriate, whereas if the network later is observed to stabilize,
multi-hour message spacing may become appropriate. Different MANET
routing protocols and different deployments of MANET routing
protocols may have different granularity requirements and bounds on
shortest and longest spacing between successive message
transmissions.
In addition, MANET routing protocol deployments typically use
bandwidth-limited wireless network interfaces, and therefore prefer
to trade off computational complexity for a saving in the number of
bits transmitted. This is practical in this case, because the
intended usages of Time TLVs, including the specified examples of
message interval time and information validity time, do not require
high-precision values of time.
The Time TLV structure, specified in this document, caters to these
characteristics by:
o encoding time-values, such as an interval or a duration, in an
8-bit field; while
o allowing these time-values to range from "very small" (e.g.,
1/1024 second) to "very long" (e.g., 45 days); and
o allowing a MANET routing protocol, or a deployment, to
parameterize this (e.g., to attain finer granularity at the
expense of a lower upper bound) through a single parameter, C.
The parameter C must be the same for all MANET routers in the same
deployment.
The TLV mechanism as specified in [RFC5444] allows associating a
"value" to either a packet, a message, or to addresses. The data
structure for doing so -- the TLV -- is identical in each of the
three cases; however, the TLV's position in a received packet allows
determining if that TLV is a "Packet TLV" (it appears in the Packet
Header, before any messages), a "Message TLV" (it appears in the TLV
Block immediately following a Message Header), or an "Address Block
TLV" (it appears in the TLV Block immediately following an Address
Block).
While TLVs may be structurally identical, that which they express may
be different. This is determined from the kind (packet, message, or
Address Block) and type of the TLV. For example, one TLV might
associate a lifetime to an address, another a content sequence number
to a message, and another a cryptographic signature to a packet. For
this reason, [RFC5444] specifies separate registries for Packet TLV
Types, Message TLV Types, and Address Block TLV Types, and it does
not specify any structure in the TLV Value field.
The TLVs defined in this document express, essentially, that "this
information will be refreshed within X seconds" and that "this
information is valid for X seconds after being received", each
allowing the "X seconds" to be specified as a function of the number
of hops from the originator of the information. This document
specifies a general format allowing expressing and encoding this as
the value field of a TLV. This representation uses a compact (8-bit)
representation of time, as message size is an issue in many MANETs,
and the offered precision and range is appropriate for MANET routing
protocols.
A TLV of this format may be used for packets, messages, or addresses.
For example, a proactive MANET routing protocol periodically
reporting link-state information could include a TLV of this format
as a Message TLV. This may indicate a different periodicity in
different scopes (possibly frequently up to a few hops, less
frequently beyond that) because some messages may be sent with
limited scope, as specified in [RFC5444]. A reactive MANET routing
protocol could include a TLV of this format as an Address Block TLV
for reporting the lifetime of routes to individual addresses.
In addition to defining the general format as outlined above, this
document requests IANA assignments for INTERVAL_TIME and
VALIDITY_TIME TLVs. These IANA assignments are requested in this
document in order to avoid interdependencies between otherwise
unrelated MANET protocols and in order to not exhaust the TLV Type
spaces by having different protocols request types for essentially
identical data structures. Only Message TLVs and Address Block TLVs
are requested, as these are those for which a need has been
demonstrated.
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
[RFC2119].
Additionally, this document uses terminology from [RFC5444], and
introduces the following terminology:
hop count - the number of hops from the message originator to the
message recipient. This is defined to equal the <msg-hop-count>
field in the <msg-header> element defined in [RFC5444], if
present, after it is incremented on reception. If the <msg-hop-
count> field is not present, or in a Packet TLV, then hop count is
defined to equal 255.
time-value - a time, measured in seconds.
time-code - an 8-bit field, representing a time-value.
3. Applicability Statement
The TLV structure described in this document is applicable whenever a
single time-value, or a time-value that varies with the number of
hops from the originator of a message, is required in a protocol
using the generalized MANET packet/message format [RFC5444].
Examples of time-values that may be included in a protocol message
are:
o The maximum time interval until the next message of the same type
is to be generated by the message's originator node.
o The validity time of the information with which the time-value is
associated.
Either of these may vary with the hop count between the originating
and receiving nodes, e.g., if messages of the same type are sent with
different hop limits as defined in [RFC5444].
Parts of this document have been generalized from material in the
proactive MANET routing protocol OLSR (Optimized Link State Routing
Protocol) [RFC3626].
4. Protocol Overview and Functioning
This document does not specify a protocol nor does it mandate
specific node or protocol behavior. Rather, it outlines mechanisms
for encoding time-values using the TLV mechanism of [RFC5444].
5. Representing Time
This document specifies a TLV structure in which time-values are each
represented in an 8-bit time-code, one or more of which may be used
in a TLV's <value> field. Of these 8 bits, the least significant 3
bits represent the mantissa (a), and the most significant 5 bits
represent the exponent (b), so that:
o time-value := (1 + a/8) * 2^b * C
o time-code := 8 * b + a
All nodes in the MANET MUST use the same value of the constant C,
which will be specified in seconds, hence so will be all time-values.
C MUST be greater than 0 seconds. Note that ascending values of the
time-code represent ascending time-values; time-values may thus be
compared by comparison of time-codes.
An algorithm for computing the time-code representing the smallest
representable time-value not less than the time-value t is:
1. find the largest integer b such that t/C >= 2^b;
2. set a := 8 * (t / (C * 2^b) - 1), rounded up to the nearest
integer;
3. if a = 8, then set b := b + 1 and set a := 0;
4. if 0 <= a <= 7, and 0 <= b <= 31, then the required time-value
can be represented by the time-code 8 * b + a, otherwise it
cannot.
The minimum time-value that can be represented in this manner is C.
The maximum time-value that can be represented in this manner is 15 *
2^28 * C, or about 4.0 * 10^9 * C. If, for example, C = 1/1024
second, then this is about 45 days.
A protocol using this time representation MUST define the value of C.
A protocol using this specification MAY specify that the all-bits
zero time-value (0) represents a time-value of zero and/or that the
all-bits one time-value (255) represents an indefinitely large time-
value.
6. General Time TLV Structure
The following data structure allows the representation of a single
time-value, or of a default time-value plus pairs of (time-values,
hop counts) for when hop-count-dependent time-values are required.
The time-values are represented as time-codes as defined in
Section 5. This <time-data> data structure is specified, using the
regular expression syntax of [RFC5444], by:
<time-data> = (<time-code><hop-count>)*<time-code>
where:
<time-code> is an 8-bit unsigned integer field containing a time-
code as defined in Section 5.
<hop-count> is an 8-bit unsigned integer field specifying a hop
count from the message originator.
A <time-data> structure thus consists of an odd number of octets;
with a repetition factor of n for the (time, hop count) pairs in the
regular expression syntax, it contains 2n+1 octets. On reception, n
is determined from the length.
A <time-data> field may be thus represented by:
<t_1><d_1><t_2><d_2> ... <t_i><d_i> ... <t_n><d_n><t_default>
<d_1>, ... <d_n>, if present, MUST be a strictly increasing sequence,
with <d_n> < 255. Then, at the receiving node's hop count from the
originator node, the time-value indicated is that represented by the
time-code:
o <t_1>, if n > 0 and hop count <= <d_1>;
o <t_i+1>, if n > 1 and <d_i> < hop count <= <d_i+1> for some i such
that 1 <= i < n;
o <t_default> otherwise, i.e. if n = 0 or hop count > <d_n>.
If included in a message without a <msg-hop-count> field in its
Message Header, or in a Packet TLV, then the form of this data
structure with a single time-code in <time-data> (i.e., n = 0) SHOULD
be used.
6.1. Single-Value Time TLVs
The purpose of a single value Time TLV is to allow a single time-
value to be determined by a node receiving an entity containing the
Time TLV, based on its hop count from the entity's originator. The
Time TLV may contain information that allows that time-value to be a
function of the hop count; thus, different receiving nodes may
determine different time-values.
A single-value Time TLV may be a Packet TLV, a Message TLV, or an
Address Block TLV.
A Time TLV that has the tismultivalue flag cleared ('0') in its <tlv-
flags> field, as defined in [RFC5444], contains a single <time-data>,
as defined above, in its <value> field. For such a Time TLV:
o The <length> field in the TLV MUST contain the value 2n+1, with n
being the number of (time-value, hop count) pairs in the Time TLV.
o The number of (time-value, hop count) pairs MUST be identified by
inspecting the <length> field in the TLV. The number of such
pairs, n, is:
* n := (<length> - 1) / 2
This MUST be an integer value.
6.2. Multi-Value Time TLVs
The purpose of a multi-value Time TLV is to associate a set of <time-
data> structures to an identically sized set of addresses, as
described in [RFC5444]. For each of these <time-data> structures, a
single time-value can be determined by a node receiving an entity
containing the Time TLV, based on its hop count from the entity's
originator. The Time TLV may contain information that allows that
time-value to be a function of the hop count, and thus different
receiving nodes may determine different time-values.
Multi-value Time TLVs MUST be Address Block TLVs. A multi-value Time
TLV MUST NOT be a Packet TLV or Message TLV.
A Time TLV that has the tismultivalue flag set ('1') in its <tlv-
flags> field, as defined in [RFC5444], contains a sequence of <time-
data> structures, as defined above, in its <value> field. For such a
Time TLV:
o The <length> field in the TLV MUST contain the value m * (2n+1),
with n being the number of (time-value, hop count) pairs in the
Time TLV, and m being number-values as defined in [RFC5444].
o The number of <time-data> structures included in the <value> field
is equal to number-values as defined in [RFC5444].
o The number of (time-value, hop count) pairs in each <time-data>
structure MUST be the same, and MUST be identified by inspecting
the <length> field in the TLV and using number-values as defined
in [RFC5444]. The number of such pairs in each <time-data>
structure, n, is:
* n := ((<length> / number-values) - 1) / 2
This MUST be an integer value. The lists of hop count values MAY
be different.
7. Message TLVs
Two Message TLVs are defined, for signaling message interval
(INTERVAL_TIME) and message validity time (VALIDITY_TIME).
7.1. INTERVAL_TIME TLV
An INTERVAL_TIME TLV is a Message TLV that defines the maximum time
before another message of the same type as this message from the same
originator should be received. This interval time MAY be specified
to depend on the hop count from the originator. (This is appropriate
if messages are sent with different hop limits so that receiving
nodes at greater hop counts have an increased interval time.)
A message MUST NOT include more than one INTERVAL_TIME TLV.
An INTERVAL_TIME TLV is an example of a Time TLV specified as in
Section 5.
7.2. VALIDITY_TIME TLV
A VALIDITY_TIME TLV is a Message TLV that defines the validity time
of the information carried in the message in which the TLV is
contained. After this time, the receiving node MUST consider the
message content to no longer be valid (unless repeated in a later
message). The validity time of a message MAY be specified to depend
on the hop count from its originator. (This is appropriate if
messages are sent with different hop limits so that receiving nodes
at greater hop counts receive information less frequently and must
treat is as valid for longer.)
A message MUST NOT include more than one VALIDITY_TIME TLV.
A VALIDITY_TIME TLV is an example of a Time TLV specified as in
Section 5.
8. Address Block TLVs
Two Address Block TLVs are defined, for signaling address
advertisement interval (INTERVAL_TIME) and address validity time
(VALIDITY_TIME).
8.1. INTERVAL_TIME TLV
An INTERVAL_TIME TLV is an Address Block TLV that defines the maximum
time before this address from the same originator should be received
again. This interval time MAY be specified to depend on the hop
count from the originator. (This is appropriate if addresses are
contained in messages sent with different hop limits so that
receiving nodes at greater hop counts have an increased interval
time.)
A protocol using this TLV and the same named Message TLV MUST specify
how to interpret the case when both are present (typically, that the
former overrides the latter for those addresses that are covered by
the former).
An INTERVAL_TIME TLV is an example of a Time TLV specified as in
Section 5.
8.2. VALIDITY_TIME TLV
A VALIDITY_TIME TLV is an Address Block TLV that defines the validity
time of the addresses to which the TLV is associated. After this
time, the receiving node MUST consider the addresses to no longer be
valid (unless these are repeated in a later message). The validity
time of an address MAY be specified to depend on the hop count from
its originator. (This is appropriate if addresses are contained in
messages sent with different hop limits so that receiving nodes at
greater hop counts receive information less frequently and must treat
is as valid for longer.)
A protocol using this TLV and the same named Message TLV MUST specify
how to interpret the case when both are present (typically, that the
former overrides the latter for those addresses that are covered by
the former).
A VALIDITY_TIME TLV is an example of a Time TLV specified as in
Section 5.
9. IANA Considerations
This specification defines two Message TLV Types, which have been
allocated from the "Assigned Message TLV Types" repository of
[RFC5444] as specified in Table 1, and two Address Block TLV Types,
which have been allocated from the "Assigned Address Block TLV Types"
repository of [RFC5444] as specified in Table 2.
IANA has assigned the same numerical value to the Message TLV Type
and Address Block TLV Type with the same name.
9.1. Expert Review: Evaluation Guidelines
For the registries for TLV Type Extensions where an Expert Review is
required, the designated expert SHOULD take the same general
recommendations into consideration as are specified by [RFC5444].
9.2. Message TLV Types
+---------------+------+-----------+--------------------------------+
| Name | Type | Type | Description |
| | | Extension | |
+---------------+------+-----------+--------------------------------+
| INTERVAL_TIME | 0 | 0 | The maximum time before |
| | | | another message of the same |
| | | | type as this message from the |
| | | | same originator should be |
| | | | received |
| Unassigned | 0 | 1-223 | Expert Review |
| | | 224-255 | Experimental Use |
| VALIDITY_TIME | 1 | 0 | The time from receipt of the |
| | | | message during which the |
| | | | information contained in the |
| | | | message is to be considered |
| | | | valid |
| Unassigned | 1 | 1-223 | Expert Review |
| | | 224-255 | Experimental Use |
+---------------+------+-----------+--------------------------------+
Table 1
9.3. Address Block TLV Types
+---------------+------+-----------+--------------------------------+
| Name | Type | Type | Description |
| | | extension | |
+---------------+------+-----------+--------------------------------+
| INTERVAL_TIME | 0 | 0 | The maximum time before |
| | | | another message of the same |
| | | | type as this message from the |
| | | | same originator and containing |
| | | | this address should be |
| | | | received |
| Unassigned | 0 | 1-223 | Expert Review |
| | | 224-255 | Experimental Use |
| VALIDITY_TIME | 1 | 0 | The time from receipt of the |
| | | | address during which the |
| | | | information regarding this |
| | | | address is to be considered |
| | | | valid |
| Unassigned | 0 | 1-223 | Expert Review |
| | | 224-255 | Experimental Use |
+---------------+------+-----------+--------------------------------+
Table 2
10. Security Considerations
This document specifies how to add data structures (TLVs) that
provide timing information to packets and messages specified using
[RFC5444]. In particular, information validity durations and
reporting intervals may be added.
The general security threats that apply are those general to
[RFC5444] and described therein, problems of integrity and
confidentiality. With regard to the former, modification of a Time
TLV can cause information to have an invalid validity time, or
expected interval time. This may cause incorrect protocol
performance. Modification or addition of timed information can add
to a protocol's workload (especially if a short validity time is
specified) and storage requirements (especially if a long validity
time is specified).
To counter these threats, the security suggestions in [RFC5444], for
the use of authentication and encryption, are appropriate.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5444] Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
"Generalized Mobile Ad Hoc Network (MANET) Packet/Message
Format", RFC 5444, February 2009.
11.2. Informative References
[RFC3626] Clausen, T. and P. Jacquet, "The Optimized Link State
Routing Protocol", RFC 3626, October 2003.
Appendix A. Acknowledgements
The authors would like to thank Brian Adamson and Justin Dean (both
NRL) and Ian Chakeres (Motorola) for their contributions, and Alan
Cullen (BAE Systems) and Jari Arkko (Ericsson, Finland) for their
careful reviews of this specification.
Authors' Addresses
Thomas Heide Clausen
LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349
EMail: T.Clausen@computer.org
URI: http://www.ThomasClausen.org/
Christopher Dearlove
BAE Systems Advanced Technology Centre
Phone: +44 1245 242194
EMail: chris.dearlove@baesystems.com
URI: http://www.baesystems.com/
ISO Plain Language
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§1What This Document Adds
The generalized packet and message format of RFC 5444 specifies a signaling format that MANET routing protocols can employ for exchanging protocol information. That format can express and associate attributes to packets, messages or addresses by way of a general TLV, type-length-value, mechanism.
This document specifies a general Time TLV structure that any MANET routing protocol can use when it needs to express either single time-values or a set of time-values with each time-value associated with a range of hop counts, as provided by the Message Header of RFC 5444. This allows a receiving node to determine a single time-value if it either knows its hop count from the originator node or the Time TLV specifies a single time-value.
A time-value is, in this context, not an absolute point in time, but an interval or a duration. An instance of a Time TLV can express an interval or a duration such as 10 seconds.
The document also specifies two Message TLV Types that use the proposed structure. These are INTERVAL_TIME and VALIDITY_TIME, specifying respectively the maximum time before another message of the same type as this message from the same originator should be received, and the duration for which the information in this message is valid after receipt. If both are present, the latter will usually be greater than the former, in order to allow for possible message loss.
Two Address Block TLV Types are specified as well, again INTERVAL_TIME and VALIDITY_TIME, defined equivalently to the two Message TLVs with the same names.
The MANET packet format of RFC 5444 lets a protocol attach small pieces of extra information to a packet, a message or a list of addresses. This document defines one such piece: a way of saying how long a time is.
§1.1Why Time Needs Its Own Encoding
The Time TLV structure is intended to be used as a component in a MANET routing protocol, for example to indicate the expected spacing between successive transmissions of a given Message Type, by including a Time TLV in transmitted messages.
Some MANET routing protocols use very short spacing for some messages and very long spacing for others, or change the message transmission rate according to observed behaviour. If a network is observed at some point to exhibit a highly dynamic topology, a sub-second message spacing could be appropriate, whereas if the network later stabilises, multi-hour spacing may become appropriate. Different MANET routing protocols and different deployments may have different granularity requirements and bounds on the shortest and longest spacing between successive message transmissions.
MANET routing protocol deployments also typically use bandwidth-limited wireless network interfaces, and therefore prefer to trade off computational complexity for a saving in the number of bits transmitted. That is practical here, because the intended uses of Time TLVs, including message interval time and information validity time, do not require high-precision values of time.
The structure caters to these characteristics by encoding time-values, such as an interval or a duration, in an 8-bit field; by allowing these time-values to range from very small, for example 1/1024 second, to very long, for example 45 days; and by allowing a routing protocol or a deployment to parameterize this, to attain finer granularity at the expense of a lower upper bound, through a single parameter, C. The parameter C must be the same for all MANET routers in the same deployment.
The TLV mechanism of RFC 5444 allows associating a value to a packet, a message or addresses. The data structure is identical in all three cases, but the TLV's position in a received packet determines what it is: a Packet TLV appears in the Packet Header before any messages, a Message TLV appears in the TLV Block immediately following a Message Header, and an Address Block TLV appears in the TLV Block immediately following an Address Block. While TLVs may be structurally identical, what they express may be different, which is why RFC 5444 specifies separate registries for Packet TLV Types, Message TLV Types and Address Block TLV Types.
Two forces shape this design. Wireless links in these networks carry few bits, so a time should not cost many bytes. And the times a routing protocol cares about do not need to be precise, because they describe how long to wait and how long to trust something, not an exact instant.
§5Encoding a Time Value in 8 Bits
Time-values are each represented in an 8-bit time-code, one or more of which may be used in a TLV's value field. Of these 8 bits, the least significant 3 bits represent the mantissa, a, and the most significant 5 bits represent the exponent, b, so that the time-value is (1 + a/8) * 2^b * C and the time-code is 8 * b + a.
All nodes in the MANET MUST use the same value of the constant C, which is specified in seconds, so all time-values are in seconds too. C MUST be greater than 0 seconds. Ascending values of the time-code represent ascending time-values, so time-values may be compared by comparing time-codes.
An algorithm for computing the time-code representing the smallest representable time-value not less than a given time-value t is given in the RFC: find the largest integer b such that t/C is at least 2^b; set a to 8 times (t divided by C times 2^b, minus 1), rounded up to the nearest integer; if a equals 8, set b to b plus 1 and set a to 0; and then, if a is between 0 and 7 and b is between 0 and 31, the required time-value can be represented by the time-code 8 * b + a, otherwise it cannot.
The minimum time-value that can be represented is C. The maximum is 15 * 2^28 * C, or about 4.0 * 10^9 * C; if C is 1/1024 second, that is about 45 days. A protocol using this time representation MUST define the value of C, and MAY specify that the all-bits zero time-value represents a time-value of zero and/or that the all-bits one time-value represents an indefinitely large time-value.
The encoding works like a rounding scheme. Five bits hold a power of two and three bits hold a fraction of it, so the byte can stand for a wide range of durations while still ordering correctly: a bigger number always means a longer time.
One number, C, is shared by everyone in the network. It sets the unit. Choosing C as a thousandth of a second gives times from a thousandth of a second up to about 45 days.
§6The Time TLV Structure
The data structure allows the representation of a single time-value, or of a default time-value plus pairs of time-values and hop counts for when hop-count-dependent time-values are required. Using the regular expression syntax of RFC 5444, it is specified as <time-data> = (<time-code><hop-count>)*<time-code>.
A time-code is an 8-bit unsigned integer field containing a time-code, and a hop-count is an 8-bit unsigned integer field specifying a hop count from the message originator. A time-data structure therefore consists of an odd number of octets: with a repetition factor of n for the (time, hop count) pairs, it contains 2n+1 octets, and on reception n is determined from the length.
The structure may be written as <t_1><d_1><t_2><d_2> ... <t_n><d_n><t_default>. The hop counts d_1 to d_n, if present, MUST be a strictly increasing sequence with d_n less than 255. At the receiving node's hop count from the originator, the indicated time-value is t_1 if n is greater than 0 and the hop count is at most d_1; t_i+1 if n is greater than 1 and d_i is less than the hop count and at most d_i+1; and t_default otherwise, that is, if n is 0 or the hop count is greater than d_n.
Two variants of the TLV exist. A single-value Time TLV, with the tismultivalue flag cleared in its tlv-flags field, contains a single time-data structure in its value field, and its length field MUST contain the value 2n+1, with n being the number of (time-value, hop count) pairs. The number of pairs is found from the length as n := (length - 1) / 2, and that MUST be an integer value.
A multi-value Time TLV, with the tismultivalue flag set, contains a sequence of time-data structures, associating a set of them with an identically sized set of addresses. Multi-value Time TLVs MUST be Address Block TLVs, and MUST NOT be a Packet TLV or a Message TLV. Their length field MUST contain the value m * (2n+1), where n is the number of (time-value, hop count) pairs and m is the number of values as defined in RFC 5444. The number of pairs in each time-data structure is found as n := ((length / number-values) - 1) / 2, and that MUST be an integer value; the lists of hop count values MAY be different.
Hop count is defined as the number of hops from the message originator to the message recipient, equal to the msg-hop-count field in the msg-header element of RFC 5444 if present, after it has been incremented on reception. If that field is not present, or in a Packet TLV, hop count is defined to equal 255. A time-data structure included in a message without a msg-hop-count field, or in a Packet TLV, SHOULD use the single time-code form, that is, n = 0.
The value of the TLV is a list of time codes, each optionally followed by the hop count at which it starts to apply. A node that knows how many hops away it is looks up the entry that covers its own hop count, and uses that time. Everything beyond the last hop count uses the final, default entry.
<time-data> = (<time-code><hop-count>)*<time-code>
<t_1><d_1><t_2><d_2> ... <t_n><d_n><t_default>
n := (length - 1) / 2 octets: 2n+1§7The Message TLVs and the Address Block TLVs
Two Message TLVs are defined, for signalling message interval and message validity time.
An INTERVAL_TIME Message TLV defines the maximum time before another message of the same type as this message from the same originator should be received. This interval time MAY be specified to depend on the hop count from the originator, which is appropriate if messages are sent with different hop limits so that receiving nodes at greater hop counts have an increased interval time. A message MUST NOT include more than one INTERVAL_TIME TLV.
A VALIDITY_TIME Message TLV defines the validity time of the information carried in the message in which the TLV is contained. After this time the receiving node MUST consider the message content to no longer be valid, unless it is repeated in a later message. The validity time of a message MAY also be specified to depend on the hop count from its originator, which is appropriate if messages are sent with different hop limits so that receiving nodes at greater hop counts receive information less frequently and must treat it as valid for longer. A message MUST NOT include more than one VALIDITY_TIME TLV.
Two Address Block TLVs are defined with the same names. An Address Block INTERVAL_TIME TLV defines the maximum time before this address from the same originator should be received again, and its interval time MAY depend on the hop count from the originator. An Address Block VALIDITY_TIME TLV defines the validity time of the addresses the TLV is associated with; after this time the receiving node MUST consider the addresses to no longer be valid, unless they are repeated in a later message, and the validity time MAY depend on the hop count.
A protocol that uses an Address Block TLV and the same named Message TLV MUST specify how to interpret the case when both are present, typically that the former overrides the latter for the addresses it covers.
In the terminology of the document, a time-value is a time measured in seconds and a time-code is the 8-bit field that represents a time-value.
Two questions are answered by the TLVs. The interval TLV says how long the sender intends to wait before sending the same kind of message again, so a receiver knows when to expect it. The validity TLV says how long the contents should be trusted after they arrive.
Both can be attached to a whole message or to a list of addresses inside a message, and both can say that the time grows with distance from the sender.
§9IANA Considerations
This specification defines two Message TLV Types, allocated from the Assigned Message TLV Types repository of RFC 5444, and two Address Block TLV Types, allocated from the Assigned Address Block TLV Types repository of RFC 5444. IANA has assigned the same numerical value to the Message TLV Type and the Address Block TLV Type with the same name.
INTERVAL_TIME is assigned type 0 and VALIDITY_TIME is assigned type 1 in both registries, with type extension 0. Type extensions 1 to 223 require Expert Review and 224 to 255 are for Experimental Use.
The two new TLVs are registered in the registries of RFC 5444, and the message version and the address list version share the same numbers: zero for the interval and one for the validity time.
§10Security Considerations
This document specifies how to add data structures, TLVs, that provide timing information to packets and messages specified using RFC 5444. In particular, information validity durations and reporting intervals may be added.
The general security threats that apply are those general to RFC 5444, problems of integrity and confidentiality. With regard to integrity, modification of a Time TLV can cause information to have an invalid validity time or expected interval time, which may cause incorrect protocol performance. Modification or addition of timed information can add to a protocol's workload, especially if a short validity time is specified, and to its storage requirements, especially if a long validity time is specified.
To counter these threats, the security suggestions in RFC 5444 for the use of authentication and encryption are appropriate.
The times inside these TLVs are only as trustworthy as the message carrying them. An attacker who changes a validity time can make the network do more work or trust stale information for longer, so the same authentication and encryption used for the rest of the message are the right protection.