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Versions: 00 draft-ietf-manet-timetlv
Mobile Ad hoc Networking (MANET) T. Clausen
Internet-Draft LIX, Ecole Polytechnique, France
Expires: August 12, 2007 C. Dearlove
BAE Systems Advanced Technology
Centre
February 8, 2007
Representing multi-value time in MANETs
draft-clausen-manet-timetlv-00
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Copyright (C) The IETF Trust (2007).
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Abstract
This document describes a general and flexible TLV (type-length-value
structure) for representing time using the generalized MANET packet/
message format. It defines two message TLVs for representing
validity and interval times for MANET routing protocols.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
4. Protocol Overview and Functioning . . . . . . . . . . . . . . 6
5. Representing Time . . . . . . . . . . . . . . . . . . . . . . 7
6. General Time TLV Structure . . . . . . . . . . . . . . . . . . 8
7. Message TLVs . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. VALIDITY_TIME TLV . . . . . . . . . . . . . . . . . . . . 10
7.2. INTERVAL_TIME TLV . . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
The generalized packet/message format [1] specifies a signaling
format which 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 value associated
with a range of distances. This allows a receiving node to determine
a single time value if either it knows its distance from the
originator node, which [1] may provide, or the Time TLV specifies a
single time value.
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.
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2. Terminology
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [2].
Additionally, this document uses terminology from [1], and introduces
the following terminology:
Distance - the number of hops from the message originator to the
message recipient, as may be indicated using the <hop-count> field
in the full message header defined in [1], after being incremented
on reception.
Time Value - a time, measured in seconds.
Time Code - an 8 bit field, representing a time value.
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3. Applicability Statement
The TLV described in this document is applicable whenever a single
time value, or a time value that varies with distance from the
originator of a message, is required in a protocol using the
generalized MANET packet/message format [1].
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 distance between the originating
and receiving nodes if messages of the same type are sent with
different hop limits as defined in [1]. Note that if using [1], then
distance information is available as the hop count field in a full
message header (after being incremented).
Parts of this document have been generalized from material in the
proactive MANET routing protocol OLSR (The Optimized Link State
Routing Protocol) [3].
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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 [1]. Protocols
using the mechanisms and TLVs specified in this document must ensure
that they do so in a coherent way.
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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 four
bits represent the mantissa (a), and the most significant four bits
represent the exponent (b), so that:
o time value = (1 + a/16) * 2^b * C
o time code = 16 * b + a
All nodes in the network MUST use the same value of C, which will be
specified in seconds, hence so will be all time values. 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 = 16 * (t / (C * 2^b) - 1), rounded up to the nearest
integer;
3. if a == 16 then set b = b + 1 and set a = 0;
4. if a and b are in the range 0 and 15 then the required time value
can be represented by the time code 16 * b + a, otherwise it can
not.
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
63488 * C.
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6. General Time TLV Structure
A Time TLV may be a packet, message or address block TLV. If it is a
packet or message TLV then it must be a single value TLV as defined
in [1]; if it is an address block TLV then it may be single value or
multivalue TLV. The specific Time TLVs specified in this document,
in Section 7 are message, and hence single value, TLVs. Note that
even a single value Time TLV may contain a multiple octet <value>
field.
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 distance from the entity's originator. The
Time TLV may contain information that allows that time value to be a
function of distance, and thus different receiving nodes may
determine different time values. If a receiving node will not be
able to determine its distance from the originating node, then the
form of this Time TLV with a single time code in a <value> field (or
single value subfield) SHOULD be used.
The <value> field of a single value Time TLV is specified, using the
regular expression syntax of [1], by:
<value> = {<time><distance>}*<time>
where:
<time> is an 8 bit field containing a time code as defined in
Section 5.
<distance> is an 8 bit field specifying a distance from the message
originator, in hops.
A single value <value> field thus consists of an odd number of
octets; with a repetition factor of n in the regular expression
syntax it contains 2n+1 octets, thus the <length> field of a single
value Time TLV, which MUST always be present, is given by:
o <length> = 2n+1
A single value <value> 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.
Then, at the receiving node's distance from the originator node, the
time value indicated is that represented by the time code:
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o <t_1>, if n > 0 and distance <= <d_1>;
o <t_i+1>, if n > 1 and <d_i> < distance <= <d_i+1> for some i such
that 1 <= i < n;
o <t_default> otherwise, i.e. if n == 0 or distance > <d_n>.
In a multivalue Time TLV, each single value subfield of the
multivalue Time TLV is defined as above. Note that [1] requires that
each single value subfield has the same length (i.e. the same value
of n) but they need not use the same values of <d_1> to <d_n>.
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7. Message TLVs
Two message TLVs are defined, for signaling message validity time
(VALIDITY_TIME) and message interval (INTERVAL_TIME).
7.1. 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 distance from its originator. (This is appropriate if
messages are sent with different hop limits, so that receiving nodes
at greater distances receive information less frequently and must
treat is as valid for longer.)
A VALIDITY_TIME TLV is an example of a Time TLV specified as in
Section 5.
7.2. 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 distance from the originator. (This is appropriate
if messages are sent with different hop limits, so that receiving
nodes at greater distances have an increased interval time.)
An INTERVAL_TIME TLV is an example of a Time TLV specified as in
Section 5.
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8. IANA Considerations
This specification defines two message TLV types, which must be
allocated from the "Assigned Message TLV Types" repository of [1].
+--------------------+-------+--------------------------------------+
| Mnemonic | Value | Description |
+--------------------+-------+--------------------------------------+
| VALIDITY_TIME | TBD | The time from receipt of the message |
| | | during which the information |
| | | contained in the message is to be |
| | | considered valid |
| | | |
| INTERVAL_TIME | TBD | The maximum time before another |
| | | message of the same type as this |
| | | message from the same originator |
| | | should be received |
+--------------------+-------+--------------------------------------+
Table 1
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9. Security Considerations
This document does not specify any security considerations.
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10. References
10.1. Normative References
[1] Clausen, T., Dean, J., Dearlove, C., and C. Adjih, "Generalized
MANET Packet/Message Format", Work In
Progress draft-ietf-manet-packetbb-03.txt, June 2006.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
10.2. Informative References
[3] Clausen, T. and P. Jacquet, "The Optimized Link State Routing
Protocol", RFC 3626, October 2003.
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Appendix A. Acknowledgements
The authors would like to thank Brian Adamson and Justin Dean (both
NRL) for their contributions.
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Authors' Addresses
Thomas Heide Clausen
LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349
Email: T.Clausen@computer.org
URI: http://www.lix.polytechnique.fr/Labo/Thomas.Clausen/
Christopher M. Dearlove
BAE Systems Advanced Technology Centre
Phone: +44 1245 242194
Email: chris.dearlove@baesystems.com
URI: http://www.baesystems.com/ocs/sharedservices/atc/
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