Hello folks,
What is the plan for releasing draft 13?
For your holiday enjoyment here is a new Internet Draft. Comments will
be appreciated.
Regards,
Charlie P.
Mobile Ad Hoc Networks [manet] C. Perkins
Internet-Draft Futurewei
Expires: June 30, 2016 December 28, 2015
Expected Transmission Time Cost Metric
draft-perkins-manet-ett-00.txt
Abstract
The Expected Transmission Time metric (ETT) is a simple cost metric
that enables selection of a route with the shortest expected time
requirement for transmissions. In conjunction with a minimum cost
per link, ETT favors routes that have the least number of hops as
well as the lowest level of damaging interference from source to
destination.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Expected Transmission Time . . . . . . . . . . . . . . . . . 2
3. Cost() and Loop_Free() functions for the ETT metric . . . . . 2
4. Units for ETT metric . . . . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
It is often desirable to identify which of several available routes
requires the least amount of time for data transmission, without the
danger of favoring noisy routes that only traverse a few hops.
The ETT metric favors routes with high bandwidth, and yet the is
simple to calculate. The Expected Transmission Time for a route is
the sum of the transmission times at each hop, so that the ETT cost
metric is additive, monotonic, and easy to calculate.
2. Expected Transmission Time
The average time consumed by transmissions over a link determines the
(ETT) for that link. The interval over which the average is computed
is AVERAGING_INTERVAL. The default value is 10 seconds, but this may
be changed locally according to local dynamic conditions.
For a route R as follows composed of links between nodes N_1 ... N_k:
N_1 <--> N_2 <--> N_3 <--> .... <--> N_k
denote the link between N_{i} and N_{i+1} by L_{i,i+1} and the
average time consumed by transmissions over link L_{i,i+1} by
ETT(L_{i,i+1}). The ETT cost for route R is the sum of the ETT costs
for each link, or in other words ETT(R) = SUM ETT(L_{i,i+1}) [i ==
1..k-1].
3. Cost() and Loop_Free() functions for the ETT metric
To be useful with AODVv2 [I-D.ietf-manet-aodvv2], it is helpful to
define functions Cost() and Loop_Free() for the ETT metric.
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The definition of the Cost() function for ETT is exactly the same as
the ETT itself. In other words, using ETT, Cost(L) = ETT(L) and
Cost(R) = ETT(R) for a link L and a route R.
For routes R1 and R2, Loop_Free(R1, R2) for ETT is defined as
follows:
LoopFree(R1,R2) := ETT(R1) < ETT(R2)
or, in other words, LoopFree(R1,R2) returns TRUE if the cost of R1 is
less than the cost of R2 (cost as measured by the ETT metric).
4. Units for ETT metric
Transmission times per bit for modern wireless media are tiny. For a
slow link operating at only 1 Mb/sec, the transmission time for a
single bit is 1 microsecond. Faster links commercially available
today for personal computers are able to transmit one bit in less
than 1 nanosecond. Already, terabit wireless transmission is
available, for instance with satellite communications. In order to
lengthen the time for which the ETT metric may be useful for route
selection in wireless networks, it is necessary to pick an extremely
unit of measurement.
For the purposes of this initial draft, it is proposed to use units
of 0.001 picosecond, and for the value of metric to be 16 bits long,
with substructure as follows.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Exponent | Significant Digits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Structure of Value Field for ETT metric
From the figure, it is seen that the range of values for the ETT
metric will be 2^31 * [0..2^11]. This will enable measurements of
ETT for links as slow as 4 milliseconds, with accuracy of better than
one part in a thousand. For routes of length up to 64 hops, the
average link speed would need to be faster, perhaps no worse than 64
microseconds/bit; this enables route selection in foreseeable
networks with that many hops.
However, should this be insufficient, either more bits of resolution
could be added (i.e., metric value of 24 bits instead of 16), or the
exponent field could be made into 6 bits or longer.
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5. Security Considerations
This document does not introduce any security mechanisms, and does
not have any impact on existing security mechanisms.
6. IANA Considerations
The routing metric defined in the document should be assigned a value
from the "Routing Metric/Constraint Type" registry [RFC6551].
7. References
7.1. Normative References
[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012,
<http://www.rfc-editor.org/info/rfc6551>.
7.2. Informative References
[I-D.ietf-manet-aodvv2]
Perkins, C., Ratliff, S., Dowdell, J., Steenbrink, L., and
V. Mercieca, "Ad Hoc On-demand Distance Vector Routing
Version 2 (AODVv2)", draft-ietf-manet-aodvv2-12 (work in
progress), October 2015.
Author's Address
Charles E. Perkins
Futurewei Inc.
2330 Central Expressway
Santa Clara, CA 95050
USA
Phone: +1-408-330-4586
Email: charliep@xxxxxxxxxxxx
Perkins Expires June 30, 2016 [Page 4]
