Hello folks,
If you would like to make comments on this draft before I submit it on
Monday, that would be appreciated.
I changed from 24 integer to a 16-bit fixed-point format that greatly
increases the range of values at the expense of some meaningless
precision. The precision is still better than one part in two thousand,
which for a bandwidth-related metric is still fabulous.
Regards,
Charlie P.
Mobile Ad Hoc Networks [manet] C. Perkins
Internet-Draft Futurewei
Expires: April 3, 2016 October 1, 2015
Transmission Duration Per Bit Cost Metric
draft-perkins-manet-tdpb-00.txt
Abstract
The Transmission Duration Per Bit metric is a simple cost metric that
enables selection of a route with the highest end-to-end bandwidth.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Total Transmission Duration Per Bit . . . . . . . . . . . . . 2
3. Cost() and Loop_Free() functions for the TDPB metric . . . . 3
4. Units for TDPB 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
offers the highest bandwidth for data transmission, regardless of
other considerations such as number of hops. However, bandwidth is
in certain ways less suitable for use as a routing metric; in
particular, the bandwidth of a path with several hops is not as easy
to calculate as cost metrics such as hop count.
Instead of bandwidth, we specify the transmission duration as a cost
metric. The route with the lowest total transmission duration per
bit is the same as the route with the highest bandwidth, and yet the
end-to-end transmission duration per bit is simple to calculate. The
total transmission duration per bit for a route is the sum of the
transmission durations at each hop, so that the TDPB cost metric is
additive, monotonic, and easy to calculate.
2. Total Transmission Duration Per Bit
The bandwidth between two neighboring nodes determines the
transmission duration per bit (TDPB) between those two nodes. If
bandwidth = B bits/second, then we define TDPB = 1 / bandwidth.
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
bandwidth over link L_{i,i+1} by B_{i,i+1}. Then the TDPB over link
L_{i,i+1} is 1 / B_{i,i+1}, which we denote as TDPB(L_{i,i+1}). The
TDPB cost for route R is the sum of the TDPB costs for each link, or
in other words TDPB(R) = SUM TDPB(L_{i,i+1}) [i == 1..k-1].
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3. Cost() and Loop_Free() functions for the TDPB metric
To be useful with AODVv2 [I-D.ietf-manet-aodvv2], it is helpful to
define functions Cost() and Loop_Free() for the TDPB metric.
The definition of the Cost() function for TDPB is exactly the same as
the TDPB itself. In other words, using TDPB, Cost(L) = TDPB(L) and
Cost(R) = TDPB(R) for a link L and a route R.
For routes R1 and R2, Loop_Free(R1, R2) for TDPB is defined as
follows:
LoopFree(R1,R2) := TDPB(R1) < TDPB(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 TDPB metric).
4. Units for TDPB 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 TDPB metric may be useful for route
selection in wireless networks, it is necessary to pick a unit of
measurement no larger than one picosecond.
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 TDBP metric
From the figure, it is seen that the range of values for the TDBP
metric will be 2^31 * [0..2^11]. This will enable measurements of
TDBP 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, which would still be workable for any foreseeable
networks with that many hops.
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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. Also see RFC
7185 [RFC7185] for related discussion.
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 (AODVv2)
Routing", draft-ietf-manet-aodvv2-11 (work in progress),
July 2015.
[RFC7185] Dearlove, C., Clausen, T., and P. Jacquet, "Link Metrics
for the Mobile Ad Hoc Network (MANET) Routing Protocol
OLSRv2 - Rationale", RFC 7185, DOI 10.17487/RFC7185, April
2014, <http://www.rfc-editor.org/info/rfc7185>.
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 April 3, 2016 [Page 4]
