Internet DRAFT - draft-perkins-manet-tdpb

draft-perkins-manet-tdpb







Mobile Ad Hoc Networks [manet]                                C. Perkins
Internet-Draft                                                 Futurewei
Expires: April 7, 2016                                   October 5, 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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   This Internet-Draft will expire on April 7, 2016.

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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 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 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; this enables route selection in 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@computer.org



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