MANET                                                           H. Rogge
Internet-Draft                                           Fraunhofer FKIE
Intended status: Experimental                                E. Baccelli
Expires: May 16, 27, 2016                                              INRIA
                                                       November 13, 24, 2015

   Packet Sequence Number based directional airtime metric for OLSRv2
                 draft-ietf-manet-olsrv2-dat-metric-09
                 draft-ietf-manet-olsrv2-dat-metric-10

Abstract

   This document specifies an directional airtime Directional Airtime (DAT) link metric for
   usage in OLSRv2.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May 16, 27, 2016.

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   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Applicability Statement . . . . . . . . . . . . . . . . . . .   4
   4.  Directional Airtime Metric Rationale  . . . . . . . . . . . .   5
   5.  Metric Functioning & Overview . . . . . . . . . . . . . . . .   6
   6.  Protocol Parameters Constants  . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Recommended Values
   7.  Protocol Parameters . . . . . . . . . . . . . . . . . . .   7
   7.  Protocol Constants . .   8
     7.1.  Recommended Values  . . . . . . . . . . . . . . . . . . .   8
   8.  Data Structures . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Initial Values  . . . . . . . . . . . . . . . . . . . . .   9
   9.  Packets and Messages  . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .  10
     9.2.  Requirements for using DAT metric in OLSRv2
           implementations . . . . . . . . . . . . . . . . . . . . .  10
     9.3.  Link Loss Data Gathering  . . . . . . . . . . . . . . . .  11
     9.4.  HELLO Message Processing  . . . . . . . . . . . . . . . .  11
   10. Timer Event Handling  . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Packet Timeout Processing  . . . . . . . . . . . . . . .  12
     10.2.  Metric Update  . . . . . . . . . . . . . . . . . . . . .  12
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     13.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Future work  . . . . . . . . . . . . . . . . . . . .  16
   Appendix B.  OLSR.org metric history  . . . . . . . . . . . . . .  17
   Appendix C.  Linkspeed stabilization  . . . . . . . . . . . . . .  18
   Appendix D.  Packet loss hysteresis . . . . . . . . . . . . . . .  18
   Appendix E.  Example DAT values . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   One of the major shortcomings of OLSR Optimized Link State Routing (OLSR)
   [RFC3626] is the lack of a granular link cost metric between OLSR
   routers.  Operational experience with OLSR networks gathered since
   its publication has revealed that wireless networks links can have
   highly variable and heterogeneous properties.  This makes a hopcount
   metric insufficient for effective OLSR routing.

   Based on this experience, OLSRv2 [RFC7181] integrates the concept of
   link metrics directly into the core specification of the routing
   protocol.  The OLSRv2 routing metric is an external process, it can
   be any kind of dimensionless additive cost function which reports to
   the OLSRv2 protocol.

   Since 2004 the OLSR.org [OLSR.org] implementation of OLSR included an
   Estimated Transmission Count (ETX) metric [MOBICOM04] as a
   proprietary extension.  While this metric is not perfect, it proved
   to be sufficient for a long time for Community Mesh Networks
   (Appendix B).  But the increasing maximum data rate of IEEE 802.11
   made the ETX metric less efficient than in the past, which is one
   reason to move to a different metric.

   This document describes a Directional Airtime routing metric for
   OLSRv2, a successor of the ETX-derived OLSR.org ETX-derived routing metric for
   OLSR.  It takes both the loss rate and the link speed into account to
   provide a more accurate picture of the links within the network.

   This experimental draft will allow OLSRv2 deployments with a metric
   defined by the IETF Manet group.  It enables easier interoperability
   tests between implementations and will also deliver an a useful baseline
   to compare other metrics to.  Appendix A contains a few possible
   steps to improve the DAT metric. Directional Airtime Metric.

2.  Terminology

   The key words "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].

   The terminology introduced in [RFC5444], [RFC7181] and [RFC6130],
   including the terms "packet", "message" and "TLV" are to be
   interpreted as described therein.

   Additionally, this document uses the following terminology and
   notational conventions:

   DAT  - Directional Airtime (Metric), the link metric described in
      this document, which is a directional variant of ETT.  It does not
      take reverse path loss into account.

   QUEUE  - a first in, first out queue of integers.

   QUEUE[TAIL]  - the most recent element in the queue.

   add(QUEUE, value)  - adds a new element to the TAIL of the queue.

   remove(QUEUE)  - removes the HEAD element of the queue

   sum(QUEUE)  - an operation which returns the sum of all elements in a
      QUEUE.

   diff_seqno(new, old)  - an operation which returns the positive
      distance between two elements of the circular sequence number
      space defined in section 5.1 of [RFC5444].  Its value is either
      (new - old) if this result is positive, or else its value is (new
      - old + 65536).

   MAX(a,b)

   MAX(a, b)  - the maximum of a and b.

   MIN(a, b)  - the minimum of a and b.

   UNDEFINED  - a value not in the normal value range of a variable.

   airtime  - the time a transmitted packet blocks the link layer, e.g.,
      a wireless link.

   ETX  - Expected Transmission Count, a link metric proportional to the
      number of transmissions to successfully send an IP packet over a
      link.

   ETT  - Estimated Travel Time, a link metric proportional to the
      amount of airtime needed to successfully transmit an IP packet
      over a link, not considering layer-2 overhead created by preamble,
      backoff time and queuing.

   DAT  - Directional Airtime Metric, the link metric described in this
      document, which is a directional variant of ETT.  It does not take
      reverse path loss into account.

3.  Applicability Statement

   The Directional Airtime Metric was designed and tested (see
   [olsrv2_paper]) in wireless IEEE 802.11 OLSRv2 [RFC7181] networks.
   These networks employ link layer retransmission to increase the
   delivery probability and multiple probability.  A dynamic rate selection algorithm selects the
   unicast data rates. rate independently for each neighbor.

   As specified in OLSRv2 OLSRv2, the metric calculates only the incoming link
   cost.  It does neither calculate the outgoing metric, nor does it
   decide the link status (heard, symmetric, lost).

   The metric works both for nodes which can send/receive [RFC5444]
   packet sequence numbers and such those which do not have this capability.
   In the absence of such sequence numbers the metric calculates the
   packet loss based on [RFC6130] HELLO message timeouts.

   The metric must learn about the unicast data rate towards each one-
   hop neighbor from an external process, either by configuration or by
   an external measurement process.  This measurement could be done by
   gathering cross-layer data from the operating system or an external
   daemon like DLEP [DLEP], but also by indirect layer-3 measurements
   like packet-pair. packet-pair (see [MOBICOM04]).

   The metric uses RFC5444 [RFC5444] multicast control traffic to determine the
   link packet loss.  The administrator should take care that link layer
   multicast transmission do not not have a higher reception probability
   than the slowest unicast transmission. transmission without retransmission.  It
   might, for example in 802.11g, be necessary to increase the data-rate
   of the multicast transmissions, e.g. set the multicast data-rate to 6
   MBit/s.

   The metric can only handle a certain range of packet loss and unicast
   data-rate.  The maximum packet loss that can be encoded into the
   metric a loss of 7 of 8 packets, packets (87.5%), without link layer
   retransmissions.  The unicast data-rate that can be encoded by this
   metric can be between 1 kBit/s and 2 GBit/s.  This metric has been
   designed for data-rates of 1 MBit/s and hundreds of MBit/s.

4.  Directional Airtime Metric Rationale

   The Directional Airtime Metric has been inspired by the publications
   on the ETX [MOBICOM03] and ETT [MOBICOM04] metric, but differs from
   both of these in several ways.

   Instead of measuring the combined loss probability of a bidirectional
   transmission of a packet over a link in both directions, the
   Directional Airtime Metric measures the incoming loss rate and
   integrates the incoming linkspeed into the metric cost.  There are
   multiple reasons for this decision:

   o  OLSRv2 [RFC7181] defines the link metric as directional costs
      between routers.

   o  Not all link layer implementations use acknowledgement mechanisms.
      Most link layer implementations who do use them use less airtime
      and a more robust modulation for the acknowledgement than the data
      transmission, which makes it more likely for the data transmission
      to be disrupted compared to the acknowledgement.

   o  Incoming packet loss and linkspeed can be measured locally,
      symmetric link loss would need an additional signaling TLV in the
      [RFC6130] HELLO and would delay metric calculation by up to one
      HELLO interval.

   The Directional Airtime Metric does not integrate the packet size
   into the link cost.  Doing so is not feasible in most link-state
   routing protocol implementations.  The routing decision of most
   operation systems don't take packet size into account.  Multiplying
   all link costs of a topology with the size of a data-plane packet
   would never change the Dijkstra result anyways.

   The queue based packet loss estimator has been tested extensively in
   the OLSR.org ETX implementation, see Appendix B.  The output is the
   average of the packet loss over a configured time period.

   The metric normally measures the loss of a link by tracking the
   incoming [RFC5444] packet sequence numbers.  Without these packet
   sequence numbers, the metric does calculate the loss of the link
   based of received and lost [RFC5444] HELLO messages.  It uses the
   incoming HELLO interval time (or if not present, the validity time)
   to decide when a HELLO is lost.

   When a neighbor router resets, its packet sequence number might jump
   to a random value.  The metric tries to detect jumps in the packet
   sequence number and removes them from the data set, because the
   already gathered link loss data should still be valid (see
   Section 9.3.  The link loss data is only removed from memory when a
   Link times out completely and its Link Set tuple is removed from the
   database.

5.  Metric Functioning & Overview

   The Directional Airtime Metric is calculated for each link set entry,
   as defined in [RFC6130] section 7.1.

   The metric processes two kinds of data into the metric value, namely
   packet loss rate and link-speed.  The link-speed is taken from an
   external process not defined in this document.  The current packet
   loss rate is defined in this document by keeping track of packet
   reception and packet loss events.  It could also be calculated by an
   external process with a compatible output.

   Multiple incoming packet loss/reception events must be combined into
   a loss rate to get a smooth metric.  Experiments with exponential
   weighted moving average (EWMA) lead to a highly fluctuating or a slow
   converging metric (or both).  To get a smoother and more controllable
   metric result, this metric uses two fixed length queues to measure
   and average the incoming packet events, one queue for received
   packets and one for the estimated number of packets sent by the other
   side of the link.

   Because the rate of incoming packets is not uniform over time, the
   queue contains a number of counters, each representing a fixed time
   interval.  Incoming packet loss and packet reception event are
   accumulated in the current queue element until a timer adds a new
   empty counter to both queues and remove the oldest counter from both.

   In addition to the packet loss stored in the queue, this metric uses
   a timer to detect a total link-loss.  For every [RFC5444] HELLO
   interval in which the metric received no packet from a neighbor, it
   scales the number of received packets in the queue based on the total
   time interval the queue represents compared to the total time of the
   lost HELLO intervals.

   The average packet loss ratio is calculated as the sum of the 'total
   packets' counters divided by the sum of the 'packets received'
   counters.  This value is then divided through the current link-speed
   and then scaled into the range of metrics allowed for OLSRv2.

   The metric value is then used as L_in_metric of the Link Set (as
   defined in section 8.1. of [RFC7181]).

   While this document does not add new RFC5444 elements to the RFC6130
   HELLO or RFC7181 TC messages, it works best when both the
   INTERVAL_TIME message TLV is present in the HELLO messages and when
   each RFC5444 packet contains an interface specific sequence number.
   It also adds a number of new data entries to be stored for each
   RFC6130 Link.

6.  Protocol Constants

   This specification defines the following constants, which define the
   range of metric values that can be encoded by the DAT metric (see
   Table 1).  They cannot be changed without making the metric outputs
   incomparable and should only be changed for MANET's with a very slow
   or very fast link layer.  See Appendix D Appendix E for example
   metric values.

   DAT_MAXIMUM_LOSS  - Fraction of the loss rate used in this routing
      metric.  Loss rate will be between 0/DAT_MAXIMUM_LOSS and
      (DAT_MAXIMUM_LOSS-1)/DAT_MAXIMUM_LOSS.

   DAT_MINIMUM_BITRATE  - Minimal bit-rate in Bit/s used by this routing
      metric.

                      +---------------------+-------+
                      |         Name        | Value |
                      +---------------------+-------+
                      |   DAT_MAXIMUM_LOSS  |   8   |
                      |                     |       |
                      | DAT_MINIMUM_BITRATE |  1000 |
                      +---------------------+-------+

                      Table 1: DAT Protocol Constants

7.  Protocol Parameters

   This specification defines the following parameters for this routing
   metric.  These parameters are:

   DAT_MEMORY_LENGTH  - Queue length for averaging packet loss.  All
      received and lost packets within the queue length are used to
      calculate the cost of the link.

   DAT_REFRESH_INTERVAL  - interval in seconds between two metric
      recalculations as described in Section 10.2.  This value SHOULD be
      smaller than a typical HELLO interval.  The interval can be a
      fraction of a second.

   DAT_HELLO_TIMEOUT_FACTOR  - multiplier relative to the HELLO_INTERVAL
      (see [RFC6130] Section 5.3.1) after which the DAT metric considers
      a HELLO as lost.

   DAT_SEQNO_RESTART_DETECTION  - threshold in number of missing packets
      (based on received packet sequence numbers) at which point the
      router considers the neighbor has restarted.  This parameter is
      only used for packet sequence number based loss estimation.  This
      number MUST be larger than DAT_MAXIMUM_LOSS.

6.1.

7.1.  Recommended Values

   The proposed values of the protocol parameters are for Community Mesh
   Networks, which mostly use immobile routers.  Using this metric for
   mobile networks might require shorter DAT_REFRESH_INTERVAL and/or
   DAT_MEMORY_LENGTH.

   DAT_MEMORY_LENGTH  := 64

   DAT_REFRESH_INTERVAL  := 1

   DAT_HELLO_TIMEOUT_FACTOR  := 1.2

   DAT_SEQNO_RESTART_DETECTION  := 256

7.  Protocol Constants

   This specification defines the following constants, which define the
   range of metric values that can be encoded by the DAT metric (see
   Table 1).  They cannot be changed without making the metric outputs
   incomparable and should only be changed for MANET's with a very slow
   or very fast linklayer.  See Appendix D Appendix E for example metric
   values.

   DAT_MAXIMUM_LOSS  - Fraction of the loss rate used in this routing
      metric.  Loss rate will be between 0/DAT_MAXIMUM_LOSS and
      (DAT_MAXIMUM_LOSS-1)/DAT_MAXIMUM_LOSS.

   DAT_MINIMUM_BITRATE  - Minimal bit-rate in Bit/s used by this routing
      metric.

                      +---------------------+-------+
                      |         Name        | Value |
                      +---------------------+-------+
                      |   DAT_MAXIMUM_LOSS  |   8   |
                      |                     |       |
                      | DAT_MINIMUM_BITRATE |  1000 |
                      +---------------------+-------+

                      Table 1: DAT Protocol Constants

8.  Data Structures

   This specification extends the Link Set of the Interface Information
   Base, as defined in [RFC6130] section 7.1, by the adding the
   following elements to each link tuple:

   L_DAT_received  - a QUEUE with DAT_MEMORY_LENGTH integer elements.
      Each entry contains the number of successfully received packets
      within an interval of DAT_REFRESH_INTERVAL.

   L_DAT_total  - a QUEUE with DAT_MEMORY_LENGTH integer elements.  Each
      entry contains the estimated number of packets transmitted by the
      neighbor, based on the received packet sequence numbers within an
      interval of DAT_REFRESH_INTERVAL.

   L_DAT_packet_time  - the time when the next RFC5444 packet should
      have arrived.

   L_DAT_hello_interval  - the interval between two hello messages of
      the links neighbor as signaled by the INTERVAL_TIME TLV [RFC5497]
      of NHDP messages [RFC6130].

   L_DAT_lost_packet_intervals  - the estimated number of HELLO
      intervals from this neighbor the metric has not received a single
      packet.

   L_DAT_rx_bitrate  - the current bitrate of incoming unicast traffic
      for this neighbor.

   L_DAT_last_pkt_seqno  - the last received packet sequence number
      received from this link.

   Methods to obtain the value of L_DAT_rx_bitrate are out of the scope
   of this specification.  Such methods may include static configuration
   via a configuration file or dynamic measurement through mechanisms
   described in a separate specification (e.g.  [DLEP]).  Any Link tuple
   with L_status = HEARD or L_status = SYMMETRIC MUST have a specified
   value of L_DAT_rx_bitrate if it is to be used by this routing metric.

   The incoming bitrate value should be stabilized by a hysteresis
   filter to improve the stability of this metric.  See Appendix B
   Appendix C for an example.

   This specification updates the L_in_metric field of the Link Set of
   the Interface Information Base, as defined in section 8.1. of
   [RFC7181])

8.1.  Initial Values

   When generating a new tuple in the Link Set, as defined in [RFC6130]
   section 12.5 bullet 3, the values of the elements specified in
   Section 8 are set as follows:

   o  L_DAT_received := 0, ..., 0.  The queue always has
      DAT_MEMORY_LENGTH elements.

   o  L_DAT_total := 0, ..., 0.  The queue always has DAT_MEMORY_LENGTH
      elements.

   o  L_DAT_packet_time := EXPIRED (no earlier RFC5444 packet received).

   o  L_DAT_hello_interval := UNDEFINED (no earlier NHDP HELLO
      received).

   o  L_DAT_lost_packet_intervals := 0 (no HELLO interval without
      packets).

   o  L_DAT_last_pkt_seqno := UNDEFINED (no earlier RFC5444 packet with
      sequence number received).

9.  Packets and Messages

   This section describes the necessary changes of [RFC7181]
   implementations with DAT metric for the processing and modification
   of incoming and outgoing [RFC5444] data.

9.1.  Definitions

   For the purpose of this section, note the following definitions:

   o  "pkt_seqno" is defined as the [RFC5444] packet sequence number of
      the received packet.

   o  "interval_time" is the time encoded in the INTERVAL_TIME message
      TLV of a received [RFC6130] HELLO message.

   o  "validity_time" is the time encoded in the VALIDITY_TIME message
      TLV of a received [RFC6130] HELLO message.

9.2.  Requirements for using DAT metric in OLSRv2 implementations

   An implementation of OLSRv2 using the metric specified by this
   document SHOULD include the following parts into its [RFC5444]
   output:

   o  an INTERVAL_TIME message TLV in each HELLO message, as defined in
      [RFC6130] section 4.3.2.

   o  an interface specific packet sequence number as defined in
      [RFC5444] section 5.1 which is incremented by 1 for each outgoing
      [RFC5444] packet on the interface.

   An implementation of OLSRv2 using the metric specified by this
   document that inserts packet sequence numbers in some, but not all
   outgoing [RFC5444] packets will make this metric ignore all packets
   without the sequence number.  Putting the INTERVAL_TIME TLV into
   some, but not all Hello messages will make the timeout based loss
   detection slower.  This will only matter in the absence of packet
   sequence numbers.

9.3.  Link Loss Data Gathering

   For each incoming [RFC5444] packet, additional processing SHOULD be
   carried out after the packet messages have been processed as
   specified in [RFC6130] and [RFC7181] as specified in this section.

   [RFC5444] packets without packet sequence number MUST NOT be
   processed in the way described in this section.

   The router updates the Link Set Tuple corresponding to the originator
   of the packet:

   1.  If L_DAT_last_pkt_seqno = UNDEFINED, then:

       1.  L_DAT_received[TAIL] := 1.

       2.  L_DAT_total[TAIL] := 1.

   2.  Otherwise:

       1.  L_DAT_received[TAIL] := L_DAT_received[TAIL] + 1.

       2.  diff := diff_seqno(pkt_seqno, L_DAT_last_pkt_seqno).

       3.  If diff > DAT_SEQNO_RESTART_DETECTION, then:

           1.  diff := 1.

       4.  L_DAT_total[TAIL] := L_DAT_total[TAIL] + diff.

   3.  L_DAT_last_pkt_seqno := pkt_seqno.

   4.  If L_DAT_hello_interval != UNDEFINED, then:

       1.  L_DAT_packet_time := current time + (L_DAT_hello_interval *
           DAT_HELLO_TIMEOUT_FACTOR).

   5.  L_DAT_lost_packet_intervals := 0.

9.4.  HELLO Message Processing

   For each incoming HELLO Message, after it has been processed as
   defined in [RFC6130] section 12, the Link Set Tuple corresponding to
   the incoming HELLO message MUST be updated.

   1.  If the HELLO message contains an INTERVAL_TIME message TLV, then:

       1.  L_DAT_hello_interval := interval_time.

   2.  Otherwise:

       1.  L_DAT_hello_interval := validity_time.

   3.  If L_DAT_last_pkt_seqno = UNDEFINED, then:

       1.  L_DAT_received[TAIL] := L_DAT_received[TAIL] + 1.

       2.  L_DAT_total[TAIL] := L_DAT_total[TAIL] + 1.

       3.  L_DAT_packet_time := current time + (L_DAT_hello_interval *
           DAT_HELLO_TIMEOUT_FACTOR).

10.  Timer Event Handling

   In addition to changes in the [RFC5444] processing/generation code,
   the DAT metric also uses two timer events.

10.1.  Packet Timeout Processing

   When L_DAT_packet_time has timed out, the following step MUST be
   done:

   1.  If L_DAT_last_pkt_seqno = UNDEFINED, then:

       1.  L_DAT_total[TAIL] := L_DAT_total[TAIL] + 1.

   2.  Otherwise:

       1.  L_DAT_lost_packet_intervals := L_DAT_lost_packet_intervals +
           1.

   3.  L_DAT_packet_time := L_DAT_packet_time + L_DAT_hello_interval.

10.2.  Metric Update

   Once every DAT_REFRESH_INTERVAL, all L_in_metric values in all Link
   Set entries MUST be recalculated:

   1.  sum_received := sum(L_DAT_received).

   2.  sum_total := sum(L_DAT_total).

   3.  If L_DAT_hello_interval != UNDEFINED and
       L_DAT_lost_packet_intervals > 0, then:

       1.  lost_time_proportion := L_DAT_hello_interval *
           L_DAT_lost_packet_intervals / DAT_MEMORY_LENGTH.

       2.  sum_received := sum_received * MAX ( 0, 1 -
           lost_time_proportion);

   4.  If sum_received < 1, then:

       1.  L_in_metric := MAXIMUM_METRIC, as defined in [RFC7181]
           section 5.6.1.

   5.  Otherwise:

       1.  loss := sum_total MIN(sum_total / sum_received. sum_received, DAT_MAXIMUM_LOSS).

       2.  If loss > DAT_MAXIMUM_LOSS, then:

           1.  loss := DAT_MAXIMUM_LOSS.

       3.  bitrate := L_DAT_rx_bitrate.

       4.  If bitrate < DAT_MINIMUM_BITRATE, then:

           1.  bitrate := DAT_MINIMUM_BITRATE.

       5. MAX(L_DAT_rx_bitrate, DAT_MINIMUM_BITRATE).

       3.  L_in_metric := (2^24 / DAT_MAXIMUM_LOSS) * loss / (bitrate /
           DAT_MINIMUM_BITRATE).

   6.  remove(L_DAT_total)

   7.  add(L_DAT_total, 0)

   8.  remove(L_DAT_received)

   9.  add(L_DAT_received, 0)

   The calculated L_in_metric value should be stabilized by a hysteresis
   function.  See Appendix C Appendix D for an example.

11.  Security Considerations

   Artificial manipulation of metrics values can drastically alter
   network performance.  In particular, advertising a higher L_in_metric
   value may decrease the amount of incoming traffic, while advertising
   lower L_in_metric may increase the amount of incoming traffic.  By
   artificially increasing or decreasing the L_in_metric values it
   advertises, a rogue router may thus attract or repulse data traffic.
   A rogue router may then potentially degrade data throughput by not
   forwarding data as it should or redirecting traffic into routing
   loops or bad links.  It might also attract traffic for "Man in the
   Middle" attacks or traffic analysis.

   An attacker might also inject packets with incorrect packet level
   sequence numbers, pretending to be somebody else.  This attack can be
   prevented by the true originator of the RFC5444 packets by adding a
   [RFC7182] ICV Packet TLV and TIMESTAMP Packet TLV to each packet.
   This allows the receiver to drop all incoming packets which have a
   forged packet source, both packets generated by the attacker or
   replayed packets.  The signature scheme security mechanism described in [RFC7183] does
   not protect the additional sequence number of the DAT metric because
   it does only sign the RFC5444 messages, not the RFC5444 packet
   header.

   Protection against "Man in the Middle" attacks are out of scope of
   this document.

12.  Acknowledgements

   The authors would like to acknowledge the network administrators from
   Freifunk Berlin [FREIFUNK] and Funkfeuer Vienna [FUNKFEUER] for
   endless hours of testing and suggestions to improve the quality of
   the original ETX metric for the OLSR.org routing daemon.

   This effort/activity is supported by the European Community Framework
   Program 7 within the Future Internet Research and Experimentation
   Initiative (FIRE), Community Networks Testbed for the Future Internet
   ([CONFINE]), contract FP7-288535.

   The authors would like to gratefully acknowledge the following people
   for intense technical discussions, early reviews and comments on the
   specification and its components (listed alphabetically): Teco Boot
   (Infinity Networks), Juliusz Chroboczek (PPS, University of Paris 7),
   Thomas Clausen, Christopher Dearlove (BAE Systems Advanced Technology
   Centre), Ulrich Herberg (Fujitsu Laboratories of America), Markus
   Kittenberger (Funkfeuer Vienna), Joseph Macker (Naval Research
   Laboratory), Fabian Nack and Stan Ratliff (Cisco Systems).

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, BCP 14, 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.

   [RFC5497]  Clausen, T. and C. Dearlove, "Representing Multi-Value
              Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497, March
              2009.

   [RFC6130]  Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
              Network (MANET) Neighborhood Discovery Protocol (NHDP)",
              RFC 6130, April 2011.

   [RFC7181]  Clausen, T., Jacquet, P., and C. Dearlove, "The Optimized
              Link State Routing Protocol version 2", RFC 7181, April
              2014.

13.2.  Informative References

   [RFC3626]  Clausen, T. and P. Jacquet, "Optimized Link State Routing
              Protocol", RFC 3626, October 2003.

   [RFC7182]  Ulrich, U., Clausen, T., and C. Dearlove, "Integrity Check
              Value and Timestamp TLV Definitions for Mobile Ad Hoc
              Networks (MANETs)", RFC 7182, April 2014.

   [RFC7183]  Ulrich, U., Dearlove, C., and T. Clausen, "Integrity
              Protection for the Neighborhood Discovery Protocol (NHDP)
              and Optimized Link State Routing Protocol Version 2
              (OLSRv2)", RFC 7183, April 2014.

   [olsrv2_paper]
              C., C., C., C., J., J., J., J., and H. H., "OLSRv2 for
              Community Networks: Using Directional Airtime Metric with
              external radios", Elsevier Computer Networks 2015 ,
              September 2015,
              <http://dx.doi.org/10.1016/j.comnet.2015.09.022>.

   [CONFINE]  "Community Networks Testbed for the Future Internet
              (CONFINE)", 2015, <http://www.confine-project.eu>.

   [DLEP]     Ratliff, S., Berry, B., Harrison, G., Jury, S., and D.
              Satterwhite, "Dynamic Link Exchange Protocol (DLEP)",
              draft-ietf-manet-dlep-17 , March 2013.

   [BATMAN]   Neumann, A., Aichele, C., Lindner, M., and S. Wunderlich,
              "Better Approach To Mobile Ad-hoc Networking
              (B.A.T.M.A.N.)", draft-wunderlich-openmesh-manet-
              routing-00 , April 2008.

   [MOBICOM03]
              De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A
              High-Throughput Path Metric for Multi-Hop Wireless
              Routing", Proceedings of the MOBICOM Conference , 2003.

   [MOBICOM04]
              Richard, D., Jitendra, P., and Z. Brian, "Routing in
              Multi-Radio, Multi-Hop Wireless Mesh Networks",
              Proceedings of the MOBICOM Conference , 2004.

   [OLSR.org]
              "The OLSR.org OLSR routing daemon", 2015,
              <http://www.olsr.org/>.

   [FREIFUNK]
              "Freifunk Wireless Community Networks", 2015,
              <http://www.freifunk.net>.

   [FUNKFEUER]
              "Austria Wireless Community Network", 2015,
              <http://www.funkfeuer.at>.

Appendix A.  Future work

   As the DAT metric proved to work reasonable well for non- or slow-
   moving ad hoc networks [olsrv2_paper], it should be considered as a
   solid first step on a way to better MANET metrics.  There are
   multiple parts of the DAT metric that need to be reviewed again in
   the context of real world deployments and can be subject to later
   improvements.

   The easiest part of the DAT metric to change and test would be the
   timings parameters.  A 1 minute interval for packet loss statistics
   might be a good compromise for some MANETs, but could easily be too
   large or to small for others.  More data is needed to verify or
   improve the current parameter selection.

   The DAT metric considers only the multicast RFC5444 packet loss for
   estimating the link loss, but it would be good to integrate unicast
   data loss into the loss estimation.  This information could be
   provided directly from the link layer.  This could increase the
   accuracy of the loss rate estimation in scenarios, where the
   assumptions regarding the ratio of multicast vs. unicast loss do not
   hold.

   The packet loss averaging algorithm could also be improved.  While
   the DAT metric provides a stable sliding time interval to average the
   incoming packet loss and not giving the recent input too much
   influence, However, first experiments suggest that the algorithm
   tends to be less agile detecting major changes of link quality.  This
   makes it less suited for mobile networks.  A more agile algorithm is
   needed for detecting major changes while filtering out random
   fluctuations regarding frame loss.  However, the current "quere of
   counters" algorithm suggested for DAT outperforms the binary queue
   algorithm and the exponential aging algorithms used for the ETX
   metric in the OLSR [RFC3626] codebase of Olsr.org.

Appendix B.  OLSR.org metric history

   The Funkfeuer [FUNKFEUER] and Freifunk networks [FREIFUNK] are OLSR-
   based [RFC3626] or B.A.T.M.A.N.  [BATMAN] based wireless community
   networks with hundreds of routers in permanent operation.  The Vienna
   Funkfeuer network in Austria, for instance, consists of 400 routers
   covering the whole city of Vienna and beyond, spanning roughly 40km
   in diameter.  It has been in operation since 2003 and supplies its
   users with Internet access.  A particularity of the Vienna Funkfeuer
   network is that it manages to provide Internet access through a city
   wide, large scale Wi-Fi MANET, with just a single Internet uplink.

   Operational experience of the OLSR project [OLSR.org] with these
   networks have revealed that the use of hop-count as routing metric
   leads to unsatisfactory network performance.  Experiments with the
   ETX metric [MOBICOM03] were therefore undertaken in parallel in the
   Berlin Freifunk network as well as in the Vienna Funkfeuer network in
   2004, and found satisfactory, i.e., sufficiently easy to implement
   and providing sufficiently good performance.  This metric has now
   been in operational use in these networks for several years.

   The ETX metric of a link is the estimated number of transmissions
   required to successfully send a packet (each packet equal to or
   smaller than MTU) over that link, until a link layer acknowledgement
   is received.  The ETX metric is additive, i.e., the ETX metric of a
   path is the sum of the ETX metrics for each link on this path.

   While the ETX metric delivers a reasonable performance, it doesn't
   handle well networks with heterogeneous links that have different
   bitrates.  Since every wireless link, when using ETX metric, is
   characterized only by its packet loss ratio, the ETX metric prefers
   long-ranged links with low bitrate (with low loss ratios) over short-
   ranged links with high bitrate (with higher but reasonable loss
   ratios).  Such conditions, when they occur, can degrade the
   performance of a network considerably by not taking advantage of
   higher capacity links.

   Because of this the OLSR.org project has implemented the Directional
   Airtime Metric for OLSRv2, which has been inspired by the Estimated
   Travel Time (ETT) metric [MOBICOM04].  This metric uses an
   unidirectional packet loss, but also takes the bitrate into account
   to create a more accurate description of the relative costs or
   capabilities of OLSRv2 links.

Appendix C.  Linkspeed stabilization

   The DAT metric describes how to generate a reasonable stable packet
   loss value from incoming packet reception/loss events, the source of
   the linkspeed used in this document is considered an external
   process.

   In the presence of a layer-2 technology with variable linkspeed it is
   likely that the raw linkspeed will be fluctuating too fast to be
   useful for the DAT metric.

   The amount of stabilization necessary for the linkspeed depends on
   the implementation of the mac-layer, especially the rate control
   algorithm.

   Experiments with the Linux 802.11 wifi stack have shown that a simple
   Median filter over a series of raw linkspeed measurements can smooth
   the calculated value without introducing intermediate linkspeed
   values you would get by using averaging or an exponential weighted
   moving average.

Appendix D.  Packet loss hysteresis

   While the DAT metric use a sliding window to calculate a reasonable
   stable frame loss, the implementation might choose to integrate an
   additional hysteresis to prevent the metric flapping between two
   values.

   In Section Section 10.2 DAT caluclates a fractional loss rate.  The
   fraction of 'loss := sum_total / sum_received' may result in minor
   fluctuations in the advertised L_in_metric due to minimal changes in
   sum_total or sum_received which can cause undesirable protocol churn.

   A hysteresis function applied to the fraction could reduce the amount
   of changes in the loss rate and help to stabilize the metric output.

Appendix E.  Example DAT values

   The DAT metric value can be expressed in terms of link speed (bit/s)
   or used airtime (s).  When using the default protocol constants (see
   Section 7), 6), DAT encodes link speeds between 119 bit/s and 2 Gbit/s.

   Table Table 2 contains a few examples for metric values and their
   meaning as a link speed:

                 +---------------------------+-----------+
                 |           Metric          |   bit/s   |
                 +---------------------------+-----------+
                 |     MINIMUM_METRIC (1)    |  2 Gbit/s |
                 |                           |           |
                 | MAXIMUM_METRIC (16776960) | 119 bit/s |
                 |                           |           |
                 |            2000           |  1 Mbit/s |
                 +---------------------------+-----------+

                      Table 2: DAT link cost examples

   A path metric value could also be expressed as a link speed, but this
   would be unintuitive and difficult to understand.  An easier way to
   transform a path metric value into a textual representation is to
   divide it by the hopcount of the path and express the path cost as
   average link speed together with the hopcount (see Table 3).

                    +---------+------+---------------+
                    |  Metric | hops | average bit/s |
                    +---------+------+---------------+
                    |    4    |  2   |    1 Gbit/s   |
                    |         |      |               |
                    | 4000000 |  6   |    3 kbit/s   |
                    +---------+------+---------------+

                      Table 3: DAT link cost examples

Authors' Addresses

   Henning Rogge
   Fraunhofer FKIE

   Email: henning.rogge@fkie.fraunhofer.de
   URI:   http://www.fkie.fraunhofer.de

   Emmanuel Baccelli
   INRIA

   Email: Emmanuel.Baccelli@inria.fr
   URI:   http://www.emmanuelbaccelli.org/