Internet Engineering Task Force (IETF) H. Rogge
Request for Comments: 7779 Fraunhofer FKIE
Category: Experimental E. Baccelli
ISSN: 2070-1721 INRIA
April 2016
Directional Airtime Metric Based on Packet Sequence Numbers for
Optimized Link State Routing Version 2 (OLSRv2)
Abstract
This document specifies a Directional Airtime (DAT) link metric for
usage in Optimized Link State Routing version 2 (OLSRv2).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7779.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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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 and Overview . . . . . . . . . . . . . . . 6
6. Protocol Constants . . . . . . . . . . . . . . . . . . . . . 7
7. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . 12
10. Timer Event Handling . . . . . . . . . . . . . . . . . . . . 12
10.1. Packet Timeout Processing . . . . . . . . . . . . . . . 12
10.2. Metric Update . . . . . . . . . . . . . . . . . . . . . 13
11. Security Considerations . . . . . . . . . . . . . . . . . . . 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1. Normative References . . . . . . . . . . . . . . . . . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Future Work . . . . . . . . . . . . . . . . . . . . 17
Appendix B. OLSR.org Metric History . . . . . . . . . . . . . . 17
Appendix C. Link-Speed Stabilization . . . . . . . . . . . . . . 18
Appendix D. Packet-Loss Hysteresis . . . . . . . . . . . . . . . 19
Appendix E. Example DAT Values . . . . . . . . . . . . . . . . . 19
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
One of the major shortcomings of 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 hop-count
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, and it
can be any kind of dimensionless additive cost function that reports
to the OLSRv2 protocol.
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Since 2004, the OLSR.org [OLSR.org] implementation of OLSR has
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 (see
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 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 specification allows OLSRv2 deployments with a metric defined by
the IETF Mobile Ad Hoc Networks (MANET) working group. It enables
easier interoperability testing between implementations and targets
to deliver a useful baseline to compare with, for experiments with
this metric as well as other metrics. Appendix A contains a few
possible steps to improve the Directional Airtime metric. Future
experiments should also determine whether the DAT metric can be
useful for other IETF protocols, both inside and outside of the MANET
working group. This could lead to either moving this document to the
Standards Track or replacing it with an improved document.
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 specified 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.
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sum(QUEUE) - An operation that returns the sum of all elements in a
QUEUE.
diff_seqno(new, old) - An operation that 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) - 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.
3. Applicability Statement
The Directional Airtime metric was designed and tested (see
[COMNET15]) in wireless IEEE 802.11 OLSRv2 networks [RFC7181]. These
networks employ link-layer retransmission to increase the delivery
probability. A dynamic rate selection algorithm selects the unicast
data rate independently for each neighbor.
As specified in OLSRv2, the metric calculates only the incoming link
cost. It neither calculates the outgoing metric, nor decides the
link status (heard, symmetric, lost).
The metric works both for nodes that can send/receive [RFC5444]
packet sequence numbers and those that do not have this capability.
In the absence of such sequence numbers, the metric calculates the
packet loss based on HELLO message [RFC6130] 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 via
gathering cross-layer data from the operating system, via an external
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daemon like Dynamic Link Exchange Protocol [DLEP], or via indirect
Layer 3 measurements like packet-pair (see [MOBICOM04]).
The metric uses [RFC5444] multicast control traffic to determine the
link packet loss. The administrator should take care that link-layer
multicast transmission do not have a higher reception probability
than the slowest unicast transmission without retransmission. For
example, with 802.11g, it might 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 is a loss of 7 of 8 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 link speed 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 that 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 link speed can be measured locally, while
symmetric link loss would need an additional signaling TLV in the
HELLO [RFC6130] 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 does not take packet size into account.
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Multiplying all link costs of a topology with the size of a data-
plane packet would never change the Dijkstra result in any way.
The queue-based packet-loss estimator specified in this document 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 on the received and lost [RFC6130] 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
previously 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 and 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 events are
accumulated in the current queue element until a timer adds a new
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empty counter to both queues and removes 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 [RFC6130] 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 HELLO
[RFC6130] or TC messages [RFC7181], 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 a MANET with a very slow
or a very fast link layer. See 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 bitrate in Bit/s used by this routing
metric.
+---------------------+-------+
| Name | Value |
+---------------------+-------+
| DAT_MAXIMUM_LOSS | 8 |
| | |
| DAT_MINIMUM_BITRATE | 1000 |
+---------------------+-------+
Table 1: DAT Protocol Constants
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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 Section 5.3.1 of [RFC6130]) after which the DAT metric
considers a HELLO as lost.
DAT_SEQNO_RESTART_DETECTION - Threshold in the 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 loss estimation based on packet sequence numbers.
This number MUST be larger than DAT_MAXIMUM_LOSS.
7.1. Recommended Values
The proposed values of the protocol parameters are for Community Mesh
Networks, which mostly use routers that are not mobile. 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
8. Data Structures
This specification extends the Link Set of the Interface Information
Base, as defined in Section 7.1 of [RFC6130], 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.
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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 from which 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 D 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 item 3 of
Section 12.5 of [RFC6130], 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.
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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 the 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 HELLO message [RFC6130].
o "validity_time" is the time encoded in the VALIDITY_TIME message
TLV of a received HELLO message [RFC6130].
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 that 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
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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 described in this section.
[RFC5444] packets without packet sequence numbers 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:
* L_DAT_received[TAIL] := 1.
* L_DAT_total[TAIL] := 1.
2. Otherwise:
* L_DAT_received[TAIL] := L_DAT_received[TAIL] + 1.
* diff := diff_seqno(pkt_seqno, L_DAT_last_pkt_seqno).
* If diff > DAT_SEQNO_RESTART_DETECTION, then:
diff := 1.
* 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:
* L_DAT_packet_time := current time + (L_DAT_hello_interval *
DAT_HELLO_TIMEOUT_FACTOR).
5. L_DAT_lost_packet_intervals := 0.
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9.4. HELLO Message Processing
For each incoming HELLO Message, after it has been processed as
defined in Section 12 of [RFC6130], 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:
L_DAT_hello_interval := interval_time.
2. Otherwise:
L_DAT_hello_interval := validity_time.
3. If L_DAT_last_pkt_seqno = UNDEFINED, then:
* L_DAT_received[TAIL] := L_DAT_received[TAIL] + 1.
* L_DAT_total[TAIL] := L_DAT_total[TAIL] + 1.
* 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:
L_DAT_total[TAIL] := L_DAT_total[TAIL] + 1.
2. Otherwise:
L_DAT_lost_packet_intervals := L_DAT_lost_packet_intervals +
1.
3. L_DAT_packet_time := L_DAT_packet_time + L_DAT_hello_interval.
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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:
* lost_time_proportion := L_DAT_hello_interval *
L_DAT_lost_packet_intervals / DAT_MEMORY_LENGTH.
* sum_received := sum_received *
MAX(0, 1 - lost_time_proportion);
4. If sum_received < 1, then:
L_in_metric := MAXIMUM_METRIC, as defined in [RFC7181],
Section 5.6.1.
5. Otherwise:
* loss := MIN(sum_total / sum_received, DAT_MAXIMUM_LOSS).
* bitrate := MAX(L_DAT_rx_bitrate, DAT_MINIMUM_BITRATE).
* 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 D for an example.
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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.
For example, by artificially attracting mesh routes and then dropping
the incoming traffic, an attacker may achieve a Denial of Service
(DoS) against other mesh nodes. Similarly, an attacker may achieve
Man-in-the-Middle (MITM) attacks or traffic analysis by concentrating
traffic being routed over a node the attacker controls (and end-to-
end encryption is not used or somehow broken). Protection mechanisms
against such MITM or DoS attacks are nevertheless out of scope of
this document.
Security threats also include potential attacks on the integrity of
the control traffic passively monitored by DAT to measure link
quality. For example, an attacker might inject packets pretending to
be somebody else and using incorrect sequence numbers. This attack
can be prevented by the true originator of the [RFC5444] packets by
adding an ICV Packet TLV and TIMESTAMP Packet TLV [RFC7182] to each
packet. This allows the receiver to drop all incoming packets that
have a forged packet source, both packets generated by the attacker,
or replayed packets. However, the security mechanism described in
[RFC7183] does not protect the sequence number used by the DAT metric
because it only signs the [RFC5444] messages, not the [RFC5444]
packet header (which contains the [RFC5444] packet sequence number).
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5444] Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
"Generalized Mobile Ad Hoc Network (MANET) Packet/Message
Format", RFC 5444, DOI 10.17487/RFC5444, February 2009,
<http://www.rfc-editor.org/info/rfc5444>.
[RFC5497] Clausen, T. and C. Dearlove, "Representing Multi-Value
Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497,
DOI 10.17487/RFC5497, March 2009,
<http://www.rfc-editor.org/info/rfc5497>.
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[RFC6130] Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
Network (MANET) Neighborhood Discovery Protocol (NHDP)",
RFC 6130, DOI 10.17487/RFC6130, April 2011,
<http://www.rfc-editor.org/info/rfc6130>.
[RFC7181] Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
"The Optimized Link State Routing Protocol Version 2",
RFC 7181, DOI 10.17487/RFC7181, April 2014,
<http://www.rfc-editor.org/info/rfc7181>.
12.2. Informative References
[RFC3626] Clausen, T., Ed. and P. Jacquet, Ed., "Optimized Link
State Routing Protocol (OLSR)", RFC 3626,
DOI 10.17487/RFC3626, October 2003,
<http://www.rfc-editor.org/info/rfc3626>.
[RFC7182] Herberg, U., Clausen, T., and C. Dearlove, "Integrity
Check Value and Timestamp TLV Definitions for Mobile Ad
Hoc Networks (MANETs)", RFC 7182, DOI 10.17487/RFC7182,
April 2014, <http://www.rfc-editor.org/info/rfc7182>.
[RFC7183] Herberg, 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, DOI 10.17487/RFC7183, April 2014,
<http://www.rfc-editor.org/info/rfc7183>.
[COMNET15] Barz, C., Fuchs, C., Kirchhoff, J., Niewiejska, J., and H.
Rogge, "OLSRv2 for Community Networks: Using Directional
Airtime Metric with external radios", Elsevier Computer
Networks 2015, DOI 10.1016/j.comnet.2015.09.022, September
2015, <http://dx.doi.org/10.1016/j.comnet.2015.09.022>.
[CONFINE] "Community Networks Testbed for the Future Internet
(CONFINE)", <http://www.confine-project.eu>.
[DLEP] Ratliff, S., Berry, B., Jury, S., Satterwhite, D., and R.
Taylor, "Dynamic Link Exchange Protocol (DLEP)", Work in
Progress, draft-ietf-manet-dlep-22, April 2016.
[BATMAN] Neumann, A., Aichele, C., Lindner, M., and S. Wunderlich,
"Better Approach To Mobile Ad-hoc Networking
(B.A.T.M.A.N.)", Work in Progress, draft-wunderlich-
openmesh-manet-routing-00, April 2008.
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RFC 7779 Directional Airtime Metric OLSRv2 April 2016
[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,
DOI 10.1145/938985.939000, 2003.
[MOBICOM04]
Draves, R., Padhye, J., and B. Zill, "Routing in Multi-
Radio, Multi-Hop Wireless Mesh Networks", Proceedings of
the MOBICOM Conference, DOI 10.1145/1023720.1023732, 2004.
[OLSR.org] "OLSR.org Wiki", <http://www.olsr.org/>.
[FREIFUNK] "Freifunk Wireless Community Networks",
<http://www.freifunk.net>.
[FUNKFEUER]
"Austria Wireless Community Network",
<http://www.funkfeuer.at>.
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Appendix A. Future Work
As the DAT metric proved to work reasonably well for non- or slow-
moving ad hoc networks [COMNET15], it should be considered 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, but it would be good to integrate the 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 does not give the recent input too much
influence, first experiments suggest that the algorithm tends to be
less agile in 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 "queue 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 based
on OLSR [RFC3626] or B.A.T.M.A.N. [BATMAN] 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 40 km
in diameter. It has been supplying its users with Internet access
since 2003. 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.
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Operational experience of the OLSR project [OLSR.org] with these
networks has revealed that the use of hop-count as a 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 does not
handle networks with heterogeneous links that have different bitrates
well. When using the ETX metric, since every wireless link is
characterized only by its packet-loss ratio, long-ranged links with
low bitrate (with low loss ratios) are preferred 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 a
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. Link-Speed Stabilization
The DAT metric specifies how to generate a reasonably stable packet-
loss rate value based on incoming packet reception/loss events, but
the source of the link speed used in this document is considered an
external process.
In the presence of a Layer 2 technology with variable link speed, it
is likely that the raw link speed will be fluctuating too fast to be
useful for the DAT metric.
The amount of stabilization necessary for the link speed depends on
the implementation of the MAC layer, especially the rate-control
algorithm.
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Experiments with the Linux 802.11 Wi-Fi stack have shown that a
simple Median filter over a series of raw link-speed measurements can
smooth the calculated value without introducing intermediate link-
speed values one would obtain by using averaging or an exponential
weighted moving average.
Appendix D. Packet-Loss Hysteresis
While the DAT metric uses a sliding window to compute a reasonably
stable frame loss, the implementation might choose to integrate an
additional hysteresis to prevent undesirable oscillations between two
values (i.e., metric flapping).
In Section 10.2, DAT calculates 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 further 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 6), DAT encodes link speeds between 119 bit/s and 2 Gbit/s.
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
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A path metric value could also be expressed as a link speed, but this
would be less intuitive. An easier way to transform a path metric
value into a textual representation is to divide it by the hop count
of the path and express the path cost as the average link speed
together with the hop count (see Table 3).
+---------+------+---------------+
| Metric | hops | average bit/s |
+---------+------+---------------+
| 4 | 2 | 1 Gbit/s |
| | | |
| 4000000 | 6 | 3 kbit/s |
+---------+------+---------------+
Table 3: DAT Link Cost Examples
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 (Freie Universitaet Berlin), and Stan
Ratliff (Cisco Systems).
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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/
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