Internet DRAFT - draft-ietf-manet-olsrv2-dat-metric
draft-ietf-manet-olsrv2-dat-metric
MANET H. Rogge
Internet-Draft Fraunhofer FKIE
Intended status: Experimental E. Baccelli
Expires: June 17, 2016 INRIA
December 15, 2015
Packet Sequence Number based directional airtime metric for OLSRv2
draft-ietf-manet-olsrv2-dat-metric-12
Abstract
This document specifies an 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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 17, 2016.
Copyright Notice
Copyright (c) 2015 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.
Rogge & Baccelli Expires June 17, 2016 [Page 1]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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 Constants . . . . . . . . . . . . . . . . . . . . . 7
7. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 8
7.1. Recommended Values . . . . . . . . . . . . . . . . . . . 8
8. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Initial Values . . . . . . . . . . . . . . . . . . . . . 10
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 . . . . . . . . . . . . . . . . . . . 13
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
14.1. Normative References . . . . . . . . . . . . . . . . . . 15
14.2. 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 . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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 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.
Rogge & Baccelli Expires June 17, 2016 [Page 2]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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 MANET working group. It enables easier interoperability
tests 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. Coming experiments should also allow
to judge if the DAT metric can be useful for other IETF protocol,
both inside and out of the MANET working group. This could lead
either to moving this draft to Standard Track or to replace 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
Rogge & Baccelli Expires June 17, 2016 [Page 3]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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) - 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 [RFC7181] networks. 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 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 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 via
gathering cross-layer data from the operating system, via an external
Rogge & Baccelli Expires June 17, 2016 [Page 4]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
daemon like DLEP [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 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, while
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
Rogge & Baccelli Expires June 17, 2016 [Page 5]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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 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 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
Rogge & Baccelli Expires June 17, 2016 [Page 6]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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 a MANET with very slow or
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 bit-rate in Bit/s used by this routing
metric.
Rogge & Baccelli Expires June 17, 2016 [Page 7]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
+---------------------+-------+
| 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.
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
Rogge & Baccelli Expires June 17, 2016 [Page 8]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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 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])
Rogge & Baccelli Expires June 17, 2016 [Page 9]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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:
Rogge & Baccelli Expires June 17, 2016 [Page 10]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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:
Rogge & Baccelli Expires June 17, 2016 [Page 11]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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.
Rogge & Baccelli Expires June 17, 2016 [Page 12]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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 := MIN(sum_total / sum_received, DAT_MAXIMUM_LOSS).
2. bitrate := 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 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
Rogge & Baccelli Expires June 17, 2016 [Page 13]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
value may decrease the amount of incoming traffic, while advertising
lower L_in_metric may increase the amount of incoming traffic.
For example, by thus 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 router 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 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. However, the security mechanism described in
[RFC7183] does not protect the sequence number used by the DAT metric
because it does only sign the RFC5444 messages, not the RFC5444
packet header (which contains the RFC5444 packet sequence number).
12. IANA Considerations
This document has no actions for IANA.
13. 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
Rogge & Baccelli Expires June 17, 2016 [Page 14]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
Laboratory), Fabian Nack (Freie Universitaet Berlin) and Stan Ratliff
(Cisco Systems).
14. References
14.1. 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.
14.2. 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.
[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 , September 2015,
<http://dx.doi.org/10.1016/j.comnet.2015.09.022>.
Rogge & Baccelli Expires June 17, 2016 [Page 15]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
[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 , October 2015.
[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 reasonably well for non- or slow-
moving ad hoc networks [COMNET15], 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.
Rogge & Baccelli Expires June 17, 2016 [Page 16]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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, 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 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.
Rogge & Baccelli Expires June 17, 2016 [Page 17]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
While the ETX metric delivers a reasonable performance, it doesn't
handle well networks with heterogeneous links that have different
bitrates. When using 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 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 specifies how to generate a reasonably stable packet
loss rate value based on incoming packet reception/loss events, but
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 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 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
Rogge & Baccelli Expires June 17, 2016 [Page 18]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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 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 less intuitive. 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
Rogge & Baccelli Expires June 17, 2016 [Page 19]
Internet-Draft Directional airtime metric for OLSRv2 December 2015
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/
Rogge & Baccelli Expires June 17, 2016 [Page 20]
