MANET H. Rogge
Internet-Draft Fraunhofer FKIE
Intended status: Informational E. Baccelli
Expires: June 11, 2010 INRIA
A. Kaplan
Cert.at/NIC.at, Internet
Verwaltungs- und
Betriebsgesellschaft m.b.H.
December 8, 2009
Packet Sequence Number based ETX Metric for Mobile Ad Hoc Networks
draft-funkfeuer-manet-olsrv2-etx-00
Abstract
This document specifies the ETX metric and its usage in OLSRv2.
Status of this Memo
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 4
4. Protocol Functioning & Overview . . . . . . . . . . . . . . . 4
5. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 5
6. Initial Values . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 6
8. Packets and Messages . . . . . . . . . . . . . . . . . . . . . 7
8.1. HELLO Message Generation . . . . . . . . . . . . . . . . . 7
8.2. HELLO Message Processing . . . . . . . . . . . . . . . . . 7
8.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 8
9. HELLO Timeout . . . . . . . . . . . . . . . . . . . . . . . . 9
10. Periodic Metric Computation . . . . . . . . . . . . . . . . . 9
11. Proposed Values for Parameters and Constants . . . . . . . . . 10
12. Security Considerations . . . . . . . . . . . . . . . . . . . 10
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
13.1. Expert Review: Evaluation Guidelines . . . . . . . . . . . 11
13.2. Address Block TLV Types . . . . . . . . . . . . . . . . . 11
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
15.1. Normative References . . . . . . . . . . . . . . . . . . . 11
15.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The Funkfeuer [FUNKFEUER] and Freifunk networks [FREIFUNK] are OLSR-
based [RFC3626] wireless community networks with hundreds of routers
in permanent operation. The Vienna Funkfeuer network in Austria
consists of 400 routers, around 600 routes, covering the whole city
of Vienna and beyond spanning roughly 40km in diameter. It has been
in operation since 2003 and supplies it's users with Internet access.
The Funkfeuer network is special in a respect that it manages to
provide Internet access through a city wide, large scale Wi-Fi mesh
cloud, having just one single uplink
Operational experience with these network has revealed that the use
of hop-count as routing metric leads to unsatisfactory network
performance (especially if going through a single uplink).
Experiments with the ETX metric [MOBICOM03] were therefore undertaken
in parallel in the Berlin Freifunk network as well as the Funkfeuer
network and found satisfactory. This metric is in operational use in
these networks for a couple of years. Even though a couple of other
metrics have been proposed and asked for by the community in the
Funkfeuer network, ETX proved a very easy to implement metric which
provides sufficiently good results.
The ETX metric of a link is the estimated number of transmissions
required to successfully send a packet (each packet smaller than MTU)
over that link, until an 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.
This document describes the ETX metric as used by the Funkfeuer
network, and specifies its usage in OLSRv2 [olsrv2]. More precisely,
this document specifies additional signaling and processing to NHDP
[nhdp] in order to establish the ETX metric value for a link.
In order to use the ETX metric for routers, this document assumes
that the suggestions in [olsrv2-metric] are incorporated into
[olsrv2].
2. Terminology
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
NOT','SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED', 'MAY',
and 'OPTIONAL' in this document are to be interpreted as described
in[RFC2119].
The terminology introduced in [RFC5444], [olsrv2], [nhdp], and
[olsrv2-metric], including the terms "packet", "message", "Address
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Block", "TLV Block","TLV", "address", "address prefix", and "address
object" are to be interpreted as described therein.
Additionally, this document uses the following terminology and
notational conventions:
LIFO - a last in, first out queue of numbers.
LIFO[current] - the most recent entry added to the queue.
push(LIFO, value) - an operation which removes the oldest entry in
the queue and place a new entry at the head of the queue.
sum(LIFO) - an operation which returns the sum of all elements in a
LIFO.
diff_seqno(new, old) - an operation which returns the positive
distance between two elements of the circular sequence number
space defined in [RFC5444]. Its value is either (new - old) if
this result is positive, or else its value is (new - old + 65536).
UNDEFINED - a constant for -1.
3. Applicability Statement
The mechanism specified in this document is used daily by hundreds of
routers in the Funkfeuer network [FUNKFEUER], as well as in similar
OLSR-based wireless community networks which use the OLSR.org
[OLSR.org] code base, such as [FREIFUNK], [AWMN], [NINUX], [GUIFI],
[OPENAIR]. Operational experience suggests that this mechanism is
viable in (at least) these kinds of networks.
The ETX metric value of a link is established by measuring the rate
of successful exchange of OLSRv2 control packets over that link,
which use the format defined in [RFC5444]. ETX metric computation is
thus based only on layer 3 signaling, and is therefore independent
from underlying link layer technologies. Moreover, ETX metric
computation does not require inspection of data traffic.
If a neighbor does not add use this ETX metric, this protocol falls
back to DEFAULT_METRIC as defined in [olsrv2-metric].
4. Protocol Functioning & Overview
Router A computes the value of the ETX metric of its link to router B
by continuously estimating the loss rates over this link, in both
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directions: from B to A (this rate is called R_etx), and from A to B
(this rate is called D_etx). Router A computes R_etx as the measured
proportion of [RFC5444] packets successfully arriving from B, and
signals this value in NHDP HELLO messages by inclusion of a R_etx
TLV. Symmetrically, router B computes the proportion of [RFC5444]
packets successfully arriving from A, and signals its value in NDHP
HELLO messages by inclusion of a R_etx TLV, which router A can then
take as D_etx value for this link.
The value of the ETX metric of the link is then ETX = R_etx * D_etx,
which corresponds to the expected number of attempts to successfully
receive and acknowledge a packet over this link. Note that this
metric is symmetric, i.e. on a link connecting router A and router B,
the ETX metric value for this link computed by router B will be the
same as the ETX metric value computed by router A.
5. Data Structures
This specification extends the Link Set per Interface Information
Base, as defined in [nhdp], with the following additional elements
for each link tuple:
L_METRIC_received_lifo - a LIFO queue with ETX_MEMORY_LENGTH integer
elements. Each entry contains the number of successfully received
[RFC5444] packets within an interval of ETX_METRIC_INTERVAL.
L_METRIC_total_lifo - a LIFO queue with ETX_MEMORY_LENGTH integer
elements. Each entry contains the estimated number of [RFC5444]
packets transmitted by the neighbor, based on the received packet
sequence numbers within an interval of ETX_METRIC_INTERVAL.
L_METRIC_last_pkt_seqno - the last received packet sequence number
received from this link.
L_METRIC_r_etx - the current r_etx value for this link to the
neighbor.
L_METRIC_d_etx - the last d_etx value received from the neighbor for
this link.
L_METRIC_hello_time - time when the next hello will be expected.
This is used to detect missing hellos by timeout.
L_METRIC_hello_interval - the interval between two hello messages of
the links neighbor.
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L_METRIC_lost_hellos - the estimated number of lost hello messages
from this neighbor, based on the value of the hello interval.
6. Initial Values
When generating a new tuple in the Link Set, the values of the
elements specified above are set as follows:
L_METRIC_received_lifo := 0, ..., 0.
L_METRIC_total_lifo := 0, ..., 0.
L_METRIC_last_pkt_seqno := UNDEFINED.
L_METRIC_r_etx := UNDEFINED.
L_METRIC_d_etx := UNDEFINED.
L_METRIC_hello_time := EXPIRED.
L_METRIC_hello_interval := UNDEFINED.
L_METRIC_lost_hellos := 0
7. Protocol Parameters
This specification uses the parameters defined in [olsrv2]. This
specification defines the following additional parameters:
ETX_MEMORY_LENGTH - ETX algorithm memory length in seconds. All
received and lost packets within this time period are used to
calculate the delivery ratio R_etx.
ETX_METRIC_INTERVAL - interval in seconds between two metric
recalculations as described in Section 10.
ETX_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.
ETX_HELLO_TIMEOUT_FACTOR - timeout factor for HELLO interval at
which point a HELLO is definitely considered lost. The value
should be between 1.0 and (2.0 * (1 - HT_MAXJITTER/
HELLO_INTERVAL)).
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ETX_PERFECT_METRIC - metric value for ETX 1.0.
8. Packets and Messages
Generated packets and messages use the format defined in [RFC5444].
The present specification adds the following constraints:
o A packet MUST contain a packet sequence number as defined in
[RFC5444]. This sequence number MUST be interface specific.
8.1. HELLO Message Generation
HELLO messages are generated as specified in [olsrv2]. In addition,
the HELLO messages must comply with the following:
o For each address included in a HELLO message with a TLV with type
LINK_STATUS and value SYMMETRIC or HEARD, a TLV of type R_etx MUST
also be included.
R_etx TLV formatting is specified in Table 1, whereby the value of
the directional link cost is encoded as TimeTLV [RFC5497] encoded
values with C = 1024.
+-------+--------------+----------+
| Type | Value Length | Value |
+-------+--------------+----------+
| R_etx | 1 octet | linkcost |
+-------+--------------+----------+
Table 1
The value of the linkcost field of an R_etx TLV in a HELLO message is
set to the L_METRIC_r_etx value of the corresponding link listed in
this HELLO message.
8.2. HELLO Message Processing
HELLO messages are first processed as specified in [olsrv2]. This
processing includes identifying (or creating) a Neighbor Tuple
corresponding to the originator of the HELLO message (the "current
Neighbor Tuple"). After this, the following MUST be performed:
1. If the IP address of this link local interface is included in the
HELLO address block and the address block contains an R_etx
address TLV, then:
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1. L_METRIC_d_etx := R_etx.
2. Otherwise:
1. L_METRIC_d_etx := UNDEFINED.
3. If the HELLO message contains an INTERVAL_TIME TLV, then:
1. L_METRIC_hello_interval := INTERVAL_TIME.
4. If L_METRIC_hello_interval != UNDEFINED, then:
1. L_METRIC_hello_time := current_time +
ETX_HELLO_TIMEOUT_FACTOR * INTERVAL_TIME.
5. L_METRIC_lost_hellos := 0.
8.3. Packet Processing
Each incoming packet is processed as defined in OLSRv2 [olsrv2].
Furthermore, the following additional processing MUST be carried out
after the package has been processed on the link set tuple
corresponding to the source of the packet:
1. If L_METRIC_last_pkt_seqno = UNDEFINED, then:
1. L_METRIC_received_lifo[current] := 1.
2. L_METRIC_total_lifo[current] := 1.
2. Otherwise:
1. L_METRIC_received_lifo[current] :=
L_METRIC_received_lifo[current] + 1.
2. diff := seq_diff(seqno, L_METRIC_last_pkt_seqno).
3. If diff > ETX_SEQNO_RESTART_DETECTION, then:
1. diff := 1.
4. L_METRIC_total_lifo[current] := L_METRIC_total_lifo[current]
+ diff.
3. L_METRIC_last_pkt_seqno := seqno.
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9. HELLO Timeout
When L_METRIC_hello_time has timed out, the following step MUST be
done:
1. L_METRIC_lost_hellos := L_METRIC_lost_hellos + 1.
2. L_METRIC_hello_time := L_METRIC_hello_time +
L_METRIC_hello_interval.
10. Periodic Metric Computation
This metric stores the number of received packets per link to a
neighbor and use the package sequence number to calculate the total
number of sent packages of the neighbor. The total number of
packages divided by the number of received packages is used as a cost
metric for the link.
If a link to a node is lost, no packets are received anymore and
neither the received not total value of packages will change. To
prevent a constant result in this case, the metric stores the number
of HELLO message interval timeouts since the last received packet
from a neighbor and use this value to reduce the received packet
count proportionally to the length of the metric memory
ETX_MEMORY_LENGTH.
Once every ETX_METRIC_INTERVAL, this protocol MUST recalculate of all
L_METRIC_r_etx in all Link Set entries:
1. sum_received := sum(L_METRIC_total_lifo).
2. sum_total := sum(L_METRIC_received_lifo).
3. If L_METRIC_hello_interval != UNDEFINED and L_METRIC_lost_hellos
> 0, then:
1. penalty := L_METRIC_hello_interval * L_METRIC_lost_hellos /
ETX_MEMORY_LENGTH.
2. sum_received := sum_received - sum_received * penalty;
4. If sum_received < 1, then:
1. L_METRIC_r_etx := UNDEFINED.
2. L_in_metric := MAXIMUM_METRIC.
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5. Otherwise:
1. L_METRIC_r_etx := sum_total / sum_received.
2. If L_METRIC_d_etc = UNDEFINED, then:
1. L_in_metric := DEFAULT_METRIC,
3. Otherwise:
1. L_in_metric := ETX_PERFECT_METRIC * L_METRIC_r_etx *
L_METRIC_d_etx.
6. push(L_METRIC_total_lifo, 0)
7. push(L_METRIC_received_lifo, 0)
11. Proposed Values for Parameters and Constants
This section proposes values for various parameters used in this
specification.
o ETX_MEMORY_LENGTH := 32 seconds
o ETX_METRIC_INTERVAL := 1 second
o ETX_SEQNO_RESTART_DETECTION := 256
o ETX_HELLO_TIMEOUT_FACTOR := 1.5
o ETX_PERFECT_METRIC := DEFAULT_METRIC * 0.1
12. Security Considerations
Artificial manipulation of metrics values can drastically alter
network performance. In particular, advertising a higher R_etx value
may decrease the amount of incoming traffic, while advertising lower
R_etx may decrease the amount of incoming traffic. By artificially
increasing or decreasing the R_etx 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.
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13. IANA Considerations
This specification defines one Address Block TLV Type, which have
been allocated from the "Assigned Address Block TLV Types" repository
of [RFC5444] as specified in Table 2.
13.1. Expert Review: Evaluation Guidelines
For the registries for TLV Type Extensions where an Expert Review is
required, the designated expert SHOULD take the same general
recommendations into consideration as are specified by [RFC5444].
13.2. Address Block TLV Types
+-------+------+---------------+------------------------------------+
| Name | Type | Type | Description |
| | | extension | |
+-------+------+---------------+------------------------------------+
| R_etx | tbd | tbd | Loss rate of incoming [RFC5444] |
| | | | packets. |
+-------+------+---------------+------------------------------------+
Table 2
14. 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
this ETX metric.
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), Thomas Clausen (LIX), Christopher Dearlove (BAE
Systems) Michael Gerharz (Fraunhofer FKIE), Ulrich Herberg (LIX),
Markus Kittenberger (Funkfeuer Vienna) and Jens Toelle (Fraunhofer
FKIE).
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
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[RFC3626] Clausen, T. and P. Jacquet, "Optimized Link State Routing
Protocol", RFC 3626, October 2003.
[RFC5444] Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
"Generalized Mobile Ad Hoc Network (MANET) Packet/Message
Format", RFC 5444, 2009.
[RFC5497] Clausen, T. and C. Dearlove, "Representing Multi-Value
Time in Mobile Ad Hoc Networks", RFC 5497, March 2009.
15.2. Informative References
[olsrv2] Clausen, T., Jacquet, P., and C. Dearlove, "The Optimized
Link State Routing Protocol version 2",
draft-ietf-manet-olsrv2-10 , September 2009.
[nhdp] Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
Network (MANET) Neighborhood Discovery Protocol (NHDP)",
draft-ietf-manet-nhdp-11 , October 2009.
[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.
[olsrv2-metric]
Dearlove, C., Clausen, T., and P. Jacquet, "Link Metrics
for OLSRv2".
[OLSR.org]
"The olsr.org OLSR daemon, http://www.olsr.org".
[FREIFUNK]
"Freifunk Wireless Community Networks,
http://www.freifunk.net".
[FUNKFEUER]
"Austria Wireless Community Network,
http://www.funkfeuer.at".
[AWMN] "Athens Wireless Community Network, http://awmn.net".
[GUIFI] "Barcelona Wireless Community Network,
http://www.guifi.net".
[NINUX] "Roma Wireless Community Network, http://www.ninux.org".
[OPENAIR] "Boston Wireless Community Network,
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http://openairboston.net/".
Authors' Addresses
Henning Rogge
Fraunhofer FKIE
Email: ietf@hrogge.de
Emmanuel Baccelli
INRIA
Email: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/
L. Aaron Kaplan
Cert.at/NIC.at, Internet Verwaltungs- und Betriebsgesellschaft m.b.H.
Email: aaron@lo-res.org
URI: http://cert.at/
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