Performance Evaluation for TCP in Tactical Mobile Ad Hoc Networks

Jonas Karlsson

, Velizar G. Dimitrov

, Andreas Kassler

, Anna Brunstrom

, Jan Nilsson

and Anders Hansson

Department of Computer Science, Karlstad University, Universitetsgatan 2, Karlstad, Sweden

Department of Communication Networks, Technical University of Soﬁa, Boulevard Kliment Ohridski 8, Soﬁa, Bulgaria

Swedish Defence Research Agency (FOI), Box 1165, SE - 581 11 Link¨oping, Sweden

Keywords:

MANET, TCP, Tactical Networks, TDMA.

Abstract:

Tactical networks are used in military and rescue operations to provide timely and accurate information to

operating teams. Tactical networks have traditionally used long distance narrow band radio links. However,

although these links provide robust real-time communication the limited bandwidth makes them less suited

for high data-rate applications. To support high-data rate TCP applications such as providing digital images

and maps, emerging tactical networks use shorter range but higher data-rate wide band radio links and multi-

hop. Due to the requirement of cheap up-front cost, most MANET research has focused on Carrier Sense

Multiple Access (CSMA) networks. However, in tactical networks, where bounded delays are important,

Time Division Multiple Access (TDMA) can give better possibility to support the Quality of Service needed

for real-time communication. The purpose of this paper is to assess and compare the throughput of three

state-of-the-art TCP versions and two routing protocols over TDMA based MANETs.

1 INTRODUCTION

During a disaster or a military campaign the regular

wired infrastructure is at best semi-functional. There-

fore, relying on an infrastructure-based system is in

most cases not possible. Prime examples of this are

disaster areas, caused by e.g. earthquakes, tsunamis

and nuclear disasters, which can render an infrastruc-

ture unusable. Tactical networks are also relatively

small networks where nodes follow one or several

group leaders (Li et al., 2012). A tactical network

should also be resilient to node failures and operate

with little or no backbone infrastructure.

One network type that promises to do this is Mo-

bile Ad Hoc Networks (MANETs). A tactical net-

work can be seen as a multi-hop MANET where node

mobility and trafﬁc follows a special pattern. In a

MANET, a node functions both as a host and as a

router and nodes can automatically organize them-

selves and form a network. However, due to limited

transmission range, intermediate nodes need to for-

ward packets to form a connected network topology.

This creates congestion and multiple points of failure.

Node mobility also leads to a highly dynamic network

topology, which is prone to frequent changes and er-

rors. This network dynamicity poses several challen-

ges on routing protocols. Two state-of-the art rout-

ing protocols for MANETs are AODV and OLSR.

AODV is a reactive routing protocol that determines

routes only when needed (Perkins et al., 2003). OLSR

is a table driven proactive protocol that regularly

exchanges topology information with its neighbors

(Clausen and Jacquet, 2009).

Although voice, positioning and short messages

are the main applications for tactical networks it is

also important to support standard applications used

on todays Internet, to e.g. provide maps or other con-

tent. The obvious solution to support standard appli-

cations would be to use standard TCP/IP. However,

TCP was designed for data delivery over wired net-

works. In a MANET, which has a substantially higher

packet loss rate and jitter compared to a wired net-

work, the performance of TCP dramatically degrades.

Furthermore, due to node mobility, routes break and

merge more frequently. This leads to a higher degree

of routing induced losses and reordered packets, com-

pared with a wired network.

MANET research has mainly focused on TCP in-

teractions with CSMA MAC based systems due to the

availability of cheap IEEE 802.11 radio cards, and the

requirement of low up front cost. However, in a tac-

tical network using a TDMA scheme has several ad-

277

Karlsson J., G. Dimitrov V., Kassler A., Brunstrom A., Nilsson J. and Hansson A..

Performance Evaluation for TCP in Tactical Mobile Ad Hoc Networks.

DOI: 10.5220/0004067502770282

In Proceedings of the International Conference on Signal Processing and Multimedia Applications and Wireless Information Networks and Systems

(WINSYS-2012), pages 277-282

ISBN: 978-989-8565-25-9

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

vantages, e.g. support of bounded delays and a stable

network under heavy trafﬁc loads. The novelty of this

paper includes a TDMA based MANET evaluation

of three well established TCP versions: TCP ELFN

(Holland and Vaidya, 2002), TCP New Reno (Floyd

et al., 2004) and TCP Westwood+ (Grieco and Mas-

colo, 2002)(Grieco and Mascolo, 2004), using both

reactive and proactive routing protocols and multiple

mobility scenarios and node densities.

The rest of the paper is structured as follows. Sec-

tion 2 describes the background and motivation for

this paper. Section 3 describes the implementation

and the motivation for the chosen topologies. Finally

Sections 4 and 5, respectively describe the experiment

results, our conclusions and future work.

2 BACKGROUND

MANETs are wireless networks that do not require

any infrastructure for their operation. Intermediate

nodes must therefore participate in the route discov-

ery and packet forwarding to other nodes. As nodes

are mobile, the network topology and link state is con-

stantly changing (Macker et al., 1999). This makes

routing a challenging task and routing protocols need

to quickly respond to topology changes.

Routing protocols in MANETs are commonly di-

vided into two categories based on how and when

routes are discovered and maintained: reactive and

proactive routing protocols. Reactive or on-demand

routing protocols establish the route to a destination

only when it is needed (Abolhasan et al., 2004). This

ensures a low routing overhead when there is a low

amount of source-destination pairs. This can lead to

an increase of routing overhead in high trafﬁc condi-

tions. A proactive routing protocol on the other hand

maintains a consistent and updated view of the net-

work by periodically propagating route updates in the

network (Clausen and Jacquet, 2009). This is costly

when there is limited amount of trafﬁc (Abolhasan

et al., 2004). The routing overhead of a proactive ap-

proach should in an ideal case not be inﬂuenced by the

trafﬁc volume, i.e. the overhead should be low when

the trafﬁc volume is large. However, studies in CSMA

networks have shown that the neighbor sensing mech-

anism of proactive routing protocols can be sensitive

to both trafﬁc and mobility, due to collisions and ra-

dio environment (Voorhaen and Blondia, 2006). De-

pending on if information is spread regularly or trig-

gered by a link loss, with high mobility, table driven

proactive routing protocols can suffer from old route

information or high overhead. In this paper we have

chosen two state-of-art routing protocols to represent

these two categories, reactive AODV and proactive

OLSR.

Another important consideration in MANETs is

the Media Access Control (MAC) layer. A TDMA

channel access method allows several stations to share

the same medium by dividing the time into small time

slots. While mobility and radio environment are the

same for TDMA as for CSMA based networks, there

are signiﬁcant differences where packets are dropped

in the two channel access methods. In static TDMA

based networks there should be no collisions; neigh-

bor sensing for proactive protocols should therefore

be more stable than in CSMA networks. Therefore

in a static network, no packets are dropped due to

collisions. However with node mobility, the TDMA

slot assignment could temporally be incorrect creat-

ing collisions and packet loss.

By design TDMA have a number of other advan-

tages over contention-basedapproaches, such as: fair-

ness, bounded delays and asymptotic behavior under

heavy trafﬁc loads. The main drawback of TDMA

scheduling is that it requires clock synchronization

between the nodes which increases the overhead and

makes nodes more complex and expensiveto build. In

addition in multi-hop environments, an interference

free slot assignment becomes computational expen-

sive.

The critical component of a TDMA protocol in

MANETs is how to assign different time slots to any

two conﬂicting nodes in a distributed way. One solu-

tion that we evaluate in this paper is DRAND (Rhee

et al., 2006). DRAND was introduced to minimize

collisions and promote bounded delay for the purpose

of real-time communication (e.g. voice communica-

tion). One of the beneﬁts of DRAND is that it can

re-compute the TDMA schedule without involving

global changes, i.e. DRAND performance is scalable

to partial topology changes.

Because of its inheritably probability based na-

ture, DRAND requires time to adapt with topology

changes and is expected to perform worse when the

number of topology changes increases. The imple-

mentation of DRAND that we used is the imple-

mentation done by the Computer Science department

of the North Carolina State University, USA (Net-

working Research Lab, 2012). The choice of using

DRAND is not for optimality to the evaluated scenar-

ios. The choice should rather be seen as one viable

way of implementing a TDMA scheme that works

both for stationary and mobile networks.

In the scenarios targeted in this paper, communi-

cations are often among groups which tend to coordi-

nate their movements, i.e. a rescue team. We have

therefore focused on using Reference Point Group

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278

Mobility Model (RPGM) that try to capture the move-

ments when nodes are inﬂuenced by a group leader

(Camp et al., 2002)(Aschenbruck et al., 2008).

In a MANET there will due to the node mobility

always be a certain amount of route breaks. This leads

to packet loss or jitter, as re-routing and link layer re-

transmission causes delay variations. The main prob-

lem for TCP in MANETs is its inability to distinguish

congestion loss from other losses and that the proper

response can be orthogonal. A congestion loss re-

quires the sender to reduce the sending rate to not

overwhelm the network. However, a loss due to a

lossy wireless channel, on the other hand, requires

the sender to quickly retransmit without necessarily

reducing the congestion window.

A great amount of research has been invested in

dealing with TCP performance issues in MANETs,

e.g. (Wang and Zhang, 2002) (Chen et al., 2003).

Most proposals are based on the idea of changing the

functionality and/or the behavior of TCP to adapt it

to the network environment. In this paper we instead

focus on three TCP variants that represent three differ-

ent design choices: TCP New Reno, TCP Westwood+

and TCP ELFN. While TCP New Reno (Floyd et al.,

2004) is one of the most common and well established

TCP variants, TCP Westwood+ and TCP ELFN have

been designed for speciﬁc environments.

TCP Westwood+ (Grieco and Mascolo, 2002) is

a sender-side-only modiﬁcation of TCP New Reno

that is intended to better handle large bandwidth de-

lay product paths with potential packet loss due to

transmission or other errors. This property makes

TCP Westwood+ very attractive to use in wireless sys-

tems. TCP Westwood+ relies on monitoring the ACK

stream for information to help set the congestion con-

trol parameters, i.e. slow start threshold (ssthresh)

and congestion window (cwnd), better. Due to mo-

bility, the ACK stream will have larger ﬂuctuations

in a MANET than in a static network. These none

congestion related ﬂuctuations can reduce TCP West-

wood+’s ability to correctly determine the available

throughput.

TCP ELFN (Explicit Link Failure Notiﬁcation)

(Holland and Vaidya, 2002) is a cross layer approach

to inform the TCP layer about route failures. The pro-

posal uses an ELFN message, which is transported by

or piggy backed on routing messages to the sender

upon a route break. The ELFN message contains

the sender, receiver addresses and ports, as well as

the TCP packet sequence number. On receiving the

ELFN message, the source responds by disabling

its retransmission timers and enters a ”frozen” state.

During the ”frozen” period, the TCP sender probes

the network to check if the route is restored. When

the route is restored, i.e. the sender starts to see ac-

knowledgments of the probe packets; the TCP sender

leaves the ”frozen” state and resumes its state as be-

fore the freeze event. The probe interval is a crucial

parameter as it determines how quickly TCP ELFN

will detect that a new route is established. However, a

too small probing interval will introduce unnecessary

overhead. In (Holland and Vaidya, 2002) the authors

propose to use a probe interval of 2 sec. Upon route

restoration, the proposals use the values of RTO and

cwnd from prior to the route failure. In the original

paper the authors used DSR (Johnson et al., 2007),

which is a reactive source routing protocol. In this

paper we are evaluating TCP ELFN with both proac-

tive and reactive routing protocols.

The authors of (Liao et al., 2002) propose a

novel reactive QoS routing protocol for TDMA based

MANETs. With the proposed protocol, the authors

show that it is possible to search for and establish

routes in a MANET supporting a given bandwidth

constraint. The bandwidth requirement is realized by

letting the route reply reserve time slots as it traverses

links on the reverse path. Our work differs in that we

evaluate TCP and different routing protocols over a

TDMA based MANET.

3 IMPLEMENTATION

The original ELFN was designed and tested with

DSR (Holland and Vaidya, 2002). In short, when-

ever DSR detects a route break, it generates a route er-

ror message which traverses back to the trafﬁc source.

With the ELFN modiﬁcation, these messages are in-

tercepted and processed at the TCP layer. For the pur-

pose of the current study and to be independent of the

underlying routing protocol we have implemented the

routing error message by using an additional ICMP

message. The main advantage of using an ICMP mes-

sage as opposed to piggy backing the information on a

routing message is that the routing protocol is decou-

pled from the TCP layer. This makes it possible to

implement a TCP layer that can operate with different

routing protocols in the same framework, e.g. IEEE

802.11s (Hiertz et al., 2010). The main drawback of

our proposal is the extra overhead of the ICMP mes-

sages (in the case of AODV). As with all approaches

that do an indirect coupling between protocol layers

there is a possibility for race conditions. In our case

this race condition is between the routing layer de-

tecting a path loss and a TCP timeout. That is, if TCP

times out due to a path loss before the routing layer

have detected and transmitted the ICMP message to

the sender, TCP ELFN might freeze in an undesirable

PerformanceEvaluationforTCPinTacticalMobileAdHocNetworks

279

state.

AODV was modiﬁed in a similar way as in (Ro-

manowicz, 2008) to work together with TCP ELFN.

When a link loss (route break) is detected AODV gen-

erates a route error message. In our modiﬁcation, the

route error message is preceded by an ICMP message

which is sent back to the trafﬁc source. Once the

TCP sender recives the ICMP message it will freeze

its timers and start the probing phase as described in

(Holland and Vaidya, 2002). When AODV has estab-

lished a new route and the TCP source have received

an acknowledgementfrom one of the probing packets,

it restores the timers. A link break is detected when

three or more HELLO messages are lost.

When using the pro-active OLSR, the ELFN op-

eration becomes much more peculiar. With OLSR,

a node is notiﬁed about any topology change (route

break) regardless of whether it is participating in a

data forwarding process or not. The implementation

has been changed to reﬂect this by removing the ex-

plicit ICMP message from the node that detects the

route break. Instead, whenever the OLSR agent at the

trafﬁc source tries to forward packets to a destination

that it does not have a route for, it sends an ELFN

message to itself. In the simulations we set the OLSR

neighbor link discovery (HELLO) interval to 1 sec-

ond and the topology control (TC) message interval

to 5 seconds.

The ELFN probe packet interval was set to two

seconds for both routing protocols. We further dis-

abled link layer feedback for both ADOV and OLSR

as initial experiments showed that link layer feedback

caused performance degradation. Preliminary analy-

sis shows that in certain situations the links became

unstable, triggering unnecessary route error messages

and route ﬂapping. This is, however, outside of the

scope of the current paper and has been left for future

work.

The topology we used was a square of 1000 me-

ters by 1000 meters without any obstacles or hetero-

geneity. Radio communication distance was 150 me-

ters and carrier sensing distance was 300 meters. The

physical layer speed was set to 1 Mbps. The reason

for selecting 1 Mbps is that it can be considered as a

challenging data rate when mobile tactical networks

need to support TCP services. Moreover, today most

mobile tactical radios rarely support more than a few

Mbps. For the mobility model we used the RPGM

mobility model with two node densities, consisting of

3 and 16 groups of 6 nodes thus forming scenarios

with 18 and 96 nodes. Group radius was set to 250

meters to ensure that internal group communication

was forwarded by at least one intermediate node. The

node speed was random in the range of 1,5 - 5,0 m/s.

The simulation time was 360 seconds and trafﬁc ﬂows

started at 60 sec, giving time for the routing protocols

to converge. There where two independent TCP ﬂows

simulating large ﬁle transfers, between two different

node pairs in all scenarios. Each experiment was

performed both with and without background trafﬁc.

When using background trafﬁc, 10% of the nodes in

a scenario where each generating a 20 Kbit/s UDP

ﬂow. The total background trafﬁc volume varied be-

tween 20 Kbit/sec (18 nodes scenario) and 180 Kbit/s

(96 nodes scenario). This setup will produce a low

amount of background trafﬁc when the node density

is low and a high amount of background trafﬁc when

the node density is high. However, when the node

density is high there is more possibility for concur-

rent non competing trafﬁc.

4 RESULTS AND ANALYSIS

In this section we will describe the results from our

experiments. The results show the aggregate aver-

age TCP throughput measured in kilobits per seconds

(Kbit/s). To compare TCP performance, we used the

ns 2.26 simulator (McCanne et al., 2012) with TCP-

New Reno, -ELFN and -Westwood+. The routing

protocols are slightly modiﬁed versions of OLSR and

AODV as described in the previous section.

The results shown in Figure 1 are from a sce-

nario with 18 nodes in 3 groups and with no back-

ground trafﬁc. The network density is low which

causes temporal route breaks and network divisions

between nodes and groups. However, due to the

low node density, the impact of the choice of rout-

ing layer is limited. The low congestion in this sce-

nario also gives the possibility to achieve a high TCP

throughput. However, both TCP New Reno and TCP

Westwood+ cannot utilize this possibility as the route

breaks limit the congestion window. Therefore both

of the approaches have similar performance. TCP

ELFN on the other hand freezes its congestion win-

dow during the route breaks and can therefore main-

tain a high congestion window.

Figure 2 refers to a high node density scenario

with 96 nodes divided in 16 groups. In this sce-

nario the connectivity is high, but the mobility com-

bined with the amount of nodes increases the stress

on both routing layer and TDMA scheduling. Due

to this the overall throughput is less. This leads to a

smaller average congestion window for all TCP ver-

sions and consequently TCP ELFN has less beneﬁts

of freezing the congestion window. What also can

be seen is that in this scenario the routing layer starts

to impact the results. Due to the increased amount

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280

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Figure 1: Low node density with no background trafﬁc.

of topology changes, a proactive protocol as OLSR

experiences a higher amount of stale routes and the

possible formation of routing loops. In this scenario

the amount of TC messages for OLSR was, as ex-

pected with a higher node density, ten fold higher then

when using 18 nodes. TCP New Reno also performs

better with AODV compared to using OLSR. On the

other hand as the route lookup is more variable with

AODV (OLSR have a route or not), TCP Westwood+

havemore difﬁcultiesto determine the availableband-

width.

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Figure 2: High node density with no background trafﬁc.

Figure 3 shows results for an 18 node density sce-

nario with 20 Kbit/s of background trafﬁc. With low

congestion, TCP ELFN outperforms both TCP West-

wood+ and TCP New Reno. In this scenario the TCP

throughput for all TCP versions is reduced as com-

pared to Figure 1, reﬂecting the lower available band-

width due to the background trafﬁc. Since we are now

sending trafﬁcbetween more node pairs and as AODV

only detects route errors on actively used routes there

is also a higher possibility for route errors. This is dif-

ferent from when using OLSR where all route errors

are detected, whether it is on an active route or not.

The amount of route error messages when us-

ing AODV also increased, with all TCP variants, by

around 50% compared to when we had no back-

ground trafﬁc.

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Figure 3: Low node density with low amount of background

trafﬁc.

The amount of congestion in the scenario with 180

Kbit/s of background trafﬁc and 96 nodes reduced the

performance of all TCP proposals. In this scenario the

network is overloaded with trafﬁc and therefore none

of the TCP proposals has a throughput higher than 50

Kbit/s, due to space constraints we have omitted the

ﬁgure.

5 CONCLUSIONS

In this paper we have investigated the TCP perfor-

mance of three TCP versions in a TDMA based

MANET using both OLSR and AODV as routing

protocols. We used reference point group mobility

model to simulate a typical tactical network scenario.

The results show that TCP ELFN achieves the overall

highest throughput. The gains are, however, reduced

in low throughput scenarios as the beneﬁt of freezing

the congestion window is less. The performance of

TCP ELFN was similar regardless of which routing

protocol was used.

Future work will further investigate the perfor-

mance of TCP ELFN in high density networks and

the possibility to enhance the performance when us-

ing OLSR.

ACKNOWLEDGEMENTS

This research was supported by grant 2008-539 from

the Knowledge Foundation, Sweden, and the Euro-

pean Regional Development Fund through the Inter-

reg IVB North Sea Region Project E-CLIC.

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