AIMD-PQ: A Path Quality based TCP-Friendly
AIMD Algorithm for Multipath Congestion Control
in Heterogeneous Wireless Networks
Jia Zhao
†
, Changqiao Xu
∗
, Jianfeng Guan
∗
and Hongke Zhang
†
†
National Engineering Laboratory for Next Generation Internet Interconnection Devices
Beijing Jiaotong University, Beijing, China
Email: {11111004, hkzhang}@bjtu.edu.cn
∗
State Key Laboratory of Networking and Switching Technology
Beijing University of Posts and Telecommunications, Beijing, China
Email: {cqxu, jfguan}@bupt.edu.cn
Abstract—Multipath congestion control algorithms are sup-
posed to be friendly to traditional single-path TCP (SP-TCP).
Existing multipath Additive Increase Multiplicative Decrease algo-
rithms (MP-AIMD) for multipath TCP (MP-TCP) are confronted
with the problems: 1) they are unfair to SP-TCP when the
multiple available paths have heterogeneity, e.g. different RTT;
2) they all use packet losses as congestion signals and induce
spurious backoffs on a wireless link with high error rate; 3)
congestion window fluctuation reduces their potential to support
smooth high-quality service such as multimedia applications. This
paper proposes a path quality based multipath AIMD (AIMD-
PQ) to tackle the above problems. We utilize the round trip
time (RTT) on each sub-path to formulate the path quality
estimation. AIMD-PQ balances the loads among its sub-paths,
moves traffic off the most congested sub-path, and triggers
Multiplicative Decrease by both packet loss and path quality
signals. Simulation results show that AIMD-PQ improves both
the TCP-friendliness and window fluctuation performance of MP-
TCP in a heterogeneous wireless network environment.
I. INTRODUCTION
Majority of data on the Internet is transmitted by using
TCP. As a fundamental transport layer protocol of the current
Internet architecture, TCP provides a reliable way for end-
to-end data transfer and a fair mechanism for bandwidth
resource pooling. With the purpose of network fairness, any
innovative transmission control protocol has to deal with its
TCP-friendliness to coexist with TCP. Traditional TCP is
designed for single-path flow control and thus loses efficiency
when there are multiple interfaces (e.g. a smartphone with
both Wi-Fi and cellular interfaces, and a server with several
high-speed interfaces in data centers). This motivates the study
on multipath transmission control protocols such as Multi-path
TCP (MP-TCP) [1] and Stream Control Transmission Protocol
(SCTP) [2]. These protocols are also supposed to be friendly
to single-path TCP (SP-TCP).
Additive Increase Multiplicative Decrease (AIMD) has been
used to design TCP-friendly congestion control algorithms [3].
An AIMD algorithm is characterized by two parameters α
and β. AIMD additively increases the congestion window by
α for a round trip time (RTT), or decreases the window size
multiplicatively to β of its previous value if a congestion signal
is received. Early versions of MP-TCP and SCTP extend the
AIMD algorithm of TCP-NewReno [4] to multipath case, and
they run the AIMD algorithm independently on each sub-
path. It is easily implemented but unfair to SP-TCP [5]. To
tackle this problem, several coupled multipath AIMD (MP-
AIMD) algorithms have been proposed in literature [6]-[10].
RFC 6356 [10] summarizes the design goals of a multipath
congestion control algorithm as: (i)achieve no less throughput
than a single-path flow would on its best sub-path; (ii) not
take up more capacity than if it was a single-path flow using
the best sub-path; (iii) move as much as possible traffic off its
most congested sub-paths. Goals (i) and (ii) guarantee fairness
between MP-TCP and SP-TCP flows on a same bottleneck.
However, coupled MP-AIMD algorithms are still entan-
gled in TCP-friendliness problems when a MP-TCP flow is
confronted with path heterogeneity, e.g. available paths for
a multi-homed mobile device in a heterogeneous wireless
network have different capacity, transmission delay, RTT,
link error rate, and packet drop rate. As discussed in [9],
the Coupled algorithm for MP-TCP use only one sub-path
when the sub-paths have different RTT, and cannot increase
its congestion window on other available sub-paths. This is
undesirable for multipath congestion control. We show in our
simulation that the Coupled algorithm has the worst perfor-
mance for TCP-friendliness of MP-TCP in a heterogeneous
wireless network environment. In [11], simulation results show
that the Modified semi-coupled algorithm [10] is still unfair
to SP-TCP when the sub-paths have different RTT.
Wireless links have high error rate. A packet loss in wireless
networks can be ascribed to either congestion or fading chan-
nels. Loss-based AIMD algorithms are designed for wireline
networks where random packet loss is negligible, and a packet
loss over a wired link can be used as congestion indication for
AIMD. However, if a loss-based AIMD algorithm is used as
congestion control over a wireless link with high loss proba-
bility, it will frequently carry out the Multiplicative Decrease
to avoid “congestion”, and thus cause spurious backoffs and
2015 IEEE Wireless Communications and Networking Conference (WCNC): - Track 3: Mobile and Wireless Networks
978-1-4799-8406-0/15/$31.00 ©2015 IEEE 1678