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10.1109/TPDS.2014.2347031, IEEE Transactions on Parallel and Distributed Systems
IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS, VOL. XX, NO. X, 2014 3
the flow rate assignment. An opportunistic scheduling-
based scheduler was proposed to send packets to multi-
ple paths while keeping the fraction of bytes transmitted
on each path.
The flow-based traffic splitting models address the
packet reordering problem by assigning the packets be-
long to the same flow onto the same path. Although the
risk of packet out-of-order may be decreased, load imbal-
ance among the communication paths frequently occurs
and the queueing delay over a single path significantly
increases. The Load Balancing for Parallel Forwarding
(LBPF) [15] model selects the path for a flow according to
the hashed result of the packet identifier in the ordinary
mode, similar to the schedule of Direct Hashing. The
FlowLet Aware Routing Engine (FLARE) [16] switch-
es packet bursts named flowlets onto available paths.
Flowlets are spaced by a minimum interval δ, chosen
to be larger than the delay difference between parallel
paths to reduce packet reordering.
The MultiPath LOss Tolerant (MPLOT) [19] transport
protocol addresses the burst data losses in wireless net-
works by introducing a hybrid proactive/reactive FEC
(Forward Error Correction) mechanism. The Encoded
Multipath Streaming (EMS) [20] model adjusts the FEC
redundancy in a progressive manner based on the infor-
mation loss rate. This study reveals the tradeoff between
loss and delay performance in real-time applications.
The work closest to ours is conducted by Bui et al. [2].
The authors formulate the multipath data distribution as
a Markov Decision Process and propose an Online Policy
Iteration (OPI) algorithm. The proposed algorithm aims
at improving the performance metrics, e.g., delay, loss,
and throughput. However, goodput is able to better indi-
cate the QoS requirements of real-time applications. Both
the traffic and network info should be comprehensively
analyzed. Towards this end, we formulate the data distri-
bution aiming to optimize the goodput performance and
develop solutions for flow rate assignment. Besides, we
take into account the burst path losses in heterogeneous
overlay networks and propose an interleaving scheme
to alleviate consecutive losses. In literature [21], we
propose a Sub-Frame Level (SFL) scheduling approach to
optimize the delay performance of high definition video
streaming over heterogeneous wireless networks.
3 SYSTEM MODEL AND PROBLEM FORMULA-
TION
We firstly describe the system overview of the proposed
GALTON, which is shown in Fig. 2. The proposed
model is an end-to-end scheduling framework and the
implementation requires the modifications on the sender
and receiver sides.
The key working component in the sender side is the
parameter control unit, which takes in the input pa-
rameters (including the path status, deadline constraint)
and outputs the scheduling parameters (including flow
assignment vector, interleaving level, and probing traffic
adaption parameters). The flow rate allocator is respon-
sible for partitioning the input traffic (which may contain
multiple deadline-constrained flows) into several sub-
flows and dispatching each of them to the available
paths. The allocated sub-flows will be temporarily stored
in the sending buffers for different communication paths.
In the process of deadline-constrained packet interleav-
ing, the packets scheduled onto the same path will be
spread out within the delay constraint.
The receiving buffer at the receiver side is used for
collecting the packets. In order to restore the original
traffic, packet-to-flow mapping and inter-packet rese-
quence are two necessary steps. The problem of traffic
load distribution involves the models of communication
path and real-time traffic, which will be described in the
rest of this section.
3.1 Communication Path Model
We consider a heterogeneous overlay network integrat-
ing multiple communication paths between two termi-
nals. The end-to-end connection can be constructed by
binding a pair of IP addresses from the source and
destination node, respectively. Each communication path
p ∈ P is considered to be an independent transport
link uncorrelated with others and this assumption can
be justified as follows
• In the environment of wired multipath networks,
a method commonly used in existing studies (e.g.,
[22][36][46]) is to detect the shared congestion paths
with the end-to-end measurement techniques (e.g.,
the cross-correlation based approaches [47], and
wavelet based schemes [48]). Then, the paths shar-
ing bottleneck links can be treated as one com-
munication path. Another method to construct the
end-to-end independent paths is to use the overlay
relay nodes to forward the traffic [49]. In this case,
the available communication paths are disjoint. The
general review of setting the independent paths can
be found in [50].
• In the context of heterogeneous wireless networks
(e.g., WLAN, Cellular, WiMAX), the last-hop wire-
less links are most likely to be the bottleneck links of
the end-to-end paths due to the limited capacity and
time-varying channel status [6][39]. These access
networks use different wireless spectrums and do
not interfere with each other. Therefore, we can
assume the available capacities and loss rates are
independent to each other [6][7].
The end-to-end communication paths are character-
ized by the following properties:
• the available bandwidth µ
p
. This metric does not
indicate the raw per-path capacity, but the time-
varying share of that bandwidth as perceived by
the end-to-end flow. It can also be viewed as the
sending rate on path p that is achievable with the
current sending buffer and round trip time.