コシュカルヘディア，池永全志，尾家祐二

社団法人電子情報通信学会
THE INSTITUTE OF ELECTRONICS,
INFORMATION AND COMMUNICATION ENGINEERS
信学技報
TECHNICAL REPORT OF IEICE
複数経路表を用いたホップバイホップ型 QoS 経路制御機構
コシュカルヘディア†
池永全志‡
尾家祐二‡
九州工業大学情報工学部
〒820-8502 福岡県飯塚市川津 680-4
E-mail: †[email protected], ‡{ike, oie}@cse.kyutech.ac.jp
あらまし
今後，インターネットにおいては，通信品質要求や優先度が異なる複数のクラスのトラヒックが混
在する状況になると考えられる．そのため，優先度が高いクラスによる通信が優先度の低いクラスの通信を圧
迫する等の相互作用を考慮したうえで網資源を効率良く利用可能な QoS 経路制御手法の実現が求められる．そ
こで本研究では，現在のインターネットにおける経路制御手法と親和性が高いホップバイホップ型のマルチク
ラス QoS 経路制御手法として，網内のノードが各トラヒッククラスに対応する複数の経路表を用いる手法を提
案する．さらに，シミュレーションによる性能評価の結果より，提案手法を用いることによって各クラスのト
ラヒックが良好な性能を得られることを示す．
キーワード
QoS，経路制御，マルチクラス，複数経路表
Multiple Routing Tables for Effective Hop-by-Hop QoS Routing
Hedia KOCHKAR†, Takeshi IKENEGA‡ , and Yuji OIE‡
†Department of Computer Science and Electronics, Kyushu Institute of Technology
680-4 Kawazu, Iizuka-shi, Fukuoka, 820-8502 Japan
E-mail: †[email protected], ‡{ike, oie}@cse.kyutech.ac.jp
Abstract Future communications networks are expected to support applications with quality of service (QoS) requirements.
Supporting QoS poses major challenges due to the large scale and complex structure of networks. Several studies have
addressed different aspects of QoS routing. In this paper, we present a new approach of QoS routing based on the concept of
multiple routing tables. Each class of traffic will have its own routing table. This scheme can prevent traffic with higher
priority from concentrating on some link, so it can improve their performances. From our extensive simulation results, we have
found that both packet loss of QoS traffic and throughput of best-effort traffic can be improved by using our scheme.
Key words QoS, Routing, multi-class, multiple routing tables
I.
INTRODUCTION
This routing may suffice in networks that provide only a single
best-effort service in which there is no guarantee about whether
With the increasing diversity of network applications, it has
and when a packet will be delivered. However it may not be
become crucial for networks such as the Internet to offer various
adequate in networks that provide Quality of Service (QoS)
services, including best-effort services and guaranteed services. In
guarantees to applications such as real time applications. These
the Internet a simple shortest path routing algorithm is employed.
applications demand a guaranteed amount of network resources
resources requirements, always select the same path. After
such as bandwidth etc. Hence, given a set of QoS requirements for
establishing a certain number of QoS flow along the shortest path,
a flow, the routers should be able to find a path satisfying these
this path will be loaded and obviously there will be an increase of
requirements.
the packet loss probability of the QoS traffic and a decrease of the
Since the current routing protocols used in IP networks do not
throughput of the best-effort traffic [1][2][3][4][5][6]. A good
support the quality-of-service that different flows require. This can
example of how to extend a conventional best-effort traffic routing
deteriorate the performance of the QoS traffic and may also
with QoS capabilities is the QOSPF routing protocol [4]. In [2] [7]
decrease the throughput of the whole network. Since the goal of
authors focused on QoS routing algorithms for connections, they
the QoS routing is to find a path that satisfies the given
did not simultaneously take class–based and hop-by-hop routing
requirements and may increase the global network resource
into consideration.
utilization, an efficient QoS routing algorithm in the current IP
networks is needed.
In this paper, we propose an extension to the OSPF routing
algorithm. The work presented here addresses the multi-class QoS
In this paper, we propose a distributed link state QoS routing
with multiple routing tables obtained upon different link costs to
routing with a standard hop-by-hop procedure. To our best
knowledge, our work is the first that raises such problem.
effectively accommodate multi-class traffic such as real time QoS
III.
traffic and best–effort traffic. Each router in the network will
PROPOSED ALGORITHM
maintain and compute multiple routing tables. For each class of
traffic, we associate a separate routing table. When a packet
The goal of QoS routing is to select paths for flows with QoS
arrives at the router, the edge router will mark the packet based on
requirements, in such manner as to increase the throughput of the
its traffic class. Based upon this mark, the router will determine
whole network. We will give in this section an overview of the
which table will be used for this packet. The significance of the
proposed multi-class routing algorithm. For ease of discussion, we
proposed scheme lies on improving the performance of traffic
assume that there are two class of traffic: QoS traffic (high
with high priority without hurting lower priority traffic’s
priority) and Best Effort traffic (low priority). Thus, two routing
performance. This scheme can prevent the QoS traffic from
tables will be used, one for each class of traffic. In order to avoid
concentrating on some links and allow at the same time to
heavy loaded links, paths for QoS flows will be selected based on
improve the best-effort traffic throughput.
the available resources. As about best-effort traffic the shortest
The rest of this paper is organized as follows. Section II
path is selected. For this, each router will maintain and compute
provides some related works. The proposed algorithm overview
two routing tables. The first one is for the shortest paths, the
will be given in section III. Experiments results are shown in
second for the QoS-based routing information. In each routing
section IV. Finally, section V. draws the conclusion.
table and for each destination, a router should store next hop
entries associated with the minimum hops or the lightest path.
II.
RELATED WORK
Upon a reception of a packet, the edge-router should mark the
packets whether it is best effort or QoS traffic. The router will then
QoS routing has attracted much attention recently. An
extensive survey can be found in [8].
forward the packet toward next hop on the lightest path or the
shortest path depending on its traffic type. The role of core-router
Open Shortest Path First (OSPF) [9] is a widely deployed link
is to check the received packet based on the mark written with it
state routing protocol that has been an Internet standard for some
and to transmit it to the adequate routing table. Our
time. The OSPF standard specifies that the routers run the shortest
implementation computes QoS paths using the widest path
path dijkstra computation on their link state database, and
selection. At a router, a dijkstra’s algorithm is used to compute
determine a shortest path to all other nodes in the network. The
paths from the router to all the destinations in the network. This
database is constructed and updated by means of link state
routing table is kept separate from the standard OSPF routing
advertisements. Recently, we can realize that given today’s
table.
emerging need for multimedia communications the traditional, the
The proposed routing for QoS traffic will shift the traffic from
well known Internet routing is no more efficient. Thus, the
one path to "better" path whenever the "better" path is found. This
traditional routing algorithm will, regardless of the flow’s
kind of shift is undesirable because it will bring routing oscillations
when the routing is based on metrics like link utilization, which
For the dynamic threshold, we will only focus during our
changes rapidly from time to time. The traffic will be routed back
simulations on the above threshold. In our approach, we can
and forth between alternate paths. Since our proposed algorithm is
substitute the total link utilization in the proposed threshold by the
based on multiple routing tables, two routing tables, and in order to
maximum link utilization in the path or by the available bandwidth.
give more freedom to the QoS traffic; high priority traffic will not
Moreover, we can use different weights in the case where different
only use their own routing table but also lower priority traffic’s. In
classes of traffic exist with different priority. Further work is needed
this way, high priority traffic will be routed in different paths and
to define and evaluate such extensions.
load balancing can be achieved. That means that not all QoS traffic
IV.
will be routed based on the QoS routing table but some percentage
SIMULATION DESIGN
of QoS traffic will be routed based on this routing table and the
rest will be routed on the shortest path. Then, the main problem is
IV.1.
Network topology
to determine how much QoS traffic has to be routed based on the
We have carried out a number of simulation experiments to
QoS routing table in order to have a good performance for both
study the performance of the proposed QoS routing algorithm. All
class of traffic. Therefore, what we need is a threshold that split QoS
simulations were performed using the ns-2 simulator [10] using the
traffic between both routing tables. We will present two new
mesh topology (3*3) with 9 nodes. All results are based on 300
schemes. The first one is based on a static threshold and the next
seconds of simulations run. The capacity of each link is set to
one is based on a dynamic threshold.
10Mb/s.
III.1. Static threshold
IV.2.
We assume the network handles two types of traffic, where the
Since two routing tables are used, the first idea that comes to
mind is to split equitably the number of QoS request between both
Traffic Model
traffic load is unevenly distributed.
tables. Thus, 50% of QoS request will be routed according to the
The first type of traffic is TCP traffic. One destination and 5
QoS routing table and the other 50% will be routed on the shortest
sources are chosen randomly. Between each source and the chosen
path. By doing so, we can avoid the oscillations problem shown
destination there are 3 TCP connections.
The second type of traffic is the UDP traffic, which represents
previously. Therefore, the traffic will be spread throughout the
network, which will make the network more balanced.
the video applications; we assume that the requested bandwidth is
set to 128Kbps. The UDP traffic is unevenly distributed, 3 pairs of
III.2. Dynamic threshold
(source, destination) are randomly chosen; between each (source,
Selecting a path for QoS traffic can have an affect the whole
destination) pair, there are 100 UDP flows. UDP flows arrive
network. Therefore, how much QoS traffic has to be routed on the
according to an exponential distribution, and its average duration
shortest path will have an impact on the lower priority traffic. For
is set to 60 seconds. All of the performance measures in the next
that, the amount of QoS traffic that is routed on the shortest path
section are functions of the arrival rate of QoS traffic, which is
has to depend on the network state and especially how much the
defined as follows:
shortest path is congested. We need a dynamic threshold, which
has to depend on the network state. Thus, we propose dynamic
arrival rate=
threshold as follows:
T
on
T on + T off
where T on represents the average duration of a UDP flow
Threshold = link_utl_Shortest_path / ( link_utl_Shortest_path
+ link_utl_QoS_path);
and
Toff the average interval time between the arrivals of two
successive flows.
Our proposed threshold depends on the link utilization of the
shortest path and the QoS path. When the shortest path link
IV.3.
Performance Metrics
utilization is high, the QoS flows will be routed based on the QoS
routing table. Otherwise, QoS flows will be routed on the shortest
path. By doing so, Best Effort traffic performance will not be hurt
since QoS traffic avoid to use shortest path when it is congested.
In our simulations, we evaluated and compared the following
two performance metrics:
Packet Loss probability for the QoS traffic: The ratio of
the total number of QoS packets dropped in the network
Fig.2 shows the TCP throughput as a function of UDP arrival rate.
to the total number of input QoS packets in the network.
It is clear that the QOSPF performs poorly because TCP traffic is
Best-Effort throughput: The ratio of the total number of
1
required for transmission.
V.
RESULTS
The main objective of our study is to improve the
Packet Loss Probability
best-effort traffic (bytes) received to the total time
0.1
0.01
0.001
performance of high priority QoS traffic, UDP traffic, without
0.0001
0
hurting the lower priority traffic.
In order to see the effectiveness of the proposed multi-table
0.2
0.4
0.6
UDP Load
qospf
ospf
0.8
1
Proposed
QoS routing scheme, we will compare its performance with two
other algorithms. The first one is the shortest path routing where
Fig.1. UDP Packets Loss rate (Random Traffic Pattern)
all the traffic, regardless of their priority, is routed on the shortest
3
routing algorithm where all the traffic is routed on the widest path,
the path with the maximum available bandwidth. During our
discussions, we will refer to the first one as “ospf”, the second as
“qospf”, the proposed algorithm as “Proposed”, the approach
using static threshold as “Proposed (50%)” and the one using the
TCP Throughput[Mbps]
path, the path with the minimum hops. The second one is the QoS
2.5
2
1.5
1
0.5
dynamic threshold as “Proposed-dynamic”.
0
0.2
0.1
V.1.
0.3
0.6
0.5
UDP Load
0.7
qospf
ospf
Impact of UDP traffic load
0.4
0.8
0.9
1
Proposed
Fig.2. TCP throughput (Random Traffic Pattern)
Fig.1 plots the UDP packet loss probability as a function of
1
that the loss probability is rather high compared with the other
schemes. When using OSPF, regardless of the flow’s resources
requirements, the same path is always selected. After establishing
a certain number of UDP flows along the shortest path, this path
will be heavily loaded and obviously there will be an increase of
Packet Loss Probability
UDP arrival rate. By applying the OSPF algorithm, we can see
0.1
0.01
ospf
0.001
qospf
the packet loss probability of the UDP traffic. But when selecting
Proposed
0.0001
the widest path, QOSPF, we can remark a decrease of the packet
0
0.2
0.4
0.6
UDP Load
0.8
1
loss probability compared with the OSPF. And this is because the
path selection is based on the high priority traffic requirement. So,
Fig.3. UDP packets Loss rate (link sharing)
UDP traffic will avoid heavily congested links and they will be
routed around them. With respect to the proposed scheme, we can
see performance quite similar to that of QOSPF since UDP traffic
are routed in both of our scheme and QOSPF in the same way.
The objective of our QoS routing algorithm is to improve the
performance of UDP traffic, by decreasing the packet loss
probability and at the same time not to hurt best-effort traffic. As
shown in Fig.1 the performance of UDP traffic is improved, we
will see now the impact of the proposed scheme on the TCP traffic.
routed in the same links where UDP traffic exists. The QOSPF
might be able to increase the throughput of an individual
connection by picking a long path with higher bandwidth; this
might reduce the throughput of other connections, and thus the
average throughput.
Moreover, paths with more hops have a
higher chance of having their throughput reduced as a result of fair
sharing with connections that are added later. So the shortest path,
minimum hops, outperforms the QOSPF as shown in Fig.2. For
the proposed scheme, we can see that a good improvement
1
hops path for TCP can conserve resources. At the same time
selecting widest path for UDP traffic makes a load balancing. As a
result, the proposed scheme can achieve resource efficiency by
limiting resources consumption while balancing the network load.
The results shown in Fig.1 and Fig.2 are based on a random
Packet Loss Probability
compared with other two algorithms. Thus selecting the minimum
0.1
0.01
ospf
0.001
Proposed
traffic pattern. However, there are some cases where the proposed
Proposed (50% UDP)
algorithm does not work well. For example, in the case when a
0.0001
0
0.2
0.4
0.6
UDP Load
number of UDP connections are sharing the same link and after a
number of UDP requests, the shared link will be congested. Thus,
0.8
1
Fig.4. UDP Packet Loss rate (Static threshold)
flows, which are routed in this link, will be shifted to a less
3
will be shifted to other path with more resource. Therefore, flows
will oscillate between alternative paths, which will cause a more
congested network. Consequently, the packets loss probability of
UDP traffic will increase. As shown in Fig.3, we can remark that
using the famous OSPF algorithm gives better performance for
TCP Throughput[Mbps]
congested. Moreover, the same scenario will happen again; flows
2.5
2
1.5
1
ospf
Proposed
0.5
UDP traffic. UDP traffic loss probability increases when the
Proposed (50% UDP)
0
QOSPF or the proposed algorithm is used.
V.2.
0.6
0.5
UDP Load
0.4
0.3
0.2
0.1
0.7
0.8
0.9
1
Fig.5. TCP Throughput (Static threshold)
Improvement of the proposed scheme
Based on the previous discussions, we can conclude that in
0.1
ospf
Proposed(50%UDP)
Proposed-Dynamic
split between different paths. Since our proposed algorithm is
based on multiple routing tables, two routing tables, and in order
to give more freedom to the QoS traffic; high priority traffic will
not only use their own routing table but also lower priority
Packet Loss Probability
order to avoid traffic oscillations, UDP traffic requests have to be
0.01
0.001
traffic’s. In this way, high priority traffic will be routed in
different paths and load balancing can be achieved.
0.0001
0.1
V.2.1.
0.2
0.3
0.4
0.6
0.5
UDP Load
0.7
1
0.9
0.8
Static threshold
We implemented this approach and we made simulations
Fig.6. UDP Packets Loss rate (Dynamic threshold)
UDP connections. As shown in Fig.4 and Fig.5 splitting UDP
traffic between different paths gives better performance for UDP
traffic than the first approach.
V.2.2.
Dynamic Threshold
By using the same traffic pattern used previously, we could
find quite similar results as using a fixed threshold (50%). This
can be explained by the fact that in the used pattern, one link is
shared only by two UDP connections. As shown in fig.6, we can
remark that using a dynamic threshold improves the performance.
put
TCP Through
Packets
loss Probability
based on a traffic pattern when a link is shared between different
ospf
Proposed (50% UDP)
Proposed-dynamic
2.4
2.2
2
1.8
1.6
0.1
0.2
0.3
0.4
0.6
0.5
UDP Load
0.7
0.8
0.9
Fig.7. TCP Throughput (Dynamic threshold)
1
Furthermore, using the dynamic threshold improves the
ACKNOWLEDGMENTS
performance of TCP traffic comparing with the other approaches
This work was supported in part by the Ministry of Education,
(See Fig.7).
Culture, Sports, Science and Technology, Japan, Grant-in-Aid for
VI.
CONCLUSION
Scientific Research (A), 15200005, 2003, and in part by the
Ministry of Public Management, Home Affairs, Posts and
In this paper, we proposed a hop-by-hop routing scheme with
Telecommunications, Japan for Research and development for
multiple routing tables. In a network that supports both QoS traffic
fostering
requiring bandwidth guarantees and best-effort traffic, which
Information and Communications R&D Promotion Scheme.
younger
excellent
IT
researchers
of
Strategic
resources are available to best-effort traffic depends on how QoS
traffic is routed. In such case QoS flows can consume all the
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