Download A performance analysis modeling of a QoS

Zeynep Yurdakul
704061015
Summary of the Term Paper
A Performance Analysis Modeling of a QoS-Enabled Home Gateway
Ssang Hee Seo1, Jung Tae Lee, and Kyung Jae Ha2
1 Pusan National University, 30, Jangjeong Dong, Geumjeong Gu, Busan, Korea,
[email protected],
2 Kyungnam University, 449, Wolyong Dong, Masan, Kyungnam, Korea
Introduction
A simple performance model of a QoS enabled home gateway is investigated on communication
systems. PC based software router is used as home gateway. The PC based home gateway is
chosen because they can be easily programmed. It is viewed as a queueing network of a home
gateway with one node. Such a simple queueing model like the M/M/1/K with First Come First
Served (FCFS) service discipline can predict a home gateway performance quite well. But, it is
difficult to assume that the service distribution is exponential and that the service discipline is
always FCFS. In this paper, M/G/1/K model is presented for a home gateway. The arrival process
to the server is assumed to be a Poisson Process and the service time distribution be arbitrary.
Software IP router running Linux 2.4 operating system is used. Traffic Control (TC),
differentiated services (DiffServ) mechanisms and Class Based Queueing (CBQ) link shared
scheduler are used in order to support Quality of Service (QoS) in Linux.
Home Gateway Architecture
Gateway is a node that serves as an entrance to another network, and vice-versa. A home gateway
is a device that interconnects various home network segments among themselves as well as
Internet through one of the broadband access technologies. QoS support on the home gateway is
an important issue.
The overall system architecture consists of home gateway between the external broadband and
the home network. The functional architecture and the path of a packet within a Linux based
home gateway in Figure 1.
Figure 1 Path of a packet in Home Gateway
Zeynep Yurdakul
704061015
1. The packet arriving from the network is placed in hardware memory on the NIC. A
hardware interrupt is issued. The processor executes the device driver and copies the
packet from the card to the main memory into a structure.
2. This structure is queued in a FIFO manner in the input queue. There exists one such input
queue per processor.
3. Before returning from the interrupt, the driver issues a soft interrupt. Once the software
interrupt is allowed to execute, the structure is dequeued and information of the ISO layer
2 is analyzed. According to the value of the type field, the packet is handed to the correct
layer 3 function.
4. The destination IP address is extracted and the route cache is inspected. If the entry is not
present in cache, a look up is performed in the routing table. The next hop information is
recorded in the structure.
5. According to the routing decision, the forwarder is then queued to the correct outgoing
interface. There exists one output queue per interface. At this point, traffic control comes
into play.
6. The packet is transferred to the hardware memory and the card is instructed to send the
packet on the network.
Below is packet processing and the queuing model of proposed home gateway. This shows that
home gateway is composed of two main parts. The first part consists of processors on network
interface cards that have functions to receive and transmit packets. The second part is the CPU of
the PC machine that forwards packets not destined for itself, and process packets destined for
itself. The basic components of the home gateway are receiver, transmitter and router with traffic
control.
Figure 2 A queueing model of home gateway
When there is an incoming packet through the i th interface (i = 1, · · · , n), the receiver of that
interface receives such a packet and stores it in the input buffer of Qi if the input buffer is not full.
In the M/G/1/K model, the receivers which do this are independent, identically distributed
stochastic variables RTi with mean rti. Once there is a packet in the output buffer of the j th
interface (j = 1, · · · , n), the transmitter of that interface picks the packet from the output buffer
and transmits it. The times for transmitters to do this are independent, identically distributed
stochastic variable TTj with mean ttj .
Through which interface a packet will be transmitted is dependent on the IP destination address
of such a packet, and is random. Thus the average time of picking and transmitting a packet, tt, is
defined as :
.
Zeynep Yurdakul
704061015
A single service facility of the home gateway serves in a weighted round robin manner. The
service times of packets of the ith network interface are arbitrary, with first and second moments
E[S] and E[s2]. They have different service times according to the type of traffic. The average
time spent in the system by each packet through the ith network interface Qi(i = 1, ..., n) is the
expected sojourn time of a packet. This value is estimated from the measured average response
time.
The system sojourn time E [ST] spent in the system by each packet through the network interface
can be defined as:
E[ST ]  rti  E[Wi ]  E[Si ]  tt  E[T ]
E [T] denotes the average response time and E [Wi] denotes the mean waiting time of a packet in
Qi (i = 1, ..., n). If the home gateway is directly connected, rti and tt are ignored.
The packets arrive according to a Poisson process with rate λ. The average service time has a
general distribution with mean E (S). The E(S) is the inverse of μ which is service rate. An arrival
will be blocked if the total number of packets in the system has reached a predetermined value K.
M/G/1/K*PS Model
Performance metrics that are used in the model:
System sojourn time : The system sojourn time is the time difference between when the source
host sends a packet and when the receiver host receives the packet.
Throughput : The throughput is estimated by taking the number of bits per second between the
total number of successful packets and the time span of measurement.
Blocking probability : The blocking probability is estimated as the drop ratio between when the
source host sends a packet and when the receiver host does not receive the packet.
The probability mass function (pmf) of the total number of packets in the system has the
following expression:
P[ N  n] 
(1   ). n
1   K 1
The probability of blocking Pb is equal to the probability that there are K packets in the system.
Pb  P[ N  K ] 
(1   ). K
(1   K 1 )
The throughput H is the rate of completed packets. When PC router reaches equilibrium, H is
equal to the rate of accepted packets;
H   (1  Pb )
The average response time E [T ] is the expected sojourn time of a packet. Following Little’s law,
E[T ] 
E[ N ]  K 1 ( K   K  1)  

H
 (1   K )(1   )
E[N] is the mean number of packets in system.
The parameters E[S] and K are estimated from the measurements:
Zeynep Yurdakul
704061015
Bandwidth(Mbps) E[S] (sec)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Bandwidth(Mbps) E [S] (sec)
0.0001916
0.0000960
0.0000640
0.0000476
0.0000383
0.0000316
0.0000273
0.0000240
0.0000213
1.0
1.1
1.2
1.3
1.4
1.5
1.7
1.8
1.9
0.0000190
0.0000173
0.0000173
0.0000146
0.0000153
0.0000126
0.0000153
0.0000110
0.0000103
K
10
10
10
10
10
10
10
10
10
Experiments
The test environment contains 3 PCs. The each one is equipped with a 100Mbps fast Ethernet
interface. The home gateway is placed between PCs.
Offered bandwidth from 0.1Mbps through 2Mbps, and the duration of 60sec were used. The three
traffics was sent to the network simultaneously.
Traffic Type
Data Size per packet (bytes)
Data Rate based on Bandwidth(%)
VoIP
100
50
UDP0
500
30
UDP1
1000
20
In order to QoS support, the different flows need different packet processing. And the rules have
to be set on the home gateway. In this experiment, the classification is based on port number.
Once the home gateway received a packet, it will extract the IP packet and will process
differently according to the type of flows.
Test Results and Comparison
The first drop of packets was started at 1.4M bandwidth. In case of 1.4M, the only UDP1 traffic
was dropped. This means that the QoS mechanism was achieved. Since the UDP1 traffic has the
lowest priority although the UDP1 traffic has the lowest sending rate. Also, UDP1 and VoIP
traffic were dropped at 1.7M. But, the UDP0 traffic was not, because of the drop of VoIP traffic.
As greater bandwidth, the drop ratio of UDP0 and UDP1 traffic were increased than that of VoIP
traffic. The average drop ratio of three traffics was similar to the average drop ratio of model.
The throughputs of each of traffic are close to the allowed bandwidth respectively. The VoIP
traffic had high throughput because of high priority. The test results have showed the home
gateway supports the differentiated services. The average throughput predicated by the model fit
well to that of three traffics.
The average response time had some difference. It is due to limited tcp and udp buffer size. The
size of buffer influences the achievable response time. So, the average waiting time was increased
by the limited buffer size. Also, average response time was similar to the average response time
of model.