SIMULATION OF THE COMBINED METHOD

International Journal of Network Security & Its Applications (IJNSA), Vol.3, No.5, Sep 2011
DOI : 10.5121/ijnsa.2011.3506 75
SIMULATION OF THE COMBINED METHOD
Ilya Levin1
and Victor Yakovlev2
1
The Department of Information Security of Systems, State University of
Telecommunication, St.Petersburg, Russia
lyowin@gmail.com
2
The Department of Information Security of Systems, State University of
Telecommunication, St.Petersburg, Russia
viyak@bk.ru
ABSTRACT
DDoS attacks have become one of the most dangerous issues in the Internet today. Because of these
attacks, legitimate users can not access the resources they need. In [1] authors proposed
a combined method for tracing and blocking the sources of DDoS-attacks. The essence of the method is
that each router marks the network packet that passes through it using a random hash function from the
set. At the receiving side this information is stored and used to filter unwanted traffic and traceback the
source of distributed attack. This article describes the simulation and its results of the combined method.
KEYWORDS
Traceback, DDoS-attacks, Attack path reconstruction
1. INTRODUCTION
With the spread of network technology has increased the number of intruders attempting to
prevent the normal operation of network applications and resources. One of the most
widespread threats on the Internet today are DDoS-attacks (Distributed Denial of Service).
DDoS-attack is directed at the violation or the complete cessation of legitimate users’ access to
networks, servers, services and other resources. To implement such an attack attackers create a
bot-network (network of infected computers) and use them for attack by sending a huge number
of packets from infected machines. Botnet can include hundreds or thousands of hosts spreaded
around the world. According to Arbor Networks’ Worldwide infrastructure security report the
size of the largest single attack increased from 10Gbps in 2005 to 100Gbps in 2010 (1000
percent increase).
Because these attacks are performed using a wide range of addresses (including spoofed IP-
addresses) it's very difficult to detect and block such attacks. Existing methods of protecting
from DDoS-attacks do not have the high speed and automaticity of work.
A large number of DDoS-attack traceback systems were proposed [6-15]. Most of them are
based on the principle of packet marking by routers [10-15]. Marks contain information about
the part of the path that packet had passed. Rarely utilized fields of IP packet header are used for
marks storing.
Marks can be made as mandatory by each router (deterministic packet mark) [12-14] and with a
certain probability (probabilistic packet mark). A victim analyzing marks in received packets
will be able to reconstruct the path a packet passed in the network and compute the attacking
host.
The proposed method of tracing DDoS-attack sources combines several methods used on the
routers (in particular, the method of mandatory packet marking using random hash functions)
and the method used on the hosts.

76
The rest of this paper is organised as follows. In Section 2, description of the proposed method
is presented. We introduce parameters of the combined method in Section 3. Section 4
demonstrates our simulation methodology. In Section 5, the simulation and results obtained
using OMNet++ tool are discussed. Finally, we will conclude our paper in Section 6.
2. DESCRIPTION OF THE METHOD
Let’s consider the way of packet marking. The mark is a hash value of the IP address of the
router iR that processes a packet and is defined as ( )( )ij
i
j RIPhh = where j is the number of
used hash function. Marks are inserted into specially designated field in the IP header. The
structure of the mark field is shown in Figure 1.
Figure 1. The structure of a mark field.
The total length of the field is M bits and it is divided into several parts. The first part hashN is
for hash function number which was chosen by the first router that sent the packet. The length
of this field is N bits. The second part is the field of marks which consists of a set of marks of
length n bits which are inserted into the packet by routers. The maximum amount of marks in
the packet is 




 −
=
n
NM
S , where   is the integer part of the fraction
n
NM −
and it’s
called the depth of the traceable path.
Our task will be to optimize the parameters N, n and S for a fixed value of M. The value of M
depends on the number of "free" bits in the IP header. The rarely utilized identification field (16
bits) and high-order bit of flags field (a total of 17 bits) are commonly used for IPv4.
Identification field is used only in case of fragmentation of the package. The average
fragmentation rate is not more than 0.25% of all packets [9]. In IPv6 for these purposes you can
use the flow type field (20 bits) whose purpose is not strictly regulated by the standard. In [15] it
is proposed to expand the field up to 25 bits.
Each router inserts its mark in the last n bits of the mark field. Marks made by previous routers
are shifted to the left. The sequence of marks made by different routers 1
jh , 2
jh ,…, S
jh by using
the same hash function will be called the mark-vector. When the attacker attacks the victim it
sends a packet, this packet arrives at the first router R1. It determines that the packet was sent
from its autonomous system (AS) and initiates the process of packet marking. The router resets
the field in the IP header, which is reserved for marks. It randomly selects number of hash
function (j) which will be used for mark generating and computes the hash 1
jh of its IP address
using the selected hash function and writes it to the least significant bits of the mark field. Then
it sends the packet to the next router. When the packet reaches the second border router R2, it
determines that the packet is not from its AS and that the packet is already marked. The router
reads the value of Nhash field (which is equal to j) and computes the hash 2
jh of its IP address.
Then it writes the hash to the mark field of the IP packet shifting the previous hash
left. Thereafter it sends the packet further. This process continues until the packet reaches the
Nhash field
M
n
N
hash of the
last router
hash of the
penultimate router

77
recipient (victim). At the end of the path the mark field will contain the hash codes of all the
border routers through which the packet had passed.
If the value of S less than the number of border routers which the packet passes through, the
left-most mark is deleted to give the opportunity to add new mark. In this case, surely the result
of the operation of the method will contain an inaccuracy.
Victim stores all the necessary information from arrived packets in the specially designed
databases. When the victim notices any abnormal activity (e.g., using intrusion detection
system), it forms a request packet. This packet contains the following fields: field D is for
measuring distance the request passed, field NH is the number of mark-vectors in the request,
victim’s IP address field, ID field is used to detect loops during the request transferring, marks
table field contains mark-vectors and the numbers of hash functions which were used.
Victim sends this request packet to the nearest router RS. Router RS receives the request and
reads the value of field D, increases this field by one and checks the appropriate column of the
mark table with hash codes of border routers with which it is connected. Sends a request to only
those routers for which there is a hash coincidence. If there is no match for any of the routers
then the request packet transferring is terminated. When a request is sent simultaneously to two
or more routers then their IP-address are stored in the option field to prevent the sending of the
request to each other in the future. Other routers send a request in the same way to other routers
increasing the value of D field by one.
When the request comes to the last border router it blocks the traffic sent to the host whose
address is specified in the request or acts in accordance with the instructions contained in the
option field of the request packet. Also it sends a response to the victim with its IP address, so
the victim could send a request to remove the restriction on its address.
Request packet must be protected cryptographically to prevent it from spoofing. Consideration
of this issue is beyond the scope of this article.
3. THE PROBABILITY OF FALSE DETECTION
The probability of false detection of the source of the attack Pfalse is one of the most important
parameters of the combined method. Parameter Pfalse shows the probability of collision of
marks from two different routers which as a result may lead to incorrect calculation of the path
and blocking legitimate user traffic instead of traffic of attacker. The value of Pfalse depends on
the length of the hash code generated by routers and the algorithm of generating mark.
In [1] it was shown that this parameter for the case when the network is a binary tree can
be calculated using the following expression
N
nfalseP
2
2
1






≈ (1)
where n is the length of hash code.
It's very difficult to calculate Pfalse when routers have more than 2 connections. In this case
combined method simulation is required.
4. SIMULATION
To test the performance and characteristics of the proposed method the program was written that
allows to emulate the network and the method. The program should do:
– generate networks of different depths;
– delete or add links between routers to simulate different situations in the network;
– provide the possibility of changing the main parameters of the algorithm, such as: N, n;

78
– display the results of the attack tracing, the number of correctly detected, falsely
detected and undetected sources of the attack;
– evaluate a large number of experiments with the networks of different topologies to
obtain statistical results.
We will consider two types of network topologies:
– Near-binary tree network (network in which each node has an average of 2 children);
– Network generated based on the Euclidean graph.
In case of near-binary tree network, building of the network starts from the first node - the
victim. Then each router is assigned randomly from one to three children. This
process continues until the network reaches the depth that has been configured. Figure 2 shows
an example of the process of generating a near-binary tree network.
We need networks of this type to check the correctness of the formulas for calculating the value
of the probability Pfalse.
Figure 2. Example of the near-binary tree network generating process.
In case of network generated based on the Euclidean graph, routers are distributed on the
surface randomly. Then calculated the average distance between all the routers. If the distance
between two routers less than a specified threshold then these two routers are interconnected.
By adjusting the threshold we can change the number of connections between nodes in the
generated network. Thus we have areas in which the routers are connected. Then these areas are
connected between each other. The result is the network which has areas of varying degrees of
connectivity. This model is well suited for real-life network topology generating. Fig. 3 shows
an example of the network obtained using the program.
The generated network consists of routers and lines connecting them. You can set any router as
an attacker in the generated network (of course, we assume that the attacker is a host which is
connected to this router). When attackers are selected the program emulates sending packets by
each of them to the victim. Each router adds its mark to a virtual packet. When the packet
reaches the victim the mark-vector is stored.
When the attack is done the sources can be traced. The program displays a summary table
showing all detected sources. It indicates the number of real sources of the attack, the number of
detected sources, the number of falsely detected and the number of undetected sources.
The program allows you to perform numbers of experiments to collect statistics. It
automatically generates new network, performs an attack, traces the source of the attack and
writes the results into the file. For each method configuration 10000 experiments were
performed. Each experiment included 500 attackers.
network depth
level S
level S-1
level i
level 1
S
V

79
The simulation results and the calculated characteristics are presented in Table 1. Obtained
experimental results are comparable to the calculated values of the probability Pfalse.
Figure 3. Example of the generated network based on Euclidean graph.
Table 1. Configuration options and method simulation results with M = 17 bits.
№
Number of
hash
functions
n,
bit
S
Pfalse
Calculated
Simulation
for tree
networks
Simulation of
networks
constructed from
the Euclidean
graphs
1 2 1 16 0,25 0,33 0,58
2 2 2 8 0,0625 0,07 0,081
3 4 1 15 0,0625 0,065 0,08
4 8 1 14 3,91*10-3
3*10-3
4,1*10-3
5 8 2 7 1,53*10-5
1,8*10-5
2,7*10-5
6 16 1 13 1,53*10-5
1,4*10-5
2*10-5
7 32 1 12 2,33*10-10
- -
8 32 2 6 5,42*10-20
- -
9 32 3 4 1,26*10-29
- -
10 64 1 11 5,42*10-20
- -
11 128 1 10 2,94*10-39
- -
12 128 2 5 8,64*10-78
- -
Because of small numbers of used hash functions options 1, 2 and 3 have a high probability of
false detection. Therefore, they are not suitable. Options 8, 9 and 12 have a small probability of
false detection but use a large number of hash functions. As a result the depth of the
investigated network is small. Options 10 and 11 provide a low probability of false detection,
greater depth of the network but because of the large number of hash functions may require
additional time for processing.
The most suitable for use are options 6 and 7. They have a small probability of false detection
and provide great depth of network investigation.

80
It's seen that theoretical results and experimental results converge well. Therefore we can
assume that the theoretical results for which we were not able to do simulation due to small
magnitude of probability are fair as well.
Despite this program can produce a large number of experiments it allows you to test the
combined method with a relatively small amount of attackers (500 attackers). To perform a
larger simulation, Omnet++ simulator was selected.
5. OMNET SIMULATION
OMNet++ is a discrete event simulator [17]. Events taking place inside the simple modules that
are implemented directly by the user using C++ language. To describe the topology of the
network NED (Network Descriptioin) language is used which is translated into C++. Because
the network model is obtained entirely written in C++ and compiled into a binary executable
file, it provides high-speed simulation even on large network models.
Figure 4. Example of the simulated network
Figure 4 shows an example of large network which consists of autonomous systems (AS). Each
AS has a random internal structure (see fig. 5). Designation of autonomous systems looks like
asij. This means that the AS is at a distance i from the Victim and this is the jth
AS at this
distance.
Figure 5. Internal structure of the AS

81
The following parameters were chosen during simulations:
– number of routers – 1500;
– number of legitimate users – 1000;
– number of attackers – 25000.
Table 2 shows the simulation results of the combined method using Omnet++. It is seen that for
large-scale networks in which the number of connections between nodes is random and often
more than two, the probability of false detection is much higher than was calculated for
configurations for which the probability of collisions of hash functions is high (options 1-4).
Table 2. Configurations and method simulation results using Omnet++ (M = 17 bits)
№
Number of
hash functions
n, bit S
Pfalse
Calculated Simulation
1 2 1 16 0,25 0.81
2 2 2 8 0,0625 0.12
3 4 1 15 0,0625 0.13
4 8 1 14 3,91*10-3
0.02
5 8 2 7 1,53*10-5
2.7*10-3
6 16 1 13 1,53*10-5
0
7 32 1 12 2,33*10-10
-
Configurations with a higher number of used hash functions have shown good coincidence
between the results obtained by calculation and simulation (options 5-6). For configurations of
the method for which the calculated probability of false detection Pfalse was less than 10-9 the
simulation was not performed.
Also, using this program dynamic characteristics of the combined method were investigated.
Figure 6. Number of requests per second that the victim receives during the attack
For each method configuration 20 experiments were performed. Figure 6 shows that traffic from
the attackers (black line) much more than traffic from legitimate users (gray line). Figure 6 and
7 were obtained during testing option 5 from Table 2. Figure 7 shows that during the attack
victim can not handle all the requests coming from legitimate users, so some of them are
dropped. Once the sources of the attack tracked and blocked, the victim able to handle all
requests from legitimate users again.

82
Figure 7. Number of requests per second that the victim could process during the attack
6. CONCLUSIONS
The results of the simulations revealed the following:
1. The combined method allows to trace and to block the sources of DDoS-attack in large-scale
networks. During testing, there was no case where the real source of the attack was not
detected. The exceptions were cases where the source of the attack was at a distance farther
than the combined method allows you to track. This should be considered when choosing a
method's parameters.
2. Were found the most appropriate configuration parameters for the method to ensure the
desired range of attack detection and satisfy the requirements of probability of false
detection.
3. Analytical conclusions were confirmed for the probability of false detection of attack sources
on networks. Also noted that for real networks in which the number of connections between
nodes is random, the probability of false detection is slightly higher and depends on the
configuration of the network.
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84
Authors
Viktor Yakovlev received the MSc degree in
Electrical Engineering from the Military
Communication College, Kiev, Soviet Union, in
1972, the PhD degree and the Doctor in Philosophy
degree from the Military Communication Academy
in 1981 and 1994 respectively. He has been
professor at the Military Communication
Academy, and at the State University of
Telecommunications, both in St. Petersburg,
Russia. His main research interests are in coding
theory, cryptography and information security
Ilya Levin received the MSc degree in Electrical
Engineering from the State University of
Telecommunications, Saint-Petersburg, Russia, in
2007. Currently he is writing a dissertation at the
same university on theme of protection from
DDoS-attacks. His main research interests are in
information security, DDoS-attacks protection
techniques.

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