Journal of Information Security, 2011, 2, 131-138
doi:10.4236/jis.2011.23013 Published Online July 2011 (
Copyright © 2011 SciRes. JIS
Effectiveness of Built-in Security Protection of
Microsoft’s Windows Server 2003 against TCP SYN
Based DDoS Attacks
Hari Krishna Vellalacheruvu, Sanjeev Kumar
Networking Security Research Lab, Department of Electrical and Computer Engineering,
The University of Texas-Pan American, Edinburg, USA
Received December 15, 2010; revised June 15, 2011; accept e d July 11, 2011
Recent DDoS attacks against several web sites operated by SONY Playstation caused wide spread outage for
several days, and loss of user account information. DDoS attacks by WikiLeaks supporters against VISA,
MasterCard, and Paypal servers made headline news globally. These DDoS attack floods are known to crash,
or reduce the performance of web based applications, and reduce the number of legitimate client connec-
tions/sec. TCP SYN flood is one of the most common DDoS attack, and latest operating systems have some
form of protection against this attack to prevent the attack in reducing the performance of web applications,
and user connections. In this paper, we evaluated the performance of the TCP-SYN attack protection pro-
vided in Microsoft’s windows server 2003. It is found that the SYN attack protection provided by the server
is effective in preventing attacks only at lower loads of SYN attack traffic, however this built-in protection is
found to be not effective against high intensity of SYN attack traffic. Measurement results in this paper can
help network operators understand the effectiveness of built-in protection mechanism that exists in millions
of Windows server 2003 against one of the most popular DDoS attacks, namely the TCP SYN attack, and
help enhance security of their network by additional means.
Keywords: Network Security, TCP SYN Based DDoS Attack, Prevention of Attacks
1. Introduction
When TCP/IP protocol suite was initially develop ed as a
part of network research development by the United
States Advanced Research Projects Agency (DARPA or
ARPA) in 1970s [1], they were unaware of the security
attacks. At that time the protocol suite designs were ba-
sically concerned with appropriate communication and
the scalability of the network. There was no proper fr ame-
work to defend against security attacks in the initial de-
sign of protocol suite. As time progressed TCP/IP gained
more popularity than any other architecture. There has al-
ways been some hacker community who have been trying
to exploit security breaches of popular TCP/IP architecture.
When ever the ha ckers expl oited the security breaches,
the TCP/IP developer community tried to fix it by mak-
ing some changes to the TCP/IP protocol suite. TCP/IP
stack is still evolving to defend against security attacks.
For example, recently Microsoft released a critical patch
to TCP/IP on 8th September 2009 [2]. This patch corre-
sponds to the zero window size of the TCP packet after
the three-way handshake is co mplete and also time stamp
code execution.
TCP implementation may permit the LISTEN state to
be entered with either all, some, or none of the pair of IP
addresses and port numbers specified by the applications.
A link can become established with any user whose de-
tails are unidentified to the server ahead of time. This
type of unbounded LISTEN is the target of SYN flood-
ing attacks due to th e way it is typically implemented by
operating systems [3].
2. Three-Way Handshake
TCP uses three-way handshake (Figure 1) to establish a
connection between any two nodes. The client sends a
SYN request with its sequence number to the server.
When a SYN is received by server for a local TCP port
Figure 1. TCP three-way handshake.
where the connection is in the LISTEN state, then the
state transitions to SYN-RECEIVED. The Transmission
control block (TCB, a data structure to store all the state
information for an individual connection) is initialized
with information from the header fields of the received
SYN segment. In second step the server responds with an
ACK to received SYN and it will also sends its own se-
quence number (SYN) to the client. In the last step, the
client responds with final ACK packet. After the last
ACK is received by the server, connection state changes
from SYN_RECEIVED to ESTABLISH state. The real
data transfer between the client and the server is initiated
after the three-way handshake is co mplete.
3. TCP SYN Flood Attack
Over Internet today, it is common for users to access data
by using application services of a remote machine. Most
of these applications like HTTP, FTP and e-mail run on
top of TCP layer. The accessibility and performance of
application services depend on how well the underlying
Transport protocol works. By some means, if the TCP
layer is made unresponsive, the person who is trying to
access these services from a remote machine may think
that the services are busy/unavailable. In recent years
increase in online shopping and online financial transac-
tions make unavailability of the web services, simply
In this attack, the attacker makes the server’s TCP
layer unresponsive by sending a large number of open
connection requests or TCP SYN packets (Figure 2).
This is known as SYN flooding or SYN Bombing,
named after specific bit in TCP header specifications.
The TCP SYN flooding weakness was discovered as
early as 1994 by Bill Cheswick and Steven Bellovin [3].
The SYN flooding attack was first publicized in 1996,
with the release of a description and exploit tool in
Phrack Magazine. By September of 1996, SYN flooding
Figure 2. TCP SYN flood attack.
attacks has been observed more frequently on the inter-
net around the world. SYN flooding was particularly
serious in comparison to other known denial of service
attacks at that time and even now. The community
quickly developed different techniques for preventing or
limiting the impact of SYN flooding attacks. Some of
these techniques like SYN Cache protection and SYN
Cookies protection have become important pieces of the
TCP implementations in certain operating systems, al-
though some significantly diverge from the TCP specifi-
cation and none of these techniques have yet been stan-
dardized or sanctioned by the IETF process. SYN
Chache is one of the most commonly used SYN flooding
prevention methods, and variants of this method is im-
plemented in m any popular com put er ope rating systems.
Suppose that an attacker directs a large number of
SYN requests rapidly to the server with spoofed source
IP addresses. In a traditional TCP 3-way hand shake, th e
server has to create a new TCB for each new connection
request it received and save the incomplete state of the
connection and the TCP options like window size,
Maximum segment size etc. Since the TCB’s are limited
for each port of the server, the TCB’s get filled up. In
traditional TCP, the server will send several retransmis-
sions for incomplete connections before the timeout pe-
riod and eventually get deleted. Even though TCB’s are
going to be unallocated after certain timeout period, if
the attacker manages to keep flooding the server so that
no TCB’s are free at any given point of time, the TCP
layer becomes unresponsive to the legitimat e clients.
One typical data structure used for communication is
the Transmission Control Block (TCB) which is created
and maintained during the lifetime of a given connection.
The TCB contains the following information according
to RFC 675 [4] (field sizes are notional only and may
vary from one implementation to another):
16 bits: Local connection name
48 bits: Local socket
48 bits: Foreign socket
16 bits: Receive window size in octets
32 bits: Receive left window edge (next sequence num-
Copyright © 2011 SciRes. JIS
ber expected)
16 bits: Receive packet buffer size of TCB (may be
less than window)
16 bits: Send window size in octets
32 bits: Send left window edge (earliest unacknow-
ledged octet)
32 bits: Next packet sequence number
16 bits: Send packet buffer size of TCB (may be less
than window)
8 bits: Connection state
The typical TCB size is sum of all fields which is 280
bits. For each connection standard transport layer allo-
cates one TCB. So the total number of connections that
can be supported by the server depends on the number of
TCB’s available in the server. A TCP synchronize (SYN)
attack is a denial-of-service attack that exploits the re-
transmission and time-out behavior of the Synchronize-
Acknowledgement (SYN-ACK) segment during the TCP
three-way handshake to create a large number of half-
open TCP connections. Depending on the TCP/IP proto-
col implementation, a large number of half-open TCP
connections could do any of the following [5]:
Use all available memory.
Use all possible entries in the TCP Transmission Con-
trol Block (TCB), an internal table used to track TCP
connections. Once the half-open connections use all
the entries, further connection attempts are responded
with a TCP connection reset.
Use all available half-open connections. Once all the
half-open connections are used, further connection at-
tempts are responded with a TCP connection reset.
4. SYN Attack Protection Performance
We measured the performance of SYN attack protection
in the real time traffic circumstances by sending the le-
gitimate client connections and SYN flood attack to the
web server at the same time. The legitimate/authentic
clients complete the there-way h andshake with the server
and then send HTTP request for a web page to the server
(Figure 3). After receiving the web page the clients close
the connection with server in traditional TCP way of
terminating the connection by exchanging FIN packets.
On the other hand the attacker’s side is made to send a
flood of TCP connection requests with spo ofed source IP
addresses to the web server with no intention to complete
the three-way hand shake with the server. The attackers
IP source address are fully randomized to overcome any
sort of filtering done on the server side.
We measured the number of legitimate client connec-
tions that can be established per second with the server
under increasing attack loads. The attack load is incre-
mented from low to high intensity in nonlinear fashion
Figure 3. Experimental setup.
from 0 Mbps to 100 Mbps in all of the following ex-
perimental results to find the connection rate behavior at
lower and higher intensity of attack traffic. The duration
of each attack load is kept for 10 minutes (600 seconds)
and the statistical readings are collected for each second.
i.e. 600 reading for each attack load.
The server CPU utilization and Memory status of the
server under different loads of SYN attack are shown in
Figures 4 and 5. The powerful quad core CPU utilization
of the server is increasing linearly as the attack load in-
creases (nonlinear) when there is no protection. The
maximum CPU utilization 41% is reached at 100 Mbps
of SYN attack load. The memory consumption is just
387MB at 100Mbps attack load which is well below the
8 GB RAM installed in the server. From the graphs
(Figures 4 and 5) it is observed that the server CPU and
Memory are not consumed completely because of the
SYN Attack.
The total number of TCP connections in SYN_RE-
CEIVED state when the server is under SYN attack is
shown in the Figure 6. Connections in SYN_RECEIVED
state is also referred as half-open TCP connections
means incomplete TCP connections The maximum num-
ber of half open connections supported by the server at
any given instant depends on the backlog size. TCP
half-open connections are increasing linearly at lower
loads of SYN attack until 7 Mbps. After this point the
number half open connections are falling at high er attack
load. The average half open connections at each attack
load shown in fig 6 is an average of 200 reading. These
reading are manually logged with the help of NETSTAT
Netstat -n -p tcp|find/c “SYN_RECEIVED”
It is observed in Figure 6 that the total number of
half-open connections in server is unstable after 7 Mbps
of SYN attack load.
Figures 7 and 8 show the average successful legiti-
mate connections establish ed with the web server when it
is under attack, and no protection is enabled at the server.
The legitimate client connections are found to decrease
rapidly with increase in TCP-SYN attack load. Without
any attack (as shown with 0 Mbps in the graphs), the
legitimate clients connectio ns are measured to be around
Copyright © 2011 SciRes. JIS
Figure 4. Server CPU utilization (without SYN attack pro-
tection) under SYN attack.
Figure 5. Memory consumption (without SYN attack pro-
tection) under SYN attack.
Figure 6. Server TCP connections in SYN_RECEIVED Stat e
(without SYN attack protection) unde r SYN Attac k.
Figure 7. Successful legitimate client connections/sec vs. the
attack load without SYN attack protection.
Figure 8. Successful legitimate client connections/sec with-
out SYN Attack Protection.
20,000 connections per second (baseline value). After 60
Mbps of SYN attack load legitimate client connections/
sec with the server are almost depleted well below 100
connections/sec. It is ob served that aroun d 5000 Connec-
tions per second are successful when the SYN attack
load intensity is 10 Mbps.
Research community proposed different techniques to
detect [6-12], Trace back [13,14] and Defend [15-21]
against the TCP SYN flooding attacks. Most of the de-
tection mechanisms proposed depend on the abnormal
traffic flow statistics in the network /Internet and the pre-
vention mechanisms depend on filtering, traffic policing
and rate limiting. These mechanisms can be implemented
in Internet core, firewalls, routers or end systems. When
a SYN attack is detected, TCP/IP in Windows Server
2003 and Windows XP lowers the number of retransmis-
Copyright © 2011 SciRes. JIS
sions of the SYN-ACK segment and does not allocate
memory or table entry resources for the connection until
the TCP three-way handshake has been completed. Mi-
crosoft provided a feature called SYN Attack Protect in
the server operating system. This feature is available in
all versions of windows server 2003 but enabled by de-
fault only in some versions of windows server 2003 op-
erating systems. The Microsoft provided definition for
this protection as follows [22].
“SYN attack protection involves reducing the amount
of retransmissions for the SYN-ACK’s, which will re-
duce the time for which resources have to remain allo-
cated. The allocation of route cache entry resources is
delayed until a connection is made and the connection
indication to application is delayed until the three-way
hand shake is completed. ”
The action taken by the SY N attack protection mecha-
nism only occurs if TcpMaxHalfOpen and TcpMaxHal-
fOpenRetried settings are exceeded. The three configur-
able threshold parameters to trigger TCP’s SYN attack
flooding protection feature are explained below [23].
1) TcpMaxHalfOpen specifies how many connections
the server can maintain in the half-open state before
TCP/IP initiates SYN flooding attack protection, by de-
fault it is 500 in windows server 2003.
2) TcpMaxHalfOpenRetried specifies how many con-
nections the server can maintain even after a connection
request has been retransmitted before TCP/IP initiates
SYN flooding attack protection by default it is 400 in
windows server 2003.
3) TcpMaxPortsExhausted specifies how many con-
nection requests the server can refuse before TCP/IP
initiates SYN flooding attack protection by default it is
100 in windows server 2003.
All the three entries mentioned are used only when
SYN flooding protection is en abled on the server, that is,
when the value of the SynAttackProtect entry is 1 and
the value of the TcpMaxConnectResponseRetransmis-
sions entry is at least 2.
The behavior of TCP/IP protocol stack in the windows
server 2003 operating system heavily depends on the
registry parameters. We recognized the research efforts
made by Microsoft in deciding these registry key pa-
rameters for the stable re sponse of server and its ser vices.
So we kept most of these parameters in the default state
or in the state recommended by the Microsoft as men-
tioned above for the stab le response of the server.
The next step is to enable the SYN attack protection
feature in windows server 2003 and observe the server
behavior under SYN attack. In the remaining part of this
chapter we will observe the server ability to provide ser-
vices to legitimate clients when SYN attack protection is
enable and compare it with the results we had when the
SYN attack protection is not active. The SYN attack
protection thresholds mentioned earlier are in the default
state/value for all the experiments we conducted in this
The network topology created for this testing is same
as shown in Figure 3. The CPU and Memory usage of
the server under SYN attack when protection enabled is
shown in the Figures 9 and 10 respectively. The CPU
utilization is nearly the same with and without pro tection.
The memory consumed by server under SYN attack is
significantly reduced when the SYN attack protection is
active. Compared to the memory resources available in
the server and the cost of memory today, it is not sig-
The successful legitimate client connections rate vs.
attack load when the server SYN attack protection en-
abled is shown in the Figures 11 and 12. It is observed
that even with protection enabled the successful connec-
tion rate is decreased as the attack load increases. The
legitimate connections are unable to establish and the
connection rate is less than 100 connections/sec after 80
Mbps attack load. This is an improvement over the pre-
vious scenario where the connections/sec fell below 100
at 60 Mbps without SYN protection. It is observed from
Figure 12 that the successful connection rate at 10 Mbps
of attack load is around 16,000 connections/sec, which is
more than two times the successful connection rate we
achieved without the SYN flood attack protection. The
successful connection rate is improved significantly for a
given attack load but at higher attack load s after 60 Mbps,
the legitimate connections are unable to be established.
Comparison of the results of these two experiments
with and without TCP-SYN protection is shown in the
Figure 13. When the TCP-SYN attack protection is used,
the new client connection rate supported by the web
server was improved by 226% under TCP SYN attack
load of 10 Mbps.
From the results presented in this paper, it is evident
that the legitimate client connection rate is improved by
the use of SYN attack protection. However SYN attack
protection is not effective at higher loads of SYN attack.
But if we increase the number of half open connection
limit on the server the successful connection rate of cli-
ents may improve [24]. A high bound for the half open
connection limit can be computed from the bandwid th of
the server’s network and the timeout used by the servers
to discard pending requests. This is kind of brute force
solution that waste lots of k ernel memory and slow down
the server response time, but it can be effective in public
servers serving large communities of clients, since such
servers have extensive hardware resources. Even if you
increase the half open conne ction limit, it is possible that
at some higher load attack traffic the hash table fills up,
Copyright © 2011 SciRes. JIS
Figure 9. Server CPU utilization (with SYN attack protec-
tion) under SYN attack.
Figure 10. Memory consumption (without SYN attack pro-
tection) under SYN attack.
Figure 11. Successful legitimate client connections/sec vs.
attack load with SYN attack protection.
Figure 12. Successful legitimate client connections/sec with
SYN attack protect (bar view).
Figure 13. Comparison of successful client connections with
and without TCP-SYN attack protection of the windows
and it could Overflow with forged connection requests.
5. Conclusions
In this paper, we evaluated the host based protection
feature provided by Microsoft against TCP-SYN based
DDoS attacks for its widely deployed Windows 2003
servers. It is observed that the built-in, host-based pro-
tection feature of Windows server 2003 has limited ef-
fectiveness in protecting against TCP-SYN based DDoS
attacks. In the absence of any attack, Windows 2003
server was found to support around 20,000 client con-
nections/sec, whereas when the TCP-SYN based DDoS
attack traffic was increased to 50Mbps, only around 1700
client connections/sec could be established, which is a
Copyright © 2011 SciRes. JIS
reduction of over 90% of legitimate client connection
rate. The experimental measurements show that the
built-in protection provided by Microsoft for its Win-
dows server 2003 is effective only for low intensity of
the TCP-SYN based DDoS attacks, but not effective
against high intensity of the DDoS attacks (exceeding 50
Mbps), and many users are not aware of this fact. This
paper conveys an important message for the network
managers that they must not rely only on the host-based
protection mechanism that exists in the Microsoft’s
server 2003, and they should deploy additional security
devices to effectively defend against DDoS attacks.
6. Acknowledgements
This work was suppo rted in part by the funding from US
National Science Foundation , G rant No: 0521585.
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