IP Traceback
Information Security Technical Update
istributed Denial of Service attack is
one of the most menacing security
threats on the Internet. In order to put
down these attacks, the real source of
the attack should be identified. IP traceback is the
function to trace the IP packets within the Internet
traffic.
D
The Internet Protocol

IP (Internet Protocol) is the primary protocol of the
Internet communication standards. It delivers
packet from the source host to the destination
device based on the information carried in the
packet header.
The IP packet is composed of the header which
carries the IP address, the destination IP address
and other meta-data required to route and deliver
the packet. Even if the source IP address is stored
in the header, address spoofing is possible by
exploiting security loopholes.
IHL
Type of
Service
Total Length
Fragment
Offset
Time To Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options
Padding
Identification
Flags
Vulnerabilities of the protocol
The TCP/IP protocol has been designed to send
data reliably but it does not secure the process1. In
fact, the authenticity of the source address carried
in IP packets is never checked by the network
routing infrastructure. Thus, a motivated attacker
can easily trigger a Denial of Service (DoS) attack.
These kinds of attacks mainly rely on forged IP
addresses or source address spoofing.

Structure of an IP packet
Version

Source address spoofing
In an IP spoofing attack, an intruder uses a forged
source IP address and establishes a one-way connection in order to execute malicious code at the
remote host2. This technique is usually used for
DoS attacks especially SYN flood attacks. The latter
is a form of DoS in which the attacker sends a
succession of SYN requests to a target’s system in
an attempt to consume enough server resources to
make the system unresponsive to legitimate traffic3.
Every connection in TCP/IP starts with a threeway-handshake. The client sends a synchronization
signal SYN to the server which acknowledges it by
sending back a SYN-ACK, and waits for the client to
send an ACK signal.
In case of an attack, the SYN-ACK is sent to a
spoofed IP address, therefore, the ACK message
never arrives and the server resources will be
blocked, degrading the service for legitimate users.
Figure 1: Structure of an IP packet header
Page 1
Scappy4
Forging a false IP address is
easy especially with the
python script “Scapy”.
Scapy is a powerful interactive packet manipulation
program. It is able to forge
or decode packets of a wide
number of protocols, send
them on the wire, capture
them, match requests and
replies.
The intended receiver uses
Wireshark to analyse the
receiving packets and verify
the information of the
forged packet.
The TCP/IP protocol does
not check the source address. Thus, the address
source that appeared on
Wireshark is not the true
source.
ble, but the identification of the source of
attack and prevention of any recurrences
are also crucial to a good practice of cyber
security.
Figure 2: SYN flood attack3
 (Distributed) Denial of Service
attacks
DoS disables network services for legitimate users. An attack starts when computers are infected with malware and
turned into botnets. These machines
become the compromised hosts. There are
two kinds of compromised hosts:
The Zombies: They launch the SYN flooding attack through uncompromised machines that communicate with the victim’s
machine
through
the
three-wayhandshake protocol. These uncompromised machines are called “reflectors”.
The Stepping Stones: They act as intermediate nodes between the attacker and
the zombies to make it harder to discover
the attacker.
IP Traceback
DDoS attack is a growing concern as it
targets a broad range of industries, from
e-commerce to financial institutions, it can
lead to a significant loss of money because
of unavailability of service. Preventive
measures against these attacks are availa-
One of the ways to achieve IP traceback is
hop-by-hop link testing. When an attack is
launched, the network administrator will
log into the closest router to the victim
and analyse the packet flow to determine
the origin of the malicious packets. This
will localise the next upstream router. The
major drawback of this simple method is
that it requires a strong interoperability
between routers, and the attack must still
be in progress while the tracing of malicious packet takes place.
Comparison of Existing IP
Traceback Techniques
IP traceback techniques can be classified
as pro-active or reactive.
A pro-active approach locates the source
after the attack by looking at the records
files and logs of the network.
A reactive approach locates the attacker
on the flight when the attack is detected
by a specialised hardware.
The comparison of traceback techniques
will focus on three illustrative methods
which belong to different classes of IP
traceback techniques. These techniques
remain at the stage of research and are
not yet released in the market. The Source
Path Isolation Engine (or hash-based)
algorithm is an in-band pro-active tech-
Page 2
DDoS attack:
Expensive
Damages
Denial of Service attack is
one of the three most
expensive cyber-attacks.
Along with malicious
insider attacks and webbased attacks, they account
for 55% of all cybercrime
costs per year.
The estimated cost damage
is $40,000 per hour and
49% of DDoS attacks last
for 6 to 24 hours according
to Incapsula poll6. Thus the
average DDoS costs is
$500,000 with some exceptionally expensive cases.
However, damages are not
only financial: loss of
customer trust, virus/malware infection and
loss of intellectual property
are other consequences of
DDoS attack.
niques. The iCaddie ICMP is the evolution
of the ICMP out-of-band traceback technique. The third one is the reactive IDIP
mechanism.
 Source
(SPIE)
Path
Isolation
Engine
SPIE, or Hashed-based IP traceback is
used to trace the origin of a single packet.
This system was proposed by Snoeren et
al5. It is a packet logging technique which
means that it involves storing packet
digests at some crucial routers. The main
issue is that the storage of saved packet
data requires a lot of memory. SPIE is of
high storage efficiency and thus reduces
the memory requirement (0.5% of the link
capacity per unit time in storage). In fact,
instead of storing the packets, it uses
auditing techniques. It computes and
stores 32-bit packet digest. Moreover, an
efficient data structure to store packet
digest is mandatory. SPIE uses Bloom
filter structure.
Another important issue of packet logs is
the risk of eavesdropping. Storing only
packet digests and not the entire packet
prevents SPIE from being misused by
attackers. Therefore, the network is protected from eavesdropping which is one of
the criteria of an effective IP traceback
system.
using them increases as the size of the
packet flow decreases. Especially, the
second one becomes impossible because
small flows have no detectable impacts on
the network. Thus, an audit option is used
in SPIE.
SPIE is also called hash-based IP traceback
because a hash of the invariant fields in
the IP header is stored in each router as a
32-bit digest. It remains stored only for a
limited duration of time because of space
constraint.
Packet digests are created by Data Generation Agent (DGA) at each router. Before a
traceback begins, an attack packet must be
detected. To determine it, an intrusion
detection system (IDS) is used. IDS provides a packet, the last hop router, the
time of attack, to the SPIE Traceback
Manager (STM) which will verify its authenticity and integrity. Upon successful
verification, STM will send the signature
information to the SPIE Collection and
Reducing Agent (SCAR) responsible for
the victim’s network area. If any match is
found, the SCAR returns a partial attack
graph of the involving routers. Then STM
will then send new queries to another
SCAR region. This process continues until
the attack path is constructed. Finally, the
STM sends the result back to the IDS.
There are two options to determine the
route of a packet flow. The first one is to
audit the flow while it passes through the
network and the second is to attempt to
infer the route based on its impact on the
state of the network. The difficulty of
Figure 3: SPIE architecture
Page 3
 iCaddie ICMP
In out-of-band pro-active schemes, the tracing
mechanism is conducted with the help of separate
packets generated at the routers when the malicious packet traverses through them.
The most used technique is the ICMP Traceback
(iTrace). As a packet traverses through the network, an ICMP (Internet Control Message Protocol)7 packet is generated by the router every
20,000 packets that pass through it. ICMP messages are contained within standard IP packets, thus
the header structure is the same as the IP one. The
iTrace payload contains useful information about
the router visited by IP packets such as router’s ID,
information about the adjacent routers, timestamp,
MAC address pair of the link traverse, etc. Thus, the
victim is able to infer the true source of the IP
packet from the information available. As most of
the DoS attacks are flooding attacks, a sufficient
amount of trace packets is likely to be generated.
This technique does not require any modification
of the existing infrastructure. However, it consumes considerable bandwidth and requires a
large number of packets to traceback an attacker. It
also has a poor handling of DDoS. In recent years,
there has been an improvement in tackling the
issues of the original scheme8.
One of the variants of the classic ICMP traceback is
iCaddie9. It has been designed by taking into account various properties:
i.
ii.
iii.
The cost and time required for upgrading
network equipment
The extra traffic load due to out-of-band IP
traceback techniques
The computational overhead of the attack
path construction process
iv.
The storage capacity to collect and store information for path construction
Like iTrace, the Caddie message is an extra ICMP
message generated by a router or an application,
called a Caddie initiator. Attached to it is the entire
packet history of one randomly selected packet,
called a Ball packet, which is forwarded by the
router. In fact, while a router is forwarding packets, it randomly selects one of the packets as a ball
packet. The Caddie message will collect the path
information about the sequence of the routers
(called Caddie propagators) identity along the way
toward the ball packet’s destination.
Figure 4: The scheme of iCaddie
However, while iTrace generates ICMP traceback
message every 20,000 packets, iCaddie works
differently. In order to reduce the number of traceback messages produced, each router maintains a
timer that indicates how long it has not received a
traceback message. If the amount exceeds a specified threshold the router will start to act as Caddie
initiator.
Then, as routers act as Caddie propagators, they
append their IP address to the Router List (RL)
along with the incoming interface and next hop
information. This list is encrypted with a MessagePage 4
Authentication Hash function (HMAC) in order to
prevent the RL elements from being modified. This
function runs with the Time-Released Key Chain
(TRKC)10. To generate a sequence of secret keys,
each router successively applies the HMAC function to a randomly selected seed, and then each
router reveals the key after a delay at the end of
each time interval. The destination of a Caddie
message can retrieve the newest key, and then
compute all the secret keys for previous time
intervals to finally compute and verify the HMACs
for every RL element in the Caddie message.
The benefit of this approach is that the number of
trace packets produced is fewer. It is independent
of the attack path and is solely dependent on the
number of attack sources. The scheme produces
fewer attack sources and false positives as the
chances of two packets digest forwarded within a
short gap of time is much smaller. More generally,
the ICMP traceback scheme is really interesting as
it can handle DDoS very well with fast recognition
and requires low interoperability between ISPs as
Caddie propagators transmit the Caddie packet like
any other packet. However, it still requires more
bandwidth than an in-band technique and the
deployment cost is non-negligible.

Reactive IDIP
Intrusion Detection System (IDS) can also be used
by reactive attack defence methods. It will alert the
system in case of attack and this one will respond
with a traceback. The defence can be handled by
the network or by the host11. For instance, the
Intrusion Detection and Identification Protocol
(IDIP) is handled by the network. Therefore, it uses
less resources.
IDIP is used to trace the real-time path and source
of intrusion12. IDIP systems are separated in IDIP
communities. Each community contains its own
system of intrusion detection and the response is
managed by the Discovery Coordinator. Each
community is divided in neighbourhoods in such a
way that only one IDIP component belongs to each
neighbourhood. This architecture allows the collection of intrusion-related information at a central
site which enables the exchange of intrusion reports to have a better understanding of the situation. In each neighbourhood, a local IDS agent
watches and sends its report to a boundary controller.
Figure 5: IDIP Community
When an attack occurs, the detector node sends an
attack report to its neighbours, which will help
trace the attack path and also send the attack
report along the attack path. But before sending it,
they will decide how to respond to the attack
(disabling the user account, installing filtering
rules, etc.), depending on the type of attack and the
response of other IDIP nodes. Then all the attack
reports are sent back to the Discovery Coordinator.
This allows it to have an overview of the situation
and modify local node decision if necessary. This
technique stops the diffusion of the attack and at
the same time rebuild the attack path.
Page 5
The efficiency of IDIP is linked to the effectiveness
of intrusion identification at different boundary
controllers. Each controller needs to have the same
intrusion detection capability as the IDS. The
automated response allows the system to react
quickly.
One of the main advantages of this technique is its
minimal dependence on the system infrastructure.
This method can trace the connection that spoofed
the source addresses. In fact, the IDIP protocol is
based on what the components have recorded
rather than network routing tables. The drawbacks
are that it requires high ISP cooperation especially
with the controller boundary and that it depends
on the reliability of the router. IDIP can successfully trace back the source unless it encounters stepping stones – a sequence of intermediate hosts that
help attacker remains anonymous.

Number of packets for tracing
Incremental
deployment
DDoS handling
Scalability
Time required for
traceback
Number of false
positives
Memory requirements
Misuse by the
attacker
Tracking until
Computational
overhead
Bandwidth usage
SPIE
iCaddie
IDIP
Many
Many
Few
Yes
Yes
Yes
No
Bad
A few
minutes
Yes
High
Yes
High
In realtime
Quick
Few
Very few
Very few
High
High
Low
No
No
No
Source
Source
Stepping
stones
High
Low
Low
Low
High
Low
Table 1 - Comparative tables of three IP traceback techniques
Comparative study
Three main approaches have been studied:
The pro-active in-band technique: The traceback
information is carried within the packet header.
Logging scheme like SPIE, can only trace packets
that have been delivered in the recent past as the
packet digests are made to expire after a certain
period of time.
The pro-active out-of-band technique: The trace
information is sent within a separate packet. This
technique requires more network bandwidth in
delivering the trace information.
The reactive IDS assisted approach: It requires a
significant amount of cooperation between ISP to
perform the traceback. The reliability of this
scheme is only up to the extent to which a router is
secured to an attacker.
Conclusion
The objective of IP traceback is to determine the
true source of DoS/DDoS attacks. IP traceback and
attack detection form an efficient collaborative
defence against DoS attacks across the Internet.
Depending on the company’s resources, there also
exist various pro-active techniques that all have
evolved from basic scheme, such as IP marking, IP
logging, ICMP-based traceback, overcoming the
shortcomings that the researchers had focused on.
Some are more prone to one aspect of the network
attack than other. Hence network administrators
should take into consideration their business
requirement and objective to implement the best
suited approach. However, it has been done at the
lab scale but hasn't yet moved out into the field.
Page 6
References
1. “Requirements for Internet Hosts – Communication Layers” R. Braden October 1989 pdf
2. “TCP/IP Vulnerabilities: IP spoofing and Denial-of-Service Attacks” A. Kak 25 April 2015 pdf
3. “SYN flood” 7 July 2015 Web. 23 July 2015
4. “Scapy” December 2014 Web. 24 July 2015
5. “Hash-Based IP Traceback” A. C. Snoeren et al. 2001 pdf.
6. “Incapsula survey – DDoS Impact Survey”T. Matthews 2014 pdf.
7. “Internet Control Message Protocol” June 2015 Web. 23 July 2015
8. “Comparative study of IP Traceback Techniques” M.Lapeyre 2015 pdf.
9. “A DoS-Resistant IP Traceback Approach” Bao-Tung Wang, 2003 pdf.
10. “Advanced and Authenticated marking Schemes for IP traceback” D.X. Song and A. Perrig 2001 pdf.
11. “Taxonomy of IP Traceback” L. Santhanam et al. 2006 pdf.
12. “Infrastructure for Intrusion Detection and Response” D. Schnackenberg et al.
Copyright Statement
All material in this document is, unless otherwise stated, the property of the Joint Universities Computer Centre (“JUCC”). Copyright
and other intellectual property laws protect these materials. Reproduction or retransmission of the materials, in whole or in part, in
any manner, without the prior written consent of the copyright holder, is a violation of copyright law.
A single copy of the materials available through this document may be made, solely for personal, non-commercial use. Individuals
must preserve any copyright or other notices contained in or associated with them. Users may not distribute such copies to others,
whether or not in electronic form, whether or not for a charge or other consideration, without prior written consent of the copyright
holder of the materials. Contact information for requests for permission to reproduce or distribute materials available through this
document are listed below:
copyright@jucc.edu.hk
Joint Universities Computer Centre Limited (JUCC)
c/o Information Technology Services
The University of Hong Kong
Pokfulam Road, Hong Kong
Page 7