Network Support for IP Traceback

Network Support for IP Traceback
226
IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 9, NO. 3, JUNE 2001
Network Support for IP Traceback
Stefan Savage, David Wetherall, Member, IEEE, Anna Karlin, and Tom Anderson
Abstract--This paper describes a technique for tracing anonymous packet flooding attacks in the Internet back toward their
source. This work is motivated by the increased frequency and
sophistication of denial-of-service attacks and by the difficulty in
tracing packets with incorrect, or "spoofed," source addresses. In
this paper, we describe a general purpose traceback mechanism
based on probabilistic packet marking in the network. Our approach allows a victim to identify the network path(s) traversed
by attack traffic without requiring interactive operational support
from Internet Service Providers (ISPs). Moreover, this traceback
can be performed "post mortem"--after an attack has completed.
We present an implementation of this technology that is incrementally deployable, (mostly) backward compatible, and can be efficiently implemented using conventional technology.
Index Terms--Computer network management, computer
network security, network servers, stochastic approximation,
wide-area networks.
I. INTRODUCTION
ENIAL-OF-SERVICE attacks consume the resources of
a remote host or network, thereby denying or degrading
service to legitimate users. Such attacks are among the hardest
security problems to address because they are simple to implement, difficult to prevent, and very difficult to trace. In the last
several years, Internet denial-of-service attacks have increased
in frequency, severity, and sophistication. Howard reports that
between the years of 1989 and 1995, the number of such attacks
reported to the Computer Emergency Response Team (CERT)
increased by 50% per year [26]. More recently, a 1999 CSFFBI
survey reports that 32% of respondents detected denial-of-service attacks directed against their sites [16]. Even more worrying, recent reports indicate that attackers have developed tools
to coordinate distributed attacks from many separate sites [14].
Unfortunately, mechanisms for dealing with denial-of-service have not advanced at the same pace. Most work in this area
has focused on tolerating attacks by mitigating their effects on
the victim [40], [2], [27], [30], [9]. This approach can provide an
effective stopgap measure, but does not eliminate the problem,
nor does it discourage attackers. The other option, and the focus
of this paper, is to trace attacks back toward their origin--ideally stopping an attacker at the source.
A perfect solution to this problem is complicated by the potential use of indirection to "launder" the true causal origin of an
D
Manuscript receivedJuly 17, 2000;revisedNovember14, 2000;approvedby
IEEE/ACMTRANSACTIONSONNETWORKINGEditor C. Diot.
S. Savage is with the Department of Computer Science and Engineering,
University of California at San Diego, La Jolla, CA 92093 USA (e-mail:
savage@cs.ucsd.edu).
D. Wetherall, A. Karlin, and T. Andersonare with the Department of Computer Science and Engineering,Universityof Washington, Seattle, WA 98195
USA.
Publisher Item Identifier S 1063-6692(01)04726-4.
attack. For example, an attack may consist of packets sent from
many different slave machines, themselves under the control of
a remote master machine. Such indirection may be achieved either explicitly (by compromising the individual slave hosts directly) or implicitly (by sending false requests to the slaves on
behalf of the victim--a so-called reflector). More challenging
still, the true origin and identity of the attacker can be similarly
concealed through chains of false computer accounts, call forwarding, and so forth. Consequently, we regard a complete solution-particularly one able to address the forensic needs of law
enforcement--as an open problem.
Instead, we address the more limited operational goal of
simply identifying the machines that directly generate attack
traffic and the network path this traffic subsequently follows.
We call this the traceback problem and it is motivated by the
operational need to control and contain attacks. In this setting,
even incomplete or approximate information is valuable because
the efficacy of measures such as packet filtering improve as they
are applied further from the victim and closer to the source.
However, even for our restricted problem, determining the
source generating attack traffic is surprisingly difficult due to the
stateless nature of Internet routing. Attackers routinely disguise
their location using incorrect, or "spoofed," IP source addresses.
As these packets traverse the Internet, their true origin is lost and
a victim is left with little useful information. While there are several ad hoc traceback techniques in use, they all have significant
drawbacks that limit their practical utility in the current Internet.
In this paper, we present a new approach to the traceback
problem that addresses the needs of both victims and network
operators. Our solution is to probabilistically mark packets with
partial path information as they arrive at routers. This approach
exploits the observation that attacks generally comprise large
numbers of packets. While each marked packet represents only
a "sample" of the path it has traversed, by combining a modest
number of such packets a victim can reconstruct the entire path.
This allows victims to locate the approximate source of attack
traffic without requiring the assistance of outside network operators. Moreover, this determination can be made even after an
attack has completed. Both facets of our solution represent substantial improvements over existing capabilities for dealing with
flooding-style denial-of-service attacks.
A key practical deployment issue with any modification of
Internet touters is to ensure that the mechanisms are efficiently
implementable, may be incrementally deployed, and are backward compatible with the existing infrastructure. We describe a
traceback algorithm that adds little or no overhead to the routers
critical forwarding path and may be incrementally deployed
to allow traceback within the subset of routers supporting our
scheme. Further, we demonstrate that we can encode the necessary path information in a way that peacefully coexists with
1063-6692/01510.00©2001IEEE
SAVAGEet al.: NETWORK SUPPORTFOR IP TRACEBACK
227
TABLE I
QUALITATIVE COMPARISON OFEXISTING SCHEMES FOR COMBATING ANONYMOUS ATTACKS ANDTHE PROBABILISTIC MARKING
APPROACH PROPOSED IN THIS PAPER
Ingress filtering
Link testing
Input debugging
Controlled flooding
Logging
ICMP Traceback
Marking
Management ] Network
overhead [ overhead
Moderate
Low
High
Low
High
Low
Low
Low
High
Low
Low
Low
existing routers, host systems, and more than 99% of today's
traffic.
The rest of this paper is organized as follows. In Section II,
we describe related work concerning IP spoofing and solutions
to the traceback problem. Section III outlines our basic approach and Section IV characterizes several abstract algorithms
for implementing it. In Section V, we detail a concrete encoding
strategy for our algorithm that can be implemented within the
current Internet environment. We also present experimental
results demonstrating the effectiveness of our solution. In
Section VI, we discuss the main limitations and weaknesses of
our proposal and potential extensions to address some of them.
Finally, we summarize our findings in Section VII.
II. RELATED W O R K
It has been long understood that the IP protocol permits
anonymous attacks. In his 1985 paper on TCPflP weaknesses,
Morris writes:
"The weakness in this scheme [the Internet Protocol]
is that the source host itself fills in the IP source host id,
and there is no provision in ... TCP/IP to discover the true
origin of a packet." [32]
In addition to denial-of-service attacks, IP spoofing can be
used in conjunction with other vulnerabilities to implement
anonymous one-way TCP channels and covert port scanning
[32], [3], [25], [46].
There have been several efforts to reduce the anonymity afforded by IP spoofing. Table I provides a subjective characterization of each of these approaches in terms of management
cost, additional network load, overhead on the router, the ability
to trace multiple simultaneous attacks, the ability trace attacks
after they have completed, and whether they are preventative or
reactive. We also characterize our proposed traceback scheme
according to the same criteria. In the remainder of this section,
we describe each previous approach in more detail.
A. Ingress Filtering
One way to address the problem of anonymous attacks is to
eliminate the ability to forge source addresses. One such approach, frequently called ingress filtering, is to configure routers
to block packets that arrive with illegitimate source addresses
[21]. This requires a router with sufficient power to examine the
source address of every packet and sufficient knowledge to distinguish between legitimate and illegitimate addresses. Conse-
Post-mortem Preventative/
capability
reactive
N/A
Preventative
Router
overhead
Moderate
Distributed
capability
High
Low
High
Low
Low
Good
Poor
Excellent
Good
N/A
N/A
Excellent
Excellent
Reactive
Reactive
Reactive
Reactive
Good
Excellent
Reactive
N/A
quently, ingress filtering is most feasible in customer networks
or at the border of Internet Service Providers (ISPs) where address ownership is relatively unambiguous and traffic load is
low. As traffic is aggregated from multiple ISPs into transit networks, there is no longer enough information to unambiguously
determine if a packet arriving on a particular interface has a
"legal" source address. Moreover, on many deployed router architectures the overhead of ingress filter becomes prohibitive on
high-speed links.
The principal problem with ingress filtering is that its effectiveness depends on widespread, if not universal, deployment.
Unfortunately, a significant fraction of ISPs, perhaps the
majority, do not implement this service--either because they
are uninformed or have been discouraged by the administrative
burden,' potential router overhead, and complications with
existing services that depend on source address spoofing (e.g.,
some versions of Mobile IP [34] and some hybrid satellite
communications architectures). A secondary problem is that
even if ingress filtering were universally deployed at the customer-to-ISP level, attackers could still forge addresses from
the hundreds or thousands of hosts within a valid customer
network [14].
It is clear that wider use of ingress filtering would dramatically improve the Internet's robustness to denial-of-service attacks. At the same time, it is prudent to assume that such a
system will never be fullproof--and therefore traceback technologies will continue to be important.
B. Link Testing
Most existing traceback techniques start from the router
closest to the victim and interactively test its upstream links
until they determine which one is used to carry the attackers
traffic. Ideally, this procedure is repeated recursively on the
upstream router until the source is reached. This technique
assumes that an attack remains active until the completion of a
trace and is therefore inappropriate for attacks that are detected
after the fact, attacks that occur intermittently, or attacks that
modulate their behavior in response to a traceback (it is prudent
to assume the attacker is fully informed). Below we describe
two varieties of link testing schemes, input debugging and
controlled flooding.
1Some modern routers ease the administrative burden of ingress filtering by
providing functionality to automatically check source addresses against the destination-based routing tables (e.g., IP verify unicast reverse-path on Cisco's
ITS). This approach is only valid if the route to and from the customer is symmetric-generally at the border of single-homed stub networks.
228
1) Input Debugging: Many routers include a feature called
input debugging, which allows an operator to filter particular
packets on some egress port and determine which ingress port
they arrived on. This capability is used to implement a trace as
follows. First, the victim must recognize that it is being attacked
and develop an attack signature that describes a common feature contained in all the attack packets. The victim communicates this signature to a network operator, frequently via telephone, who then installs a corresponding input debugging filter
on the victim's upstream egress port. This filter reveals the associated input port, and hence which upstream router originated
the traffic. The process is then repeated recursively on the upstream router, until the originating site is reached or the trace
leaves the ISP's border (and hence its administrative control
over the routers). In the latter case, the upstream ISP must be
contacted and the procedure repeats itself. While such tracing
is frequently performed manually, several ISPs have developed
tools to automatically trace attacks across their own networks.
One such system, called CenterTrack, provides an improvement
over hop-by-hop backtracking by dynamically rerouting all of
the victim's traffic to flow through a centralized tracking router
[43]. Once this reroute is complete, a network operator can then
use input debugging at the tracking router to investigate where
the attack enters the ISP network.
The most obvious problem with the input debugging
approach, even with automated tools, is its considerable
management overhead. Communicating and coordinating
with network operators at multiple ISPs requires the time,
attention, and commitment of both the victim and the remote
personnel--many of whom have no direct economic incentive
to provide aid. If the appropriate network operators are not
available, if they are unwilling to assist, or if they do not
have the appropriate technical skills and capabilities, then a
traceback may be slow or impossible to complete [22].
2) Controlled Flooding: Burch and Cheswick have developed a link-testing traceback technique that does not require
any support from network operators [6]. We call this technique
controlled flooding because it tests links by flooding them with
large bursts of traffic and observing how this perturbs traffic
from the attacker. Using a pregenerated "map" of Internet
topology, the victim coerces selected hosts along the upstream
route into iteratively flooding each incoming link on the router
closest to the victim. Since router buffers are shared, packets
traveling across the loaded link including any sent by the
attacker--have an increased probability of being dropped. By
observing changes in the rate of packets received from the
attacker, the victim can therefore infer which link they arrived
from. As with other link testing schemes, the basic procedure
is then applied recursively on the next upstream router until the
source is reached.
While the scheme is both ingenious and pragmatic, it has
several drawbacks and limitations. Most problematic among
these is that controlled flooding is itself a denial-of-service
attack---exploiting vulnerabilities in unsuspecting hosts to
achieve its ends. This drawback alone makes it unsuitable for
routine use. Also, controlled flooding requires the victim to
have a good topological map of large sections of the Internet
in addition to an associated list of "willing" flooding hosts. As
IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 9, NO. 3, JUNE 2001
Burch and Cheswick note, controlled flooding is also poorly
suited for tracing distributed denial-of-service attacks because
the link-testing mechanism is inherently noisy and it can be
difficult to discern the set of paths being exploited when multiple upstream links are contributing to the attack. Finally, like
all link-testing schemes, controlled flooding is only effective at
tracing an ongoing attack and cannot be used "post mortem."
C. Logging
An approach suggested in [38] and [43] is to log packets at
key routers and then use data mining techniques to determine
the path that the packets traversed. This scheme has the useful
property that it can trace an attack long after the attack has completed. However, it also has obvious drawbacks, including potentially enormous resource requirements (possibly addressed
by sampling) and a large scale interprovider database integration problem. We are unaware of any commercial organizations
using a fully operational traceback approach based on logging. 2
D. ICMP Traceback
Since the first writing of this paper, a new traceback proposal has emerged based on the use of explicit router-generated ICMP traceback messages [4]. The principle idea in this
scheme is for every router to sample, with low probability (e.g.,
1/20 000), one of the packets it is forwarding and copy the contents into a special ICMP Traceback message including information about the adjacent routers along the path to the destination. During a flooding-style attack, the victim host can then use
these messages to reconstruct a path back to the attacker. This
scheme has many benefits compared to previous work and is
in many ways similar to the packet marking approach we have
taken. However, there are several disadvantages in the current
design that complicate its use. Among these: ICMP traffic is
increasingly differentiated and may itself be filtered in a network under attack; the ICMP Traceback message relies on an
input debugging capability (i.e., the ability to associate a packet
with the input port and/or MAC address on which it arrived)
that is not available in some router architectures; if only some of
the routers participate it seems difficult to positively "connect"
traceback messages from participating routers separated by a
nonparticipating router; and finally, it requires a key distribution infrastructure to deal with the problem of attackers sending
false ICMP Traceback messages. That said, we believe that the
scheme is promising and that hybrid approaches combining it
with some of the algorithms we propose are likely to be quite
effective.
III. OVERVIEW
Burch and Cheswick mention the possibility of tracing
flooding attacks by "marking" packets, either probabilistically
or deterministically, with the addresses of the touters they
traverse [6]. The victim uses the information in the marked
packets to trace an attack back to its source. This approach
has not been previously explored in any depth, but has many
2Historically, the T3-NFSNETdid log network-to-networktraffic statistics
and these were used on at least one occasion to trace IP spoofingattacks to an
upstream provider [45].
SAVAGEet aL: NETWORK SUPPORT FOR IP TRACEBACK
A~
\
R5
A2
A3
ii
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**~a6
R7
R4'
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fat
V
Fig. 1. Network as seen from the victim of an attack, V. Routers are
represented by R~, and potential attackers by A~. The dotted line represents a
particular attack path between an attacker and the victim.
potential advantages. It does not require interactive cooperation
with ISPs and therefore avoids the high management overhead
of input debugging. Unlike controlled flooding, it does not
require significant additional network traffic and can potentially be used to track multiple attacks. Moreover, like logging,
packet marking can be used to trace attacks "post mortem" long after the attack has stopped. Finally, we have found that
marking algorithms can be implemented without incurring any
significant overhead on network routers. The remainder of
this paper focuses on fully exploring and characterizing this
approach.
A. Definitions
Fig. 1 depicts the network as seen from a victim V. For the
purposes of this paper, V may be a single host under attack, or a
network border device such as a firewall or intrusion detection
system that represents many such hosts. Every potential attack
origin Ai is a leaf in a tree rooted at V and every router Ri is an
internal node along a path between some Ai and V. The attack
path from Ai is the unique ordered list of routers between Ai
and V. For instance, if an attack originates from A2, then to
reach V it must first traverse the path R6, R3, R2, and R l - - a s
shown by the dotted line in Fig. 1.
The exact traceback problem is to determine the attack path
and the associated attack origin for each attacker. However,
solving this problem is complicated by several practical limitations. The exact attack origin may never be revealed (even
MAC source addresses may be spoofed) and a wily attacker
may send false signals to "invent" additional routers in the
traceback path. We address these issues in Section VI, but
for now we restrict our discussion to solving a more limited
problem. We define the approximate traceback problem as
finding a candidate attack path for each attacker that contains
the true attack path as a suffix. We call this the valid suffix of
the candidate path. For example, (-/Ys, R6, R3, R2, R1) is a
valid approximate solution to Fig. 1 because it contains the
true attack path as a suffix. We say a solution to this problem is
robust if an attacker cannot prevent the victim from discovering
candidate paths containing the valid suffix.
229
All marking algorithms have two components: a marking procedure executed by routers in the network and apath reconstruction procedure implemented by the victim. A router "marks"
one or more packets by augmenting them with additional information about the path they are traveling. The victim attempts to
reconstruct the attack path using only the information in these
marked packets. The convergence time of an algorithm is the
number of packets that the victim must observe to reconstruct
the attack path.
B. Basic Assumptions
The design space of possible marking algorithms is large, and
to place our work in context we identify the assumptions that
motivate and constrain our design.
• An attacker may generate any packet.
• Multiple attackers may conspire.
• Attackers may be aware they are being traced.
• Packets may be lost or reordered.
• Attackers send numerous packets.
• The route between attacker and victim is fairly stable.
• Routers are both CPU and memory limited.
• Routers are not widely compromised.
The first four assumptions represent conservative assessments of the abilities of the modern attackers and limitations
of the network. Designing a traceback system for the Internet
environment is extremely challenging because there is very
little that can be trusted. In particular, the attackers ability
to create arbitrary packets significantly constrains potential
solutions. When a router receives a packet, it has no way to tell
whether that packet has been marked by an upstream router
or if the attacker simply has forged this information. In fact,
the only invariant that we can depend on is that a packet from
the attacker must traverse all of the routers between it and the
victim.
The remaining assumptions reflect the basis for our design
and deserve additional discussion. First, denial-of-service attacks are only effective so tong as they occupy the resources of
the victim. Consequently, most attacks are comprised of thousands or millions of packets. Our approach relies on this property because we mark each packet with only a small piece of
path state and the victim must observe many such packets to
reconstruct the complete path back the the attacker. If many attacks emerge that require only a single packet to disable a host
(e.g., ping-of-death [ 11]), then this assumption may not hold (although we note that even these attacks require multiple packets
to keep a machine down).
Second, measurement evidence suggests that while Internet
routes do change, it is extremely rare for packets to follow many
different paths over the short timescales of a traceback operation
(seconds in our system) [33]. This assumption greatly simplifies
the role of the victim, since it can therefore limit its consideration to a single primary path for each attacker. If the Internet
evolves to allow significant degrees of multipath routing, then
this assumption may not hold.
Third, while there have been considerable improvements
in router implementation technology, link speeds have also
increased dramatically. Consequently, we assert that any viable
230
IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 9, NO. 3, JUNE 2001
Marking procedure at router R:
for each packet w, append R to w
Path reconstruction procedure at victim v:
for any packet w from attacker
extract path (Ri..Rj) from the suffix of w
Fig. 2.
Node append algorithm.
implementation must have low per-packet overhead and must
not require per-flow state. Significantly simpler schemes than
ours can be implemented if we assume that routers are not
resource constrained.
Finally, since a compromised router can effectively eliminate
any information provided by upstream routers, it is effectively
indistinguishable from an attacker. In such circumstances, the
security violation at the router must be addressed first, before
any further traceback is attempted. In normal circumstances, we
believe this is an acceptable design point. However, if nonmalicious, but information hiding, routing infrastructures become
popular, such as described in [23], [36], then this issue may need
to be revisited.
IV. BASIC MARKING ALGORITHMS
In this section, we describe a series of marking algorithms-starting from the most simple and advancing in
complexity. Each algorithm attempts to solve the approximate
traceback problem in a manner consistent with our assumptions.
A. Node Append
The simplest marking algorithm----conceptually similar to the
IP Record Route option [35]----is to append each node's address
to the end of the packet as it travels through the network from
attacker to victim (see Fig. 2). Consequently, every packet received by the victim arrives with a complete ordered list of the
routers it traversed---a built-in attack path.
The node append algorithm is both robust and extremely
quick to converge (a single packet), however, it has several
serious limitations. Principal among these is the infeasibly high
router overhead incurred by appending data to packets in flight.
Moreover, since the length of the path is not known a priori, it
is impossible to ensure that there is sufficient unused space in
the packet for the complete list. This can lead to unnecessary
fragmentation and bad interactions with services such as MTU
discovery [31]. This problem cannot be solved by reserving
"enough" space, as the attacker can completely fill any such
space with false, or misleading, path information.
B. Node Sampling
To reduce both the router overhead and the per-packet space
requirement, we can sample the path one node at a time instead of recording the entire path. A single static "node" field
is reserved in the packet header--large enough to hold a single
router address (i.e., 32 bits for IPv4). Upon receiving a packet,
each router chooses to write its address in the node field with
some probability p. After enough packets have been sent, the
Marking procedure at router R:
for each packet w
let x be a random number from [0.. 1)
if x < p then,
write R into w.node
Path reconstruction procedure at victim v:
let NodeTbl be a table of tuples (node,count)
for each packet w from attacker
z := lookup w.node in NodeTbl
if z != NIL then
increment z.count
else
insert tuple (w.node, l) in NodeTbl
sort NodeTbl by count
extract path (Ri..Rj) from ordered node fields in NodeTbl
Fig. 3.
Node sampling algorithm.
victim will have received at least one sample for every router in
the attack path. As stated in Section III, we assume that the attacker sends enough packets and the route is stable enough that
this sampling can converge.
Although it might seem impossible to reconstruct an ordered
path given only an unordered collection of node samples, it
turns out that with a sufficient number of trials, the order can be
deduced from the relative number of samples per node. Since
routers are arranged serially, the probability that a packet will
be marked by a router and then left unmolested by all downstream routers is a strictly decreasing function of the distance
to the victim. If we constrain p to be identical at each router,
then the probability of receiving a marked packet from a router
d hops away is p(1 - p ) a - 1 . Since this function is monotonic in
the distance from the victim, ranking each router by the number
of samples it contributes will tend to produce the accurate attack
path. The full algorithm is shown in Fig. 3.
Putting aside for the moment the difficulty in changing the IP
header to add a 32-bit node field, this algorithm is efficient to
implement because it only requires the addition of a write and
checksum update to the forwarding path. Current high-speed
routers already must perform these operations efficiently to update the time-to-live field on each hop. Moreover, ifp > 0.5 then
this algorithm is robust against a single attacker because there
is no way for an attacker to insert a "false" router into the paths
valid suffix by contributing more samples than a downstream
router, nor to reorder valid routers in the path by contributing
more samples than the difference between any two downstream
routers.
However, there are also two serious limitations. First, inferring the total router order from the distribution of samples is
a slow process. Routers far away from the victim contribute
relatively few samples (especially since p must be large) and
random variability can easily lead to misordering unless a very
large number of samples are observed. For instance, if d -15 and p -- 0.51, the receiver must receive more than 42000
packets on average before it receives a single sample from the
furthest router. To guarantee that the order is correct with 95%
certainty requires more than seven times that number.
Second, if there are multiple attackers, then multiple routers
may exist at the same distance--and hence be sampled with
SAVAGEet al.: NETWORK SUPPORT FOR IP TRACEBACK
the sample probability. Therefore, this technique is not robust
against multiple attackers.
C. Edge Sampling
A straightforward solution to these problems is to explicitly
encode edges in the attack path rather than simply individual
nodes. To do this, we would need to reserve two static addresssized fields, start and end, in each packet to represent the routers
at each end of a link, as well as an additional small field to
represent the distance of an edge sample from the victim.
When a router decides to mark a packet, it writes its own address into the start field and writes a zero into the distance field.
Otherwise, if the distance field is already zero this indicates that
the packet was marked by the previous router. In this case, the
router writes its own address into the end field--thereby representing the edge between itself and the previous router--and
increments the distance field to one. Finally, if the router does
not mark the packet, then it always increments the distance field.
This somewhat baroque signaling mechanism allows edge sampiing to be incrementally deployed---edges are constructed only
between participating routers.
The mandatory increment is critical to minimize spoofing
by an attacker. When the packet arrives at the victim its distance field represents the number of hops traversed since the
edge it contains was sampled. 3 Any packets written by the attacker will necessarily have a distance greater or equal to the
length of the true attack path. Therefore, a single attacker is unable to forge any edge between themselves and the victim (for
a distributed attack, of course, this applies only to the closest
attacker) and the victim does not have to worry about "chaff'
while reconstructing the valid suffix of the attack path. Consequently, since we no longer use the sampling rank approach to
distinguish "false" samples, we are free to use arbitrary values
for the marking probability p.
The victim uses the edges sampled in these packets to create a
graph (much as in Fig. 1) leading back to the source, or sources,
of attack. The full algorithm is described in Fig. 4. Because the
probability of receiving a sample is geometrically smaller the
further away it is from the victim, the time for this algorithm to
converge is dominated by the the time to receive a sample from
the furthest router, 1/p(1 - p)a-1 in expectation, for a router
d hops away. However, there is also a small probability that we
will receive a sample from the furthest router, but not from some
nearer router. We can bound this effect to a factor of ln(d) by
the following argument. We conservatively assume that samples
from all of the d routers appear with the same likelihood as the
furthest router. Since these probabilities are disjoint, the probability that a given packet will deliver a sample from some router
is at least dp(1 - p)a-1. Finally, as per the well-known coupon
collector problem, the expected number of trials required to select one of each of d equiprobable items is d( In (d) + O (1))4 [20].
3It is important that distance field is updated using a saturating addition. If the
distance field were allowed to wrap, then the attacker could spoof edges close
to the victim by sending packets with a distance value close to the maximum.
4More exactly, the expression is d(ln(d) + 7), where "7 represents Euler's
constant. For simplicity, we ignore this small constant when describing the expectation, although we include its effect when we perform calculations.
231
Marking procedure at router R:
for each packet w
let x be a random number from [0.. 1)
if x < p then
write R into w.start and 0 into w.distance
else
if w.distance = 0 then
write R into w.end
increment w.distance
Path reconstruction procedure at victim v:
let G be a tree with root v
let edges in G be tuples (start,end,distance)
for each packet w from attacker
if w.distance = 0 then
insert edge (w.start,v,0) into G
else
insert edge (w.start,w.end,w.distance) into G
remove any edge (x,y,d) with d ¢ distance from x to v in G
extract path (Ri..Rj) by enumerating acyclic paths in G
Fig. 4. Edgesampling algorithm.
Therefore, the number of packets X required for the victim to
reconstruct a path of length d has the following bounded expectation:
ln(d)
E(X) < p(1 _p)d-l"
For example, if p = 1/10, and the attack path has a length
of 10, then a victim can typically reconstruct this path after
receiving 75 packets from the attacker. While this choice of
p = 1/d is optimal, the convergence time is not overly sensitive to this parameter for the path lengths that occur in the
Internet. So long as p < 1/d, the results are generally within
a small constant of optimal. In the rest of this paper, we will
use p = 1/25 since few paths exceed this length [7], [44], [17].
For comparison, the previous example converges with only 108
packets using p = 1/25.
This same algorithm can efficiently discern multiple attacks
because attackers from different sources produce disjoint edges
in the tree structure used during reconstruction. The number of
packets needed to reconstruct each path is independent, so the
number of packets needed to reconstruct all paths is a linear
function of the number of attackers. Finally, edge sampling is
also robust. That is, it is impossible for any edge closer than
the closest attacker to be spoofed, due to the robust distance determination. Conversely, in a distributed attack this also means
that it is impossible to trust the contents of any edge further away
than the closest attacker. As with the ICMP Traceback approach
[4], an additional mechanism incorporating a shared secret is required to completely address the problem of attackers spoofing
edges.
Of course, a significant practical limitation of this approach
is that it requires additional space in the IP packet header and
therefore is not backward compatible. In the next section, we
discuss a modified version of edge sampling that addresses this
problem, albeit at some cost in performance and a reduction in
robustness during large distributed attacks.
232
Reuters
in path
a
IEEE/ACM TRANSACTIONSON NETWORKING,VOL. 9, NO. 3, JUNE 2001
Marked
packets
Path reconstruction
at victim
a~b
e aeb
Address
INIlIIIII|HIIHI
Hash(Address)
IIHllmlII|HIIHI
Bitlnterleave
b~c
L ...........................,
IlUHIliilHIIIIII
IILIIIIIIHII
0
*lll]mc*d
llWd
k,,~.L..
......
.............Reconstructed
i
...........
" .... d ........................
path
Fig. 5. Edge data can be communicated in half the space by sending the XOR of
the two nodes (i.e., router IP addresses) making up an edge, rather than sending
each node separately. Over time the victim receives the messages d, c • d,
b • c, and a • b. By XORing these messages together, the original path can be
reconstructed.
V. ENCODING
k-1
Send k fragments into network
ISSUES
The edge-sampling algorithm requires 72 bits of space in
every IP packet (two 32-b IP addresses and 8 bits for distance
to represent the theoretical maximum number of hops allowed
using IP). It would be possible to directly encode these values
into an MPLS label stack [37], to enable traceback within a
single homogeneous ISP network. However, our focus is on a
heterogeneous environment based purely on IP datagrams. One
obvious approach is to store the edge sample data in an IP option, but this is a poor choice for many of the same reasons
that the node append algorithm is infeasible--appending additional data to a packet in flight is expensive and there may
not be sufficient space to append this data. We could also send
this data out-of-band--in a separate packet--but this would add
both router and network overhead plus the complexity of a new
and incompatible protocol.
Instead, we have developed a modified version of edge sampling that dramatically reduces the space requirement in return
for a modest increase in convergence time and a reduction in robustness to multiple attackers. Following an analysis of our algorithm, we explore the practical implementation issues and discuss one concrete encoding of this scheme based on overloading
the 16-b IP identification field used for fragmentation. Any solution involving such overloading necessarily requires compromises and we stress that our solution reflects only one design
point among many potential implementation tradeoffs for this
class of algorithm and does not necessarily reflect an optimal
balance among them.
A. Compressed Edge Fragment Sampling
We use three techniques to reduce per-packet storage requirements while preserving robustness. First, we encode each edge
in half the space by representing it as the exclusive-or (XOR) of
the two IP addresses making up the edge, as depicted in Fig. 5.
When some router decides to mark a packet, it writes its address
a into the packet. The following router, b, notices that the distance field is 0 and (assuming it does not mark the packet itself)
reads a from the packet, XORs this value with its own address,
and writes the resulting value, a ® b, into the packet. We call the
Fig. 6. Each router calculates a uniform hash of its IP address once, at startup,
using a well-known function. This hash is interleaved with the original IP
address (the original address on odd bits, the hash on even bits). The resulting
quantity is then broken into k fragments, which the router selects among
randomly when marking a packet. Although it is not shown, each of these
fragments is further labeled with its offset. The next downstream router uses
this offset to select the appropriate fragment to XOR thereby encoding part
of an edge.
resulting value the edge-id for the edge between a and b. The
edge-ids in the packets received by the victim always contain
the XOR of two adjacent reuters, except for samples from reuters
one hop away from the victim, which arrive unmodified. Since
b (~ a @ b = a, marked packets from the final router can be used
to decode the previous edge id, and so on, hop-by-hop until we
reach the first router.
Our second modification further reduces our per-packet space
requirements by subdividing each edge-id into some number k
of smaller nonoverlapping fragments. When a router decides to
mark a packet, it selects one of these fragments at random and
stores it in the packet. We use a few additional bits (log 2 k) to
store the offset of this fragment within the original address--this
is necessary to ensure that different fragments from an edge-id
can be recombined in the correct order. If enough packets are
sent by the attacker, the victim will eventually receive all fragments from all edge-ids.
Finally, unlike full IP addresses, edge-id fragments are not
unique and multiple fragments from different edge-ids may have
the same value. If there are multiple attackers, a victim may
receive multiple edge fragments with the same offset and distance. To reduce the probability of accidentally reconstructing
a "false" edge-id by combining fragments from different paths,
we add a simple error detection code to our algorithm. We increase the size of each router address, and hence each edge-id,
by bit-interleaving its IP address with a random hash of itself
(depicted in Fig. 6). As described earlier, this value is split into
fragments, each fragment is selected randomly and stored with
an offset, and downstream routers use XOR to combine fragments at the same offset to make up edge-id fragments. The
victim constructs candidate edge-ids by combining all combinations of fragments at each distance with disjoint offset values.
As shown in Fig. 7, a candidate edge-id is only accepted if the
hash portion matches the data portion for each of its two nodes.
As we increase the size of the hash, the probability of a collision
is reduced. We describe the full procedure in Fig. 8.
The expected number of packets for this algorithm to converge is similar to the edge sampling approach, except now we
need k fragments for each edge-id, rather than just one, a total
of kd fragments. If we again assume conservatively that each
of these fragments is delivered equiprobably with probability
SAVAGEet
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NETWORK SUPPORTFOR IP TRACEBACK
233
Marking procedure at router R:
Combine k fragments from network
%.
.J
0
let R' = Bitlntereave(R, Hash(R))
let k be the number of non-overlapping fragments in R'
for each packet w
let x be a random number from [0..1)
if x < p then
let o be a random integer from [0..k - 1]
let f be the fragment of R' at offset o
write f into w.frag
write 0 into w.distance
write o into w.offset
else
if w.distance = 0 then
let f be the fragment of R' at offset w.offset
write f @ w.frag into w.frag
increment w.distance
k-1
IlUilnmlUlllllll
lll|lllll|ll
BitDeinterteave
Address?
Hash(Address).
Hash(Address?)
~ ~ / N o
Reject
/
Address
Fig. 7. When reconstructing a candidate edge, the victim combines k
fragments to produce a bit string. By de-interleaving this string, the address
portion and the hash portion are extracted. We recalculate the hash over this
address portion using the same hash function used by the router. If the resulting
hash is the same as the hash portion extracted, then the address is accepted as
Path reconstruction procedure at victim v:
let FragTbl be a table of tuples (flag,offset,distance)
let G be a tree with root v
let edges in G be tuples (start,end,distance)
let m a x d := 0
let last := v
for each packet w from attacker
valid. This procedure protects against accidentally combining fragments of
different edges.
FragTbl.Insert(w.frag,w.offset,w.distance)
p(1 - p)d-1, the expected number of packets required for path
reconstruction is bounded by
E(X) <
k. ln(kd)
;(1 - ; V
For example, if there are eight fragments per edge-id, an attacker
is ten hops away, and p = 1/25, then a victim can reconstruct
the full path after receiving slightly less than 1300 packets on
average. Using techniques similar to those used to show sharp
concentration results for the coupon collectors problem, we can
further show that the approximate the number of packets required to ensure that a path can be reconstructed with probability 1 - 1 / c is
k" ln(kdc)
p(1 - p)a-1
packets. To completely reconstruct the previous path with 95%
certainty should require no more than 2150 packets. Many denial-of-service attacks send this many packets in a few seconds.
Finally, we explore the robustness of this algorithm with
respect to multiple attackers. For a random hash of length h,
the probability of accepting an arbitrarily constructed candidate
edge-id is 1/2 h. In the event that there are m attackers, then
at any particular distance d, in the worst case there may be up
to m distinct routersP Consequently, the probability that any
edge-id at distance d is accepted incorrectly is at most
1-(1
if w.distance > m a x d then
m a x d := w.distance
for d := 0 to m a x d
for all ordered combinations of fragments at distance d
construct edge z
if d ¢ 0 then
z := z @ last
if Hash(EvenBits(z)) = OddBits(z) then
insert edge (z,EvenBits(z),d) into G
last := EvenBits(z);
remove any edge (x,y,d) with d :fi distance from x to v in G
extract path (RI..Rj) by enumerating acyclic paths in G
- l ~ m k] 2 h
since there are m k possible combinations of fragments in the
worst case. For h = 32 and k = 4 this means that 100 distinct
5In practice, the number of distinct routers is likely to be smaller for the portion of the path closest to the receiver, since many attackers will still share significant portions of their attack path with one another.
Fig. 8.
Compressed edge fragment sampling algorithm.
routers at the same distance (i.e., disjoint attack paths) will be
resolved with no errors with a probability of better than 97%.
For h = 32 and k = 8 (the values we use for our implementation), the same certainty can only be provided for ten distinct
routers at the same distance. Our use of the XOR function further complicates reconstruction since all combinations of XOR
values must be tried as attack paths diverge. This is somewhat
mitigated as the probability of propagating an error from a single
edge all the way to the attacker is is extremely small because the
resulting edge-id, when xORed with the previous edge-id, must
again produce a correct hash.
The most significant drawback to this scheme is the large
number of combinations that must be considered as the multiple
attack paths diverge. While these combinations can be computed
off-line, for large values of k and m even this can become intractable. For example, even with k = 8 and m = 10, if the separate attack paths diverge such that there are ten completely independent edges per attacker, this will require roughly a billion
combinations to be considered. Consequently, there is a design
tension in the size of k - per-packet space overhead is reduced
by a larger k, while computational overhead and robustness benefits from a smaller k.
234
IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 9, NO. 3, JUNE 2001
/
IP header
total length
ver Jhlen TOS
N ~
~
~i/~lt.gSI
offset
i
time to live protocol
/
:~eader checksum
sourceIP address
destinationIP ad~ess
/
/
Io.se*l
0
Fig. 9.
distance I
23
78
edge fragment
i
15
Encoding edge fragments into the IP identification field.
B. IP Header Encoding
To allow for practical deployment requires that we "overload" existing header fields in a manner that will have minimal impact on existing users. This is a difficult task, especially
given that even after prodigious effort we require 16 bits of
space. Nonetheless, we believe it possible to obtain this space
by overloading the 16-b IP identification field. This field is currently used to differentiate IP fragments that belong to different
packets. We describe our proposed encoding below, and then
discuss the issues of backward-compatibility that it raises. However, we note that because the issue of backward-compatible encoding is largely separate from our traceback algorithms, we
could adopt any reasonable encoding that comes to light.
Fig. 9 depicts our choice for partitioning the identification
field: three offset bits to represent eight possible fragments, five
bits to represent the distance, and eight bits for the edge fragment. We use a 32-b hash, which doubles the size of each router
address to 64 bits. This implies that eight separate fragments
are needed to represent each edge---each fragment indicated by
a unique offset value. Finally, five bits is sufficient to represent
31 hops, which is more than almost all Internet paths [7], [44],
[17]. 6
The observant reader will note that this layout is chosen to
allow the highest performance software implementation of our
algorithm, which already had a low per-packet router overhead.
In the common case, the only modification to the packet is to
increment its distance field. Because of its alignment within the
packet, this increment precisely offsets the required decrement
of the time-to-live field implemented by each router [1]. Consequently, the header checksum does not need to be altered at
all and the header manipulation overhead could be even lower
than in current software-based reuters--simply an addition to
the distance field, a decrement to the TI'L field, and a comparison to check if either has overflowed. In the worst case, our
algorithm must read the IP identification field, lookup an edge
fragment and XOR it, and fold the write-back into the existing
6It is also reasonable to turn off marking on any reuters that cannot be directly connected to an attacking host (e.g., core reuters). This both reduces the
convergence time, and increases the "reach" of the distance field.
checksum update procedure (a few ALU operations). Of course,
for modern ASIC-based reuters these optimizations are unnecessary.
As we reuse the IP identification field, we must address issues
of backward compatibility for IP fragment traffic. Ultimately,
there is no perfect solution to this problem and we are forced to
make compromises that disadvantage fragmented traffic. Fortunately, recent measurements suggest that less than 0.25% of
packets are fragmented [42], [10]. Moreover, it has long been
understood that network-layer fragmentation is detrimental to
end-to-end performance [28] so modern network stacks implement automatic MTU discovery to prevent fragmentation regardless of the underlying media [31 ]. Consequently, we believe
that our encoding will interoperate seamlessly with existing protocol implementations in the vast majority of cases.
However, there is a small but real fraction of legitimate traffic
that is fragmented, and we wish to ensure that it is not affected
by our modifications to the extent that this is possible. Normally
if a packet is fragmented, its identification field is copied to each
fragment so the receiver can faithfully reassemble the fragments
into the original packet. Our marking procedure can violate this
property in one of two ways: by writing different values into
the identification fields of fragments from the same datagram or
by writing the same values into the identification fields of fragments from different datagrams. These two problems present
different challenges and have different solutions.
First, a datagram may be fragmented upstream from a
marking router. If the fragment is subsequently marked and
future fragments from the same datagram are not marked
consistently then reassembly may fail or data may be corrupted.
While the simplest solution to this problem is to simply not
mark fragments, an adversary would quickly learn to evade
traceback by exploiting this limitation. In fact, some current
denial-of-service attacks already use IP fragments to exploit
errors in host IP reassembly functions [ 12]. Instead, we propose
an alternative marking mechanism for fragments. We use
a separate marking probability, q, for fragments. When we
decide to mark a fragment, we prepend a new ICMP "echo
reply" header, along with the full edge data--truncating the
tail of the packet. This ICMP packet is considered "marked"
and its distance field is set to zero, thereby guaranteeing that
the distance field reflects the number of edges traversed on
the way to the victim. The packet is consequently "lost" from
the standpoint of the receiver, but the edge information is
delivered in a way that does not impact legacy hosts. Because
we can use the full edge sampling algorithm, q can be more
than an order of magnitude smaller than p and yet achieve the
same convergence time. This solution increases the loss rate
of fragmented flows somewhat (more substantially for longer
paths) but preserves the integrity of the data in these flows.
A more insidious problem is presented by fragmentation that
occurs downstream from a marking router. If a marked packet is
fragmented, but one of the fragments is lost, then the remaining
fragments may linger in the victims reassembly buffer for an
extended period [5]. Future packets marked by the same router
can have the same IP identification value and consequently may
be incorrectly reassembled with the previous fragments. One
possibility is to leave this problem to be dealt with by higher
SAVAGE et
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NETWORK SUPPORT FOR IP TRACEBACK
235
A. Backward Compatibility
4500 4000= 3500
~
3000-
~25002000
~,500:
"6
1000
500
0
Median
'
•
i
5
0
.
.
.
.
t
10
.
.
.
.
i
.
.
.
.
t5
i
20
.
.
.
.
~
25
.
.
.
.
i
•
-
30
Path length
Fig. 10. Experimental results for number of packets needed to reconstruct
paths of varying lengths. The marking probability p is set to 1/25. Each path
length result represents the results of 1000 independent simulation runs.
layer checksums. However, not all higher layer protocols employ checksums, and in any case it is dangerous to rely on such
checksums because they are typically designed only for low
residual error rates. Another solution is to set the Don't Fragment flag on every marked packet. Along rare paths that require fragmentation, this solution will degrade communication
between hosts not using MTU path discovery, and may filter
marked packets if a reduced MTU edge is close to the victim,
but it will never lead to data corruption.
C. Assessment
We have implemented the marking and reconstruction
portions of our algorithm and have tested it using a simulator
that creates random paths and originates attacks. In Fig. 10, we
graph the mean, median, and 95th percentile for the number of
packets required to reconstruct paths of varying lengths over
1000 random test runs for each length value. We assume a
marking probability of 1/25. Note that while the convergence
time is theoretically exponential in the path length, all three
lines appear linear due to the finite path length and appropriate
choice of marking probability.
We see that most paths can be resolved with between one
and two thousand packets, and even the longest paths can be
resolved with a very high likelihood within 4000 packets. To
put these numbers in context, most flooding-style denial of service attacks send many hundreds or thousands of packets each
second. The analytic bounds we described earlier are conservative, but in our experience they are no more than 30% higher
than our experimental results.
VI. LIMITATIONSAND FUTURE WORK
There are still a number of limitations and loose ends in our
approach. We discuss the most important of these here:
• backward compatibility;
• distributed attacks;
• path validation;
• approaches for determining the attack origin.
The IP header encoding as we have described it has several
practical limitations. It negatively impacts users that require
fragmented IP datagrams and is incompatible with parts of
IPsec [29] (the authentication header provides cryptographic
protection for the identification field and therefore the field
cannot be safely modified by routers). These problems are
hardly unique to our traceback technique and are inherent
limitations that come about from attempting to coexist with
or co-opt protocol features that did not anticipate a new use.
One way to partially address this issue, originally proposed
by Hawkinson, is to selectively enable traceback support in
response to operational needs. A "request for traceback" from
a particular network could be encoded as a BGP attribute in
the networks route advertisement. Routers receiving such an
advertisement would enable traceback support on packets destined for that network. Since a network requesting such support
is presumably already suffering under an attack, any minor
service degradation for fragmented flows would be acceptable.
Finally, our scheme does not address implementation in IPv6,
the proposed successor to IPv4, which does not have an identification field [19]. While we do not attempt to propose a complete encoding here, we believe that the same techniques we
have proposed could also be employed within IPv6, perhaps by
overloading the 24-bflow label field (without any further modifications this would result in roughly a factor of three increase
in the number of packets required to reconstruct a path).
B. Distributed Attacks
For moderate distributed attacks, the implementation we
have described has serious limitations due to the difficulty
in correctly grouping fragments together. Consequently, the
probability of misattributing an edge, as well as the amount of
state needed to evaluate this decision, increases very quickly
with the fanout of an attack. There is ongoing work by several
groups to develop improved marking algorithms to address this
deficiency. Song and Perrig leverage the additional assumption
of a network topology map to compress the representation of
edge state--thereby vastly improving the robustness against
distributed attack [39]. Dean, Franklin, and Stubblefield also
improve robustness by replacing our ad hoc XOR-based marking
approach with one based on algebraic coding theory [ 18]. There
is significant future work in designing alternative encoding
methods that scale their robustness as they receive more data.
C. Path Validation
Some number of the packets sent by the attacker are unmarked by intervening routers. The victim cannot differentiate
between these packets and genuine marked packets. Therefore
an attacker could insert "fake" edges by carefully manipulating
the identification fields in the packets it sends. While the distance field prevents an attacker from spoofing edges between it
and the victim--what we call the valid suffix- nothing prevents
the attacker from spoofing extra edges past the end of the true
attack path.
There are several ways to identify the valid suffix within a
path generated by the reconstruction procedure. With minimal
236
knowledge of Internet topology, one can differentiate between
routers that belong to transit networks (e.g., ISPs) and those
which belong to stub networks (e.g., enterprise networks). Generally speaking, a valid path will never enter a stub network and
then continue into a transit network. Moreover, simple testing
tools such as traceroute should enable a victim to determine if
two networks do, in fact, connect. More advanced network maps
[8], [24] can resolve this issue in an increasing number of cases.
A more general mechanism is to provide each router with a
time-varying "secret" that is used to authenticate each marked
packet (minimally, one bit in the IP header). When the victim
wants to validate a router in the path, it could contact the associated network (possibly out of band, via telephone or e-mail)
and obtain the secret(s) used by the router at the time of the attack. To guard against replay, the secret must be varied relatively
quickly and hashed with the packet contents. Since the attacker
will not know the routers secret, the forged edge-id fragments
will not contain a proper authentication code. By eliminating
edge-ids for which the the constituent fragments can not be validated, the candidate attack path can be pruned to only include
the valid suffix. This rough idea is developed much further in
Song and Perrig's traceback proposal [39].
D. Attack Origin Detection
While our IP-level traceback algorithm could be an important
part of the solution for stopping denial-of-service attacks, it
is by no means a complete solution. Our algorithm attempts
to determine the approximate origin of attack traffic--in
particular, the earliest traceback-capable router involved in
forwarding attack traffic from the source that directly generated
it. As mentioned earlier, there are a number of reasons why
this may differ from the true source of the attack: attackers
can hide their true identities by "laundering" attacks through
third parties, either indirectly (e.g., smurf attacks [13] or DNS
reflectors [15]) or directly via compromised "stepping stone"
machines or IP-in-IP tunnels. While there is on-going work
on following attackers through intermediate hosts [47], [41],
there are still significant challenges in developing a generally
applicable and universally deployable solution to this problem.
One interesting possibility enabled by the packet marking
approach is to extend traceback across "laundering points."
For example, identifying marks could be copied from a DNS
request packet into the associated DNS reply, thereby allowing
the victim to trace the full causal path. However, this would also
increase the required path length to be reconstructed in such
cases--possibly exceeding the limited space in the length field.
Even in absence of such "laundering," our approach does not
reveal the actual host originating the attack. Moreover, since
hosts can forge both their IP source address and MAC address
the origin of a packet may never be explicitly visible. On shared
media such as FDDI rings, this problem can only be solved
by explicit testing. However, on point-to-point media, the input
port a packet arrives on is frequently enough to determine its
true origin. On other media, there may be a MAC address, cell
number, channel, or other hint that would help to locate the attack origin. In principle, our algorithm could be modified to report this information by occasionally marking packets with a
special edge-id representing a link between the router and the
IEEE/ACMTRANSACTIONSONNETWORKING,VOL.9, NO. 3, JUNE2001
input port on which the packet arrived (or other "hint" information). We have not explored the design of such a feature in any
depth.
Finally, traceback is only effective at finding the source of attack traffic, not necessarily the attacker themselves. Stopping an
attack may be sufficient to eliminate an immediate problem, but
long term disincentives may require a legal remedy and therefore the forensic means to determine an attackers identity. Even
with perfect traceback support, unambiguously identifying a
sufficiently skilled and cautious attacker is likely to require cooperation from law enforcement and telecommunications organizations.
VII. CONCLUSION
In this paper, we have argued that denial-of-service attacks
motivate the development of improved traceback capabilities
and we have explored traceback algorithms based on packet
marking in the network. We have shown that this class of algorithm, best embodied in edge sampling, can enable efficient
and robust multiparty traceback that can be incrementally deployed and efficiently implemented. As well, we have developed variant algorithms that sacrifice convergence time and robustness for reduced per-packet space requirements. Finally, we
have suggested one potential deployment strategy using such an
algorithm based on overloading existing IP header fields and
we have demonstrated that this implementation is capable of
fully tracing an attack after having received only a few thousand
packets. We believe our solution represents a valuable first step
toward an automated network-wide traceback facility. Several
areas remain to be addressed in future work, such as improving
robustness under distributed attacks and tracing past points of
indirection such as reflectors.
ACKNOWLEDGMENT
This paper has benefited from conversations with many
different people--far more than can be acknowledged completely here. Still, the authors would like to particularly thank
B. Cheswick and H. Burch for early access to their work in
this area, S. McCreary and K.C. Claffy for access to their
packet trace data, A. Wolman for help with jgraph, P. Pardyak,
M. Swift, N. Spring, G. Bartels, R. Grimm, and G. Voelker
for commenting on early drafts of the paper, and constructive
feedback from V. Paxson, C. Partridge, J. Hawkinson, R. Stone,
J. Mogul, R. Moskowitz, G. Minshall, T. Li, C. Villamizar,
S. Corbato, and countless others. Finally, the authors thank the
anonymous reviewers for their efforts in improving this work.
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Stefan Savage received the Ph.D. degree from the University of Washington,
Seattle.
He is currently an Assistant Professor at the University of Califomia at San
Diego (UCSD). His previous research has spanned a number of areas, including
real-time systems, OS kernel structure, disk arrays, and concurrency control.
He has focused solely on problems in wide-area networking for the last several
years.
David Wetherall (M'89) received the Ph.D. degree in computer science from
the Massachusetts Institute of Technology (MIT), Cambridge, in 1998.
He is currently a Member of the Faculty of Computer Science and Engineering at the University of Washington (UW), Seattle. He has conducted computer systems research for ten years and authored papers on topics ranging from
distributed systems to internetworking to programming languages. His thesis
research helped to pioneer the field of Active Networks, in which flexible network infrastructures are used to enable rapid service innovation.
Anna Karlin received the Ph.D. degree in computer science from Stanford University, Stanford, CA, in 1987.
After a one and half year postdoctoral position at Princeton University,
Princeton, NJ, she joined Digital Equipment Corporations Systems Research
Center as a Research Scientist. In 1994, she was a Visiting Professor with
the University of Washington, Seattle, where she became a Member of the
Faculty in 1996. She has been a full Professor since 1998. Her research is
concerned with the design and analysis of algorithms, with a primary focus on
probabilistic and online algorithms. She is a member of the National Research
Councils Computer Science and Telecommunications Board, and is on the
editorial board for SIAM Journal on Computing.
Tom Anderson is currently an Associate Professor of computer science and
engineering at the University of Washington, Seattle. His research has spanned
a wide spectrum of topics, from multiprocessor scheduling, to high-speed switch
design, to tools for software engineering, to scalable and fault tolerant cluster
software, to merged logic and DRAM chip designs, to his most recent focus on
Interact reliability and security. He has co-authored a dozen award papers at top
conferences.
Dr. Anderson has received a National Science Foundation Presidential Faculty Fellowship and a Sloan Research Fellowship.
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