SRUTI '05 Paper
[SRUTI '05 Technical Program]
Stress Testing Traffic to Infer Its Legitimacy
Nick Duffield and Balachander Krishnamurthy
AT&T Labs-Research, 180 Park Avenue, Florham Park, New Jersey, 07932, USA
Adaptation in the face of performance degradation is the hallmark of
well-behaved network traffic. For sufficiently robust applications,
we propose distinguishing good from bad traffic on the basis of the
response to artificial performance impairments. We explain the basic
requirements for our scheme, and show how it could be generically applied
at different levels in the protocol stack.
This paper proposes a new measurement-based methodology for detecting
badly behaving network entities. The methodology can potentially be
applied across a range of protocol levels. The key idea is to stress
test an entity by impairing its network performance, and then to
observe the response of the entity to the impairment. Our work is
primarily oriented closer to the edge rather than the middle of the
The usefulness of the method rests on two assumptions:
- Differentiation: The response to impairment is
different for bad entities than good entities, and can hence be used
to classify entities as good or bad.
- Recovery: good entities can recover from impairment
in the sense that the performance degradation they suffer remains
within acceptable levels.
In practice, these two assumptions are coupled. Suppose we impair the
performance of a protocol that is used by an application. We require
the underlying protocol to be more sensitive and responsive to
impairment that the application, i.e., it must adapt to the impairment
before the application suffers an unacceptable performance
degradation. Impairment must be sufficiently short-lived to be
interpreted as a transient, forcing retry, leading to recovery. The
nature of this response is used to distinguish bad from good.
For some applications, e.g., online gaming and others that are
highly sensitive to loss and delay, no impairment is
tolerable. We assume that these applications can be identified
(e.g. on the basis of TCP/UDP port numbers) and excluded from our
scheme. More generally, the impairments induced by stress testing must
not cause performance degradation to exceed acceptable limits, e.g.,
as specified in a Service Level Agreement (SLA).
We now describe the broad characteristics of impairments, propose some
schemes for good/bad classification, and discuss the costs of
In many cases, applications will experience performance impairments
even in the absence of stress testing, for example due to packet loss,
delay, network rerouting or resource contention at endpoints. We call
these impairments ambient. We assume that applications are
well-designed in the sense that they a reasonably good at adapting to
or otherwise withstanding ambient impairments. If this assumption
holds, then it makes sense for impairments introduced by stress
testing to conform as closely as possible to the characteristics of
the ambient impairments. For then applications need only recover from
impairments of the type (even if not the intensity) of those that they
are known to adapt to. Another reason for stress impairments from to
conform to ambiance is to make it harder for an adversarial
bad application to detect that it is being stress tested.
This begs the question of the extent to which ambient impairments can
be characterized and then reproduced in stress testing. We will discuss this
further for specific applications in Section 3. Here we
enumerate some general parameters for impairment. In our model, the
stress testing takes the form of a sequence of individual stress
events, characterized by the following parameters:
- Frequency: of stress events during stress testing
- Event duration: the duration of an single stress event
- Stress duration: the duration of a set of stress
events. Note that the stress duration may itself be adaptive in the
sense that stress testing is terminated once sufficient information has
been extracted from responses for classification, or if it is
decided that no such classification can be made.
- Granularity: at which stress take place. For example,
a stress test might target all packets from an IP
address, or a subnet, or using a particular UDP/TCP port.
How should these parameters be determined? The aggressiveness of the
stress test should be commensurate with the perceived threat. The
frequency of bad traffic may be bootstrapped by learning directly from
the identification scheme that is used in the stress test, although
their will be a transient period in which such estimates are
unreliable. One way to seed an initial the threat level is to monitor
the intensity of bad traffic taking place, e.g., attacks on ``dark''
address space as seen in a honeypot . A generic scheme to set
parameters for the stress test is described in Section 2.3
We use the following three outcome model to describe the response of an
entity to a stress event:
- Cease: the entity ceases all activity. This may be due
to a bad entity having turned its attention elsewhere, or any entity
suffering an unrecoverable error as a result of impairment. Whatever
the reason, the current stress test of the entity is terminated.
- Evolve to Good: adaptation to impairment in the manner
expected of a good entity. The current stress test of the entity is
- Evolve to Bad: any other result, for example, not
responding to impairment in any manner. The entity is flagged as
``suspicious''. Stress testing may continue.
How should the outcome of one or several stress events be interpreted?
The greater the number of stress events, the greater the certainty of
the classification based upon the outcomes. This motivates a cautious
approach in which only multiple suspicious outcomes would result in an
entity being designated bad, in which case some further action, e.g.,
blocking, is taken. This may be accomplished in a number of ways; here
- Fixed horizon classification: a fixed number
n of stress events is generated in sequence. An entity is classified as
bad if the number of suspicious outcomes exceeds some level . The history of outcomes is discarded for each new stress test.
Impairment may also be used as means of control; in this case the
intensity of impairments (as determined by frequency and/or duration of
stress events) can be increased in response to the level of
suspicion. At the same time, it is desirable not to penalize an
entity unduly for suspicious behavior far in the past. Thus one
penalizes bursts of suspicion, while admitting ambient low level
of false positives. This motivates:
- Queue-based classification:
Suspicion flags are enqueued in an infinite buffer which drains at
some rate. The impairment intensity is a function of the buffer
We use the term cost to denote the potential undesirable
impact of the impairment method on applications. We identify two types
- The impairment cost represents the performance degradation
experienced by good applications during impairments, and during
their response to impairments. This cost
includes, for example, the impact of packet loss or delay due to
impairment on an application's performance.
- The identification costs are those of actions taken on
the basis of the identification of applications as good or bad.
Examples of such costs include the the false positive rate (the
frequency with which good applications are misidentified as bad)
and the false negative rate (the frequency with which bad
applications are misidentified as good).
Impairment may be combined with other classification methods. In
this case, the relevant identification costs are those of the
For a number of reasons, both impairment and identification costs may
vary with time. As noted, a stress test may adapt to the responses of
the target, either to increase the effectiveness of classification, or
to control entities classified as bad. Adaptation takes the form of
changing the intensity of the impairment, or possibly removing it
altogether. This results in a concomitant changes in the false
negative and positive rates.
We now examine the key set of protocols on a case by case basis. The
main issues are: the possible responses as a result of impairment
(apart from normal response and no response), how the responses may be
interpreted at the client side, are they recoverable, and if so how
(where will the adaptation be seen if at all). Impairment and
monitoring has to take place at the same protocol level to facilitate
tracking of the reaction. Our guiding principle is that our scheme
becomes difficult to use at a given protocol level if there are a
variety of adaptation mechanisms possible or used by traffic using
that protocol that cannot be distinguished. Differentiation typically
requires moving to a higher layer.
Note that in some protocols/layers impairments already being used or
subsumed by other mechanisms. Where appropriate we mention them. For
example, in BGP the notion of impairment is not so crucial as the
peers exchanging routes and/or traffic have prearranged thresholds.
UDP does not provide any adaptation mechanisms in itself. However,
higher level applications and protocols may select UDP as their
transport, but implement their own adaptation to impairment. In some
cases the adaptation to be evident at the transport layer, involving
reduction of the sending rate (to avoid congestion) and the repetition
of packets (for reliability). The precise details of the adaptation
will depend on the higher level protocol or application in question.
The UDP port number can be useful in identifying the application. More
subtle adaptations may only be visible in the transport payload, and
again their location and interpretation would rely on accurate
identification of the application in question.
For a specific example, we consider the Realtime Transport Protocol
RTP  running over UDP. Within RTP, The RTP control protocol
(RTCP) provides feedback on the quality of data distribution. Under
impairment due to stress testing, receivers will notify senders of
reduced quality via RTCP. A good sender is expected to modify its
transmissions accordingly. However, the nature may appear quite subtle
at the network level , e.g., reducing the number of layers sent from a
layered encoding of audio or video. The precise nature of this
adaptation is determined according to the policy of the end
TCP is equipped with congestion avoidance mechanisms: a well-behaved
TCP adapts to congestion by decreasing its congestion window before
growing it again to explore the available bandwidth. The TCP sender
detects congestion when acknowledgements are not forthcoming from the
receiver, due to loss and/or delay of packets in transit. Thus a TCP
connection should both adapt to impairment, and also recover from it.
Stress testing of TCP is accomplished by dropping and/or delaying
packets, and observing the connection's response. Impairment must not
be so intense as to seriously degrade throughput. A
connection which either adapts either too slowly, or not at all, is
regarded as bad. Thus, depending on the thresholds for classification.
TCP sender that is too aggressive may be regarded as bad.
We give more detail on how the framework might be implemented for
impairment by packet loss. We require to be able to monitor and
possibly impair both directions of a TCP connection. One way to
achieve this is to insert the desired functionality below the TCP
layer in a receiving host. Impairment is achieved by selectively
dropping packets incoming from a sender. Using the terminology from
Section 2, a normal TCP connection is expected to
``evolve to good'' as it adapts to dropped packets.
A number of approaches to passively characterizing TCP connections
have been proposed; see
[4, 5, 19].
One aim of this work has is to compare measured characteristics with
theoretical baselines. Deviations from these baselines would then be
used as triggers for routers to impose limits, e.g. by restricting the
bandwidth available to a connection in order to restore a fair
distribution of bandwidth. Our aim is somewhat different: to identify
and penalize bad connections in advance of their impact on other traffic.
 proposed three categories: flows that are not TCP friendly,
flows that use a disproportionate amount of bandwidth, and flows that
are unresponsive to packet loss. The last case matches best with our
framework;  mentions the idea of introducing packet loss
impairments, and observing whether reduction in throughput (if any)
conforms to expectations.
A more detailed yardstick against which to measure the sender's
response to impairment is an estimate of the sender's congestion
window obtained by monitoring the receiver-to-sender ACKs. This has
been proposed in
, which uses the ACKs to drive transitions in a
finite state machine (FSM) that represents the sender. State
transitions due to sender timeouts may be inferred by monitoring
sender-to-receiver retransmissions. Parallel FSMs may be run for
different flavors of TCP. Losses occurring between the monitoring point
and the sender lead to estimation uncertainties, although the impact
can be detected and corrected for to some degree.
We outline how classification of bad senders might proceed if using
the congestion window estimation scheme. A sender is flagged as
suspicious if, it sends a packet that would not be allowed by TCP
under the current estimate of the senders congestion window. The
sender is flagged as bad if it sends multiple suspicious packets,
e.g., as determined by one of the classification schemes in
Section 2.2. In principle, this classification could
be made of any connection, whether impaired or not. The advantage
of stress testing is that by active soliciting a response from the
sender, it enables the identification of bad senders in advance of
circumstances where their bad behavior may be problematic. In this
example stress testing helps identify overaggressive TCP senders in
times of low network congestion, rather than in congestion periods in
which they could have a disproportionate impact on other connections.
This is somewhat different to the approach of , which
proposed characterization of higher rate flows during periods of
congestion. Once a sender is classified as bad, some action may
be taken against it (e.g. dropping, blocking, or otherwise
deprioritizing) at the monitoring point, or by reconfiguring
routers in the traffic's path.
We propose to evaluate this approach in future work. Initial
evaluations will use controlled TCP-like senders that can be
configured to act in good or bad fashion. The existing active TCP
inference tool tbit  can likely be
modified for this purpose.
Stress testing is intended for connections that are already
established. On the other hand dropping packets at the start of a
connection, specifically the SYN and SYN-ACK packets used in the
three-way handshake that establishes the connection, should be used
sparingly or not at all, since the performance impact of preventing
connections establish altogether will be far greater.
Although the operational details may differ, a similar approach to
stress testing can be adopted with other congestion avoiding transport
protocols, including the Stream Control Transmission Protocol (SCTP)  and the Datagram Congestion
Control Protocol (DCCP) .
Although we have described an example in which monitoring and
impairment takes place at a connection endpoint, the same could be
accomplished in the middle of the network. (The methods of
 were originally designed for this context).
Here, keeping track of the identification status of potentially a huge
number of connections is a challenging. But since their are only
limited number of potential states in the classification schemes of
Section 2.2, one may use compression schemes such as
Bloom filters  in order to reduced the
space needed. For example, in the fixed horizon classification scheme
one may maintain a Bloom filter each number of suspicious outcomes,
and store keys of connections in it. The current state is then yielded
by the highest level Bloom filter that declare a match on the
connection key. However, Bloom filters are subject to false positives,
which increases the identification costs.
bgp: impairment: dropping a connection, ignoring announcement and
withdrawals. notion of trust (e.g. peers exchange lists of speakers).
bgp: existing assumptions of trust relations seem to preclude need to
impair: everyone is on the white list.
A disproportionate amount of DNS traffic consists of UDP exchanges
involving port 53; apart from zone transfers there is very little TCP
traffic. Traditionally DNS clients are configured to send recursive
queries as their resolver libraries often do not follow referrals.
Local DNS servers can handle iterative referrals that might come back
from authoritative DNS servers. One weakness with impairment in DNS is
that many clients may ask multiple name servers in parallel for
non-authoritative queries. Authoritative DNS servers however can
impair requests by referring the request to non-existent servers to
see if the query comes back. Root servers are ill equipped to
participate given how busy they are with the already large fraction of
illegitimate, albeit non-attack, queries 
that they receive.
The Simple Mail Transfer Protocol has reasonable potential for
impairment trials. Although email has the reputation for being
delivered instantaneously, the dependence on this has significantly
eroded due to the popularity of instant messaging and the rise of
email spam. A non-trivial number of email messages are delayed for a
variety of reasons and the retry mechanism built into the application
(with retrials carried out over several days) ensures that the mail
will be eventually delivered.
Our assumption here is that spammers are not likely to retry sending
the mail as it would require human intervention. Robots would have to
be able to parse the reply and separate simple bounce messages from
vacation programs and impairment triggered responses. However,
depending on the importance of the mail, the human (non-spammer)
sender may retransmit the mail. Each such retransmission dramatically
lowers the probability of the sender being a spammer. Many sites
maintain a whitelist (good senders), blacklist (bad senders), and
graylist (as yet unclassified senders). Retransmitters could be moved
from the graylist to the whitelist. The related step of moving
non-retransmitters to a blacklist is however trickier as is the problem
of dealing with mailing lists.
A serendipitous benefit for users who have been selected for
impairment is that they faced a one-time delay but future
communications are likely to improve. A spammer trying to
subvert the mechanism would have to retransmit a very large
number of messages in trying to game this mechanism with a
low probability of guaranteed success.
At the HTTP layer, redirection would involve a 3xx level response
(redirection class response code such as
307 Temporary Redirect
but to a blank page. Alternately the server could send back a
408 Request Timeout or a
503 Service Unavailable along
Retry-After header indicating the number of seconds
after which the client should retry the request. Here, this value
should be as low as possible; say 1 second. The assumption again is
that a user may retry but an attack program is not likely to react the
same way. With the Web server under our control, it is possible to
monitor the frequency of access and retries as input to further
impairment actions. Interestingly, the expected adaptation on the
part of the user is to retry whereas in TCP the expectation is the
opposite: backing off.
We benefit due to the additional and more detailed semantic
information available at the Web server. For example, it is possible
to distinguish between normal user requests, and those that emanate
from spiders. Thus, a Web server operating under impairment rules, can
make judicious choices of URLs whose access trigger impairment guided
by its access patterns and popularity.
Three broad categories of P2P protocols have emerged: the original
central index model of Napster, the flooding of neighbors with
separation of search and download phases model exemplified by Gnutella
and KaZaa, and the most popular tracker process and segmented
downloading model of BitTorrent. There are at least two schools of
thought: one that presumes virtually all the content exchanged on P2P
networks are of dubious legality and the other that views that there
is a steadily increasing share of significant amounts of legal
content being exchanged. For example, Linux Kernel releases are
now routinely copied via BitTorrent.
One impairment technique already prevalent is content pollution:
insertion of white noise in audio files or unexpected content in large
video files, to frustrate the illegal downloaders .
Another prevalent technique-that of choking or throttling
connections-is to reduce freeloading and to look for other peers who
might be able to share wanted content faster. We consider the latter
Elimination of freeriding that began in eMule and was extended in
BitTorrent revolves around the use of tit-for-tat mechanisms by giving
poor service to nodes that do not dedicate enough of their bandwidth
to uploading. Here impairment is done by deliberately scheduling poor
sharers at the rear end of the queue. Another impairment technique
available in protocols like BitTorrent is an early choke
mechanism whereby nodes who are constantly looking for better
connected nodes can drop one of the existing (poor) connections. While
this is done for a selfish purpose, it can address concerns about
nodes that may pretend to have poor connectivity. However, the false
positives will be detrimental to the choked node.
The absence of both local history (between a pair of nodes) and global
history (across nodes) leads to false positives (well behaved nodes
are mistakenly impaired) and false negatives (nodes that don't share
enough receive good treatment). Similar to the TCP case where we
argued for trying impairments on long lasting connections only,
we believe such impairments should only be tried when there is
enough information about the other nodes.
Variants to our proposed impairment notion have been proposed both in
the middle of the network and at the edge in the past. In the TCP
arena, the notion of penalty box has been advanced, as a way of
restricting bandwidth usage by misbehaving flows . At
user-level applications such as Email it is possible to challenge
email senders with a simple puzzle to be solved in order to
distinguish human senders from programs. It is important to note a key
distinction between proposals made in the past and ours: the primary
issue was fairness in the past with a different quality of service
offered to misbehaving flows arising out of, presumably accidental,
misconfigurations. We are more interested in identifying malicious
and deliberate senders of unwanted traffic. Once an email puzzle is
solved the sender is permanently classified as good by being
added to a whitelist thus leading to potential abuse by forging
addresses of whitelist members.
Honeypots [9, 15], a resource whose value lies in its
unauthorized use, advertise unused address space and examine traffic
that arrive there. Honeypots can help identify suspicious IP
addresses . Some honeypots listen
passively, while others actually interact with the traffic by
responding to connection set up attempts. At the other extreme, there
are honeypots that can emulate a kernel keeping the attacker busy with
fake responses. In the past, tarpits  have been deployed
to actually waste resources of suspicious attack sources. However, the
key difference is that virtually all traffic arriving at the dark
address space is known to be unwanted. In the domain of electronic
mail, the MAPS  black list of abusers of email allowed any
system administrator to drop all email coming from these
domains. Again, the impairment here is after the identification of the
(potentially) offensive source.
The throttling of connections suspected not to share an equal fraction
of their bandwidth for uploading in P2P networks, is clearly a form of
impairment. However, such tit-for-tat mechanisms is on a one-to-one
basis and is based on immediate recent history of transactions. Such
a mechanism requires evaluation of all of the node's neighbors. The
impairment can be temporary as choked connections can be unchoked
later on a case-by-case basis. The granularity of impairment is at the
connection level rather than on packets. Even the semantic queuing of
suspected poor sharers by placing them towards the tail end of the
queue can lead to starvation of some nodes indicating absence of
In this section we discuss strengths and limitations of stress
testing: the need to control impairment cost, the scope for
countermeasures, and the trade-offs in using
application level semantics for identification.
Impairment costs must not be so large as to make stress testing
unacceptable for good network users. Other approaches that have been
proposed include doing limited flooding of links that are suspected to
be bearing attack traffic  and the more recent proposal to
attempt to generate challenges to suspected email senders .
In the former case there is a risk of doing more damage than warranted
or beneficial while in the latter zombie machines generating email
may get challenges. We focus on keeping impairment within an SLA
or similar limits.
The total impairment experienced by an entity has two components: the
ambient impairment and the impairment due to stress testing.
The total costs must be kept within acceptable limits. This
requires two things.
Firstly, the acceptable limit must be known. This
may be expressed in terms of SLAs between a
service provider and its customers, that governing the allowable
performance degradation and penalties for deviations therefrom.
However such limits may need to be applied conservatively, since
customers are commonly responsive to degradations beyond an
``effective'' SLA corresponding to the level of service that they
usually receive, which may be better than the worst degradation
allowed under the SLA.
Secondly, the level of ambient impairment must be known. Depending on
the application, this may be known from application level statistics,
e.g. as maintained by Web servers, or from network measurements. In the
latter case, one challenge is to characterize ambient impairments at
the same level of granularity as the stress test itself. Although
commonly reported loss statistics, such as those reported via SNMP, are
aggregated at the link level, a more recent method, Trajectory
Sampling , allows the estimation of loss rate (and sometimes
packet delays) at the level of individual hosts and links.
A strength of stress testing is that the scope for
countermeasures from a bed entity is limited. Firstly, the impairments
in a well-designed stress test are not distinguishable from ambient
impairments, and thus it is difficult to determine that stress testing
is taking place. This likely entails using a full spectrum of likely
impairments (e.g. including both loss and latency) in a suitably
randomized manner that leaves no signature. Secondly, the method is potentially ubiquitous, making
reverse blacklisting by bad entities harder. Thirdly, response to
impairment in an aggressive manner makes it more likely that the entity
will be flagged as suspicious or even identified as bad.
In principle a detailed understanding of application semantics may be
used to construct detailed models of expected user behavior, and
departures therefrom detected. However, this approach becomes more
problematic the further up the protocol stack one goes. Applications
may be deployed in multiple hosts and in different variants; thus it
is necessary to either modify applications or write appropriate
dynamically linked libraries. At the network layer, with the small
number of transport protocols, it is easier to implement impairments.
The trade-off is that with knowledge of application-specification
semantics it is easier to construct narrow and specific impairments to
identify potential attackers. For example, a Web server could track
access patterns on its site and selectively redirect requests for
rarely requested URLs when the number of requests exceed a threshold
or when they come from BGP prefixes never seen before .
However, this requires state maintenance about URL access patterns and
tuning when the site changes.
Below we list various factors to be kept in mind while considering impairment.
They include where impairment should be done, directionality of traffic,
for how long, and how meaningful thresholds on false positives and false
negatives can be generated.
Impairment is done most readily at the receiving network's ingress
point or at the application host. It does not involve any resources in
the middle of the network.
Should impairment be done on incoming traffic only or can we consider
outgoing traffic too? Depending on the application it may well
make sense to impair outgoing traffic. For example, if there is a
suspected virus loose behind a network triggering large volumes of
email or outbound portscans, it might be reasonable to selectively
drop some transactions as a pre-throttling measure. An added benefit
in dealing with outgoing traffic is that there is a better prior
history of patterns to make informed and selective impairments.
Depending on the infrastructure being protected, the cost of false
negatives and false positives, the duration and nature of impairment
will vary within applications. A DNS attack against a typical site is
a lot less problematic than against a CDN infrastructure which depends
on DNS for its entire service. In such instances, cost of a false
negative is high and it may pay to be more aggressive. Thus, where there is a potential of amplification of attacks,
frequency of impairment attempts can go up.
In some cases, the parameters of the stress test can be set based on a
cost analysis related to that in . Suppose that the false
positive and false negative rate for identification are
known and a function of test parameters . This knowledge can
come from analysis of a model of the test, or from empirical
observations, or both. Suppose a proportion p of the total traffic
is believed to be bad traffic. p may be a current estimate based in
the stress testing itself, or as determined by independent
measurements. Then the parameters are chosen so as to minimize
the total cost . Note that the
test parameters automatically adapt to changes in the measured or
expected value of p.
We have outlined a new technique, impairment via stress testing,
to distinguish good and bad traffic. The technique is low cost,
tailored to different applicable layers, and thresholdable to ensure
no service level agreements are violated. The natural concerns of
false positives and negatives are discussed via tunable parameters
governing the stress causing events. The technique is more natural at
the transport layer than others although we can take advantage of the
broader leeway available in SMTP, HTTP, and P2P applications. A key
advantage of stress tests is that attackers cannot easily detect them
and will likely become more visible by trying to counter the measure.
We would like to thank the anonymous referees for their useful
feedback and pointers to earlier related work.
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Stress Testing Traffic to Infer Its Legitimacy
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