USENIX 2006 Annual Technical Conference Refereed Paper|
[USENIX 2006 Annual Technical Conference Technical Program]
Stealth Probing: Efficient Data-Plane Security for IP Routing
Ioannis Avramopoulos and Jennifer Rexford
IP routing is notoriously vulnerable to accidental misconfiguration
and malicious attack. Although secure routing protocols are an
important defense, the data plane must be part of any complete
solution. Existing proposals for secure (link-level) forwarding are
heavy-weight, requiring cryptographic operations at each hop in a
path. Instead, we propose a light-weight data-plane
mechanism (called stealth probing) that monitors the
availability of paths in a secure fashion, while enabling the
management plane to home in on the location of adversaries by
combining the results of probes from different vantage points (called
Byzantine tomography). We illustrate how stealth probing and
Byzantine tomography can be applied in today's routing architecture,
without requiring support from end hosts or internal routers.
Most research and standards activity in secure IP routing has focused
on the routing protocols, rather than the forwarding of data packets.
In this paper, we
propose an operationally viable approach to providing data-plane
security. As an
example of the threats we address, consider an attacker that breaks
into one or more routers, or a disgruntled network operator with
easy access to the routers. The adversary can easily create an
unnoticed disruption by installing access control lists (ACLs) that
selectively discard data traffic, while leaving the routing protocol
intact and allowing probe traffic through. Since the routing protocol
does not verify the operation of the data plane, and because the probes
are delivered successfully, this attack is extremely difficult to diagnose.
Threat Model: We consider a network where a subset of the routers
and links (unknown to the defending entity) are controlled by an adversary.
Using these routers and links, the
adversary can eavesdrop on
tamper with the packet contents, impersonate host
services, misdirect network traffic,
or render the packet-delivery service unavailable
by means of routing-protocol and data-plane attacks. A remote
adversary may also deplete data-plane resources through
denial-of-service attacks; because other techniques, such as fair
queuing and packet filtering, can protect from these
attacks, we do not consider them further.
Our primary goal is to secure routing through data-plane
countermeasures that detect routing-protocol and data-plane attacks
that disrupt packet delivery, assuming arbitrary (or Byzantine)
behavior by the adversary. Ensuring the availability of the network
despite the presence of adversaries prevents financial losses and
other detrimental societal impacts that these adversaries could
otherwise inflict. We do not prevent traffic misdirection attacks
(and, thus, we avoid any associated overheads such as
cryptographically enforcing a route), though we use network encryption
of the data traffic to counter their consequences. Encryption,
furthermore, safeguards against eavesdropping, the tampering of packet
contents, and the impersonation of host services.
Layering of Countermeasures:
In an operationally viable secured routing system, we argue
that we should consider all three dimensions of IP routing, i.e.,
the data, control, and management planes:
The data plane supports packet-forwarding functionality, such as
destination-based forwarding, filtering, and tunneling. The control
plane implements the routing protocols that discover the topology
and select routes. The management plane monitors the network and
configures the routers.
The management and control planes have been the focus of most
countermeasures to Byzantine failures.
Management Plane: As a management-plane countermeasure,
operators can apply Best Common Practices (BCPs) for securing their
infrastructure and filtering suspicious route announcements. These
BCPs reduce the likelihood of attacks but cannot prevent them
entirely; in addition, an end-to-end path often
traverses multiple networks, including some that do not apply BCPs.
Control Plane: Secure routing
protocols (14,10,18,8) ensure that valid
routing advertisements correctly identify the links between non-faulty
routers (or Autonomous Systems). However, these protocols do not
prevent false announcements that faulty routers are connected to other
faulty routers, as in collusion (or wormhole) attacks (18,8).
Wormholes along with malicious ACLs can create invisible black holes
for the data traffic. Because routing protocols do not verify
forwarding behavior, even if perfectly secure routing protocols were
deployed, availability could still be compromised.
This risk can be mitigated by incorporating secure forwarding
functionality in the routing system.
Data Plane: Secure forwarding protocols such
as (4,2) and the protocol
in (13) provide availability monitoring and secure fault
localization at the link level. The fine granularity leads to
high overhead and complexity (e.g., path-specific authentication and,
in certain cases (4,2), the distribution of
pairwise router keys) inappropriate for a generic forwarding paradigm.
Although useful for failure recovery, fault localization should not
overburden the data plane. We advocate that availability monitoring
and fault localization should be cleanly separated, into the data
plane ( stealth probing) and management plane ( Byzantine
Stealth Probing: The stealth-probing protocol we propose in this
paper is a data-plane availability monitor. It determines whether a
router-to-router path is operational, even if an adversary controls
intermediate routers and tries to evade detection.
Stealth probing creates an encrypted tunnel between two end-routers
and diverts both the data and probe traffic into the tunnel. Since
the data and probe packets are indistinguishable, the adversary cannot
drop data packets without dropping the probes as well, making
it difficult to evade detection. Rather than
requiring ubiquitous deployment, stealth probing could be
deployed ``as needed'' to protect critical traffic between selected
Stealth probing offers several key practical advantages. First,
stealth probing is incrementally deployable. Because of its
end-router-to-end-router design, stealth probing does not require
support from legacy routers in the core of an ISP network or
intermediate ASes in an interdomain path. Networks that adopt stealth
probing will see immediate benefits even in limited deployment
scenarios. Second, stealth probing is backward compatible
with the existing infrastructure, since the tunnels do not require
any support from the internal routers. Finally, stealth probing is
incentive compatible. Service providers can use the encrypted
tunnels to provide other value-added services, such as secure Virtual
Private Networks (VPNs), to customers. Encrypted tunnels also protect
users from a broader range of attacks such as eavesdropping,
tampering, traffic analysis, and misdirection.
Stealth probing is a secure data-plane monitoring tool that relies on
the efficient symmetric cryptographic protection of the IPsec protocol
suite, applied in an end-router-to-end-router fashion. In this
section, we first discuss the limitations of other approaches to
secure data-plane monitoring, followed by an overview of stealth
probing. Then, we describe how stealth probing works in greater
2. Stealth Probing
Stealth probing addresses the problem of securely deciding
whether a node-to-node path correctly delivers data packets from one end
of the path to the other. An adversary that is present at one or more
intermediate nodes of the path must not be able to coerce a false decision.
Furthermore, the overhead of the decision process must be practical for
deployment in operational networks.
Consider two routers and . Lets assume for simplicity that
is a source of data traffic for which is a sink. We want to verify
that this traffic is flowing properly in the forward
Probing One approach to meet our objective is for router to
send to one or more ICMP echo requests and infer the fate
of data traffic based on the receipt of ICMP echo replies. This
method is non-intrusive since it reaches a
decision with a small number of probes. However, if an adversary
is present in the path between and , he can selectively drop
data packets and avoid detection by selectively forwarding echo
requests and replies.
Cumulative network-layer ACKs In a second approach,
explicitly acknowledges receipt of a bundle of data packets from
by a cumulative ACK that contains a count of the received
if an adversary is present in the path between and , he can
drop data packets and avoid detection by forging destination
ACKs. So, let's further assume that and share a
secret key. Using this key, we can prevent this attack by
requiring to authenticate data packets by means of a message
authentication code (MAC) and to authenticate
ACKs in the same way. However, packet counts are insufficient to
determine the timeliness of data delivery and, therefore,
this scheme is vulnerable to an adversary that delays
packets. Furthermore, packet counts are insufficient to detect
selective attacks that target individual IP addresses (or
Transport-layer ACKs A third approach is to use a secure
layer protocol such as TLS (Transport Layer Security) (5).
However, because this scheme cannot differentiate between host
and router failures, it would suffer from ``false alarms''
due to host failures that would complicate fault localization by
the management plane.
Traceroute A fourth approach is
that adopted by traceroute that uses ICMP
``time exceeded'' and ``port unreachable'' messages to either
determine the full path from a source to a destination or identify
the last router before a black hole. Traceroute has fine
link-level detection granularity but cannot prevent the
preferential treatment of its packets by an adversary who can
in this way avoid detection.
In addition, many ISPs disable their routers from sending ICMP
2.1 Limitations of Strawman Designs
Stealth probing has a ``minimalist'' design: It enables
recovery from routing attacks and
misconfigurations by offering secure path-level failure detection
capability, keeping the data-plane support to a minimum.
The idea in stealth probing is to use probes to reach a secure decision
on the fate of data traffic by establishing an encrypted and authenticated
tunnel between two routers in the traffic's path and diverting both
the data traffic and the probes into this tunnel. Encryption
conceals probing traffic so that it is indistinguishable from
data traffic, and authentication makes the tampering of data
traffic detectable. Probing can be either active or passive:
2.2 Minimal Secure Data Plane Monitor
Stealth probing has the following primary benefits:
- Active probing uses ICMP echo requests and replies. The size of
echo requests is concealed using padding to decrease the number of
distinct data-packet sizes. Echo request sizes are then chosen to
match data-packet sizes, and inter-probing intervals are randomly
- In passive probing, the tunnel entry and exit points agree on an
efficient (non-cryptographic) hash function to be applied on the
immutable fields of each packet--before encryption at the entry point
and after decryption at the exit point. If the image of the hash is
less than an agreed-upon value, the corresponding data packet serves
as an implicit probe that the tunnel exit point must acknowledge. This
method is akin to trajectory sampling (6); a Bloom
filter may be used to compress the ACKs, similar to how hashes are
compressed in (17).
Stealth probing has the following secondary benefits:
- Because stealth probing is an end-router-to-end-router
failure detection mechanism, intermediate routers of a monitored
path do not need to explicitly support the stealth probe.
Therefore, stealth probing can be deployed across legacy
routers and over interdomain paths.
- Stealth probing is non-intrusive. The processing
requirements at tunnel endpoints (outlined in Section 2.3)
are simple and the probing overhead is minimal. Intermediate routers
do not process tunneled packets as they are tunnel agnostic.
- By measuring the round-trip-times of probing traffic, attacks
that delay packets are detectable.
- By hiding the source and destination IP addresses of the data
traffic, encryption prevents attacks that target individual IP
- By making the TCP mechanism opaque, encryption mitigates
attacks that exploit the TCP mechanism.
- The use of tunnels permits selectivity in the traffic
that is protected. The management plane can configure packet
classifiers that identify the critical traffic and direct only the
matching packets into the encrypted tunnels.
- Encryption at the edge routers of a network infrastructure (even
if selectively applied) (a) prevents the eavesdropping of unencrypted
host-to-host communications, (b) prevents traffic-analysis attacks that
host-to-host encryption does not prevent, for example, by hiding the source
and destination addresses of data traffic, (c) precludes the adversary
from impersonating the services of the receiving host, (d) renders
misdirection attacks that divert traffic to adversarially controlled locations
for eavesdropping and traffic analysis ineffective, and (e) enables ISPs to
offer value-added services like VPNs.
- Stealth probing enforces fate sharing between data traffic and
probes, which is broadly useful for troubleshooting network problems.
For example, simple ICMP echo requests and replies may be treated
differently from data packets either because of MTU size limits or
packet filters that discard traffic based on the protocol or port
numbers. Stealth probing avoids this problem by tunneling all traffic
and matching the packet sizes of data and probe traffic (e.g., due to
the padding step in active probing or the random packet sampling
in passive probing).
- Tunnels are broadly useful for controlling the
flow of traffic in an AS (e.g., for traffic engineering).
Stealth probing requires the endpoints of a path to share a
secret and use this secret to create an IPsec tunnel. This
section charts the workings of the IPsec protocol suite and the
process that directs packets into tunnels.
IPsec protocol suite: IPsec provides end-to-end cryptographic
protection at the IP layer. The communicating parties--the tunnel
end-points--use the Internet Key Exchange (IKE) protocol (7)
to negotiate the establishment of a Security Association (SA). IKE
relies on preshared secret keys or the public keys of an associated
Public Key Infrastructure (PKI). In intradomain routing, key exchange
can be assumed by a domain's authority; in interdomain routing, key
exchange should not depend on a single central authority. Due to its
end-to-end design, stealth probing does not depend on such authority.
Following the SA establishment, IP packets are protected using an
Encapsulating Security Payload (ESP) module (9). Using
tunnel-mode ESP, the tunnel entry point adds an outer IP header to
each packet, followed by the ESP header and trailer. ESP provides
encryption using a standard encryption algorithm and ensures
authenticity and integrity using a standard MAC. The tunnel exit
point removes the outer IP header and restores the inner IP packet by
inverting the encryption. Stealth probing, therefore, relies only on
efficient symmetric cryptographic primitives. Thus, packet processing
can proceed at the line speeds of core routers. Commercial routers
increasingly offer such encryption capabilities.
Directing packets into tunnels: The management plane configures
packet classifiers to specify which traffic should enter the tunnel,
based on the five-tuples of source and destination address prefixes,
port numbers, and protocol numbers. Tunnels are deployed across the
network to match this specification (see Section 3).
For protected packets, a longest-prefix-match table lookup will
determine the tunnel exit point, based on a packet's destination
address. A simple table lookup will then retrieve the associated
encryption key needed to encapsulate the packet.
In this section, we present two deployment scenarios for stealth
probing. First, we describe how an ISP can deploy stealth probing to
secure its own infrastructure. Then, we discuss how a pair of edge
networks can deploy stealth probing to secure the path through
untrusted ASes in the Internet.
Identifying tunnel endpoints:
An ISP network typically has a periphery (i.e., edge routers
that aggregate customer, transit, and peering traffic) and a
core that interconnects the edge routers. The edge routers are an
apt location to deploy stealth probing to leverage the benefits of an
end-to-end design. First, core routers can be tunnel-agnostic and
need only support simple destination-based forwarding and, second,
processing requirements are distributed over a large number of edge
The management plane can configure five-tuples to identify the
protected traffic, as discussed in Section 2.3.
The tunnel exit point corresponds to the next-hop attribute of the
chosen BGP route for the destination prefix. A longest prefix match
on a packet's destination address will determine the tunnel exit point
(i.e., the egress router), and a simple table lookup returns the
appropriate encryption key. In terms of scale, a large ISP network
might have a few hundred edge routers, resulting in a few hundred keys
and a few hundred tunnels per ingress point (i.e., one per egress
router). Compared to the standard forwarding-table lookup that must
be performed for each IP packet, the overhead of retrieving the keys
is low; in fact, the forwarding table could store a pointer to the
appropriate key for each prefix.
Byzantine tomography: In a network under attack, stealth probes
detect the dysfunctional paths. Armed with this knowledge, the
management plane can identify the compromised routers and recover from
the attack. In the simplest case, the management plane can
reconfigure or reboot the compromised routers, or reinstall the
routers' operating system. Fine-grained detection of the compromised
routers is useful to avoid the unnecessary downtime caused by false
Byzantine tomography estimates the compromised routers by
combining stealth probing output from multiple vantage points.
Byzantine tomography generalizes the notion of network tomography,
which identifies the loss rates of network links using end-to-end
probing traffic, by assuming that (the unknown) malicious routers may
lie about their collected measurements. Byzantine tomography
minimizes, over all possible faulty compositions, the number of faulty
routers that explain the faulty paths observed in stealth probing.
Algorithmically, this is an instance of the Minimum Hitting Set (MHS)
problem: If is the set of routers in the network and is the
collection of paths (subsets of ) that are faulty, a hitting
set for is a subset of such that contains at least
one element from each path in . MHS can be solved using one of the
algorithms presented in (11,3).
The adversary's goal is to disorient the management plane into false
detections. For example, the adversary can instruct the compromised
routers to spuriously report certain paths as dysfunctional. If we
routers to cryptographically sign their stealth-probing reports, a
compromised router could not forge a bogus report for another router.
As such, these reports could only identify paths that include the faulty
router, making these reports accurate because the path does indeed
contain a compromised router!
3. Deployment Scenarios
An adversary could also try to thwart the management system by selectively
discarding packets traversing a small number of paths, making it
difficult for Byzantine tomography to have fault reports from enough
vantage points to identify the compromised routers. However, in doing
so, the adversary also confines the scope of attacks. The more selective
the adversary is in dropping
packets (to evade detection), the less extensive the damage of the
attack. In addition, even if Byzantine tomography cannot uniquely
identify the faulty routers, the network operators could easily take
corrective action based on a set of suspected routers. For
example, the operators could reconfigure the remaining routers to
select paths that circumvent the suspected routers, or reboot each
of the suspected routers.
Securing interdomain routing is arguably harder than securing
intradomain routing for two reasons. First, without a trusted central
authority, key distribution is more challenging. Second, the
compromised routers might reside in a remote AS outside the control of
the communicating edge networks, making fault localization and fault
recovery more challenging. An interdomain deployment can address
these challenges through a small-scale key distribution (between
selected edge networks) and coarse-grain rerouting (through techniques
commonly used for intelligent route control).
Incremental deployability: ASes willing to deploy stealth
probing over interdomain paths can engage in bilateral or small-scale
multilateral agreements, and exchange pairwise keys either manually or
by small-scale PKIs. ISPs have an incentive to join small
groups, both to provide value-added services (such as multi-site VPNs)
and to securely detect connectivity problems (to ensure higher
availability for their services). Because stealth probing can be
deployed across tunnel-agnostic legacy routers, early adopters will
see an immediate benefit without requiring the participation of
intermediate ASes. In fact, stealth probing enables the participating
ASes to provide secure service, despite the presence of untrusted ASes
in the rest of the Internet. The economic return to the early adopters
can provide an incentive for other ASes to join these groups. As more
ASes join these groups, scalable key distribution could be addressed
through a larger PKI or a distributed trust model.
Circumventing the compromised routers: Although securely
detecting routing failures is an important capability in its own
right, the ability to bypass the affected routers is important as
well. However, in interdomain routing, the communicating edge networks
might not be able to identify the specific routers (or ASes) that have
been compromised. Instead, the tunnel end points can adapt by
directing the tunneled traffic on a different path, in the hope of
circumventing the compromised routers, following techniques used in
intelligent route control (1). For example, consider two
stub ASes, and , and assume that is
-multihomed and is -multihomed. (For simplicity, also
assume that each of and has a single border router.)
can choose among
different BGP paths to
forward traffic from to . Choosing any of the
outgoing links is straightforward for . Furthermore, any of the
incoming links to can be chosen as follows:
advertises a different primary prefix to each of its
providers, and destination addresses from each of these prefixes are
used to terminate tunnels between the border routers of
and . can, thus, direct traffic via any of the
incoming links to by choosing the remote tunnel end-point
address accordingly. selects the reverse path to in the
same manner. In this setting, stealth probing can detect which of the
paths contain compromised routers, and the edge
networks can switch to a working path.
3.2 Interdomain Routing
4. Related Work
Encryption to make data and control traffic
indistinguishable was first suggested by Perlman (16), who
proposed hop-by-hop encryption between neighboring routers to hide
beaconing traffic and prevent ``man-in-the-middle'' attacks on
the topology-discovery process. The novelty of stealth
probing is in applying this general idea to the paths between
end-routers to identify data-plane problems in a secure
fashion. Perlman also proposed recovery from routing
attacks using multipath routing and disjoint paths. Stealth probing is
well-suited for monitoring the quality of active paths in order to dynamically
recompute the active path set.
The Fatih (13) secure data-plane monitor can adjust detection
granularity from link-level to path-level for lower overhead.
However, (13) does not propose a fault-localization mechanism
to compensate for the reduced detection level, and the scheme also
requires synchronized clocks. In our proposal, fault localization is
attained using Byzantine tomography and we do not rely on clock
Secure traceroute (15) is a link-level detection scheme that
could conceivably be applied at the path level. Secure traceroute is
based on secret identifiers embedded in packets that single out those
packets as probes, which elicit responses for detecting reachability.
However, this scheme may fail to detect attacks that target low-rate
components of the aggregate traffic in a path or attacks that exploit
the TCP mechanism. By encrypting traffic, stealth probing prevents
those attacks. Secure traceroute could conceivably be extended into a
hybrid scheme where the sender initiates link-level detection only after
path-level probing suggests a problem. However, an adversary could
easily thwart the on-demand link-level detection by limiting the
duration of attacks; in addition, such a hybrid scheme would still
require pairwise keys between the routers. In contrast, stealth
probing, combined with Byzantine tomography, can pinpoint even
short-lived attacks and does not require per-hop keys.
Other recent proposals, such as Listen (18) and
Feedback-Based Routing (19), detect data-plane attacks by
monitoring traffic at the TCP level. However, these techniques would
falsely detect an unavailable path to a prefix as workable, if an
adversary impersonates hosts in the monitored prefix.
In this paper, we presented stealth probing and Byzantine tomography
as effective ways to protect against network-availability attacks
without overburdening the data plane. We also showed how these
techniques can be applied in the Internet's existing routing system,
without changing the end hosts or the internal routers.
In the future, we will explore ``clean-slate'' secure routing system
particular, we will study whether more flexible path-selection
schemes, such as source routing,
are necessary, or whether
coarse-grained path selection is sufficient for secure routing. We will
also explore the many security benefits of encrypting the data
traffic between edge networks, and study how to balance the
trade-offs between host-based and network-based encryption for
providing secure Internet services.
The authors would like to thank Constantinos Dovrolis, Nick Feamster,
Barath Raghavan, Alex Snoeren, and the anonymous reviewers for their
invaluable feedback. Ioannis Avramopoulos has been supported by a
grant from the New Jersey Center for Wireless and Internet Security
and a wireless testbed project (ORBIT) grant from the National Science
Foundation. Jennifer Rexford was supported by Homeland Security
Advanced Research Project Agency grant 1756303.
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