A NICE Way to Test OpenFlow Applications Marco Canini , Daniele Venzano

A NICE Way to Test OpenFlow Applications Marco Canini , Daniele Venzano
A NICE Way to Test OpenFlow Applications
Marco Canini⋆ , Daniele Venzano⋆ , Peter Perešı́ni⋆ , Dejan Kostić⋆ , and Jennifer Rexford†
The emergence of OpenFlow-capable switches enables
exciting new network functionality, at the risk of programming errors that make communication less reliable.
The centralized programming model, where a single controller program manages the network, seems to reduce
the likelihood of bugs. However, the system is inherently
distributed and asynchronous, with events happening at
different switches and end hosts, and inevitable delays
affecting communication with the controller. In this paper, we present efficient, systematic techniques for testing unmodified controller programs. Our NICE tool applies model checking to explore the state space of the entire system—the controller, the switches, and the hosts.
Scalability is the main challenge, given the diversity of
data packets, the large system state, and the many possible event orderings. To address this, we propose a novel
way to augment model checking with symbolic execution of event handlers (to identify representative packets that exercise code paths on the controller). We also
present a simplified OpenFlow switch model (to reduce
the state space), and effective strategies for generating
event interleavings likely to uncover bugs. Our prototype tests Python applications on the popular NOX platform. In testing three real applications—a MAC-learning
switch, in-network server load balancing, and energyefficient traffic engineering—we uncover eleven bugs.
While lowering the barrier for introducing new functionality into the network, Software Defined Networking
(SDN) also raises the risks of software faults (or bugs).
Even today’s networking software—written and extensively tested by equipment vendors, and constrained
(at least somewhat) by the protocol standardization
process—can have bugs that trigger Internet-wide outages [1, 2]. In contrast, programmable networks will offer a much wider range of functionality, through software
created by a diverse collection of network operators and
Princeton University
third-party developers. The ultimate success of SDN,
and enabling technologies like OpenFlow [3], depends
on having effective ways to test applications in pursuit
of achieving high reliability. In this paper, we present
NICE, a tool that efficiently uncovers bugs in OpenFlow
programs, through a combination of model checking and
symbolic execution. Building on our position paper [4]
that argues for automating the testing of OpenFlow applications, we introduce several new contributions summarized in Section 1.3.
1.1 Bugs in OpenFlow Applications
An OpenFlow network consists of a distributed collection of switches managed by a program running on a
logically-centralized controller, as illustrated in Figure 1.
Each switch has a flow table that stores a list of rules
for processing packets. Each rule consists of a pattern
(matching on packet header fields) and actions (such as
forwarding, dropping, flooding, or modifying the packets, or sending them to the controller). A pattern can require an “exact match” on all relevant header fields (i.e.,
a microflow rule), or have “don’t care” bits in some fields
(i.e., a wildcard rule). For each rule, the switch maintains traffic counters that measure the bytes and packets
processed so far. When a packet arrives, a switch selects
the highest-priority matching rule, updates the counters,
and performs the specified action(s). If no rule matches,
the switch sends the packet header to the controller and
awaits a response on what actions to take. Switches also
send event messages, such as a “join” upon joining the
network, or “port change” when links go up or down.
The OpenFlow controller (un)installs rules in the
switches, reads traffic statistics, and responds to events.
For each event, the controller program defines a handler,
which may install rules or issue requests for traffic statistics. Many OpenFlow applications1 are written on the
NOX controller platform [5], which offers an OpenFlow
1 In this paper, we use the terms “OpenFlow application” and “controller program” interchangeably.
Install rule
Host A
Host B
Switch 1
Switch 2
Figure 1: An example of OpenFlow network traversed by
a packet. In a plausible scenario, due to delays between
controller and switches, the packet does not encounter an
installed rule in the second switch.
API for Python and C++ applications. These programs
can perform arbitrary computation and maintain arbitrary
state. A growing collection of controller applications
support new network functionality [6–11], over OpenFlow switches available from several different vendors.
Our goal is to create an efficient tool for systematically
testing these applications. More precisely, we seek to
discover violations of (network-wide) correctness properties due to bugs in the controller programs.
On the surface, the centralized programming model
should reduce the likelihood of bugs. Yet, the system
is inherently distributed and asynchronous, with events
happening at multiple switches and inevitable delays affecting communication with the controller. To reduce
overhead and delay, applications push as much packethandling functionality to the switches as possible. A
common programming idiom is to respond to a packet
arrival by installing a rule for handling subsequent packets in the data plane. Yet, a race condition can arise if
additional packets arrive while installing the rule. A program that implicitly expects to see just one packet may
behave incorrectly when multiple arrive [4]. In addition,
many applications install rules at multiple switches along
a path. Since rules are not installed atomically, some
switches may apply new rules before others install theirs.
Figure 1 shows an example where a packet reaches an
intermediate switch before the relevant rule is installed.
This can lead to unexpected behavior, where an intermediate switch directs a packet to the controller. As a result, an OpenFlow application that works correctly most
of the time can misbehave under certain event orderings.
1.2 Challenges of Testing OpenFlow Apps
Testing OpenFlow applications is challenging because
the behavior of a program depends on the larger environment. The end-host applications sending and receiving traffic—and the switches handling packets, installing
rules, and generating events—all affect the program running on the controller. The need to consider the larger environment leads to an extremely large state space, which
“explodes” along three dimensions:
Large space of switch state: Switches run their own
programs that maintain state, including the many packetprocessing rules and associated counters and timers. Further, the set of packets that match a rule depends on the
presence or absence of other rules, due to the “match the
highest-priority rule” semantics. As such, testing OpenFlow applications requires an effective way to capture
the large state space of the switch.
Large space of input packets: Applications are dataplane driven, i.e., programs must react to a huge space
of possible packets. The OpenFlow specification allows switches to match on source and destination MAC
addresses, IP addresses, and TCP/UDP port numbers,
as well as the switch input port; future generations of
switches will match on even more fields. The controller
can perform arbitrary processing based on other fields,
such as TCP flags or sequence numbers. As such, testing OpenFlow applications requires effective techniques
to deal with large space of inputs.
Large space of event orderings: Network events,
such as packet arrivals and topology changes, can happen
at any switch at any time. Due to communication delays,
the controller may not receive events in order, and rules
may not be installed in order across multiple switches.
Serializing rule installation, while possible, would significantly reduce application performance. As such, testing OpenFlow applications requires efficient strategies to
explore a large space of event orderings.
To simplify the problem, we could require programmers to use domain-specific languages that prevent certain classes of bugs. However, the adoption of new languages is difficult in practice. Not surprisingly, most
OpenFlow applications are written in general-purpose
languages, like Python, Java. Alternatively, developers
could create abstract models of their applications, and
use formal-methods techniques to prove properties about
the system. However, these models are time-consuming
to create and easily become out-of-sync with the real implementation. In addition, existing model-checking tools
like SPIN [12] and Java PathFinder (JPF) [13] cannot be
directly applied because they require explicit developer
inputs to resolve the data-dependency issues and sophisticated modeling techniques to leverage domain-specific
information. They also suffer state-space explosion, as
we show in Section 7. Instead, we argue that testing
tools should operate directly on unmodified OpenFlow
applications, and leverage domain-specific knowledge to
improve scalability.
1.3 NICE Research Contributions
To address these scalability challenges, we present NICE
(No bugs In Controller Execution)—a tool that tests unmodified controller programs by automatically generating carefully-crafted streams of packets under many possible event interleavings. To use NICE, the programmer
Traces of
Figure 2: Given an OpenFlow program, a network topology, and correctness properties, NICE performs a statespace search and outputs traces of property violations.
supplies the controller program, and the specification of
a topology with switches and hosts. The programmer can
instruct NICE to check for generic correctness properties
such as no forwarding loops or no black holes, and optionally write additional, application-specific correctness
properties (i.e., Python code snippets that make assertions about the global system state). By default, NICE
systematically explores the space of possible system behaviors, and checks them against the desired correctness
properties. The programmer can also configure the desired search strategy. In the end, NICE outputs property
violations along with the traces to deterministically reproduce them. The programmer can also use NICE as a
simulator to perform manually-driven, step-by-step system executions or random walks on system states.
Our design uses explicit state, software model checking [13–16] to explore the state space of the entire system—the controller program, the OpenFlow
switches, and the end hosts—as discussed in Section 2.
However, applying model checking “out of the box” does
not scale. While simplified models of the switches and
hosts help, the main challenge is the event handlers in
the controller program. These handlers are data dependent, forcing model checking to explore all possible inputs (which doesn’t scale) or a set of “important” inputs provided by the developer (which is undesirable).
Instead, we extend model checking to symbolically execute [17, 18] the handlers, as discussed in Section 3.
By symbolically executing the packet-arrival handler,
NICE identifies equivalence classes of packets—ranges
of header fields that determine unique paths through the
code. NICE feeds the network a representative packet
from each class by adding a state transition that injects
the packet. To reduce the space of event orderings, we
propose several domain-specific search strategies that
generate event interleavings that are likely to uncover
bugs in the controller program, as discussed in Section 4.
Bringing these ideas together, NICE combines model
checking (to explore system execution paths), symbolic
execution (to reduce the space of inputs), and search
strategies (to reduce the space of event orderings). The
programmer can specify correctness properties as snippets of Python code that operate on system state, or se-
lect from a library of common properties, as discussed in
Section 5. Our NICE prototype tests unmodified applications written in Python for the popular NOX platform,
as discussed in Section 6. Our performance evaluation in
Section 7 shows that: (i) even on small examples, NICE
is five times faster than approaches that apply state-ofthe-art tools, (ii) our OpenFlow-specific search strategies reduce the state space by up to 20 times, and (iii)
the simplified switch model brings a 7-fold reduction on
its own. In Section 8, we apply NICE to three real OpenFlow applications and uncover 11 bugs. Most of the bugs
we found are design flaws, which are inherently less numerous than simple implementation bugs. In addition,
at least one of these applications was tested using unit
tests. Section 9 discusses the trade-off between testing
coverage and the overhead of symbolic execution. Section 10 discusses related work, and Section 11 concludes
the paper with a discussion of future research directions.
Model Checking OpenFlow Applications
The execution of a controller program depends on the underlying switches and end hosts; the controller, in turn,
affects the behavior of these components. As such, testing is not just a simple matter of exercising every path
through the controller program—we must consider the
state of the larger system. The needs to systematically
explore the space of system states, and check correctness
in each state, naturally lead us to consider model checking techniques. To apply model checking, we need to
identify the system states and the transitions from one
state to another. After a brief review of model checking, we present a strawman approach for applying model
checking to OpenFlow applications, and proceed by describing changes that make it more tractable.
2.1 Background on Model Checking
Modeling the state space. A distributed system consists of multiple components that communicate asynchronously over message channels, i.e., first-in, first-out
buffers (e.g., see Chapter 2 of [19]). Each component has
a set of variables, and the component state is an assignment of values to these variables. The system state is the
composition of the component states. To capture in-flight
messages, the system state also includes the contents of
the channels. A transition represents a change from one
state to another (e.g., due to sending a message). At any
given state, each component maintains a set of enabled
transitions, i.e., the state’s possible transitions. For each
state, the enabled system transitions are the union of enabled transitions at all components. A system execution
corresponds to a sequence of these transitions, and thus
specifies a possible behavior of the system.
Model-checking process. Given a model of the state
space, performing a search is conceptually straightfor-
ward. Figure 5 (non boxed-in text) shows the pseudocode of the model-checking loop. First, the model
checker initializes a stack of states with the initial state of
the system. At each step, the checker chooses one state
from the stack and one of its enabled transitions. After
executing that transition, the checker tests the correctness properties on the newly reached state. If the new
state violates a correctness property, the checker saves
the error and the execution trace. Otherwise, the checker
adds the new state to the set of explored states (unless
the state was added earlier) and schedules the execution
of all transitions enabled in this state (if any). The model
checker can run until the stack of states is empty, or until
detecting the first error.
2.2 Transition Model for OpenFlow Apps
Model checking relies on having a model of the system,
i.e., a description of the state space. This requires us to
identify the states and transitions for each component—
the controller program, the OpenFlow switches, and the
end hosts. However, we argue that applying existing
model-checking techniques imposes too much work on
the developer and leads to an explosion in the state space.
2.2.1 Controller Program
Modeling the controller as a transition system seems relatively straightforward. A controller program is structured as a set of event handlers (e.g., packet arrival and
switch join/leave for the MAC-learning application in
Figure 3), that interact with the switches using a standard interface, and these handlers execute atomically. As
such, we can model the state of the program as the values
of its global variables (e.g., ctrl state in Figure 3),
and treat each event handler as a transition. To execute a
transition, the model checker can simply invoke the associated event handler. For example, receiving a packet-in
message from a switch enables the packet in transition, and the model checker can execute the transition by
invoking the corresponding event handler.
However, the behavior of event handlers is often datadependent. In line 7 of Figure 3, for instance, the
packet in handler assigns mactable only for unicast source MAC addresses, and either installs a forwarding rule or floods a packet depending on whether or not
the destination MAC address is known. This leads to different system executions. Unfortunately, model checking does not cope well with data-dependent applications
(e.g., see Chapter 1 of [19]). Since enumerating all possible inputs is intractable, a brute-force solution would
require developers to specify a set of “relevant” inputs
based on their knowledge of the application. Hence, a
controller transition would be modeled as a pair consisting of an event handler and a concrete input. This
is clearly undesirable. NICE overcomes this limitation
ctrl state = {} # State of the controller is a global variable (a hashtable)
def packet in(sw id, inport, pkt, bufid): # Handles packet arrivals
mactable = ctrl state[sw id]
is bcast src = pkt.src[0] & 1
is bcast dst = pkt.dst[0] & 1
if not is bcast src:
mactable[pkt.src] = inport
if (not is bcast dst) and (mactable.has key(pkt.dst)):
outport = mactable[pkt.dst]
if outport != inport:
match = {DL SRC: pkt.src, DL DST: pkt.dst, ←֓
DL TYPE: pkt.type, IN PORT: inport}
actions = [OUTPUT, outport]
install rule(sw id, match, actions, soft timer=5, ←֓
hard timer=PERMANENT) # 2 lines optionally
send packet out(sw id, pkt, bufid) # combined in 1 API
flood packet(sw id, pkt, bufid)
17 def switch join(sw id, stats): # Handles when a switch joins
if not ctrl state.has key(sw id):
ctrl state[sw id] = {}
20 def switch leave(sw id): # Handles when a switch leaves
if ctrl state.has key(sw id):
del ctrl state[sw id]
Figure 3: Pseudo-code of a MAC-learning switch, based
on the pyswitch application. The packet in handler
learns the input port associated with each non-broadcast
source MAC address; if the destination MAC address is
known, the handler installs a forwarding rule and instructs
the switch to send the packet according to that rule; and
otherwise floods the packet. The switch join/leave events
initialize/delete a table mapping addresses to switch ports.
by using symbolic execution to automatically identify the
relevant inputs, as discussed in Section 3.
2.2.2 OpenFlow Switches
To test the controller program, the system model must
include the underlying switches. Yet, switches run complex software, and this is not the code we intend to test.
A strawman approach for modeling the switch is to start
with an existing reference OpenFlow switch implementation (e.g., [20]), define the switch state as the values
of all variables, and identify transitions as the portions
of the code that process packets or exchange messages
with the controller. However, the reference switch software has a large amount of state (e.g., several hundred
KB), not including the buffers containing packets and
OpenFlow messages awaiting service; this aggravates the
state-space explosion problem. Importantly, such a large
program has many sources of nondeterminism and it is
difficult to identify them automatically [16].
Instead, we create a switch model that omits inessential details. Indeed, creating models of some parts of the
system is common to many standard approaches for applying model checking. Further, in our case, this is a onetime effort that does not add burden on the user. Following the OpenFlow specification [21], we view a switch as
a set of communication channels, transitions that handle
data packets and OpenFlow messages, and a flow table.
Simple communication channels: Each channel is
a first-in, first-out buffer. Packet channels have an
optionally-enabled fault model that can drop, duplicate,
or reorder packets, or fail the link. The channel with the
controller offers reliable, in-order delivery of OpenFlow
messages, except for optional switch failures. We do not
run the OpenFlow protocol over SSL on top of TCP/IP,
allowing us to avoid intermediate protocol encoding/decoding and the substantial state in the network stack.
Two simple transitions: The switch model supports
process pkt and process of transitions—for processing data packets and OpenFlow messages, respectively. We enable these transitions if at least one packet
channel or the OpenFlow channel is non empty, respectively. A final simplification we make is in the
process pkt transition. Here, the switch dequeues
the first packet from each packet channel, and processes
all these packets according to the flow table. So, multiple packets at different channels are processed as a single
transition. This optimization is safe because the model
checker already systematically explores the possible orderings of packet arrivals at the switch.
Merging equivalent flow tables: A flow table can easily have two states that appear different but are semantically equivalent, leading to a larger search space than
necessary. For example, consider a switch with two microflow rules. These rules do not overlap—no packet
would ever match both rules. As such, the order of these
two rules is not important. Yet, simply storing the rules
as a list would cause the model checker to treat two different orderings of the rules as two distinct states. Instead, as often done in model checking, we construct a
canonical representation of the flow table that derives a
unique order of rules with overlapping patterns.
2.2.3 End Hosts
Modeling the end hosts is tricky, because hosts run arbitrary applications and protocols, have large state, and
have behavior that depends on incoming packets. We
could require the developer to provide the host programs, with a clear indication of the transitions between
states. Instead, NICE provides simple programs that act
as clients or servers for a variety of protocols including
Ethernet, ARP, IP, and TCP. These models have explicit
transitions and relatively little state. For instance, the default client has two basic transitions—send (initially enabled; can execute C times, where C is configurable) and
receive—and a counter of sent packets. The default
server has the receive and the send reply transitions; the latter is enabled by the former. A more realistic refinement of this model is the mobile host that includes the move transition that moves the host to a new
<switch, port> location. The programmer can also customize the models we provide, or create new models.
Symbolic Execution of Event Handlers
To systematically test the controller program, we must
explore all of its possible transitions. Yet, the behavior
of an event handler depends on the inputs (e.g., the MAC
addresses of packets in Figure 3). Rather than explore
all possible inputs, NICE identifies which inputs would
exercise different code paths through an event handler.
Systematically exploring all code paths naturally leads us
to consider symbolic execution (SE) techniques. After a
brief review of symbolic execution, we describe how we
apply symbolic execution to controller programs. Then,
we explain how NICE combines model checking and
symbolic execution to explore the state space effectively.
3.1 Background on Symbolic Execution
Symbolic execution runs a program with symbolic variables as inputs (i.e., any values). The symbolic-execution
engine tracks the use of symbolic variables and records
the constraints on their possible values. For example, in
line 4 of Figure 3, the engine learns that is bcast src
is “pkt.src[0] & 1”. At any branch, the engine
queries a constraint solver for two assignments of symbolic inputs—one that satisfies the branch predicate and
one that satisfies its negation (i.e., takes the “else”
branch)— and logically forks the execution to follow the
feasible paths. For example, the engine determines that
to reach line 7 of Figure 3, the source MAC address must
have its eighth bit set to zero.
Unfortunately, symbolic execution does not scale well
because the number of code paths can grow exponentially with the number of branches and the size of the inputs. Also, symbolic execution does not explicitly model
the state space, which can cause repeated exploration
of the same system state2 . In addition, despite exploring all code paths, symbolic execution does not explore
all system execution paths, such as different event interleavings. Techniques exist that can add artificial branching points to a program to inject faults or explore different event orderings [18, 22], but at the expense of
extra complexity. As such, symbolic execution is not
a sufficient solution for testing OpenFlow applications.
Instead, NICE uses model checking to explore system
execution paths (and detect repeated visits to the same
state [23]), and symbolic execution to determine which
inputs would exercise a particular state transition.
3.2 Symbolic Execution of OpenFlow Apps
Applying symbolic execution to the controller event handlers is relatively straightforward, with two exceptions.
2 Unless
expensive and possibly undecidable state-equivalence
checks are performed.
First, to handle the diverse inputs to the packet in
handler, we construct symbolic packets. Second, to minimize the size of the state space, we choose a concrete
(rather than symbolic) representation of controller state.
Symbolic packets. The main input to the packet in
handler is the incoming packet. To perform symbolic
execution, NICE must identify which (ranges of) packet
header fields determine the path through the handler.
Rather than view a packet as a generic array of symbolic
bytes, we introduce symbolic packets as our symbolic
data type. A symbolic packet is a group of symbolic integer variables that each represents a header field. To reduce the overhead for the constraint solver, we maintain
each header field as a lazily-initialized, individual symbolic variable (e.g., a MAC address is a 6-byte variable),
which reduces the number of variables. Yet, we still allow byte- and bit-level accesses to the fields. We also apply domain knowledge to further constrain the possible
values of header fields (e.g., the MAC and IP addresses
used by the hosts and switches in the system model, as
specified by the input topology).
Concrete controller state. The execution of the event
handlers also depends on the controller state. For example, the code in Figure 3 reaches line 9 only for unicast destination MAC addresses stored in mactable.
Starting with an empty mactable, symbolic execution
cannot find an input packet that forces the execution of
line 9; yet, with a non-empty table, certain packets could
trigger line 9 to run, while others would not. As such,
we must incorporate the global variables into the symbolic execution. We choose to represent the global variables in a concrete form. We apply symbolic execution
by using these concrete variables as the initial state and
by marking as symbolic the packets and statistics arguments to the handlers. The alternative of treating the controller state as symbolic would require a sophisticated
type-sensitive analysis of complex data structures (e.g.,
[23]), which is computationally expensive and difficult
for an untyped language like Python.
3.3 Combining SE with Model Checking
With all of NICE’s parts in place, we now describe
how we combine model checking (to explore system execution paths) and symbolic execution (to reduce the
space of inputs). At any given controller state, we
want to identify the packets that each client should
send—specifically, the set of packets that exercise all
feasible code paths on the controller in that state.
To do so, we create a special client transition called
discover packets that symbolically executes the
packet in handler. Figure 4 shows the unfolding of
controller’s state-space graph.
Symbolic execution of the handler starts from the
initial state defined by (i) the concrete controller state
State client1
State client1
discover_packets transition:
Controller state
sw_id, inport
of packet_in
New relevant
[pkt1, pkt2]
Enable new
client1 send(pkt1)
client1 send(pkt2)
Figure 4: Example of how NICE identifies relevant packets
and uses them as new enabled send packet transitions of
client1 . For clarity, the circled states refer to the controller
state only.
(e.g., State 0 in Figure 4) and (ii) a concrete “context” (i.e., the switch and input port that identify the
client’s location). For every feasible code path in the
handler, the symbolic-execution engine finds an equivalence class of packets that exercise it. For each equivalence class, we instantiate one concrete packet (referred
to as the relevant packet) and enable a corresponding
send transition for the client. While this example focuses on the packet in handler, we apply similar techniques to deal with traffic statistics, by introducing a special discover stats transition that symbolically executes the statistics handler with symbolic integers as arguments. Other handlers, related to topology changes,
operate on concrete inputs (e.g., the switch and port ids).
Figure 5 shows the pseudo-code of our search-space
algorithm, which extends the basic model-checking loop
in two main ways.
Initialization (lines 3-5): For each client, the algorithm (i) creates an empty map for storing the relevant
packets for a given controller state and (ii) enables the
discover packets transition.
Checking process (lines 12-18): Upon reaching a
new state, the algorithm checks for each client (line
15) whether a set of relevant packets already exists.
If not, it enables the discover packets transition.
In addition, it checks (line 17) if the controller has
a process stats transition enabled in the newlyreached state, meaning that the controller is awaiting a
response to a previous query for statistics. If so, the algorithm enables the discover stats transition.
Invoking the discover packets (lines 26-31) and
discover stats (lines 32-35) transitions allows the
system to evolve to a state where new transitions become possible—one for each path in the packet-arrival
or statistics handler. This allows the model checker to
reach new controller states, allowing symbolic execution
to again uncover new classes of inputs that enable additional transitions, and so on.
state stack = []; explored states = []; errors = []
initial state = create initial state()
3 for client in initial state.clients
client.packets = {}
client.enable transition(discover packets)
for t in initial state.enabled transitions:
state stack.push([initial state, t])
8 while len(state stack) > 0:
state, transition = choose(state stack)
next state = run(state, transition)
ctrl = next state.ctrl # Reference to controller in next state
ctrl state = state(ctrl) # Stringified controller state in next state
for client in state.clients:
if not client.packets.has key(ctrl state):
client.enable transition(discover packets, ctrl)
if process stats in ctrl.enabled transitions:
ctrl.enable transition(discover stats, state, sw id)
check properties(next state)
if next state not in explored states:
explored states.add(next state)
for t in next state.enabled transitions:
state stack.push([next state, t])
except PropertyViolation as e:
errors.append([e, trace])
26 def discover packets transition(client, ctrl):
sw id, inport = switch location of(client)
new packets = SymbolicExecution(ctrl, packet in,
context=[sw id, inport])
client.packets[state(ctrl)] = new packets
for packet in client.packets[state(ctrl)]:
client.enable transition(send, packet)
def discover stats transition(ctrl, state, sw id):
new stats = SymbolicExecution(ctrl, process stats,
context=[sw id])
for stats in new stats:
ctrl.enable transition(process stats, stats)
Figure 5: Pseudo-code of the state-space search algorithm
used in NICE for finding errors. The highlighted parts, including the special “discover” transitions, are our additions
to the basic model-checking loop.
By symbolically executing the controller event handlers, NICE can automatically infer the test inputs for
enabling model checking without developer input, at the
expense of some limitations in coverage of the system
state space which we discuss later in Section 9.
OpenFlow-Specific Search Strategies
Even with our optimizations from the last two sections,
the model checker cannot typically explore the entire
state space, since this may be prohibitively large or even
infinite. Thus, we propose domain-specific heuristics
that substantially reduce the space of event orderings
while focusing on scenarios that are likely to uncover
bugs. Most of the strategies operate on the event interleavings produced by model checking, except for PKTSEQ which reduces the state-space explosion due to the
transitions uncovered by symbolic execution.
PKT-SEQ: Relevant packet sequences. The effect of
discovering new relevant packets and using them as new
enabled send transitions is that each end-host generates a potentially-unbounded tree of packet sequences.
To make the state space finite and smaller, this heuristic reduces the search space by bounding the possible
end host transitions (indirectly, bounding the tree) along
two dimensions, each of which can be fine-tuned by the
user. The first is merely the maximum length of the sequence, or in other words, the depth of the tree. Effectively, this also places a hard limit to the issue of infinite execution trees due to symbolic execution. The second is the maximum number of outstanding packets, or in
other words, the length of a packet burst. For example,
if client1 in Figure 4 is allowed only a 1-packet burst,
this heuristic would disallow both send(pkt2 ) in State
2 and send(pkt1 ) in State 3. Effectively, this limits
the level of “packet concurrency” within the state space.
To introduce this limit, we assign each end host with a
counter c; when c = 0, the end host cannot send any
more packet until the counter is replenished. As we are
dealing with communicating end hosts, we adopt as default behavior to increase c by one unit for every received
packet. However, this behavior can be modified in more
complex end host models, e.g., to mimic the TCP flow
and congestion controls.
NO-DELAY: Instantaneous rule updates. When using this simple heuristic, NICE treats each communication between a switch and the controller as a single
atomic action (i.e., not interleaved with any other transitions). In other words, the global system runs in “lock
step.” This heuristic is useful during the early stages of
development to find basic design errors, rather than race
conditions or other concurrency-related problems. For
instance, this heuristic would allow the developer to realize that installing a rule prevents the controller from
seeing other packets that are important for program correctness. For example, a MAC-learning application that
installs forwarding rules based only on the destination
MAC address would prevent the controller from seeing
some packets with new source MAC addresses.
UNUSUAL: Uncommon delays and reorderings.
With this heuristic, NICE only explores event orderings
with unusual and unexpected delays, with the goal of uncovering race conditions. For example, if an event handler in the controller installs rules in switches 1, 2, and
3, the heuristic explores transitions that reverse the order
by allowing switch 3 to install its rule first, followed by
switch 2 and then switch 1. This heuristic uncovers bugs
like the example in Figure 1.
FLOW-IR: Flow independence reduction.
OpenFlow applications treat different groups of packets
independently; that is, the handling of one group is not
affected by the presence or absence of another. In this
case, NICE can reduce the search space by exploring
only one relative ordering between the events affecting
each group. To use this heuristic, the programmer provides isSameFlow, a Python function that takes two
packets (and the switch and input port) as arguments and
returns whether the packets belong to the same group.
For example, in some scenarios different microflows are
independent, whereas other programs may treat packets
with different destination MAC addresses independently.
Summary. PKT-SEQ is complementary to other strategies in that it only reduces the number of send transitions rather than the possible kind of event orderings.
PKT-SEQ is enabled by default and used in our experiments (unless otherwise noted). The other heuristics can
be selectively enabled.
Specifying Application Correctness
Correctness is not an intrinsic property of a system—a
specification of correctness states what the system should
(or should not) do, whereas the implementation determines what it actually does. NICE allows programmers
to specify correctness properties as Python code snippets,
and provides a library of common properties (e.g., no forwarding loops or blackholes).
5.1 Customizable Correctness Properties
Testing correctness involves asserting safety properties
(“something bad never happens”) and liveness properties (“eventually something good happens”), defined
more formally in Chapter 3 of [19]. Checking for safety
properties is relatively easy, though sometimes writing
an appropriate predicate over all state variables is tedious. As a simple example, a predicate could check
that the collection of flow rules does not form a forwarding loop or a black hole. Checking for liveness properties is typically harder because of the need to consider
a possibly infinite system execution. In NICE, we make
the inputs finite (e.g., a finite number of packets, each
with a finite set of possible header values), allowing us
to check some liveness properties. For example, NICE
could check that, once two hosts exchange at least one
packet in each direction, no further packets go to the controller (a property we call “StrictDirectPaths”). Checking
this liveness property requires knowledge not only of the
system state, but also which transitions have executed.
To check both safety and liveness properties, NICE allows correctness properties to (i) access the system state,
(ii) register callbacks invoked by NICE to observe important transitions in system execution, and (iii) maintain local state. In our experience, these features offer
enough expressiveness for specifying correctness properties. For ease of implementation, these properties are
represented as snippets of Python code that make as-
sertions about global system state. NICE invokes these
snippets after each transition. For example, to check the
StrictDirectPaths property, the code snippet would have
local state variables that keep track of whether a pair of
hosts has exchanged at least one packet in each direction, and would flag a violation if a subsequent packet
triggers a packet in event at the controller. When a
correctness check signals a violation, the tool records the
execution trace that recreates the problem.
5.2 Library of Correctness Properties
NICE provides a library of correctness properties applicable to a wide range of OpenFlow applications. A programmer can select properties from a list, as appropriate
for the application. Writing these correctness modules
can be challenging because the definitions must be robust to communication delays between the switches and
the controller. Many of the definitions must intentionally
wait until a “safe” time to test the property to prevent
natural delays from erroneously triggering a violation of
the property. Providing these modules as part of NICE
can relieve the developers from the challenges of specifying correctness properties precisely, though creating
any custom modules would require similar care.
• NoForwardingLoops: This property asserts that packets do not encounter forwarding loops, and is implemented by checking that each packet goes through any
given <switch, input port> pair at most once.
• NoBlackHoles: This property states that no packets
should be dropped in the network, and is implemented
by checking that every packet that enters the network ultimately leaves the network or is consumed by the controller itself (for simplicity, we disable optional packet
drops and duplication on the channels). To account for
flooding, the property enforces a zero balance between
the packet copies and packets consumed.
• DirectPaths: This property checks that, once a packet
has successfully reached its destination, future packets
of the same flow do not go to the controller. Effectively,
this checks that the controller successfully establishes a
direct path to the destination as part of handling the first
packet of a flow. This property is useful for many OpenFlow applications, though it does not apply to the MAClearning switch, which requires the controller to learn
how to reach both hosts before it can construct unicast
forwarding paths in either direction.
• StrictDirectPaths: This property checks that, after two
hosts have successfully delivered at least one packet of
a flow in each direction, no successive packets reach the
controller. This checks that the controller has established
a direct path in both directions between the two hosts.
• NoForgottenPackets: This property checks that all
switch buffers are empty at the end of system execution. A program can easily violate this property by for-
getting to tell the switch how to handle a packet. This
can eventually consume all the available buffer space for
packets awaiting controller instruction; after a timeout,
the switch may discard these buffered packets. A shortrunning program may not run long enough for the queue
of awaiting-controller-response packets to fill, but the
NoForgottenPackets property easily detects these bugs.
Implementation Highlights
We have built a prototype implementation of NICE written in Python so as to seamlessly support OpenFlow controller programs for the popular NOX controller platform
(which provides an API for Python).
As a result of using Python, we face the challenge of
doing symbolic execution for a dynamic, untyped language. This task turned out to be quite challenging from
an implementation perspective. To avoid modifying the
Python interpreter, we implement a derivative technique
of symbolic execution called concolic execution [24]3 ,
which executes the code with concrete instead of symbolic inputs. Alike symbolic execution, it collects constraints along code paths and tries to explore all feasible
paths. Another consequence of using Python is that we
incur a significant performance overhead, which is the
price for favoring usability. We plan to improve performance in a future release of the tool.
NICE consists of three parts: (i) a model checker,
(ii) a concolic-execution engine, and (iii) a collection
of models including the simplified switch and several end
hosts. We now briefly highlight some of the implementation details of the first two parts: the model checker and
concolic engine, which run as different processes.
Model checker details.
To checkpoint and restore
system state, NICE takes the approach of remembering
the sequence of transitions that created the state and restores it by replaying such sequence, while leveraging
the fact that the system components execute deterministically. State-matching is doing by comparing and storing
hashes of the explored states. The main benefit of this approach is that it reduces memory consumption and, secondarily, it is simpler to implement. Trading computation for memory is a common approach for other modelchecking tools (e.g., [15, 16]). To create state hashes,
NICE serializes the state via the cPickle module and applies the built-in hash function to the resulting string.
Concolic execution details. A key step in concolic execution is tracking the constraints on symbolic variables
during code execution. To achieve this, we first implement a new “symbolic integer” data type that tracks assignments, changes and comparisons to its value while
behaving like a normal integer from the program point
of view. We also implement arrays (tuples in Python terminology) of these symbolic integers. Second, we reuse
3 Concolic
stands for concrete + symbolic.
the Python modules that naturally serve for debugging
and disassembling the byte-code to trace the program execution through the Python interpreter.
Further, before running the code symbolically, we normalize and instrument it since, in Python, the execution can be traced at best with single code-line granularity. Specifically, we convert the source code into its
abstract syntax tree (AST) representation and then manipulate this tree through several recursive passes that
perform the following transformations: (i) we split composite branch predicates into nested if statements to work
around shortcut evaluation, (ii) we move function calls
before conditional expressions to ease the job for the STP
constraint solver [25], (iii) we instrument branches to
inform the concolic engine on which branch is taken,
(iv) we substitute the built-in dictionary with a special
stub that exposes the constraints, and (v) we intercept
and remove sources of nondeterminism (e.g., seeding the
pseudo-random number generator). The AST tree is then
converted back to source code for execution.
Performance Evaluation
Here we present an evaluation of how effectively NICE
copes with the large state space in OpenFlow.
Experimental setup. We run the experiments on the
simple topology of Figure 1, where the end hosts behave
as follows: host A sends a “layer-2 ping” packet to host
B which replies with a packet to A. The controller runs
the MAC-learning switch program of Figure 3. We report the numbers of transitions and unique states, and the
execution time as we increase the number of concurrent
pings (a pair of packets). We run all our experiments on a
machine set up with Linux 2.6.32 x86 64 that has 64 GB
of RAM and a clock speed of 2.6 GHz. Our prototype
implementation does not yet make use of multiple cores.
Benefits of simplified switch model. We first perform a
full search of the state space using NICE as a depth-first
search model checker (NICE-MC, without symbolic execution) and compare to NO-SWITCH-REDUCTION:
doing model-checking without a canonical representation of the switch state. Effectively, this prevents the
model checker from recognizing that it is exploring semantically equivalent states. These results, shown in
Table 1, are obtained without using any of our search
strategies. We compute ρ, a metric of state-space reduction due to using the simplified switch model, as
We observe the following:
• In both samples, the number of transitions and of
unique states grow roughly exponentially (as expected).
However, using the simplified switch model, the unique
states explored in NICE-MC only grow with a rate
that is about half the one observed for NO-SWITCHREDUCTION.
Unique states
CPU time
0.94 [s]
47.27 [s]
36 [m]
30 [h]
Unique states
CPU time
1.93 [s]
208.63 [s]
318 [m]
Reduction [%]
Table 1: Dimensions of exhaustive search in NICE-MC vs. model-checking without a canonical representation of the
switch state, which prevents recognizing equivalent states. Symbolic execution is turned off in both cases. NO-SWITCHREDUCTION did not finish with five pings in four days.
• The efficiency in state-space reduction ρ scales with the
problem size (number of pings), and is substantial (factor
of seven for three pings).
NO−DELAY transitions
FLOW−IR transitions
Heuristic-based search strategies. Figure 6 illustrates
the contribution of NO-DELAY and FLOW-IR in reducing the search space relative to the metrics reported for
the full search (NICE-MC). We omit the results for UNUSUAL as they are similar. The state space reduction is
again significant; about factor of four for three pings. In
summary, our switch model and these heuristics result in
a 28-fold state space reduction for three pings.
Figure 6: Relative state-space search reduction of our
heuristic-based search strategies vs. NICE-MC.
Comparison to other model checkers. Next, we contrast NICE-MC with two state-of-the-art model checkers, SPIN [12] and JPF [13]. We create system models in
PROMELA and Java that replicate as closely as possible
the system tested in NICE. Due to space limitations, we
only briefly summarize the results and refer to [26] for
the details:
8.1 MAC-learning Switch (PySwitch)
• As expected, by using an abstract model of the system,
SPIN performs a full search more efficiently than NICE.
Of course, state-space explosion still occurs: e.g., with
7 pings, SPIN runs of out memory. This validates our
decision to maintain hashes of system states instead of
keeping entire system states.
• SPIN’s partial-order reduction (POR)4 , decreases the
growth rate of explored transitions by only 18%. This is
because even the finest granularity at which POR can be
applied does not distinguish between independent flows.
• Taken “as is”, JPF is already slower than NICE by a
factor of 290 with 3 pings. The reason is that JPF uses
Java threads to represent system concurrency. However,
JPF leads to too many possible thread interleavings to
explore even in our small example.
• Even with our extra effort in rewriting the Java model
to explicitly expose possible transitions, JPF is 5.5 times
slower than NICE using 4 pings.
These results suggest that NICE, in comparison to the
other model-checkers, strikes a good balance between (i)
capturing system concurrency at the right level of granularity, (ii) simplifying the state space and (iii) allowing
testing of unmodified controller programs.
is a well-known technique for avoiding exploring unnecessary orderings of transitions (e.g., [27]).
Number of pings
Experiences with Real Applications
In this section, we report on our experiences applying NICE to three real applications—a MAC-learning
switch, a server load-balancer, and energy-aware traffic
engineering—and uncovering eleven bugs.
Our first application is the pyswitch software included
in the NOX distribution (98 LoC). The application implements MAC learning, coupled with flooding to unknown destinations, common in Ethernet switches. Realizing this functionality seems straightforward (e.g., the
pseudo-code in Figure 3), yet NICE automatically detects three violations of correctness properties.
BUG-I: Host unreachable after moving. This fairly
subtle bug is triggered when a host B moves from one location to another. Before B moves, host A starts streaming to B, which causes the controller to install a forwarding rule. When B moves, the rule stays in the switch as
long as A keeps sending traffic, because the soft timeout
does not expire. As such, the packets do not reach B’s
new location. This serious correctness bug violates the
NoBlackHoles property. If the rule had a hard timeout,
the application would eventually flood packets and reach
B at its new location; then, B would send return traffic
that would trigger MAC learning, allowing future packets to follow a direct path to B. While this “bug fix” prevents persistent packet loss, the network still experiences
transient loss until the hard timeout expires. Designing
a new NoBlackHoles property that is robust to transient
loss is part of our ongoing work.
BUG-II: Delayed direct path. The pyswitch also violates the StrictDirectPaths property, leading to suboptimal performance. The violation arises after a host A
sends a packet to host B, and B sends a response packet
to A. This is because pyswitch installs a forwarding
rule in one direction—from the sender (B) to the destination (A), in line 13 of Figure 3. The controller does
not install a forwarding rule for the other direction until
seeing a subsequent packet from A to B. For a threeway packet exchange (e.g., a TCP handshake), this performance bug directs 50% more traffic than necessary
to the controller. Anecdotally, fixing this bug can easily introduce another one. The naı̈ve fix is to add another install rule call, with the addresses and ports
reversed, after line 14, for forwarding packets from A
to B. However, since the two rules are not installed
atomically, installing the rules in this order can allow the
packet from B to reach A before the switch installs the
second rule. This can cause a subsequent packet from
A to reach the controller unnecessarily. A correct fix
would install the rule for traffic from A first, before allowing the packet from B to A to traverse the switch.
With this “fix”, the resulting program satisfies the StrictDirectPaths property.
BUG-III: Excess flooding. When we test pyswitch
on a topology that contains a cycle, the program violates
the NoForwardingLoops property. This is not surprising,
since pyswitch does not construct a spanning tree.
8.2 Web Server Load Balancer
Data centers rely on load balancers to spread incoming
requests over service replicas. Previous work created a
load-balancer application that uses wildcard rules to divide traffic based on the client IP addresses to achieve
a target load distribution [9]. The application can dynamically adjust the load distribution by installing new
wildcard rules; during the transition, old transfers complete at their existing servers while new requests are handled according to the new distribution. We test this application with one client and two servers connected to a
single switch. The client opens a TCP connection to a
virtual IP address corresponding to the two replicas. In
addition to the default correctness properties, we create
an application-specific property FlowAffinity that verifies
that all packets of a single TCP connection go to the same
server replica. Here we report on the bugs NICE found
in the original code (1209 LoC), which had already been
unit tested to some extent.
BUG-IV: Next TCP packet always dropped after reconfiguration. Having observed a violation of the NoForgottenPackets property, we identified a bug where the
application neglects to handle the “next” packet of each
flow—for both ongoing transfers and new requests—
after a change in the load-balancing policy. Despite correctly installing the forwarding rule for each flow, the
application does not instruct the switch to forward the
packet that triggered the packet in handler. Since
the TCP sender ultimately retransmits the lost packet,
the program does successfully handle each Web request,
making it hard to notice the bug. The bug degrades performance and, for a long execution trace, would ulti-
mately exhaust the switch’s space for buffering packets
awaiting controller action.
BUG-V: Some TCP packets dropped after reconfiguration. After fixing BUG-IV, NICE detected another
NoForgottenPackets violation due to a race condition. In
switching from one load-balancing policy to another, the
application sends multiple updates to the switch for each
existing rule: (i) a command to remove the existing forwarding rule followed by (ii) commands to install one
or more rules (one for each group of affected client IP
addresses) that direct packets to the controller. Since
these commands are not executed atomically, packets arriving between the first and second step do not match
either rule. The OpenFlow specification prescribes that
packets that do not match any rule should go to the controller. Although the packets go to the controller either
way, these packets arrive with a different “reason code”
(i.e., NO MATCH). As written, the packet in handler
ignores such (unexpected) packets, causing the switch to
hold them until the buffer fills. This appears as packet
loss to the end hosts. To fix this bug, the program should
reverse the two steps, installing the new rules (perhaps at
a lower priority) before deleting the existing ones.
BUG-VI: ARP packets forgotten during address resolution. Another NoForgottenPackets violation uncovered two bugs that are similar in spirit to the previous
one. The controller program handles client ARP requests
on behalf of the server replicas. Despite sending the correct reply, the program neglects to discard the ARP request packets from the switch buffer. A similar problem
occurs for server-generated ARP messages.
BUG-VII: Duplicate SYN packets during transitions.
A FlowAffinity violation detected a subtle bug that arises
only when a connection experiences a duplicate (e.g., retransmitted) SYN packet while the controller changes
from one load-balancing policy to another. During the
transition, the controller inspects the “next” packet of
each flow, and assumes a SYN packet implies the flow
is new and should follow the new load-balancing policy.
Under duplicate SYN packets, some packets of a connection (arriving before the duplicate SYN) may go to one
server, and the remaining packets to another, leading to
a broken connection. The authors of [9] acknowledged
this possibility (see footnote #2 in their paper), but only
realized this was a problem after careful consideration.
8.3 Energy-Efficient Traffic Engineering
OpenFlow enables a network to reduce energy consumption [10,28] by selectively powering down links and redirecting traffic to alternate paths during periods of lighter
load. REsPoNse [28] pre-computes several routing tables (the default is two), and makes an online selection
for each flow. The NOX implementation (374 LoC) has
an always-on routing table (that can carry all traffic un-
der low demand) and an on-demand table (that serves additional traffic under higher demand). Under high load,
the flows should probabilistically split evenly over the
two classes of paths. The application learns the link
utilizations by querying the switches for port statistics.
Upon receiving a packet of a new flow, the packet in
handler chooses the routing table, looks up the list of
switches in the path, and installs a rule at each hop.
For testing with NICE, we install a network topology
with three switches in a triangle, one sender host at one
switch and two receivers at another switch. The third
switch lies on the on-demand path. We define the following application-specific correctness property:
• UseCorrectRoutingTable: This property checks that
the controller program, upon receiving a packet from an
ingress switch, issues the installation of rules to all and
just the switches on the appropriate path for that packet,
as determined by the network load. Enforcing this property is important, because if it is violated, the network
might be configured to carry more traffic than it physically can, degrading the performance of end-host applications running on top of the network.
NICE found several bugs in this application:
BUG-VIII: The first packet of a new flow is dropped.
A violation of NoForgottenPackets revealed a bug that is
almost identical to BUG-IV. The packet in handler
installed a rule but neglected to instruct the switch to forward the packet that triggered the event.
BUG-IX: The first few packets of a new flow can be
dropped. After fixing BUG-VIII, NICE detected another NoForgottenPackets violation at the second switch
in the path. Since the packet in handler installs an
end-to-end path when the first packet of a flow enters the
network, the program implicitly assumes that intermediate switches would never direct packets to the controller.
However, with communication delays in installing the
rules, the packet could reach the second switch before
the rule is installed. Although these packets trigger
packet in events, the handler implicitly ignores them,
causing the packets to buffer at the intermediate switch.
This bug is hard to detect because the problem only arises
under certain event orderings. Simply installing the rules
in the reverse order, from the last switch to the first, is
not sufficient—differences in the delays for installing the
rules could still cause a packet to encounter a switch that
has not (yet) installed the rule. A correct “fix” should either handle packets arriving at intermediate switches, or
use “barriers” (where available) to ensure that rule installation completes at all intermediate hops before allowing
the packet to depart the ingress switch.
BUG-X: Only on-demand routes used under high
load. NICE detects a CorrectRoutingTableUsed violation that prevents on-demand routes from being used
properly. The program updates an extra routing table in
PKT-SEQ only
23 / 0.02
18 / 0.01
11 / 0.01
386 / 3.41
22 / 0.05
48 / 0.05
297k / 1h
23 / 0.03
21 / 0.03
2893 / 35.2
98 / 0.67
23 / 0.02
18 / 0.01
16 / 0.01
1661 / 9.66
23 / 0.02
18 / 0.01
11 / 0.01
321 / 1.1
21 / 0.02
31 / 0.04
23 / 0.02
18 / 0.01
11 / 0.01
64 / 0.19
60 / 0.18
49 / 0.07
26.5k / 5m
23 / 0.02
21 / 0.02
2367 / 25.6
25 / 0.03
48 / 0.06
191k / 39m
22 / 0.02
17 / 0.02
23 / 0.03
21 / 0.03
2893 / 35.2
98 / 0.67
Table 2: Comparison of the number of transitions / running
time to the first violation that uncovered each bug. Time is
in seconds unless otherwise noted.
the port-statistic handler (when the network’s perceived
energy state changes) to either always-on or on-demand,
in an effort to let the remainder of the code simply reference this extra table when deciding where to route a
flow. Unfortunately, this made it impossible to split flows
equally between always-on and on-demand routes, and
the code directed all new flows over on-demand routes
under high load. A “fix” was to abandon the extra table
and choose the routing table on per-flow basis.
BUG-XI: Packets can be dropped when the load reduces. After fixing BUG-IX, NICE detected another violation of the NoForgottenPackets. When the load reduces, the program recomputes the list of switches in
each always-on path. Under delays in installing rules,
a switch not on these paths may send a packet to the controller, which ignores the packet because it fails to find
this switch in any of those lists.
8.4 Overhead of Running NICE
In Table 2, we summarize how many seconds NICE took
(and how many state transitions were explored) to discover the first property violation that uncovered each
bug, under four different search strategies. Note the numbers are generally small because NICE quickly produces
simple test cases that trigger the bugs. One exception,
BUG-VII, is found in 1 hour by doing a PKT-SEQ-only
search but UNUSUAL can detect it in just 5 minutes. Our
search strategies are also generally faster than PKT-SEQonly to trigger property violations, except in one case
(BUG-IV). Also, note that there are no false positives in
our case studies—every property violation is due to the
manifestation of a bug—and only in few cases (BUGV, BUG-VII, BUG-X and BUG-XI) the heuristic-based
strategies experience false negatives. Expectedly, NODELAY, which does not consider rule installation delays,
misses race condition bugs (27% missed bugs). BUGVII is missed by FLOW-IR because the duplicate SYN
is treated as a new independent flow (9% missed bugs).
Finally, the reader may find that some of the bugs
we found—like persistently leaving some packets in the
switch buffer—are relatively simple and their manifesta-
tions could be detected with run-time checks performed
by the controller platform. However, the programmer
would not know what caused them. For example, a runtime check that flags a “no forgotten packets” error due
to BUG-IV or BUG-V would not tell the programmer
what was special about this particular system execution
that triggered the error. Subtle race conditions are very
hard to diagnose, so having a (preferably small) example
trace—like NICE produces—is crucial.
Coverage vs. Overhead Trade-Offs
Testing is inherently incomplete, walking a fine line between good coverage and low overhead. As part of our
ongoing work, we want to explore further how to best
leverage symbolic execution in NICE. We here discuss
some limitations of our current approach.
Concrete execution on the switch: In identifying the
equivalence classes of packets, the algorithm in Figure 5 implicitly assumes the packets reach the controller.
However, depending on the rules already installed in the
switch, some packets in a class may reach the controller
while others do not. This leads to two limitations. First,
if no packets in an equivalence class would go to the
controller, generating a representative packet from this
class was unnecessary. This leads to some loss in efficiency. Second, if some (but not all) packets go to the
controller, we may miss an opportunity to test a code path
through the handler by inadvertently generating a packet
that stays in the “fast path” through the switches. This
leads to some loss in both efficiency and coverage. We
could overcome these limitations by extending symbolic
execution to include our simplified switch model and
performing “symbolic packet forwarding” across multiple switches. We chose not to pursue this approach
because (i) symbolic execution of the flow-table code
would lead to a path-explosion problem, (ii) including
these variables would increase the overhead of the constraint solver, and (iii) rules that modify packet headers
would further complicate the symbolic analysis. Still, we
are exploring “symbolic forwarding” as future work.
Concrete global controller variables: In symbolically
executing each event handler, NICE could miss complex dependencies between handler invocations. This is
a byproduct of our decision to represent controller variables in a concrete form. In some cases, one call to a handler could update the variables in a way that affects the
symbolic execution of a second call (to the same handler,
or a different one). Symbolic execution of the second
handler would start from the concrete global variables,
and may miss an opportunity to recognize additional constraints on packet header fields. We could overcome this
limitation by running symbolic execution across multiple handler invocations, at the expense of a significant
explosion in the number of code paths. Or, we could re-
visit our decision to represent controller variables in a
concrete form. As future work, we are considering ways
to efficiently represent global variables symbolically.
Infinite execution trees in symbolic execution: Despite its many advantages, symbolic execution can lead
to infinite execution trees [23]. In our context, an infinite state space arises if each state has at least one input that modifies the controller state. This is an inherent limitation of symbolic execution, whether applied
independently or in conjunction with model checking.
To address this limitation, we explicitly bound the state
space by limiting the size of the input (e.g., a limit on the
number of packets) and devise OpenFlow-specific search
strategies that explore the system state space efficiently.
These heuristics offer a tremendous improvement in efficiency, at the expense of some loss in coverage.
Related Work
Bug finding. While model checking [12–16] and symbolic execution [17, 18, 24] are automatic techniques,
a drawback is that they typically require a closed system, i.e., a system (model) together with its environment. Typically, the creation of such environment is a
manual process (e.g., [22]). NICE re-uses the idea of
model checking—systematic state-space exploration—
and combines it with the idea of symbolic execution—
exhaustive path coverage—to avoid pushing the burden
of modeling the environment on the user. Also, NICE is
the first to demonstrate the applicability of these techniques for testing the dynamic behavior of OpenFlow
networks. Finally, NICE makes a contribution in managing state-space explosion for this specific domain.
Khurshid et al. [23] enable a model checker to perform symbolic execution. Both our and their work share
the spirit of using symbolic variables to represent data
from very large domains. Our approach differs in that
it uses symbolic execution in a selective way for uncovering possible transitions given a certain controller state.
As a result, we (i) reduce state-space explosion due to
feasible code paths because not all code is symbolically
executed, and (ii) enable matching of concrete system
states to further reduce the search of the state space.
OpenFlow and network testing. Frenetic [29] is a
domain-specific language for OpenFlow that aims to
eradicate a large class of programming faults. Using Frenetic requires the network programmer to learn extensions to Python to support the higher-layer abstractions.
OFRewind [30] enables recording and replay of events
for troubleshooting problems in production networks due
to closed-source network devices. However, it does not
automate the testing of OpenFlow controller programs.
Mai et al. [31] use static analysis of network devices
forwarding information bases to uncover problems in the
data plane. FlowChecker [32] applies symbolic model
checking techniques on a manually-constructed network
model based on binary decision diagrams to detect misconfigurations in OpenFlow forwarding tables. We view
these works as orthogonal to ours since they both aim to
analyze a snapshot of the data plane.
Bishop et al. [33] examine the problem of testing the
specification of end host protocols. NICE tests the network itself, in a new domain of software defined networks. Kothari et al. [34] use symbolic execution and
developer input to identify protocol manipulation attacks for network protocols. In contrast, NICE combines
model checking with symbolic execution to identify relevant test inputs for injection into the model checker.
We built NICE, a tool for automating the testing of OpenFlow applications that combines model checking and
concolic execution in a novel way to quickly explore
the state space of unmodified controller programs written for the popular NOX platform. Further, we devised
a number of new, domain-specific techniques for mitigating the state-space explosion that plagues approaches
such as ours. We contrast NICE with an approach that
applies off-the-shelf model checkers to the OpenFlow
domain, and demonstrate that NICE is five times faster
even on small examples. We applied NICE to implementations of three important applications, and found
11 bugs. A release of NICE is publicly available at
We thank our shepherd Arvind Krishnamurthy and the anonymous reviewers who provided excellent feedback. We are
grateful to Stefan Bucur, Olivier Crameri, Johannes Kinder,
Viktor Kuncak, Sharad Malik, and Darko Marinov for useful discussions and comments on earlier drafts of this work.
The research leading to these results has received funding from
the European Research Council under the European Union’s
Seventh Framework Programme (FP7/2007-2013) / ERC grant
agreement 259110.
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