Enabling End-host Network Functions

Enabling End-host Network Functions
Enabling End-host Network Functions
Hitesh Ballani, Paolo Costa, Christos Gkantsidis, Matthew P. Grosvenor,
Thomas Karagiannis, Lazaros Koromilas, and Greg O’Shea
Microsoft Research
Cambridge, UK
Many network functions executed in modern datacenters, e.g., load balancing, application-level QoS, and
congestion control, exhibit three common properties at
the data plane: they need to access and modify state, to
perform computations, and to access application semantics — this is critical since many network functions are
best expressed in terms of application-level messages.
In this paper, we argue that the end hosts are a natural
enforcement point for these functions and we present
Eden, an architecture for implementing network functions at end hosts with minimal network support.
Eden comprises three components, a centralized controller, an enclave at each end host, and Eden-compliant
applications called stages. To implement network functions, the controller configures stages to classify their
data into messages and the enclaves to apply action
functions based on a packet’s class. Our Eden prototype includes enclaves implemented both in the OS
kernel and on programmable NICs. Through case studies, we show how application-level classification and the
ability to run actual programs on the data-path allows
Eden to efficiently support a broad range of network
functions at the network’s edge.
Software Defined Networking; SDN; Network Management; Data-plane programming; Network Functions
Recent years have seen a lot of innovation in functionality deployed across datacenter networks. Network functions range from management tasks like load balancing [65, 4, 26, 1, 40] and Quality of Service (QoS) [9,
39, 10, 33, 52, 61, 6, 28] to data-plane tasks like (centralized) congestion control and network scheduling [48, 27,
64, 45, 30] to application-specific tasks like replica selection [17]. Today, such functions are implemented using
a mishmash of techniques and at a variety of places—at
network switches using SDN and OpenFlow, at physical or virtual middleboxes using NFV, and at end hosts
through Open vSwitch or custom implementations.
Despite their differences, three common requirements
characterize a large fraction of network functions: i)
they maintain state at the data plane, ii) they perform
computation at the data plane, and iii) they operate on
application semantics. The last feature is particularly
important since many network functions are best expressed in terms of application data units or “messages”.
For example, a load balancing function for memcached,
a popular key-value store, may put its messages into
two “classes” (GETs and PUTs) and treat them differently. It may even use message-specific details like the
key being accessed for load balancing [40].
Traditional network management approaches implement network functions completely decoupled from applications. They infer the application message a packet
belongs to using deep packet inspection or through other
heuristics. Instead, we propose messages and classes as
first-order abstractions at the network layer. A message refers to an (arbitrary) application data unit while
a class is the set of messages (and consequent network
packets) to which the same network function should be
applied. Applications can provide the class and message
information for any traffic they generate.
In this paper, we present Eden, an architecture for
implementing network functions at end hosts. End hosts
are a natural enforcement point for such functions—
CCS Concepts
•Networks → Programmable networks; Network
management; Data center networks; Cloud computing;
∗Work performed while an intern with Microsoft Research; currently at Cambridge University, UK
†Work performed while an intern with Microsoft Research; currently at University of Crete, Greece
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DOI: http://dx.doi.org/10.1145/2785956.2787493
Overall, this paper makes the following contributions:
they have plentiful resources that allow for complex
computation and large amounts of state to be maintained, and they are ideally placed for fine-grained visibility into application semantics. Finally, in single administrator environments like enterprises and datacenters, some part of end hosts can be trusted. We show
that a large number of diverse and interesting network
functions can be efficiently realized at the end hosts
with minimal support from the network.
Eden comprises three components: a logically centralized controller, stages, and end host enclaves. A
stage is any application or library that is Eden-compliant.
Stages bridge the gap between network functions expressed in terms of application-level messages and the
enclave operating on packets. To achieve this, stages
classify their network traffic, associating application messages with a class and a message identifier that is carried
with it down the host’s network stack. The enclave resides along the end host network stack, either in the
OS or the NIC. It extends and replaces functionality
typically performed by the end host virtual switch. An
enclave has a set of match-action tables that, based on
a packet’s class, determine an action function to apply.
The action function can modify both the packet and
the enclave’s global state. Enclaves and stages each expose an API through which they can be programmed.
Enclaves and stages, taken together, enable applicationaware data plane programmability.
Given a network function, the controller can implement it by programming stages and enclaves across the
network. Hence, Eden achieves a careful division of
functionality; the controller provides global visibility,
stages provide application visibility while enclaves provide a pragmatic enforcement point at the data plane.
A key challenge posed by our design is efficient and
safe execution of action functions at enclaves while allowing for the functions to be dynamically updated by
the controller without impacting data plane performance.
With Eden, action functions are written in a high-level
domain specific language using F# code quotations.
They are compiled to bytecode which is then interpreted
through a stack-based interpreter within the enclave.
This approach allows Eden to execute the same computation across multiple platforms and avoids the complexities of dynamically loading code in the OS or the
NIC. Indeed, our interpreter can execute the same action function inside the OS or in a programmable NIC.
We have implemented the Eden enclave across two
platforms: Hyper-V and Netronome’s programmable
NICs [46]. We evaluate Eden through case studies across
these platforms. These studies highlight how the Eden
architecture can implement diverse functions spanning
application-aware load-balancing, quality of service and
weighted path selection. We show that Eden’s interpreter-based data plane computation incurs reasonable
overheads with negligible impact on application performance metrics.
• We highlight that a large class of network functions
feature three key requirements: data-plane computation, data-plane state, and operate on application
semantics (§2).
• We design and implement Eden, an architecture that
enables end host network functions through data plane
programmability (§3).
• We present a flexible scheme for application-level classification of network traffic (§3.3).
• We present a language, compiler and runtime for action functions. The compiler decouples state management from the function, thus providing a clean
programming abstraction to administrators (§3.4).
The idea of end hosts participating in the implementation of network functions is not new [49, 16, 21, 34,
59, 57]. These state of the art approaches, however,
still encourage a low-level, packet-based API for programming the data plane, often a variant of OpenFlow.
This ignores end host capabilities and restricts the functions that can be implemented. Instead, Eden adopts a
different philosophy by introducing a data plane interface that is wide and rich enough to allow for general,
application-informed data-plane computation.
Network Functions
This paper is motivated by the observation that three
common data-plane requirements underlie many network functions. First, they need to create, access and
modify state, in many cases on a per-packet basis. This
allows for “stateful network functions” where a packet
influences the processing of subsequent packets. Second, they require computation on the data path.
Finally, they require visibility into application semantics. This introduces a mismatch – data-plane elements
like the end host network stack and switches operate
at the granularity of packets. Instead, each application
has its own application data unit or “message”. For
example, for memcached, a popular key-value store, a
message is a GET or a PUT request or response. For
a HTTP library, a message is an HTTP request or response. These messages are fragmented into packets
before being sent across the network. Many network
functions however are best expressed in terms of application messages. Implementing such network functions
thus requires a mapping between messages and the consequent network packets.
To make the discussion concrete, we now use a few network functions proposed in recent literature as casestudies to highlight the three requirements mentioned
above. For each example, we present a pseudo-code
sketch for the network function; in Section 5, we show
how the functions can be expressed in Eden’s language.
Require: Global program state- pathMatrix, which
gives the list of paths and their weights for each
source destination pair.
pathMatrix:[src, dst] -> {[Path1, Weight1],
. . .}
1: fun WCMP (packet) {
2: Choose a path in a weighted random fashion from
pathMatrix[p.src, p.dst])
Figure 1: Example of an asymmetric topology.
3: Send packet
4: }
Load balancing
Require: Each packet is tagged with its message identifier
Many recent proposals focus on load balancing of traffic
across network paths and across destinations in datacenters. We begin with a simple path-level load balancing example.
1: fun messageWCMP (packet) {
2: msg = packet.message
3: if packet belongs to new message then
4: Choose a path in a weighted random fashion from
Datacenters today use ECMP to load balance traffic among multiple equal cost paths. However, ECMP
assumes that the underlying topology is balanced and
regular which is often not true in practice [65]. For example, in Figure 1, ECMP would distribute the traffic
at switch A evenly across the two upstream links even
though subsequent links along the paths have very different capacities. This can result in unfairness and poor
tail performance. Motivated by this, Zhou et al. [65]
proposed WCMP, a weighted version of ECMP that
balances traffic across links in a weighted fashion. In
the example in Figure 1, WCMP can be used to balance traffic across the two upstream links of A in a 10:1
ratio. While the authors present a switch-based implementation of WCMP in [65], we focus on the higherorder network function associated with WCMP and for
now, sidestep where this function is implemented.
pathMatrix[p.src, p.dst]
cachedPaths[msg] = path
6: Use cachedPaths[msg] as this packet’s path
7: Send packet
8: }
Figure 2:
Network functions implementing
weighted load balancing at packet- and message
Computation. The function performs a simple computation — it chooses the packet’s path from the set of
possible paths in a weighted random fashion.
Application semantics. The WCMP function chooses
paths on a packet-by-packet basis. However, as we show
in Section 5, this can lead to packet re-ordering across
the network which can adversely impact the performance of connection oriented protocols like TCP. Flowlevel weighted load balancing [65] can address this problem, at the expense of some load imbalance.
More broadly, datacenter operators can balance the
trade-off between application performance and load across
network links through “message-level load balancing”
whereby all packets of the same application message are
sent along the same path. For example, for memcached
traffic, all packets corresponding to a GET or a PUT
operation are sent along the same path. This is implemented by the messageWCMP function in Figure 2 which
needs to know the message a given packet belongs to.
WCMP requires two main pieces of functionality: (a)
for each source-destination pair, computation of paths
and weights, and (b) picking a path for packets. The
former requires information about the network topology
which, in turn, requires a coordination point with global
visibility. With SDN-based management approaches,
such functionality is typically implemented at a centralized controller, possibly using high-level programming
abstractions [25, 62, 24, 54]. Here, we focus on the latter operation that needs to operate at the data path.
The first snippet in Figure 2 implements the WCMP
data-plane function. It takes a packet as input and
chooses its network path based on the weights of the
links between the packet’s source and destination. These
weights are obtained periodically from the controller.
Next, we discuss the state maintained by this function
and the computation it performs.
The previous discussion also applies to other schemes
for load balancing in datacenters [4, 26, 1]. For example, Ananta [4] load balances incoming connections
across a pool of application servers. This requires a
NAT-like functionality along the network path which,
in turn, requires both computation and state. Several proposals explicitly use application semantics as
State. The function accesses global state—it reads the
pathMatrix variable that, for each source and destination pair, contains the list of paths between them and
their weights. A path’s weight is the probability with
which it should be used and is a product of the normalized weights of all links along the path.
Require: Each packet is tagged with its message, size
and tenant information.
Global state: queueMap, a map that returns the rate
limited queue for each tenant
Require: Each packet is tagged with its message and
size information.
Global state: msgTable:[msg] -> bytesSent and
1: fun Pulsar (packet) {
2: msg = packet.message
3: if msg is a “READ” message then
4: Set size to msg.size //if read, policing is based
fun PIAS (packet) {
msg = packet.message
if packet belongs to a new message then
Initialize msgTable[msg] to packet.size
Increment msgTable[msg] by packet.size
Set packet priority according to priorityThresholds
8: Send packet
9: }
on operation size
5: else
6: Set size to packet.size //policing is based on the
packet size
7: Send packet to queue queueMap[packet.tenant] and
charge it size bytes
8: }
Figure 3: Pulsar’s rate control function
Figure 4: Network function implementing a generalized version of PIAS’ dynamic priority logic.
well. For example, Sinbad [17] maximizes performance
by choosing endpoints for write operations in HDFS,
and Facebook’s mcrouter [40] achieves low latency and
failover through a distributed middle layer that routes
memcached requests based on their key.
Computation. The function ensures that the packet
is rate limited based on the type of message (i.e., IO
operation) that it belongs to. It also queues the packet
at the rate limiter corresponding to the tenant generating the packet. This ensures aggregate tenant-level
guarantees instead of per-VM guarantees.
Datacenter QoS
Application semantics. For a packet, the function
needs to determine its message, the message size, and
the tenant it belongs to.
Many recent proposals provide quality of service (QoS)
guarantees for traffic across the datacenter network. We
begin with a recent QoS solution.
Pulsar [6] gives tenant-level end-to-end bandwidth
guarantees across multiple resources, such as network
and storage. A tenant refers to a collection of virtual
machines (VMs) owned by the same user. Enforcing
such guarantees at the network layer poses two key
challenges. First, it requires determining the tenant a
given packet belongs to. Second, since the guarantee
spans non-network resources, it requires knowledge of
the operation being performed. For example, consider
a packet corresponding to a request to a storage server.
Network rate limiters pace packets based on their size.
However, depending on whether the packet corresponds
to a READ or a WRITE request to a storage server,
the packet size may not correctly represent its impact
on the storage server—read packets are small on the
forward path from the client generating the request to
the server, but their cost on the server and the reverse
network path can be high depending on the read size.
Today, information about the type of an operation and
its size is not available to network rate limiters at end
hosts and at switches.
Figure 3 shows a network function implementing Pulsar’s rate control. We describe the state it maintains,
the computation it performs and the application semantics it requires.
We note that the same features apply to other schemes
for datacenter QoS that offer performance guarantees
to tenants and applications [9, 39, 10, 33, 52, 61].
Datacenter flow scheduling
Many recent proposals aim to optimize application performance in terms of reduced user-perceived latency by
carefully scheduling flows across the network. For example, PIAS [8] minimizes flow completion time without application support by mimicking the shortest flow
first scheduling strategy. With PIAS, flows start in the
highest priority but are demoted to lower priorities as
their size increases.
While PIAS [8] targets flow-level prioritization, the
concept can be generalized and applied at a message
level. A message could be a flow, a GET/PUT, etc. Figure 4 shows a network function implementing messagelevel PIAS scheduling.
State. The function uses two pieces of global state.
First, priorityThresholds provide flow-size thresholds
that define the priority for a flow given its current size.
These thresholds need to be calculated periodically based
on the datacenter’s overall traffic load. Second, the
function maintains a msgTable to track the current size
of all active messages.
State. The function accesses global state—it reads
from the queueMap to determine the rate limited queue
a packet should go to.
Computation. When a packet for a new message arrives, the function initializes a messageTable entry for
it. When a packet for an existing message arrives, the
Load Balancing
Replica Selection
Datacenter QoS
Flow scheduling
and congestion control
Stateful firewall
WCMP [65]
Message-based WCMP
Ananta [47]
Conga [1]
Duet [26]
mcrouter [40]
Pulsar [6]
Storage QoS [61, 58]
Network QoS [9, 51, 38, 33]
PIAS [8]
QJump [28]
Centralized [48, 27]
congestion control
Explicit rate control
(D3 [64], PASE [45], PDQ [30])
IDS(e.g. [19])
Port knocking [13]
(out of the box)
Table 1: Example network functions, their data-plane requirements and whether they need network
support beyond commodity features like network priorities. Eden can support many of these functions
out of the box. (3∗ refers to functions that would benefit from application semantics. For example,
Conga uses flowlets that approximate application messages.)
message’s current size is updated. Finally, we use the
current message size to determine the packet’s priority.
memory, can provide fine-grained visibility into application semantics, and by definition, are on the data path.
Application semantics. PIAS achieves flow scheduling without application support. However, in closedenvironments like datacenters, it is possible to modify
applications or the OS to directly provide information
about the size of a flow [64, 22, 30]. This would allow for shortest flow first scheduling. In Section 5, we
show that this approach, by using application information, can provide increased accuracy compared to PIAS’
application-agnostic scheduling.
Placing network functionality at end hosts provides
also additional benefits. First, enforcing a function at
the source of traffic instead of distributed enforcement
along its path makes it easier to achieve consistent behavior even when the function is updated. On the contrary, achieving consistent updates in the network requires complex mechanisms to ensure that all switches
along the path make consistent decisions [55]. Second,
implementing network functions at end hosts leads to a
natural partitioning of both the state and computation
overhead of enforcement because hosts are only responsible for a subset of the traffic a switch would need to
process. Finally, certain network properties such as flow
RTT or loss and network events like flow termination
are readily available at the hosts; instead, such properties need to be inferred, typically through heuristics
inside the network.
We note that the same features apply to other schemes
for optimizing application performance by reducing flow
completion times [30, 45], meeting flow deadlines [64] or
similar task-level metrics [18, 22].
End hosts: A natural enforcement point
Table 1 extends the previous discussion to other categories of common network functions like congestion control and stateful firewalling. The table is certainly not
exhaustive; instead, it illustrates how data-plane state,
computation and application semantics are common requirements across a large selection of network functions.
Given these requirements, a natural question is where
should such functionality be implemented. Traditionally, network functions have been implemented at switches
and middleboxes; this however reflects legacy originating from the Internet, where ISPs only control their
equipment and end hosts are not trusted. By contrast,
in closed settings owned and operated by a single entity, like datacenters and enterprises, end hosts (or some
parts of the end hosts) are trusted. Thus, end hosts are
well-suited to implement network functions—they have
high computational capabilities, significant amounts of
Of course, implementing network functionality at end
hosts is not a panacea. As shown in Table 1, some functions do require network support beyond what is offered
by commodity equipment. For example, deadline-based
congestion control protocols like D3 [64] and PDQ [30]
rely on explicit, fine-grained feedback from the network.
In general, network switches have better visibility of instantaneous aggregate network behavior and are needed
to implement network functions that rely on such information. Furthermore, despite the resources available at
end hosts, some network functions, such as compression
or encryption, may be too resource intensive for general
purpose processors and may require custom hardware.
In summary, a variety of network functions can indeed
be implemented with a modicum of computation and
Unmodified App Layer
App Layer
state at the end host data plane. This paper proposes
an architecture to enable such functions.
Match-Action Table
Metadata info
Design overview
Eden comprises three components (Figure 5):
1). Controller. It provides a logically centralized coordination point where any part of the network function
logic requiring global visibility resides (§3.2).
2). Stage. Applications are a first-order citizen in
Eden. Any application, library or even kernel module
in the end-host stack that is Eden-compliant is called
a “stage” (§3.3). Stages can classify packets based on
stage-specific semantics, and this classification is carried
through the end host stack to the Eden enclave where it
is used to determine the enclave rule(s) to apply. Stages
are programmed using the classification API which allows the controller to learn their classification abilities
and to configure them with classification rules.
3). Enclave. Eden provides a programmable dataplane through an enclave at each end host (§3.4). The
enclave may be implemented at the software network
stack inside the OS or the hypervisor, or using programmable hardware like FPGAs and programmable
NICs. The enclave is programmed by the controller using the enclave API. An enclave program comprises a
set of “match-action” rules. However, instead of matching on packet headers, packets are matched based on the
class associated with each packet. Finally, instead of a
pre-defined set of actions, the action part of the matchaction rule is an action function comprising logic that
can access and modify enclave and class state.
Packetization (socket)
This section provides an overview of the Eden architecture. Eden targets environments in which end hosts
are owned by a single administrative domain and can
therefore be trusted. In this paper, we focus on datacenters, although our approach can also be applied to
other scenarios such as enterprises [21, 34].
Packet Transmission
Figure 5: The Eden architecture.
rest of this paper, we focus on how data plane functions can be implemented at stages and enclaves, and
the APIs that the Eden controller uses to program such
Eden stages
As shown in Table 1, many network functions require
application semantics as they operate at the granularity
of application messages. By contrast, the Eden enclave
operates at the granularity of packets. Traditional approaches infer the application message a given packet
belongs to using deep packet inspection or through other
heuristics. Eden adopts a different tack by allowing applications to explicitly share this information.
Applications, libraries (e.g., an HTTP library) and
services (e.g., NFS or SMB service) that run either in
user space or at the OS can opt to classify network traffic generated by them, i.e., identify messages and their
class(es). A message refers to an (arbitrary) application data unit while a class refers to the set of packets to which the same action function should be applied.
For example, a memcached-specific load balancing function may pick the destination server based on whether
a packet corresponds to a GET or a PUT message. To
allow such functions to be implemented at the Eden
enclave, the memcached application needs to mark all
its traffic as belonging to one of two classes, GETs and
PUTs, and to identify each GET/PUT message separately. The class information is carried through the
end-host stack to the enclave and is used to determine
the action function to apply.
A stage specifies the application-specific fields that
can be used for classification, i.e., fields that can categorize messages into classes. It also specifies meta-data,
i.e., other application-specific data, it can generate. To
make the discussion concrete, Table 2 gives examples
of the classification and meta-data capabilities of three
stages. For instance, an Eden-compliant memcached
Eden controller
A network function is conceptually a combination of
a control-plane function residing at the controller and
a data-plane function. Given a network function, all
computation that either needs global network visibility
or needs to be invoked at coarse timescales can reside in
the control function. By contrast, any computation that
needs to be invoked on a per-packet basis or needs to
access rapidly changing state should be part of the data
function. For our WCMP example (§2.1.1), the control
plane function involves (periodically) determining the
weight associated with each network link based on the
current topology and traffic matrix. These weights are
used to compute the pathMatrix variable used in the
WCMP data plane function shown in Figure 2.
Recent work in the SDN literature mostly focuses on
controller-based algorithms [24] and languages for expressing control plane functions [25, 62]. Hence, in the
<msg_type, key>
<msg_type, url>
Table 2:
dst_ip, dst_port,
proto >
{msg id, msg type,
key, msg size}
{msg id, msg type,
url, msg size}
{msg id}
Table 3: Stage API, used by the controller to
program data-plane stages.
Classification capabilities of a few
r1 : <GET, - >
r1 : <PUT, - >
→ [GET, {msg_id, msg_size}]
→ [PUT, {msg_id, msg_size}]
r2 : <*, - >
→ [DEFAULT, {msg_id, msg_size}]
getStageInfo ()
returns the fields that the stage can use to classify data,
and the meta-data fields it can output
createStageRule (Rule set s, Classifier c, Class id i,
Meta-data set m)
creates classification rule <c> → [i, {m}] in rule set s
returns a unique identifer for the rule
removeStageRule (Rule set s, Rule id r)
belonging to classes named “GET” and “PUT”, and are
associated with meta-data including a unique message
identifier and the message size. Rule-set r2 simply puts
all messages into the “DEFAULT” class and associates
them with a message identifier. Finally, rule-set r3 classifies any GET requests for the key named “a” into the
“GETA” class, any other request for key “a” into the “A”
class, and all other requests to the “OTHER” class.
External to a stage, a class is referred to using its fully
qualified name: stage.rule-set.class_name. Thus,
the class named “GET” in rule-set r1 is referred to as
memcached.r1.GET. Given the rule-sets above, a PUT
request for key “a” would be classified as belonging to
three classes, memcached.r1.PUT, memcached.r2.DEFAULT,
and memcached.r3.A.
r3 : <GET, "a" > → [GETA, {msg_id, msg_size}]
r3 : <*, "a" > → [A, {msg_id, msg_size}]
r3 : <*, * >
→ [OTHER, {msg_id, msg_size}]
Figure 6: Examples of classification rules and
client can classify its messages based on whether it is a
GET or a PUT (msg_type) and the key being accessed
by the message. Furthermore, the stage can associate
the message with meta-data comprising a unique (message) identifier, the message type, the key being accessed, and the message size.
Beyond applications, the Eden enclave itself can also
classify traffic. It operates at the granularity of packets,
and like Open vSwitch, it can classify packets based on
different network headers, including the IP five-tuple.
This is shown in the last line of Table 2. Thus, when
classification is done at the granularity of TCP flows,
each transport connection is a message.
Stage-Controller interface. The controller can program stages using the stage API shown in Table 3.
Through getStageInfo, the controller can discover a
stage’s classification abilities, what fields can be used as
classifiers and the meta-data it can generate. The controller can also create and remove classification rules at
stages (S1,S2 calls in Table 3).
Eden enclave
The enclave resides along the end host network stack,
and operates on packets being sent and received. The
enclave comprises two components:
1). A set of tables with match-action rules that, depending on a packet’s class, determine the action function to apply.
2). A runtime that can execute the action functions.
Classification rules. A stage maintains classification rules, written as <classifier> → [class_name,
{meta-data}]. The classifier is an expression that
defines the class, while the meta-data specifies information, including a message identifier, that should be associated with messages belonging to this class. The message identifier allows the enclave to later recover packets
carrying data for the same message. Classification rules
are arranged into rule-sets such that a message matches
at most one rule in each rule-set. Rule-sets are needed
since different network functions may require stages to
classify their data differently. A given message can be
marked as belonging to many classes, one for each ruleset.
Figure 6 lists a few example classification rules for the
memcached stage. Rule-set r1 comprises two rules, the
first one matches all GET messages while the second
matches PUT messages. The messages are marked as
Match-action tables
In allowing data-plane programmability through enclaves,
we wanted to achieve flexibility without sacrificing forwarding performance. We chose a match-action model
for the enclave for two reasons. First, it is possible
to efficiently implement lookup tables using TCAMs
when the enclave is implemented in hardware. Secondly,
thanks to the popularity of the OpenFlow community,
match-action programming is already a familiar model
for programmers to express network functions.
As shown in Table 4, each rule matches on class names
and produces an action function which is written in a
domain specific language. This is in contrast to the predefined set of actions with today’s SDN design. Match-
<match on class name>
1 fun(packet : Packet, msg : Message, _global :
Global) ->
let msg_size = msg.Size + packet.Size
msg.Size <- msg_size
let priorities = _global.Priorities
let rec search index =
if index >= priorities.Length then 0
elif msg_size <=
else search (index + 1)
packet.Priority <14
let desired = msg.Priority
if desired < 1 then desired
else search 0
f (pkt, . . .)
Table 4: Match-action table in the enclave
ing on class names allows for two types of functions.
First, packet header based classification at the enclave
enables functions that simply need to operate at, say,
the granularity of a TCP flow. Second, classification by
stages means that the enclave can implement functions
that operate on a higher-level grouping of packets as
defined by applications. Next, we describe the action
function language and how enclaves execute the functions.
Action functions
Action functions are written in a domain specific language (DSL) built using F# code quotations.1 The use
of a DSL makes it easier to check safety properties of
the function. For example, action functions cannot perform pointer arithmetic and they need to rely on limited
stack and heap space. We discuss the choice of the specific language in Section 6. As summarized below, the
language provides features that allow expressing basic
computations including loops, and to manipulate both
packets and any state at the enclave.
The action function language is a subset of F#. This
subset does not include objects, exceptions, and floating point operations. Such features benefit large code
bases; instead, we expect small functions running in
the enclave, and hence, the overhead of implementing
those features offers little value. Floating point operations are typically not supported in the kernel, and
they are also not supported by the programmable NICs
we experimented with. The subset of F# that we support includes basic arithmetic operations, assignments,
function definitions, and basic control operations.
Figure 7: Program for priority selection
For each packet, the program updates the size of the
flow (lines 2–3) and then searches in the _global.Priorities
array to find the priority corresponding to the flow size
(using search, lines 6–11). Background flows can specify a low priority class (using flow.Priority), and the
program will respect that (lines 14–15).
An action function can have the following side-effects:
(i) It can modify the packet variable, thus allowing
the function to change header fields, (ii) It can modify
the msg variable, including the class name and metadata associated with the message. This can be used
to control routing decisions for the packet, including
dropping it, sending it to a specific queue associated
with rate limits, sending it to a specific match-action
table or forwarding it to the controller.
We note that while the programmer accesses packet
headers, as well as message and global state using the
packet, msg, and _global function arguments respectively, this is just a convenient illusion for the programmer. It also helps during development and debugging.
As we describe later, Eden’s compiler rewrites the use
of these function arguments.
Action Function example. We use an example to illustrate Eden’s action functions. Figure 7 shows how
the PIAS function discussed in Section 2 (pseudo code
in Figure 4) is written in our action function language.
The function takes three parameters, packet, msg and
_global. packet refers to the actual packet.
It is important to understand message in the context
of the program. For an action function, a message refers
to packets that should be treated as a unit by it. A message could be a TCP flow, a GET/PUT request, or a
HTTP page. As mentioned above, the enclave runtime
determines a packet’s message identifier. The msg parameter thus lets the function access state shared for
the processing of all packets of the same message (Section 3.4.4 details how this state is maintained). Finally,
the _global parameter gives access to state shared by
all instances of the action function that may run in parallel.
Interpreter-based execution
A key challenge posed by executing action functions on
the data plane is ensuring they can run across platforms
and they can be updated dynamically by the controller
without affecting forwarding performance. The latter
requirement is particularly problematic, especially for
today’s programmable NIC platforms that do not support dynamic injection of code. Eden sidesteps these
issues by using an interpreter to execute action functions. The interpreter uses a stack and heap for keeping
action function state.
Additionally, the use of interpreted execution enables
monitoring the execution of the action function and
guaranteeing that it can read and modify only the state
related to that program; i.e., state not associated with
the action function is inaccessible to the program. In
particular, a faulty action function will result in terminating the execution of that program, but will not affect
the rest of the system. Of course, we do rely on correct
execution of the interpreter, but we believe that it is
easier to guarantee the correct execution of the interpreter than to verify every possible action function.
The use of the interpreter, instead of compiling to native code, hurts performance when executing the action
functions. However, as we demonstrate in Section 5,
this is a small penalty for the convenience of injecting
code at runtime, and for the ability to use the same
bytecode across platforms.
[<State(lifetime = Granularity.Packet)>]
type Packet () =
[<HeaderMap("IPv4", "TotalLength")>]
[<HeaderMap("IPv6", "PayloadLength")>]
member val Size = 0L with get, set
Figure 8: Example of annotations used by the
priority selection policy. The HeaderMap annotations specify the field in the header that corresponds to the property. While the Packet type
appears to be a generic class, we expect them to
just contain properties of basic types with associated attributes. We also expect the properties
to provide default initializers.
[<HeaderMap("802.1q", "PriorityCodePoint")>]
member val Priority = 0 with get, set
Eden compiler
We compile action functions to bytecode which is then
interpreted inside the enclave. Given an action function,
the most challenging aspect of the compilation process
is determining its input dependencies and reflecting any
changes made by the function to packet fields and the
state maintained by the runtime. For instance, for the
PIAS action function in Figure 7, the compiler needs
to recover the function’s input parameters. This includes packet information like its size, message-specific
state (msg parameter which is used to determine message size) and global state (_global parameter used to
determine the priority thresholds). For the output, it
needs to determine how to update the packet headers
(e.g., 802.1q priority field in the header) or other state
(e.g., message size).
This is done using three kinds of type annotations
for state variables; we expect the programmer to specify these. First, the lifetime of each state variable, i.e.,
whether it exists for the duration of the message or till
the function is being used in the enclave. Figure 8 highlights an example of the lifetime annotations (we omit
the rest of the annotations due to space constraints).
This lifetime information allows us to decide where to
store the state requested. For convenience of state management, the enclave runtime provides the ability to
store message-specific state, and makes it available upon
function invocation.
Second, the access permissions of a state variable, i.e.,
whether the function can update its value or whether it
is read-only. The access permissions allows us to determine the concurrency level for the function; in Figure 7
the function can update the message size and, hence,
we will process at most one packet per message concurrently. This restriction does not apply to the global
state which is read-only for the function.
Finally, the mapping to packet header values; for example, the mapping of the packet size field in IPv4
headers to the “size” variable inside the function in Figure 7. Currently, we provide a fixed header parsing
scheme, and are investigating more extensible parsing
approaches using ideas from P4 [14].
Apart from resolving input and output dependencies,
the rest of the compilation process, mainly the trans-
lation of the abstract syntax tree to bytecode, is more
straightforward. Value types are preferentially placed
in the stack and otherwise in the local variables and
the heap. More complicated types, such as arrays, are
placed in the program heap (when necessary by copying
the values from the flow or function state). The layout
is then used to generate the bytecode of the program. In
the current version, we perform a number of optimizations such as recognizing tail recursion and compiling it
as a loop.
Concurrency model. As discussed, the programmer
specifies the lifetime and access model of the state used
by the action function with type annotations (Figure 8).
The authoritative state is maintained in the enclave,
and the annotations determine the concurrency model
for the action functions. The enclave creates a consistent copy of the state needed by the program in the heap
and stack. If the action function requires read-only access to message and global state, the enclave is allowed
to schedule multiple invocations of that program in parallel (i.e., the program is allowed to write only to packet
state). The state stored by the enclave may change during program execution; however, the program will not
see the change.
If the action function requires write-access to the state
maintained at the message level, then only one packet
from that message can be processed in parallel. Further,
if the action function requires write-access to global
state, then only one parallel invocation of the action
function is allowed. These restrictions are necessary because, after program termination, the enclave must update the authoritative state with the changes, and pass
updated values to the next invocation of the same action
function. An alternative approach that we have not ex-
(c) overseeing program invocation. Overall, the enclave,
including the interpreter, is about 16K lines of C code.
plored would be to introduce synchronization primitives
in the language. This however might lead to a performance penalty while accessing shared state. Overall,
the programs should restrict writable state to the finest
granularity possible.
Stages associate class and metadata information with
(application) messages; packets generated as a result
of those messages will be carrying the extra information and processed by the interpreter accordingly. The
implementation of that mechanism in its pure form requires a large number of changes in the current network
stack, and application and library code. As a proof of
concept, we have extended the socket interface to implement an additional send primitive that accepts class and
metadata information (our extension works for sockets
using TCP). At the kernel, we record the sequence number of the sender along with the extra information.
At the bottom of the network stack, we intercept
packets before they get transmitted. When we detect
that they contain data with the appropriate sequence
numbers, we update the relevant message state, and
schedule the packet for processing inside the enclave.
The mechanism above allows us to associate class and
metadata information at a per-packet granularity. We
believe that a generic mechanism for associating metadata with events that traverse application and kernel
network stacks could be of general use (see also [11]).
Enclave-Controller interface
The controller can program enclaves through the enclave API. This allows the controller to create, delete
and query both tables and individual match-action rules
in an enclave. We omit the API details for brevity.
Network support
Eden’s end-host based approach to network functions
requires two features from the network: the ability to
specify a packet’s priority and its route. For the former, we use 802.1q tags. For the latter, recent proposals have shown how existing technologies like MPLS
and VLANs can be used to achieve source routing in
datacenters [44, 16]. Here, end hosts specify the path
of a network packet as a label in the packet header, e.g.,
through an MPLS label or VLAN tag, and switches perform label-based forwarding. In our experiments, we
use VLAN tagging for route control.
Such source routing requires the controller to configure the label forwarding tables at switches. For MPLS,
this can be achieved through a distributed control protocol like LDP [5] while for VLANs, Spain [44] describes
a solution involving multiple spanning trees. Hence,
label-based forwarding and the corresponding control
protocol is the primary functionality Eden requires of
the underlying network. Additionally, most existing
switches already support statistics gathering capabilities (e.g SNMP) and priority-based queuing.
Our Eden implementation targets traditional OS network stacks and programmable NICs, and we thus deploy two separate testbeds.
Our software testbed comprises five machines, four
Intel XEON W3530 (2.8GHz/12GB RAM), and one
Intel XEON E7450 (2.4GHz/32GB). All machines use
10GbE Mellanox network cards, and are connected through
an Arista 7050QX-32S-R switch. The enclave runs inside a Windows network filter driver. We added a new
ioctl that allows the application to communicate metadata to the driver, and used that to implement the
(socket-like) message send call.
Our programmable NIC testbed consists of four Intel Xeon E5-1620 (3.70GHz/16GB). Each machine is
equipped with a dual-port 10GbE Netronome NFE-3240
programmable NIC [46]. These NICs descend from Intel’s IXP network processors [31]. The NICs include a
40 core, 8 way multi-threaded CPU (320 threads), and
4GB of memory. We perform CAM operations on main
memory (other models of the processor can offload them
to a large TCAM). The network switch is a 40x10Gbps
Blade RackSwitch.
We have extended the NIC firmware to be able to run
the Eden interpreter. Beyond that, we can also execute
control programs and keep state in the NIC. Currently,
we have limited support for stages in our programmable
NIC testbed.
We now provide more information about the implementation of the enclave and stages including current limitations. We also describe our evaluation testbed.
Interpreter and enclave
The Eden interpreter supports relatively small programs
that use limited (operand) stack and heap space. The
execution is stack based, similar in spirit to the Java
Virtual Machine (JVM); we support many of the standard JVM op-codes. Apart from op-codes that specify
basic load and store, arithmetic, branches, and conditionals, we have also implemented as op-codes a limited
set of basic functions, such as picking random numbers and accessing a high-frequency clock in the system. Unlike general VMs, we do not support many
features such as exceptions, objects, and just-in-time
compilation. An alternative design would be to base
the interpreter on a register machine (as in [41]).
Section 3.4.4 mentioned a number of facilities expected by the enclave that we have implemented: (a)
managing flow and function specific state, (b) preparing
program inputs and consuming program outputs, and
FCT (usec)
FCT (usec)
FCT (usec)
FCT (usec)
Figure 9: Average flow completion times (FCT) for small (left) and intermediate (right) flows with
95% confidence intervals. The top bars show the average while the bottom the 95th percentile.
1MB) and background. Small flows have the highest
priority, followed by the intermediate ones.
Figure 9 shows the average and the 95th -percentile
of the flow completion times for small and intermediate flows when executing the two functions. The figure shows results when running natively in the OS and
through the Eden interpreter across ten runs of the experiments. Baseline highlights flow completion times
without any prioritization. We report two Baseline figures, native and Eden. The latter shows the overhead
of running the classification and data-plane functions,
but ignoring the interpreter output before packets are
transmitted (i.e., the priority marks are not inserted in
the packet header). Essentially, the overhead reflects
running the Eden enclave over vanilla TCP.
As expected, enabling prioritization significantly reduces the flow completion times; for small flows, flow
completion times reduce from 363µs to 274µs on the average, and from 1.6ms to 1ms at the 95th percentile, an
overall a reduction of 25%-40%. Similar trends are observed for intermediate flows as well. At the same time,
background traffic manages to saturate the rest of the
link capacity. SFF, by utilizing application knowledge,
typically provides slightly better performance with less
variability. In SFF, the mapping of flows to classes occurs when the flow starts, and flows do not change priorities over time.
While similar observations have been reported in previous work, what is important in our context is that the
performance of the native implementation of the policy
and the interpreted one are similar. In all cases the
differences are not statistically significant.
Our evaluation of Eden spans three axes: i) It highlights Eden’s performance while implementing diverse
network functions, ii) it covers functions implemented
in an OS network stack and in programmable NICs,
and iii) it provides micro-benchmarks evaluating Eden’s
data-plane overheads. We consider the three case studies from Section 2.
Case Study 1: Flow Scheduling
Our first case study describes how Eden implements
network functions that schedule flows so as to reduce
flow completion times. We examine two such functions: PIAS, whose logic was discussed in Figure 4 and
shortest flow first scheduling (SFF). PIAS requires dataplane computation and state to enable tracking flow
sizes and tagging packets with priorities depending on
thresholds based on the distribution of flow sizes. SFF,
instead, does not track traffic flow sizes but it requires
applications to provide the flow size to the Eden enclave,
so that the correct priority is enforced.
To assess the impact of Eden on the performance of
these functions, we compare Eden with a “native” implementation. The latter implements a hard-coded function within the Eden enclave instead of using the interpreter, similar to a typical implementation through
a customised layer in the OS [8]. Figure 7 shows the
PIAS action function in our language.
The workload driving the experiments is based on a
realistic request-response workload, with responses reflecting the flow size distribution found in search applications [2, 8]. Such applications generate traffic mostly
comprising small flows of a few packets with high rate
of flows starting and terminating. In our set-up, one
worker responds to requests generating load at roughly
70%, while other sources generate background traffic at
the same time. Priority thresholds were set up for three
classes of flows: small (<10KB), intermediate (10KB-
Case Study 2: Load-balancing
The second case study examines how Eden enables perpacket WCMP on top of our programmable NIC testbed.
The WCMP function has been discussed in Section 2
(Figure 2). We arranged the topology of our testbed to
emulate the topology of Figure 1, with two hosts connected through two paths, one 10Gbps and one 1Gbps.
Throughput (MB/sec)
Throughput (Mb/sec)
Figure 11: Average READ vs. WRITE throughput when requests run in isolation, simultaneously, and when READ requests are ratecontrolled based on the request size. 95% confidence intervals are <0.2% of the mean values.
Figure 10: Aggregate throughput for ECMP and
WCMP. Confidence intervals are within 2% of
the values shown.
overhead (%)
The programmable NICs run our custom firmware
that implements the enclave and the Eden interpreter.
The interpreted program controls how packets are sourcerouted through the two paths. We use per-packet balancing, instead of per-flow balancing to highlight Eden’s
ability to apply per-packet functions at high rate. In the
default case, paths are selected for long-running TCP
flows with equal weights, thus implementing ECMP. For
WCMP, we enforce a ratio 10:1.
Figure 10 shows the average throughput achieved for
the two functions with a native program implemented
directly on the programmable NICs and with Eden. For
both ECMP and WCMP, Eden’s overheads are negligible with the difference between Eden and native not
being statistically significant. In ECMP, TCP throughput is dominated by the capacity of the slowest path,
and throughput peaks at just over 2Gbps as expected.
Instead, with per-packet WCMP, TCP throughput is
around 7.8Gbps, 3x better than ECMP. The throughput is lower than the full 11Gbps which is our topology’s
min cut due to in-network reordering of packets [29].
Modifying TCP’s congestion control algorithm can alleviate such issues [53]; however, the goal of this casestudy is to highlight how Eden can easily implement
network functions such as weighted selection of paths
without any impact on performance–in this case with
unmodified applications and while running vanilla TCP.
average 95th-perc.
Figure 12: CPU overheads incurred by Eden
components compared to running the vanilla
TCP stack. API refers to passing metadata information to the enclave.
The previous case studies have focused on the overheads
incurred by Eden with respect to application metrics.
Here, we quantify Eden’s CPU overheads and memory
footprint of the enclave.
Figure 12 quantifies Eden overheads while running
the SFF policy (case study 1). The workload comprises
12 long-running TCP flows in one of our testbed nodes,
and Eden can saturate the 10Gbps capacity of its link.
The figure shows how the different components of Eden
contribute to the CPU load. We consider these overheads reasonable. Note however, that saturating the
link capacity assumes that the cycle budget of the interpreted program allows for such speeds. As we discuss
in the following section, Eden intentionally avoids posing any restrictions in the cycle budget of data-plane
In the examples discussed in the paper, the (operand)
stack and heap space of the interpreter are in the order
of 64 and 256 bytes respectively.
Case Study 3: Datacenter QoS
We now examine Pulsar’s rate control as described in
Figure 3. The experiment involves two tenants running
our custom application that generates 64K IOs. One of
the tenants generates READ requests while the other
one WRITEs to a storage server backed by a RAM disk
drive. The storage server is connected to our testbed
through a 1Gbps link.
The results in Figure 11 show that when WRITE requests compete with READs, their throughput drops by
72%. As previously discussed, this reflects the asymmetry of IO operations; READs are small on the forward
path and manage to fill the queues in shared resources.
Instead, we account for this mismatch through Pulsar’s
rate control, by charging READ requests based on the
request size and WRITEs on the packet size. This ensures equal throughput between the two operations.
The previous sections discussed Eden’s design and demonstrated its expressiveness through case studies. Here, we
briefly complement the discussion of our design choices
and highlight interesting issues for future research.
Choice of language and environment. Beyond the
benefits discussed in Section 3.4 (i.e., expressiveness,
portability, safety, dynamically-injected programs), the
choice of the DSL and the interpreted environment were
by networking devices. Eden avoids this issue by focusing on data plane functions executed by end hosts only.
Recently, Jeyakumar et al. proposed the use of tiny programs that can be carried in packets and executed by
routers [32]. This provides a flexible mechanism to exchange state information between the network and the
end hosts. Eden can benefit from such information, but
is agnostic to how it is obtained.
Today, programmable networks are implemented using Software-Defined Networking (SDN) [23, 37]. SDN
decouples the control plane from the data plane and
requires a way for the control plane to communicate
with the data plane programmatically. The most prominent API for such control- and data-plane interaction
is OpenFlow [42]. Eden focuses on implementing functionality at the ends with a wider and richer data-plane
interface, and goes one step further to take advantage
of application knowledge.
P4 [14] proposes a language for programming packet
processors, hence, generalizing OpenFlow [42]. P4 targets modern switching chips, e.g., [15], and is tailored to
a specific abstract forwarding model comprising parse,
match and action. This results in a restricted programming model that limits its expressivity; for example,
to policies that can be encoded through exact matches.
Using P4 to encode a policy involving comparison operations, like the flow scheduling example used throughout this paper, would be challenging.
The idea of end hosts participating in network functionality has been explored in enterprise and data center networks [49, 16, 21, 34, 59, 57]. For example, Open
vSwitch [50] extends the OpenFlow [42] programming
model to end hosts. However, even in these proposals, the data-plane interface is narrow—it can be programmed to match (mostly) on packet headers and apply a fixed set of actions. They can thus implement
application-agnostic network functions that operate at
coarse time-scales. Instead, Eden can accommodate
network functions that are dynamic, require application
semantics, and operate even at per-packet time-scales.
Network exception handlers [34] was one of the first
proposals to incorporate application semantics into management policies. Akin to program exception handlers,
end hosts register exception conditions with network
controllers, which in turn inform end hosts when exceptions trigger. Corresponding actions at end hosts would
then be parameterized with application context. Applications are a first-order citizen in Eden as well; however,
Eden goes beyond static actions such as rate limiting,
by allowing for functions to be evaluated at end hosts
with minimal controller involvement.
Network function virtualization (NFV) targets virtualizing arbitrary network functions, traditionally implemented at specialized physical devices. Eden can be
seen as an end host pragmatic and reasoned point in
the NFV space.
The Click software router [36] allows the construction
of modular data-plane processing pipelines. It empha-
further motivated by fast prototyping. In particular,
the use of F# code quotations is convenient because
it facilitates retrieval of the abstract syntax tree of the
program, which is the input to our compiler. Additionally, F# allowed us to experiment with various ways of
expressing the action functions, and to run and debug
the programs locally (that can even take place with the
F# interpreter without invoking the compiler and the
enclave interpreter).
Action function composition. In this paper, we assumed a fixed order of network function execution. This
is effective when the functions run in isolation, or when
their composition can be determined trivially. Network functions, however, can interact in arbitrary ways,
hence, it is an open question to define the semantics of
function composition. One option is to impose a hierarchy. This can be achieved, for example, by imposing
that application specific functions precede flow specific
ones or vice versa, or apply priorities to functions which
define the execution order.
OS vs NIC program execution. Eden’s interpreter
can run both in the kernel and in a programmable NIC.
When both are available, deciding where functions run
is an open question. While it may seem that processing should always be offloaded, the cost of offloading
may be prohibitive in some cases. For example, when
the associated metadata is large, or when the interaction with the local controller is frequent. Automatically
partitioning of a high-level policy across different execution points would thus be very useful.
Cycle budget and memory requirements. Achieving
line rates of 10Gbps+ in modern datacenter networks
results in a tight cycle budget for data plane computations. Techniques like IO batching and offloading
are often employed to reduce the processing overhead.
While Eden’s action functions express per-packet computation, they can be extended to allow for computation
over a batch of packets. If the batch contains packets
from multiple messages, the enclave will have to preprocess it and split it into messages.
While the enclave can, in principle, limit the amount
or resources (memory and computational cycles) used
by an action function, we chose not to restrict the complexity of the computation or the amount of state maintained by them. Instead, we believe that the system administrator should decide whether to run a function or
not, irrespective of its overheads. Indeed, deeming such
overheads acceptable may depend on exogenous factors
such as the expected load in the system.
Related work
The idea of a programmable data plane dates back to
active networking. Active networks exposed an API
that allows control over router and switch resources,
enabling processing, storage and packet queue management [60, 56, 63, 12, 3]. However, it has proven difficult
to realize a generic programming model implemented
sizes the flexible chaining of building blocks called elements. Eden also operates in the data-plane, but leverages application information to enables application-level
message-oriented policies. The enclaves share similarities with Click’s elements. Eden’s enclave offers the
convenience of a higher-level environment to network
policy developers. The elements, however, are more appropriate for performance critical tasks (e.g., rate limiting functionality).
Eden enables rapid prototyping of network functions
in the data plane. Other efforts to enable innovation in
the data plane include PLUG [20] and SwitchBlade [7].
These efforts focus on implementing full protocols like
IPv6 on programmable hardware like FPGAs.
Similarly to packet filters [43, 41], the Eden interpreter processes packets in a constrained execution environment inside the kernel according to user-defined programs. However, the Eden interpreter supports richer
packet processing to accommodate more expressive action functions.
The idea of associating metadata with network traffic as it gets processed in the network stack is similar to Mbuf tags [35]. This work also proposes a userlevel mechanism to attach metadata to packets. Eden
aims to start the discussion of extending the interface
to higher level libraries and applications.
Concluding remarks
This paper proposes Eden, an architecture for implementing network functions at end hosts. This involves
applications (stages) classifying application data into
messages and annotating the messages with meta-data.
Enclaves use the message and meta-data information to
apply generic stateful actions on network packets.
Network operators express desired actions in a highlevel language based on F# code quotations; their programs are compiled to bytecode suitable for execution
by an interpreter that operates in the enclave. The enclave compiler and runtime takes care of managing any
state in the enclave, freeing the network programmer
to focus on the functionality to be achieved. The combination of stages, enclave and the interpreted action
language allows expressing a variety of functions in a
natural way and with a small performance penalty.
We are indebted to our colleague Andy Slowey for providing technical support with our experimental setup.
We are also grateful to Rolf Neugebauer, Stuart Wray,
and their colleagues at Netronome for their help with
the NFP3240 NICs. Finally, we thank Dimitrios Vytiniotis, Z. Morley Mao and the anonymous SIGCOMM
reviewers for their feedback.
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