LWIG Working Group C. Gomez
Internet-Draft UPC
Intended status: Informational J. Crowcroft
Expires: September 30, 2019 University of Cambridge
M. Scharf
Hochschule Esslingen
March 29, 2019
TCP Usage Guidance in the Internet of Things (IoT)
draft-ietf-lwig-tcp-constrained-node-networks-07
Abstract
This document provides guidance on how to implement and use the
Transmission Control Protocol (TCP) in Constrained-Node Networks
(CNNs), which are a characterstic of the Internet of Things (IoT).
Such environments require a lightweight TCP implementation and may
not make use of optional functionality. This document explains a
number of known and deployed techniques to simplify a TCP stack as
well as corresponding tradeoffs. The objective is to help embedded
developers with decisions on which TCP features to use.
Status of This Memo
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This Internet-Draft will expire on September 30, 2019.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . 4
3. Characteristics of CNNs relevant for TCP . . . . . . . . . . 4
3.1. Network and link properties . . . . . . . . . . . . . . . 4
3.2. Usage scenarios . . . . . . . . . . . . . . . . . . . . . 5
3.3. Communication and traffic patterns . . . . . . . . . . . 6
4. TCP implementation and configuration in CNNs . . . . . . . . 6
4.1. Path properties . . . . . . . . . . . . . . . . . . . . . 7
4.1.1. Maximum Segment Size (MSS) . . . . . . . . . . . . . 7
4.1.2. Explicit Congestion Notification (ECN) . . . . . . . 8
4.1.3. Explicit loss notifications . . . . . . . . . . . . . 9
4.2. TCP guidance for single-MSS windows and buffers . . . . . 9
4.2.1. Single-MSS stacks - benefits and issues . . . . . . . 9
4.2.2. TCP options for single-MSS stacks . . . . . . . . . . 9
4.2.3. Delayed Acknowledgments for single-MSS stacks . . . . 10
4.2.4. RTO estimation for single-MSS stacks . . . . . . . . 11
4.3. General recommendations for TCP in CNNs . . . . . . . . . 11
4.3.1. Loss recovery and congestion/flow control . . . . . . 11
4.3.1.1. Selective Acknowledgments (SACK) . . . . . . . . 12
4.3.2. Delayed Acknowledgments . . . . . . . . . . . . . . . 12
5. TCP usage recommendations in CNNs . . . . . . . . . . . . . . 13
5.1. TCP connection initiation . . . . . . . . . . . . . . . . 13
5.2. Number of concurrent connections . . . . . . . . . . . . 13
5.3. TCP connection lifetime . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
8. Annex. TCP implementations for constrained devices . . . . . 16
8.1. uIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.2. lwIP . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.3. RIOT . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.4. TinyOS . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.5. FreeRTOS . . . . . . . . . . . . . . . . . . . . . . . . 18
8.6. uC/OS . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. Annex. Changes compared to previous versions . . . . . . . . 20
9.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 20
9.2. Changes between -01 and -02 . . . . . . . . . . . . . . . 20
9.3. Changes between -02 and -03 . . . . . . . . . . . . . . . 20
9.4. Changes between -03 and -04 . . . . . . . . . . . . . . . 21
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9.5. Changes between -04 and -05 . . . . . . . . . . . . . . . 21
9.6. Changes between -05 and -06 . . . . . . . . . . . . . . . 21
9.7. Changes between -06 and -07 . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The Internet Protocol suite is being used for connecting Constrained-
Node Networks (CNNs) to the Internet, enabling the so-called Internet
of Things (IoT) [RFC7228]. In order to meet the requirements that
stem from CNNs, the IETF has produced a suite of new protocols
specifically designed for such environments (see e.g. [RFC8352]).
New IETF protocol stack components include the IPv6 over Low-power
Wireless Personal Area Networks (6LoWPAN) adaptation layer, the IPv6
Routing Protocol for Low-power and lossy networks (RPL) routing
protocol, and the Constrained Application Protocol (CoAP).
As of the writing, the main current transport layer protocols in IP-
based IoT scenarios are UDP and TCP. However, TCP has been
criticized (often, unfairly) as a protocol for the IoT. In fact,
some TCP features are not optimal for IoT scenarios, such as
relatively long header size, unsuitability for multicast, and always-
confirmed data delivery. However, many typical claims on TCP
unsuitability for IoT (e.g. a high complexity, connection-oriented
approach incompatibility with radio duty-cycling, and spurious
congestion control activation in wireless links) are not valid, can
be solved, or are also found in well accepted IoT end-to-end
reliability mechanisms (see [IntComp] for a detailed analysis).
At the application layer, CoAP was developed over UDP [RFC7252].
However, the integration of some CoAP deployments with existing
infrastructure is being challenged by middleboxes such as firewalls,
which may limit and even block UDP-based communications. This the
main reason why a CoAP over TCP specification has been developed
[RFC8323].
Other application layer protocols not specifically designed for CNNs
are also being considered for the IoT space. Some examples include
HTTP/2 and even HTTP/1.1, both of which run over TCP by default
[RFC7230] [RFC7540], and the Extensible Messaging and Presence
Protocol (XMPP) [RFC6120]. TCP is also used by non-IETF application-
layer protocols in the IoT space such as the Message Queue Telemetry
Transport (MQTT) and its lightweight variants.
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TCP is a sophisticated transport protocol that includes optional
functionality (e.g. TCP options) that may improve performance in
some environments. However, many optional TCP extensions require
complex logic inside the TCP stack and increase the codesize and the
memory requirements. Many TCP extensions are not required for
interoperability with other standard-compliant TCP endpoints. Given
the limited resources on constrained devices, careful "tuning" of the
TCP implementation can make an implementation more lightweight.
This document provides guidance on how to implement and use TCP in
CNNs. The overarching goal is to offer simple measures to allow for
lightweight TCP implementation and suitable operation in such
environments. A TCP implementation following the guidance in this
document is intended to be compatible with a TCP endpoint that is
compliant to the TCP standards, albeit possibly with a lower
performance. This implies that such a TCP client would always be
able to connect with a standard-compliant TCP server, and a
corresponding TCP server would always be able to connect with a
standard-compliant TCP client.
This document assumes that the reader is familiar with TCP. A
comprehensive survey of the TCP standards can be found in [RFC7414].
Similar guidance regarding the use of TCP in special environments has
been published before, e.g., for cellular wireless networks
[RFC3481].
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Characteristics of CNNs relevant for TCP
3.1. Network and link properties
CNNs are defined in [RFC7228] as networks whose characteristics are
influenced by being composed of a significant portion of constrained
nodes. The latter are characterized by significant limitations on
processing, memory, and energy resources, among others [RFC7228].
The first two dimensions pose constraints on the complexity and on
the memory footprint of the protocols that constrained nodes can
support. The latter requires techniques to save energy, such as
radio duty-cycling in wireless devices [RFC8352], as well as
minimization of the number of messages transmitted/received (and
their size).
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[RFC7228] lists typical network constraints in CNN, including low
achievable bitrate/throughput, high packet loss and high variability
of packet loss, highly asymmetric link characteristics, severe
penalties for using larger packets, limits on reachability over time,
etc. CNN may use wireless or wired technologies (e.g., Power Line
Communication), and the transmission rates are typically low (e.g.
below 1 Mbps).
For use of TCP, one challenge is that not all technologies in CNN may
be aligned with typical Internet subnetwork design principles
[RFC3819]. For instance, constrained nodes often use physical/link
layer technologies that have been characterized as 'lossy', i.e.,
exhibit a relatively high bit error rate. Dealing with corruption
loss is one of the open issues in the Internet [RFC6077].
3.2. Usage scenarios
There are different deployment and usage scenarios for CNNs. Some
CNNs follow the star topology, whereby one or several hosts are
linked to a central device that acts as a router connecting the CNN
to the Internet. CNNs may also follow the multihop topology
[RFC6606]. One key use case for the use of TCP is a model where
constrained devices connect to unconstrained servers in the Internet.
But it is also possible that both TCP endpoints run on constrained
devices.
In constrained environments, there can be different types of devices
[RFC7228]. For example, there can be devices with single combined
send/receive buffer, devices with a separate send and receive buffer,
or devices with a pool of multiple send/receive buffers. In the
latter case, it is possible that buffers also be shared for other
protocols.
When a CNN comprising one or more constrained devices and an
unconstrained device communicate over the Internet using TCP, the
communication possibly has to traverse a middlebox (e.g. a firewall,
NAT, etc.). Figure 1 illustrates such scenario. Note that the
scenario is asymmetric, as the unconstrained device will typically
not suffer the severe constraints of the constrained device. The
unconstrained device is expected to be mains-powered, to have high
amount of memory and processing power, and to be connected to a
resource-rich network.
Assuming that a majority of constrained devices will correspond to
sensor nodes, the amount of data traffic sent by constrained devices
(e.g. sensor node measurements) is expected to be higher than the
amount of data traffic in the opposite direction. Nevertheless,
constrained devices may receive requests (to which they may respond),
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commands (for configuration purposes and for constrained devices
including actuators) and relatively infrequent firmware/software
updates.
+---------------+
o o <-------- TCP communication -----> | |
o o | |
o o | Unconstrained |
o o +-----------+ | device |
o o o ------ | Middlebox | ------- | |
o o +-----------+ | (e.g. cloud) |
o o o | |
+---------------+
constrained devices
Figure 1: TCP communication between a constrained device and an
unconstrained device, traversing a middlebox.
3.3. Communication and traffic patterns
IoT applications are characterized by a number of different
communication patterns. The following non-comprehensive list
explains some typical examples:
o Unidirectional transfers: An IoT device (e.g. a sensor) can send
(repeatedly) updates to the other endpoint. Not in every case
there is a need for an application response back to the IoT
device.
o Request-response patterns: An IoT device receiving a request from
the other endpoint, which triggers a response from the IoT device.
o Bulk data transfers: A typical example for a long file transfer
would be an IoT device firmware update.
A typical communication pattern is that a constrained device
communicates with an unconstrained device (cf. Figure 1). But it is
also possible that constrained devices communicate amongst
themselves.
4. TCP implementation and configuration in CNNs
This section explains how a TCP stack can deal with typical
constraints in CNN. The guidance in this section relates to the TCP
implementation and its configuration.
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4.1. Path properties
4.1.1. Maximum Segment Size (MSS)
Assuming that IPv6 is used, and for the sake of lightweight
implementation and operation, unless applications require handling
large data units (i.e. leading to an IPv6 datagram size greater than
1280 bytes), it may be desirable to limit the MTU to 1280 bytes in
order to avoid the need to support Path MTU Discovery [RFC8201].
An IPv6 datagram size exceeding 1280 bytes can be avoided by setting
the TCP MSS not larger than 1220 bytes. This assumes that the remote
sender will use no TCP options, aside from possibly the MSS option,
which is only used in the initial TCP SYN packet. In order to
accommodate unrequested TCP options that may be used by some TCP
implementations, a constrained device may advertise an MSS not larger
than 1200 bytes.
Note that setting the MTU to 1280 bytes is possible for link layer
technologies in the CNN space, even if some of them are characterized
by a short data unit payload size, e.g. up to a few tens or hundreds
of bytes. For example, the maximum frame size in IEEE 802.15.4 is
127 bytes. 6LoWPAN defined an adaptation layer to support IPv6 over
IEEE 802.15.4 networks. The adaptation layer includes a
fragmentation mechanism, since IPv6 requires the layer below to
support an MTU of 1280 bytes [RFC2460], while IEEE 802.15.4 lacked
fragmentation mechanisms. 6LoWPAN defines an IEEE 802.15.4 link MTU
of 1280 bytes [RFC4944]. Other technologies, such as Bluetooth LE
[RFC7668], ITU-T G.9959 [RFC7428] or DECT-ULE [RFC8105], also use
6LoWPAN-based adaptation layers in order to enable IPv6 support.
These technologies do support link layer fragmentation. By
exploiting this functionality, the adaptation layers that enable IPv6
over such technologies also define an MTU of 1280 bytes.
On the other hand, there exist technologies also used in the CNN
space, such as Master Slave / Token Passing (TP) [RFC8163],
Narrowband IoT (NB-IoT) [RFC8376] or IEEE 802.11ah
[I-D.delcarpio-6lo-wlanah], that do not suffer the same degree of
frame size limitations as the technologies mentioned above. The MTU
for MS/TP is recommended to be 1500 bytes [RFC8163], the MTU in NB-
IoT is 1600 bytes, and the maximum frame payload size for IEEE
802.11ah is 7991 bytes.
While IPv6 is the main IP version used in IP-based IoT environments,
some IoT scenarios use IPv4. In IPv4, the MTU is 576 bytes. In
order to avoid exceeding the IPv4 MTU, the MSS needs to be set to a
value not larger than 536 bytes. Similarly to the recommendations
given above for IPv6, a constrained device using IPv4 may advertise
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an MSS not larger than 516 bytes in order to accommodate unrequested
TCP options.
Finally, note that using larger MSS (to a suitable extent) may be
beneficial, especially when transferring large payloads, as it
reduces the number of packets (and packet headers) required for a
given payload.
4.1.2. Explicit Congestion Notification (ECN)
Explicit Congestion Notification (ECN) [RFC3168] ECN allows a router
to signal in the IP header of a packet that congestion is arising,
for example when a queue size reaches a certain threshold. An ECN-
enabled TCP receiver will echo back the congestion signal to the TCP
sender by setting a flag in its next TCP ACK. The sender triggers
congestion control measures as if a packet loss had happened.
The document [RFC8087] outlines the principal gains in terms of
increased throughput, reduced delay, and other benefits when ECN is
used over a network path that includes equipment that supports
Congestion Experienced (CE) marking. In the context of CNNs, a
remarkable feature of ECN is that congestion can be signalled without
incurring packet drops (which will lead to retransmissions and
consumption of limited resources such as energy and bandwitdh).
ECN can further reduce packet losses since congestion control
measures can be applied earlier [RFC2884]. Less lost packets implies
that the number of retransmitted segments decreases, which is
particularly beneficial in CNNs, where energy and bandwidth resources
are typically limited. Also, it makes sense to try to avoid packet
drops for transactional workloads with small data sizes, which are
typical for CNNs. In such traffic patterns, it is more difficult to
detect packet loss without retransmission timeouts (e.g., as there
may be no three duplicate ACKs). Any retransmission timeout slows
down the data transfer significantly. In addition, if the
constrained device uses power saving techniques, a retransmission
timeout will incur a wake-up action, in contrast to ACK clock-
triggered sending. When the congestion window of a TCP sender has a
size of one segment, the TCP sender resets the retransmit timer, and
the sender will only be able to send a new packet when the retransmit
timer expires [RFC3168]. Effectively, the TCP sender reduces at that
moment its sending rate from 1 segment per Round Trip Time (RTT) to 1
segment per RTO, which can result in a very low throughput. In
addition to better throughput, ECN can also help reducing latency and
jitter.
ECN can be incrementally deployed in the Internet. Guidance on
configuration and usage of ECN is provided in [RFC7567]. Given the
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benefits, more and more TCP stacks in the Internet support ECN, and
it specifically makes sense to leverage ECN in controlled
environments such as CNNs. Note, however, that supporting ECN
increases implementation complexity.
4.1.3. Explicit loss notifications
There has been a significant body of research on solutions capable of
explicitly indicating whether a TCP segment loss is due to
corruption, in order to avoid activation of congestion control
mechanisms [ETEN] [RFC2757]. While such solutions may provide
significant improvement, they have not been widely deployed and
remain as experimental work. In fact, as of today, the IETF has not
standardized any such solution.
4.2. TCP guidance for single-MSS windows and buffers
This section discusses TCP stacks that focus on transferring a single
MSS. More general guidance is provided in Section 4.3.
4.2.1. Single-MSS stacks - benefits and issues
A TCP stack can reduce the memory requirements by advertising a TCP
window size of one MSS, and also transmit at most one MSS of
unacknowledged data. In that case, both congestion and flow control
implementation is quite simple. Such a small receive and send window
may be sufficient for simple message exchanges in the CNN space.
However, only using a window of one MSS can significantly affect
performance. A stop-and-wait operation results in low throughput for
transfers that exceed the length of one MSS, e.g., a firmware
download. Furthermore, a single-MSS solution relies solely on timer-
based loss recovery, therefore missing the performance gain of Fast
Retransmit and Fast Recovery (which require a larger window size, see
Subsection 4.3.1).
If CoAP is used over TCP with the default setting for NSTART in
[RFC7252], a CoAP endpoint is not allowed to send a new message to a
destination until a response for the previous message sent to that
destination has been received. This is equivalent to an application-
layer window size of 1. For this use of CoAP, a maximum TCP window
of one MSS will be sufficient.
4.2.2. TCP options for single-MSS stacks
A TCP implementation needs to support, at a minimum, TCP options 2, 1
and 0. These are, respectively, the Maximum Segment Size (MSS)
option, the No-Operation option, and the End Of Option List marker
[RFC0793]. None of these are a substantial burden to support. These
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options are sufficient for interoperability with a standard-compliant
TCP endpoint, albeit many TCP stacks support additional options and
can negotiate their use. A TCP implementation is permitted to
silently ignore all other TCP options.
A TCP implementation for a constrained device that uses a single-MSS
TCP receive or transmit window size may not benefit from supporting
the following TCP options: Window scale [RFC7323], TCP Timestamps
[RFC7323], Selective Acknowledgments (SACK) and SACK-Permitted
[RFC2018]. Also other TCP options may not be required on a
constrained device with a very lightweight implementation. With
regard to the Window scale option, note that it is only useful if a
window size greater than 64 kB is needed.
One potentially relevant TCP option in the context of CNNs is TCP
Fast Open (TFO) [RFC7413]. As described in Section 5.3, TFO can be
used to address the problem of traversing middleboxes that perform
early filter state record deletion.
4.2.3. Delayed Acknowledgments for single-MSS stacks
TCP Delayed Acknowledgments are meant to reduce the number of ACKs
sent within a TCP connection, thus reducing network overhead, but
they may increase the time until a sender may receive an ACK. In
general, usefulness of Delayed ACKs depends heavily on the usage
scenario. There can be interactions with stacks that use single-MSS
windows.
A device that advertises a single-MSS receive window should avoid use
of Delayed ACKs in order to avoid contributing unnecessary delay (of
up to 500 ms) to the RTT [RFC5681], which limits the throughput and
can increase the data delivery time.
A device that can send at most one MSS of data is significantly
affected if the receiver uses Delayed ACKs, e.g., if a TCP server or
receiver is outside the CNN. One known workaround is to split the
data to be sent into two segments of smaller size. A standard
compliant TCP receiver will acknowledge the second MSS of data, which
can improve throughput. This "split hack" works if the TCP receiver
uses Delayed ACKs, but the downside is the overhead of sending two IP
packets instead of one.
Similar issues happen when the sender uses the Nagle algorithm.
Disabling the algorithm will not have impact if the sender can only
handle stop-and-wait operation.
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4.2.4. RTO estimation for single-MSS stacks
The Retransmission Timeout (RTO) estimation is one of the fundamental
TCP algorithms. There is a fundamental trade-off: A short,
aggressive RTO behavior reduces wait time before retransmissions, but
it also increases the probability of spurious timeouts. The latter
lead to unnecessary waste of potentially scarce resources in CNNs
such as energy and bandwidth. In contrast, a conservative timeout
can result in long error recovery times and thus needlessly delay
data delivery.
[RFC6298] describes the standard TCP RTO algorithm. If a TCP sender
uses very small window size, and it cannot use Fast Retransmit/Fast
Recovery or SACK, the Retransmission Timeout (RTO) algorithm has a
larger impact on performance than for a more powerful TCP stack. In
that case, RTO algorithm tuning may be considered, although careful
assessment of possible drawbacks is recommended
[I-D.ietf-tcpm-rto-consider].
As an example, an adaptive RTO algorithm for CoAP over UDP has been
defined [I-D.ietf-core-cocoa] that has been found to perform well in
CNN scenarios [Commag].
4.3. General recommendations for TCP in CNNs
This section summarizes some widely used techniques to improve TCP,
with a focus on their use in CNNs. The TCP extensions discussed here
are useful in a wide range of network scenarios, including CNNs.
This section is not comprehensive. A comprehensive survey of TCP
extensions is published in [RFC7414].
4.3.1. Loss recovery and congestion/flow control
Devices that have enough memory to allow a larger (i.e. more than 3
MSS of data) TCP window size can leverage a more efficient loss
recovery than the timer-based approach used for smaller TCP window
size (see Subsection 3.2.1) by using Fast Retransmit and Fast
Recovery [RFC5681], at the expense of slightly greater complexity and
TCB size. Assuming that Delayed ACKs are used by the receiver, the
mentioned algorithms work efficiently for window sizes of at least 5
MSS: If in a given TCP transmission of segments 1, 2, 3, 4, 5, and 6
the segment 2 gets lost, the sender should get an ACK for segment 1
when 3 arrives and duplicate acknowledgements when 4, 5, and 6
arrive. It will retransmit segment 2 when the third duplicate ACK
arrives. In order to have segment 2, 3, 4, 5, and 6 sent, the window
has to be at least 5 MSS. With an MSS of 1220 byte, a buffer of the
size of 5 MSS would require 6100 bytes.
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For bulk data transfers further TCP improvements may also be useful,
such as limited transmit [RFC3042].
4.3.1.1. Selective Acknowledgments (SACK)
If a device with less severe memory and processing constraints can
afford advertising a TCP window size of several MSS, it makes sense
to support the SACK option to improve performance. SACK allows a
data receiver to inform the data sender of non-contiguous data blocks
received, thus a sender (having previously sent the SACK-Permitted
option) can avoid performing unnecessary retransmissions, saving
energy and bandwidth, as well as reducing latency. SACK is
particularly useful for bulk data transfers. The receiver supporting
SACK will need to manage the reception of possible out-of-order
received segments, requiring sufficient buffer space. SACK adds
8*n+2 bytes to the TCP header, where n denotes the number of data
blocks received, up to 4 blocks. For a low number of out-of-order
segments, the header overhead penalty of SACK is compensated by
avoiding unnecessary retransmissions.
4.3.2. Delayed Acknowledgments
For certain traffic patterns, Delayed ACKs may have a detrimental
effect, as already noted in Section 4.2.3. Advanced TCP stacks may
use heuristics to determine the maximum delay for an ACK. For CNNs,
the recommendation depends on the expected communication patterns.
When traffic over a CNN is expected to mostly be unidirectional
messages with a size typically up to one MSS, and the time between
two consecutive message transmissions is greater than the delayed ACK
timeout, it may make sense to use a small timeout or disable delayed
ACKs at the receiver. This avoids incurring additional delay, as
well as the energy consumption of the sender (which might e.g. keep
its radio interface in receive mode) during that time. Note that
disabling delayed ACKs may only be possible if the peer device is
administered by the same entity managing the constrained device. For
request-response traffic, enabling delayed ACKs is recommended, in
order to allow combining a response with the ACK into a single
segment, thus increasing efficiency.
In contrast, Delayed ACKs allow to reduce the number of ACKs in bulk
transfer type of traffic, e.g. for firmware/software updates or for
transferring larger data units containing a batch of sensor readings.
Note that, in many scenarios, the peer that a constrained device
communicates with will be a general purpose system that communicates
with both constrained and unconstrained devices. Since delayed ACKs
are often configured through system-wide parameters, delayed ACKs
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behavior at the peer will be the same regardless of the nature of the
endpoints it talks to. Such a peer will typically have delayed ACKs
enabled.
5. TCP usage recommendations in CNNs
This section discusses how a TCP stack can be used by applications
that are developed for CNN scenarios. These remarks are by and large
independent of how TCP is exactly implemented.
5.1. TCP connection initiation
In the constrained device to unconstrained device scenario
illustrated above, a TCP connection is typically initiated by the
constrained device, in order for this device to support possible
sleep periods to save energy.
5.2. Number of concurrent connections
TCP endpoints with a small amount of memory may only support a small
number of connections. Each TCP connection requires storing a number
of variables in the Transmission Control Block (TCB). Depending on
the internal TCP implementation, each connection may result in
further memory overhead, and connections may compete for scarce
resources (e.g. further memory overhead for send and receive buffers,
etc).
A careful application design may try to keep the number of concurrent
connections as small as possible. A client can for instance limit
the number of simultaneous open connections that it maintains to a
given server. Multiple connections could for instance be used to
avoid the "head-of-line blocking" problem in an application transfer.
However, in addition to comsuming resources, using multiple
connections can also cause undesirable side effects in congested
networks. For example, the HTTP/1.1 specification encourages clients
to be conservative when opening multiple connections [RFC7230].
Furthermore, each new connection will start with a 3-way handshake,
therefore increasing message overhead.
Being conservative when opening multiple TCP connections is of
particular importance in Constrained-Node Networks.
5.3. TCP connection lifetime
In order to minimize message overhead, it makes sense to keep a TCP
connection open as long as the two TCP endpoints have more data to
send. If applications exchange data rather infrequently, i.e., if
TCP connections would stay idle for a long time, the idle time can
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result in problems. For instance, certain middleboxes such as
firewalls or NAT devices are known to delete state records after an
inactivity interval. RFC 5382 specifies a minimum value for such
interval of 124 minutes. A mean TCP NAT binding timeout of 386
minutes has been reported, while in some cases, inactivity timeouts
are in the order of a few minutes [HomeGateway]. The timeout
duration used by a middlebox implementation may not be known to the
TCP endpoints.
In CNNs, such middleboxes may e.g. be present at the boundary between
the CNN and other networks. If the middlebox can be optimized for
CNN use cases, it makes sense to increase the initial value for
filter state inactivity timers to avoid problems with idle
connections. Apart from that, this problem can be dealt with by
different connection handling strategies, each having pros and cons.
One approach for infrequent data transfer is to use short-lived TCP
connections. Instead of trying to maintain a TCP connection for long
time, possibly short-lived connections can be opened between two
endpoints, which are closed if no more data needs to be exchanged.
For use cases that can cope with the additional messages and the
latency resulting from starting new connections, it is recommended to
use a sequence of short-lived connections, instead of maintaining a
single long-lived connection.
The message and latency overhead that stems from using a sequence of
short-lived connections could be reduced by TCP Fast Open (TFO)
[RFC7413], which is an experimental TCP extension, at the expense of
increased implementation complexity and increased TCP Control Block
(TCB) size. TFO allows data to be carried in SYN (and SYN-ACK)
segments, and to be consumed immediately by the receiving endpoint.
This reduces the message and latency overhead compared to the
traditional three-way handshake to establish a TCP connection. For
security reasons, the connection initiator has to request a TFO
cookie from the other endpoint. The cookie, with a size of 4 or 16
bytes, is then included in SYN packets of subsequent connections.
The cookie needs to be refreshed (and obtained by the client) after a
certain amount of time. Nevertheless, TFO is more efficient than
frequently opening new TCP connections with the traditional three-way
handshake, as long as the cookie can be reused in subsequent
connections. However, as stated in RFC 7413, TFO deviates from the
standard TCP semantics, since the data in the SYN could be replayed
to an application in some rare circumstances. Applications should
not use TFO unless they can tolerate this issue, e.g., by using
Transport Layer Security (TLS) [RFC7413]. A comprehensive discussion
on TFO can be found at RFC 7413.
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Another approach is to use long-lived TCP connections with
application-layer heartbeat messages. Various application protocols
support such heartbeat messages (e.g. CoAP over TCP [RFC8323]).
Periodic application-layer heartbeats can prevent early filter state
record deletion in middleboxes. If the TCP binding timeout for a
middlebox to be traversed by a given connection is known, middlebox
filter state deletion will be avoided if the heartbeat period is
lower than the middlebox TCP binding timeout. Otherwise, the
implementer needs to take into account that middlebox TCP binding
timeouts fall in a wide range of possible values [HomeGateway]. One
specific advantage of Heartbeat messages is that they also allow
aliveness checks at the application level. In general, it makes
sense to realize aliveness checks at the highest protocol layer
possible that is meaningful to the application, in order to maximize
the depth of the aliveness check. In addition, timely detection of a
dead peer may allow savings in terms of TCB memory use. However, the
transmission of heartbeat messages consumes resources. This aspect
needs to be assessed carefully, considering the characteristics of
each specific CNN.
A TCP implementation may also be able to send "keep-alive" segments
to test a TCP connection. According to [RFC1122], "keep-alives" are
an optional TCP mechanism that is turned off by default, i.e., an
application must explicitly enable it for a TCP connection. The
interval between "keep-alive" messages must be configurable and it
must default to no less than two hours. With this large timeout, TCP
keep-alive messages might not always be useful to avoid deletion of
filter state records in some middleboxes. However, sending TCP keep-
alive probes more frequently risks draining power on energy-
constrained devices.
6. Security Considerations
Best current practise for securing TCP and TCP-based communication
also applies to CNN. As example, use of Transport Layer Security
(TLS) is strongly recommended if it is applicable.
There are also TCP options which can improve TCP security. One
example is the TCP Authentication Option (TCP-AO) [RFC5925].
However, this option adds overhead and complexity. TCP-AO typically
has a size of 16-20 bytes.
For the mechanisms discussed in this document, the corresponding
considerations apply. For instance, if TFO is used, the security
considerations of [RFC7413] apply.
Constrained devices are expected to support smaller TCP window sizes
than less limited devices. In such conditions, segment
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retransmission triggered by RTO expiration is expected to be
relatively frequent, due to lack of (enough) duplicate ACKs,
especially when a constrained device uses a single-MSS window size.
For this reason, constrained devices running TCP may appear as
particularly appealing victims of the so-called "shrew" Denial of
Service (DoS) attack [shrew], whereby one or more sources generate a
packet spike targetted to coincide with consecutive RTO-expiration-
triggered retry attempts of a victim node. Note that the attack may
be performed by Internet-connected devices, including constrained
devices in the same CNN as the victim, as well as remote ones.
Mitigation techniques include RTO randomization and attack blocking
by routers able to detect shrew attacks based on their traffic
pattern.
7. Acknowledgments
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grants CAS15/00336 and and CAS18/00170, and by European
Regional Development Fund (ERDF) and the Spanish Government through
project TEC2016-79988-P, AEI/FEDER, UE. Part of his contribution to
this work has been carried out during his stays as a visiting scholar
at the Computer Laboratory of the University of Cambridge.
The authors appreciate the feedback received for this document. The
following folks provided comments that helped improve the document:
Carsten Bormann, Zhen Cao, Wei Genyu, Ari Keranen, Abhijan
Bhattacharyya, Andres Arcia-Moret, Yoshifumi Nishida, Joe Touch, Fred
Baker, Nik Sultana, Kerry Lynn, Erik Nordmark, Markku Kojo, Hannes
Tschofenig, David Black, Yoshifumi Nishida, Ilpo Jarvinen, Emmanuel
Baccelli, and Stuart Cheshire. Simon Brummer provided details, and
kindly performed RAM and ROM usage measurements, on the RIOT TCP
implementation. Xavi Vilajosana provided details on the OpenWSN TCP
implementation. Rahul Jadhav kindly performed code size measurements
on the Contiki-NG and lwIP 2.1.2 TCP implementations. He also
provided details on the uIP TCP implementation.
8. Annex. TCP implementations for constrained devices
This section overviews the main features of TCP implementations for
constrained devices. The survey is limited to open source stacks
with small footprint. It is not meant to be all-encompassing. For
more powerful embedded systems (e.g., with 32-bit processors), there
are further stacks that comprehensively implement TCP. On the other
hand, please be aware that this Annex is based on information
available as of the writing.
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8.1. uIP
uIP is a TCP/IP stack, targetted for 8 and 16-bit microcontrollers,
which pioneered TCP/IP implementations for constrained devices. uIP
has been deployed with Contiki and the Arduino Ethernet shield. A
code size of ~5 kB (which comprises checksumming, IP, ICMP and TCP)
has been reported for uIP [Dunk].
uIP uses the same global buffer for both incoming and outgoing
traffic, which has a size of a single packet. In case of a
retransmission, an application must be able to reproduce the same
user data that had been transmitted. Multiple connections are
supported, but need to share the global buffer.
The MSS is announced via the MSS option on connection establishment
and the receive window size (of one MSS) is not modified during a
connection. Stop-and-wait operation is used for sending data. Among
other optimizations, this allows to avoid sliding window operations,
which use 32-bit arithmetic extensively and are expensive on 8-bit
CPUs.
Contiki uses the "split hack" technique (see Section 4.2.3) to avoid
Delayed ACKs for senders using a single segment.
The code size of the TCP implementation in Contiki-NG has been
measured to be of 3.2 kB on CC2538DK, cross-compiling on Linux.
8.2. lwIP
lwIP is a TCP/IP stack, targetted for 8- and 16-bit microcontrollers.
lwIP has a total code size of ~14 kB to ~22 kB (which comprises
memory management, checksumming, network interfaces, IP, ICMP and
TCP), and a TCP code size of ~9 kB to ~14 kB [Dunk].
In contrast with uIP, lwIP decouples applications from the network
stack. lwIP supports a TCP transmission window greater than a single
segment, as well as buffering of incoming and outcoming data. Other
implemented mechanisms comprise slow start, congestion avoidance,
fast retransmit and fast recovery. SACK and Window Scale support has
been recently added to lwIP.
8.3. RIOT
The RIOT TCP implementation (called GNRC TCP) has been designed for
Class 1 devices [RFC 7228]. The main target platforms are 8- and
16-bit microcontrollers, with 32-bit platforms also supported. GNRC
TCP offers a similar function set as uIP, but it provides and
maintains an independent receive buffer for each connection. In
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contrast to uIP, retransmission is also handled by GNRC TCP. GNRC
TCP uses a single-MSS window size, which simplifies the
implementation. The application programmer does not need to know
anything about the TCP internals, therefore GNRC TCP can be seen as a
user-friendly uIP TCP implementation.
The MSS is set on connections establishment and cannot be changed
during connection lifetime. GNRC TCP allows multiple connections in
parallel, but each TCB must be allocated somewhere in the system. By
default there is only enough memory allocated for a single TCP
connection, but it can be increased at compile time if the user needs
multiple parallel connections.
The RIOT TCP implementation offers an optional POSIX socket wrapper
that enables POSIX compliance, if needed.
Further details on RIOT and GNRC can be found in the literature
[RIOT], [GNRC].
8.4. TinyOS
TinyOS was important as platform for early constrained devices.
TinyOS has an experimental TCP stack that uses a simple nonblocking
library-based implementation of TCP, which provides a subset of the
socket interface primitives. The application is responsible for
buffering. The TCP library does not do any receive-side buffering.
Instead, it will immediately dispatch new, in-order data to the
application and otherwise drop the segment. A send buffer is
provided by the application. Multiple TCP connections are possible.
Recently there has been little further work on the stack.
8.5. FreeRTOS
FreeRTOS is a real-time operating system kernel for embedded devices
that is supported by 16- and 32-bit microprocessors. Its TCP
implementation is based on multiple-segment window size, although a
'Tiny-TCP' option, which is a single-MSS variant, can be enabled.
Delayed ACKs are supported, with a 20-ms Delayed ACK timer as a
technique intended 'to gain performance'.
8.6. uC/OS
uC/OS is a real-time operating system kernel for embedded devices,
which is maintained by Micrium. uC/OS is intended for 8-, 16- and
32-bit microprocessors. The uC/OS TCP implementation supports a
multiple-segment window size.
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8.7. Summary
+---+---------+--------+----+------+--------+-----+
|uIP|lwIP orig|lwIP 2.1|RIOT|TinyOS|FreeRTOS|uC/OS|
+------+-------------+---+---------+--------+----+------+--------+-----+
|Memory|Code size(kB)| <5|~9 to ~14| 38 | <7 | N/A | <9.2 | N/A |
| | |(a)| (T1) | (T4) |(T3)| | (T2) | |
+------+-------------+---+---------+--------+----+------+--------+-----+
| | Single-Segm.|Yes| No | No | Yes| No | No | No |
| +-------------+---+---------+--------+----+------+--------+-----+
| | Slow start | No| Yes | Yes | No | Yes | No | Yes |
| T +-------------+---+---------+--------+----+------+--------+-----+
| C |Fast rec/retx| No| Yes | Yes | No | Yes | No | Yes |
| P +-------------+---+---------+--------+----+------+--------+-----+
| | Keep-alive | No| No | Yes | No | No | Yes | Yes |
| +-------------+---+---------+--------+----+------+--------+-----+
| f | Win. Scale | No| No | Yes | No | No | Yes | No |
| e +-------------+---+---------+--------+----+------+--------+-----+
| a | TCP timest.| No| No | Yes | No | No | Yes | No |
| t +-------------+---+---------+--------+----+------+--------+-----+
| u | SACK | No| No | Yes | No | No | Yes | No |
| r +-------------+---+---------+--------+----+------+--------+-----+
| e | Del. ACKs | No| Yes | Yes | No | No | Yes | Yes |
| s +-------------+---+---------+--------+----+------+--------+-----+
| | Socket | No| No |Optional|(I) |Subset| Yes | Yes |
| +-------------+---+---------+--------+----+------+--------+-----+
| |Concur. Conn.|Yes| Yes | Yes | Yes| Yes | Yes | Yes |
+------+-------------+---+---------+--------+----+------+--------+-----+
| TLS supported | No| No | Yes | Yes| Yes | Yes | Yes |
+--------------------+---+---------+--------+----+------+--------+-----+
(T1) = TCP-only, on x86 and AVR platforms
(T2) = TCP-only, on ARM Cortex-M platform
(T3) = TCP-only, on ARM Cortex-M0+ platform (NOTE: RAM usage for the same platform
is ~2.5 kB for one TCP connection plus ~1.2 kB for each additional connection)
(T4) = TCP-only, on CC2538DK, cross-compiling on Linux
(a) = includes IP, ICMP and TCP on x86 and AVR platforms. The Contiki-NG TCP implementation has a code size of 3.2 kB on CC2538DK, cross-compiling on Linux
(I) = optional POSIX socket wrapper which enables POSIX compliance if needed
Mult. = Multiple
N/A = Not Available
Figure 2: Summary of TCP features for differrent lightweight TCP
implementations. None of the implementations considered in this
Annex support ECN or TFO.
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9. Annex. Changes compared to previous versions
RFC Editor: To be removed prior to publication
9.1. Changes between -00 and -01
o Changed title and abstract
o Clarification that communcation with standard-compliant TCP
endpoints is required, based on feedback from Joe Touch
o Additional discussion on communication patters
o Numerous changes to address a comprehensive review from Hannes
Tschofenig
o Reworded security considerations
o Additional references and better distinction between normative and
informative entries
o Feedback from Rahul Jadhav on the uIP TCP implementation
o Basic data for the TinyOS TCP implementation added, based on
source code analysis
9.2. Changes between -01 and -02
o Added text to the Introduction section, and a reference, on
traditional bad perception of TCP for IoT
o Added sections on FreeRTOS and uC/OS
o Updated TinyOS section
o Updated summary table
o Reorganized Section 4 (single-MSS vs multiple-MSS window size),
some content now also in new Section 5
9.3. Changes between -02 and -03
o Rewording to better explain the benefit of ECN
o Additional context information on the surveyed implementations
o Added details, but removed "Data size" raw, in the summary table
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o Added discussion on shrew attacks
9.4. Changes between -03 and -04
o Addressing the remaining TODOs
o Alignment of the wording on TCP "keep-alives" with related
discussions in the IETF transport area
o Added further discussion on delayed ACKs
o Removed OpenWSN subsection from the Annex
9.5. Changes between -04 and -05
o Addressing comments by Yoshifumi Nishida
o Removed mentioning MD5 as an example (comment by David Black)
o Added memory footprint details of TCP implementations (Contiki-NG
and lwIP 2.1.2) provided by Rahul Jadhav in the Annex
o Addressed comments by Ilpo Jarvinen throughout the whole document
o Improved the RIOT section in the Annex, based on feedback from
Emmanuel Baccelli
9.6. Changes between -05 and -06
o Incorporated suggestions by Stuart Cheshire
9.7. Changes between -06 and -07
o Addressed comments by Gorry Fairhurst
10. References
10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
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[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC3042] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
TCP's Loss Recovery Using Limited Transmit", RFC 3042,
DOI 10.17487/RFC3042, January 2001,
<https://www.rfc-editor.org/info/rfc3042>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, DOI 10.17487/RFC3819, July 2004,
<https://www.rfc-editor.org/info/rfc3819>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
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[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
10.2. Informative References
[Commag] A. Betzler, C. Gomez, I. Demirkol, J. Paradells, "CoAP
Congestion Control for the Internet of Things", IEEE
Communications Magazine, June 2016.
[Dunk] A. Dunkels, "Full TCP/IP for 8-Bit Architectures", 2003.
[ETEN] R. Krishnan et al, "Explicit transport error notification
(ETEN) for error-prone wireless and satellite networks",
Computer Networks 2004.
[GNRC] M. Lenders et al., "Connecting the World of Embedded
Mobiles: The RIOTApproach to Ubiquitous Networking for the
IoT", 2018.
[HomeGateway]
Haetoenen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
Sarolahti, P., and M. Kojo, "An Experimental Study of Home
Gateway Characteristics", Proceedings of the 10th ACM
SIGCOMM conference on Internet measurement 2010.
[I-D.delcarpio-6lo-wlanah]
Vega, L., Robles, I., and R. Morabito, "IPv6 over
802.11ah", draft-delcarpio-6lo-wlanah-01 (work in
progress), October 2015.
[I-D.ietf-core-cocoa]
Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", draft-ietf-
core-cocoa-03 (work in progress), February 2018.
[I-D.ietf-tcpm-rto-consider]
Allman, M., "Retransmission Timeout Requirements", draft-
ietf-tcpm-rto-consider-08 (work in progress), February
2019.
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[IntComp] C. Gomez, A. Arcia-Moret, J. Crowcroft, "TCP in the
Internet of Things: from ostracism to prominence", IEEE
Internet Computing, January-February 2018.
[RFC2757] Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N.
Vaidya, "Long Thin Networks", RFC 2757,
DOI 10.17487/RFC2757, January 2000,
<https://www.rfc-editor.org/info/rfc2757>.
[RFC2884] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
Explicit Congestion Notification (ECN) in IP Networks",
RFC 2884, DOI 10.17487/RFC2884, July 2000,
<https://www.rfc-editor.org/info/rfc2884>.
[RFC3481] Inamura, H., Ed., Montenegro, G., Ed., Ludwig, R., Gurtov,
A., and F. Khafizov, "TCP over Second (2.5G) and Third
(3G) Generation Wireless Networks", BCP 71, RFC 3481,
DOI 10.17487/RFC3481, February 2003,
<https://www.rfc-editor.org/info/rfc3481>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC6077] Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
Briscoe, "Open Research Issues in Internet Congestion
Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,
<https://www.rfc-editor.org/info/rfc6077>.
[RFC6092] Woodyatt, J., Ed., "Recommended Simple Security
Capabilities in Customer Premises Equipment (CPE) for
Providing Residential IPv6 Internet Service", RFC 6092,
DOI 10.17487/RFC6092, January 2011,
<https://www.rfc-editor.org/info/rfc6092>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing",
RFC 6606, DOI 10.17487/RFC6606, May 2012,
<https://www.rfc-editor.org/info/rfc6606>.
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[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414,
DOI 10.17487/RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>.
[RFC7428] Brandt, A. and J. Buron, "Transmission of IPv6 Packets
over ITU-T G.9959 Networks", RFC 7428,
DOI 10.17487/RFC7428, February 2015,
<https://www.rfc-editor.org/info/rfc7428>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.
[RFC7668] Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,
<https://www.rfc-editor.org/info/rfc7668>.
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>.
[RFC8105] Mariager, P., Petersen, J., Ed., Shelby, Z., Van de Logt,
M., and D. Barthel, "Transmission of IPv6 Packets over
Digital Enhanced Cordless Telecommunications (DECT) Ultra
Low Energy (ULE)", RFC 8105, DOI 10.17487/RFC8105, May
2017, <https://www.rfc-editor.org/info/rfc8105>.
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[RFC8163] Lynn, K., Ed., Martocci, J., Neilson, C., and S.
Donaldson, "Transmission of IPv6 over Master-Slave/Token-
Passing (MS/TP) Networks", RFC 8163, DOI 10.17487/RFC8163,
May 2017, <https://www.rfc-editor.org/info/rfc8163>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[RFC8352] Gomez, C., Kovatsch, M., Tian, H., and Z. Cao, Ed.,
"Energy-Efficient Features of Internet of Things
Protocols", RFC 8352, DOI 10.17487/RFC8352, April 2018,
<https://www.rfc-editor.org/info/rfc8352>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.
[RIOT] E. Baccelli et al., "RIOT: an Open Source Operating
Systemfor Low-end Embedded Devices in the IoT", 2018.
[shrew] A. Kuzmanovic, E. Knightly, "Low-Rate TCP-Targeted Denial
of Service Attacks", SIGCOMM'03 2003.
Authors' Addresses
Carles Gomez
UPC
C/Esteve Terradas, 7
Castelldefels 08860
Spain
Email: carlesgo@entel.upc.edu
Gomez, et al. Expires September 30, 2019 [Page 26]
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Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge, CB3 0FD
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
Michael Scharf
Hochschule Esslingen
Flandernstr. 101
Esslingen 73732
Germany
Email: michael.scharf@hs-esslingen.de
Gomez, et al. Expires September 30, 2019 [Page 27]