LWIG Working Group C. Gomez
Internet-Draft UPC/i2CAT
Intended status: Informational J. Crowcroft
Expires: April 17, 2018 University of Cambridge
M. Scharf
Nokia
October 14, 2017
TCP Usage Guidance in the Internet of Things (IoT)
draft-ietf-lwig-tcp-constrained-node-networks-01
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 17, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . . . . . . . . . . . 4
3.3. Communication and traffic patterns . . . . . . . . . . . 5
4. TCP over CNNs . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. TCP connection initiation . . . . . . . . . . . . . . . . 6
4.2. Maximum Segment Size (MSS) . . . . . . . . . . . . . . . 6
4.3. Window Size . . . . . . . . . . . . . . . . . . . . . . . 7
4.4. RTO estimation . . . . . . . . . . . . . . . . . . . . . 8
4.5. TCP connection lifetime . . . . . . . . . . . . . . . . . 8
4.5.1. Long TCP connection lifetime . . . . . . . . . . . . 8
4.5.2. Short TCP connection lifetime . . . . . . . . . . . . 9
4.6. Explicit congestion notification . . . . . . . . . . . . 9
4.7. TCP options . . . . . . . . . . . . . . . . . . . . . . . 10
4.8. Delayed Acknowledgments . . . . . . . . . . . . . . . . . 11
4.9. Explicit loss notifications . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
7. Annex. TCP implementations for constrained devices . . . . . 12
7.1. uIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.2. lwIP . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.3. RIOT . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.4. OpenWSN . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.5. TinyOS . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Annex. Changes compared to previous versions . . . . . . . . 15
8.1. Changes compared to -00 . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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
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stem from CNNs, the IETF has produced a suite of new protocols
specifically designed for such environments (see e.g.
[I-D.ietf-lwig-energy-efficient]).
At the application layer, the Constrained Application Protocol (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 is being developed [I-D.ietf-core-coap-tcp-tls].
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
[RFC7540] [RFC2616], 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.
TCP is a sophisticated transport protocol that includes many optional
functionality and TCP options that improve performance. Many
optional TCP extensions require complex logic inside the TCP stack
and increase the codesize and the RAM requirements. However, 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].
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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
[I-D.ietf-lwig-energy-efficient], as well as minimization of the
number of messages transmitted/received (and their size).
[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.
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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),
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:
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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 over CNNs
4.1. 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.
4.2. Maximum Segment Size (MSS)
Some link layer technologies in the CNN space 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) [I-D.ietf-lpwan-overview] 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-
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IoT is 1600 bytes, and the maximum frame payload size for IEEE
802.11ah is 7991 bytes.
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 [RFC1981].
An IPv6 datagram size exceeding 1280 bytes can be avoided by setting
the TCP MSS not larger than 1220 bytes. (Note: IP version 6 is
assumed.)
4.3. Window Size
A TCP stack can reduce the RAM 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 lengths of one MSS, e.g., a firmware
download. In addition, there can be interactions with the delayed
acknowledgements (see Section 4.8).
Devices that have enough memory to allow larger TCP window size can
leverage a more efficient error recovery using Fast Retransmit and
Fast Recovery [RFC5681]. These 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 acknowledgement 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 five. With an MSS
of 1220 byte, a buffer of the size of 5 MSS would require 6100 byte.
For bulk data transfers further TCP improvements may also be useful,
such as limited transmit [RFC3402].
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.
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4.4. RTO estimation
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 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.
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].
4.5. TCP connection lifetime
[[Note: future revisions will better separate what a TCP stack should
support, or not, and how the TCP stack should be used by
applications, e.g., whether to close connections or not.]]
4.5.1. Long TCP connection lifetime
In CNNs, in order to minimize message overhead, a TCP connection
should be kept open as long as the two TCP endpoints have more data
to exchange or it is envisaged that further segment exchanges will
take place within an interval of two hours since the last segment has
been sent. A greater interval may be used in scenarios where
applications exchange data infrequently.
TCP keep-alive messages [RFC1122] may be supported by a server, to
check whether a TCP connection is active, in order to release state
of inactive connections. This may be useful for servers running on
memory-constrained devices.
Since the keep-alive timer may not be set to a value lower than two
hours [RFC1122], TCP keep-alive messages are not useful to guarantee
that filter state records in middleboxes such as firewalls will not
be deleted after an inactivity interval typically in the order of a
few minutes [RFC6092]. In scenarios where such middleboxes are
present, alternative measures to avoid early deletion of filter state
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records (which might lead to frequent establishment of new TCP
connections between the two involved endpoints) include increasing
the initial value for the filter state inactivity timers (if
possible), and using application layer heartbeat messages.
4.5.2. Short TCP connection lifetime
A different approach to addressing the problem of traversing
middleboxes that perform early filter state record deletion relies on
using TCP Fast Open (TFO) [RFC7413]. In this case, instead of trying
to maintain a TCP connection for long time, possibly short-lived
connections can be opened between two endpoints while incurring low
overhead. In fact, TFO allows data to be carried in SYN (and SYN-
ACK) packets, and to be consumed immediately by the receceiving
endpoint, thus reducing overhead compared with the traditional three-
way handshake required to establish a TCP connection.
For security reasons, TFO requires the TCP endpoint that will open
the TCP connection (which in CNNs will typically be the constrained
device) to request a 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 (by using the traditional three-way handshake) for
transmitting new data, as long as the cookie update rate is well
below the data new connection rate.
4.6. Explicit congestion notification
Explicit Congestion Notification (ECN) [RFC3168] may be used in CNNs.
ECN allows a router to signal in the IP header of a packet that
congestion is arising, for example when queue size reaches a certain
threshold. If such a packet encapsulates a TCP data packet, 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. In
that case, when the congestion window of a TCP sender has a size of
one segment, the TCP sender resets the retransmit timer, and 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 default RTO.
ECN can reduce packet losses, since congestion control measures can
be applied earlier than after the reception of three duplicate ACKs
(if the TCP sender window is large enough) or upon TCP sender RTO
expiration [RFC2884]. Therefore, the number of retries decreases,
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which is particularly beneficial in CNNs, where energy and bandwidth
resources are typically limited. Furthermore, latency and jitter are
also reduced.
ECN is particularly appropriate in CNNs, since in these environments
transactional type interactions are a dominant traffic pattern. As
transactional data size decreases, the probability of detecting
congestion by the presence of three duplicate ACKs decreases. In
contrast, ECN can still activate congestion control measures without
requiring three duplicate ACKs.
4.7. TCP options
A TCP implementation needs to support options 0, 1 and 2 [RFC0793].
These 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 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 [RFC1323], TCP Timestamps
[RFC1323], Selective Acknowledgements (SACK) and SACK-Permitted
[RFC2018]. Also other TCP options may not be required on a
constrained device with a very lightweight implementation.
If a device with less severe memory and processing constraints can
afford advertising a TCP window size of several MSSs, 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.
Another potentially relevant TCP option in the context of CNNs is
(TFO) [RFC7413]. As described in Section 4.5.2, TFO can be used to
address the problem of traversing middleboxes that perform early
filter state record deletion.
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4.8. Delayed Acknowledgments
TCP Delayed Acknowledgements reduce the number of transferred bytes
within a TCP connection, but they may increase the time until a
sender may receive an ACK. For certain traffic patterns Delayed
Acknowledgements may have a detrimental effect. Advanced TCP stacks
may use heuristics to determine the maximum delay for an ACK. For
CNNs, the recommendation depends on the expected communication
patterns.
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 then immediately acknowledge the second
segment, 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.
Also for larger windows, it may make sense to use a small timeout or
disable delayed ACKs when traffic over a CNN is expected to mostly be
small messages with a size typically below one MSS. For request-
response traffic between a constrained device and a peer (e.g.
backend infrastructure) that uses delayed ACKs, the maximum ACK rate
of the peer will be typically of one ACK every 200 ms (or even
lower). If in such conditions the peer device is administered by the
same entity managing the constrained device, it is recommended to
disable delayed ACKs at the peer side.
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.
4.9. 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.
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5. 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. Examples
include the TCP MD5 signature option [RFC2385] and the TCP
Authentication Option (TCP-AO) [RFC5925]. However, both options add
overhead and complexity. The TCP MD5 signature option adds 18 bytes
to every segment of a connection. 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.
6. Acknowledgments
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336 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 stay 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, and
Hannes Tschofenig. Simon Brummer provided details on the RIOT TCP
implementation. Xavi Vilajosana provided details on the OpenWSN TCP
implementation. Rahul Jadhav provided details on the uIP TCP
implementation.
7. Annex. TCP implementations for constrained devices
This section overviews the main features of TCP implementations for
constrained devices.
7.1. uIP
uIP is a TCP/IP stack, targetted for 8 and 16-bit microcontrollers.
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].
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uIP uses same buffer both incoming and outgoing traffic, with 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.
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 4.8) to avoid
delayed ACKs for senders using a single MSS.
7.2. 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 have been
recently added to lwIP.
7.3. 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. GNRC TCP offers a similar function set as
uIP, but it provides and maintains an independent receive buffer for
each connection. In 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.
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7.4. OpenWSN
The TCP implementation in OpenWSN is mostly equivalent to the uIP TCP
implementation. OpenWSN TCP implementation only supports the minimum
state machine functionality required. For example, it does not
perform retransmissions.
7.5. TinyOS
TODO: To be verified
TinyOS has an experimental TCP stack that uses a simple nonblocking
library-based implementation of TCP. 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 so that the TCP implementation can automatically retransmit
missing segments.
7.6. Summary
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+-------+---------+---------+------+---------+--------+
| uIP |lwIP orig|lwIP 2.0 | RIOT | OpenWSN | TinyOS |
+--------+----------------+-------+---------+---------+------+---------+--------+
| | Data size | * | * | * | * | * | * |
| Memory +----------------+-------+---------+---------+------+---------+--------+
| | Code size (kB) | < 5 |~9 to ~14| * | * | * | * |
+--------+----------------+-------+---------+---------+------+---------+--------+
| |Window size(MSS)| 1 | Multiple| Multiple| 1 | 1 |Multiple|
| +----------------+-------+---------+---------+------+---------+--------+
| | Slow start | No | Yes | Yes | No | No | Yes |
| T +----------------+-------+---------+---------+------+---------+--------+
| C | Fast rec/retx | No | Yes | Yes | No | No | Yes |
| P +----------------+-------+---------+---------+------+---------+--------+
| | Keep-alive | No | * | * | No | No | No |
| +----------------+-------+---------+---------+------+---------+--------+
| f | TFO | No | No | * | No | No | No |
| e +----------------+-------+---------+---------+------+---------+--------+
| a | ECN | No | No | * | No | No | No |
| t +----------------+-------+---------+---------+------+---------+--------+
| u | Window Scale | No | No | Yes | No | No | No |
| r +----------------+-------+---------+---------+------+---------+--------+
| e | TCP timestamps | No | No | Yes | No | No | No |
| s +----------------+-------+---------+---------+------+---------+--------+
| | SACK | No | No | Yes | No | No | No |
| +----------------+-------+---------+---------+------+---------+--------+
| | Delayed ACKs | No | Yes | Yes | No | No | No |
+--------+----------------+-------+---------+---------+------+---------+--------+
Figure 2: Summary of TCP features for differrent lightweight TCP
implementations.
TODO: Add information about RAM requirements (in addition to
codesize)
8. Annex. Changes compared to previous versions
RFC Editor: To be removed prior to publication
8.1. Changes compared to -00
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
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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. References
9.1. 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>.
[RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
1992, <https://www.rfc-editor.org/info/rfc1323>.
[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>.
[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>.
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[RFC3402] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Two: The Algorithm", RFC 3402, DOI 10.17487/RFC3402,
October 2002, <https://www.rfc-editor.org/info/rfc3402>.
[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>.
[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>.
9.2. 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.
[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.
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[I-D.ietf-core-coap-tcp-tls]
Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
draft-ietf-core-coap-tcp-tls-09 (work in progress), May
2017.
[I-D.ietf-core-cocoa]
Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", draft-ietf-
core-cocoa-01 (work in progress), March 2017.
[I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-07 (work in progress), October 2017.
[I-D.ietf-lwig-energy-efficient]
Gomez, C., Kovatsch, M., Tian, H., and Z. Cao, "Energy-
Efficient Features of Internet of Things Protocols",
draft-ietf-lwig-energy-efficient-07 (work in progress),
March 2017.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <https://www.rfc-editor.org/info/rfc1981>.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, DOI 10.17487/RFC2385, August
1998, <https://www.rfc-editor.org/info/rfc2385>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/info/rfc2616>.
[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>.
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[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>.
[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>.
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[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>.
[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>.
[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>.
[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>.
Authors' Addresses
Carles Gomez
UPC/i2CAT
C/Esteve Terradas, 7
Castelldefels 08860
Spain
Email: carlesgo@entel.upc.edu
Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge, CB3 0FD
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
Michael Scharf
Nokia
Lorenzstrasse 10
Stuttgart, 70435
Germany
Email: michael.scharf@nokia.com
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