draft-ietf-lwig-tcp-constrained-node-networks-13.txt   rfc9006.txt 
LWIG Working Group C. Gomez Internet Engineering Task Force (IETF) C. Gomez
Internet-Draft UPC Request for Comments: 9006 UPC
Intended status: Informational J. Crowcroft Category: Informational J. Crowcroft
Expires: May 3, 2021 University of Cambridge ISSN: 2070-1721 University of Cambridge
M. Scharf M. Scharf
Hochschule Esslingen Hochschule Esslingen
October 30, 2020 March 2021
TCP Usage Guidance in the Internet of Things (IoT) TCP Usage Guidance in the Internet of Things (IoT)
draft-ietf-lwig-tcp-constrained-node-networks-13
Abstract Abstract
This document provides guidance on how to implement and use the This document provides guidance on how to implement and use the
Transmission Control Protocol (TCP) in Constrained-Node Networks Transmission Control Protocol (TCP) in Constrained-Node Networks
(CNNs), which are a characteristic of the Internet of Things (IoT). (CNNs), which are a characteristic of the Internet of Things (IoT).
Such environments require a lightweight TCP implementation and may Such environments require a lightweight TCP implementation and may
not make use of optional functionality. This document explains a not make use of optional functionality. This document explains a
number of known and deployed techniques to simplify a TCP stack as number of known and deployed techniques to simplify a TCP stack as
well as corresponding tradeoffs. The objective is to help embedded well as corresponding trade-offs. The objective is to help embedded
developers with decisions on which TCP features to use. developers with decisions on which TCP features to use.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
This Internet-Draft will expire on May 3, 2021. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9006.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
2. Characteristics of CNNs relevant for TCP . . . . . . . . . . 4 2. Characteristics of CNNs Relevant for TCP
2.1. Network and link properties . . . . . . . . . . . . . . . 4 2.1. Network and Link Properties
2.2. Usage scenarios . . . . . . . . . . . . . . . . . . . . . 5 2.2. Usage Scenarios
2.3. Communication and traffic patterns . . . . . . . . . . . 6 2.3. Communication and Traffic Patterns
3. TCP implementation and configuration in CNNs . . . . . . . . 6 3. TCP Implementation and Configuration in CNNs
3.1. Addressing path properties . . . . . . . . . . . . . . . 7 3.1. Addressing Path Properties
3.1.1. Maximum Segment Size (MSS) . . . . . . . . . . . . . 7 3.1.1. Maximum Segment Size (MSS)
3.1.2. Explicit Congestion Notification (ECN) . . . . . . . 8 3.1.2. Explicit Congestion Notification (ECN)
3.1.3. Explicit loss notifications . . . . . . . . . . . . . 9 3.1.3. Explicit Loss Notifications
3.2. TCP guidance for single-MSS stacks . . . . . . . . . . . 9 3.2. TCP Guidance for Single-MSS Stacks
3.2.1. Single-MSS stacks - benefits and issues . . . . . . . 9 3.2.1. Single-MSS Stacks -- Benefits and Issues
3.2.2. TCP options for single-MSS stacks . . . . . . . . . . 10 3.2.2. TCP Options for Single-MSS Stacks
3.2.3. Delayed Acknowledgments for single-MSS stacks . . . . 10 3.2.3. Delayed Acknowledgments for Single-MSS Stacks
3.2.4. RTO calculation for single-MSS stacks . . . . . . . . 11 3.2.4. RTO Calculation for Single-MSS Stacks
3.3. General recommendations for TCP in CNNs . . . . . . . . . 12 3.3. General Recommendations for TCP in CNNs
3.3.1. Loss recovery and congestion/flow control . . . . . . 12 3.3.1. Loss Recovery and Congestion/Flow Control
3.3.1.1. Selective Acknowledgments (SACK) . . . . . . . . 13 3.3.1.1. Selective Acknowledgments (SACKs)
3.3.2. Delayed Acknowledgments . . . . . . . . . . . . . . . 13 3.3.2. Delayed Acknowledgments
3.3.3. Initial Window . . . . . . . . . . . . . . . . . . . 14 3.3.3. Initial Window
4. TCP usage recommendations in CNNs . . . . . . . . . . . . . . 14 4. TCP Usage Recommendations in CNNs
4.1. TCP connection initiation . . . . . . . . . . . . . . . . 14 4.1. TCP Connection Initiation
4.2. Number of concurrent connections . . . . . . . . . . . . 15 4.2. Number of Concurrent Connections
4.3. TCP connection lifetime . . . . . . . . . . . . . . . . . 15 4.3. TCP Connection Lifetime
5. Security Considerations . . . . . . . . . . . . . . . . . . . 17 5. Security Considerations
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 6. IANA Considerations
7. Annex. TCP implementations for constrained devices . . . . . 18 7. References
7.1. uIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 7.1. Normative References
7.2. lwIP . . . . . . . . . . . . . . . . . . . . . . . . . . 19 7.2. Informative References
7.3. RIOT . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Appendix A. TCP Implementations for Constrained Devices
7.4. TinyOS . . . . . . . . . . . . . . . . . . . . . . . . . 20 A.1. uIP
7.5. FreeRTOS . . . . . . . . . . . . . . . . . . . . . . . . 20 A.2. lwIP
7.6. uC/OS . . . . . . . . . . . . . . . . . . . . . . . . . . 20 A.3. RIOT
7.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 21 A.4. TinyOS
8. Annex. Changes compared to previous versions . . . . . . . . 22 A.5. FreeRTOS
8.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 22 A.6. uC/OS
8.2. Changes between -01 and -02 . . . . . . . . . . . . . . . 22 A.7. Summary
8.3. Changes between -02 and -03 . . . . . . . . . . . . . . . 22 Acknowledgments
8.4. Changes between -03 and -04 . . . . . . . . . . . . . . . 23 Authors' Addresses
8.5. Changes between -04 and -05 . . . . . . . . . . . . . . . 23
8.6. Changes between -05 and -06 . . . . . . . . . . . . . . . 23
8.7. Changes between -06 and -07 . . . . . . . . . . . . . . . 23
8.8. Changes between -07 and -08 . . . . . . . . . . . . . . . 23
8.9. Changes between -08 and -09 . . . . . . . . . . . . . . . 23
8.10. Changes between -09 and -10 . . . . . . . . . . . . . . . 24
8.11. Changes between -10 and -11 . . . . . . . . . . . . . . . 24
8.12. Changes between -11 and -12 . . . . . . . . . . . . . . . 24
8.13. Changes between -12 and -13 . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction 1. Introduction
The Internet Protocol suite is being used for connecting Constrained- The Internet Protocol suite is being used for connecting Constrained-
Node Networks (CNNs) to the Internet, enabling the so-called Internet Node Networks (CNNs) to the Internet, enabling the so-called Internet
of Things (IoT) [RFC7228]. In order to meet the requirements that of Things (IoT) [RFC7228]. In order to meet the requirements that
stem from CNNs, the IETF has produced a suite of new protocols stem from CNNs, the IETF has produced a suite of new protocols
specifically designed for such environments (see e.g. [RFC8352]). specifically designed for such environments (see, e.g., [RFC8352]).
New IETF protocol stack components include the IPv6 over Low-power New IETF protocol stack components include the IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPAN) adaptation layer Wireless Personal Area Networks (6LoWPANs) adaptation layer
[RFC4944][RFC6282][RFC6775], the IPv6 Routing Protocol for Low-power [RFC4944][RFC6282][RFC6775], the IPv6 Routing Protocol for Low-Power
and lossy networks (RPL) routing protocol [RFC6550], and the and Lossy Networks (RPL) [RFC6550], and the Constrained Application
Constrained Application Protocol (CoAP) [RFC7252]. Protocol (CoAP) [RFC7252].
As of the writing, the main current transport layer protocols in IP- As of this writing, the main transport-layer protocols in IP-based
based IoT scenarios are UDP and TCP. TCP has been criticized, often IoT scenarios are UDP and TCP. TCP has been criticized, often
unfairly, as a protocol that is unsuitable for the IoT. It is true unfairly, as a protocol that is unsuitable for the IoT. It is true
that some TCP features, such as relatively long header size, that some TCP features, such as relatively long header size,
unsuitability for multicast, and always-confirmed data delivery, are unsuitability for multicast, and always-confirmed data delivery, are
not optimal for IoT scenarios. However, many typical claims on TCP not optimal for IoT scenarios. However, many typical claims on TCP
unsuitability for IoT (e.g. a high complexity, connection-oriented unsuitability for IoT (e.g., a high complexity, connection-oriented
approach incompatibility with radio duty-cycling, and spurious approach incompatibility with radio duty-cycling and spurious
congestion control activation in wireless links) are not valid, can congestion control activation in wireless links) are not valid, can
be solved, or are also found in well accepted IoT end-to-end be solved, or are also found in well-accepted IoT end-to-end
reliability mechanisms (see [IntComp] for a detailed analysis). reliability mechanisms (see a detailed analysis in [IntComp]).
At the application layer, CoAP was developed over UDP [RFC7252]. At the application layer, CoAP was developed over UDP [RFC7252].
However, the integration of some CoAP deployments with existing However, the integration of some CoAP deployments with existing
infrastructure is being challenged by middleboxes such as firewalls, infrastructure is being challenged by middleboxes such as firewalls,
which may limit and even block UDP-based communications. This is the which may limit and even block UDP-based communications. This is the
main reason why a CoAP over TCP specification has been developed main reason why a CoAP over TCP specification has been developed
[RFC8323]. [RFC8323].
Other application layer protocols not specifically designed for CNNs Other application-layer protocols not specifically designed for CNNs
are also being considered for the IoT space. Some examples include 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 HTTP/2 and even HTTP/1.1, both of which run over TCP by default
[RFC7230] [RFC7540], and the Extensible Messaging and Presence [RFC7230] [RFC7540], and the Extensible Messaging and Presence
Protocol (XMPP) [RFC6120]. TCP is also used by non-IETF application- Protocol (XMPP) [RFC6120]. TCP is also used by non-IETF application-
layer protocols in the IoT space such as the Message Queuing layer protocols in the IoT space such as the Message Queuing
Telemetry Transport (MQTT) [MQTT] and its lightweight variants. Telemetry Transport (MQTT) [MQTT] and its lightweight variants.
TCP is a sophisticated transport protocol that includes optional TCP is a sophisticated transport protocol that includes optional
functionality (e.g. TCP options) that may improve performance in functionality (e.g., TCP options) that may improve performance in
some environments. However, many optional TCP extensions require some environments. However, many optional TCP extensions require
complex logic inside the TCP stack and increase the code size and the complex logic inside the TCP stack and increase the code size and the
memory requirements. Many TCP extensions are not required for memory requirements. Many TCP extensions are not required for
interoperability with other standard-compliant TCP endpoints. Given interoperability with other standard-compliant TCP endpoints. Given
the limited resources on constrained devices, careful selection of the limited resources on constrained devices, careful selection of
optional TCP features can make an implementation more lightweight. optional TCP features can make an implementation more lightweight.
This document provides guidance on how to implement and configure This document provides guidance on how to implement and configure TCP
TCP, as well as on how TCP is advisable to be used by applications, and guidance on how applications should use TCP in CNNs. The
in CNNs. The overarching goal is to offer simple measures to allow overarching goal is to offer simple measures to allow for lightweight
for lightweight TCP implementation and suitable operation in such TCP implementation and suitable operation in such environments. A
environments. A TCP implementation following the guidance in this TCP implementation following the guidance in this document is
document is intended to be compatible with a TCP endpoint that is intended to be compatible with a TCP endpoint that is compliant to
compliant to the TCP standards, albeit possibly with a lower the TCP standards, albeit possibly with a lower performance. This
performance. This implies that such a TCP client would always be implies that such a TCP client would always be able to connect with a
able to connect with a standard-compliant TCP server, and a standard-compliant TCP server, and a corresponding TCP server would
corresponding TCP server would always be able to connect with a always be able to connect with a standard-compliant TCP client.
standard-compliant TCP client.
This document assumes that the reader is familiar with TCP. A This document assumes that the reader is familiar with TCP. A
comprehensive survey of the TCP standards can be found in [RFC7414]. comprehensive survey of the TCP standards can be found in RFC 7414
Similar guidance regarding the use of TCP in special environments has [RFC7414]. Similar guidance regarding the use of TCP in special
been published before, e.g., for cellular wireless networks environments has been published before, e.g., for cellular wireless
[RFC3481]. networks [RFC3481].
2. Characteristics of CNNs relevant for TCP 2. Characteristics of CNNs Relevant for TCP
2.1. Network and link properties 2.1. Network and Link Properties
CNNs are defined in [RFC7228] as networks whose characteristics are CNNs are defined in [RFC7228] as networks whose characteristics are
influenced by being composed of a significant portion of constrained influenced by being composed of a significant portion of constrained
nodes. The latter are characterized by significant limitations on nodes. The latter are characterized by significant limitations on
processing, memory, and energy resources, among others [RFC7228]. processing, memory, and energy resources, among others [RFC7228].
The first two dimensions pose constraints on the complexity and on The first two dimensions pose constraints on the complexity and
the memory footprint of the protocols that constrained nodes can memory footprint of the protocols that constrained nodes can support.
support. The latter requires techniques to save energy, such as The latter requires techniques to save energy, such as radio duty-
radio duty-cycling in wireless devices [RFC8352], as well as cycling in wireless devices [RFC8352] and the minimization of the
minimization of the number of messages transmitted/received (and number of messages transmitted/received (and their size).
their size).
[RFC7228] lists typical network constraints in CNN, including low [RFC7228] lists typical network constraints in CNNs, including low
achievable bitrate/throughput, high packet loss and high variability achievable bitrate/throughput, high packet loss and high variability
of packet loss, highly asymmetric link characteristics, severe of packet loss, highly asymmetric link characteristics, severe
penalties for using larger packets, limits on reachability over time, penalties for using larger packets, limits on reachability over time,
etc. CNN may use wireless or wired technologies (e.g., Power Line etc. CNNs may use wireless or wired technologies (e.g., Power Line
Communication), and the transmission rates are typically low (e.g. Communication), and the transmission rates are typically low (e.g.,
below 1 Mbps). below 1 Mbps).
For use of TCP, one challenge is that not all technologies in CNN may For use of TCP, one challenge is that not all technologies in a CNN
be aligned with typical Internet subnetwork design principles may be aligned with typical Internet subnetwork design principles
[RFC3819]. For instance, constrained nodes often use physical/link [RFC3819]. For instance, constrained nodes often use physical- /
layer technologies that have been characterized as 'lossy', i.e., link-layer technologies that have been characterized as 'lossy',
exhibit a relatively high bit error rate. Dealing with corruption i.e., exhibit a relatively high bit error rate. Dealing with
loss is one of the open issues in the Internet [RFC6077]. corruption loss is one of the open issues in the Internet [RFC6077].
2.2. Usage scenarios 2.2. Usage Scenarios
There are different deployment and usage scenarios for CNNs. Some There are different deployment and usage scenarios for CNNs. Some
CNNs follow the star topology, whereby one or several hosts are CNNs follow the star topology, whereby one or several hosts are
linked to a central device that acts as a router connecting the CNN linked to a central device that acts as a router connecting the CNN
to the Internet. Alternatively, CNNs may also follow the multihop to the Internet. Alternatively, CNNs may also follow the multihop
topology [RFC6606]. topology [RFC6606].
In constrained environments, there can be different types of devices In constrained environments, there can be different types of devices
[RFC7228]. For example, there can be devices with single combined [RFC7228]. For example, there can be devices with a single combined
send/receive buffer, devices with a separate send and receive buffer, send/receive buffer, a separate send and receive buffer, or a pool of
or devices with a pool of multiple send/receive buffers. In the multiple send/receive buffers. In the latter case, it is possible
latter case, it is possible that buffers are also shared for other that buffers are also shared for other protocols.
protocols.
One key use case for TCP in CNNs is a model where constrained devices One key use case for TCP in CNNs is a model where constrained devices
connect to unconstrained servers in the Internet. But it is also connect to unconstrained servers in the Internet. But it is also
possible that both TCP endpoints run on constrained devices. In the possible that both TCP endpoints run on constrained devices. In the
first case, communication possibly has to traverse a middlebox (e.g. first case, communication will possibly traverse a middlebox (e.g., a
a firewall, NAT, etc.). Figure 1 illustrates such a scenario. Note firewall, NAT, etc.). Figure 1 illustrates such a scenario. Note
that the scenario is asymmetric, as the unconstrained device will that the scenario is asymmetric, as the unconstrained device will
typically not suffer the severe constraints of the constrained typically not suffer the severe constraints of the constrained
device. The unconstrained device is expected to be mains-powered, to device. The unconstrained device is expected to be mains-powered,
have high amount of memory and processing power, and to be connected have a high amount of memory and processing power, and be connected
to a resource-rich network. to a resource-rich network.
Assuming that a majority of constrained devices will correspond to Assuming that a majority of constrained devices will correspond to
sensor nodes, the amount of data traffic sent by constrained devices sensor nodes, the amount of data traffic sent by constrained devices
(e.g. sensor node measurements) is expected to be higher than the (e.g., sensor node measurements) is expected to be higher than the
amount of data traffic in the opposite direction. Nevertheless, amount of data traffic in the opposite direction. Nevertheless,
constrained devices may receive requests (to which they may respond), constrained devices may receive requests (to which they may respond),
commands (for configuration purposes and for constrained devices commands (for configuration purposes and for constrained devices
including actuators) and relatively infrequent firmware/software including actuators), and relatively infrequent firmware/software
updates. updates.
+---------------+ +---------------+
o o <-------- TCP communication -----> | | o o <-------- TCP communication -----> | |
o o | | o o | |
o o | Unconstrained | o o | Unconstrained |
o o +-----------+ | device | o o +-----------+ | device |
o o o ------ | Middlebox | ------- | | o o o ------ | Middlebox | ------- | |
o o +-----------+ | (e.g. cloud) | o o +-----------+ | (e.g., cloud) |
o o o | | o o o | |
+---------------+ +---------------+
constrained devices Constrained devices
Figure 1: TCP communication between a constrained device and an Figure 1: TCP Communication between a Constrained Device and an
unconstrained device, traversing a middlebox. Unconstrained Device, Traversing a Middlebox
2.3. Communication and traffic patterns 2.3. Communication and Traffic Patterns
IoT applications are characterized by a number of different IoT applications are characterized by a number of different
communication patterns. The following non-comprehensive list communication patterns. The following non-comprehensive list
explains some typical examples: explains some typical examples:
o Unidirectional transfers: An IoT device (e.g. a sensor) can send Unidirectional transfers: An IoT device (e.g., a sensor) can
(repeatedly) updates to the other endpoint. There is not always a (repeatedly) send updates to the other endpoint. There is not
need for an application response back to the IoT device. always a need for an application response back to the IoT device.
o Request-response patterns: An IoT device receiving a request from Request-response patterns: An IoT device receiving a request from
the other endpoint, which triggers a response from the IoT device. the other endpoint, which triggers a response from the IoT device.
o Bulk data transfers: A typical example for a long file transfer Bulk data transfers: A typical example for a long file transfer
would be an IoT device firmware update. would be an IoT device firmware update.
A typical communication pattern is that a constrained device A typical communication pattern is that a constrained device
communicates with an unconstrained device (cf. Figure 1). But it is communicates with an unconstrained device (cf. Figure 1). But it is
also possible that constrained devices communicate amongst also possible that constrained devices communicate amongst
themselves. themselves.
3. TCP implementation and configuration in CNNs 3. TCP Implementation and Configuration in CNNs
This section explains how a TCP stack can deal with typical This section explains how a TCP stack can deal with typical
constraints in CNN. The guidance in this section relates to the TCP constraints in CNN. The guidance in this section relates to the TCP
implementation and its configuration. implementation and its configuration.
3.1. Addressing path properties 3.1. Addressing Path Properties
3.1.1. Maximum Segment Size (MSS) 3.1.1. Maximum Segment Size (MSS)
Assuming that IPv6 is used, and for the sake of lightweight Assuming that IPv6 is used, and for the sake of lightweight
implementation and operation, unless applications require handling implementation and operation, unless applications require handling
large data units (i.e. leading to an IPv6 datagram size greater than large data units (i.e., leading to an IPv6 datagram size greater than
1280 bytes), it may be desirable to limit the IP datagram size to 1280 bytes), it may be desirable to limit the IP datagram size to
1280 bytes in order to avoid the need to support Path MTU Discovery 1280 bytes in order to avoid the need to support Path MTU Discovery
[RFC8201]. In addition, an IP datagram size of 1280 bytes avoids [RFC8201]. In addition, an IP datagram size of 1280 bytes avoids
incurring IPv6-layer fragmentation [RFC8900]. incurring IPv6-layer fragmentation [RFC8900].
An IPv6 datagram size exceeding 1280 bytes can be avoided by setting An IPv6 datagram size exceeding 1280 bytes can be avoided by setting
the TCP MSS not larger than 1220 bytes. Note that it is already a the TCP MSS to 1220 bytes or less. Note that it is already a
requirement that TCP implementations consume payload space instead of requirement for TCP implementations to consume payload space instead
increasing datagram size when including IP or TCP options in an IP of increasing datagram size when including IP or TCP options in an IP
packet to be sent [RFC6691]. Therefore, it is not required to packet to be sent [RFC6691]. Therefore, it is not required to
advertise an MSS smaller than 1220 bytes in order to accommodate TCP advertise an MSS smaller than 1220 bytes in order to accommodate TCP
options. options.
Note that setting the MTU to 1280 bytes is possible for link layer 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 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 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 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 127 bytes. 6LoWPAN defined an adaptation layer to support IPv6 over
IEEE 802.15.4 networks. The adaptation layer includes a IEEE 802.15.4 networks. The adaptation layer includes a
fragmentation mechanism, since IPv6 requires the layer below to fragmentation mechanism, since IPv6 requires the layer below to
support an MTU of 1280 bytes [RFC8200], while IEEE 802.15.4 lacked support an MTU of 1280 bytes [RFC8200], while IEEE 802.15.4 lacks
fragmentation mechanisms. 6LoWPAN defines an IEEE 802.15.4 link MTU fragmentation mechanisms. 6LoWPAN defines an IEEE 802.15.4 link MTU
of 1280 bytes [RFC4944]. Other technologies, such as Bluetooth LE of 1280 bytes [RFC4944]. Other technologies, such as Bluetooth low
[RFC7668], ITU-T G.9959 [RFC7428] or DECT-ULE [RFC8105], also use energy [RFC7668], ITU-T G.9959 [RFC7428], or Digital Enhanced
6LoWPAN-based adaptation layers in order to enable IPv6 support. Cordless Telecommunications (DECT) Ultra Low Energy (ULE) [RFC8105],
These technologies do support link layer fragmentation. By 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 exploiting this functionality, the adaptation layers that enable IPv6
over such technologies also define an MTU of 1280 bytes. over such technologies also define an MTU of 1280 bytes.
On the other hand, there exist technologies also used in the CNN On the other hand, there exist technologies also used in the CNN
space, such as Master Slave / Token Passing (TP) [RFC8163], space, such as Master Slave (MS) / Token Passing (TP) [RFC8163],
Narrowband IoT (NB-IoT) [RFC8376] or IEEE 802.11ah Narrowband IoT (NB-IoT) [RFC8376], or IEEE 802.11ah [6LO-WLANAH],
[I-D.delcarpio-6lo-wlanah], that do not suffer the same degree of that do not suffer the same degree of frame size limitations as the
frame size limitations as the technologies mentioned above. The MTU technologies mentioned above. It is recommended that the MTU for MS/
for MS/TP is recommended to be 1500 bytes [RFC8163], the MTU in NB- TP be 1500 bytes [RFC8163]; the MTU in NB-IoT is 1600 bytes, and the
IoT is 1600 bytes, and the maximum frame payload size for IEEE maximum frame payload size for IEEE 802.11ah is 7991 bytes.
802.11ah is 7991 bytes.
Using larger MSS (to a suitable extent) may be beneficial in some Using a larger MSS (to a suitable extent) may be beneficial in some
scenarios, especially when transferring large payloads, as it reduces scenarios, especially when transferring large payloads, as it reduces
the number of packets (and packet headers) required for a given the number of packets (and packet headers) required for a given
payload. However, the characteristics of the constrained network payload. However, the characteristics of the constrained network
need to be considered. In particular, in a lossy network where need to be considered. In particular, in a lossy network where
unreliable fragment delivery is used, the amount of data that TCP unreliable fragment delivery is used, the amount of data that TCP
unnecessarily retransmits due to fragment loss increases (and unnecessarily retransmits due to fragment loss increases (and
throughput decreases) quickly with the MSS. This happens because the throughput decreases) quickly with the MSS. This happens because the
loss of a fragment leads to the loss of the whole fragmented packet loss of a fragment leads to the loss of the whole fragmented packet
being transmitted. Unnecessary data retransmission is particularly being transmitted. Unnecessary data retransmission is particularly
harmful in CNNs due to the resource constraints of such environments. harmful in CNNs due to the resource constraints of such environments.
Note that, while the original 6LoWPAN fragmentation mechanism Note that, while the original 6LoWPAN fragmentation mechanism
[RFC4944] does not offer reliable fragment delivery, fragment [RFC4944] does not offer reliable fragment delivery, fragment
recovery functionality for 6LoWPAN or 6Lo environments is being recovery functionality for 6LoWPAN or 6Lo environments has been
standardized as of the writing [I-D.ietf-6lo-fragment-recovery]. standardized [RFC8931].
3.1.2. Explicit Congestion Notification (ECN) 3.1.2. Explicit Congestion Notification (ECN)
Explicit Congestion Notification (ECN) [RFC3168] ECN allows a router ECN [RFC3168] allows a router to signal in the IP header of a packet
to signal in the IP header of a packet that congestion is arising, that congestion is rising, for example, when a queue size reaches a
for example when a queue size reaches a certain threshold. An ECN- certain threshold. An ECN-enabled TCP receiver will echo back the
enabled TCP receiver will echo back the congestion signal to the TCP congestion signal to the TCP sender by setting a flag in its next TCP
sender by setting a flag in its next TCP ACK. The sender triggers Acknowledgment (ACK). The sender triggers congestion control
congestion control measures as if a packet loss had happened. measures as if a packet loss had happened.
The document [RFC8087] outlines the principal gains in terms of RFC 8087 [RFC8087] outlines the principal gains in terms of increased
increased throughput, reduced delay, and other benefits when ECN is throughput, reduced delay, and other benefits when ECN is used over a
used over a network path that includes equipment that supports network path that includes equipment that supports Congestion
Congestion Experienced (CE) marking. In the context of CNNs, a Experienced (CE) marking. In the context of CNNs, a remarkable
remarkable feature of ECN is that congestion can be signalled without feature of ECN is that congestion can be signaled without incurring
incurring packet drops (which will lead to retransmissions and packet drops (which will lead to retransmissions and consumption of
consumption of limited resources such as energy and bandwidth). limited resources such as energy and bandwidth).
ECN can further reduce packet losses since congestion control ECN can further reduce packet losses since congestion control
measures can be applied earlier [RFC2884]. Fewer lost packets measures can be applied earlier [RFC2884]. Fewer lost packets
implies that the number of retransmitted segments decreases, which is implies that the number of retransmitted segments decreases, which is
particularly beneficial in CNNs, where energy and bandwidth resources particularly beneficial in CNNs, where energy and bandwidth resources
are typically limited. Also, it makes sense to try to avoid packet are typically limited. Also, it makes sense to try to avoid packet
drops for transactional workloads with small data sizes, which are drops for transactional workloads with small data sizes, which are
typical for CNNs. In such traffic patterns, it is more difficult and typical for CNNs. In such traffic patterns, it is more difficult and
often impossible to detect packet loss without retransmission often impossible to detect packet loss without retransmission
timeouts (e.g., as there may be no three duplicate ACKs). Any timeouts (e.g., as there may not be three duplicate ACKs). Any
retransmission timeout slows down the data transfer significantly. retransmission timeout slows down the data transfer significantly.
In addition, if the constrained device uses power saving techniques, In addition, if the constrained device uses power-saving techniques,
a retransmission timeout will incur a wake-up action, in contrast to a retransmission timeout will incur a wake-up action, in contrast to
ACK clock- triggered sending. When the congestion window of a TCP ACK clock-triggered sending. When the congestion window of a TCP
sender has a size of one segment and a TCP ACK with an ECN signal sender has a size of one segment and a TCP ACK with an ECN signal
(ECE flag) arrives at the TCP sender, the TCP sender resets the (ECN-Echo (ECE) flag) arrives at the TCP sender, the TCP sender
retransmit timer, and the sender will only be able to send a new resets the retransmit timer, and the sender will only be able to send
packet when the retransmit timer expires. Effectively, the TCP a new packet when the retransmit timer expires. Effectively, at that
sender reduces at that moment its sending rate from 1 segment per moment, the TCP sender reduces its sending rate from 1 segment per
Round Trip Time (RTT) to 1 segment per Retransmission Timeout (RTO) Round-Trip Time (RTT) to 1 segment per Retransmission Timeout (RTO)
and reduces the sending rate further on each ECN signal received in and reduces the sending rate further on each ECN signal received in
subsequent TCP ACKs. Otherwise, if an ECN signal is not present in a subsequent TCP ACKs. Otherwise, if an ECN signal is not present in a
subsequent TCP ACK the TCP sender resumes the normal ACK-clocked subsequent TCP ACK, the TCP sender resumes the normal ACK-clocked
transmission of segments [RFC3168]. transmission of segments [RFC3168].
ECN can be incrementally deployed in the Internet. Guidance on ECN can be incrementally deployed in the Internet. Guidance on
configuration and usage of ECN is provided in [RFC7567]. Given the configuration and usage of ECN is provided in RFC 7567 [RFC7567].
benefits, more and more TCP stacks in the Internet support ECN, and Given the benefits, more and more TCP stacks in the Internet support
it specifically makes sense to leverage ECN in controlled ECN, and it makes sense to specifically leverage ECN in controlled
environments such as CNNs. As of the writing, there is on-going work environments such as CNNs. As of this writing, there is ongoing work
to extend the types of TCP packets that are ECN-capable, including to extend the types of TCP packets that are ECN capable, including
pure ACKs [I-D.ietf-tcpm-generalized-ecn]. Such a feature may pure ACKs [TCPM-ECN]. Such a feature may further increase the
further increase the benefits of ECN in CNN environments. Note, benefits of ECN in CNN environments. Note, however, that supporting
however, that supporting ECN increases implementation complexity. ECN increases implementation complexity.
3.1.3. Explicit loss notifications 3.1.3. Explicit Loss Notifications
There has been a significant body of research on solutions capable of There has been a significant body of research on solutions capable of
explicitly indicating whether a TCP segment loss is due to explicitly indicating whether a TCP segment loss is due to
corruption, in order to avoid activation of congestion control corruption, in order to avoid activation of congestion control
mechanisms [ETEN] [RFC2757]. While such solutions may provide mechanisms [ETEN] [RFC2757]. While such solutions may provide
significant improvement, they have not been widely deployed and significant improvement, they have not been widely deployed and
remain as experimental work. In fact, as of today, the IETF has not remain as experimental work. In fact, as of today, the IETF has not
standardized any such solution. standardized any such solution.
3.2. TCP guidance for single-MSS stacks 3.2. TCP Guidance for Single-MSS Stacks
This section discusses TCP stacks that allow transferring a single This section discusses TCP stacks that allow transferring a single
MSS. More general guidance is provided in Section 3.3. MSS. More general guidance is provided in Section 3.3.
3.2.1. Single-MSS stacks - benefits and issues 3.2.1. Single-MSS Stacks -- Benefits and Issues
A TCP stack can reduce the memory requirements by advertising a TCP 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 window size of 1 MSS and also transmit, at most, 1 MSS of
unacknowledged data. In that case, both congestion and flow control unacknowledged data. In that case, both congestion and flow control
implementation are quite simple. Such a small receive and send implementation are quite simple. Such a small receive and send
window may be sufficient for simple message exchanges in the CNN window may be sufficient for simple message exchanges in the CNN
space. However, only using a window of one MSS can significantly space. However, only using a window of 1 MSS can significantly
affect performance. A stop-and-wait operation results in low affect performance. A stop-and-wait operation results in low
throughput for transfers that exceed the length of one MSS, e.g., a throughput for transfers that exceed the length of 1 MSS, e.g., a
firmware download. Furthermore, a single-MSS solution relies solely firmware download. Furthermore, a single-MSS solution relies solely
on timer-based loss recovery, therefore missing the performance gain on timer-based loss recovery, therefore missing the performance gain
of Fast Retransmit and Fast Recovery (which require a larger window of Fast Retransmit and Fast Recovery (which requires a larger window
size, see Section 3.3.1). size; see Section 3.3.1).
If CoAP is used over TCP with the default setting for NSTART in If CoAP is used over TCP with the default setting for NSTART in RFC
[RFC7252], a CoAP endpoint is not allowed to send a new message to a 7252 [RFC7252], a CoAP endpoint is not allowed to send a new message
destination until a response for the previous message sent to that to a destination until a response for the previous message sent to
destination has been received. This is equivalent to an application- that destination has been received. This is equivalent to an
layer window size of 1 data unit. For this use of CoAP, a maximum application-layer window size of 1 data unit. For this use of CoAP,
TCP window of one MSS may be sufficient, as long as the CoAP message a maximum TCP window of 1 MSS may be sufficient, as long as the CoAP
size does not exceed one MSS. An exception in CoAP over TCP, though, message size does not exceed 1 MSS. An exception in CoAP over TCP,
is the Capabilities and Settings Message (CSM) that must be sent at though, is the Capabilities and Settings Message (CSM) that must be
the start of the TCP connection. The first application message sent at the start of the TCP connection. The first application
carrying user data is allowed to be sent immediately after the CSM message carrying user data is allowed to be sent immediately after
message. If the sum of the CSM size plus the application message the CSM message. If the sum of the CSM size plus the application
size exceeds the MSS, a sender using a single-MSS stack will need to message size exceeds the MSS, a sender using a single-MSS stack will
wait for the ACK confirming the CSM before sending the application need to wait for the ACK confirming the CSM before sending the
message. application message.
3.2.2. TCP options for single-MSS stacks 3.2.2. TCP Options for Single-MSS Stacks
A TCP implementation needs to support, at a minimum, TCP options 2, 1 A TCP implementation needs to support, at a minimum, TCP options 2,
and 0. These are, respectively, the Maximum Segment Size (MSS) 1, and 0. These are, respectively, the MSS option, the No-Operation
option, the No-Operation option, and the End Of Option List marker option, and the End Of Option List marker [RFC0793]. None of these
[RFC0793]. None of these are a substantial burden to support. These are a substantial burden to support. These options are sufficient
options are sufficient for interoperability with a standard-compliant for interoperability with a standard-compliant TCP endpoint, albeit
TCP endpoint, albeit many TCP stacks support additional options and many TCP stacks support additional options and can negotiate their
can negotiate their use. A TCP implementation is permitted to use. A TCP implementation is permitted to silently ignore all other
silently ignore all other TCP options. TCP options.
A TCP implementation for a constrained device that uses a single-MSS A TCP implementation for a constrained device that uses a single-MSS
TCP receive or transmit window size may not benefit from supporting TCP receive or transmit window size may not benefit from supporting
the following TCP options: Window scale [RFC7323], TCP Timestamps the following TCP options: Window Scale [RFC7323], TCP Timestamps
[RFC7323], Selective Acknowledgments (SACK) and SACK-Permitted [RFC7323], Selective Acknowledgment (SACK) [RFC2018], and SACK-
[RFC2018]. Also other TCP options may not be required on a Permitted [RFC2018]. Also, other TCP options may not be required on
constrained device with a very lightweight implementation. With a constrained device with a very lightweight implementation. With
regard to the Window scale option, note that it is only useful if a regard to the Window Scale option, note that it is only useful if a
window size greater than 64 kB is needed. window size greater than 64 kB is needed.
Note that a TCP sender can benefit from the TCP Timestamps option Note that a TCP sender can benefit from the TCP Timestamps option
[RFC7323] in detecting spurious RTOs. The latter are quite likely to [RFC7323] in detecting spurious RTOs. The latter are quite likely to
occur in CNN scenarios due to a number of reasons (e.g. route changes occur in CNN scenarios due to a number of reasons (e.g., route
in a multihop scenario, link layer retries, etc.). The header changes in a multihop scenario, link-layer retries, etc.). The
overhead incurred by the Timestamps option (of up to 12 bytes) needs header overhead incurred by the Timestamps option (of up to 12 bytes)
to be taken into account. needs to be taken into account.
3.2.3. Delayed Acknowledgments for single-MSS stacks 3.2.3. Delayed Acknowledgments for Single-MSS Stacks
TCP Delayed Acknowledgments are meant to reduce the number of ACKs TCP Delayed Acknowledgments are meant to reduce the number of ACKs
sent within a TCP connection, thus reducing network overhead, but sent within a TCP connection, thus reducing network overhead, but
they may increase the time until a sender may receive an ACK. In they may increase the time until a sender may receive an ACK. In
general, usefulness of Delayed ACKs depends heavily on the usage general, usefulness of Delayed ACKs depends heavily on the usage
scenario (see Section 3.3.2). There can be interactions with single- scenario (see Section 3.3.2). There can be interactions with single-
MSS stacks. MSS stacks.
When traffic is unidirectional, if the sender can send at most one When traffic is unidirectional, if the sender can send at most 1 MSS
MSS of data or the receiver advertises a receive window not greater of data or the receiver advertises a receive window not greater than
than the MSS, Delayed ACKs may unnecessarily contribute delay (up to the MSS, Delayed ACKs may unnecessarily contribute delay (up to 500
500 ms) to the RTT [RFC5681], which limits the throughput and can ms) to the RTT [RFC5681], which limits the throughput and can
increase data delivery time. Note that, in some cases, it may not be increase data delivery time. Note that, in some cases, it may not be
possible to disable Delayed ACKs. One known workaround is to split possible to disable Delayed ACKs. One known workaround is to split
the data to be sent into two segments of smaller size. A standard the data to be sent into two segments of smaller size. A standard-
compliant TCP receiver may immediately acknowledge the second MSS of compliant TCP receiver may immediately acknowledge the second MSS of
data, which can improve throughput. However, this 'split hack' may data, which can improve throughput. However, this "split hack" may
not always work since a TCP receiver is required to acknowledge every not always work since a TCP receiver is required to acknowledge every
second full-sized segment, but not two consecutive small segments. second full-sized segment, but not two consecutive small segments.
The overhead of sending two IP packets instead of one is another The overhead of sending two IP packets instead of one is another
downside of the 'split hack'. downside of the "split hack".
Similar issues may happen when the sender uses the Nagle algorithm, Similar issues may happen when the sender uses the Nagle algorithm,
since the sender may need to wait for an unnecessarily delayed ACK to since the sender may need to wait for an unnecessarily Delayed ACK to
send a new segment. Disabling the algorithm will not have impact if send a new segment. Disabling the algorithm will not have impact if
the sender can only handle stop-and-wait operation at the TCP level. the sender can only handle stop-and-wait operation at the TCP level.
For request-response traffic, when the receiver uses Delayed ACKs, a For request-response traffic, when the receiver uses Delayed ACKs, a
response to a data message can piggyback an ACK, as long as the response to a data message can piggyback an ACK, as long as the
latter is sent before the Delayed ACK timer expires, thus avoiding latter is sent before the Delayed ACK timer expires, thus avoiding
unnecessary ACKs without payload. Disabling Delayed ACKs at the unnecessary ACKs without payload. Disabling Delayed ACKs at the
request sender allows an immediate ACK for the data segment carrying request sender allows an immediate ACK for the data segment carrying
the response. the response.
3.2.4. RTO calculation for single-MSS stacks 3.2.4. RTO Calculation for Single-MSS Stacks
The RTO calculation is one of the fundamental TCP algorithms The RTO calculation is one of the fundamental TCP algorithms
[RFC6298]. There is a fundamental trade-off: A short, aggressive RTO [RFC6298]. There is a fundamental trade-off: a short, aggressive RTO
behavior reduces wait time before retransmissions, but it also behavior reduces wait time before retransmissions, but it also
increases the probability of spurious timeouts. The latter lead to increases the probability of spurious timeouts. The latter leads to
unnecessary waste of potentially scarce resources in CNNs such as unnecessary waste of potentially scarce resources in CNNs such as
energy and bandwidth. In contrast, a conservative timeout can result energy and bandwidth. In contrast, a conservative timeout can result
in long error recovery times and thus needlessly delay data delivery. in long error recovery times and, thus, needlessly delay data
delivery.
If a TCP sender uses a very small window size, and it cannot benefit If a TCP sender uses a very small window size, and it cannot benefit
from Fast Retransmit/Fast Recovery or SACK, the RTO algorithm has a from Fast Retransmit and Fast Recovery or SACK, the RTO algorithm has
large impact on performance. In that case, RTO algorithm tuning may a large impact on performance. In that case, RTO algorithm tuning
be considered, although careful assessment of possible drawbacks is may be considered, although careful assessment of possible drawbacks
recommended [I-D.ietf-tcpm-rto-consider]. is recommended [RFC8961].
As an example, adaptive RTO algorithms defined for CoAP over UDP have As an example, adaptive RTO algorithms defined for CoAP over UDP have
been found to perform well in CNN scenarios [Commag] been found to perform well in CNN scenarios [Commag] [CORE-FASOR].
[I-D.ietf-core-fasor].
3.3. General recommendations for TCP in CNNs 3.3. General Recommendations for TCP in CNNs
This section summarizes some widely used techniques to improve TCP, This section summarizes some widely used techniques to improve TCP,
with a focus on their use in CNNs. The TCP extensions discussed here with a focus on their use in CNNs. The TCP extensions discussed here
are useful in a wide range of network scenarios, including CNNs. are useful in a wide range of network scenarios, including CNNs.
This section is not comprehensive. A comprehensive survey of TCP This section is not comprehensive. A comprehensive survey of TCP
extensions is published in [RFC7414]. extensions is published in RFC 7414 [RFC7414].
3.3.1. Loss recovery and congestion/flow control 3.3.1. Loss Recovery and Congestion/Flow Control
Devices that have enough memory to allow a larger (i.e. more than 3 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 MSS of data) TCP window size can leverage a more efficient loss
recovery than the timer-based approach used for smaller TCP window recovery than the timer-based approach used for a smaller TCP window
size (see Section 3.2.1) by using Fast Retransmit and Fast Recovery size (see Section 3.2.1) by using Fast Retransmit and Fast Recovery
[RFC5681], at the expense of slightly greater complexity and [RFC5681], at the expense of slightly greater complexity and
Transmission Control Block (TCB) size. Assuming that Delayed ACKs Transmission Control Block (TCB) size. Assuming that Delayed ACKs
are used by the receiver, a window size of up to 5 MSS is required are used by the receiver, a window size of up to 5 MSS is required
for Fast Retransmit and Fast Recovery to work efficiently: If in a for Fast Retransmit and Fast Recovery to work efficiently: in a given
given TCP transmission of full-sized segments 1, 2, 3, 4, and 5, TCP transmission of full-sized segments 1, 2, 3, 4, and 5, if segment
segment 2 gets lost, and the ACK for segment 1 is held by the Delayed 2 gets lost, and the ACK for segment 1 is held by the Delayed ACK
ACK timer, then the sender should get an ACK for segment 1 when 3 timer, then the sender should get an ACK for segment 1 when 3 arrives
arrives and duplicate ACKs when segments 4, 5, and 6 arrive. It will and duplicate ACKs when segments 4, 5, and 6 arrive. It will
retransmit segment 2 when the third duplicate ACK arrives. In order retransmit segment 2 when the third duplicate ACK arrives. In order
to have segments 2, 3, 4, 5, and 6 sent, the window has to be of at to have segments 2, 3, 4, 5, and 6 sent, the window has to be of at
least 5 MSS. With an MSS of 1220 bytes, a buffer of a size of 5 MSS least 5 MSS. With an MSS of 1220 bytes, a buffer of a size of 5 MSS
would require 6100 bytes. would require 6100 bytes.
The example in the previous paragraph did not use a further TCP The example in the previous paragraph did not use a further TCP
improvement such as Limited Transmit [RFC3042]. The latter may also improvement such as Limited Transmit [RFC3042]. The latter may also
be useful for any transfer that has more than one segment in flight. be useful for any transfer that has more than one segment in flight.
Small transfers tend to benefit more from Limited Transmit, because Small transfers tend to benefit more from Limited Transmit, because
they are more likely to not receive enough duplicate ACKs. Assuming they are more likely to not receive enough duplicate ACKs. Assuming
the example in the previous paragraph, Limited Transmit allows the example in the previous paragraph, Limited Transmit allows
sending 5 MSS with a congestion window (cwnd) of 3 segments, plus two sending 5 MSS with a congestion window (cwnd) of three segments, plus
additional segments for the first two duplicate ACKs. With Limited two additional segments for the first two duplicate ACKs. With
Transmit, even a cwnd of 2 segments allows sending 5 MSS, at the Limited Transmit, even a cwnd of two segments allows sending 5 MSS,
expense of additional delay contributed by the Delayed ACK timer for at the expense of additional delay contributed by the Delayed ACK
the ACK that confirms segment 1. timer for the ACK that confirms segment 1.
When a multiple-segment window is used, the receiver will need to When a multiple-segment window is used, the receiver will need to
manage the reception of possible out-of-order received segments, manage the reception of possible out-of-order received segments,
requiring sufficient buffer space. Note that even when a 1-MSS requiring sufficient buffer space. Note that even when a window of 1
window is used, out-of-order arrival should also be managed, as the MSS is used, out-of-order arrival should also be managed, as the
sender may send multiple sub-MSS packets that fit in the window. (On sender may send multiple sub-MSS packets that fit in the window. (On
the other hand, the receiver is free to simply drop out-of-order the other hand, the receiver is free to simply drop out-of-order
segments, thus forcing retransmissions). segments, thus forcing retransmissions.)
3.3.1.1. Selective Acknowledgments (SACK) 3.3.1.1. Selective Acknowledgments (SACKs)
If a device with less severe memory and processing constraints can If a device with less severe memory and processing constraints can
afford advertising a TCP window size of several MSS, it makes sense afford advertising a TCP window size of several MSSs, it makes sense
to support the SACK option to improve performance. SACK allows a to support the SACK option to improve performance. SACK allows a
data receiver to inform the data sender of non-contiguous data blocks data receiver to inform the data sender of non-contiguous data blocks
received, thus a sender (having previously sent the SACK-Permitted received, thus a sender (having previously sent the SACK-Permitted
option) can avoid performing unnecessary retransmissions, saving option) can avoid performing unnecessary retransmissions, saving
energy and bandwidth, as well as reducing latency. In addition, SACK energy and bandwidth, as well as reducing latency. In addition, SACK
often allows for faster loss recovery when there is more than one often allows for faster loss recovery when there is more than one
lost segment in a window of data, since SACK recovery may complete lost segment in a window of data, since SACK recovery may complete
with less RTTs. SACK is particularly useful for bulk data transfers. with less RTTs. SACK is particularly useful for bulk data transfers.
A receiver supporting SACK will need to keep track of the data blocks A receiver supporting SACK will need to keep track of the data blocks
that need to be received. The sender will also need to keep track of that need to be received. The sender will also need to keep track of
which data segments need to be resent after learning which data which data segments need to be resent after learning which data
blocks are missing at the receiver. SACK adds 8*n+2 bytes to the TCP blocks are missing at the receiver. SACK adds 8*n+2 bytes to the TCP
header, where n denotes the number of data blocks received, up to 4 header, where n denotes the number of data blocks received, up to
blocks. For a low number of out-of-order segments, the header four blocks. For a low number of out-of-order segments, the header
overhead penalty of SACK is compensated by avoiding unnecessary overhead penalty of SACK is compensated by avoiding unnecessary
retransmissions. When the sender discovers the data blocks that have retransmissions. When the sender discovers the data blocks that have
already been received, it needs to also store the necessary state to already been received, it needs to also store the necessary state to
avoid unnecessary retransmission of data segments that have already avoid unnecessary retransmission of data segments that have already
been received. been received.
3.3.2. Delayed Acknowledgments 3.3.2. Delayed Acknowledgments
For certain traffic patterns, Delayed ACKs may have a detrimental For certain traffic patterns, Delayed ACKs may have a detrimental
effect, as already noted in Section 3.2.3. Advanced TCP stacks may effect, as already noted in Section 3.2.3. Advanced TCP stacks may
use heuristics to determine the maximum delay for an ACK. For CNNs, use heuristics to determine the maximum delay for an ACK. For CNNs,
the recommendation depends on the expected communication patterns. the recommendation depends on the expected communication patterns.
When traffic over a CNN is expected to mostly be unidirectional When traffic over a CNN is expected mostly to be unidirectional
messages with a size typically up to one MSS, and the time between messages with a size typically up to 1 MSS, and the time between two
two consecutive message transmissions is greater than the Delayed ACK consecutive message transmissions is greater than the Delayed ACK
timeout, it may make sense to use a smaller timeout or disable timeout, it may make sense to use a smaller timeout or disable
Delayed ACKs at the receiver. This avoids incurring additional Delayed ACKs at the receiver. This avoids incurring additional
delay, as well as the energy consumption of the sender (which might delay, as well as the energy consumption of the sender (which might,
e.g. keep its radio interface in receive mode) during that time. e.g., keep its radio interface in receive mode) during that time.
Note that disabling Delayed ACKs may only be possible if the peer Note that disabling Delayed ACKs may only be possible if the peer
device is administered by the same entity managing the constrained device is administered by the same entity managing the constrained
device. For request-response traffic, enabling Delayed ACKs is device. For request-response traffic, enabling Delayed ACKs is
recommended at the server end, in order to allow combining a response recommended at the server end, in order to allow combining a response
with the ACK into a single segment, thus increasing efficiency. In with the ACK into a single segment, thus increasing efficiency. In
addition, if a client issues requests infrequently, disabling Delayed addition, if a client issues requests infrequently, disabling Delayed
ACKs at the client allows an immediate ACK for the data segment ACKs at the client allows an immediate ACK for the data segment
carrying the response. carrying the response.
In contrast, Delayed ACKs allow to reduce the number of ACKs in bulk In contrast, Delayed ACKs allow for a reduced number of ACKs in bulk
transfer type of traffic, e.g. for firmware/software updates or for transfer types of traffic, e.g., for firmware/software updates or for
transferring larger data units containing a batch of sensor readings. transferring larger data units containing a batch of sensor readings.
Note that, in many scenarios, the peer that a constrained device Note that, in many scenarios, the peer that a constrained device
communicates with will be a general purpose system that communicates communicates with will be a general purpose system that communicates
with both constrained and unconstrained devices. Since delayed ACKs with both constrained and unconstrained devices. Since Delayed ACKs
are often configured through system-wide parameters, delayed ACKs are often configured through system-wide parameters, the behavior of
behavior at the peer will be the same regardless of the nature of the Delayed ACKs at the peer will be the same regardless of the nature of
endpoints it talks to. Such a peer will typically have delayed ACKs the endpoints it talks to. Such a peer will typically have Delayed
enabled. ACKs enabled.
3.3.3. Initial Window 3.3.3. Initial Window
RFC 5681 specifies a TCP Initial Window (IW) of roughly 4 kB [RFC5681] specifies a TCP Initial Window (IW) of roughly 4 kB.
[RFC5681]. Subsequently, RFC 6928 defined an experimental new value Subsequently, RFC 6928 [RFC6928] defines an experimental new value
for the IW, which in practice will result in an IW of 10 MSS for the IW, which in practice will result in an IW of 10 MSS.
[RFC6928]. The latter is nowadays used in many TCP implementations. Nowadays, the latter is used in many TCP implementations.
Note that a 10-MSS IW was recommended for resource-rich environments Note that a 10-MSS IW was recommended for resource-rich environments
(e.g. broadband environments), which are significantly different from (e.g., broadband environments), which are significantly different
CNNs. In CNNs, many application layer data units are relatively from CNNs. In CNNs, many application-layer data units are relatively
small (e.g. below one MSS). However, larger objects (e.g. large small (e.g., below 1 MSS). However, larger objects (e.g., large
files containing sensor readings, firmware updates, etc.) may also files containing sensor readings, firmware updates, etc.) may also
need to be transferred in CNNs. If such a large object is need to be transferred in CNNs. If such a large object is
transferred in CNNs, with an IW setting of 10 MSS, there is transferred in CNNs, with an IW setting of 10 MSS, there is
significant buffer overflow risk, since many CNN devices support significant buffer overflow risk, since many CNN devices support
network or radio buffers of a size smaller than 10 MSS. In order to network or radio buffers of a size smaller than 10 MSS. In order to
avoid such problem, in CNNs the IW needs to be carefully set, based avoid such a problem, the IW needs to be carefully set in CNNs, based
on device and network resource constraints. In many cases, a safe IW on device and network resource constraints. In many cases, a safe IW
setting will be smaller than 10 MSS. setting will be smaller than 10 MSS.
4. TCP usage recommendations in CNNs 4. TCP Usage Recommendations in CNNs
This section discusses how TCP can be used by applications that are This section discusses how TCP can be used by applications that are
developed for CNN scenarios. These remarks are by and large developed for CNN scenarios. These remarks are by and large
independent of how TCP is exactly implemented. independent of how TCP is exactly implemented.
4.1. TCP connection initiation 4.1. TCP Connection Initiation
In the constrained device to unconstrained device scenario In the scenario of a constrained device to an unconstrained device
illustrated above, a TCP connection is typically initiated by the illustrated above, a TCP connection is typically initiated by the
constrained device, in order for this device to support possible constrained device, in order for the device to support possible sleep
sleep periods to save energy. periods to save energy.
4.2. Number of concurrent connections 4.2. Number of Concurrent Connections
TCP endpoints with a small amount of memory may only support a small TCP endpoints with a small amount of memory may only support a small
number of connections. Each TCP connection requires storing a number number of connections. Each TCP connection requires storing a number
of variables in the TCB. Depending on the internal TCP of variables in the TCB. Depending on the internal TCP
implementation, each connection may result in further memory implementation, each connection may result in further memory
overhead, and connections may compete for scarce resources (e.g. overhead, and connections may compete for scarce resources (e.g.,
further memory overhead for send and receive buffers, etc). further memory overhead for send and receive buffers, etc.).
A careful application design may try to keep the number of concurrent A careful application design may try to keep the number of concurrent
connections as small as possible. A client can for instance limit connections as small as possible. A client can, for instance, limit
the number of simultaneous open connections that it maintains to a the number of simultaneous open connections that it maintains to a
given server. Multiple connections could for instance be used to given server. Multiple connections could, for instance, be used to
avoid the "head-of-line blocking" problem in an application transfer. avoid the "head-of-line blocking" problem in an application transfer.
However, in addition to consuming resources, using multiple However, in addition to consuming resources, using multiple
connections can also cause undesirable side effects in congested connections can also cause undesirable side effects in congested
networks. For example, the HTTP/1.1 specification encourages clients networks. For example, the HTTP/1.1 specification encourages clients
to be conservative when opening multiple connections [RFC7230]. to be conservative when opening multiple connections [RFC7230].
Furthermore, each new connection will start with a 3-way handshake, Furthermore, each new connection will start with a three-way
therefore increasing message overhead. handshake, therefore increasing message overhead.
Being conservative when opening multiple TCP connections is of Being conservative when opening multiple TCP connections is of
particular importance in Constrained-Node Networks. particular importance in Constrained-Node Networks.
4.3. TCP connection lifetime 4.3. TCP Connection Lifetime
In order to minimize message overhead, it makes sense to keep a TCP 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 connection open as long as the two TCP endpoints have more data to
send. If applications exchange data rather infrequently, i.e., if send. If applications exchange data rather infrequently, i.e., if
TCP connections would stay idle for a long time, the idle time can TCP connections would stay idle for a long time, the idle time can
result in problems. For instance, certain middleboxes such as result in problems. For instance, certain middleboxes such as
firewalls or NAT devices are known to delete state records after an firewalls or NAT devices are known to delete state records after an
inactivity interval. RFC 5382 specifies a minimum value for such inactivity interval. RFC 5382 [RFC5382] specifies a minimum value
interval of 124 minutes. Measurement studies have reported that TCP for such an interval of 124 minutes. Measurement studies have
NAT binding timeouts are highly variable across devices, with a reported that TCP NAT binding timeouts are highly variable across
median around 60 minutes, the shortest timeout being around 2 devices, with the median being around 60 minutes, the shortest
minutes, and more than 50% of the devices with a timeout shorter than timeout being around 2 minutes, and more than 50% of the devices with
the aforementioned minimum timeout of 124 minutes [HomeGateway]. The a timeout shorter than the aforementioned minimum timeout of 124
timeout duration used by a middlebox implementation may not be known minutes [HomeGateway]. The timeout duration used by a middlebox
to the TCP endpoints. implementation may not be known to the TCP endpoints.
In CNNs, such middleboxes may e.g. be present at the boundary between In CNNs, such middleboxes may, e.g., be present at the boundary
the CNN and other networks. If the middlebox can be optimized for between the CNN and other networks. If the middlebox can be
CNN use cases, it makes sense to increase the initial value for optimized for CNN use cases, it makes sense to increase the initial
filter state inactivity timers to avoid problems with idle value for filter state inactivity timers to avoid problems with idle
connections. Apart from that, this problem can be dealt with by connections. Apart from that, this problem can be dealt with by
different connection handling strategies, each having pros and cons. different connection-handling strategies, each having pros and cons.
One approach for infrequent data transfer is to use short-lived TCP One approach for infrequent data transfer is to use short-lived TCP
connections. Instead of trying to maintain a TCP connection for a connections. Instead of trying to maintain a TCP connection for a
long time, possibly short-lived connections can be opened between two long time, it is possible that short-lived connections can be opened
endpoints, which are closed if no more data needs to be exchanged. between two endpoints, which are closed if no more data needs to be
For use cases that can cope with the additional messages and the exchanged. For use cases that can cope with the additional messages
latency resulting from starting new connections, it is recommended to and the latency resulting from starting new connections, it is
use a sequence of short-lived connections, instead of maintaining a recommended to use a sequence of short-lived connections instead of
single long-lived connection. maintaining a single long-lived connection.
The message and latency overhead that stems from using a sequence of The message and latency overhead that stems from using a sequence of
short-lived connections could be reduced by TCP Fast Open (TFO) short-lived connections could be reduced by TCP Fast Open (TFO)
[RFC7413], which is an experimental TCP extension, at the expense of [RFC7413], which is an experimental TCP extension, at the expense of
increased implementation complexity and increased TCP Control Block increased implementation complexity and increased TCB size. TFO
(TCB) size. TFO allows data to be carried in SYN (and SYN-ACK) allows data to be carried in SYN (and SYN-ACK) segments and to be
segments, and to be consumed immediately by the receiving endpoint. consumed immediately by the receiving endpoint. This reduces the
This reduces the message and latency overhead compared to the message and latency overhead compared to the traditional three-way
traditional three-way handshake to establish a TCP connection. For handshake to establish a TCP connection. For security reasons, the
security reasons, the connection initiator has to request a TFO connection initiator has to request a TFO cookie from the other
cookie from the other endpoint. The cookie, with a size of 4 or 16 endpoint. The cookie, with a size of 4 or 16 bytes, is then included
bytes, is then included in SYN packets of subsequent connections. in SYN packets of subsequent connections. The cookie needs to be
The cookie needs to be refreshed (and obtained by the client) after a refreshed (and obtained by the client) after a certain amount of
certain amount of time. While a given cookie is used for multiple time. While a given cookie is used for multiple connections between
connections between the same two endpoints, the latter may become the same two endpoints, the latter may become vulnerable to privacy
vulnerable to privacy threats. In addition, a valid cookie may be threats. In addition, a valid cookie may be stolen from a
stolen from a compromised host and may be used to perform SYN flood compromised host and may be used to perform SYN flood attacks, as
attacks, as well as amplified reflection attacks to victim hosts (see well as amplified reflection attacks to victim hosts (see Section 5
Section 5 of RFC 7413). Nevertheless, TFO is more efficient than of [RFC7413]). Nevertheless, TFO is more efficient than frequently
frequently opening new TCP connections with the traditional three-way opening new TCP connections with the traditional three-way handshake,
handshake, as long as the cookie can be reused in subsequent as long as the cookie can be reused in subsequent connections.
connections. However, as stated in RFC 7413, TFO deviates from the However, as stated in [RFC7413], TFO deviates from the standard TCP
standard TCP semantics, since the data in the SYN could be replayed semantics, since the data in the SYN could be replayed to an
to an application in some rare circumstances. Applications should application in some rare circumstances. Applications should not use
not use TFO unless they can tolerate this issue, e.g., by using TFO unless they can tolerate this issue, e.g., by using TLS
Transport Layer Security (TLS) [RFC7413]. A comprehensive discussion [RFC7413]. A comprehensive discussion on TFO can be found in RFC
on TFO can be found at RFC 7413. 7413 [RFC7413].
Another approach is to use long-lived TCP connections with Another approach is to use long-lived TCP connections with
application-layer heartbeat messages. Various application protocols application-layer heartbeat messages. Various application protocols
support such heartbeat messages (e.g. CoAP over TCP [RFC8323]). support such heartbeat messages (e.g., CoAP over TCP [RFC8323]).
Periodic application-layer heartbeats can prevent early filter state Periodic application-layer heartbeats can prevent early filter state
record deletion in middleboxes. If the TCP binding timeout for a record deletion in middleboxes. If the TCP binding timeout for a
middlebox to be traversed by a given connection is known, middlebox middlebox to be traversed by a given connection is known, middlebox
filter state deletion will be avoided if the heartbeat period is filter state deletion will be avoided if the heartbeat period is
lower than the middlebox TCP binding timeout. Otherwise, the lower than the middlebox TCP binding timeout. Otherwise, the
implementer needs to take into account that middlebox TCP binding implementer needs to take into account that middlebox TCP binding
timeouts fall in a wide range of possible values [HomeGateway], and timeouts fall in a wide range of possible values [HomeGateway], and
it may be hard to find a proper heartbeat period for application- it may be hard to find a proper heartbeat period for application-
layer heartbeat messages. layer heartbeat messages.
One specific advantage of Heartbeat messages is that they also allow One specific advantage of heartbeat messages is that they also allow
aliveness checks at the application level. In general, it makes liveness checks at the application level. In general, it makes sense
sense to realize aliveness checks at the highest protocol layer to realize liveness checks at the highest protocol layer possible
possible that is meaningful to the application, in order to maximize that is meaningful to the application, in order to maximize the depth
the depth of the aliveness check. In addition, timely detection of a of the liveness check. In addition, timely detection of a dead peer
dead peer may allow savings in terms of TCB memory use. However, the may allow savings in terms of TCB memory use. However, the
transmission of heartbeat messages consumes resources. This aspect transmission of heartbeat messages consumes resources. This aspect
needs to be assessed carefully, considering the characteristics of needs to be assessed carefully, considering the characteristics of
each specific CNN. each specific CNN.
A TCP implementation may also be able to send "keep-alive" segments A TCP implementation may also be able to send "keep-alive" segments
to test a TCP connection. According to [RFC1122], "keep-alives" are to test a TCP connection. According to [RFC1122], keep-alives are an
an optional TCP mechanism that is turned off by default, i.e., an optional TCP mechanism that is turned off by default, i.e., an
application must explicitly enable it for a TCP connection. The application must explicitly enable it for a TCP connection. The
interval between "keep-alive" messages must be configurable and it interval between keep-alive messages must be configurable, and it
must default to no less than two hours. With this large timeout, TCP 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 keep-alive messages might not always be useful to avoid deletion of
filter state records in some middleboxes. However, sending TCP keep- filter state records in some middleboxes. However, sending TCP keep-
alive probes more frequently risks draining power on energy- alive probes more frequently risks draining power on energy-
constrained devices. constrained devices.
5. Security Considerations 5. Security Considerations
Best current practice for securing TCP and TCP-based communication Best current practices for securing TCP and TCP-based communication
also applies to CNN. As example, use of Transport Layer Security also applies to CNN. As an example, use of TLS [RFC8446] is strongly
(TLS) [RFC8446] is strongly recommended if it is applicable. recommended if it is applicable. However, note that TLS protects
However, note that TLS protects only the contents of the data only the contents of the data segments.
segments.
There are TCP options which can actually protect the transport layer. There are TCP options that can actually protect the transport layer.
One example is the TCP Authentication Option (TCP-AO) [RFC5925]. One example is the TCP Authentication Option (TCP-AO) [RFC5925].
However, this option adds overhead and complexity. TCP-AO typically However, this option adds overhead and complexity. TCP-AO typically
has a size of 16-20 bytes. An implementer needs to asses the trade- has a size of 16-20 bytes. An implementer needs to asses the trade-
off between security and performance when using TCP-AO, considering off between security and performance when using TCP-AO, considering
the characteristics (in terms of energy, bandwidth and computational the characteristics (in terms of energy, bandwidth, and computational
power) of the environment where TCP will be used. power) of the environment where TCP will be used.
For the mechanisms discussed in this document, the corresponding For the mechanisms discussed in this document, the corresponding
considerations apply. For instance, if TFO is used, the security considerations apply. For instance, if TFO is used, the security
considerations of [RFC7413] apply. considerations of RFC 7413 [RFC7413] apply.
Constrained devices are expected to support smaller TCP window sizes Constrained devices are expected to support smaller TCP window sizes
than less limited devices. In such conditions, segment than less-limited devices. In such conditions, segment
retransmission triggered by RTO expiration is expected to be retransmission triggered by RTO expiration is expected to be
relatively frequent, due to lack of (enough) duplicate ACKs, relatively frequent, due to lack of (enough) duplicate ACKs,
especially when a constrained device uses a single-MSS especially when a constrained device uses a single-MSS
implementation. For this reason, constrained devices running TCP may implementation. For this reason, constrained devices running TCP may
appear as particularly appealing victims of the so-called "shrew" appear as particularly appealing victims of the so-called "shrew"
Denial of Service (DoS) attack [shrew], whereby one or more sources Denial-of-Service (DoS) attack [SHREW], whereby one or more sources
generate a packet spike targeted to coincide with consecutive RTO- generate a packet spike targeted to coincide with consecutive RTO-
expiration-triggered retry attempts of a victim node. Note that the expiration-triggered retry attempts of a victim node. Note that the
attack may be performed by Internet-connected devices, including attack may be performed by Internet-connected devices, including
constrained devices in the same CNN as the victim, as well as remote constrained devices in the same CNN as the victim, as well as remote
ones. Mitigation techniques include RTO randomization and attack ones. Mitigation techniques include RTO randomization and attack
blocking by routers able to detect shrew attacks based on their blocking by routers able to detect shrew attacks based on their
traffic pattern. traffic pattern.
6. Acknowledgments 6. IANA Considerations
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
projects TEC2016-79988-P, PID2019-106808RA-I00, AEI/FEDER, UE, and by
Generalitat de Catalunya Grant 2017 SGR 376. 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, Stuart Cheshire, Gorry Fairhurst, Ingemar Johansson, Ted
Lemon, and Michael Tuexen. 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.
7. 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.
7.1. 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, IPv4, ICMP and TCP)
has been reported for uIP [Dunk]. Later versions of uIP implement
IPv6 as well.
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 3.2.3) 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.
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, IPv4, ICMP and
TCP), and a TCP code size of ~9 kB to ~14 kB [Dunk]. Both IPv4 and
IPv6 are supported in lwIP since v2.0.0.
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.
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, 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
contrast to uIP, retransmission is also handled by GNRC TCP. For
simplicity, GNRC TCP uses a single-MSS 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].
7.4. TinyOS
TinyOS was important as a 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.
7.5. 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'.
7.6. 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.
7.7. 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 different lightweight TCP
implementations. None of the implementations considered in this
Annex support ECN or TFO.
8. Annex. Changes compared to previous versions
RFC Editor: To be removed prior to publication
8.1. Changes between -00 and -01
o Changed title and abstract
o Clarification that communication 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
8.2. 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
8.3. 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
o Added discussion on shrew attacks
8.4. 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 section from the Annex
8.5. 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
8.6. Changes between -05 and -06
o Incorporated suggestions by Stuart Cheshire
8.7. Changes between -06 and -07
o Addressed comments by Gorry Fairhurst
8.8. Changes between -07 and -08
o Addressed WGLC comments by Ilpo Jarvinen, Markku Kojo and Ingemar
Johansson throughout the document, including the addition of a new
section on Initial Window considerations.
8.9. Changes between -08 and -09
o Addressed second round of comments by Ilpo Jarvinen and Markku
Kojo, based on the previous draft update.
8.10. Changes between -09 and -10
o Addressed comments by Erik Kline.
o Addressed a comment by Markku Kojo on advice given in RFC 6691.
8.11. Changes between -10 and -11
o Addressed a comment by Ted Lemon on MSS advice.
8.12. Changes between -11 and -12
o Addressed comments from IESG and various directorates.
8.13. Changes between -12 and -13
o Fixed two typos.
o Addressed a comment by Barry Leiba. This document has no IANA actions.
9. References 7. References
9.1. Normative References 7.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981, RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>. <https://www.rfc-editor.org/info/rfc793>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989, DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>. <https://www.rfc-editor.org/info/rfc1122>.
skipping to change at page 25, line 47 skipping to change at line 871
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management", Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015, BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>. <https://www.rfc-editor.org/info/rfc7567>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200, (IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017, DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>. <https://www.rfc-editor.org/info/rfc8200>.
9.2. Informative References 7.2. Informative References
[Commag] A. Betzler, C. Gomez, I. Demirkol, J. Paradells, "CoAP [6LO-WLANAH]
Congestion Control for the Internet of Things", IEEE Del Carpio Vega, L., Robles, M., and R. Morabito, "IPv6
Communications Magazine, June 2016. over 802.11ah", Work in Progress, Internet-Draft, draft-
delcarpio-6lo-wlanah-01, 19 October 2015,
<https://tools.ietf.org/html/draft-delcarpio-6lo-wlanah-
01>.
[Dunk] A. Dunkels, "Full TCP/IP for 8-Bit Architectures", 2003. [Commag] Betzler, A., Gomez, C., Demirkol, I., and J. Paradells,
"CoAP Congestion Control for the Internet of Things", IEEE
Communications Magazine, Vol. 54, Issue 7, pp. 154-160,
DOI 10.1109/MCOM.2016.7509394, July 2016,
<https://doi.org/10.1109/MCOM.2016.7509394>.
[ETEN] R. Krishnan et al, "Explicit transport error notification [CORE-FASOR]
(ETEN) for error-prone wireless and satellite networks", Jarvinen, I., Kojo, M., Raitahila, I., and Z. Cao, "Fast-
Computer Networks 2004. Slow Retransmission Timeout and Congestion Control
Algorithm for CoAP", Work in Progress, Internet-Draft,
draft-ietf-core-fasor-01, 19 October 2020,
<https://tools.ietf.org/html/draft-ietf-core-fasor-01>.
[GNRC] M. Lenders et al., "Connecting the World of Embedded [Dunk] Dunkels, A., "Full TCP/IP for 8-Bit Architectures",
Mobiles: The RIOTApproach to Ubiquitous Networking for the MobiSys '03, pp. 85-98, DOI 10.1145/1066116.106611, May
IoT", 2018. 2003, <https://doi.org/10.1145/1066116.106611>.
[ETEN] Krishnan, R., Sterbenz, J., Eddy, W., and C. Partridge,
"Explicit transport error notification (ETEN) for error-
prone wireless and satellite networks", Computer Networks,
DOI 10.1016/j.comnet.2004.06.012, June 2004,
<https://doi.org/10.1016/j.comnet.2004.06.012>.
[GNRC] Lenders, M., Kietzmann, P., Hahm, O., Petersen, H.,
Gündoğa, C., Baccelli, E., Schleiser, K., Schmidt, T., and
M. Wählisch, "Connecting the World of Embedded Mobiles:
The RIOT Approach to Ubiquitous Networking for the IoT",
arXiv:1801.02833v1 [cs.NI], January 2018.
[HomeGateway] [HomeGateway]
Haetoenen, S., Nyrhinen, A., Eggert, L., Strowes, S., Haetoenen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
Sarolahti, P., and M. Kojo, "An Experimental Study of Home Sarolahti, P., and M. Kojo, "An Experimental Study of Home
Gateway Characteristics", Proceedings of the 10th ACM Gateway Characteristics", Proceedings of the 10th ACM
SIGCOMM conference on Internet measurement 2010. SIGCOMM conference on Internet measurement, pp. 260-266,
DOI 10.1145/1879141.1879174, November 2010,
[I-D.delcarpio-6lo-wlanah] <https://doi.org/10.1145/1879141.1879174>.
Vega, L., Robles, I., and R. Morabito, "IPv6 over
802.11ah", draft-delcarpio-6lo-wlanah-01 (work in
progress), October 2015.
[I-D.ietf-6lo-fragment-recovery]
Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
ietf-6lo-fragment-recovery-21 (work in progress), March
2020.
[I-D.ietf-core-fasor]
Jarvinen, I., Kojo, M., Raitahila, I., and Z. Cao, "Fast-
Slow Retransmission Timeout and Congestion Control
Algorithm for CoAP", draft-ietf-core-fasor-01 (work in
progress), October 2020.
[I-D.ietf-tcpm-generalized-ecn]
Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets",
draft-ietf-tcpm-generalized-ecn-05 (work in progress),
November 2019.
[I-D.ietf-tcpm-rto-consider]
Allman, M., "Requirements for Time-Based Loss Detection",
draft-ietf-tcpm-rto-consider-17 (work in progress), July
2020.
[IntComp] C. Gomez, A. Arcia-Moret, J. Crowcroft, "TCP in the [IntComp] Gomez, C., Arcia-Moret, A., and J. Crowcroft, "TCP in the
Internet of Things: from ostracism to prominence", IEEE Internet of Things: from Ostracism to Prominence", IEEE
Internet Computing, January-February 2018. Internet Computing, Vol. 22, Issue 1, pp. 29-41,
DOI 10.1109/MIC.2018.112102200, January 2018,
<https://doi.org/10.1109/MIC.2018.112102200>.
[MQTT] ISO/IEC 20922:2016, "Message Queuing Telemetry Transport [MQTT] ISO/IEC, "Information technology -- Message Queuing
(MQTT) v3.1.1", 2016. Telemetry Transport (MQTT) v3.1.1", ISO/IEC 20922:2016,
June 2016.
[RFC2757] Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N. [RFC2757] Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N.
Vaidya, "Long Thin Networks", RFC 2757, Vaidya, "Long Thin Networks", RFC 2757,
DOI 10.17487/RFC2757, January 2000, DOI 10.17487/RFC2757, January 2000,
<https://www.rfc-editor.org/info/rfc2757>. <https://www.rfc-editor.org/info/rfc2757>.
[RFC2884] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of [RFC2884] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
Explicit Congestion Notification (ECN) in IP Networks", Explicit Congestion Notification (ECN) in IP Networks",
RFC 2884, DOI 10.17487/RFC2884, July 2000, RFC 2884, DOI 10.17487/RFC2884, July 2000,
<https://www.rfc-editor.org/info/rfc2884>. <https://www.rfc-editor.org/info/rfc2884>.
skipping to change at page 27, line 35 skipping to change at line 954
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89, Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, DOI 10.17487/RFC3819, July 2004, RFC 3819, DOI 10.17487/RFC3819, July 2004,
<https://www.rfc-editor.org/info/rfc3819>. <https://www.rfc-editor.org/info/rfc3819>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 "Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>. <https://www.rfc-editor.org/info/rfc4944>.
[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<https://www.rfc-editor.org/info/rfc5382>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925, Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>. June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6077] Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B. [RFC6077] Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
Briscoe, "Open Research Issues in Internet Congestion Briscoe, "Open Research Issues in Internet Congestion
Control", RFC 6077, DOI 10.17487/RFC6077, February 2011, Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,
<https://www.rfc-editor.org/info/rfc6077>. <https://www.rfc-editor.org/info/rfc6077>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
skipping to change at page 30, line 10 skipping to change at line 1072
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., [RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020, BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/info/rfc8900>. <https://www.rfc-editor.org/info/rfc8900>.
[RIOT] E. Baccelli et al., "RIOT: an Open Source Operating [RFC8931] Thubert, P., Ed., "IPv6 over Low-Power Wireless Personal
Systemfor Low-end Embedded Devices in the IoT", 2018. Area Network (6LoWPAN) Selective Fragment Recovery",
RFC 8931, DOI 10.17487/RFC8931, November 2020,
<https://www.rfc-editor.org/info/rfc8931>.
[shrew] A. Kuzmanovic, E. Knightly, "Low-Rate TCP-Targeted Denial [RFC8961] Allman, M., "Requirements for Time-Based Loss Detection",
of Service Attacks", SIGCOMM'03 2003. BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,
<https://www.rfc-editor.org/info/rfc8961>.
[RIOT] Baccelli, E., Gündoğa, C., Hahm, O., Kietzmann, P.,
Lenders, M., Petersen, H., Schleiser, K., Schmidt, T., and
M. Wählisch, "RIOT: An Open Source Operating System for
Low-End Embedded Devices in the IoT", IEEE Internet of
Things Journal, Vol. 5, Issue 6,
DOI 10.1109/JIOT.2018.2815038, March 2018,
<https://doi.org/10.1109/JIOT.2018.2815038>.
[SHREW] Nyrhinen, A. and E. Knightly, "Low-Rate TCP-Targeted
Denial of Service Attacks (The Shrew vs. the Mice and
Elephants)", SIGCOMM'03, DOI 10.1145/863955.863966, August
2003, <https://doi.org/10.1145/863955.863966>.
[TCPM-ECN] Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets",
Work in Progress, Internet-Draft, draft-ietf-tcpm-
generalized-ecn-07, 16 February 2021,
<https://tools.ietf.org/html/draft-ietf-tcpm-generalized-
ecn-07>.
Appendix A. 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 a 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.
A.1. uIP
uIP is a TCP/IP stack, targeted 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, IPv4, ICMP, and
TCP) has been reported for uIP [Dunk]. Later versions of uIP
implement IPv6 as well.
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 1 MSS) is not modified during a
connection. Stop-and-wait operation is used for sending data. Among
other optimizations, this allows for the avoidance of sliding window
operations, which use 32-bit arithmetic extensively and are expensive
on 8-bit CPUs.
Contiki uses the "split hack" technique (see Section 3.2.3) 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 3.2 kB on CC2538DK, cross-compiling on Linux.
A.2. lwIP
lwIP is a TCP/IP stack, targeted 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, IPv4, ICMP, and
TCP) and a TCP code size of ~9 kB to ~14 kB [Dunk]. Both IPv4 and
IPv6 are supported in lwIP since v2.0.0.
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 the buffering of incoming and outgoing 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.
A.3. RIOT
The RIOT TCP implementation (called "GNRC TCP") has been designed for
Class 1 devices [RFC7228]. 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
contrast to uIP, retransmission is also handled by GNRC TCP. For
simplicity, GNRC TCP uses a single-MSS 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 Portable Operating
System Interface (POSIX) socket wrapper that enables POSIX
compliance, if needed.
Further details on RIOT and GNRC can be found in [RIOT] and [GNRC].
A.4. TinyOS
TinyOS was important as a platform for early constrained devices.
TinyOS has an experimental TCP stack that uses a simple non-blocking
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 or otherwise drop the segment. A send buffer is provided
by the application. Multiple TCP connections are possible.
Recently, there has been little work on the stack.
A.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".
A.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.
A.7. Summary
None of the implementations considered in this Annex support ECN or
TFO.
+==========+=====+======+==========+======+========+==========+=====+
| | uIP |lwIP | lwIP 2.1 | RIOT | TinyOS | FreeRTOS |uC/OS|
| | |orig | | | | | |
+==========+=====+======+==========+======+========+==========+=====+
| Code | <5 |~9 to | 38 | <7 | N/A | <9.2 | N/A |
| Size | | ~14 | | | | | |
| (kB) | | | | | | | |
+----------+-----+------+----------+------+--------+----------+-----+
| Memory | (a) | (T1) | (T4) | (T3) | N/A | (T2) | N/A |
+==========+=====+======+==========+======+========+==========+=====+
| TCP |
| Features |
+==========+=====+======+==========+======+========+==========+=====+
| Single- | Yes | No | No | Yes | No | No | No |
| Segm. | | | | | | | |
+----------+-----+------+----------+------+--------+----------+-----+
| Slow | No | Yes | Yes | No | Yes | No | Yes |
| start | | | | | | | |
+----------+-----+------+----------+------+--------+----------+-----+
| Fast | No | Yes | Yes | No | Yes | No | Yes |
| rec/retx | | | | | | | |
+----------+-----+------+----------+------+--------+----------+-----+
| Keep- | No | No | Yes | No | No | Yes | Yes |
| alive | | | | | | | |
+----------+-----+------+----------+------+--------+----------+-----+
| Win. | No | No | Yes | No | No | Yes | No |
| Scale | | | | | | | |
+----------+-----+------+----------+------+--------+----------+-----+
| TCP | No | No | Yes | No | No | Yes | No |
| timest. | | | | | | | |
+----------+-----+------+----------+------+--------+----------+-----+
| SACK | No | No | Yes | No | No | Yes | No |
+----------+-----+------+----------+------+--------+----------+-----+
| Del. | No | Yes | Yes | No | No | Yes | Yes |
| ACKs | | | | | | | |
+----------+-----+------+----------+------+--------+----------+-----+
| Socket | No | No | Optional | (I) | Subset | Yes | Yes |
+----------+-----+------+----------+------+--------+----------+-----+
| Concur. | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
| Conn. | | | | | | | |
+==========+=====+======+==========+======+========+==========+=====+
| TLS supp | No | No | Yes | Yes | Yes | Yes | Yes |
| orted | | | | | | | |
+==========+=====+======+==========+======+========+==========+=====+
Table 1: Summary of TCP Features for Different Lightweight TCP
Implementations
Legend:
(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 that enables POSIX compliance
if needed
Mult.: Multiple
N/A: Not Available
Acknowledgments
The work of Carles Gomez has been funded in part by the Spanish
Government (Ministerio de Educacion, Cultura y Deporte) through Jose
Castillejo grants CAS15/00336 and CAS18/00170; the European Regional
Development Fund (ERDF); the Spanish Government through projects
TEC2016-79988-P, PID2019-106808RA-I00, AEI/FEDER, and UE; and the
Generalitat de Catalunya Grant 2017 SGR 376. 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 Keränen, Abhijan
Bhattacharyya, Andres Arcia-Moret, Yoshifumi Nishida, Joe Touch, Fred
Baker, Nik Sultana, Kerry Lynn, Erik Nordmark, Markku Kojo, Hannes
Tschofenig, David Black, Ilpo Jarvinen, Emmanuel Baccelli, Stuart
Cheshire, Gorry Fairhurst, Ingemar Johansson, Ted Lemon, and Michael
Tüxen. Simon Brummer provided details and kindly performed Random
Access Memory (RAM) and Read-Only Memory (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.
Authors' Addresses Authors' Addresses
Carles Gomez Carles Gomez
UPC UPC
C/Esteve Terradas, 7 C/Esteve Terradas, 7
Castelldefels 08860 08860 Castelldefels
Spain Spain
Email: carlesgo@entel.upc.edu Email: carlesgo@entel.upc.edu
Jon Crowcroft Jon Crowcroft
University of Cambridge University of Cambridge
JJ Thomson Avenue JJ Thomson Avenue
Cambridge, CB3 0FD Cambridge
CB3 0FD
United Kingdom United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk Email: jon.crowcroft@cl.cam.ac.uk
Michael Scharf Michael Scharf
Hochschule Esslingen Hochschule Esslingen
University of Applied Sciences
Flandernstr. 101 Flandernstr. 101
Esslingen 73732 73732 Esslingen am Neckar
Germany Germany
Email: michael.scharf@hs-esslingen.de Email: michael.scharf@hs-esslingen.de
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