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Versions: (draft-gomez-lwig-tcp-constrained-node-networks) 00 01 02 03 04 05 06 07 08

LWIG Working Group                                              C. Gomez
Internet-Draft                                                       UPC
Intended status: Informational                              J. Crowcroft
Expires: December 6, 2019                        University of Cambridge
                                                               M. Scharf
                                                    Hochschule Esslingen
                                                            June 4, 2019


           TCP Usage Guidance in the Internet of Things (IoT)
            draft-ietf-lwig-tcp-constrained-node-networks-08

Abstract

   This document provides guidance on how to implement and use the
   Transmission Control Protocol (TCP) in Constrained-Node Networks
   (CNNs), which are a characterstic of the Internet of Things (IoT).
   Such environments require a lightweight TCP implementation and may
   not make use of optional functionality.  This document explains a
   number of known and deployed techniques to simplify a TCP stack as
   well as corresponding tradeoffs.  The objective is to help embedded
   developers with decisions on which TCP features to use.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 6, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions used in this document . . . . . . . . . . . . . .   4
   3.  Characteristics of CNNs relevant for TCP  . . . . . . . . . .   4
     3.1.  Network and link properties . . . . . . . . . . . . . . .   4
     3.2.  Usage scenarios . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Communication and traffic patterns  . . . . . . . . . . .   6
   4.  TCP implementation and configuration in CNNs  . . . . . . . .   6
     4.1.  Addressing path properties  . . . . . . . . . . . . . . .   7
       4.1.1.  Maximum Segment Size (MSS)  . . . . . . . . . . . . .   7
       4.1.2.  Explicit Congestion Notification (ECN)  . . . . . . .   8
       4.1.3.  Explicit loss notifications . . . . . . . . . . . . .   9
     4.2.  TCP guidance for single-segment stacks  . . . . . . . . .   9
       4.2.1.  Single-segment stacks - benefits and issues . . . . .   9
       4.2.2.  TCP options for single-segment stacks . . . . . . . .  10
       4.2.3.  Delayed Acknowledgments for single-segment stacks . .  10
       4.2.4.  RTO calculation for single-segment stacks . . . . . .  11
     4.3.  General recommendations for TCP in CNNs . . . . . . . . .  11
       4.3.1.  Loss recovery and congestion/flow control . . . . . .  12
         4.3.1.1.  Selective Acknowledgments (SACK)  . . . . . . . .  12
       4.3.2.  Delayed Acknowledgments . . . . . . . . . . . . . . .  13
       4.3.3.  Initial Window  . . . . . . . . . . . . . . . . . . .  13
   5.  TCP usage recommendations in CNNs . . . . . . . . . . . . . .  14
     5.1.  TCP connection initiation . . . . . . . . . . . . . . . .  14
     5.2.  Number of concurrent connections  . . . . . . . . . . . .  14
     5.3.  TCP connection lifetime . . . . . . . . . . . . . . . . .  15
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   8.  Annex. TCP implementations for constrained devices  . . . . .  18
     8.1.  uIP . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.2.  lwIP  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.3.  RIOT  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.4.  TinyOS  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.5.  FreeRTOS  . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.6.  uC/OS . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.7.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  Annex. Changes compared to previous versions  . . . . . . . .  22
     9.1.  Changes between -00 and -01 . . . . . . . . . . . . . . .  22
     9.2.  Changes between -01 and -02 . . . . . . . . . . . . . . .  22
     9.3.  Changes between -02 and -03 . . . . . . . . . . . . . . .  22



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     9.4.  Changes between -03 and -04 . . . . . . . . . . . . . . .  23
     9.5.  Changes between -04 and -05 . . . . . . . . . . . . . . .  23
     9.6.  Changes between -05 and -06 . . . . . . . . . . . . . . .  23
     9.7.  Changes between -06 and -07 . . . . . . . . . . . . . . .  23
     9.8.  Changes between -07 and -08 . . . . . . . . . . . . . . .  23
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     10.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   The Internet Protocol suite is being used for connecting Constrained-
   Node Networks (CNNs) to the Internet, enabling the so-called Internet
   of Things (IoT) [RFC7228].  In order to meet the requirements that
   stem from CNNs, the IETF has produced a suite of new protocols
   specifically designed for such environments (see e.g.  [RFC8352]).
   New IETF protocol stack components include the IPv6 over Low-power
   Wireless Personal Area Networks (6LoWPAN) adaptation layer
   [RFC4944][RFC6282][RFC6775], the IPv6 Routing Protocol for Low-power
   and lossy networks (RPL) routing protocol [RFC6550], and the
   Constrained Application Protocol (CoAP) [RFC7252].

   As of the writing, the main current transport layer protocols in IP-
   based IoT scenarios are UDP and TCP.  However, TCP has been
   criticized (often, unfairly) as a protocol for the IoT.  In fact,
   some TCP features are not optimal for IoT scenarios, such as
   relatively long header size, unsuitability for multicast, and always-
   confirmed data delivery.  However, many typical claims on TCP
   unsuitability for IoT (e.g. a high complexity, connection-oriented
   approach incompatibility with radio duty-cycling, and spurious
   congestion control activation in wireless links) are not valid, can
   be solved, or are also found in well accepted IoT end-to-end
   reliability mechanisms (see [IntComp] for a detailed analysis).

   At the application layer, CoAP was developed over UDP [RFC7252].
   However, the integration of some CoAP deployments with existing
   infrastructure is being challenged by middleboxes such as firewalls,
   which may limit and even block UDP-based communications.  This is the
   main reason why a CoAP over TCP specification has been developed
   [RFC8323].

   Other application layer protocols not specifically designed for CNNs
   are also being considered for the IoT space.  Some examples include
   HTTP/2 and even HTTP/1.1, both of which run over TCP by default
   [RFC7230] [RFC7540], and the Extensible Messaging and Presence
   Protocol (XMPP) [RFC6120].  TCP is also used by non-IETF application-




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   layer protocols in the IoT space such as the Message Queue Telemetry
   Transport (MQTT) and its lightweight variants.

   TCP is a sophisticated transport protocol that includes optional
   functionality (e.g.  TCP options) that may improve performance in
   some environments.  However, many optional TCP extensions require
   complex logic inside the TCP stack and increase the codesize and the
   memory requirements.  Many TCP extensions are not required for
   interoperability with other standard-compliant TCP endpoints.  Given
   the limited resources on constrained devices, careful selection of
   optional TCP features can make an implementation more lightweight.

   This document provides guidance on how to implement and configure
   TCP, as well as on how TCP is advisable to be used by applications,
   in CNNs.  The overarching goal is to offer simple measures to allow
   for lightweight TCP implementation and suitable operation in such
   environments.  A TCP implementation following the guidance in this
   document is intended to be compatible with a TCP endpoint that is
   compliant to the TCP standards, albeit possibly with a lower
   performance.  This implies that such a TCP client would always be
   able to connect with a standard-compliant TCP server, and a
   corresponding TCP server would always be able to connect with a
   standard-compliant TCP client.

   This document assumes that the reader is familiar with TCP.  A
   comprehensive survey of the TCP standards can be found in [RFC7414].
   Similar guidance regarding the use of TCP in special environments has
   been published before, e.g., for cellular wireless networks
   [RFC3481].

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Characteristics of CNNs relevant for TCP

3.1.  Network and link properties

   CNNs are defined in [RFC7228] as networks whose characteristics are
   influenced by being composed of a significant portion of constrained
   nodes.  The latter are characterized by significant limitations on
   processing, memory, and energy resources, among others [RFC7228].
   The first two dimensions pose constraints on the complexity and on
   the memory footprint of the protocols that constrained nodes can
   support.  The latter requires techniques to save energy, such as
   radio duty-cycling in wireless devices [RFC8352], as well as



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   minimization of the number of messages transmitted/received (and
   their size).

   [RFC7228] lists typical network constraints in CNN, including low
   achievable bitrate/throughput, high packet loss and high variability
   of packet loss, highly asymmetric link characteristics, severe
   penalties for using larger packets, limits on reachability over time,
   etc.  CNN may use wireless or wired technologies (e.g., Power Line
   Communication), and the transmission rates are typically low (e.g.
   below 1 Mbps).

   For use of TCP, one challenge is that not all technologies in CNN may
   be aligned with typical Internet subnetwork design principles
   [RFC3819].  For instance, constrained nodes often use physical/link
   layer technologies that have been characterized as 'lossy', i.e.,
   exhibit a relatively high bit error rate.  Dealing with corruption
   loss is one of the open issues in the Internet [RFC6077].

3.2.  Usage scenarios

   There are different deployment and usage scenarios for CNNs.  Some
   CNNs follow the star topology, whereby one or several hosts are
   linked to a central device that acts as a router connecting the CNN
   to the Internet.  CNNs may also follow the multihop topology
   [RFC6606].

   In constrained environments, there can be different types of devices
   [RFC7228].  For example, there can be devices with single combined
   send/receive buffer, devices with a separate send and receive buffer,
   or devices with a pool of multiple send/receive buffers.  In the
   latter case, it is possible that buffers also be shared for other
   protocols.

   One key use case for the use of TCP in CNNs is a model where
   constrained devices connect to unconstrained servers in the Internet.
   But it is also possible that both TCP endpoints run on constrained
   devices.  In the first case, communication possibly has to traverse a
   middlebox (e.g. a firewall, NAT, etc.).  Figure 1 illustrates such
   scenario.  Note that the scenario is asymmetric, as the unconstrained
   device will typically not suffer the severe constraints of the
   constrained device.  The unconstrained device is expected to be
   mains-powered, to have high amount of memory and processing power,
   and to be connected to a resource-rich network.

   Assuming that a majority of constrained devices will correspond to
   sensor nodes, the amount of data traffic sent by constrained devices
   (e.g. sensor node measurements) is expected to be higher than the
   amount of data traffic in the opposite direction.  Nevertheless,



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   constrained devices may receive requests (to which they may respond),
   commands (for configuration purposes and for constrained devices
   including actuators) and relatively infrequent firmware/software
   updates.

                                                      +---------------+
           o     o <-------- TCP communication -----> |               |
          o     o                                     |               |
             o     o                                  | Unconstrained |
       o        o              +-----------+          |    device     |
           o     o   o  ------ | Middlebox |  ------- |               |
            o   o              +-----------+          |  (e.g. cloud) |
          o    o  o                                   |               |
                                                      +---------------+
      constrained devices


      Figure 1: TCP communication between a constrained device and an
               unconstrained device, traversing a middlebox.

3.3.  Communication and traffic patterns

   IoT applications are characterized by a number of different
   communication patterns.  The following non-comprehensive list
   explains some typical examples:

   o  Unidirectional transfers: An IoT device (e.g. a sensor) can send
      (repeatedly) updates to the other endpoint.  Not in every case
      there is a need for an application response back to the IoT
      device.

   o  Request-response patterns: An IoT device receiving a request from
      the other endpoint, which triggers a response from the IoT device.

   o  Bulk data transfers: A typical example for a long file transfer
      would be an IoT device firmware update.

   A typical communication pattern is that a constrained device
   communicates with an unconstrained device (cf.  Figure 1).  But it is
   also possible that constrained devices communicate amongst
   themselves.

4.  TCP implementation and configuration in CNNs

   This section explains how a TCP stack can deal with typical
   constraints in CNN.  The guidance in this section relates to the TCP
   implementation and its configuration.




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4.1.  Addressing path properties

4.1.1.  Maximum Segment Size (MSS)

   Assuming that IPv6 is used, and for the sake of lightweight
   implementation and operation, unless applications require handling
   large data units (i.e. leading to an IPv6 datagram size greater than
   1280 bytes), it may be desirable to limit the MTU to 1280 bytes in
   order to avoid the need to support Path MTU Discovery [RFC8201].  In
   addition, an MTU of 1280 bytes avoids incurring IPv6-layer
   fragmentation.

   An IPv6 datagram size exceeding 1280 bytes can be avoided by setting
   the TCP MSS not larger than 1220 bytes.  This assumes that the remote
   sender will use no TCP options, aside from possibly the MSS option,
   which is only used in the initial TCP SYN packet.

   In order to accommodate unrequested TCP options that may be used by
   some TCP implementations, a constrained device may advertise an MSS
   smaller than 1220 bytes (e.g. not larger than 1200 bytes).  Note
   that, in many implementations, TCP options generally consume payload
   space instead of increasing datagram size, therefore this suggestion
   might be overcautious and its suitability will depend on each
   specific scenario.

   Note that setting the MTU to 1280 bytes is possible for link layer
   technologies in the CNN space, even if some of them are characterized
   by a short data unit payload size, e.g. up to a few tens or hundreds
   of bytes.  For example, the maximum frame size in IEEE 802.15.4 is
   127 bytes.  6LoWPAN defined an adaptation layer to support IPv6 over
   IEEE 802.15.4 networks.  The adaptation layer includes a
   fragmentation mechanism, since IPv6 requires the layer below to
   support an MTU of 1280 bytes [RFC2460], while IEEE 802.15.4 lacked
   fragmentation mechanisms.  6LoWPAN defines an IEEE 802.15.4 link MTU
   of 1280 bytes [RFC4944].  Other technologies, such as Bluetooth LE
   [RFC7668], ITU-T G.9959 [RFC7428] or DECT-ULE [RFC8105], also use
   6LoWPAN-based adaptation layers in order to enable IPv6 support.
   These technologies do support link layer fragmentation.  By
   exploiting this functionality, the adaptation layers that enable IPv6
   over such technologies also define an MTU of 1280 bytes.

   On the other hand, there exist technologies also used in the CNN
   space, such as Master Slave / Token Passing (TP) [RFC8163],
   Narrowband IoT (NB-IoT) [RFC8376] or IEEE 802.11ah
   [I-D.delcarpio-6lo-wlanah], that do not suffer the same degree of
   frame size limitations as the technologies mentioned above.  The MTU
   for MS/TP is recommended to be 1500 bytes [RFC8163], the MTU in NB-




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   IoT is 1600 bytes, and the maximum frame payload size for IEEE
   802.11ah is 7991 bytes.

   While many IP-based IoT environments use IPv6, IPv4 can also be in
   use.  In IPv4, the minimum MTU is 576 bytes.  In order to avoid
   exceeding the IPv4 MTU, the MSS needs to be set to a value not larger
   than the IPv4 MTU minus 40 bytes.  Similarly to the recommendations
   given above for IPv6, a constrained device using IPv4 may advertise
   an even smaller MSS in order to accommodate unrequested TCP options.

   Finally, note that using larger MSS (to a suitable extent) may be
   beneficial, especially when transferring large payloads, as it
   reduces the number of packets (and packet headers) required for a
   given payload.

4.1.2.  Explicit Congestion Notification (ECN)

   Explicit Congestion Notification (ECN) [RFC3168] ECN allows a router
   to signal in the IP header of a packet that congestion is arising,
   for example when a queue size reaches a certain threshold.  An ECN-
   enabled TCP receiver will echo back the congestion signal to the TCP
   sender by setting a flag in its next TCP ACK.  The sender triggers
   congestion control measures as if a packet loss had happened.

   The document [RFC8087] outlines the principal gains in terms of
   increased throughput, reduced delay, and other benefits when ECN is
   used over a network path that includes equipment that supports
   Congestion Experienced (CE) marking.  In the context of CNNs, a
   remarkable feature of ECN is that congestion can be signalled without
   incurring packet drops (which will lead to retransmissions and
   consumption of limited resources such as energy and bandwitdh).

   ECN can further reduce packet losses since congestion control
   measures can be applied earlier [RFC2884].  Less lost packets implies
   that the number of retransmitted segments decreases, which is
   particularly beneficial in CNNs, where energy and bandwidth resources
   are typically limited.  Also, it makes sense to try to avoid packet
   drops for transactional workloads with small data sizes, which are
   typical for CNNs.  In such traffic patterns, it is more difficult and
   often impossible to detect packet loss without retransmission
   timeouts (e.g., as there may be no three duplicate ACKs).  Any
   retransmission timeout slows down the data transfer significantly.
   In addition, if the constrained device uses power saving techniques,
   a retransmission timeout will incur a wake-up action, in contrast to
   ACK clock- triggered sending.  When the congestion window of a TCP
   sender has a size of one segment and a TCP ACK with an ECN signal
   (ECE flag) arrives at the TCP sender, the TCP sender resets the
   retransmit timer, and the sender will only be able to send a new



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   packet when the retransmit timer expires.  Effectively, the TCP
   sender reduces at that moment its sending rate from 1 segment per
   Round Trip Time (RTT) to 1 segment per RTO 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 ACK
   the TCP sender resumes the normal ACK-clocked transmission of
   segments [RFC3168].

   ECN can be incrementally deployed in the Internet.  Guidance on
   configuration and usage of ECN is provided in [RFC7567].  Given the
   benefits, more and more TCP stacks in the Internet support ECN, and
   it specifically makes sense to leverage ECN in controlled
   environments such as CNNs.  Note, however, that supporting ECN
   increases implementation complexity.

4.1.3.  Explicit loss notifications

   There has been a significant body of research on solutions capable of
   explicitly indicating whether a TCP segment loss is due to
   corruption, in order to avoid activation of congestion control
   mechanisms [ETEN] [RFC2757].  While such solutions may provide
   significant improvement, they have not been widely deployed and
   remain as experimental work.  In fact, as of today, the IETF has not
   standardized any such solution.

4.2.  TCP guidance for single-segment stacks

   This section discusses TCP stacks that allow transferring only a
   single segment.  More general guidance is provided in Section 4.3.

4.2.1.  Single-segment stacks - benefits and issues

   A TCP stack can reduce the memory requirements by advertising a TCP
   window size of one MSS, and also transmit at most one MSS of
   unacknowledged data.  In that case, both congestion and flow control
   implementation are quite simple.  Such a small receive and send
   window may be sufficient for simple message exchanges in the CNN
   space.  However, only using a window of one MSS can significantly
   affect performance.  A stop-and-wait operation results in low
   throughput for transfers that exceed the length of one MSS, e.g., a
   firmware download.  Furthermore, a single-segment solution relies
   solely on timer-based loss recovery, therefore missing the
   performance gain of Fast Retransmit and Fast Recovery (which require
   a larger window size, see Subsection 4.3.1).

   If CoAP is used over TCP with the default setting for NSTART in
   [RFC7252], a CoAP endpoint is not allowed to send a new message to a
   destination until a response for the previous message sent to that



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   destination has been received.  This is equivalent to an application-
   layer window size of 1 data unit.  For this use of CoAP, a maximum
   TCP window of one MSS may be sufficient, as long as the CoAP message
   size does not exceed one MSS.

4.2.2.  TCP options for single-segment stacks

   A TCP implementation needs to support, at a minimum, TCP options 2, 1
   and 0.  These are, respectively, the Maximum Segment Size (MSS)
   option, the No-Operation option, and the End Of Option List marker
   [RFC0793].  None of these are a substantial burden to support.  These
   options are sufficient for interoperability with a standard-compliant
   TCP endpoint, albeit many TCP stacks support additional options and
   can negotiate their use.  A TCP implementation is permitted to
   silently ignore all other TCP options.

   A TCP implementation for a constrained device that uses a single-
   segment TCP receive or transmit window size may not benefit from
   supporting the following TCP options: Window scale [RFC7323], TCP
   Timestamps [RFC7323], Selective Acknowledgments (SACK) and SACK-
   Permitted [RFC2018].  Also other TCP options may not be required on a
   constrained device with a very lightweight implementation.  With
   regard to the Window scale option, note that it is only useful if a
   window size greater than 64 kB is needed.

   Note that a TCP sender can benefit from the TCP Timestamps option
   [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
   in a multihop scenario, link layer retries, etc.).  The header
   overhead incurred by the Timestamps option (of up to 12 bytes) needs
   to be taken into account.

   One potentially relevant TCP option in the context of CNNs is TCP
   Fast Open (TFO) [RFC7413].  As described in Section 5.3, TFO can be
   used to address the problem of traversing middleboxes that perform
   early filter state record deletion.

4.2.3.  Delayed Acknowledgments for single-segment stacks

   TCP Delayed Acknowledgments are meant to reduce the number of ACKs
   sent within a TCP connection, thus reducing network overhead, but
   they may increase the time until a sender may receive an ACK.  In
   general, usefulness of Delayed ACKs depends heavily on the usage
   scenario (see subsection 4.3.2).  There can be interactions with
   single-segment stacks.

   When traffic is unidirectional, if the sender can send at most one
   MSS of data or the receiver advertises a receive window not greater



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   than the MSS, Delayed ACKs may unnecessarily contribute delay (up to
   500 ms) to the RTT [RFC5681], which limits the throughput and can
   increase data delivery time.  Note that, in some cases, it may not be
   possible to disable Delayed ACKs.  One known workaround is to split
   the data to be sent into two segments of smaller size.  A standard
   compliant TCP receiver will acknowledge the second MSS of data, which
   can improve throughput.  However, this 'split hack' may not always
   work since a TCP receiver is required to acknowledge every second
   full-sized segment, but not two consecutive small segments.
   Furthermore, the overhead of sending two IP packets instead of one is
   another downside of the 'split hack'.

   Similar issues happen when the sender uses the Nagle algorithm.
   Disabling the algorithm will not have impact if the sender can only
   handle stop-and-wait operation.

   For request-response traffic, when the receiver uses Delayed ACKs, a
   response to a data message can piggyback an ACK, as long as the
   latter is sent before the Delayed ACK timer expires, thus avoiding
   unnecessary pure ACKs.  Disabling Delayed ACKs at the sender allows
   an immediate ACK for the data segment carrying the response.

4.2.4.  RTO calculation for single-segment stacks

   The Retransmission Timeout (RTO) calculation is one of the
   fundamental TCP algorithms [RFC6298].  There is a fundamental trade-
   off: A short, aggressive RTO behavior reduces wait time before
   retransmissions, but it also increases the probability of spurious
   timeouts.  The latter lead to unnecessary waste of potentially scarce
   resources in CNNs such as energy and bandwidth.  In contrast, a
   conservative timeout can result in long error recovery times and thus
   needlessly delay data delivery.

   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
   large impact on performance.  In that case, RTO algorithm tuning may
   be considered, although careful assessment of possible drawbacks is
   recommended [I-D.ietf-tcpm-rto-consider].

   As an example, an adaptive RTO algorithm for CoAP over UDP has been
   defined that has been found to perform well in CNN scenarios
   [Commag].

4.3.  General recommendations for TCP in CNNs

   This section summarizes some widely used techniques to improve TCP,
   with a focus on their use in CNNs.  The TCP extensions discussed here
   are useful in a wide range of network scenarios, including CNNs.



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   This section is not comprehensive.  A comprehensive survey of TCP
   extensions is published in [RFC7414].

4.3.1.  Loss recovery and congestion/flow control

   Devices that have enough memory to allow a larger (i.e. more than 3
   MSS of data) TCP window size can leverage a more efficient loss
   recovery than the timer-based approach used for smaller TCP window
   size (see Subsection 3.2.1) by using Fast Retransmit and Fast
   Recovery [RFC5681], at the expense of slightly greater complexity and
   TCB size.  Assuming that Delayed ACKs are used by the receiver, a
   window size of up to 5 MSS is required for Fast Retransmit and Fast
   Recovery to work efficiently: If in a given TCP transmission of
   segments 1, 2, 3, 4, 5, and 6 segment 2 gets lost, and the ACK for
   segment 1 is held by the Delayed ACK timer, then the sender should
   get an ACK for segment 1 when 3 arrives and duplicate ACKs when
   segments 4, 5, and 6 arrive.  It will 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 least 5 MSS.  With an MSS of
   1220 bytes, a buffer of a size of 5 MSS would require 6100 bytes.

   Further TCP improvements such as Limited Transmit [RFC3042] may also
   be useful for any transfer that has more than one segment in flight.
   Small transfers tend to benefit more from Limited Transmit, because
   they are more likely to not receive enough duplicate ACKs.  Assuming
   the example in the previous paragraph, Limited Transmit allows
   sending 5 MSS with a congestion window (cwnd) of 3 segments, plus two
   additional segments for each one of the first two duplicate ACKs.

   When a multiple-segment window is used, the receiver will need to
   manage the reception of possible out-of-order received segments,
   requiring sufficient buffer space.

4.3.1.1.  Selective Acknowledgments (SACK)

   If a device with less severe memory and processing constraints can
   afford advertising a TCP window size of several MSS, it makes sense
   to support the SACK option to improve performance.  SACK allows a
   data receiver to inform the data sender of non-contiguous data blocks
   received, thus a sender (having previously sent the SACK-Permitted
   option) can avoid performing unnecessary retransmissions, saving
   energy and bandwidth, as well as reducing latency.  In addition, SACK
   often allows for faster loss recovery when there is more than one
   lost segment in a window of data, since with SACK recovery requires
   less RTTs.  SACK is particularly useful for bulk data transfers.  A
   receiver supporting SACK will need to keep track of the SACK blocks
   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



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   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
   blocks.  For a low number of out-of-order segments, the header
   overhead penalty of SACK is compensated by avoiding unnecessary
   retransmissions.  When the sender discovers the data blocks that have
   already been received, it needs to also store the necessary state to
   avoid unnecessary retransmission of data segments that have already
   been received.

4.3.2.  Delayed Acknowledgments

   For certain traffic patterns, Delayed ACKs may have a detrimental
   effect, as already noted in Section 4.2.3.  Advanced TCP stacks may
   use heuristics to determine the maximum delay for an ACK.  For CNNs,
   the recommendation depends on the expected communication patterns.

   When traffic over a CNN is expected to mostly be unidirectional
   messages with a size typically up to one MSS, and the time between
   two consecutive message transmissions is greater than the Delayed ACK
   timeout, it may make sense to use a small timeout or disable Delayed
   ACKs at the receiver.  This avoids incurring additional delay, as
   well as the energy consumption of the sender (which might e.g. keep
   its radio interface in receive mode) during that time.  Note that
   disabling Delayed ACKs may only be possible if the peer device is
   administered by the same entity managing the constrained device.  For
   request-response traffic, enabling Delayed ACKs is recommended, in
   order to allow combining a response with the ACK into a single
   segment, thus increasing efficiency.  In this case, disabling Delayed
   ACKs at the sender allows an immediate ACK for the data segment
   carrying the response.

   In contrast, Delayed ACKs allow to reduce the number of ACKs in bulk
   transfer type of traffic, e.g. for firmware/software updates or for
   transferring larger data units containing a batch of sensor readings.

   Note that, in many scenarios, the peer that a constrained device
   communicates with will be a general purpose system that communicates
   with both constrained and unconstrained devices.  Since delayed ACKs
   are often configured through system-wide parameters, delayed ACKs
   behavior at the peer will be the same regardless of the nature of the
   endpoints it talks to.  Such a peer will typically have delayed ACKs
   enabled.

4.3.3.  Initial Window

   RFC 5681 specifies a TCP Initial Window (IW) of roughly 4 kB
   [RFC5681].  Subsequently, RFC 6928 defined an experimental new value




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   for the IW, which in practice will result in an IW of 10 MSS
   [RFC6928].  The latter is nowadays used in many TCP implementations.

   Note that a 10-MSS IW was recommended for resource-rich environments
   (e.g. broadband environments), which are significantly different from
   CNNs.  In CNNs, many application layer data units are relatively
   small (e.g. below one MSS).  However, larger objects (e.g. large
   files containing sensor readings, firmware updates, etc.) may also
   need to be transferred in CNNs.  If such a large object is
   transferred in CNNs, with an IW setting of 10 MSS, there is
   significant buffer overflow risk.  In order to avoid such problem, in
   CNNs the IW needs to be carefully set, based on device and network
   resource constraints.  In many cases, a safe IW setting will be
   smaller than 10 MSS.

5.  TCP usage recommendations in CNNs

   This section discusses how TCP can be used by applications that are
   developed for CNN scenarios.  These remarks are by and large
   independent of how TCP is exactly implemented.

5.1.  TCP connection initiation

   In the constrained device to unconstrained device scenario
   illustrated above, a TCP connection is typically initiated by the
   constrained device, in order for this device to support possible
   sleep periods to save energy.

5.2.  Number of concurrent connections

   TCP endpoints with a small amount of memory may only support a small
   number of connections.  Each TCP connection requires storing a number
   of variables in the Transmission Control Block (TCB).  Depending on
   the internal TCP implementation, each connection may result in
   further memory overhead, and connections may compete for scarce
   resources (e.g. further memory overhead for send and receive buffers,
   etc).

   A careful application design may try to keep the number of concurrent
   connections as small as possible.  A client can for instance limit
   the number of simultaneous open connections that it maintains to a
   given server.  Multiple connections could for instance be used to
   avoid the "head-of-line blocking" problem in an application transfer.
   However, in addition to consuming resources, using multiple
   connections can also cause undesirable side effects in congested
   networks.  For example, the HTTP/1.1 specification encourages clients
   to be conservative when opening multiple connections [RFC7230].




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   Furthermore, each new connection will start with a 3-way handshake,
   therefore increasing message overhead.

   Being conservative when opening multiple TCP connections is of
   particular importance in Constrained-Node Networks.

5.3.  TCP connection lifetime

   In order to minimize message overhead, it makes sense to keep a TCP
   connection open as long as the two TCP endpoints have more data to
   send.  If applications exchange data rather infrequently, i.e., if
   TCP connections would stay idle for a long time, the idle time can
   result in problems.  For instance, certain middleboxes such as
   firewalls or NAT devices are known to delete state records after an
   inactivity interval.  RFC 5382 specifies a minimum value for such
   interval of 124 minutes.  Measurement studies have reported that TCP
   NAT binding timeouts are highly variable across devices, with a
   median around 60 minutes, the shortest timeout being around 2
   minutes, and more than 50% of the devices with a timeout shorter than
   the aforementioned minimum timeout of 124 minutes [HomeGateway].  The
   timeout duration used by a middlebox implementation may not be known
   to the TCP endpoints.

   In CNNs, such middleboxes may e.g. be present at the boundary between
   the CNN and other networks.  If the middlebox can be optimized for
   CNN use cases, it makes sense to increase the initial value for
   filter state inactivity timers to avoid problems with idle
   connections.  Apart from that, this problem can be dealt with by
   different connection handling strategies, each having pros and cons.

   One approach for infrequent data transfer is to use short-lived TCP
   connections.  Instead of trying to maintain a TCP connection for long
   time, possibly short-lived connections can be opened between two
   endpoints, which are closed if no more data needs to be exchanged.
   For use cases that can cope with the additional messages and the
   latency resulting from starting new connections, it is recommended to
   use a sequence of short-lived connections, instead of maintaining a
   single long-lived connection.

   The message and latency overhead that stems from using a sequence of
   short-lived connections could be reduced by TCP Fast Open (TFO)
   [RFC7413], which is an experimental TCP extension, at the expense of
   increased implementation complexity and increased TCP Control Block
   (TCB) size.  TFO allows data to be carried in SYN (and SYN-ACK)
   segments, and to be consumed immediately by the receiving endpoint.
   This reduces the message and latency overhead compared to the
   traditional three-way handshake to establish a TCP connection.  For
   security reasons, the connection initiator has to request a TFO



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   cookie from the other endpoint.  The cookie, with a size of 4 or 16
   bytes, is then included in SYN packets of subsequent connections.
   The cookie needs to be refreshed (and obtained by the client) after a
   certain amount of time.  Nevertheless, TFO is more efficient than
   frequently opening new TCP connections with the traditional three-way
   handshake, as long as the cookie can be reused in subsequent
   connections.  However, as stated in RFC 7413, TFO deviates from the
   standard TCP semantics, since the data in the SYN could be replayed
   to an application in some rare circumstances.  Applications should
   not use TFO unless they can tolerate this issue, e.g., by using
   Transport Layer Security (TLS) [RFC7413].  A comprehensive discussion
   on TFO can be found at RFC 7413.

   Another approach is to use long-lived TCP connections with
   application-layer heartbeat messages.  Various application protocols
   support such heartbeat messages (e.g.  CoAP over TCP [RFC8323]).
   Periodic application-layer heartbeats can prevent early filter state
   record deletion in middleboxes.  If the TCP binding timeout for a
   middlebox to be traversed by a given connection is known, middlebox
   filter state deletion will be avoided if the heartbeat period is
   lower than the middlebox TCP binding timeout.  Otherwise, the
   implementer needs to take into account that middlebox TCP binding
   timeouts fall in a wide range of possible values [HomeGateway], and
   it may be hard to find a proper heartbeat period for application-
   layer heartbeat messages.

   One specific advantage of Heartbeat messages is that they also allow
   aliveness checks at the application level.  In general, it makes
   sense to realize aliveness checks at the highest protocol layer
   possible that is meaningful to the application, in order to maximize
   the depth of the aliveness check.  In addition, timely detection of a
   dead peer may allow savings in terms of TCB memory use.  However, the
   transmission of heartbeat messages consumes resources.  This aspect
   needs to be assessed carefully, considering the characteristics of
   each specific CNN.

   A TCP implementation may also be able to send "keep-alive" segments
   to test a TCP connection.  According to [RFC1122], "keep-alives" are
   an optional TCP mechanism that is turned off by default, i.e., an
   application must explicitly enable it for a TCP connection.  The
   interval between "keep-alive" messages must be configurable and it
   must default to no less than two hours.  With this large timeout, TCP
   keep-alive messages might not always be useful to avoid deletion of
   filter state records in some middleboxes.  However, sending TCP keep-
   alive probes more frequently risks draining power on energy-
   constrained devices.





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6.  Security Considerations

   Best current practise for securing TCP and TCP-based communication
   also applies to CNN.  As example, use of Transport Layer Security
   (TLS) is strongly recommended if it is applicable.

   There are also TCP options which can improve TCP security.  One
   example is the TCP Authentication Option (TCP-AO) [RFC5925].
   However, this option adds overhead and complexity.  TCP-AO typically
   has a size of 16-20 bytes.

   For the mechanisms discussed in this document, the corresponding
   considerations apply.  For instance, if TFO is used, the security
   considerations of [RFC7413] apply.

   Constrained devices are expected to support smaller TCP window sizes
   than less limited devices.  In such conditions, segment
   retransmission triggered by RTO expiration is expected to be
   relatively frequent, due to lack of (enough) duplicate ACKs,
   especially when a constrained device uses a single-segment
   implementation.  For this reason, constrained devices running TCP may
   appear as particularly appealing victims of the so-called "shrew"
   Denial of Service (DoS) attack [shrew], whereby one or more sources
   generate a packet spike targetted to coincide with consecutive RTO-
   expiration-triggered retry attempts of a victim node.  Note that the
   attack may be performed by Internet-connected devices, including
   constrained devices in the same CNN as the victim, as well as remote
   ones.  Mitigation techniques include RTO randomization and attack
   blocking by routers able to detect shrew attacks based on their
   traffic pattern.

7.  Acknowledgments

   Carles Gomez has been funded in part by the Spanish Government
   (Ministerio de Educacion, Cultura y Deporte) through the Jose
   Castillejo grants CAS15/00336 and and CAS18/00170, and by European
   Regional Development Fund (ERDF) and the Spanish Government through
   project TEC2016-79988-P, AEI/FEDER, UE.  Part of his contribution to
   this work has been carried out during his stays as a visiting scholar
   at the Computer Laboratory of the University of Cambridge.

   The authors appreciate the feedback received for this document.  The
   following folks provided comments that helped improve the document:
   Carsten Bormann, Zhen Cao, Wei Genyu, Ari Keranen, Abhijan
   Bhattacharyya, Andres Arcia-Moret, Yoshifumi Nishida, Joe Touch, Fred
   Baker, Nik Sultana, Kerry Lynn, Erik Nordmark, Markku Kojo, Hannes
   Tschofenig, David Black, Yoshifumi Nishida, Ilpo Jarvinen, Emmanuel
   Baccelli, Stuart Cheshire, Gorry Fairhurst, and Ingemar Johansson.



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   Simon Brummer provided details, and kindly performed RAM and ROM
   usage measurements, on the RIOT TCP implementation.  Xavi Vilajosana
   provided details on the OpenWSN TCP implementation.  Rahul Jadhav
   kindly performed code size measurements on the Contiki-NG and lwIP
   2.1.2 TCP implementations.  He also provided details on the uIP TCP
   implementation.

8.  Annex.  TCP implementations for constrained devices

   This section overviews the main features of TCP implementations for
   constrained devices.  The survey is limited to open source stacks
   with small footprint.  It is not meant to be all-encompassing.  For
   more powerful embedded systems (e.g., with 32-bit processors), there
   are further stacks that comprehensively implement TCP.  On the other
   hand, please be aware that this Annex is based on information
   available as of the writing.

8.1.  uIP

   uIP is a TCP/IP stack, targetted for 8 and 16-bit microcontrollers,
   which pioneered TCP/IP implementations for constrained devices. uIP
   has been deployed with Contiki and the Arduino Ethernet shield.  A
   code size of ~5 kB (which comprises checksumming, IP, ICMP and TCP)
   has been reported for uIP [Dunk].

   uIP uses the same global buffer for both incoming and outgoing
   traffic, which has a size of a single packet.  In case of a
   retransmission, an application must be able to reproduce the same
   user data that had been transmitted.  Multiple connections are
   supported, but need to share the global buffer.

   The MSS is announced via the MSS option on connection establishment
   and the receive window size (of one MSS) is not modified during a
   connection.  Stop-and-wait operation is used for sending data.  Among
   other optimizations, this allows to avoid sliding window operations,
   which use 32-bit arithmetic extensively and are expensive on 8-bit
   CPUs.

   Contiki uses the "split hack" technique (see Section 4.2.3) to avoid
   Delayed ACKs for senders using a single segment.

   The code size of the TCP implementation in Contiki-NG has been
   measured to be of 3.2 kB on CC2538DK, cross-compiling on Linux.








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8.2.  lwIP

   lwIP is a TCP/IP stack, targetted for 8- and 16-bit microcontrollers.
   lwIP has a total code size of ~14 kB to ~22 kB (which comprises
   memory management, checksumming, network interfaces, IP, ICMP and
   TCP), and a TCP code size of ~9 kB to ~14 kB [Dunk].

   In contrast with uIP, lwIP decouples applications from the network
   stack. lwIP supports a TCP transmission window greater than a single
   segment, as well as buffering of incoming and outcoming data.  Other
   implemented mechanisms comprise slow start, congestion avoidance,
   fast retransmit and fast recovery.  SACK and Window Scale support has
   been recently added to lwIP.

8.3.  RIOT

   The RIOT TCP implementation (called GNRC TCP) has been designed for
   Class 1 devices [RFC 7228].  The main target platforms are 8- and
   16-bit microcontrollers, with 32-bit platforms also supported.  GNRC
   TCP offers a similar function set as uIP, but it provides and
   maintains an independent receive buffer for each connection.  In
   contrast to uIP, retransmission is also handled by GNRC TCP.  For
   simplicity, GNRC TCP uses a single-segment implementation.  The
   application programmer does not need to know anything about the TCP
   internals, therefore GNRC TCP can be seen as a user-friendly uIP TCP
   implementation.

   The MSS is set on connections establishment and cannot be changed
   during connection lifetime.  GNRC TCP allows multiple connections in
   parallel, but each TCB must be allocated somewhere in the system.  By
   default there is only enough memory allocated for a single TCP
   connection, but it can be increased at compile time if the user needs
   multiple parallel connections.

   The RIOT TCP implementation offers an optional POSIX socket wrapper
   that enables POSIX compliance, if needed.

   Further details on RIOT and GNRC can be found in the literature
   [RIOT], [GNRC].

8.4.  TinyOS

   TinyOS was important as platform for early constrained devices.
   TinyOS has an experimental TCP stack that uses a simple nonblocking
   library-based implementation of TCP, which provides a subset of the
   socket interface primitives.  The application is responsible for
   buffering.  The TCP library does not do any receive-side buffering.
   Instead, it will immediately dispatch new, in-order data to the



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   application and otherwise drop the segment.  A send buffer is
   provided by the application.  Multiple TCP connections are possible.
   Recently there has been little further work on the stack.

8.5.  FreeRTOS

   FreeRTOS is a real-time operating system kernel for embedded devices
   that is supported by 16- and 32-bit microprocessors.  Its TCP
   implementation is based on multiple-segment window size, although a
   'Tiny-TCP' option, which is a single-segment variant, can be enabled.
   Delayed ACKs are supported, with a 20-ms Delayed ACK timer as a
   technique intended 'to gain performance'.

8.6.  uC/OS

   uC/OS is a real-time operating system kernel for embedded devices,
   which is maintained by Micrium. uC/OS is intended for 8-, 16- and
   32-bit microprocessors.  The uC/OS TCP implementation supports a
   multiple-segment window size.

8.7.  Summary






























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                        +---+---------+--------+----+------+--------+-----+
                        |uIP|lwIP orig|lwIP 2.1|RIOT|TinyOS|FreeRTOS|uC/OS|
   +------+-------------+---+---------+--------+----+------+--------+-----+
   |Memory|Code size(kB)| <5|~9 to ~14|   38   | <7 | N/A  |  <9.2  | N/A |
   |      |             |(a)|   (T1)  |  (T4)  |(T3)|      |  (T2)  |     |
   +------+-------------+---+---------+--------+----+------+--------+-----+
   |      | Single-Segm.|Yes|    No   |   No   | Yes|  No  |   No   |  No |
   |      +-------------+---+---------+--------+----+------+--------+-----+
   |      |  Slow start | No|   Yes   |   Yes  | No | Yes  |   No   | Yes |
   |  T   +-------------+---+---------+--------+----+------+--------+-----+
   |  C   |Fast rec/retx| No|   Yes   |   Yes  | No | Yes  |   No   | Yes |
   |  P   +-------------+---+---------+--------+----+------+--------+-----+
   |      |  Keep-alive | No|    No   |   Yes  | No |  No  |  Yes   | Yes |
   |      +-------------+---+---------+--------+----+------+--------+-----+
   |  f   |  Win. Scale | No|    No   |   Yes  | No |  No  |  Yes   |  No |
   |  e   +-------------+---+---------+--------+----+------+--------+-----+
   |  a   |  TCP timest.| No|    No   |   Yes  | No |  No  |  Yes   |  No |
   |  t   +-------------+---+---------+--------+----+------+--------+-----+
   |  u   |      SACK   | No|    No   |   Yes  | No |  No  |  Yes   |  No |
   |  r   +-------------+---+---------+--------+----+------+--------+-----+
   |  e   |  Del. ACKs  | No|   Yes   |   Yes  | No |  No  |  Yes   | Yes |
   |  s   +-------------+---+---------+--------+----+------+--------+-----+
   |      |     Socket  | No|    No   |Optional|(I) |Subset|  Yes   | Yes |
   |      +-------------+---+---------+--------+----+------+--------+-----+
   |      |Concur. Conn.|Yes|   Yes   |   Yes  | Yes| Yes  |  Yes   | Yes |
   +------+-------------+---+---------+--------+----+------+--------+-----+
   |    TLS supported   | No|    No   |   Yes  | Yes| Yes  |  Yes   | Yes |
   +--------------------+---+---------+--------+----+------+--------+-----+

     (T1)  = TCP-only, on x86 and AVR platforms
     (T2)  = TCP-only, on ARM Cortex-M platform
     (T3)  = TCP-only, on ARM Cortex-M0+ platform (NOTE: RAM usage for the same platform
             is ~2.5 kB for one TCP connection plus ~1.2 kB for each additional connection)
     (T4)  = TCP-only, on CC2538DK, cross-compiling on Linux
     (a)   = includes IP, ICMP and TCP on x86 and AVR platforms. The Contiki-NG TCP implementation has a code size of 3.2 kB on CC2538DK, cross-compiling on Linux
     (I)   = optional POSIX socket wrapper which enables POSIX compliance if needed
     Mult. = Multiple
     N/A   = Not Available


     Figure 2: Summary of TCP features for differrent lightweight TCP
     implementations.  None of the implementations considered in this
                         Annex support ECN or TFO.








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9.  Annex.  Changes compared to previous versions

   RFC Editor: To be removed prior to publication

9.1.  Changes between -00 and -01

   o  Changed title and abstract

   o  Clarification that communcation with standard-compliant TCP
      endpoints is required, based on feedback from Joe Touch

   o  Additional discussion on communication patters

   o  Numerous changes to address a comprehensive review from Hannes
      Tschofenig

   o  Reworded security considerations

   o  Additional references and better distinction between normative and
      informative entries

   o  Feedback from Rahul Jadhav on the uIP TCP implementation

   o  Basic data for the TinyOS TCP implementation added, based on
      source code analysis

9.2.  Changes between -01 and -02

   o  Added text to the Introduction section, and a reference, on
      traditional bad perception of TCP for IoT

   o  Added sections on FreeRTOS and uC/OS

   o  Updated TinyOS section

   o  Updated summary table

   o  Reorganized Section 4 (single-MSS vs multiple-MSS window size),
      some content now also in new Section 5

9.3.  Changes between -02 and -03

   o  Rewording to better explain the benefit of ECN

   o  Additional context information on the surveyed implementations

   o  Added details, but removed "Data size" raw, in the summary table




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   o  Added discussion on shrew attacks

9.4.  Changes between -03 and -04

   o  Addressing the remaining TODOs

   o  Alignment of the wording on TCP "keep-alives" with related
      discussions in the IETF transport area

   o  Added further discussion on delayed ACKs

   o  Removed OpenWSN subsection from the Annex

9.5.  Changes between -04 and -05

   o  Addressing comments by Yoshifumi Nishida

   o  Removed mentioning MD5 as an example (comment by David Black)

   o  Added memory footprint details of TCP implementations (Contiki-NG
      and lwIP 2.1.2) provided by Rahul Jadhav in the Annex

   o  Addressed comments by Ilpo Jarvinen throughout the whole document

   o  Improved the RIOT section in the Annex, based on feedback from
      Emmanuel Baccelli

9.6.  Changes between -05 and -06

   o  Incorporated suggestions by Stuart Cheshire

9.7.  Changes between -06 and -07

   o  Addressed comments by Gorry Fairhurst

9.8.  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
      subsection on Initial Window considerations.

10.  References

10.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.



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   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,
              <https://www.rfc-editor.org/info/rfc2018>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://www.rfc-editor.org/info/rfc2460>.

   [RFC3042]  Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
              TCP's Loss Recovery Using Limited Transmit", RFC 3042,
              DOI 10.17487/RFC3042, January 2001,
              <https://www.rfc-editor.org/info/rfc3042>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,
              <https://www.rfc-editor.org/info/rfc3819>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.




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   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <https://www.rfc-editor.org/info/rfc6928>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,
              <https://www.rfc-editor.org/info/rfc7323>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

10.2.  Informative References

   [Commag]   A. Betzler, C. Gomez, I. Demirkol, J. Paradells, "CoAP
              Congestion Control for the Internet of Things", IEEE
              Communications Magazine, June 2016.

   [Dunk]     A. Dunkels, "Full TCP/IP for 8-Bit Architectures", 2003.

   [ETEN]     R. Krishnan et al, "Explicit transport error notification
              (ETEN) for error-prone wireless and satellite networks",
              Computer Networks 2004.

   [GNRC]     M. Lenders et al., "Connecting the World of Embedded
              Mobiles: The RIOTApproach to Ubiquitous Networking for the
              IoT", 2018.

   [HomeGateway]
              Haetoenen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
              Sarolahti, P., and M. Kojo, "An Experimental Study of Home
              Gateway Characteristics", Proceedings of the 10th ACM
              SIGCOMM conference on Internet measurement 2010.

   [I-D.delcarpio-6lo-wlanah]
              Vega, L., Robles, I., and R. Morabito, "IPv6 over
              802.11ah", draft-delcarpio-6lo-wlanah-01 (work in
              progress), October 2015.






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   [I-D.ietf-tcpm-rto-consider]
              Allman, M., "Retransmission Timeout Requirements", draft-
              ietf-tcpm-rto-consider-08 (work in progress), February
              2019.

   [IntComp]  C. Gomez, A. Arcia-Moret, J. Crowcroft, "TCP in the
              Internet of Things: from ostracism to prominence", IEEE
              Internet Computing, January-February 2018.

   [RFC2757]  Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N.
              Vaidya, "Long Thin Networks", RFC 2757,
              DOI 10.17487/RFC2757, January 2000,
              <https://www.rfc-editor.org/info/rfc2757>.

   [RFC2884]  Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
              Explicit Congestion Notification (ECN) in IP Networks",
              RFC 2884, DOI 10.17487/RFC2884, July 2000,
              <https://www.rfc-editor.org/info/rfc2884>.

   [RFC3481]  Inamura, H., Ed., Montenegro, G., Ed., Ludwig, R., Gurtov,
              A., and F. Khafizov, "TCP over Second (2.5G) and Third
              (3G) Generation Wireless Networks", BCP 71, RFC 3481,
              DOI 10.17487/RFC3481, February 2003,
              <https://www.rfc-editor.org/info/rfc3481>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [RFC6077]  Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
              Briscoe, "Open Research Issues in Internet Congestion
              Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,
              <https://www.rfc-editor.org/info/rfc6077>.

   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,
              <https://www.rfc-editor.org/info/rfc6092>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.







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   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/info/rfc6282>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Routing",
              RFC 6606, DOI 10.17487/RFC6606, May 2012,
              <https://www.rfc-editor.org/info/rfc6606>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7414]  Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
              Zimmermann, "A Roadmap for Transmission Control Protocol
              (TCP) Specification Documents", RFC 7414,
              DOI 10.17487/RFC7414, February 2015,
              <https://www.rfc-editor.org/info/rfc7414>.

   [RFC7428]  Brandt, A. and J. Buron, "Transmission of IPv6 Packets
              over ITU-T G.9959 Networks", RFC 7428,
              DOI 10.17487/RFC7428, February 2015,
              <https://www.rfc-editor.org/info/rfc7428>.







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   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <https://www.rfc-editor.org/info/rfc7567>.

   [RFC7668]  Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
              Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,
              <https://www.rfc-editor.org/info/rfc7668>.

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,
              <https://www.rfc-editor.org/info/rfc8087>.

   [RFC8105]  Mariager, P., Petersen, J., Ed., Shelby, Z., Van de Logt,
              M., and D. Barthel, "Transmission of IPv6 Packets over
              Digital Enhanced Cordless Telecommunications (DECT) Ultra
              Low Energy (ULE)", RFC 8105, DOI 10.17487/RFC8105, May
              2017, <https://www.rfc-editor.org/info/rfc8105>.

   [RFC8163]  Lynn, K., Ed., Martocci, J., Neilson, C., and S.
              Donaldson, "Transmission of IPv6 over Master-Slave/Token-
              Passing (MS/TP) Networks", RFC 8163, DOI 10.17487/RFC8163,
              May 2017, <https://www.rfc-editor.org/info/rfc8163>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

   [RFC8352]  Gomez, C., Kovatsch, M., Tian, H., and Z. Cao, Ed.,
              "Energy-Efficient Features of Internet of Things
              Protocols", RFC 8352, DOI 10.17487/RFC8352, April 2018,
              <https://www.rfc-editor.org/info/rfc8352>.





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   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
              <https://www.rfc-editor.org/info/rfc8376>.

   [RIOT]     E. Baccelli et al., "RIOT: an Open Source Operating
              Systemfor Low-end Embedded Devices in the IoT", 2018.

   [shrew]    A. Kuzmanovic, E. Knightly, "Low-Rate TCP-Targeted Denial
              of Service Attacks", SIGCOMM'03 2003.

Authors' Addresses

   Carles Gomez
   UPC
   C/Esteve Terradas, 7
   Castelldefels  08860
   Spain

   Email: carlesgo@entel.upc.edu


   Jon Crowcroft
   University of Cambridge
   JJ Thomson Avenue
   Cambridge, CB3 0FD
   United Kingdom

   Email: jon.crowcroft@cl.cam.ac.uk


   Michael Scharf
   Hochschule Esslingen
   Flandernstr. 101
   Esslingen  73732
   Germany

   Email: michael.scharf@hs-esslingen.de














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