LWIG Working Group                                              C. Gomez
Internet-Draft                                                 UPC/i2CAT
Intended status: Informational                              J. Crowcroft
Expires: December 31, 2017                       University of Cambridge
                                                               M. Scharf
                                                           June 29, 2017

                   TCP over Constrained-Node Networks


   This document provides a profile for the Transmission Control
   Protocol (TCP) over Constrained-Node Networks (CNNs).  The
   overarching goal is to offer simple measures to allow for lightweight
   TCP implementation and suitable operation in such environments.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
   2.  Characteristics of CNNs relevant for TCP  . . . . . . . . . .   3
   3.  Scenario  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  TCP over CNNs . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  TCP connection initiation . . . . . . . . . . . . . . . .   4
     4.2.  Maximum Segment Size (MSS)  . . . . . . . . . . . . . . .   5
     4.3.  Window Size . . . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  RTO estimation  . . . . . . . . . . . . . . . . . . . . .   6
     4.5.  TCP connection lifetime . . . . . . . . . . . . . . . . .   7
       4.5.1.  Long TCP connection lifetime  . . . . . . . . . . . .   7
       4.5.2.  Short TCP connection lifetime . . . . . . . . . . . .   7
     4.6.  Explicit congestion notification  . . . . . . . . . . . .   8
     4.7.  TCP options . . . . . . . . . . . . . . . . . . . . . . .   8
     4.8.  Delayed Acknowledgments . . . . . . . . . . . . . . . . .   9
     4.9.  Explicit loss notifications . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Annex. TCP implementations for constrained devices  . . . . .  10
     7.1.  uIP . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.2.  lwIP  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.3.  RIOT  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.4.  OpenWSN . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.5.  TinyOS  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.6.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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 protocols
   specifically designed for such environments

   At the application layer, the Constrained Application Protocol (CoAP)
   was developed over UDP [RFC7252].  However, the integration of some
   CoAP deployments with existing infrastructure is being challenged by
   middleboxes such as firewalls, which may limit and even block UDP-

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   based communications.  This the main reason why a CoAP over TCP
   specification is being developed [I-D.tschofenig-core-coap-tcp-tls].

   On the other hand, other application layer protocols not specifically
   designed for CNNs are also being considered for the IoT space.  Some
   examples include HTTP/2 and even HTTP/1.1, both of which run over TCP
   by default [RFC7540][RFC2616], and the Extensible Messaging and
   Presence Protocol (XMPP) [RFC 6120].  TCP is also used by non-IETF
   application-layer protocols in the IoT space such as MQTT and its
   lightweight variants [MQTTS].

   This document provides a profile for TCP over CNNs.  The overarching
   goal is to offer simple measures to allow for lightweight TCP
   implementation and suitable operation in such environments.

1.1.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
   document are to be interpreted as described in [RFC2119]

2.  Characteristics of CNNs relevant for TCP

   CNNs are defined in [RFC7228] as networks whose characteristics are
   influenced by being composed of a significant portion of constrained
   nodes.  The latter are characterized by significant limitations on
   processing, memory, and energy resources, among others [RFC7228].
   The first two dimensions pose constraints on the complexity and on
   the memory footprint of the protocols that constrained nodes can
   support.  The latter requires techniques to save energy, such as
   radio duty-cycling in wireless devices
   [I-D.ietf-lwig-energy-efficient], as well as minimization of the
   number of messages transmitted/received (and their size).

   Constrained nodes often use physical/link layer technologies that
   have been characterized as 'lossy'.  Many such technologies are
   wireless, therefore exhibiting a relatively high bit error rate.
   However, some wired technologies used in the CNN space are also lossy
   (e.g.  Power Line Communication).  Transmission rates of CNN radio or
   wired interfaces are typically low (e.g. below 1 Mbps).

   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

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3.  Scenario

   The main scenario for use of TCP over CNNs comprises a constrained
   device and an unconstrained device that communicate over the Internet
   using TCP, possibly traversing a middlebox (e.g. a firewall, NAT,
   etc.).  Figure 1 illustrates such scenario.  Note that the scenario
   is asymmetric, as the unconstrained device will typically not suffer
   the severe constraints of the constrained device.  The unconstrained
   device is expected to be mains-powered, to have high amount of memory
   and processing power, and to be connected to a resource-rich network.

   Assuming that a majority of constrained devices will correspond to
   sensor nodes, the amount of data traffic sent by constrained devices
   (e.g. sensor node measurements) is expected to be higher than the
   amount of data traffic in the opposite direction.  Nevertheless,
   constrained devices may receive requests (to which they may respond),
   commands (for configuration purposes and for constrained devices
   including actuators) and relatively infrequent firmware/software

         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.

4.  TCP over CNNs

4.1.  TCP connection initiation

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

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4.2.  Maximum Segment Size (MSS)

   Some link layer technologies in the CNN space are characterized by a
   short data unit payload size, e.g. up to a few tens or hundreds of
   bytes.  For example, the maximum frame size in IEEE 802.15.4 is 127

   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.

   For devices using technologies with a link MTU of 1280 bytes (e.g.
   defined by a 6LoWPAN-based adaptation layer), in order to avoid IP
   layer fragmentation, the TCP MSS must not be set to a value greater
   than 1220 bytes in CNNs, and it must not be set to a value leading to
   an IPv6 datagram size exceeding 1280 bytes.  (Note: IP version 6 is

   On the other hand, there exist technologies also used in the CNN
   space, such as Master Slave / Token Passing (TP) [RFC8163],
   Narrowband IoT (NB-IoT) [I-D.ietf-lpwan-overview] or IEEE 802.11ah
   [I-D.delcarpio-6lo-wlanah], that do not suffer the same degree of
   frame size limitations as the technologies mentioned above.  The MTU
   for MS/TP is recommended to be 1500 bytes [RFC8163], the MTU in NB-
   IoT is 1600 bytes, and the maximum frame payload size for IEEE
   802.11ah is 7991 bytes.  Over such technologies, the TCP MSS may be
   set to a value greater than 1220 bytes, as long as IPv6 datagram size
   does not exceed the MTU for each technology.  One consideration in
   this regard is that, when a node supports an MTU greater than 1280
   bytes, it 'SHOULD' then support Path MTU (PMTU) discovery [RFC1981].
   (Note that, as explained in RFC 1981, a minimal IPv6 implementation
   may 'choose to omit implementation of Path MTU Discovery').  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.

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4.3.  Window Size

   A TCP stack can reduce the implementation complexity by advertising a
   TCP window size of one MSS, and also transmit at most one MSS of
   unacknowledged data, at the cost of decreased performance.  This size
   for receive and send window is appropriate for simple message
   exchanges in the CNN space, reduces implementation complexity and
   memory requirements, and reduces overhead (see section 4.7).

   A TCP window size of one MSS follows the same rationale as the
   default setting for NSTART in [RFC7252], leading to equivalent
   operation when CoAP is used over TCP.

   For devices that can afford greater TCP window size, it may be useful
   to allow window sizes of at least five MSSs, in order to allow Fast
   Retransmit and Fast Recovery [RFC5681].

4.4.  RTO estimation

   If a TCP sender uses very small window size and cannot use Fast
   Retransmit/Fast Recovery or SACK, the RTO algorithm has a larger
   impact on performance than for a more powerful TCP stack.  In that
   case, RTO algorithm tuning may be considered, although careful
   assessment of possible drawbacks is recommended.  A fundamental
   trade-off exists between responsiveness and correctness of RTOs
   [I-D.ietf-tcpm-rto-consider].  A more aggressive RTO behavior reduces
   wait time before retransmissions, but it also increases the
   probability of incurring spurious timeouts.  The latter lead to
   unnecessary waste of potentially scarce resources in CNNs such as
   energy and bandwidth.

   On a related note, there has been recent activity in the area of
   defining an adaptive RTO algorithm for CoAP (over UDP).  As shown in
   experimental studies, the RTO estimator for CoAP defined in
   [I-D.ietf-core-cocoa] (hereinafter, CoCoA RTO) outperforms state-of-
   art algorithms designed as improvements to RFC 6298 [RFC6298] for
   TCP, in terms of packet delivery ratio, settling time after a burst
   of messages, and fairness (the latter is specially relevant in
   multihop networks connected to the Internet through a single device,
   such as a 6LoWPAN Border Router (6LBR) configured as a RPL root)
   [Commag].  In fact, CoCoA RTO has been designed specifically
   considering the challenges of CNNs, in contrast with the RFC 6298

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4.5.  TCP connection lifetime

   [[Note: future revisions will better separate what a TCP stack should
   support, or not, and how the TCP stack should be used by
   applications, e.g., whether to close connections or not.]]

4.5.1.  Long TCP connection lifetime

   In CNNs, in order to minimize message overhead, a TCP connection
   should be kept open as long as the two TCP endpoints have more data
   to exchange or it is envisaged that further segment exchanges will
   take place within an interval of two hours since the last segment has
   been sent.  A greater interval may be used in scenarios where
   applications exchange data infrequently.

   TCP keep-alive messages [RFC1122] may be supported by a server, to
   check whether a TCP connection is active, in order to release state
   of inactive connections.  This may be useful for servers running on
   memory-constrained devices.

   Since the keep-alive timer may not be set to a value lower than two
   hours [RFC1122], TCP keep-alive messages are not useful to guarantee
   that filter state records in middleboxes such as firewalls will not
   be deleted after an inactivity interval typically in the order of a
   few minutes [RFC6092].  In scenarios where such middleboxes are
   present, alternative measures to avoid early deletion of filter state
   records (which might lead to frequent establishment of new TCP
   connections between the two involved endpoints) include increasing
   the initial value for the filter state inactivity timers (if
   possible), and using application layer heartbeat messages.

4.5.2.  Short TCP connection lifetime

   A different approach to addressing the problem of traversing
   middleboxes that perform early filter state record deletion relies on
   using TCP Fast Open (TFO) [RFC7413].  In this case, instead of trying
   to maintain a TCP connection for long time, possibly short-lived
   connections can be opened between two endpoints while incurring low
   overhead.  In fact, TFO allows data to be carried in SYN (and SYN-
   ACK) packets, and to be consumed immediately by the receceiving
   endpoint, thus reducing overhead compared with the traditional three-
   way handshake required to establish a TCP connection.

   For security reasons, TFO requires the TCP endpoint that will open
   the TCP connection (which in CNNs will typically be the constrained
   device) to request a cookie from the other endpoint.  The cookie,
   with a size of 4 or 16 bytes, is then included in SYN packets of
   subsequent connections.  The cookie needs to be refreshed (and

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   obtained by the client) after a certain amount of time.
   Nevertheless, TFO is more efficient than frequently opening new TCP
   connections (by using the traditional three-way handshake) for
   transmitting new data, as long as the cookie update rate is well
   below the data new connection rate.

4.6.  Explicit congestion notification

   Explicit Congestion Notification (ECN) [RFC3168] may be used in CNNs.
   ECN allows a router to signal in the IP header of a packet that
   congestion is arising, for example when queue size reaches a certain
   threshold.  If such a packet encapsulates a TCP data packet, an ECN-
   enabled TCP receiver will echo back the congestion signal to the TCP
   sender by setting a flag in its next TCP ACK.  The sender triggers
   congestion control measures as if a packet loss had happened.  In
   that case, when the congestion window of a TCP sender has a size of
   one segment, the TCP sender resets the retransmit timer, and will
   only be able to send a new packet when the retransmit timer expires
   [RFC3168].  Effectively, the TCP sender reduces at that moment its
   sending rate from 1 segment per Round Trip Time (RTT) to 1 segment
   per default RTO.

   ECN can reduce packet losses, since congestion control measures can
   be applied earlier than after the reception of three duplicate ACKs
   (if the TCP sender window is large enough) or upon TCP sender RTO
   expiration [RFC2884].  Therefore, the number of retries decreases,
   which is particularly beneficial in CNNs, where energy and bandwidth
   resources are typically limited.  Furthermore, latency and jitter are
   also reduced.

   ECN is particularly appropriate in CNNs, since in these environments
   transactional type interactions are a dominant traffic pattern.  As
   transactional data size decreases, the probability of detecting
   congestion by the presence of three duplicate ACKs decreases.  In
   contrast, ECN can still activate congestion control measures without
   requiring three duplicate ACKs.

4.7.  TCP options

   A TCP implementation needs to support options 0, 1 and 2 [RFC793].  A
   TCP implementation for a constrained device that uses a single-MSS
   TCP receive or transmit window size may not benefit from supporting
   the following TCP options: Window scale [RFC1323], TCP Timestamps
   [RFC1323], Selective Acknowledgements (SACK) and SACK-Permitted
   [RFC2018].  Other TCP options should not be used, in keeping with the
   principle of lightweight operation.

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   Other TCP options should not be supported by a constrained device, in
   keeping with the principle of lightweight implementation and

   If a device, with less severe memory and processing constraints, can
   afford advertising a TCP window size of several MSSs, it may 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.  The receiver supporting SACK
   will need to manage the reception of possible out-of-order received
   segments, requiring sufficient buffer space.

   SACK adds 8*n+2 bytes to the TCP header, where n denotes the number
   of data blocks received, up to 4 blocks.  For a low number of out-of-
   order segments, the header overhead penalty of SACK is compensated by
   avoiding unnecessary retransmissions.

   Another potentially relevant TCP option in the context of CNNs is
   (TFO) [RFC7413].  As described in section 4.5.2, TFO can be used to
   address the problem of traversing middleboxes that perform early
   filter state record deletion.

4.8.  Delayed Acknowledgments

   A device that advertises a single-MSS receive window needs to avoid
   use of delayed ACKs in order to avoid contributing unnecessary delay
   (of up to 500 ms) to the RTT [RFC5681].

   When traffic over a CNN is expected to be mostly of transactional
   type, with transaction size typically below one MSS, delayed ACKs are
   not recommended.  For transactional-type traffic between a
   constrained device and a peer (e.g. backend infrastructure) that uses
   delayed ACKs, the maximum ACK rate of the peer will be typically of
   one ACK every 200 ms (or even lower).  If in such conditions the peer
   device is administered by the same entity managing the constrained
   device, it is recommended to disable delayed ACKs at the peer side.

   On the other hand, 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

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4.9.  Explicit loss notifications

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

5.  Security Considerations

   If TFO is used, the security considerations of RFC 7413 apply.

   There exist TCP options which improve TCP security.  Examples include
   the TCP MD5 signature option [RFC2385] and the TCP Authentication
   Option (TCP-AO) [RFC5925].  However, both options add overhead and
   complexity.  The TCP MD5 signature option adds 18 bytes to every
   segment of a connection.  TCP-AO typically has a size of 16-20 bytes.

6.  Acknowledgments

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

   The authors appreciate the feedback received for this document.  The
   following folks provided comments that helped improve the document:
   Carsten Bormann, Zhen Cao, Wei Genyu, Michael Scharf, Ari Keranen,
   Abhijan Bhattacharyya, Andres Arcia-Moret, Yoshifumi Nishida, Joe
   Touch, Fred Baker, Nik Sultana, Kerry Lynn, and Erik Nordmark.  Simon
   Brummer provided details on the RIOT TCP implementation.  Xavi
   Vilajosana provided details on the OpenWSN TCP implementation.

7.  Annex.  TCP implementations for constrained devices

   This section overviews the main features of TCP implementations for
   constrained devices.

7.1.  uIP

   uIP is a TCP/IP stack, targetted for 8 and 16-bit microcontrollers.
   uIP has been deployed with Contiki and the Arduino Ethernet shield.

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   A code size of ~5 kB (which comprises checksumming, IP, ICMP and TCP)
   has been reported for uIP [Dunk].

   uIP provides a global buffer for incoming packets, of single-packet
   size.  A buffer for outgoing data is not provided.  In case of a
   retransmission, an application must be able to reproduce the same
   packet that had been transmitted.

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

7.2.  lwIP

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

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

7.3.  RIOT

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

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

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7.4.  OpenWSN

   The TCP implementation in OpenWSN is mostly equivalent to the uIP TCP
   implementation.  OpenWSN TCP implementation only supports the minimum
   state machine functionality required.  For example, it does not
   perform retransmissions.

7.5.  TinyOS


7.6.  Summary

                             |  uIP  |lwIP orig|lwIP 2.0 | RIOT | OpenWSN | TinyOS |
   |        |   Data size    |   *   |    *    |    *    |  *   |    *    |   *    |
   | Memory +----------------+-------+---------+---------+------+---------+--------+
   |        | Code size (kB) |  < 5  |~9 to ~14|    *    |  *   |    *    |   *    |
   |        |Window size(MSS)|  1    | Multiple| Multiple|  1   |    1    |   *    |
   |        +----------------+-------+---------+---------+------+---------+--------+
   |        |  Slow start    |  No   |   Yes   |   Yes   |  No  |   No    |   *    |
   |   T    +----------------+-------+---------+---------+------+---------+--------+
   |   C    | Fast rec/retx  |  No   |   Yes   |   Yes   |  No  |   No    |   *    |
   |   P    +----------------+-------+---------+---------+------+---------+--------+
   |        |  Keep-alive    |  No   |    *    |    *    |  No  |   No    |   *    |
   |        +----------------+-------+---------+---------+------+---------+--------+
   |   f    |     TFO        |  No   |    No   |    *    |  No  |   No    |   *    |
   |   e    +----------------+-------+---------+---------+------+---------+--------+
   |   a    |     ECN        |  No   |    No   |    *    |  No  |   No    |   *    |
   |   t    +----------------+-------+---------+---------+------+---------+--------+
   |   u    | Window Scale   |  No   |    No   |   Yes   |  No  |   No    |   *    |
   |   r    +----------------+-------+---------+---------+------+---------+--------+
   |   e    | TCP timestamps |  No   |    No   |   Yes   |  No  |   No    |   *    |
   |   s    +----------------+-------+---------+---------+------+---------+--------+
   |        |    SACK        |  No   |    No   |   Yes   |  No  |   No    |   *    |
   |        +----------------+-------+---------+---------+------+---------+--------+
   |        | Delayed ACKs   |  No   |   Yes   |   Yes   |  No  |   No    |   *    |

     Figure 2: Summary of TCP features for differrent lightweight TCP

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8.  References

8.1.  Normative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC1323]  Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
              1992, <http://www.rfc-editor.org/info/rfc1323>.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <http://www.rfc-editor.org/info/rfc1981>.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, DOI 10.17487/RFC2385, August
              1998, <http://www.rfc-editor.org/info/rfc2385>.

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,

   [RFC2757]  Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N.
              Vaidya, "Long Thin Networks", RFC 2757,
              DOI 10.17487/RFC2757, January 2000,

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   [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,

   [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,

   [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,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

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

   [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,

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

   [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,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

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   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [RFC7428]  Brandt, A. and J. Buron, "Transmission of IPv6 Packets
              over ITU-T G.9959 Networks", RFC 7428,
              DOI 10.17487/RFC7428, February 2015,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [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,

   [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, <http://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, <http://www.rfc-editor.org/info/rfc8163>.

8.2.  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.

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

              Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
              "CoAP Simple Congestion Control/Advanced", draft-ietf-
              core-cocoa-01 (work in progress), March 2017.

              Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
              overview-04 (work in progress), June 2017.

              Gomez, C., Kovatsch, M., Tian, H., and Z. Cao, "Energy-
              Efficient Features of Internet of Things Protocols",
              draft-ietf-lwig-energy-efficient-07 (work in progress),
              March 2017.

              Allman, M., "Retransmission Timeout Requirements", draft-
              ietf-tcpm-rto-consider-05 (work in progress), March 2017.

              Bormann, C., Lemay, S., Technologies, Z., and H.
              Tschofenig, "A TCP and TLS Transport for the Constrained
              Application Protocol (CoAP)", draft-tschofenig-core-coap-
              tcp-tls-05 (work in progress), November 2015.

   [MQTTS]    U. Hunkeler, H.-L. Truong, A. Stanford-Clark, "MQTT-S: A
              Publish/Subscribe Protocol For Wireless Sensor Networks",

Authors' Addresses

   Carles Gomez
   C/Esteve Terradas, 7
   Castelldefels  08860

   Email: carlesgo@entel.upc.edu

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

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

   Michael Scharf
   Lorenzstrasse 10
   Stuttgart, 70435

   Email: michael.scharf@nokia.com

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