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Versions: 00 01 02 03 draft-ietf-lwig-tcp-constrained-node-networks

LWIG Working Group                                              C. Gomez
Internet-Draft                                                 UPC/i2CAT
Intended status: Best Current Practice                      J. Crowcroft
Expires: May 4, 2017                             University of Cambridge
                                                        October 31, 2016

                   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.

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
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   This Internet-Draft will expire on May 4, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

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

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-
   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

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   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

   Constrained nodes are characterized by significant limitations on
   processing, memory, and energy resources [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

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.

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   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.

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 [I-D.ietf-6lo-dect-ule], also use
   6LoWPAN-based adaptation layers in order to enable IPv6 support.

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   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.  (Note: IP version 6 is assumed.)  In any
   case, the TCP MSS must not be set to a value leading to an IPv6
   datagram size exceeding 1280 bytes.

   If a link layer technology used by a constrained device offers a link
   layer MTU greater than 1280 bytes, it is still useful to set the MSS
   so that IPv6 datagram size does not exceed 1280 bytes, in order to
   avoid issues due to Internet links that do not support greater MTUs.

4.3.  Window Size

   This document recommends that constrained devices advertise a TCP
   window size of one MSS, and also transmit at most one MSS of
   unacknowledged data.  This value 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 use window sizes of at least five MSSs, in order to allow Fast
   Retransmit and Fast Recovery [RFC5681].

4.4.  RTO estimation

   Traditionally, TCP has used the well known RTO estimation algorithm
   defined in [RFC6298].  However, experimental studies have shown that
   another algorithm such as the RTO estimator defined in
   [I-D.ietf-core-cocoa] (hereinafter, CoCoA RTO) outperforms state-of-
   art algorithms designed as improvements to RFC 6298 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 RTO.

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   TBD: devise approaches for use of CoCoA for TCP by constrained
   devices, as currently its use would conflict with several TCP MUSTs
   (RFC 6298, Karn algorithm, etc.).  If the unconstrained device is
   dedicated for communication with constrained devices, CoCoA would be
   suitable as well.

4.5.  TCP connection lifetime

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,

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   with a size of 4 or 16 bytes, is then included in SYN packets of
   subsequent connections.  The cookie needs to be refreshed (and
   obtained by the client) after a certain amount of time.
   Nevertheless, TFO is more efficient than frequently opening new TCP
   connections (by using the traditional three-way handshake) for
   transmitting new data, as long as the cookie update rate is well
   below the data new connection rate.

4.6.  Explicit congestion notification

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

   ECN can reduce packet losses, since congestion control measures can
   be applied earlier than after the reception of three duplicate ACKs
   (if the TCP sender window is large enough) or upon TCP sender RTO
   expiration [RFC2884].  Therefore, the number of retries decreases,
   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 will benefit from not supporting,
   and ignoring if received, 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.

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].

   Since traffic over CNNs is expected to be mostly of transactional
   type, with transaction size typically below one MSS, delayed ACKs are
   not recommended for TCP over CNNs.  (Note: delayed ACKs could be
   useful to reduce the number of ACKs in bulk transfer type of traffic,
   e.g. for firmware/software updates; however, it is assumed that these
   will be infrequent in comparison with transactional type exchanges.)

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.

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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.  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, Simon Brummer.

7.  Annex.  TCP implementations for constrained devices

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

7.1.  uIP

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

   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

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   implemented mechanisms comprise slow start, congestion avoidance,
   fast retransmit and fast recovery.  SACK and Window Scale are not

7.3.  RIOT


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>.

   [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,

   [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,

   [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,

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

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   [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,

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.

              Mariager, P., Petersen, J., Shelby, Z., Logt, M., and D.
              Barthel, "Transmission of IPv6 Packets over DECT Ultra Low
              Energy", draft-ietf-6lo-dect-ule-07 (work in progress),
              October 2016.

              Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
              "CoAP Simple Congestion Control/Advanced", draft-ietf-
              core-cocoa-00 (work in progress), October 2016.

              Gomez, C., Kovatsch, M., Tian, H., and Z. Cao, "Energy-
              Efficient Features of Internet of Things Protocols",
              draft-ietf-lwig-energy-efficient-05 (work in progress),
              October 2016.

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Internet-Draft                TCP over CNNs                 October 2016

              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

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

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

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