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

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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.  TCP over CNNs . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Maximum Segment Size (MSS)  . . . . . . . . . . . . . . .   3
     3.2.  Window Size . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  RTO estimation  . . . . . . . . . . . . . . . . . . . . .   4
     3.4.  Keep-alive and TCP connection lifetime  . . . . . . . . .   4
     3.5.  Explicit congestion notification  . . . . . . . . . . . .   5
     3.6.  TCP options . . . . . . . . . . . . . . . . . . . . . . .   5
     3.7.  Explicit loss notifications . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 UDP-based
   communications.  This is one of the main reasons 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].  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.

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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).

   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.  TCP over CNNs

3.1.  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], do support link
   layer fragmentation.  By exploiting this functionality, the
   adaptation layers to enable IPv6 over such technologies also support
   an MTU of 1280 bytes.

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   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.

3.2.  Window Size

   As per this document, the TCP window size MUST have a size of one
   segment.  This value is appropriate for simple message exchanges in
   the CNN space, reduces implementation complexity and memory
   requirements, and reduces overhead (see section 3.6).

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

3.3.  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.bormann-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.  Therefore, as
   per this document, CoCoA RTO SHOULD be used in TCP over CNNs.
   Alternatively, implementors MAY choose the RTO estimation algorithm
   defined in RFC 6298.  One of the two RTO algorithms MUST be

3.4.  Keep-alive and TCP connection lifetime

   In CNNs, 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.

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   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.

3.5.  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 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, which will not happen as
   per section 3.2 of this document) 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

   ECN is also appropriate in CNNs, since in these environments
   transactional type interactions are a dominant traffic pattern.
   Exploiting other possible congestion signals such as the reception of
   three duplicate ACKs would require the use of greater TCP window
   sizes than the one specified in this document.

3.6.  TCP options

   Because this specification mandates a TCP window size of one segment,
   the following TCP options MUST NOT be supported in CNNs: Window scale
   [RFC1323], TCP Timestamps [RFC1323], and Selective Acknowledgements
   (SACK) [RFC2018].  Other TCP options SHOULD NOT be used, in keeping
   with the principle of lightweight operation.

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3.7.  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.  Security Considerations


5.  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.  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.

6.  References

6.1.  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, <>.

   [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, <>.

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   [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,

   [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,

   [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,

   [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,

6.2.  Informative References

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

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

              Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
              "CoAP Simple Congestion Control/Advanced", draft-bormann-
              core-cocoa-04 (work in progress), July 2016.

              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.

              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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              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


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


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