TCP over Constrained-Node Networks
draft-gomez-lwig-tcp-constrained-node-networks-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Authors | Carles Gomez , Jon Crowcroft | ||
| Last updated | 2016-10-31 | ||
| Replaced by | draft-ietf-lwig-tcp-constrained-node-networks, RFC 9006 | ||
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draft-gomez-lwig-tcp-constrained-node-networks-00
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
draft-gomez-lwig-tcp-constrained-node-networks-00
Abstract
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.
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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.
This document is subject to BCP 78 and the IETF Trust's Legal
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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
[I-D.ietf-lwig-energy-efficient].
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
[RFC6606].
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
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 [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
implemented.
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
reduced.
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
TBD
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,
<http://www.rfc-editor.org/info/rfc1122>.
[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,
<http://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,
<http://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, <http://www.rfc-editor.org/info/rfc2460>.
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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,
<http://www.rfc-editor.org/info/rfc2616>.
[RFC2757] Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N.
Vaidya, "Long Thin Networks", RFC 2757,
DOI 10.17487/RFC2757, January 2000,
<http://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,
<http://www.rfc-editor.org/info/rfc2884>.
[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,
<http://www.rfc-editor.org/info/rfc3168>.
[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,
<http://www.rfc-editor.org/info/rfc4944>.
[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,
<http://www.rfc-editor.org/info/rfc6092>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<http://www.rfc-editor.org/info/rfc6298>.
[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,
<http://www.rfc-editor.org/info/rfc6606>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
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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,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7428] Brandt, A. and J. Buron, "Transmission of IPv6 Packets
over ITU-T G.9959 Networks", RFC 7428,
DOI 10.17487/RFC7428, February 2015,
<http://www.rfc-editor.org/info/rfc7428>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
[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,
<http://www.rfc-editor.org/info/rfc7668>.
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.
[I-D.bormann-core-cocoa]
Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", draft-bormann-
core-cocoa-04 (work in progress), July 2016.
[I-D.ietf-6lo-dect-ule]
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.
[I-D.ietf-lwig-energy-efficient]
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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[I-D.tschofenig-core-coap-tcp-tls]
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",
2008.
Authors' Addresses
Carles Gomez
UPC/i2CAT
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
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