Internet DRAFT - draft-gomez-frag-lpwan-considerations
draft-gomez-frag-lpwan-considerations
LPWAN Working Group C. Gomez
Internet-Draft UPC
Intended status: Informational J. Crowcroft
Expires: February 27, 2020 University of Cambridge
August 26, 2019
Fragmentation in LPWAN considerations: CoAP Block vs SCHC fragmentation
draft-gomez-frag-lpwan-considerations-00
Abstract
The SCHC adaptation layer provides header compression and
fragmentation functionality between IPv6 and an underlying LPWAN
technology. SCHC fragmentation has been specifically designed for
the characteristics of LPWANs. However, when CoAP is used at the
application layer, there exists an alternative approach for
fragmentation, which is using the CoAP Block option. This document
aims at illustrating the advantages and limitations of each approach
for transferring larger payloads.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Header overhead . . . . . . . . . . . . . . . . . . . . . . . 3
4. L2 MTU supported . . . . . . . . . . . . . . . . . . . . . . 3
5. Reliability modes . . . . . . . . . . . . . . . . . . . . . . 4
6. CoAP RTO calculation . . . . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The Static Context Header Compression and fragmentation (SCHC)
framework provides an adaptation layer that has been specifically
designed to enable support of IPv6 over Low Power Wide Area Network
(LPWAN) technologies [I-D.ietf-lpwan-ipv6-static-context-hc]. SCHC
comprises header compression and fragmentation functionality. The
latter is needed when SCHC is used over technologies such as LoRaWAN
or Sigfox [RFC8376], and might be needed over further LPWAN
technologies.
On the other hand, considering the significant resource constraints
in many LPWAN scenarios, the Constrained Application Protocol (CoAP)
is a suitable candidate application-layer protocol for use in LPWAN.
CoAP has been specified over both UDP and TCP [RFC7252][RFC8323].
For CoAP over UDP, the Block option can be used in order to perform
application-layer fragmentation [RFC7959]. In this document, CoAP
over UDP is assumed.
Therefore, when CoAP and SCHC are used in LPWAN, there exist two
possible approaches for fragmentation: SCHC-level fragmentation and
CoAP-level fragmentation. This document aims at systematically
analyzing the characteristics, advantages and limitations of both
approaches.
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2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Header overhead
The SCHC specification defines a fragmentation header that is
prepended to each fragment. The SCHC fragmentation header has been
defined in a flexible way, yet with the aim to enable very short
fragmentation header sizes. Fragmentation header sizes are then
actually specified per LPWAN technology in corresponding SCHC-over-
foo documents. Fragmentation header sizes of 1-2 bytes are currently
being used over technologies such as Sigfox or LoRaWAN. When SCHC
fragmentation is used, each fragment will carry a SCHC fragmentation
header. The first fragment will also carry the SCHC compressed IPv6/
UDP header, which may have a size of 1 byte.
When using CoAP blockwise transfers, each block will carry a CoAP
header comprising the base header (4 bytes, although perhaps it can
be reduced with SCHC CoAP header compression), the option format (2
bytes), the Block option value (1-3 bytes) and the payload marker (1
byte). In addition, each block will be encapsulated in a different
IPv6 datagram. A SCHC compressed IPv6/UDP header may have a size of
1 byte.
4. L2 MTU supported
Assuming that the L2 word [I-D.ietf-lpwan-ipv6-static-context-hc] of
the underlying LPWAN technology is 1 byte, and a SCHC fragmentation
header of 1 byte, SCHC fragmentation can support underlying LPWAN
technologies with a maximum link layer data unit payload of even only
2 bytes.
The smallest CoAP layer payload size that can be supported with
Blockwise transfers is 16 bytes. Note that the whole header
overhead, comprising IPv6, UDP and CoAP headers, even when SCHC
compression is applied, is added to the CoAP layer payload. While
some LPWAN technologies can, at least in some cases, carry a CoAP
Block, note that it is not possible with RFC 7959 to transport a CoAP
Block over Sigfox (with uplink and downlink L2 MTU values of 12 and 8
bytes, respectively) or over LoRaWAN DR0 in the US band (with an L2
MTU of 11 bytes). Assuming that SCHC header compression is used,
CoAP Blockwise transfers can only be supported over technologies with
an L2 MTU of at least ~25 bytes.
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5. Reliability modes
SCHC offers a gradation of reliability modes: No-ACK, ACK-Always, and
ACK-on-Error. In No-ACK there is no feedback from the receiver to
the sender, and there is no retransmission of fragments. In ACK-
Always, the receiver inconditionally generates an ACK after a window
of fragments. In ACK-on-Error, the receiver only generates an ACK
after a window of fragments if at least one of the fragments of the
window has been lost (an exception to this behavior is the last
window, for which an ACK-Always behavior is used). Note that each
mode involves a different message overhead.
CoAP blockwise transfers follow a stop-and-wait behavior by default.
Note that congestion control defined in the basic CoAP specification
applies, and NON messages are not very useful in blockwise transfers,
as they increase the probability of non-delivery [RFC 7959].
6. CoAP RTO calculation
CoAP CON messages require an Acknowledgment (ACK) by the receiving
endpoint. After sending a CON message, a sender waits for such ACK
for Retransmission TimeOut (RTO) time. Upon RTO expiration, the CoAP
sender retransmits the message. The CoAP main specification does not
define a mechanism for adaptively calculating the RTO based on the
Round Trip Time (RTT). However, it is expected that future
specifications will do so [I-D.ietf-core-cocoa]. The RTT comprises
the time since the CoAP CON message is passed to its lower layer
until an ACK is received.
When SCHC-level fragmentation is used, the RTT seen by CoAP depends
significantly on the corresponding IPv6 datagram size. CoAP has no
visibility of how many fragments are needed to carry the datagram.
Therefore, simple RTO schemes may be inaccurate if CON messages have
a variable size. Such inaccuracy may lead to either too long RTO
values (involving unnecessarily large delay) or too short ones
(leading to spurious retries, which may consume scarce transmission
resources).
In contrast, CoAP-level blockwise transfers may exploit the per-block
RTT samples, as in fact, each block is carried by a CoAP message, and
retries are carried out message-wise. Therefore CoAP blockwise
transfers will result in accurate RTT estimation (as long as an
adaptive RTO scheme based on RTT samples is used).
Whether the better RTO accuracy of CoAP blockwise transfers may
compensate the advantages of SCHC fragmentation (i.e. lower header
overhead, better support for payload size constrained L2
technologies, richer reliability approaches) needs to be determined
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by means of further study. Note that this open question does not
apply for CoAP NON messages.
7. Security Considerations
TBD
8. Acknowledgments
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Ciencia, Innovacion y Universidades) through the Jose
Castillejo grant CAS18/00170 and by European Regional Development
Fund (ERDF) and the Spanish Government through project
TEC2016-79988-P, AEI/FEDER, UE. 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.
9. References
9.1. Normative References
[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
Zuniga, "Static Context Header Compression (SCHC) and
fragmentation for LPWAN, application to UDP/IPv6", draft-
ietf-lpwan-ipv6-static-context-hc-21 (work in progress),
July 2019.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References
[I-D.ietf-core-cocoa]
Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", draft-ietf-
core-cocoa-03 (work in progress), February 2018.
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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,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.
Authors' Addresses
Carles Gomez
UPC
C/Esteve Terradas, 7
Castelldefels 08860
Spain
Email: carlesgo@entel.upc.edu
Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge, CB3 0FD
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
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