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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


   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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on February 27, 2020.

Copyright Notice

   Copyright (c) 2019 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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   to this document.  Code Components extracted from this document must
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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

   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

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2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
   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

   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


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

              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,

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

9.2.  Informative References

              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,

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,

   [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,

   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,

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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