SCHC Compound ACK
draft-ietf-lpwan-schc-compound-ack-02

lpwan Working Group                                           JC. Zuniga
Internet-Draft                                                    SIGFOX
Intended status: Standards Track                                C. Gomez
Expires: 13 June 2022                                         S. Aguilar
                                    Universitat Politècnica de Catalunya
                                                              L. Toutain
                                                          IMT-Atlantique
                                                             S. Céspedes
                                                              D. Wistuba
                                          NIC Labs, Universidad de Chile
                                                        10 December 2021

                           SCHC Compound ACK
                 draft-ietf-lpwan-schc-compound-ack-02

Abstract

   The present document describes an extension to the SCHC (Static
   Header Compression and Fragmentation) protocol [RFC8724].  It defines
   a SCHC Compound ACK message format, which is intended to reduce the
   number of downlink transmissions (i.e., SCHC ACKs) by accumulating
   bitmaps of several windows in a single SCHC message (i.e., the SCHC
   Compound ACK).

   The message format is generic, and can be used, for instance, by any
   of the four LWPAN technologies defined in [RFC8376], being Sigfox,
   LoRaWAN, NB-IoT and IEEE 802.15.4w.

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 13 June 2022.

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Copyright Notice

   Copyright (c) 2021 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SCHC Compound ACK . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  SCHC Compound ACK Message Format  . . . . . . . . . . . .   3
     3.2.  SCHC Compound ACK Behaviour . . . . . . . . . . . . . . .   4
       3.2.1.  Sender Behaviour  . . . . . . . . . . . . . . . . . .   5
       3.2.2.  Receiver Behaviour  . . . . . . . . . . . . . . . . .   5
     3.3.  SCHC Compound ACK Examples  . . . . . . . . . . . . . . .   5
     3.4.  SCHC Compound ACK YANG Data Model . . . . . . . . . . . .   6
       3.4.1.  SCHC YANG Data Model Extension  . . . . . . . . . . .   6
       3.4.2.  SCHC YANG Tree Extension  . . . . . . . . . . . . . .   9
   4.  SCHC Compound ACK Parameters  . . . . . . . . . . . . . . . .   9
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Generic Framework for Static Context Header Compression and
   Fragmentation (SCHC) specification [RFC8724] describes two
   mechanisms: i) a protocol header compression scheme, and ii) a frame
   fragmentation and loss recovery functionality.  Either can be used on
   top of radio technologies such as the four LWPAN defined in
   [RFC8376], being Sigfox, LoRaWAN, NB-IoT and IEEE 802.15.4w.  These
   LPWANs have similar characteristics such as star-oriented topologies,
   network architecture, connected devices with built-in applications,
   etc.

   SCHC offers a great level of flexibility to accommodate all these
   LPWAN technologies.  Even though there are a great number of
   similarities between them, some differences exist with respect to the

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   transmission characteristics, payload sizes, etc.  Hence, there are
   optimal parameters and modes of operation that can be used when SCHC
   is used on top of a specific LPWAN technology.

   The present document describes an extension to the SCHC protocol.  It
   defines a SCHC Compound ACK format, which is intended to reduce the
   number of downlink transmissions (i.e., SCHC ACKs) in the ACK-on-
   Error mode of SCHC.  The SCHC Compound ACK extends the SCHC ACK
   message format so that it can contain several bitmaps, each bitmap
   being identified by its corresponding window number.

   The SCHC Compound ACK:

   *  provides feedback only for windows with fragment losses,

   *  has a variable size that depends on the number of windows with
      fragment losses being reported in the single Compound SCHC ACK,

   *  includes the window number (i.e., W) of each bitmap,

   *  has the same SCHC ACK format defined in [RFC8724] when only one
      window with losses is reported,

   *  might not cover all windows with fragment losses of a SCHC Packet,

   *  and is distinguishable from the SCHC Receiver-Abort.

2.  Terminology

   It is assumed that the reader is familiar with the terms and
   mechanisms defined in [RFC8376] and in [RFC8724].

3.  SCHC Compound ACK

   The SCHC Compound ACK is a SCHC ACK message that can contain several
   bitmaps, each bitmap being identified by its corresponding window
   number.

   The SCHC Compound ACK groups the window number (W) with its
   corresponding bitmap.  Windows do not need to be contiguous.
   However, the window numbers and corresponding bitmaps included in the
   SCHC Compound ACK message MUST be ordered from the lowest-numbered to
   the highest-numbered window.

3.1.  SCHC Compound ACK Message Format

   Figure 1 shows the regular SCHC ACK format when all fragments have
   been correctly received (C=1), as defined in [RFC8724].

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                  |- SCHC ACK Header --|
                  + -------+---+------ + ------------ +
                  | RuleID | W | C=b'1 | b'0-pad(opt) |
                  + ------ + - + ----- + ------------ +

      Figure 1: SCHC Success ACK message format, as defined in RFC8724

   In case SCHC fragment losses are found in any of the windows of the
   SCHC Packet, the SCHC Compound ACK MAY be used.  The SCHC Compound
   ACK message format is shown in Figure 2.

   The window numbered 00, if present in the SCHC Compound ACK, MUST be
   placed between the Rule ID and the C bit to avoid confusion with
   padding bits.

    |--SCHC ACK Header--| W = w1 |...|      W = wi      |
    +-------------------+ ------ +...+ ------- + ------ +------------+
    |RuleID|W=b'w1|C=b'0| Bitmap |...|  W=b'wi | Bitmap |b'0-pad(opt)|
    +------+------+-----+ ------ +...+ ------- + ------ +------------+

       Losses are found in windows W = w1,...,wi; where w1<w2<...<wi

                 Figure 2: SCHC Compound ACK message format

   The SCHC Compound ACK MUST NOT use the Compressed Bitmap format for
   intermediate windows/bitmaps (i.e., bitmaps that are not the last
   one), and therefore intermediate bitmaps fields MUST be of size
   WINDOW_SIZE.  Hence, the SCHC Compound ACK MAY use a Compressed
   Bitmap format only for the last bitmap.  The optional usage of this
   Compressed Bitmap for the last bitmap MUST be specified by the SCHC
   technology-specific profile.

   If a SCHC sender gets a SCHC Compound ACK with invalid W's, such as
   duplicate W values or W values not sent yet, it MUST discard the
   whole SCHC Compound ACK message.

   Each different SCHC LPWAN technology profile MUST specify how the
   SCHC Compound ACK is different from the Receiver-Abort message as per
   [RFC8724], e.g., the Receiver-Abort message is padded with 1s with an
   extra byte appended at the end, while the SCHC Compound ACK is
   0-padded.

3.2.  SCHC Compound ACK Behaviour

   Besides the ACK-on-Error behaviour described in section 8.4.3 of
   [RFC8724], when implementing the SCHC Compound ACK the following
   applies:

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3.2.1.  Sender Behaviour

   On receiving a SCHC Compound ACK:

   *  The fragment sender MUST resend SCHC Fragment messages containing
      all the tiles of all the windows that are reported missing in the
      SCHC Compound ACK.

   *  If the last of these SCHC Fragment messages reported missing is
      not an All-1 SCHC Fragment, then the fragment sender MAY either,
      send in addition a SCHC ACK REQ with the W field corresponding to
      the last window, continue the transmission of the remaining
      fragments to be transmitted, or repeat the All-1 fragment to
      confirm that all fragments have been correctly received.

3.2.2.  Receiver Behaviour

   On receiving a SCHC ACK REQ:

   *  If the receiver knows of any windows with missing tiles for the
      packet being reassembled, it MAY return a SCHC Compound ACK for
      the missing fragments, starting from the lowest-numbered window.

   On receiving an All-1 SCHC Fragment:

   *  If the receiver knows of any windows with missing tiles for the
      packet being reassembled, it MUST return a SCHC Compound ACK for
      the missing fragments, starting from the lowest-numbered window.

3.3.  SCHC Compound ACK Examples

   Figure 3 shows an example transmission of a SCHC Packet in ACK-on-
   Error mode using the SCHC Compound ACK.  In the example, the SCHC
   Packet is fragmented in 14 tiles, with N=3, WINDOW_SIZE=7, M=2 and
   two lost SCHC fragments.  Only 1 compound SCHC ACK is generated.

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           Sender                Receiver
             |-----W=0, FCN=6 ----->|
             |-----W=0, FCN=5 ----->|
             |-----W=0, FCN=4 ----->|
             |-----W=0, FCN=3 ----->|
             |-----W=0, FCN=2 --X-->|
             |-----W=0, FCN=1 ----->|
             |-----W=0, FCN=0 ----->| Bitmap: 1111011
         (no ACK)
             |-----W=1, FCN=6 ----->|
             |-----W=1, FCN=5 ----->|
             |-----W=1, FCN=4 ----->|
             |-----W=1, FCN=3 ----->|
             |-----W=1, FCN=2 ----->|
             |-----W=1, FCN=1 --X-->|
             |-- W=1, FCN=7 + RSC ->| Integrity check: failure
             |<--- Compound ACK ----| [C=0, W=0 - Bitmap:1111011,
             |-----W=0, FCN=2 ----->|        W=1 - Bitmap:1111101]
             |-----W=1, FCN=1 ----->| Integrity check: success
             |<--- ACK, W=1, C=1 ---| C=1
           (End)

            Figure 3: SCHC Compound ACK message sequence example

    |-- SCHC ACK Header ---|- W=00 --|----- W=01 -----|
    + -------------------- + ------- + ---- + ------- + ------------- +
    | RuleID | W=00  | C=0 | 1111011 | W=01 | 1111011 | b'0-pad (opt) |
    + ------ + ------ + -- + ------- + ---- + ------- + ------------- +

       Figure 4: SCHC Compound ACK message format example: Losses are
                         found in windows 00 and 01

3.4.  SCHC Compound ACK YANG Data Model

   The present document also extends the SCHC YANG data model defined in
   [I-D.ietf-lpwan-schc-yang-data-model] by including a new leaf in the
   Ack-on-Error fragmentation mode to describe both the option to use
   the SCHC Compound ACK, as well as its bitmap format.

3.4.1.  SCHC YANG Data Model Extension

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   <CODE BEGINS> file "ietf-compound-ack@2021-12-10.yang"
   module ietf-schc-compound-ack {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-schc-compound-ack";
     prefix schc-compound-ack;

     import ietf-schc {
         prefix schc;
     }

     organization
       "IETF IPv6 over Low Power Wide-Area Networks (lpwan) working group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/lpwan/about/>
        WG List:  <mailto:lp-wan@ietf.org>
        Editor:   Laurent Toutain
          <mailto:laurent.toutain@imt-atlantique.fr>
        Editor:   Juan Carlos Zuniga
          <mailto:j.c.zuniga@ieee.org>";
     description
        "
        Copyright (c) 2021 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.
        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject to
        the license terms contained in, the Simplified BSD License set
        forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).
        This version of this YANG module is part of RFC XXXX
        (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
        for full legal notices.
        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
        'MAY', and 'OPTIONAL' in this document are to be interpreted as
        described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
        they appear in all capitals, as shown here.

        ***************************************************************

        This module extends the ietf-schc module to include the
        Compound ACK behavior for ACK-on-Error as defined in RFC YYYY.
        It introduces a new leaf for ACK-on-Error defining the format
        of the SCHC Compound ACK, adding the possibility to send
        several bitmaps in a single SCHC ACK message.";

     revision 2021-12-08 {
       description

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         "Initial version for RFC YYYY ";
       reference
         "RFC YYYY: SCHC Compound ACK";
     }

     identity bitmap-format-base-type {
       description
         "Define how the bitmap is formed in ACK messages.";
     }

     identity bitmap-RFC8724 {
       base bitmap-format-base-type;
       description
         "Bitmap by default as defined in RFC8724.";
     }

     identity bitmap-compound-ack {
       base bitmap-format-base-type;
       description
         "Compound ACK.";
     }

     typedef bitmap-format-type {
       type identityref {
         base bitmap-format-base-type;
       }
       description
         "type used in rules";
     }

     augment "/schc:schc/schc:rule/schc:nature/schc:fragmentation/schc:mode/schc:ack-on-error" {
       leaf bitmap-format {
           when "derived-from(../schc:fragmentation-mode, 'schc:fragmentation-mode-ack-on-error')";
           type schc-compound-ack:bitmap-format-type;
           default "schc-compound-ack:bitmap-RFC8724";
           description
                 "How the bitmaps are included in the Ack message.";
       }
       description
         "added to SCHC rules";
     }

   }
   <CODE ENDS>

          Figure 5: SCHC YANG Data Model - Compound ACK extension

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3.4.2.  SCHC YANG Tree Extension

  augment /schc:schc/schc:rule/schc:nature/schc:fragmentation/schc:mode/schc:ack-on-error:
    +--rw bitmap-format?   schc-compound-ack:bitmap-format-type

          Figure 6: SCHC YANG Tree - Compound ACK extension

4.  SCHC Compound ACK Parameters

   This section lists the parameters related to the SCHC Compound ACK
   usage that need to be defined in the Profile, in addition to the ones
   listed in Annex D of [RFC8724].

   *  Usage or not of the SCHC Compound ACK message.

   *  Usage or not of the compressed bitmap format in the last window of
      the SCHC Compound ACK message.

   *  Differentiation between SCHC Receiver-Abort and SCHC Compound ACK
      message, e.g., Receiver-Abort message padded with 1s with an extra
      byte appended at the end, while the SCHC Compound ACK is 0-padded.

5.  Security considerations

   The current document specifies a message format extension for SCHC.
   Hence, the same Security Considerations defined in [RFC8724] apply.

6.  Acknowledgements

   Carles Gomez has been funded in part by the Spanish Government
   through the Jose Castillejo CAS15/00336 grant, the TEC2016-79988-P
   grant, and the PID2019-106808RA-I00 grant, and by Secretaria
   d'Universitats i Recerca del Departament d'Empresa i Coneixement de
   la Generalitat de Catalunya 2017 through grant SGR 376.

   Sergio Aguilar has been funded by the ERDF and the Spanish Government
   through project TEC2016-79988-P and project PID2019-106808RA-I00,
   AEI/FEDER, EU.

   Sandra Cespedes has been funded in part by the ANID Chile Project
   FONDECYT Regular 1201893 and Basal Project FB0008.

   Diego Wistuba has been funded by the ANID Chile Project FONDECYT
   Regular 1201893.

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   The authors would like to thank Rafael Vidal, Julien Boite, Renaud
   Marty, Antonis Platis, Dominique Barthel and Pascal Thubert for their
   very useful comments, reviews and implementation design
   considerations.

7.  Normative References

   [I-D.ietf-lpwan-schc-yang-data-model]
              Minaburo, A. and L. Toutain, "Data Model for Static
              Context Header Compression (SCHC)", Work in Progress,
              Internet-Draft, draft-ietf-lpwan-schc-yang-data-model-04,
              February 2021, <http://www.ietf.org/internet-drafts/draft-
              ietf-lpwan-schc-yang-data-model-04.txt>.

   [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>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,
              <https://www.rfc-editor.org/info/rfc8724>.

Authors' Addresses

   Juan Carlos Zúñiga
   SIGFOX
   Montreal  QC
   Canada

   Email: JuanCarlos.Zuniga@sigfox.com
   URI:   http://www.sigfox.com/

   Carles Gomez
   Universitat Politècnica de Catalunya
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain

   Email: carlesgo@entel.upc.edu

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   Sergio Aguilar
   Universitat Politècnica de Catalunya
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain

   Email: sergio.aguilar.romero@upc.edu

   Laurent Toutain
   IMT-Atlantique
   2 rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex
   France

   Email: Laurent.Toutain@imt-atlantique.fr

   Sandra Céspedes
   NIC Labs, Universidad de Chile
   Av. Almte. Blanco Encalada 1975
   Santiago
   Chile

   Email: scespedes@niclabs.cl

   Diego Wistuba
   NIC Labs, Universidad de Chile
   Av. Almte. Blanco Encalada 1975
   Santiago
   Chile

   Email: wistuba@niclabs.cl

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