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SCHC Compound ACK
draft-ietf-lpwan-schc-compound-ack-14

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9441.
Authors Juan-Carlos Zúñiga , Carles Gomez , Sergio Aguilar , Laurent Toutain , Sandra Cespedes , Diego S. Wistuba La Torre
Last updated 2023-03-16 (Latest revision 2023-03-13)
RFC stream Internet Engineering Task Force (IETF)
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Reviews
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Alexander Pelov
Shepherd write-up Show Last changed 2023-02-22
IESG IESG state Became RFC 9441 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Éric Vyncke
Send notices to pthubert@cisco.com, a@ackl.io
IANA IANA review state IANA OK - No Actions Needed
draft-ietf-lpwan-schc-compound-ack-14
lpwan Working Group                                           JC. Zuniga
Internet-Draft                                                     Cisco
Updates: 8724 (if approved)                                     C. Gomez
Intended status: Standards Track                              S. Aguilar
Expires: 13 September 2023          Universitat Politecnica de Catalunya
                                                              L. Toutain
                                                          IMT-Atlantique
                                                             S. Cespedes
                                                    Concordia University
                                                              D. Wistuba
                                          NIC Labs, Universidad de Chile
                                                           12 March 2023

                           SCHC Compound ACK
                 draft-ietf-lpwan-schc-compound-ack-14

Abstract

   The present document updates the SCHC (Static Context Header
   Compression and fragmentation) protocol RFC8724.  It defines a SCHC
   Compound ACK message format and procedure, which are intended to
   reduce the number of response transmissions (i.e., SCHC ACKs) in the
   ACK-on-Error mode, by accumulating bitmaps of several windows in a
   single SCHC message (i.e., the SCHC Compound ACK).

   Both message format and procedure are generic, so they can be used,
   for instance, by any of the four Low Power Wide Area Networks
   (LPWANs) 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 13 September 2023.

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Copyright Notice

   Copyright (c) 2023 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 . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  SCHC Compound ACK Message Format  . . . . . . . . . . . .   4
     3.2.  SCHC Compound ACK Behaviour . . . . . . . . . . . . . . .   7
       8.4.3.  ACK-on-Error Mode . . . . . . . . . . . . . . . . . .   8
   4.  SCHC Compound ACK Examples  . . . . . . . . . . . . . . . . .  16
   5.  SCHC Compound ACK YANG Data Model . . . . . . . . . . . . . .  17
     5.1.  SCHC YANG Data Model Extension  . . . . . . . . . . . . .  17
     5.2.  SCHC YANG Tree Extension  . . . . . . . . . . . . . . . .  19
   6.  SCHC Compound ACK Parameters  . . . . . . . . . . . . . . . .  20
   7.  Security considerations . . . . . . . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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 Low Power Wide Area
   Networks (LPWANs) listed 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.

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   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
   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.

   In ACK-on-Error mode in [RFC8724] the SCHC Packet is fragmented into
   pieces called tiles, with all tiles of the same size except for the
   last one, which can be smaller.  Successive tiles are grouped in
   windows of fixed size.  A SCHC Fragment carries one or several
   contiguous tiles, which may span multiple windows.  When sending all
   tiles from all windows, the last tile is sent in an All-1 SCHC
   Fragment.  The SCHC receiver, after receiving the All-1 SCHC Fragment
   will send a SCHC ACK reporting on the reception of exactly one window
   of tiles.  In case of SCHC Fragment losses, a bitmap is added to the
   failure SCHC ACK, where each bit in the bitmap corresponds to a tile
   in the window.  If SCHC Fragment losses span multiple windows, the
   SCHC receiver will send one failure SCHC ACK per window with losses.

   The present document updates the SCHC protocol for frame
   fragmentation and loss recovery.  It defines a SCHC Compound ACK
   format and procedure, which is intended to reduce the number of
   response transmissions (i.e., SCHC ACKs) in the ACK-on-Error mode of
   SCHC.  The SCHC Compound ACK extends the failure SCHC ACK message
   format so that it can contain several bitmaps, each bitmap being
   identified by its corresponding window number.  The SCHC Compound ACK
   is backwards compatible with the SCHC ACK as defined in [RFC8724],
   and introduces flexibility, as the receiver has the capability to
   respond to the All-0 SCHC Fragment, providing more downlink
   opportunities, and therefore adjusting to the delay requirements of
   the application.

2.  Terminology

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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

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3.  SCHC Compound ACK

   The SCHC Compound ACK is a failure SCHC ACK message that can contain
   several bitmaps, each bitmap being identified by its corresponding
   window number.  In [RFC8724], the failure SCHC ACK message only
   contain one bitmap corresponding to one window.  The SCHC Compound
   ACK extends this format allowing more windows to be acknowledged in a
   single ACK, reducing the total number of failure SCHC ACK messages,
   specially when fragment losses are present in intermediate windows.

   The SCHC Compound ACK MAY be used in fragmentation modes that use
   windows and that allow reporting the bitmaps of multiple windows at
   the same time, and MUST NOT be used otherwise.

   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,

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

   *  and is distinguishable from the SCHC Receiver-Abort.

3.1.  SCHC Compound ACK Message Format

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

                  |-- SCHC ACK Header --|
                           |--T-|---M--| 1 |
                  +--------+----+------+---+~~~~~~~~~~~~~~~~~~
                  | RuleID |DTag|   W  |C=1| padding as needed
                  +--------+----+------+---+~~~~~~~~~~~~~~~~~~

      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 and Figure 3.

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     |--- SCHC ACK Header --|- W=w1 -|...|---- W=wi -----|
            |--T-|---M--|-1-|        |...|---M--|        |---M--|
     +------+----+------+---+--------+...+------+--------+------+~~~~~+
     |RuleID|DTag| W=w1 |C=0| Bitmap |...| W=wi | Bitmap |00..00| pad |
     +------+----+------+---+--------+...+------+--------+------+~~~~~+
                               next L2 Word boundary ->|<-- L2 Word ->|

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

                 Figure 2: SCHC Compound ACK message format

   The SCHC Compound ACK groups the window number (W) with its
   corresponding bitmap.  Window numbers do not need to be contiguous.
   However, the window numbers and its corresponding bitmaps included in
   the SCHC Compound ACK message MUST be ordered from the lowest-
   numbered to the highest-numbered window.  Hence, if the bitmap of
   window number zero is present in the SCHC Compound ACK message, it
   MUST always be the first one in order and its W number MUST be placed
   in the SCHC ACK Header.

   If M or more padding bits would be needed after the last bitmap in
   the message to fill the last L2 Word, M bits at 0 MUST be appended
   after the last bitmap, and then padding is applied as needed (see
   Figure 2).  Since window number 0, if present in the message, is
   placed as w1, the M bits set to zero can’t be confused with window
   number 0, and therefore they signal the end of the SCHC Compound ACK
   message.

   Figure 3 shows the case when the required padding bits are strictly
   less than M bits.  In this case, the layer-2 MTU (Maximum
   Transmission Unit) does not leave room for any extra window value,
   let alone any bitmap, thereby signaling the end of the SCHC Compound
   ACK message.

     |--- SCHC ACK Header --|- W=w1 -|...|---- W=wi -----|
            |--T-|---M--|-1-|        |...|---M--|        |---M--|
     +------+----+------+---+--------+...+------+--------+~~~+
     |RuleID|DTag| W=w1 |C=0| Bitmap |...| W=wi | Bitmap |pad|
     +------+----+------+---+--------+...+------+--------+~~~+
                                     next L2 Word boundary ->|
        Losses are found in windows W = w1,...,wi; where w1<w2<...<wi

        Figure 3: SCHC Compound ACK message format with less than M
                                padding bits

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   The SCHC Compound ACK MUST NOT use the Compressed Bitmap format for
   intermediate windows/bitmaps (i.e., bitmaps that are not the last one
   of the SCHC Compound ACK message), 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 in the
   message.  The optional usage of this Compressed Bitmap for the last
   bitmap MUST be specified by the SCHC technology-specific profile.

   The case where the last bitmap is effectively compressed corresponds
   to Figure 3, with the last bitmap ending, by construction, on an L2
   Word boundary, therefore resulting in no padding at all.

   Figure 4 illustrates a bitmap compression example of a SCHC Compound
   ACK, where the bitmap of the last window (wi) indicates that the
   first tile has not been correctly received.  Because the compression
   algorithm resulted in effective compression, no padding is needed.

     |--- SCHC ACK Header --|- W=w1 -|...|-------- W=wi -------|
            |--T-|---M--|-1-|        |...|---M--|
     +------+----+------+---+--------+...+------+--------------+
     |RuleID|DTag| W=w1 |C=0| Bitmap |...| W=wi |0 1 1 1 1 1 1 |
     +------+----+------+---+--------+...+------+--------------+
                            next L2 Word boundary ->|

                    SCHC Compound ACK with uncompressed Bitmap

     |--- SCHC ACK Header --|- W=w1 -|...|-- W=wi --|
            |--T-|---M--|-1-|        |...|---M--|
     +------+----+------+---+--------+...+------+---+
     |RuleID|DTag| W=w1 |C=0| Bitmap |...| W=wi |0 1|
     +------+----+------+---+--------+...+------+---+
                            next L2 Word boundary ->|

           Transmitted SCHC Compound ACK with compressed Bitmap

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

     Figure 4: SCHC Compound ACK message format with compressed bitmap

   Figure 5 illustrates another bitmap compression example of a SCHC
   Compound ACK, where the bitmap of the last window (wi) indicates that
   the second and the fourth tile have not been correctly received.  In
   this example, the compression algorithm does not result in effective
   compression of the last bitmap.  Besides, because more than M bits of
   padding would be needed to fill the last L2 Word, M bits at 0 are
   appended to the message before padding is applied.

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    |--- SCHC ACK Header --|-W=w1-|...|-------- W=wi -------|
           |--T-|---M--|-1-|      |...|---M--|
    +------+----+------+---+------+...+------+--------------+
    |RuleID|DTag| W=w1 |C=0|Bitmap|...| W=wi |1 0 1 0 1 1 1 |
    +------+----+------+---+------+...+------+--------------+
                       next L2 Word boundary ->|
                    SCHC Compound ACK with uncompressed Bitmap

    |--- SCHC ACK Header --|-W=w1-|...|-------- W=wi -------|
           |--T-|---M--|-1-|      |...|---M--|              |---M--|
    +------+----+------+---+------+...+------+--------------+------+~~~+
    |RuleID|DTag| W=w1 |C=0|Bitmap|...| W=wi |1 0 1 0 1 1 1 |00..00|pad|
    +------+----+------+---+------+...+------+--------------+------+~~~+
                       next L2 Word boundary ->|<------ L2 Word ------>|
                     Transmitted SCHC Compound ACK

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

     Figure 5: SCHC Compound ACK message format with compressed bitmap

   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.

   Note: because it has a C bit reset to 0, the SCHC Compound ACK is
   distinguishable from the Receiver-Abort message [RFC8724], which has
   a C bit set to 1.

3.2.  SCHC Compound ACK Behaviour

   The SCHC ACK-on-Error behaviour is described in section 8.4.3 of
   [RFC8724].  The present document slightly modifies this behaviour,
   since in the baseline SCHC specification a SCHC ACK reports only one
   bitmap for the reception of exactly one window of tiles.  The present
   SCHC Compound ACK specification extends the SCHC ACK message format
   so that it can contain several bitmaps, each bitmap being identified
   by its corresponding window number.

   The SCHC ACK format, as presented in [RFC8724], can be considered a
   special SCHC Compound ACK case, in which it reports only the tiles of
   one window.  Therefore, the SCHC Compound ACK is backwards compatible
   with the SCHC ACK format presented in [RFC8724].  The receiver can
   suspect if the sender does not support the SCHC Compound ACK, if the
   sender does not resend any tiles from windows that are not the first
   one in the SCHC Compound ACK and more ACKs are needed.

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   Also, some flexibility is introduced with respect to [RFC8724], in
   that the receiver has the capability to respond to the All-0 with a
   SCHC Compound ACK or not, depending on certain parameters, like
   network conditions, sender buffer/chache size, supported application
   delay.  Note that even though the protocol allows for such
   flexibility, the actual decision criteria is not specified in this
   document.  The application MUST set expiration timer values according
   to when the feedback is expected to be received, e.g., after the
   All-0 or after the All-1.

   The following Section 8.4.3 (and its subsections) replaces the
   complete sections 8.4.2 (and its subsections) of RFC 8724.

8.4.3.  ACK-on-Error Mode

   The ACK-on-Error mode supports L2 technologies that have variable MTU
   and out-of-order delivery.  It requires an L2 that provides a
   feedback path from the reassembler to the fragmenter.  See Appendix F
   for a discussion on using ACK-on-Error mode on quasi-bidirectional
   links.

   In ACK-on-Error mode, windows are used.

   All tiles except the last one and the penultimate one MUST be of
   equal size, hereafter called "regular".  The size of the last tile
   MUST be smaller than or equal to the regular tile size.  Regarding
   the penultimate tile, a Profile MUST pick one of the following two
   options:

   *  The penultimate tile size MUST be the regular tile size, or

   *  the penultimate tile size MUST be either the regular tile size or
      the regular tile size minus one L2 Word.

   A SCHC Fragment message carries one or several contiguous tiles,
   which may span multiple windows.  A SCHC Compound ACK reports on the
   reception of one window of tiles or several windows of tiles, each
   one identified by its window number.

   See Figure 23 for an example.

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           +---------------------------------------------...-----------+
           |                       SCHC Packet                         |
           +---------------------------------------------...-----------+

   Tile#   | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 |     | 0 | 4 |3|
   Window# |-------- 0 --------|-------- 1 --------|- 2  ... 27 -|- 28-|

   SCHC Fragment msg   |-----------|

       Figure 23: SCHC Packet Fragmented in Tiles, ACK-on-Error Mode

   The W field is wide enough that it unambiguously represents an
   absolute window number.  The fragment receiver sends SCHC Compound
   ACKs to the fragment sender about windows for which tiles are
   missing.  No SCHC Compound ACK is sent by the fragment receiver for
   windows that it knows have been fully received.

   The fragment sender retransmits SCHC Fragments for tiles that are
   reported missing.  It can advance to next windows even before it has
   ascertained that all tiles belonging to previous windows have been
   correctly received, and it can still later retransmit SCHC Fragments
   with tiles belonging to previous windows.  Therefore, the sender and
   the receiver may operate in a decoupled fashion.  The fragmented SCHC
   Packet transmission concludes when:

   *  integrity checking shows that the fragmented SCHC Packet has been
      correctly reassembled at the receive end, and this information has
      been conveyed back to the sender, or

   *  too many retransmission attempts were made, or

   *  the receiver determines that the transmission of this fragmented
      SCHC Packet has been inactive for too long.

   Each Profile MUST specify which RuleID value(s) corresponds to SCHC
   F/R messages operating in this mode.

   The W field MUST be present in the SCHC F/R messages.

   Each Profile, for each RuleID value, MUST define:

   *  the tile size (a tile does not need to be multiple of an L2 Word,
      but it MUST be at least the size of an L2 Word),

   *  the value of M,

   *  the value of N,

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   *  the value of WINDOW_SIZE, which MUST be strictly less than 2^N,

   *  the size and algorithm for the RCS field,

   *  the value of T,

   *  the value of MAX_ACK_REQUESTS,

   *  the expiration time of the Retransmission Timer,

   *  the expiration time of the Inactivity Timer,

   *  if the last tile is carried in a Regular SCHC Fragment or an All-1
      SCHC Fragment Section 8.4.3.1), and

   *  if the penultimate tile MAY be one L2 Word smaller than the
      regular tile size.  In this case, the regular tile size MUST be at
      least twice the L2 Word size.

   *  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.

   For each active pair of RuleID and DTag values, the sender MUST
   maintain:

   *  one Attempts counter, and

   *  one Retransmission Timer.

   For each active pair of RuleID and DTag values, the receiver MUST
   maintain:

   *  one Inactivity Timer, and

   *  one Attempts counter.

8.4.3.1.  Sender Behavior

   At the beginning of the fragmentation of a new SCHC Packet:

   *  the fragment sender MUST select a RuleID and DTag value pair for
      this SCHC Packet.  A Rule MUST NOT be selected if the values of M
      and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot
      be fragmented in (2^M) * WINDOW_SIZE tiles or less.

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   *  the fragment sender MUST initialize the Attempts counter to 0 for
      that RuleID and DTag value pair.

   A Regular SCHC Fragment message carries in its payload one or more
   tiles.  If more than one tile is carried in one Regular SCHC
   Fragment:

   *  the selected tiles MUST be contiguous in the original SCHC Packet,
      and

   *  they MUST be placed in the SCHC Fragment Payload adjacent to one
      another, in the order they appear in the SCHC Packet, from the
      start of the SCHC Packet toward its end.

   Tiles that are not the last one MUST be sent in Regular SCHC
   Fragments specified in Section 8.3.1.1.  The FCN field MUST contain
   the tile index of the first tile sent in that SCHC Fragment.

   In a Regular SCHC Fragment message, the sender MUST fill the W field
   with the window number of the first tile sent in that SCHC Fragment.

   A Profile MUST define if the last tile of a SCHC Packet is sent:

   *  in a Regular SCHC Fragment, alone or as part of a multi-tiles
      Payload,

   *  alone in an All-1 SCHC Fragment, or

   *  with any of the above two methods.

   In an All-1 SCHC Fragment message, the sender MUST fill the W field
   with the window number of the last tile of the SCHC Packet.

   The fragment sender MUST send SCHC Fragments such that, all together,
   they contain all the tiles of the fragmented SCHC Packet.

   The fragment sender MUST send at least one All-1 SCHC Fragment.

   In doing the two items above, the sender MUST ascertain that the
   receiver will not receive the last tile through both a Regular SCHC
   Fragment and an All-1 SCHC Fragment.

   The fragment sender MUST listen for SCHC Compound ACK messages after
   having sent:

   *  an All-1 SCHC Fragment, or

   *  a SCHC ACK REQ.

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   A Profile MAY specify other times at which the fragment sender MUST
   listen for SCHC Compound ACK messages.  For example, this could be
   after sending a complete window of tiles.

   Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC
   ACK REQ:

   *  it MUST increment the Attempts counter, and

   *  it MUST reset the Retransmission Timer.

   On Retransmission Timer expiration:

   *  if the Attempts counter is strictly less than MAX_ACK_REQUESTS,
      the fragment sender MUST send either the All-1 SCHC Fragment or a
      SCHC ACK REQ with the W field corresponding to the last window,

   *  otherwise, the fragment sender MUST send a SCHC Sender-Abort, and
      it MAY exit with an error condition.

   All message receptions being discussed in the rest of this section
   are to be understood as "matching the RuleID and DTag pair being
   processed", even if not spelled out, for brevity.

   On receiving a SCHC Compound ACK:

   *  if one of the W field in the SCHC Compound ACK corresponds to the
      last window of the SCHC Packet:

      -  if the C bit is set, the sender MAY exit successfully.

      -  otherwise:

         o  if the Profile mandates that the last tile be sent in an
            All-1 SCHC Fragment:

            +  if the SCHC Compound ACK shows no missing tile at the
               receiver, the sender:

               *  MUST send a SCHC Sender-Abort, and

               *  MAY exit with an error condition.

            +  otherwise:

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

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               *  if the last of these SCHC Fragment messages 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, or repeat the
                  All-1 SCHC Fragment to ask the receiver confirmation
                  that all tiles have been correctly received.

               *  in doing the two items above, the sender MUST
                  ascertain that the receiver will not receive the last
                  tile through both a Regular SCHC Fragment and an All-1
                  SCHC Fragment.

         o  otherwise:

            +  if the SCHC Compound ACK shows no missing tile at the
               receiver, the sender MUST send the All-1 SCHC Fragment

            +  otherwise:

               *  the fragment sender MUST send SCHC Fragment messages
                  containing all the tiles that are reported missing in
                  the SCHC Compound ACK.

               *  the fragment sender MUST then send either the All-1
                  SCHC Fragment or a SCHC ACK REQ with the W field
                  corresponding to the last window.

   *  otherwise, the fragment sender:

      -  MUST send SCHC Fragment messages containing the tiles that are
         reported missing in the SCHC Compound ACK.

      -  then, it MAY send a SCHC ACK REQ with the W field corresponding
         to the last window.

   See Figure 43/> for one among several possible examples of a Finite
   State Machine implementing a sender behavior obeying this
   specification.

8.4.3.2.  Receiver Behavior

   On receiving a SCHC Fragment with a RuleID and DTag pair not being
   processed at that time:

   *  the receiver SHOULD check if the DTag value has not recently been
      used for that RuleID value, thereby ensuring that the received
      SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
      transmission.  The initial value of the Inactivity Timer is the

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      RECOMMENDED lifetime for the DTag value at the receiver.  If the
      SCHC Fragment is determined to be such a remnant, the receiver MAY
      silently ignore it and discard it.

   *  the receiver MUST start a process to assemble a new SCHC Packet
      with that RuleID and DTag value pair.  The receiver MUST start an
      Inactivity Timer for that RuleID and DTag value pair.  It MUST
      initialize an Attempts counter to 0 for that RuleID and DTag value
      pair.  If the receiver is under-resourced to do this, it MUST
      respond to the sender with a SCHC Receiver-Abort.

   On reception of any SCHC F/R message for the RuleID and DTag pair
   being processed, the receiver MUST reset the Inactivity Timer
   pertaining to that RuleID and DTag pair.

   All message receptions being discussed in the rest of this section
   are to be understood as "matching the RuleID and DTag pair being
   processed", even if not spelled out, for brevity.

   On receiving a SCHC Fragment message, the receiver determines what
   tiles were received, based on the payload length and on the W and FCN
   fields of the SCHC Fragment.

   *  if the FCN is All-1, if a Payload is present, the full SCHC
      Fragment Payload MUST be assembled including the padding bits.
      This is because the size of the last tile is not known by the
      receiver; therefore, padding bits are indistinguishable from the
      tile data bits, at this stage.  They will be removed by the SCHC
      C/D sublayer.  If the size of the SCHC Fragment Payload exceeds or
      equals the size of one regular tile plus the size of an L2 Word,
      this SHOULD raise an error flag.

   *  otherwise, tiles MUST be assembled based on the a priori known
      tile size.

      -  If allowed by the Profile, the end of the payload MAY contain
         the last tile, which may be shorter.  Padding bits are
         indistinguishable from the tile data bits, at this stage.

      -  The payload may contain the penultimate tile that, if allowed
         by the Profile, MAY be exactly one L2 Word shorter than the
         regular tile size.

      -  Otherwise, padding bits MUST be discarded.  This is possible
         because:

         o  the size of the tiles is known a priori,

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         o  tiles are larger than an L2 Word, and

         o  padding bits are always strictly less than an L2 Word.

   On receiving a SCHC All-0 SCHC Fragment:

   *  if the receiver knows of any windows with missing tiles for the
      packet being reassembled (and if network conditions are known to
      be conducive), it MAY return a SCHC Compound ACK for the missing
      tiles, starting from the lowest-numbered window.

   On receiving a SCHC ACK REQ or 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 tiles, starting from the lowest-numbered window.

   *  otherwise:

      -  if it has received at least one tile, it MUST return a SCHC
         Compound ACK for the highest-numbered window it currently has
         tiles for,

      -  otherwise, it MUST return a SCHC Compound ACK for window
         numbered 0.

   A Profile MAY specify other times and circumstances at which a
   receiver sends a SCHC Compound ACK, and which window the SCHC
   Compound ACK reports about in these circumstances.

   Upon sending a SCHC Compound ACK, the receiver MUST increase the
   Attempts counter.

   After receiving an All-1 SCHC Fragment, a receiver MUST check the
   integrity of the reassembled SCHC Packet at least every time it
   prepares for sending a SCHC Compound ACK for the last window.

   Upon receiving a SCHC Sender-Abort, the receiver MAY exit with an
   error condition.

   Upon expiration of the Inactivity Timer, the receiver MUST send a
   SCHC Receiver-Abort, and it MAY exit with an error condition.

   On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST
   send a SCHC Receiver-Abort, and it MAY exit with an error condition.

   Reassembly of the SCHC Packet concludes when:

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   *  a Sender-Abort has been received, or

   *  the Inactivity Timer has expired, or

   *  the Attempts counter has exceeded MAX_ACK_REQUESTS, or

   *  at least an All-1 SCHC Fragment has been received and integrity
      checking of the reassembled SCHC Packet is successful.

   See Figure 44 for one among several possible examples of a Finite
   State Machine implementing a receiver behavior obeying this
   specification.  The example provided is meant to match the sender
   Finite State Machine of Figure 43.

4.  SCHC Compound ACK Examples

   Figure 7 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.

           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 + RCS ->| 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 7: SCHC Compound ACK message sequence example

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    |--- SCHC ACK Header --|- W=00 --|----- W=01 -----|
           |--T-|---M--|-1-|         |---M--|         |---M--|
    +------+----+------+---+---------+------+---------+------+-----+
    |RuleID|DTag| W=00 |C=0| 1111011 | W=01 | 1111101 |  00  | pad |
    +------+----+------+---+---------+------+---------+------+-----+
                            next L2 Word boundary ->|<-- L2 Word ->|

       Figure 8: SCHC Compound ACK message format example: Losses are
                         found in windows 00 and 01

5.  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.

5.1.  SCHC YANG Data Model Extension

   <CODE BEGINS> file "ietf-lpwan-schc-compound-ack@2022-12-02.yang"
   module ietf-lpwan-schc-compound-ack {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:"
        +"ietf-lpwan-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

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        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 2022-12-02 {
       description
         "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

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         "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"
              +" SCHC ACK message.";
       }

           leaf last-bitmap-compression {
           when "derived-from(../schc:fragmentation-mode,"
           +" 'schc:fragmentation-mode-ack-on-error')";
           type boolean;
           default true;
           description
               "when true ultimate bitmap in the SCHC ACK message
                can be compressed";
           }

       description
         "added to SCHC rules";
     }

   }
   <CODE ENDS>

          Figure 9: SCHC YANG Data Model - Compound ACK extension

5.2.  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 10: SCHC YANG Tree - Compound ACK extension

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6.  SCHC Compound ACK Parameters

   This section lists the parameters related to the SCHC Compound ACK
   usage that need to be defined in the Profile.  This list MUST be
   appended to the list of SCHC parameters under "Decision to use SCHC
   fragmentation mechanism or not.  If yes, the document must describe:"
   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.

7.  Security considerations

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

8.  IANA Considerations

   This document has no IANA actions.

9.  Acknowledgements

   Carles Gomez has been funded in part by the Spanish Government
   through the TEC2016-79988-P grant, and the PID2019-106808RA-I00 grant
   (funded by MCIN / AEI / 10.13039/501100011033), 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 (funded by MCIN / AEI / 10.13039/501100011033).

   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.

   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.

10.  References

10.1.  Normative References

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   [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-21,
              February 2021, <http://www.ietf.org/internet-drafts/draft-
              ietf-lpwan-schc-yang-data-model-14.txt>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

10.2.  Informative References

   [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

   Juan Carlos Zuniga
   Cisco
   Montreal  QC
   Canada
   Email: juzuniga@cisco.com

   Carles Gomez
   Universitat Politecnica de Catalunya
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain
   Email: carles.gomez@upc.edu

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   Sergio Aguilar
   Universitat Politecnica 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 Cespedes
   Concordia University
   1455 De Maisonneuve Blvd. W.
   Montreal QC, H3G 1M8
   Canada
   Email: sandra.cespedes@concordia.ca

   Diego Wistuba
   NIC Labs, Universidad de Chile
   Av. Almte. Blanco Encalada 1975
   Santiago
   Chile
   Email: wistuba@niclabs.cl

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