SCHC Compound ACK
draft-ietf-lpwan-schc-compound-ack-05
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.
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Authors | Juan-Carlos Zúñiga , Carles Gomez , Sergio Aguilar , Laurent Toutain , Sandra Cespedes , Diego S. Wistuba La Torre | ||
Last updated | 2022-07-08 (Latest revision 2022-03-21) | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Formats | |||
Reviews |
TSVART Last Call review
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by Wesley Eddy
Ready w/nits
YANGDOCTORS Early review
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by Ebben Aries
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Additional resources | Mailing list discussion | ||
Stream | WG state | WG Document | |
Document shepherd | Alexander Pelov | ||
IESG | IESG state | Became RFC 9441 (Proposed Standard) | |
Consensus boilerplate | Yes | ||
Telechat date | (None) | ||
Responsible AD | (None) | ||
Send notices to | pthubert@cisco.com, a@ackl.io |
draft-ietf-lpwan-schc-compound-ack-05
lpwan Working Group JC. Zuniga Internet-Draft Cisco Updates: RFC8724 (if approved) C. Gomez Intended status: Standards Track S. Aguilar Expires: 9 January 2023 Universitat Politècnica de Catalunya L. Toutain IMT-Atlantique S. Céspedes Concordia University D. Wistuba NIC Labs, Universidad de Chile 8 July 2022 SCHC Compound ACK draft-ietf-lpwan-schc-compound-ack-05 Abstract The present document describes an extension to 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 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 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 9 January 2023. Zuniga, et al. Expires 9 January 2023 [Page 1] Internet-Draft SCHC Compound ACK July 2022 Copyright Notice Copyright (c) 2022 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 . . . . . . . . . . . . 4 3.2. SCHC Compound ACK Behaviour . . . . . . . . . . . . . . . 6 8.4.3. ACK-on-Error Mode . . . . . . . . . . . . . . . . . . 7 4. SCHC Compound ACK Examples . . . . . . . . . . . . . . . . . 15 5. SCHC Compound ACK YANG Data Model . . . . . . . . . . . . . . 16 5.1. SCHC YANG Data Model Extension . . . . . . . . . . . . . 16 5.2. SCHC YANG Tree Extension . . . . . . . . . . . . . . . . 18 6. SCHC Compound ACK Parameters . . . . . . . . . . . . . . . . 18 7. Security considerations . . . . . . . . . . . . . . . . . . . 19 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 9. Normative References . . . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 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. Zuniga, et al. Expires 9 January 2023 [Page 2] Internet-Draft SCHC Compound ACK July 2022 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. The present document describes an extension to 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 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, * 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 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 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. Hence, if the bitmap of window number Zuniga, et al. Expires 9 January 2023 [Page 3] Internet-Draft SCHC Compound ACK July 2022 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. 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]. |-- 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. 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. |--- 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 Figure 3 shows the case when the required padding bits are strictly less than M bits. In this case, the length of the message learnt from the underlying layer does not leave room for any extra window value, let alone any bitmap, thereby signaling the end of the SCHC Compound ACK message. Zuniga, et al. Expires 9 January 2023 [Page 4] Internet-Draft SCHC Compound ACK July 2022 |--- 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 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 Losses are found in windows W = w1,...,wi; where w1<w2<...<wi Figure 4: SCHC Compound ACK message format with compressed bitmap Zuniga, et al. Expires 9 January 2023 [Page 5] Internet-Draft SCHC Compound ACK July 2022 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. |--- 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. Zuniga, et al. Expires 9 January 2023 [Page 6] Internet-Draft SCHC Compound ACK July 2022 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]. 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. Note that even though the protocol allows for such flexibility, the actual decision criteria is not specified in this document. The following sections describe the differences between the baseline SCHC specification and the present SCHC protocol extension specification. New text is between ** NEW TEXT **. Old text is -- OLD TEXT --. New text replaces old text. 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 --exactly-- one window of tiles ** or several windows of tiles, each one identified by its window number**. See Figure 23 for an example. Zuniga, et al. Expires 9 January 2023 [Page 7] Internet-Draft SCHC Compound ACK July 2022 +---------------------------------------------...-----------+ | 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, Zuniga, et al. Expires 9 January 2023 [Page 8] Internet-Draft SCHC Compound ACK July 2022 * 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. * **if the SCHC Compound ACK is used.** 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. * the fragment sender MUST initialize the Attempts counter to 0 for that RuleID and DTag value pair. Zuniga, et al. Expires 9 January 2023 [Page 9] Internet-Draft SCHC Compound ACK July 2022 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. Zuniga, et al. Expires 9 January 2023 [Page 10] Internet-Draft SCHC Compound ACK July 2022 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. Zuniga, et al. Expires 9 January 2023 [Page 11] Internet-Draft SCHC Compound ACK July 2022 * if the last of these SCHC Fragment messages is not an All-1 SCHC Fragment, then the fragment sender --MUST in addition send after it a SCHC ACK REQ with the W field corresponding to the last window.-- **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: Zuniga, et al. Expires 9 January 2023 [Page 12] Internet-Draft SCHC Compound ACK July 2022 * 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 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. Zuniga, et al. Expires 9 January 2023 [Page 13] Internet-Draft SCHC Compound ACK July 2022 - Otherwise, padding bits MUST be discarded. This is possible because: o the size of the tiles is known a priori, 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.**--lowest-numbered such 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. Zuniga, et al. Expires 9 January 2023 [Page 14] Internet-Draft SCHC Compound ACK July 2022 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: * 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) Zuniga, et al. Expires 9 January 2023 [Page 15] Internet-Draft SCHC Compound ACK July 2022 Figure 7: SCHC Compound ACK message sequence example |--- 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-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 Zuniga, et al. Expires 9 January 2023 [Page 16] Internet-Draft SCHC Compound ACK July 2022 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 2022-02-08 { 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 Zuniga, et al. Expires 9 January 2023 [Page 17] Internet-Draft SCHC Compound ACK July 2022 "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 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, 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. Zuniga, et al. Expires 9 January 2023 [Page 18] Internet-Draft SCHC Compound ACK July 2022 7. Security considerations The current document specifies a message format extension for SCHC. Hence, the same Security Considerations defined in [RFC8724] apply. 8. 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. 9. 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>. Zuniga, et al. Expires 9 January 2023 [Page 19] Internet-Draft SCHC Compound ACK July 2022 Authors' Addresses Juan Carlos Zúñiga Cisco Montreal QC Canada Email: juzuniga@cisco.com Carles Gomez Universitat Politècnica de Catalunya C/Esteve Terradas, 7 08860 Castelldefels Spain Email: carlesgo@entel.upc.edu 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 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 Zuniga, et al. Expires 9 January 2023 [Page 20]