| Internet-Draft | SCHC Compound ACK | February 2022 |
| Zuniga, et al. | Expires 12 August 2022 | [Page] |
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, which is intended to reduce the number of response 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.¶
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 12 August 2022.¶
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.¶
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 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, 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:¶
It is assumed that the reader is familiar with the terms and mechanisms defined in [RFC8376] and in [RFC8724].¶
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.¶
Figure 1 shows the regular SCHC ACK format when all fragments have been correctly received (C=1), as defined in [RFC8724].¶
|- SCHC ACK Header --|
+ -------+---+------ + ------------ +
| RuleID | W | C=b'1 | b'0-pad(opt) |
+ ------ + - + ----- + ------------ +
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
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.¶
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 following sections describe the differences between the baseline SCHC specification and the present SCHC protocol extension specification.¶
OLD TEXT ([RFC8724], section 8.4.3.1) - On receiving a SCHC ACK:¶
NEW TEXT - On receiving a SCHC Compound ACK:¶
OLD TEXT ([RFC8724], section 8.4.3.2) - On receiving a SCHC ACK REQ or an All-1 SCHC Fragment:¶
NEW TEXT: On receiving an All-0 SCHC Fragment:¶
NEW TEXT: On receiving a SCHC ACK REQ or an All-1 SCHC Fragment:¶
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.¶
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)
|-- SCHC ACK Header ---|- W=00 --|----- W=01 -----| + -------------------- + ------- + ---- + ------- + ------------- + | RuleID | W=00 | C=0 | 1111011 | W=01 | 1111101 | b'0-pad (opt) | + ------ + ------ + -- + ------- + ---- + ------- + ------------- +
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.¶
<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 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
"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>
augment /schc:schc/schc:rule/schc:nature/schc:fragmentation/schc:mode/schc:ack-on-error:
+--rw bitmap-format? schc-compound-ack:bitmap-format-type
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].¶
The current document specifies a message format extension for SCHC. Hence, the same Security Considerations defined in [RFC8724] apply.¶
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.¶