Internet-Draft | SCHC Compound ACK | January 2023 |
Zuniga, et al. | Expires 23 July 2023 | [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 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.¶
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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 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:¶
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].¶
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 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.¶
Figure 1 shows the regular SCHC ACK format when all fragments have been correctly received (C=1), 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.¶
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.¶
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.¶
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.¶
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.¶
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].¶
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.¶
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:¶
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.¶
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:¶
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:¶
For each active pair of RuleID and DTag values, the sender MUST maintain:¶
For each active pair of RuleID and DTag values, the receiver MUST maintain:¶
At the beginning of the fragmentation of a new SCHC Packet:¶
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:¶
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 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:¶
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:¶
On Retransmission Timer expiration:¶
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:¶
otherwise:¶
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:¶
otherwise:¶
otherwise:¶
otherwise, the fragment sender:¶
See Figure 43/> for one among several possible examples of a Finite State Machine implementing a sender behavior obeying this specification.¶
On receiving a SCHC Fragment with a RuleID and DTag pair not being processed at that time:¶
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.¶
otherwise, tiles MUST be assembled based on the a priori known tile size.¶
Otherwise, padding bits MUST be discarded. This is possible because:¶
**On receiving a SCHC All-0 SCHC Fragment:¶
On receiving a SCHC ACK REQ or an All-1 SCHC Fragment:¶
otherwise:¶
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:¶
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.¶
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.¶
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.¶
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.¶
This document has no IANA actions.¶
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.¶