lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Standards Track L. Toutain
Expires: April 25, 2019 IMT-Atlantique
C. Gomez
Universitat Politècnica de Catalunya
D. Barthel
Orange Labs
JC. Zuniga
SIGFOX
October 22, 2018

LPWAN Static Context Header Compression (SCHC) and fragmentation for IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-17

Abstract

This document defines the Static Context Header Compression (SCHC) framework, which provides both header compression and fragmentation functionalities. SCHC has been designed for Low Power Wide Area Networks (LPWAN).

SCHC compression is based on a common static context stored in both the LPWAN device and the network side. This document defines a header compression mechanism and its application to compress IPv6/UDP headers.

This document also specifies a fragmentation and reassembly mechanism that is used to support the IPv6 MTU requirement over the LPWAN technologies. Fragmentation is needed for IPv6 datagrams that, after SCHC compression or when such compression was not possible, still exceed the layer-2 maximum payload size.

The SCHC header compression and fragmentation mechanisms are independent of the specific LPWAN technology over which they are used. This document defines generic functionalities and offers flexibility with regard to parameter settings and mechanism choices. Technology-specific and product-specific settings and choices are expected to be grouped into Profiles specified in other documents.

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/.

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This Internet-Draft will expire on April 25, 2019.

Copyright Notice

Copyright (c) 2018 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Introduction

This document defines the Static Context Header Compression (SCHC) framework, which provides both header compression and fragmentation functionalities. SCHC has been designed for Low Power Wide Area Networks (LPWAN).

Header compression is needed for efficient Internet connectivity to the node within an LPWAN network. Some LPWAN networks properties can be exploited to get an efficient header compression:

SCHC compression uses a context in which information about header fields is stored. This context is static: the values of the header fields do not change over time. This avoids complex resynchronization mechanisms. Indeed, downlink is often more restricted/expensive, perhaps completely unavailable [RFC8376]. A compression protocol that relies on feedback is not compatible with the characteristics of such LPWANs.

In most cases, a small context identifier is enough to represent the full IPv6/UDP headers. The SCHC header compression mechanism is independent of the specific LPWAN technology over which it is used.

LPWAN technologies impose some strict limitations on traffic. For instance, devices are sleeping most of the time and may receive data during short periods of time after transmission to preserve battery. LPWAN technologies are also characterized by a greatly reduced data unit and/or payload size (see [RFC8376]). However, some LPWAN technologies do not provide fragmentation functionality; to support the IPv6 MTU requirement of 1280 bytes [RFC8200], they require a fragmentation protocol at the adaptation layer below IPv6. Accordingly, this document defines an fragmentation/reassembly mechanism for LPWAN technologies to supports the IPv6 MTU. Its implementation is optional. If not interested, the reader can safely skip its description.

This document defines generic functionality and offers flexibility with regard to parameters settings and mechanism choices. Technology-specific settings and product-specific and choices are expected to be grouped into Profiles specified in other documents.

2. Requirements Notation

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.

3. LPWAN Architecture

LPWAN technologies have similar network architectures but different terminologies. Using the terminology defined in [RFC8376], we can identify different types of entities in a typical LPWAN network, see Figure 1:

o Devices (Dev) are the end-devices or hosts (e.g. sensors, actuators, etc.). There can be a very high density of devices per radio gateway.

o The Radio Gateway (RGW), which is the end point of the constrained link.

o The Network Gateway (NGW) is the interconnection node between the Radio Gateway and the Internet.

o LPWAN-AAA Server, which controls the user authentication and the applications.

o Application Server (App)

                                           +------+
 ()   ()   ()       |                      |LPWAN-|
  ()  () () ()     / \       +---------+   | AAA  |
() () () () () () /   \======|    ^    |===|Server|  +-----------+
 ()  ()   ()     |           | <--|--> |   +------+  |APPLICATION|
()  ()  ()  ()  / \==========|    v    |=============|   (App)   |
  ()  ()  ()   /   \         +---------+             +-----------+
 Dev        Radio Gateways         NGW

Figure 1: LPWAN Architecture

4. Terminology

This section defines the terminology and acronyms used in this document.

Note that the SCHC acronym is pronounced like “sheek” in English (or “chic” in French). Therefore, this document writes “a SCHC Packet” instead of “an SCHC Packet”.

Additional terminology for the optional SCHC Fragmentation / Reassembly mechanism (SCHC F/R) is found in Section 8.2.

5. SCHC overview

SCHC can be characterized as an adaptation layer between IPv6 and the underlying LPWAN technology. SCHC comprises two sublayers (i.e. the Compression sublayer and the Fragmentation sublayer), as shown in Figure 2.

             +----------------+
             |      IPv6      |
          +- +----------------+            
          |  |   Compression  |  
    SCHC <   +----------------+   
          |  |  Fragmentation |
          +- +----------------+         
             |LPWAN technology|
             +----------------+

Figure 2: Protocol stack comprising IPv6, SCHC and an LPWAN technology

As per this document, when a packet (e.g. an IPv6 packet) needs to be transmitted, header compression is first applied to the packet. The resulting packet after header compression (whose header may or may not actually be smaller than that of the original packet) is called a SCHC Packet. If the SCHC Packet needs to be fragmented by the optional SCHC Fragmentation, fragmentation is then applied to the SCHC Packet. The SCHC Packet or the SCHC Fragments are then transmitted over the LPWAN. The reciprocal operations take place at the receiver. This process is illustrated in Figure 3.

A packet (e.g. an IPv6 packet) 
         |                                           ^
         v                                           |
+------------------+                      +--------------------+
| SCHC Compression |                      | SCHC Decompression |
+------------------+                      +--------------------+
         |                                           ^
         |   If no fragmentation (*)                 |
         +-------------- SCHC Packet  -------------->|
         |                                           |
         v                                           |
+--------------------+                       +-----------------+
| SCHC Fragmentation |                       | SCHC Reassembly |
+--------------------+                       +-----------------+
      |     ^                                     |     ^
      |     |                                     |     |
      |     +-------------- SCHC ACK -------------+     |
      |                                                 |
      +-------------- SCHC Fragments -------------------+

        SENDER                                    RECEIVER


*: the decision to use Fragmentation or not is left to each Profile.

Figure 3: SCHC operations at the SENDER and the RECEIVER

5.1. SCHC Packet format

The SCHC Packet is composed of the Compressed Header followed by the payload from the original packet (see Figure 4). The Compressed Header itself is composed of the Rule ID and a Compression Residue, which is the output of the compression actions of the Rule that was applied (see Section 7). The Compression Residue may be empty. Both the Rule ID and the Compression Residue potentially have a variable size, and generally are not a mutiple of bytes in size.

|  Rule ID +  Compression Residue |
+---------------------------------+--------------------+ 
|      Compressed Header          |      Payload       |
+---------------------------------+--------------------+

Figure 4: SCHC Packet

5.2. Functional mapping

Figure 5 below maps the functional elements of Figure 3 onto the LPWAN architecture elements of Figure 1.

     Dev                                                 App
+----------------+                                  +--------------+
| APP1 APP2 APP3 |                                  |APP1 APP2 APP3|
|                |                                  |              |
|       UDP      |                                  |     UDP      |
|      IPv6      |                                  |    IPv6      |   
|                |                                  |              |  
|SCHC C/D and F/R|                                  |              |
+--------+-------+                                  +-------+------+
         |   +--+     +----+     +-----------+              .
         +~~ |RG| === |NGW | === |   SCHC    |... Internet ..
             +--+     +----+     |F/R and C/D|
                                 +-----------+

Figure 5: Architecture

SCHC C/D and SCHC F/R are located on both sides of the LPWAN transmission, i.e. on the Dev side and on the Network side.

The operation in the Uplink direction is as follows. The Device application uses IPv6 or IPv6/UDP protocols. Before sending the packets, the Dev compresses their headers using SCHC C/D and, if the SCHC Packet resulting from the compression needs to be fragmented by SCHC, SCHC F/R is performed (see Section 8). The resulting SCHC Fragments are sent to an LPWAN Radio Gateway (RG) which forwards them to a Network Gateway (NGW). The NGW sends the data to a SCHC F/R for re-assembly (if needed) and then to the SCHC C/D for decompression. After decompression, the packet can be sent over the Internet to one or several LPWAN Application Servers (App).

The SCHC F/R and C/D on the Network side can be located in the NGW, or somewhere else as long as a tunnel is established between them and the NGW. Note that, for some LPWAN technologies, it MAY be suitable to locate the SCHC F/R functionality nearer the NGW, in order to better deal with time constraints of such technologies.

The SCHC C/D and F/R on both sides MUST share the same set of Rules.

The SCHC C/D and F/R process is symmetrical, therefore the description of the Downlink direction is symmetrical to the one above.

6. Rule ID

Rule IDs are identifiers used to select the correct context either for Compression/Decompression or for Fragmentation/Reassembly.

The size of the Rule IDs is not specified in this document, as it is implementation-specific and can vary according to the LPWAN technology and the number of Rules, among others. It is defined in Profiles.

The Rule IDs are used:

7. Compression/Decompression

Compression with SCHC is based on using context, i.e. a set of Rules to compress or decompress headers. SCHC avoids context synchronization, which consumes considerable bandwidth in other header compression mechanisms such as RoHC [RFC5795]. Since the content of packets is highly predictable in LPWAN networks, static contexts MAY be stored beforehand to omit transmitting some information over the air. The contexts MUST be stored at both ends, and they can be learned by a provisioning protocol or by out of band means, or they can be pre-provisioned. The way the contexts are provisioned is out of the scope of this document.

7.1. SCHC C/D Rules

The main idea of the SCHC compression scheme is to transmit the Rule ID to the other end instead of sending known field values. This Rule ID identifies a Rule that provides the closest match to the original packet values. Hence, when a value is known by both ends, it is only necessary to send the corresponding Rule ID over the LPWAN network. The manner by which Rules are generated is out of the scope of this document. The Rules MAY be changed at run-time but the mechanism is out of scope of this document.

The context contains a list of Rules (see Figure 6). Each Rule itself contains a list of Field Descriptions composed of a Field Identifier (FID), a Field Length (FL), a Field Position (FP), a Direction Indicator (DI), a Target Value (TV), a Matching Operator (MO) and a Compression/Decompression Action (CDA).

  /-----------------------------------------------------------------\
  |                         Rule N                                  |
 /-----------------------------------------------------------------\|
 |                       Rule i                                    ||
/-----------------------------------------------------------------\||
|  (FID)            Rule 1                                        |||
|+-------+--+--+--+------------+-----------------+---------------+|||
||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+--+------------+-----------------+---------------+|||
||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+--+------------+-----------------+---------------+|||
||...    |..|..|..|   ...      | ...             | ...           ||||
|+-------+--+--+--+------------+-----------------+---------------+||/
||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
|+-------+--+--+--+------------+-----------------+---------------+|/
|                                                                 |
\-----------------------------------------------------------------/

Figure 6: A Compression/Decompression Context

A Rule does not describe how to parse a packet header to find each field. This MUST be known from the compressor/decompressor. Rules only describe the compression/decompression behavior for each header field. In a Rule, the Field Descriptions are listed in the order in which the fields appear in the packet header.

A Rule also describes what is sent in the Compression Residue. The Compression Residue is assembled by concatenating the residues for each field, in the order the Field Descriptions appear in the Rule.

The Context describes the header fields and its values with the following entries:

7.2. Rule ID for SCHC C/D

Rule IDs are sent by the compression function in one side and are received for the decompression function in the other side. In SCHC C/D, the Rule IDs are specific to a Dev. Hence, multiple Dev instances MAY use the same Rule ID to define different header compression contexts. To identify the correct Rule ID, the SCHC C/D needs to associate the Rule ID with the Dev identifier to find the appropriate Rule to be applied.

7.3. Packet processing

The compression/decompression process follows several steps:

7.4. Matching operators

Matching Operators (MOs) are functions used by both SCHC C/D endpoints involved in the header compression/decompression. They are not typed and can be applied to integer, string or any other data type. The result of the operation can either be True or False. MOs are defined as follows:

7.5. Compression Decompression Actions (CDA)

The Compression Decompression Action (CDA) describes the actions taken during the compression of headers fields, and inversely, the action taken by the decompressor to restore the original value.

/--------------------+-------------+----------------------------\
|  Action            | Compression | Decompression              |
|                    |             |                            |
+--------------------+-------------+----------------------------+
|not-sent            |elided       |use value stored in context |
|value-sent          |send         |build from received value   |
|mapping-sent        |send index   |value from index on a table |
|LSB                 |send LSB     |TV, received value          |
|compute-length      |elided       |compute length              |
|compute-checksum    |elided       |compute UDP checksum        |
|DevIID              |elided       |build IID from L2 Dev addr  |
|AppIID              |elided       |build IID from L2 App addr  |
\--------------------+-------------+----------------------------/

Figure 7: Compression and Decompression Actions

Figure 7 summarizes the basic actions that can be used to compress and decompress a field. The first column shows the action’s name. The second and third columns show the reciprocal compression/decompression behavior for each action.

Compression is done in the order that the Field Descriptions appear in a Rule. The result of each Compression/Decompression Action is appended to the accumulated Compression Residue in that same order. The receiver knows the size of each compressed field, which can be given by the Rule or MAY be sent with the compressed header.

7.5.1. processing variable-length fields

If the field is identified in the Field Description as being of variable size, then the size of the Compression Residue value (using the unit defined in the FL) MUST first be sent as follows:

If a field is not present in the packet but exists in the Rule and its FL is specified as being variable, size 0 MUST be sent to denote its absence.

7.5.2. not-sent CDA

The not-sent action is generally used when the field value is specified in a Rule and therefore known by both the Compressor and the Decompressor. This action SHOULD be used with the “equal” MO. If MO is “ignore”, there is a risk to have a decompressed field value different from the original field that was compressed.

The compressor does not send any Compression Residue for a field on which not-sent compression is applied.

The decompressor restores the field value with the Target Value stored in the matched Rule identified by the received Rule ID.

7.5.3. value-sent CDA

The value-sent action is generally used when the field value is not known by both the Compressor and the Decompressor. The value is sent as a residue in the compressed message header. Both Compressor and Decompressor MUST know the size of the field, either implicitly (the size is known by both sides) or by explicitly indicating the length in the Compression Residue, as defined in Section 7.5.1. This action is generally used with the “ignore” MO.

7.5.4. mapping-sent CDA

The mapping-sent action is used to send an index (the index into the Target Value list of values) instead of the original value. This action is used together with the “match-mapping” MO.

On the compressor side, the match-mapping Matching Operator searches the TV for a match with the header field value and the mapping-sent CDA appends the corresponding index to the Compression Residue to be sent. On the decompressor side, the CDA uses the received index to restore the field value by looking up the list in the TV.

The number of bits sent is the minimal size for coding all the possible indices.

7.5.5. LSB CDA

The LSB action is used together with the “MSB(x)” MO to avoid sending the most significant part of the packet field if that part is already known by the receiving end. The number of bits sent is the original header field length minus the length specified in the MSB(x) MO.

The compressor sends the Least Significant Bits (e.g. LSB of the length field). The decompressor concatenates the x most significant bits of Target Value and the received residue.

If this action needs to be done on a variable length field, the size of the Compression Residue in bytes MUST be sent as described in Section 7.5.1.

7.5.6. DevIID, AppIID CDA

These actions are used to process respectively the Dev and the App Interface Identifiers (DevIID and AppIID) of the IPv6 addresses. AppIID CDA is less common since most current LPWAN technologies frames contain a single L2 address, which is the Dev’s address.

The IID value MAY be computed from the Device ID present in the L2 header, or from some other stable identifier. The computation is specific to each Profile and MAY depend on the Device ID size.

In the downlink direction (Dw), at the compressor, the DevIID CDA may be used to generate the L2 addresses on the LPWAN, based on the packet’s Destination Address.

7.5.7. Compute-*

Some fields may be elided during compression and reconstructed during decompression. This is the case for length and checksum, so:

8. Fragmentation/Reassembly

8.1. Overview

In LPWAN technologies, the L2 MTU typically ranges from tens to hundreds of bytes. Some of these technologies do not have an internal fragmentation/reassembly mechanism.

The SCHC Fragmentation/Reassembly (SCHC F/R) functionality is offered as an option for such LPWAN technologies to cope with the IPv6 MTU requirement of 1280 bytes [RFC8200]. It is optional to implement. If it is not needed, its description can be safely ignored.

This specification includes several SCHC F/R modes, which allow for a range of reliability options such as optional SCHC Fragment retransmission. More modes may be defined in the future.

The same SCHC F/R mode MUST be used for all SCHC Fragments of the same fragmented SCHC Packet. This document does not make any decision with regard to which mode(s) will be used over a specific LPWAN technology. This will be defined in Profiles.

SCHC F/R uses the knowledge of the L2 Word size (see Section 4) to encode some messages. Therefore, SCHC MUST know the L2 Word size. SCHC F/R usually generates SCHC Fragments and SCHC ACKs that are multiples of L2 Words. The padding overhead is kept to the absolute minimum (see Section 9).

8.2. SCHC F/R Tools

This subsection describes the different tools that are used to enable the SCHC F/R functionality defined in this document. These tools include the SCHC F/R messages, tiles, windows, counters, timers and header fields.

The tools are described here in a generic manner. Their application to each SCHC F/R mode is found in Section 8.4.

8.2.1. Messages

The messages that can be used by SCHC F/R are the following:

8.2.2. Tiles, Windows, Bitmaps, Timers, Counters

8.2.2.1. Tiles

The SCHC Packet is fragmented into pieces, hereafter called tiles. The tiles MUST be contiguous.

See Figure 8 for an example.

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

Figure 8: a SCHC Packet fragmented in tiles

Each SCHC Fragment message carries at least one tile in its Payload, if the Payload field is present.

8.2.2.2. Windows

Some SCHC F/R modes may handle successive tiles in groups, called windows.

If windows are used

See Figure 9 for an example.

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

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

Figure 9: a SCHC Packet fragmented in tiles grouped in 28 windows, with WINDOW_SIZE = 5

When windows are used

8.2.2.3. Bitmaps

Each bit in the Bitmap for a window corresponds to a tile in the window. Each Bitmap has therefore WINDOW_SIZE bits. The bit at the left-most position corresponds to the tile numbered WINDOW_SIZE - 1. Consecutive bits, going right, correspond to sequentially decreasing tile numbers. In Bitmaps for windows that are not the last one of a SCHC Packet, the bit at the right-most position corresponds to the tile numbered 0. In the Bitmap for the last window, the bit at the right-most position corresponds either to the tile numbered 0 or to a tile that is sent/received as “the last one of the SCHC Packet” without explicitely stating its number (see Section 8.3.1.2).

At the receiver

WINDOW_SIZE finely controls the size of the Bitmap sent in the SCHC ACK message, which may be critical to some LPWAN technologies.

8.2.2.4. Timers and counters

Some SCHC F/R modes can use the following timers and counters

8.2.3. Integrity Checking

The reassembled SCHC Packet is checked for integrity at the receive end. Integrity checking is performed by computing a MIC at the sender side and transmitting it to the receiver for comparison with the locally computed MIC.

The MIC supports UDP checksum elision by SCHC C/D (see Section 10.11).

The CRC32 polynomial 0xEDB88320 (i.e. the reverse representation of the polynomial used e.g. in the Ethernet standard [RFC3385]) is RECOMMENDED as the default algorithm for computing the MIC. Nevertheless, other MIC lengths or other algorithms MAY be required by the Profile.

Note that the concatenation of the complete SCHC Packet and the potential padding bits of the last SCHC Fragment does not generally constitute an integer number of bytes. For implementers to be able to use byte-oriented CRC libraries, it is RECOMMENDED that the concatenation of the complete SCHC Packet and the last fragment potential padding bits be zero-extended to the next byte boundary and that the MIC be computed on that byte array. A Profile MAY specify another behaviour.

8.2.4. Header Fields

The SCHC F/R messages use the following fields (see the related formats in Section 8.3):

8.3. SCHC F/R Message Formats

This section defines the SCHC Fragment formats, the SCHC ACK format, the SCHC ACK REQ format and the SCHC Abort formats.

8.3.1. SCHC Fragment format

A SCHC Fragment conforms to the general format shown in Figure 10. It comprises a SCHC Fragment Header and a SCHC Fragment Payload. The SCHC Fragment Payload carries one or several tile(s).

+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
| Fragment Header |   Fragment Payload    | padding (as needed)
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~

Figure 10: SCHC Fragment general format. Presence of a padding field is optional

8.3.1.1. Regular SCHC Fragment

The Regular SCHC Fragment format is shown in Figure 11. Regular SCHC Fragments are generally used to carry tiles that are not the last one of a SCHC Packet. The DTag field and the W field are optional.

 |--- SCHC Fragment Header ----|
           |-- T --|-M-|-- N --|
 +-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
 | Rule ID | DTag  | W |  FCN  | Fragment Payload | padding (as needed)
 +-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~

Figure 11: Detailed Header Format for Regular SCHC Fragments

The FCN field MUST NOT contain all bits set to 1.

If the size of the SCHC Fragment Payload does not nicely complement the SCHC Header size in a way that would make the SCHC Fragment a multiple of the L2 Word, then padding bits MUST be added.

The Fragment Payload of a SCHC Fragment with FCN == 0 (called an All-0 SCHC Fragment) MUST be at least the size of an L2 Word. The rationale is that, even in the presence of padding, an All-0 SCHC Fragment needs to be distinguishable from the SCHC ACK REQ message, which has the same header but has no payload (see Section 8.3.3).

8.3.1.2. All-1 SCHC Fragment

The All-1 SCHC Fragment format is shown in Figure 12. The All-1 SCHC Fragment is generally used to carry the very last tile of a SCHC Packet and a MIC, or a MIC only. The DTag field, the W field and the Payload are optional.

|-------- SCHC Fragment Header -------|
          |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag  | W | 11..1 |  MIC  | Frag Payload | pad. (as needed)
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
                        (FCN)

Figure 12: Detailed format for the All-1 SCHC Fragment

If the size of the SCHC Fragment Payload does not nicely complement the SCHC Header size in a way that would make the SCHC Fragment a multiple of the L2 Word, then padding bits MUST be added.

The All-1 SCHC Fragment message MUST be distinguishable by size from a SCHC Sender-Abort message (see Section 8.3.4.1) that has the same T, M and N values. This is trivially achieved by having the MIC larger than an L2 Word, or by having the Payload larger than an L2 Word. This is also naturally achieved if the SCHC Sender-Abort Header is a multiple of L2 Words.

8.3.2. SCHC ACK format

The SCHC ACK message MUST obey the format shown in Figure 13. The DTag field, the W field and the Compressed Bitmap field are optional. The Compressed Bitmap field can only be present in SCHC F/R modes that use windows.

|---- SCHC ACK Header ----|
            |-- T --|-M-|1|
+---- ... --+- ... -+---+-+~~~~~~~~~~~~~~~~~~
|  Rule ID  |  DTag | W |1| padding as needed                (success)
+---- ... --+- ... -+---+-+~~~~~~~~~~~~~~~~~~

+---- ... --+- ... -+---+-+------ ... ------+~~~~~~~~~~~~~~~
|  Rule ID  |  DTag | W |0|Compressed Bitmap| pad. as needed (failure)
+---- ... --+- ... -+---+-+------ ... ------+~~~~~~~~~~~~~~~
                         C

Figure 13: Format of the SCHC ACK message

The SCHC ACK Header contains a C bit (see Section 8.2.4).

If the C bit is set to 1 (integrity check successful), no Bitmap is carried and padding bits MUST be appended as needed to fill up the last L2 Word.

If the C bit is set to 0 (integrity check not performed or failed) and if windows are used,

If the C bit is 1 or windows are not used, the C bit MUST be followed by padding bits as needed to fill up the last L2 Word.

See Section 8.2.2.3 for a description of the Bitmap.

The representation of the Bitmap that is transmitted MUST be the compressed version specified in Section 8.3.2.1, in order to reduce the SCHC ACK message size.

8.3.2.1. Bitmap Compression

For transmission, the Compressed Bitmap in the SCHC ACK message is defined by the following algorithm (see Figure 14 for a follow-along example):

When one or more bits have effectively been dropped off as a result of the above algorithm, the SCHC ACK message is a multiple of L2 Words, no padding bits will be appended.

Because the SCHC Fragment sender knows the size of the original Bitmap, it can reconstruct the original Bitmap from the Compressed Bitmap received in the SCH ACK message.

Figure 14 shows an example where L2 Words are actually bytes and where the original Bitmap contains 17 bits, the last 15 of which are all set to 1.

|---- SCHC ACK Header ----|--------      Bitmap     --------|
            |-- T --|-M-|1|
+---- ... --+- ... -+---+-+---------------------------------+
|  Rule ID  |  DTag | W |0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+---- ... --+- ... -+---+-+---------------------------------+
                         C      |
        next L2 Word boundary ->|

Figure 14: Tentative SCHC ACK message with Bitmap before compression

Figure 15 shows that the last 14 bits are not sent.

|---- SCHC ACK Header ----|CpBmp|
            |-- T --|-M-|1|
+---- ... --+- ... -+---+-+-----+
|  Rule ID  |  DTag | W |0|1 0 1|
+---- ... --+- ... -+---+-+-----+
                         C      |
        next L2 Word boundary ->|

Figure 15: Actual SCHC ACK message with Compressed Bitmap, no padding

Figure 16 shows an example of a SCHC ACK with tile numbers ranging from 6 down to 0, where the Bitmap indicates that the second and the fourth tile of the window have not been correctly received.

|---- SCHC ACK Header ----|--- Bitmap --|
            |-- T --|-M-|1|6 5 4 3 2 1 0| (tile #)
+-----------+-------+---+-+-------------+
|  Rule ID  |  DTag | W |0|1 0 1 0 1 1 1|      with Original Bitmap
+-----------+-------+---+-+-------------+
                         C
    next L2 Word boundary ->|<-- L2 Word -->|

+-----------+-------+---+-+-------------+~~~+
|  Rule ID  |  DTag | W |0|1 0 1 0 1 1 1|Pad|  transmitted SCHC ACK
+-----------+-------+---+-+-------------+~~~+
                         C
    next L2 Word boundary ->|<-- L2 Word -->|

Figure 16: Example of a SCHC ACK message, missing tiles, with padding

Figure 17 shows an example of a SCHC ACK with FCN ranging from 6 down to 0, where integrity check has not been performed or has failed and the Bitmap indicates that there is no missing tile in that window.

|---- SCHC ACK Header ----|--- Bitmap --|
            |-- T --|-M-|1|6 5 4 3 2 1 0| (tile #)
+-----------+-------+---+-+-------------+
|  Rule ID  |  DTag | W |0|1 1 1 1 1 1 1|      with Original Bitmap
+-----------+-------+---+-+-------------+
                         C
    next L2 Word boundary ->|

+---- ... --+- ... -+---+-+-+
|  Rule ID  |  DTag | W |0|1|                  transmitted SCHC ACK
+---- ... --+- ... -+---+-+-+
                         C
    next L2 Word boundary ->|

Figure 17: Example of a SCHC ACK message, no missing tile, no padding

8.3.3. SCHC ACK REQ format

The SCHC ACK REQ is used by a sender to explicitely request a SCHC ACK from the receiver. Its format is described in Figure 18. The DTag field and the W field are optional.

|---- SCHC ACK REQ Header ----|
          |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag  | W |  0..0 | padding (as needed)      (no payload)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~

Figure 18: SCHC ACK REQ detailed format

The size of the SCHC ACK REQ header is generally not a multiple of the L2 Word size. Therefore, a SCHC ACK REQ generally needs padding bits.

Note that the SCHC ACK REQ has the same header as an All-0 SCHC Fragment (see Section 8.3.1.1) but it doesn’t have a payload. A receiver can distinguish the former form the latter by the message length, even in the presence of padding. This is possible because

8.3.4. SCHC Abort formats

8.3.4.1. SCHC Sender-Abort

When a SCHC Fragment sender needs to abort an on-going fragmented SCHC Packet transmission, it sends a SCHC Sender-Abort message to the SCHC Fragment receiver.

The SCHC Sender-Abort format is described in Figure 19. The DTag field and the W field are optional.

|---- Sender-Abort Header ----|
          |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag  | W | 11..1 | padding (as needed)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~

Figure 19: SCHC Sender-Abort format

If the W field is present,

The size of the SCHC Sender-Abort header is generally not a multiple of the L2 Word size. Therefore, a SCHC Sender-Abort generally needs padding bits.

Note that the SCHC Sender-Abort has the same header as an All-1 SCHC Fragment (see Section 8.3.1.2), but that it does not include a MIC nor a payload. The receiver distinguishes the former from the latter by the message length, even in the presence of padding. This is possible through different combinations

The SCHC Sender-Abort MUST NOT be acknowledged.

8.3.4.2. SCHC Receiver-Abort

When a SCHC Fragment receiver needs to abort an on-going fragmented SCHC Packet transmission, it transmits a SCHC Receiver-Abort message to the SCHC Fragment sender.

The SCHC Receiver-Abort format is described in Figure 20. The DTag field and the W field are optional.

|--- Receiver-Abort Header ---|
              |--- T ---|-M-|1|
+---- ... ----+-- ... --+---+-+-+-+-+-+-+-+-+-+-+-+-+
|   Rule ID   |   DTag  | W |1| 1..1|      1..1     |
+---- ... ----+-- ... --+---+-+-+-+-+-+-+-+-+-+-+-+-+
                             C
            next L2 Word boundary ->|<-- L2 Word -->|

Figure 20: SCHC Receiver-Abort format

If the W field is present,

Note that the SCHC Receiver-Abort has the same header as a SCHC ACK message. The bits that follow the SCHC Receiver-Abort Header MUST be as follows

Such a bit pattern never occurs in a regular SCHC ACK. This is how the fragment sender recognizes a SCHC Receiver-Abort.

A SCHC Receiver-Abort is aligned to L2 Words, by design. Therefore, padding MUST NOT be appended.

The SCHC Receiver-Abort MUST NOT be acknowledged.

8.4. SCHC F/R modes

This specification includes several SCHC F/R modes, which allow for

More modes may be defined in the future.

8.4.1. No-ACK mode

The No-ACK mode has been designed under the assumption that data unit out-of-sequence delivery does not occur between the entity performing fragmentation and the entity performing reassembly. This mode supports LPWAN technologies that have a variable MTU.

In No-ACK mode, there is no feedback communication from the fragment receiver to the fragment sender. The sender just transmits all the SCHC Fragments blindly.

Padding is kept to a minimum: only the last SCHC Fragment is padded as needed.

The tile sizes are not required to be uniform. Windows are not used. The Retransmission Timer is not used. The Attempts counter is not used.

Each Profile MUST specify which Rule ID value(s) is (are) allocated to this mode. For brevity, the rest of Section 8.4.1 only refers to Rule ID values that are allocated to this mode.

The W field MUST NOT be present in the SCHC F/R messages. SCHC ACK MUST NOT be sent. SCHC ACK REQ MUST NOT be sent. SCHC Sender-Abort MAY be sent. SCHC Receiver-Abort MUST NOT be sent.

The value of N (size of the FCN field) is RECOMMENDED to be 1.

Each Profile, for each Rule ID value, MUST define

Each Profile, for each Rule ID value, MAY define

The receiver, for each pair of Rule ID and optional DTag values, MUST maintain

8.4.1.1. Sender behaviour

At the beginning of the fragmentation of a new SCHC Packet, the fragment sender MUST select a Rule ID and optional DTag value pair for this SCHC Packet. For brevity, the rest of Section 8.4.1 only refers to SCHC F/R messages bearing the Rule ID and optional DTag values hereby selected.

Each SCHC Fragment MUST contain exactly one tile in its Payload. The tile MUST be at least the size of an L2 Word. The sender MUST transmit the SCHC Fragments messages in the order that the tiles appear in the SCHC Packet. Except for the last tile of a SCHC Packet, each tile MUST be of a size that complements the SCHC Fragment Header so that the SCHC Fragment is a multiple of L2 Words without the need for padding bits. Except for the last one, the SCHC Fragments MUST use the Regular SCHC Fragment format specified in Section 8.3.1.1. The last SCHC Fragment MUST use the All-1 format specified in Section 8.3.1.2.

The MIC MUST be computed on the reassembled SCHC Packet concatenated with the padding bits of the last SCHC Fragment. The rationale is that the SCHC Reassembler has no way of knowing where the payload of the last SCHC Fragment ends. Indeed, this requires decompressing the SCHC Packet, which is out of the scope of the SCHC Reassembler.

The sender MAY transmit a SCHC Sender-Abort.

Figure 35 shows an example of a corresponding state machine.

8.4.1.2. Receiver behaviour

On receiving Regular SCHC Fragments,

On receiving an All-1 SCHC Fragment,

On expiration of the Inactivity Timer, the receiver MUST drop the SCHC Packet being reassembled and it MUST release all resources associated with this Rule ID and optional DTag values.

On receiving a SCHC Sender-Abort, the receiver MAY release all resources associated with this Rule ID and optional DTag values.

The MIC computed at the receiver MUST be computed over the reassembled SCHC Packet and over the padding bits that were received in the SCHC Fragment carrying the last tile.

Figure 36 shows an example of a corresponding state machine.

8.4.2. ACK-Always

The ACK-Always mode has been designed under the following assumptions

In ACK-Always mode, windows are used. An acknowledgement, positive or negative, is fed by the fragment receiver back to the fragment sender at the end of the transmission of each window of SCHC Fragments.

The tiles are not required to be of uniform size. Padding is kept to a minimum: only the last SCHC Fragment is padded as needed.

In a nutshell, the algorithm is the following: after a first blind transmission of all the tiles of a window, the fragment sender iterates retransmitting the tiles that are reported missing until the fragment receiver reports that all the tiles belonging to the window have been correctly received, or until too many attempts were made. The fragment sender only advances to the next window of tiles when it has ascertained that all the tiles belonging to the current window have been fully and correctly received. This results in a lock-step behaviour between the sender and the receiver, at the window granularity.

Each Profile MUST specify which Rule ID value(s) is (are) allocated to this mode. For brevity, the rest of Section 8.4.1 only refers to Rule ID values that are allocated to this mode.

The W field MUST be present and its size M MUST be 1 bit. WINDOW_SIZE MUST be equal to MAX_WIND_FCN + 1.

Each Profile, for each Rule ID value, MUST define

The sender, for each active pair of Rule ID and optional DTag values, MUST maintain

The receiver, for each pair of Rule ID and optional DTag values, MUST maintain

8.4.2.1. Sender behaviour

At the beginning of the fragmentation of a new SCHC Packet, the fragment sender MUST select a Rule ID and DTag value pair for this SCHC Packet. For brevity, the rest of Section 8.4.2 only refers to SCHC F/R messages bearing the Rule ID and optional DTag values hereby selected.

Each SCHC Fragment MUST contain exactly one tile in its Payload. All tiles with the number 0 in their window, as well as the last tile, MUST be at least the size of an L2 Word.

In all SCHC Fragment messages, the W field MUST be filled with the least significant bit of the window number that the sender is currently processing.

If a SCHC Fragment carries a tile that is not the last one of the SCHC Packet,

The SCHC Fragment that carries the last tile MUST be an All-1 SCHC Fragment, described in Section 8.3.1.2.

The bits on which the MIC is computed MUST be the SCHC Packet concatenated with the potential padding bits that are appended to the Payload of the SCHC Fragment that carries the last tile.

The fragment sender MUST start by processing the window numbered 0.

In a “blind transmission” phase, it MUST transmit all the tiles composing the window, in decreasing tile number.

Then, it enters an “equalization phase” in which it MUST initialize an Attempts counter to 0, it MUST start a Retransmission Timer and it MUST expect to receive a SCHC ACK. Then,

At any time,

Figure 37 shows an example of a corresponding state machine.

8.4.2.2. Receiver behaviour

On receiving a SCHC Fragment with a Rule ID and optional DTag pair not being processed at that time

In the rest of this section, “local W bit” means the least significant bit of the window counter of the receiver.

On reception of any SCHC F/R message, the receiver MUST reset the Inactivity Timer.

Entering an “acceptance phase”, the receiver MUST first initialise an empty Bitmap for this window, then

In the “equalization phase”:

In the “clean-up phase”:

At any time, on expiration of the Inactivity Timer, on receiving a SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the receiver MUST send a SCHC Receiver-Abort, it MUST release all resource associated with this SCHC Packet and it MAY exit the receive process for that SCHC Packet.

The MIC computed at the receiver MUST be computed over the reassembled SCHC Packet and over the padding bits that were received in the SCHC Fragment carrying the last tile.

Figure 38 shows an example of a corresponding state machine.

8.4.3. ACK-on-Error

The ACK-on-Error mode supports LPWAN technologies that have variable MTU and out-of-order delivery.

In ACK-on-Error mode, windows are used. All tiles MUST be of equal size, except for the last one, which MUST be of the same size or smaller than the preceding ones. WINDOW_SIZE MUST be equal to MAX_WIND_FCN + 1.

A SCHC Fragment message carries one or more tiles, which may span multiple windows. A SCHC ACK reports on the reception of exactly one window of tiles.

See Figure 21 for an example.

         +---------------------------------------------...-----------+
         |                       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 21: a 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 feeds SCHC ACKs back to the fragment sender about windows that it misses tiles of. No SCHC ACK is fed back 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 can still later retransmit SCHC Fragments with tiles belonging to previous windows. Therefore, the sender and the receiver may operate in a fully decoupled fashion. The fragmented SCHC Packet transmission concludes when

Each Profile MUST specify which Rule ID value(s) is (are) allocated to this ACK-on-Error mode. For brevity, the rest of Section 8.4.3 only refers to SCHC F/R messages with Rule ID values that are allocated to this mode.

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

Each Profile, for each Rule ID value, MUST define

The sender, for each active pair of Rule ID and optional DTag values, MUST maintain

The receiver, for each pair of Rule ID and optional DTag values, MUST maintain

8.4.3.1. Sender behaviour

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

For brevity, the rest of Section 8.4.3 only refers to SCHC F/R messages bearing the Rule ID and optional DTag values hereby selected.

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

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

If a SCHC Fragment carries more than one tile, or carries one tile that is not the last one of the SCHC Packet,

The bits on which the MIC is computed MUST be the SCHC Packet concatenated with the padding bits that are appended to the Payload of the SCHC Fragment that carries the last tile.

The fragment sender MAY send the last tile as the Payload of an All-1 SCHC Fragment.

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.

Note that the last tile of a SCHC Packet can be sent in different ways, depending on Profiles and implementations

However, the last tile MUST NOT have ever been sent both in a Regular SCHC Fragment and in a All-1 SCHC Fragment.

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

A Profile MAY specify other times at which the fragment sender MUST listen for SCHC ACK messages.

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

On Retransmission Timer expiration

On receiving a SCHC ACK,

See Figure 39 for one among several possible examples of a Finite State Machine implementing a sender behaviour obeying this specification.

8.4.3.2. Receiver behaviour

On receiving a SCHC Fragment with a Rule ID and optional DTag pair not being processed at that time

On reception of any SCHC F/R message, the receiver MUST reset the Inactivity Timer.

On reception of a SCHC Fragment message, the receiver MUST assemble the received tiles based on the W and FCN fields of the SCHC Fragment.

On reception of a SCHC ACK REQ or of an All-1 SCHC Fragment,

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

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

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

On reception of a SCHC Sender-Abort, the receiver MUST release all resource associated with this SCHC Packet.

On expiration of the Inactivity Timer, the receiver MUST send a SCHC Receiver-Abort and it MUST release all resource associated with this SCHC Packet.

On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST send a SCHC Receiver-Abort and it MUST release all resource associated with this SCHC Packet.

Reassembly of the SCHC Packet concludes when

The MIC computed at the receiver MUST be computed over the reassembled SCHC Packet and over the padding bits that were received in the SCHC Fragment carrying the last tile.

See Figure 40 for one among several possible examples of a Finite State Machine implementing a receiver behaviour obeying this specification, and that is meant to match the sender Finite State Machine of Figure 39.

9. Padding management

SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does not have any alignment prerequisite. The size of SCHC Packets can be any number of bits. If the layer below SCHC constrains the payload to align to some boundary, called L2 Words (for example, bytes), SCHC will meet that constraint and produce messages with the correct alignement. This may entail adding extra bits, called padding bits.

When padding occurs, the number of appended bits MUST be strictly less than the L2 Word size.

Padding happens at most once for each Packet during SCHC Compression and optional SCHC Fragmentation (see Figure 2). If a SCHC Packet is sent unfragmented (see Figure 22), it is padded as needed for transmission. If a SCHC Packet is fragmented, it is not padded in itself, only the SCHC Fragments are padded as needed for transmission. Some SCHC F/R modes only pad the very last SCHC Fragment.

A packet (e.g. an IPv6 packet)
         |                                           ^ (padding bits
         v                                           |       dropped)
+------------------+                      +--------------------+
| SCHC Compression |                      | SCHC Decompression |
+------------------+                      +--------------------+
         |                                           ^
         |   If no fragmentation                     |
         +---- SCHC Packet + padding as needed ----->|
         |                                           | (MIC checked
         v                                           |  and removed)
+--------------------+                       +-----------------+
| SCHC Fragmentation |                       | SCHC Reassembly |
+--------------------+                       +-----------------+
     |       ^                                   |       ^
     |       |                                   |       |
     |       +------------- SCHC ACK ------------+       |
     |                                                   |
     +------- SCHC Fragments + padding as needed---------+

        SENDER                                    RECEIVER

Figure 22: SCHC operations, including padding as needed

Each Profile MUST specify the size of the L2 Word. The L2 Word might actually be a single bit, in which case at most zero bits of padding will be appended to any message, i.e. no padding will take place at all.

A Profile MAY define the value of the padding bits. The RECOMMENDED value is 0.

10. SCHC Compression for IPv6 and UDP headers

This section lists the different IPv6 and UDP header fields and how they can be compressed.

10.1. IPv6 version field

This field always holds the same value. Therefore, in the Rule, TV is set to 6, MO to “equal” and CDA to “not-sent”.

10.2. IPv6 Traffic class field

If the DiffServ field does not vary and is known by both sides, the Field Descriptor in the Rule SHOULD contain a TV with this well-known value, an “equal” MO and a “not-sent” CDA.

Otherwise (e.g. ECN bits are to be transmitted), two possibilities can be considered depending on the variability of the value:

10.3. Flow label field

If the Flow Label field does not vary and is known by both sides, the Field Descriptor in the Rule SHOULD contain a TV with this well-known value, an “equal” MO and a “not-sent” CDA.

Otherwise, two possibilities can be considered:

10.4. Payload Length field

This field can be elided for the transmission on the LPWAN network. The SCHC C/D recomputes the original payload length value. In the Field Descriptor, TV is not set, MO is set to “ignore” and CDA is “compute-IPv6-length”.

If the payload length needs to be sent and does not need to be coded in 16 bits, the TV can be set to 0x0000, the MO set to MSB(16-s) where ‘s’ is the number of bits to code the maximum length, and CDA is set to LSB.

10.5. Next Header field

If the Next Header field does not vary and is known by both sides, the Field Descriptor in the Rule SHOULD contain a TV with this Next Header value, the MO SHOULD be “equal” and the CDA SHOULD be “not-sent”.

Otherwise, TV is not set in the Field Descriptor, MO is set to “ignore” and CDA is set to “value-sent”. Alternatively, a matching-list MAY also be used.

10.6. Hop Limit field

The field behavior for this field is different for Uplink and Downlink. In Uplink, since there is no IP forwarding between the Dev and the SCHC C/D, the value is relatively constant. On the other hand, the Downlink value depends of Internet routing and MAY change more frequently. One neat way of processing this field is to use the Direction Indicator (DI) to distinguish both directions:

10.7. IPv6 addresses fields

As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit long fields; one for the prefix and one for the Interface Identifier (IID). These fields SHOULD be compressed. To allow for a single Rule being used for both directions, these values are identified by their role (DEV or APP) and not by their position in the header (source or destination).

10.7.1. IPv6 source and destination prefixes

Both ends MUST be synchronized with the appropriate prefixes. For a specific flow, the source and destination prefixes can be unique and stored in the context. It can be either a link-local prefix or a global prefix. In that case, the TV for the source and destination prefixes contain the values, the MO is set to “equal” and the CDA is set to “not-sent”.

If the Rule is intended to compress packets with different prefix values, match-mapping SHOULD be used. The different prefixes are listed in the TV, the MO is set to “match-mapping” and the CDA is set to “mapping-sent”. See Figure 24

Otherwise, the TV contains the prefix, the MO is set to “equal” and the CDA is set to “value-sent”.

10.7.2. IPv6 source and destination IID

If the DEV or APP IID are based on an LPWAN address, then the IID can be reconstructed with information coming from the LPWAN header. In that case, the TV is not set, the MO is set to “ignore” and the CDA is set to “DevIID” or “AppIID”. Note that the LPWAN technology generally carries a single identifier corresponding to the DEV. Therefore AppIID cannot be used.

For privacy reasons or if the DEV address is changing over time, a static value that is not equal to the DEV address SHOULD be used. In that case, the TV contains the static value, the MO operator is set to “equal” and the CDA is set to “not-sent”. [RFC7217] provides some methods that MAY be used to derive this static identifier.

If several IIDs are possible, then the TV contains the list of possible IIDs, the MO is set to “match-mapping” and the CDA is set to “mapping-sent”.

It MAY also happen that the IID variability only expresses itself on a few bytes. In that case, the TV is set to the stable part of the IID, the MO is set to “MSB” and the CDA is set to “LSB”.

Finally, the IID can be sent in extenso on the LPWAN. In that case, the TV is not set, the MO is set to “ignore” and the CDA is set to “value-sent”.

10.8. IPv6 extensions

No Rule is currently defined that processes IPv6 extensions. If such extensions are needed, their compression/decompression Rules can be based on the MOs and CDAs described above.

10.9. UDP source and destination port

To allow for a single Rule being used for both directions, the UDP port values are identified by their role (DEV or APP) and not by their position in the header (source or destination). The SCHC C/D MUST be aware of the traffic direction (Uplink, Downlink) to select the appropriate field. The following Rules apply for DEV and APP port numbers.

If both ends know the port number, it can be elided. The TV contains the port number, the MO is set to “equal” and the CDA is set to “not-sent”.

If the port variation is on few bits, the TV contains the stable part of the port number, the MO is set to “MSB” and the CDA is set to “LSB”.

If some well-known values are used, the TV can contain the list of these values, the MO is set to “match-mapping” and the CDA is set to “mapping-sent”.

Otherwise the port numbers are sent over the LPWAN. The TV is not set, the MO is set to “ignore” and the CDA is set to “value-sent”.

10.10. UDP length field

The UDP length can be computed from the received data. In that case, the TV is not set, the MO is set to “ignore” and the CDA is set to “compute-length”.

If the payload is small, the TV can be set to 0x0000, the MO set to “MSB” and the CDA to “LSB”.

In other cases, the length SHOULD be sent and the CDA is replaced by “value-sent”.

10.11. UDP Checksum field

The UDP checksum operation is mandatory with IPv6 [RFC8200] for most packets but recognizes that there are exceptions to that default behavior.

For instance, protocols that use UDP as a tunnel encapsulation may enable zero-checksum mode for a specific port (or set of ports) for sending and/or receiving. [RFC8200] also stipulates that any node implementing zero-checksum mode must follow the requirements specified in “Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums” [RFC6936].

6LoWPAN Header Compression [RFC6282] also authorizes to send UDP datagram that are deprived of the checksum protection when an upper layer guarantees the integrity of the UDP payload and pseudo-header all the way between the compressor that elides the UDP checksum and the decompressor that computes again it. A specific example of this is when a Message Integrity Check (MIC) protects the compressed message all along that path with a strength that is identical or better to the UDP checksum.

In a similar fashion, this specification allows a SCHC compressor to elide the UDP checks when another layer guarantees an identical or better integrity protection for the UDP payload and the pseudo-header. In this case, the TV is not set, the MO is set to “ignore” and the CDA is set to “compute-checksum”.

In particular, when SCHC fragmentation is used, a fragmentation MIC of 2 bytes or more provides equal or better protection than the UDP checksum; in that case, if the compressor is collocated with the fragmentation point and the decompressor is collocated with the packet reassembly point, then compressor MAY elide the UDP checksum. Whether and when the UDP Checksum is elided is to be specified in the Profile.

Since the compression happens before the fragmentation, implementors should understand the risks when dealing with unprotected data below the transport layer and take special care when manipulating that data.

In other cases, the checksum SHOULD be explicitly sent. The TV is not set, the MO is set to “ignore” and the CDA is set to “value-sent”.

11. IANA Considerations

This document has no request to IANA.

12. Security considerations

12.1. Security considerations for SCHC Compression/Decompression

A malicious header compression could cause the reconstruction of a wrong packet that does not match with the original one. Such a corruption MAY be detected with end-to-end authentication and integrity mechanisms. Header Compression does not add more security problem than what is already needed in a transmission. For instance, to avoid an attack, never re-construct a packet bigger than some configured size (with 1500 bytes as generic default).

12.2. Security considerations for SCHC Fragmentation/Reassembly

This subsection describes potential attacks to LPWAN SCHC F/R and suggests possible countermeasures.

A node can perform a buffer reservation attack by sending a first SCHC Fragment to a target. Then, the receiver will reserve buffer space for the IPv6 packet. Other incoming fragmented SCHC Packets will be dropped while the reassembly buffer is occupied during the reassembly timeout. Once that timeout expires, the attacker can repeat the same procedure, and iterate, thus creating a denial of service attack. The (low) cost to mount this attack is linear with the number of buffers at the target node. However, the cost for an attacker can be increased if individual SCHC Fragments of multiple packets can be stored in the reassembly buffer. To further increase the attack cost, the reassembly buffer can be split into SCHC Fragment-sized buffer slots. Once a packet is complete, it is processed normally. If buffer overload occurs, a receiver can discard packets based on the sender behavior, which MAY help identify which SCHC Fragments have been sent by an attacker.

In another type of attack, the malicious node is required to have overhearing capabilities. If an attacker can overhear a SCHC Fragment, it can send a spoofed duplicate (e.g. with random payload) to the destination. If the LPWAN technology does not support suitable protection (e.g. source authentication and frame counters to prevent replay attacks), a receiver cannot distinguish legitimate from spoofed SCHC Fragments. Therefore, the original IPv6 packet will be considered corrupt and will be dropped. To protect resource-constrained nodes from this attack, it has been proposed to establish a binding among the SCHC Fragments to be transmitted by a node, by applying content-chaining to the different SCHC Fragments, based on cryptographic hash functionality. The aim of this technique is to allow a receiver to identify illegitimate SCHC Fragments.

Further attacks MAY involve sending overlapped fragments (i.e. comprising some overlapping parts of the original IPv6 datagram). Implementers SHOULD make sure that the correct operation is not affected by such event.

In ACK-on-Error, a malicious node MAY force a SCHC Fragment sender to resend a SCHC Fragment a number of times, with the aim to increase consumption of the SCHC Fragment sender’s resources. To this end, the malicious node MAY repeatedly send a fake ACK to the SCHC Fragment sender, with a Bitmap that reports that one or more SCHC Fragments have been lost. In order to mitigate this possible attack, MAX_ACK_RETRIES MAY be set to a safe value which allows to limit the maximum damage of the attack to an acceptable extent. However, note that a high setting for MAX_ACK_RETRIES benefits SCHC Fragment reliability modes, therefore the trade-off needs to be carefully considered.

13. Acknowledgements

Thanks to Carsten Bormann, Philippe Clavier, Diego Dujovne, Eduardo Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez Bernal, Antony Markovski, Alexander Pelov, Charles Perkins, Edgar Ramos, Shoichi Sakane, and Pascal Thubert for useful design consideration and comments.

14. References

14.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)", RFC 7217, DOI 10.17487/RFC7217, April 2014.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

14.2. Informative References

[RFC3385] Sheinwald, D., Satran, J., Thaler, P. and V. Cavanna, "Internet Protocol Small Computer System Interface (iSCSI) Cyclic Redundancy Check (CRC)/Checksum Considerations", RFC 3385, DOI 10.17487/RFC3385, September 2002.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007.
[RFC5795] Sandlund, K., Pelletier, G. and L-E. Jonsson, "The RObust Header Compression (ROHC) Framework", RFC 5795, DOI 10.17487/RFC5795, March 2010.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, September 2011.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936, DOI 10.17487/RFC6936, April 2013.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, February 2014.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017.
[RFC8376] Farrell, S., "Low-Power Wide Area Network (LPWAN) Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018.

Appendix A. SCHC Compression Examples

This section gives some scenarios of the compression mechanism for IPv6/UDP. The goal is to illustrate the behavior of SCHC.

The most common case using the mechanisms defined in this document will be a LPWAN Dev that embeds some applications running over CoAP. In this example, three flows are considered. The first flow is for the device management based on CoAP using Link Local IPv6 addresses and UDP ports 123 and 124 for Dev and App, respectively. The second flow will be a CoAP server for measurements done by the Device (using ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to beta::1/64. The last flow is for legacy applications using different ports numbers, the destination IPv6 address prefix is gamma::1/64.

Figure 23 presents the protocol stack for this Device. IPv6 and UDP are represented with dotted lines since these protocols are compressed on the radio link.

 Management   Data
+----------+---------+---------+
|   CoAP   |  CoAP   | legacy  |
+----||----+---||----+---||----+
.   UDP    .  UDP    |   UDP   |
................................
.   IPv6   .  IPv6   .  IPv6   .
+------------------------------+
|    SCHC Header compression   |
|      and fragmentation       |
+------------------------------+
|      LPWAN L2 technologies   |
+------------------------------+
         DEV or NGW

Figure 23: Simplified Protocol Stack for LP-WAN

Note that in some LPWAN technologies, only the Devs have a device ID. Therefore, when such technologies are used, it is necessary to statically define an IID for the Link Local address for the SCHC C/D.

Rule 0
 +----------------+--+--+--+---------+--------+------------++------+
 | Field          |FL|FP|DI| Value   | Match  | Comp Decomp|| Sent |
 |                |  |  |  |         | Opera. | Action     ||[bits]|
 +----------------+--+--+--+---------+---------------------++------+
 |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
 |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
 |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
 |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
 |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
 |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
 |IPv6 DEVprefix  |64|1 |Bi|FE80::/64| equal  | not-sent   ||      |
 |IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
 |IPv6 APPprefix  |64|1 |Bi|FE80::/64| equal  | not-sent   ||      |
 |IPv6 AppIID     |64|1 |Bi|::1      | equal  | not-sent   ||      |
 +================+==+==+==+=========+========+============++======+
 |UDP DEVport     |16|1 |Bi|123      | equal  | not-sent   ||      |
 |UDP APPport     |16|1 |Bi|124      | equal  | not-sent   ||      |
 |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
 |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
 +================+==+==+==+=========+========+============++======+

 Rule 1
 +----------------+--+--+--+---------+--------+------------++------+
 | Field          |FL|FP|DI| Value   | Match  | Action     || Sent |
 |                |  |  |  |         | Opera. | Action     ||[bits]|
 +----------------+--+--+--+---------+--------+------------++------+
 |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
 |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
 |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
 |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
 |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
 |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
 |IPv6 DEVprefix  |64|1 |Bi|[alpha/64, match- |mapping-sent||   1  |
 |                |  |  |  |fe80::/64] mapping|            ||      |
 |IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
 |IPv6 APPprefix  |64|1 |Bi|[beta/64,| match- |mapping-sent||   2  |
 |                |  |  |  |alpha/64,| mapping|            ||      |
 |                |  |  |  |fe80::64]|        |            ||      |
 |IPv6 AppIID     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
 +================+==+==+==+=========+========+============++======+
 |UDP DEVport     |16|1 |Bi|5683     | equal  | not-sent   ||      |
 |UDP APPport     |16|1 |Bi|5683     | equal  | not-sent   ||      |
 |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
 |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
 +================+==+==+==+=========+========+============++======+

 Rule 2
 +----------------+--+--+--+---------+--------+------------++------+
 | Field          |FL|FP|DI| Value   | Match  | Action     || Sent |
 |                |  |  |  |         | Opera. | Action     ||[bits]|
 +----------------+--+--+--+---------+--------+------------++------+
 |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
 |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
 |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
 |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
 |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
 |IPv6 Hop Limit  |8 |1 |Up|255      | ignore | not-sent   ||      |
 |IPv6 Hop Limit  |8 |1 |Dw|         | ignore | value-sent ||   8  |
 |IPv6 DEVprefix  |64|1 |Bi|alpha/64 | equal  | not-sent   ||      |
 |IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
 |IPv6 APPprefix  |64|1 |Bi|gamma/64 | equal  | not-sent   ||      |
 |IPv6 AppIID     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
 +================+==+==+==+=========+========+============++======+
 |UDP DEVport     |16|1 |Bi|8720     | MSB(12)| LSB        ||   4  |
 |UDP APPport     |16|1 |Bi|8720     | MSB(12)| LSB        ||   4  |
 |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
 |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
 +================+==+==+==+=========+========+============++======+

Figure 24: Context Rules

All the fields described in the three Rules depicted on Figure 24 are present in the IPv6 and UDP headers. The DevIID-DID value is found in the L2 header.

The second and third Rules use global addresses. The way the Dev learns the prefix is not in the scope of the document.

The third Rule compresses port numbers to 4 bits.

Appendix B. Fragmentation Examples

This section provides examples for the different fragment reliability modes specified in this document.

Figure 25 illustrates the transmission in No-ACK mode of a SCHC Packet that needs 11 SCHC Fragments. FCN is 1 bit wide.

        Sender               Receiver
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-------FCN=0-------->|
          |-----FCN=1 + MIC --->| Integrity check: success
        (End)      

Figure 25: Transmission in No-ACK mode of a SCHC Packet carried by 11 SCHC Fragments

In the following examples, N (the size of the FCN field) is 3 bits. Therefore, the All-1 FCN value is 7.

Figure 26 illustrates the transmission in ACK-on-Error mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, MAX_WIND_FCN=6 and no lost SCHC Fragment.

        Sender               Receiver
          |-----W=0, FCN=6----->|
          |-----W=0, FCN=5----->|
          |-----W=0, FCN=4----->|
          |-----W=0, FCN=3----->|
          |-----W=0, FCN=2----->|
          |-----W=0, FCN=1----->|
          |-----W=0, FCN=0----->|
      (no ACK)
          |-----W=1, FCN=6----->|
          |-----W=1, FCN=5----->|
          |-----W=1, FCN=4----->|
          |--W=1, FCN=7 + MIC-->| Integrity check: success
          |<-- ACK, W=1, C=1 ---| C=1
        (End)

Figure 26: Transmission in ACK-on-Error mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, MAX_WIND_FCN=6 and no lost SCHC Fragment.

Figure 27 illustrates the transmission in ACK-on-Error mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, MAX_WIND_FCN=6 and three lost SCHC Fragments.

         Sender             Receiver
          |-----W=0, FCN=6----->|
          |-----W=0, FCN=5----->|
          |-----W=0, FCN=4--X-->|
          |-----W=0, FCN=3----->|
          |-----W=0, FCN=2--X-->|
          |-----W=0, FCN=1----->|
          |-----W=0, FCN=0----->|        6543210
          |<-- ACK, W=0, C=0 ---| Bitmap:1101011
          |-----W=0, FCN=4----->|
          |-----W=0, FCN=2----->|   
      (no ACK)     
          |-----W=1, FCN=6----->|
          |-----W=1, FCN=5----->|
          |-----W=1, FCN=4--X-->|
          |- W=1, FCN=7 + MIC ->| Integrity check: failure
          |<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
          |-----W=1, FCN=4----->| Integrity check: success
          |<-- ACK, W=1, C=1 ---| C=1
        (End)

Figure 27: Transmission in ACK-on-Error mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, MAX_WIND_FCN=6 and three lost SCHC Fragments.

Figure 28 shows an example of a transmission in ACK-on-Error mode of a SCHC Packet fragmented in 73 tiles, with N=5, MAX_WIND_FCN=27, M=2 and 3 lost SCHC Fragments.

      Sender               Receiver
       |-----W=0, FCN=27----->| 4 tiles sent
       |-----W=0, FCN=23----->| 4 tiles sent
       |-----W=0, FCN=19----->| 4 tiles sent
       |-----W=0, FCN=15--X-->| 4 tiles sent (not received)
       |-----W=0, FCN=11----->| 4 tiles sent
       |-----W=0, FCN=7 ----->| 4 tiles sent
       |-----W=0, FCN=3 ----->| 4 tiles sent
       |-----W=1, FCN=27----->| 4 tiles sent
       |-----W=1, FCN=23----->| 4 tiles sent
       |-----W=1, FCN=19----->| 4 tiles sent
       |-----W=1, FCN=15----->| 4 tiles sent
       |-----W=1, FCN=11----->| 4 tiles sent
       |-----W=1, FCN=7 ----->| 4 tiles sent
       |-----W=1, FCN=3 --X-->| 4 tiles sent (not received)
       |-----W=2, FCN=27----->| 4 tiles sent
       |-----W=2, FCN=23----->| 4 tiles sent
   ^   |-----W=2, FCN=19----->| 1 tile sent
   |   |-----W=2, FCN=18----->| 1 tile sent
   |   |-----W=2, FCN=17----->| 1 tile sent
       |-----W=2, FCN=16----->| 1 tile sent
   s   |-----W=2, FCN=15----->| 1 tile sent
   m   |-----W=2, FCN=14----->| 1 tile sent
   a   |-----W=2, FCN=13--X-->| 1 tile sent (not received)
   l   |-----W=2, FCN=12----->| 1 tile sent
   l   |---W=2, FCN=31 + MIC->| Integrity check: failure
   e   |<--- ACK, W=0, C=0 ---| C=0, Bitmap:1111111111110000111111111111
   r   |-----W=0, FCN=15----->| 1 tile sent
       |-----W=0, FCN=14----->| 1 tile sent
   L   |-----W=0, FCN=13----->| 1 tile sent
   2   |-----W=0, FCN=12----->| 1 tile sent
       |<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000
   M   |-----W=1, FCN=3 ----->| 1 tile sent
   T   |-----W=1, FCN=2 ----->| 1 tile sent
   U   |-----W=1, FCN=1 ----->| 1 tile sent
       |-----W=1, FCN=0 ----->| 1 tile sent
   |   |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001
   |   |-----W=2, FCN=13----->| Integrity check: success
   V   |<--- ACK, W=2, C=1 ---| C=1
     (End)

Figure 28: ACK-on-Error mode with variable MTU.

In this example, the L2 MTU becomes reduced just before sending the “W=2, FCN=19” fragment, leaving space for only 1 tile in each forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are carried by a total of 25 SCHC Fragments, the last 9 being of smaller size.

Note 1: Bitmaps are shown prior to compression for transmission

Note 2: other sequences of events (e.g. regarding when ACKs are sent by the Receiver) are also allowed by this specification. Profiles may restrict this flexibility.

Figure 29 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with N=3, MAX_WIND_FCN=6 and no loss.

        Sender               Receiver
          |-----W=0, FCN=6----->|
          |-----W=0, FCN=5----->|
          |-----W=0, FCN=4----->|
          |-----W=0, FCN=3----->|
          |-----W=0, FCN=2----->|
          |-----W=0, FCN=1----->|
          |-----W=0, FCN=0----->|
          |<-- ACK, W=0, C=0 ---| Bitmap:1111111
          |-----W=1, FCN=6----->|
          |-----W=1, FCN=5----->|   
          |-----W=1, FCN=4----->|
          |--W=1, FCN=7 + MIC-->| Integrity check: success
          |<-- ACK, W=1, C=1 ---| C=1
        (End)

Figure 29: Transmission in ACK-Always mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with N=3, MAX_WIND_FCN=6 and no loss.

Figure 30 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3, MAX_WIND_FCN=6 and three lost SCHC Fragments.

        Sender               Receiver
          |-----W=0, FCN=6----->|
          |-----W=0, FCN=5----->|
          |-----W=0, FCN=4--X-->|
          |-----W=0, FCN=3----->|
          |-----W=0, FCN=2--X-->|
          |-----W=0, FCN=1----->|
          |-----W=0, FCN=0----->|        6543210
          |<-- ACK, W=0, C=0 ---| Bitmap:1101011
          |-----W=0, FCN=4----->|
          |-----W=0, FCN=2----->|
          |<-- ACK, W=0, C=0 ---| Bitmap:1111111
          |-----W=1, FCN=6----->|
          |-----W=1, FCN=5----->|
          |-----W=1, FCN=4--X-->|
          |--W=1, FCN=7 + MIC-->| Integrity check: failure
          |<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
          |-----W=1, FCN=4----->| Integrity check: success
          |<-- ACK, W=1, C=1 ---| C=1
        (End)

Figure 30: Transmission in ACK-Always mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3, MAX_WIND_FCN=6 and three lost SCHC Fragments.

Figure 31 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, MAX_WIND_FCN=6, three lost SCHC Fragments and only one retry needed to recover each lost SCHC Fragment.

          Sender                Receiver
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4--X-->|
             |-----W=0, FCN=3--X-->|
             |-----W=0, FCN=2--X-->|
             |--W=0, FCN=7 + MIC-->| Integrity check: failure
             |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
             |-----W=0, FCN=4----->| Integrity check: failure
             |-----W=0, FCN=3----->| Integrity check: failure
             |-----W=0, FCN=2----->| Integrity check: success
             |<-- ACK, W=0, C=1 ---| C=1
           (End)

Figure 31: Transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, MAX_WIND_FCN=6, three lost SCHC Fragments.

Figure 32 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, MAX_WIND_FCN=6, three lost SCHC Fragments, and the second SCHC ACK lost.

          Sender                Receiver
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4--X-->|
             |-----W=0, FCN=3--X-->|
             |-----W=0, FCN=2--X-->|
             |--W=0, FCN=7 + MIC-->| Integrity check: failure
             |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
             |-----W=0, FCN=4----->| Integrity check: failure
             |-----W=0, FCN=3----->| Integrity check: failure
             |-----W=0, FCN=2----->| Integrity check: success
             |<-X-ACK, W=0, C=1 ---| C=1
    timeout  |                     |
             |--- W=0, ACK REQ --->| ACK REQ
             |<-- ACK, W=0, C=1 ---| C=1
           (End)

Figure 32: Transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, MAX_WIND_FCN=6, three lost SCHC Fragments, and the second SCHC ACK lost.

Figure 33 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with N=3, MAX_WIND_FCN=6, with three lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.

           Sender                Receiver
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4--X-->|
             |-----W=0, FCN=3--X-->|
             |-----W=0, FCN=2--X-->|
             |--W=0, FCN=7 + MIC-->| Integrity check: failure
             |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
             |-----W=0, FCN=4----->| Integrity check: failure
             |-----W=0, FCN=3----->| Integrity check: failure
             |-----W=0, FCN=2--X-->|
      timeout|                     |
             |--- W=0, ACK REQ --->| ACK REQ
             |<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101
             |-----W=0, FCN=2----->| Integrity check: success
             |<-- ACK, W=0, C=1 ---| C=1
           (End)

Figure 33: Transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with N=3, MAX_WIND_FCN=6, with three lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.

Figure 34 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5, MAX_WIND_FCN=23 and two lost SCHC Fragments.

      Sender               Receiver
        |-----W=0, FCN=23----->|
        |-----W=0, FCN=22----->|
        |-----W=0, FCN=21--X-->|
        |-----W=0, FCN=20----->|
        |-----W=0, FCN=19----->|
        |-----W=0, FCN=18----->|
        |-----W=0, FCN=17----->|
        |-----W=0, FCN=16----->|
        |-----W=0, FCN=15----->|
        |-----W=0, FCN=14----->|
        |-----W=0, FCN=13----->|
        |-----W=0, FCN=12----->|
        |-----W=0, FCN=11----->|
        |-----W=0, FCN=10--X-->|
        |-----W=0, FCN=9 ----->|
        |-----W=0, FCN=8 ----->|
        |-----W=0, FCN=7 ----->|
        |-----W=0, FCN=6 ----->|
        |-----W=0, FCN=5 ----->|
        |-----W=0, FCN=4 ----->|
        |-----W=0, FCN=3 ----->|
        |-----W=0, FCN=2 ----->|
        |-----W=0, FCN=1 ----->|
        |-----W=0, FCN=0 ----->|
        |                      |
        |<--- ACK, W=0, C=0 ---| Bitmap:110111111111101111111111
        |-----W=0, FCN=21----->|
        |-----W=0, FCN=10----->|
        |<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
        |-----W=1, FCN=23----->|
        |-----W=1, FCN=22----->|
        |-----W=1, FCN=21----->|
        |--W=1, FCN=31 + MIC-->| Integrity check: success
        |<--- ACK, W=1, C=1 ---| C=1
      (End)

Figure 34: Transmission in ACK-Always mode of a SCHC Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5, MAX_WIND_FCN=23 and two lost SCHC Fragments.

Appendix C. Fragmentation State Machines

The fragmentation state machines of the sender and the receiver, one for each of the different reliability modes, are described in the following figures:

             +===========+
+------------+  Init     |                                      
|  FCN=0     +===========+                                      
|  No Window                                       
|  No Bitmap                                                      
|                   +-------+           
|          +========+==+    | More Fragments                 
|          |           | <--+ ~~~~~~~~~~~~~~~~~~~~                          
+--------> |   Send    |      send Fragment (FCN=0)                            
           +===+=======+                                                                      
               |  last fragment
               |  ~~~~~~~~~~~~                               
               |  FCN = 1                               
               v  send fragment+MIC
           +============+                                             
           |    END     |                                             
           +============+

Figure 35: Sender State Machine for the No-ACK Mode

                      +------+ Not All-1
           +==========+=+    | ~~~~~~~~~~~~~~~~~~~
           |            + <--+ set Inactivity Timer
           |  RCV Frag  +-------+
           +=+===+======+       |All-1 &
   All-1 &   |   |              |MIC correct
 MIC wrong   |   |Inactivity    |
             |   |Timer Exp.    |
             v   |              |
  +==========++  |              v
  |   Error   |<-+     +========+==+
  +===========+        |    END    |
                       +===========+

Figure 36: Receiver State Machine for the No-ACK Mode

              +=======+  
              | INIT  |       FCN!=0 & more frags
              |       |       ~~~~~~~~~~~~~~~~~~~~~~
              +======++  +--+ send Window + frag(FCN)
                 W=0 |   |  | FCN-
  Clear lcl_bm       |   |  v set lcl_bm
       FCN=max value |  ++==+========+
                     +> |            |
+---------------------> |    SEND    |
|                       +==+===+=====+
|      FCN==0 & more frags |   | last frag
|    ~~~~~~~~~~~~~~~~~~~~~ |   | ~~~~~~~~~~~~~~~
|               set lcl_bm |   | set lcl_bm
|   send wnd + frag(all-0) |   | send wnd+frag(all-1)+MIC
|       set Retrans_Timer  |   | set Retrans_Timer
|                          |   |
|Recv_wnd == wnd &         |   |  
|lcl_bm==recv_bm &         |   |  +-----------------------+
|more frag                 |   |  | lcl_bm!=rcv-bm        |
|~~~~~~~~~~~~~~~~~~~~~~    |   |  | ~~~~~~~~~             |
|Stop Retrans_Timer        |   |  | Attempt++             v
|clear lcl_bm              v   v  |                +=====+=+
|window=next_window   +====+===+==+===+            |Resend |
+---------------------+               |            |Missing|
                 +----+     Wait      |            |Frag   |
not expected wnd |    |    Bitmap     |            +=======+
~~~~~~~~~~~~~~~~ +--->+               ++Retrans_Timer Exp  |          
    discard frag      +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
                         | |   | ^  ^  |reSend(empty)All-* |   
                         | |   | |  |  |Set Retrans_Timer  |
                         | |   | |  +--+Attempt++          |
MIC_bit==1 &             | |   | +-------------------------+
Recv_window==window &    | |   |   all missing frags sent
             no more frag| |   |   ~~~~~~~~~~~~~~~~~~~~~~
 ~~~~~~~~~~~~~~~~~~~~~~~~| |   |   Set Retrans_Timer                
       Stop Retrans_Timer| |   |    
 +=============+         | |   |
 |     END     +<--------+ |   |
 +=============+           |   | Attempt > MAX_ACK_REQUESTS
            All-1 Window & |   | ~~~~~~~~~~~~~~~~~~
             MIC_bit ==0 & |   v Send Abort
          lcl_bm==recv_bm  | +=+===========+
              ~~~~~~~~~~~~ +>|    ERROR    |
                Send Abort   +=============+

Figure 37: Sender State Machine for the ACK-Always Mode

 Not All- & w=expected +---+   +---+w = Not expected
 ~~~~~~~~~~~~~~~~~~~~~ |   |   |   |~~~~~~~~~~~~~~~~
 Set lcl_bm(FCN)       |   v   v   |discard
                      ++===+===+===+=+      
+---------------------+     Rcv      +--->* ABORT
|  +------------------+   Window     |
|  |                  +=====+==+=====+  
|  |       All-0 & w=expect |  ^ w =next & not-All
|  |     ~~~~~~~~~~~~~~~~~~ |  |~~~~~~~~~~~~~~~~~~~~~
|  |    set lcl_bm(FCN)     |  |expected = next window
|  |      send lcl_bm       |  |Clear lcl_bm
|  |                        |  |    
|  | w=expected & not-All   |  |
|  | ~~~~~~~~~~~~~~~~~~     |  |
|  |     set lcl_bm(FCN)+-+ |  | +--+ w=next & All-0
|  |     if lcl_bm full | | |  | |  | ~~~~~~~~~~~~~~~
|  |     send lcl_bm    | | |  | |  | expected = nxt wnd
|  |                    v | v  | |  | Clear lcl_bm
|  |w=expected& All-1 +=+=+=+==+=++ | set lcl_bm(FCN)
|  |  ~~~~~~~~~~~  +->+    Wait   +<+ send lcl_bm
|  |    discard    +--|    Next   |   
|  | All-0  +---------+  Window   +--->* ABORT  
|  | ~~~~~  +-------->+========+=++        
|  | snd lcl_bm  All-1 & w=next| |  All-1 & w=nxt
|  |                & MIC wrong| |  & MIC right      
|  |          ~~~~~~~~~~~~~~~~~| | ~~~~~~~~~~~~~~~~~~
|  |            set lcl_bm(FCN)| |set lcl_bm(FCN)
|  |                send lcl_bm| |send lcl_bm
|  |                           | +----------------------+
|  |All-1 & w=expected         |                        |
|  |& MIC wrong                v   +---+ w=expected &   |
|  |~~~~~~~~~~~~~~~~~~~~  +====+=====+ | MIC wrong      |
|  |set lcl_bm(FCN)       |          +<+ ~~~~~~~~~~~~~~ |
|  |send lcl_bm           | Wait End |   set lcl_bm(FCN)|
|  +--------------------->+          +--->* ABORT       |
|                         +===+====+=+-+ All-1&MIC wrong|
|                             |    ^   | ~~~~~~~~~~~~~~~|
|      w=expected & MIC right |    +---+   send lcl_bm  |
|      ~~~~~~~~~~~~~~~~~~~~~~ |                         |
|       set lcl_bm(FCN)       | +-+ Not All-1           |
|        send lcl_bm          | | | ~~~~~~~~~           |
|                             | | |  discard            |
|All-1&w=expected & MIC right | | |                     |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v +----+All-1         |
|set lcl_bm(FCN)            +=+=+=+=+==+ |~~~~~~~~~     |
|send lcl_bm                |          +<+Send lcl_bm   |
+-------------------------->+    END   |                |
                            +==========+<---------------+

       --->* ABORT
            ~~~~~~~
            Inactivity_Timer = expires
        When DWL
          IF Inactivity_Timer expires
             Send DWL Request
             Attempt++
                            

Figure 38: Receiver State Machine for the ACK-Always Mode

                  +=======+
                  |       |
                  | INIT  |
                  |       |       FCN!=0 & more frags
                  +======++       ~~~~~~~~~~~~~~~~~~~~~~
     Frag RuleID trigger |   +--+ Send cur_W + frag(FCN);
     ~~~~~~~~~~~~~~~~~~~ |   |  | FCN--;
  cur_W=0; FCN=max_value;|   |  | set [cur_W, cur_Bmp]
    clear [cur_W, Bmp_n];|   |  v
          clear rcv_Bmp  |  ++==+==========+         **BACK_TO_SEND
                         +->+              |     cur_W==rcv_W &
      **BACK_TO_SEND        |     SEND     |     [cur_W,Bmp_n]==rcv_Bmp
+-------------------------->+              |     & more frags
|  +----------------------->+              |     ~~~~~~~~~~~~
|  |                        ++===+=========+     cur_W++;
|  |      FCN==0 & more frags|   |last frag      clear [cur_W, Bmp_n]
|  |  ~~~~~~~~~~~~~~~~~~~~~~~|   |~~~~~~~~~
|  |        set cur_Bmp;     |   |set [cur_W, Bmp_n];
|  |send cur_W + frag(All-0);|   |send cur_W + frag(All-1)+MIC;
|  |        set Retrans_Timer|   |set Retrans_Timer
|  |                         |   | +-----------------------------------+
|  |Retrans_Timer expires &  |   | |cur_W==rcv_W&[cur_W,Bmp_n]!=rcv_Bmp|
|  |more Frags               |   | |  ~~~~~~~~~~~~~~~~~~~              |
|  |~~~~~~~~~~~~~~~~~~~~     |   | |  Attempts++; W=cur_W              |
|  |stop Retrans_Timer;      |   | | +--------+             rcv_W==Wn &|
|  |[cur_W,Bmp_n]==cur_Bmp;  v   v | |        v     [Wn,Bmp_n]!=rcv_Bmp|
|  |cur_W++            +=====+===+=+=+==+   +=+=========+   ~~~~~~~~~~~|
|  +-------------------+                |   | Resend    |   Attempts++;|
+----------------------+   Wait x ACK   |   | Missing   |         W=Wn |
+--------------------->+                |   | Frags(W)  +<-------------+
|         rcv_W==Wn &+-+                |   +======+====+
| [Wn,Bmp_n]!=rcv_Bmp| ++=+===+===+==+==+          |
|      ~~~~~~~~~~~~~~|  ^ |   |   |  ^             |
|        send (cur_W,+--+ |   |   |  +-------------+
|        ALL-0-empty)     |   |   |     all missing frag sent(W)
|                         |   |   |     ~~~~~~~~~~~~~~~~~
|  Retrans_Timer expires &|   |   |     set Retrans_Timer
|            No more Frags|   |   |
|           ~~~~~~~~~~~~~~|   |   |
|      stop Retrans_Timer;|   |   |
|(re)send frag(All-1)+MIC |   |   |
+-------------------------+   |   |
                 cur_W==rcv_W&|   |
       [cur_W,Bmp_n]==rcv_Bmp&|   | Attempts > MAX_ACK_REQUESTS
  No more Frags & MIC flag==OK|   | ~~~~~~~~~~
            ~~~~~~~~~~~~~~~~~~|   | send Abort
 +=========+stop Retrans_Timer|   |  +===========+
 |   END   +<-----------------+   +->+   ERROR   |
 +=========+                         +===========+

Figure 39: Sender State Machine for the ACK-on-Error Mode

This is an example only. The specification in Section 8.4.3.1 is open to very different sequencing of operations.

                 +=======+        New frag RuleID received
                 |       |        ~~~~~~~~~~~~~
                 | INIT  +-------+cur_W=0;clear([cur_W,Bmp_n]);
                 +=======+       |sync=0
                                 |
    Not All* & rcv_W==cur_W+---+ | +---+
      ~~~~~~~~~~~~~~~~~~~~ |   | | |  (E)
      set[cur_W,Bmp_n(FCN)]|   v v v   |
                          ++===+=+=+===+=+
   +----------------------+              +--+ All-0&Full[cur_W,Bmp_n]
   |           ABORT *<---+  Rcv Window  |  | ~~~~~~~~~~
   |  +-------------------+              +<-+ cur_W++;set Inact_timer;
   |  |                +->+=+=+=+=+=+====+    clear [cur_W,Bmp_n]
   |  | All-0 empty(Wn)|    | | | ^ ^
   |  | ~~~~~~~~~~~~~~ +----+ | | | |rcv_W==cur_W & sync==0;
   |  | sendACK([Wn,Bmp_n])   | | | |& Full([cur_W,Bmp_n])
   |  |                       | | | |& All* || last_miss_frag
   |  |                       | | | |~~~~~~~~~~~~~~~~~~~~~~
   |  |    All* & rcv_W==cur_W|(C)| |sendACK([cur_W,Bmp_n]);
   |  |              & sync==0| | | |cur_W++; clear([cur_W,Bmp_n])
   |  |&no_full([cur_W,Bmp_n])| |(E)|
   |  |      ~~~~~~~~~~~~~~~~ | | | |              +========+
   |  | sendACK([cur_W,Bmp_n])| | | |              | Error/ |
   |  |                       | | | |   +----+     | Abort  |
   |  |                       v v | |   |    |     +===+====+
   |  |                   +===+=+=+=+===+=+ (D)        ^
   |  |                +--+    Wait x     |  |         |
   |  | All-0 empty(Wn)+->| Missing Frags |<-+         |
   |  | ~~~~~~~~~~~~~~    +=============+=+            |
   |  | sendACK([Wn,Bmp_n])             +--------------+
   |  |                                       *ABORT
   v  v
  (A)(B)
                                    (D) All* || last_miss_frag
    (C) All* & sync>0                   & rcv_W!=cur_W & sync>0
        ~~~~~~~~~~~~                    & Full([rcv_W,Bmp_n])
        Wn=oldest[not full(W)];         ~~~~~~~~~~~~~~~~~~~~
        sendACK([Wn,Bmp_n])             Wn=oldest[not full(W)];
                                        sendACK([Wn,Bmp_n]);sync--

                              ABORT-->* Uplink Only &
                                        Inact_Timer expires
    (E) Not All* & rcv_W!=cur_W         || Attempts > MAX_ACK_REQUESTS
        ~~~~~~~~~~~~~~~~~~~~            ~~~~~~~~~~~~~~~~~~~~~
        sync++; cur_W=rcv_W;            send Abort
        set[cur_W,Bmp_n(FCN)]


  (A)(B)
   |  |
   |  | All-1 & rcv_W==cur_W & MIC!=OK        All-0 empty(Wn)
   |  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~     +-+  ~~~~~~~~~~
   |  | sendACK([cur_W,Bmp_n],MIC=0)     | v  sendACK([Wn,Bmp_n])
   |  |                      +===========+=++
   |  +--------------------->+   Wait End   +-+
   |                         +=====+=+====+=+ | All-1
   |     rcv_W==cur_W & MIC==OK    | |    ^   | & rcv_W==cur_W
   |     ~~~~~~~~~~~~~~~~~~~~~~    | |    +---+ & MIC!=OK
   |  sendACK([cur_W,Bmp_n],MIC=1) | |          ~~~~~~~~~~~~~~~~~~~
   |                               | | sendACK([cur_W,Bmp_n],MIC=0);
   |                               | |          Attempts++
   |All-1 & Full([cur_W,Bmp_n])    | |
   |& MIC==OK & sync==0            | +-->* ABORT
   |~~~~~~~~~~~~~~~~~~~            v
   |sendACK([cur_W,Bmp_n],MIC=1) +=+=========+
   +---------------------------->+    END    |
                                 +===========+


         ABORT -->* Uplink Only &
                    Inact_Timer = expires
                    || Attempts > MAX_ACK_REQUESTS
                    ~~~~~~~~~~~~~~~~~~~~~
                    send Abort

Figure 40: Receiver State Machine for the ACK-on-Error Mode

Appendix D. SCHC Parameters

This section lists the information that need to be provided in the LPWAN technology-specific documents.

This section lists the parameters that need to be defined in the Profile.

A Profile MAY define a delay to be added between each SCHC message transmission to respect local regulations or other constraints imposed by the applications.

Appendix E. Supporting multiple window sizes for fragmentation

For ACK-Always or ACK-on-Error, implementers MAY opt to support a single window size or multiple window sizes. The latter, when feasible, may provide performance optimizations. For example, a large window size SHOULD be used for packets that need to be carried by a large number of SCHC Fragments. However, when the number of SCHC Fragments required to carry a packet is low, a smaller window size, and thus a shorter Bitmap, MAY be sufficient to provide feedback on all SCHC Fragments. If multiple window sizes are supported, the Rule ID MAY be used to signal the window size in use for a specific packet transmission.

Note that the same window size MUST be used for the transmission of all SCHC Fragments that belong to the same SCHC Packet.

Appendix F. Downlink SCHC Fragment transmission

For downlink transmission of a fragmented SCHC Packet in ACK-Always mode, the SCHC Fragment receiver MAY support timer-based SCHC ACK retransmission. In this mechanism, the SCHC Fragment receiver initializes and starts a timer (the Inactivity Timer is used) after the transmission of a SCHC ACK, except when the SCHC ACK is sent in response to the last SCHC Fragment of a packet (All-1 fragment). In the latter case, the SCHC Fragment receiver does not start a timer after transmission of the SCHC ACK.

If, after transmission of a SCHC ACK that is not an All-1 fragment, and before expiration of the corresponding Inactivity timer, the SCHC Fragment receiver receives a SCHC Fragment that belongs to the current window (e.g. a missing SCHC Fragment from the current window) or to the next window, the Inactivity timer for the SCHC ACK is stopped. However, if the Inactivity timer expires, the SCHC ACK is resent and the Inactivity timer is reinitialized and restarted.

The default initial value for the Inactivity timer, as well as the maximum number of retries for a specific SCHC ACK, denoted MAX_ACK_RETRIES, are not defined in this document, and need to be defined in a Profile. The initial value of the Inactivity timer is expected to be greater than that of the Retransmission timer, in order to make sure that a (buffered) SCHC Fragment to be retransmitted can find an opportunity for that transmission.

When the SCHC Fragment sender transmits the All-1 fragment, it starts its Retransmission Timer with a large timeout value (e.g. several times that of the initial Inactivity timer). If a SCHC ACK is received before expiration of this timer, the SCHC Fragment sender retransmits any lost SCHC Fragments reported by the SCHC ACK, or if the SCHC ACK confirms successful reception of all SCHC Fragments of the last window, the transmission of the fragmented SCHC Packet is considered complete. If the timer expires, and no SCHC ACK has been received since the start of the timer, the SCHC Fragment sender assumes that the All-1 fragment has been successfully received (and possibly, the last SCHC ACK has been lost: this mechanism assumes that the retransmission timer for the All-1 fragment is long enough to allow several SCHC ACK retries if the All-1 fragment has not;been received by the SCHC Fragment receiver, and it also assumes that it is unlikely that several ACKs become all lost).

Appendix G. Note

Carles Gomez has been funded in part by the Spanish Government (Ministerio de Educacion, Cultura y Deporte) through the Jose Castillejo grant CAS15/00336, and by the ERDF and the Spanish Government through project TEC2016-79988-P. Part of his contribution to this work has been carried out during his stay as a visiting scholar at the Computer Laboratory of the University of Cambridge.

Authors' Addresses

Ana Minaburo Acklio 1137A avenue des Champs Blancs 35510 Cesson-Sevigne Cedex, France EMail: ana@ackl.io
Laurent Toutain IMT-Atlantique 2 rue de la Chataigneraie CS 17607 35576 Cesson-Sevigne Cedex, France EMail: Laurent.Toutain@imt-atlantique.fr
Carles Gomez Universitat Politècnica de Catalunya C/Esteve Terradas, 7 08860 Castelldefels Spain EMail: carlesgo@entel.upc.edu
Dominique Barthel Orange Labs 28 chemin du Vieux Chêne 38243 Meylan France EMail: dominique.barthel@orange.com
Juan Carlos Zuniga SIGFOX 425 rue Jean Rostand Labege 31670 France EMail: JuanCarlos.Zuniga@sigfox.com