LPWAN Static Context Header Compression (SCHC) and fragmentation for IPv6 and UDPAcklio2bis rue de la Chataigneraie35510 Cesson-Sevigne CedexFranceana@ackl.ioIMT-Atlantique2 rue de la ChataigneraieCS 1760735576 Cesson-Sevigne CedexFranceLaurent.Toutain@imt-atlantique.frUniversitat Politècnica de CatalunyaC/Esteve Terradas, 708860 CastelldefelsSpaincarlesgo@entel.upc.edulpwan Working GroupThis document defines the Static Context Header Compression (SCHC) framework, which provides header compression and
fragmentation functionality. SCHC has been tailored for Low Power Wide Area Networks (LPWAN).SCHC compression is based on a common static context stored in LPWAN devices and in the network. This document applies SCHC
compression to IPv6/UDP headers. This document also specifies a fragmentation and reassembly mechanism that is used to
support the IPv6 MTU requirement over LPWAN technologies. Fragmentation is mandatory for IPv6 datagrams that, after SCHC
compression or when it has not been possible to apply such compression, still exceed the layer two maximum payload size.The SCHC header compression mechanism is independent of the specific LPWAN technology over which it will be used.
Note that this document defines generic functionality. This document purposefully offers flexibility with regard to parameter
settings and mechanism choices, that are expected to be made in other, technology-specific, documents.This document defines a header compression scheme and fragmentation functionality, both specially tailored for Low Power Wide Area Networks (LPWAN).Header compression is needed to efficiently bring Internet connectivity to the node
within an LPWAN network. Some LPWAN networks properties can be exploited to get an efficient header compression:The topology is star-oriented which means that all packets follow the same path. For the necessity of this draft, the architecture is simple and is described as Devices (Dev) exchanging information with LPWAN Application Servers (App) through Network Gateways (NGW).The traffic flows can be known in advance since devices embed built-in applications. New applications cannot be easily installed in LPWAN devices, as they would in computers or smartphones.The Static Context Header Compression (SCHC) is defined for this environment.
SCHC uses a context, where header information is kept in the header format order. This context is
static: the values of the header fields do not change over time. This avoids
complex resynchronization mechanisms, that would be incompatible
with LPWAN characteristics. 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,
among others, by a very reduced data unit and/or payload size
. However, some of these technologies
do not provide fragmentation functionality, therefore the only option for them to support the IPv6 MTU requirement of 1280
bytes is to use a fragmentation protocol at the adaptation layer, below IPv6.
In response to this need, this document also defines a fragmentation/reassembly
mechanism, which supports the IPv6 MTU requirement over LPWAN
technologies. Such functionality has been designed under the assumption that data unit out-of-sequence delivery will not
happen between the entity performing fragmentation and the entity performing reassembly.Note that this document defines generic functionality and purposefully offers flexibility with regard to parameter settings
and mechanism choices, that are expected to be made in other, technology-specific documents.LPWAN technologies have similar network architectures but different
terminology. We can identify different types of entities in a
typical LPWAN network, see :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)This section defines the terminology and acronyms used in this document.Abort. A SCHC fragment format to signal the other end-point that the on-going fragment transmission is stopped and
finished.ACK (Acknowledgment). A SCHC fragment format used to report the success or unsuccess reception of a set of SCHC fragments.All-0. The SCHC fragment format for the last frame of a window that is not the last one of a packet (see Window in this
glossary).All-1. The SCHC fragment format for the last frame of the packet.All-0 empty. An All-0 SCHC fragment without a payload. It is used to request the ACK with the encoded Bitmap when the
Retransmission Timer expires, in a window that is not the last one of a packet.All-1 empty. An All-1 SCHC fragment without a payload. It is used to request the ACK with the encoded Bitmap when the
Retransmission Timer expires in the last window of a packet.App: LPWAN Application. An application sending/receiving IPv6 packets to/from the Device.APP-IID: Application Interface Identifier. Second part of the IPv6 address that identifies the application server
interface.Bi: Bidirectional, a rule entry that applies to headers of packets travelling in both directions (Up and Dw).Bitmap: a field of bits in an acknowledgment message that tells the sender which SCHC fragments of a window were correctly
received.C: Checked bit. Used in an acknowledgment (ACK) header to determine if the MIC locally computed by the receiver matches (1)
the received MIC or not (0).CDA: Compression/Decompression Action. Describes the reciprocal pair of actions that are performed at the compressor to
compress a header field and at the decompressor to recover the original header field value.Compress Residue. The bytes that need to be sent after applying the SCHC compression over each header fieldContext: A set of rules used to compress/decompress headers.Dev: Device. A node connected to the LPWAN. A Dev SHOULD implement SCHC.Dev-IID: Device Interface Identifier. Second part of the IPv6 address that identifies the device interface.DI: Direction Indicator. This field tells which direction of packet travel (Up, Dw or Bi) a rule applies to. This allows
for assymmetric processing.DTag: Datagram Tag. This SCHC fragmentation header field is set to the same value for all SCHC fragments carrying the same
IPv6 datagram.Dw: Dw: Downlink direction for compression/decompression in both sides, from SCHC C/D in the network to SCHC C/D in the
Dev.FCN: Fragment Compressed Number. This SCHC fragmentation header field carries an efficient representation of a larger-sized
fragment number.Field Description. A line in the Rule Table.FID: Field Identifier. This is an index to describe the header fields in a Rule.FL: Field Length is the length of the field in bits for fixed values or a type (variable, token length, …) for length
unknown at the rule creation. The length of a header field is defined in the specific protocol standard.FP: Field Position is a value that is used to identify the position where each instance of a field appears in the header.SCHC Fragment: A data unit that carries a subset of a SCHC packet. SCHC Fragmentation is needed when the size of a SCHC
packet exceeds the available payload size of the underlying L2 technology data unit.IID: Interface Identifier. See the IPv6 addressing architecture Inactivity Timer. A timer used after receiving a SCHC fragment to detect when there is an error and there is no possibility
to continue an on-going SCHC fragmented packet transmission.L2: Layer two. The immediate lower layer SCHC interfaces with. It is provided by an underlying LPWAN technology.MIC: Message Integrity Check. A SCHC fragmentation header field computed over an IPv6 packet before fragmentation, used
for error detection after IPv6 packet reassembly.MO: Matching Operator. An operator used to match a value contained in a header field with a value contained in a Rule.Retransmission Timer. A timer used by the SCHC fragment sender during an on-going SCHC fragmented packet transmission to
detect possible link errors when waiting for a possible incoming ACK.Rule: A set of header field values.Rule entry: A row in the rule that describes a header field.Rule ID: An identifier for a rule, SCHC C/D in both sides share the same Rule ID for a specific packet. A set of Rule IDs
are used to support SCHC fragmentation functionality.SCHC C/D: Static Context Header Compression Compressor/Decompressor. A mechanism used in both sides, at the Dev and at the
network to achieve Compression/Decompression of headers. SCHC C/D uses SCHC rules to perform compression and decompression.SCHC packet: A packet (e.g. an IPv6 packet) whose header has been compressed as per the header compression mechanism
defined in this document. If the header compression process is unable to actually compress the packet header, the packet
with the uncompressed header is still called a SCHC packet (in this case, a Rule ID is used to indicate that the packet
header has not been compressed).TV: Target value. A value contained in the Rule that will be matched with the value of a header field.Up: Uplink direction for compression/decompression in both sides, from the Dev SCHC C/D to the network SCHC C/D.W: Window bit. A SCHC fragment header field used in Window mode ({Frag}), which carries the same value for all
SCHC fragments of a window.Window: A subset of the SCHC fragments needed to carry a packet ({Frag}).SCHC can be abstracted as an adaptation layer below IPv6 and the underlying LPWAN technology. SCHC that comprises two
sublayers (i.e. the Compression sublayer and the Fragmentation sublayer), as shown in .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 actually be smaller than that of the original packet
or not) is called a SCHC packet. Subsequently, and if the SCHC packet size exceeds the layer 2 (L2) MTU, fragmentation is
then applied to the SCHC packet. This process is illustrated by Rule ID are identifiers used to select either the correct context to be used for Compression/Decompression functionalities or
for SCHC Fragmentation or after trying to do SCHC C/D and SCHC fragmentation the packet is sent as is. The size of the Rule
ID 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.The Rule IDs identifiers are:
* In the SCHC C/D context the Rule used to keep the Field Description of the header packet.In SCHC Fragmentation to identify the specific modes and settings. In bidirectional SCHC fragmentation at least two Rules
ID are needed.And at least one Rule ID MAY be reserved to the case where no SCHC C/D nor SCHC fragmentation were possible.In order to perform header compression, this document defines a mechanism called Static Context Header Compression (SCHC),
which is based on using context, i.e. a set of rules to compress or decompress headers. SCHC avoids context
synchronization, which is the most bandwidth-consuming operation in
other header compression mechanisms such as RoHC . Since the nature of packets are 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 either be learned by a provisioning protocol, by out of band means,
or they can be pre-provisioned. The way the contexts are provisioned on both ends is out of the scope of this document. The figure represents the architecture for SCHC (Static Context Header Compression) Compression / Fragmentation
where SCHC C/D (Compressor/Decompressor) and SCHC Fragmentation are performed. It is based on
terminology.
SCHC Compression / Fragmentation is located on both sides of the transmission in the Dev and in the Network side.
In the Uplink direction, the Device application packets use IPv6 or IPv6/UDP protocols. Before sending these packets, the Dev
compresses their headers using SCHC C/D and if the SCHC packet resulting from the compression exceeds the maximum payload
size of the underlying LPWAN technology, SCHC fragmentation is performed, see . The resulting SCHC fragments are sent
as one or more L2 frames to an LPWAN Radio Gateway (RG) which forwards the frame(s) to a Network Gateway (NGW).The NGW sends the data to an SCHC Fragmentation and then to the SCHC C/D for decompression. The SCHC C/D in the Network side
can be located in the Network Gateway (NGW) or somewhere else as long as a tunnel is established between the NGW and the SCHC
Compression / Fragmentation. Note that, for some LPWAN technologies, it MAY be suitable to locate SCHC fragmentation and
reassembly functionality nearer the NGW, in order to better deal with time constraints of such technologies.
The SCHC C/Ds on both sides MUST share the same set of Rules. After decompression, the packet can be sent over the Internet
to one or several LPWAN Application Servers (App).The SCHC Compression / Fragmentation process is symmetrical, therefore the same description applies to the reverse direction.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.
How Rules are generated is out of the scope of this document. The rule MAY be changed
but it will be specified in another document.The context contains a list of rules (cf. ). Each Rule
contains itself a list of Fields 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).The Rule does not describe how to delineate each field in the original packet header. This
MUST be known from the compressor/decompressor. The rule only describes the
compression/decompression behavior for each header field. In the rule, the Fields Descriptions are listed in the order in
which the fields appear in the packet header.The Rule also describes the Compression Residue sent regarding the order of the Fields Descriptions in the Rule.The Context describes the header fields and its values with the following entries:Field ID (FID) is a unique value to define the header field.Field Length (FL) represents the length of the field in bits for fixed values or a type (variable, token length, …) for Field
Description length unknown at the rule creation. The length of a header field is defined in the specific protocol standard.Field Position (FP): indicating if several instances of a field exist in the headers which one is targeted. The default
position is 1.A direction indicator (DI) indicating the packet direction(s) this Field Description applies to. Three values are possible: UPLINK (Up): this Field Description is only applicable to packets sent by the Dev to the App,DOWNLINK (Dw): this Field Description is only applicable to packets sent from the App to the Dev,BIDIRECTIONAL (Bi): this Field Description is applicable to packets travelling both Up and Dw.Target Value (TV) is the value used to make the match with the packet header field. The Target Value can be of any type
(integer, strings, etc.). For instance, it can be a single value or a more complex structure (array, list, etc.), such as a
JSON or a CBOR structure.Matching Operator (MO) is the operator used to match the Field Value and the Target Value. The Matching Operator may
require some parameters. MO is only used during the compression phase. The set of MOs defined in this document can be found
in .Compression Decompression Action (CDA) describes the compression and decompression processes to be performed after the MO
is applied. The CDA MAY require some parameters to be processed. CDAs are used in both the compression and the
decompression functions. The set of CDAs defined in this document can be found in .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 correlate the Rule ID with the Dev
identifier to find the appropriate Rule to be applied.The compression/decompression process follows several steps:Compression Rule selection: The goal is to identify which Rule(s) will be used to compress the packet’s headers. When
doing decompression, in the network side the SCHC C/D needs to find the correct Rule based on the L2 address and in this
way, it can use the Dev-ID and the Rule-ID. In the Dev side, only the Rule ID is needed to identify the correct Rule since
the Dev only holds Rules that apply to itself. The Rule will be selected by matching the Fields Descriptions to the packet
header as described below. When the selection of a Rule is done, this Rule is used to compress the header.
The detailed steps for compression Rule selection are the following: The first step is to choose the Fields Descriptions by their direction, using the direction indicator (DI). A Field
Description that does not correspond to the appropriate DI will be ignored, if all the fields of the packet do not have a
Field Description with the correct DI the Rule is discarded and SCHC C/D proceeds to explore the next Rule.When the DI has matched, then the next step is to identify the fields according to Field Position (FP). If the Field
Position does not correspond, the Rule is not used and the SCHC C/D proceeds to consider the next Rule.Once the DI and the FP correspond to the header information, each field’s value of the packet is then compared to the
corresponding Target Value (TV) stored in the Rule for that specific field using the matching operator (MO).If all the fields in the packet’s header satisfy all the matching operators (MO) of a Rule (i.e. all MO results are
True), the fields of the header are then compressed according to the Compression/Decompression Actions (CDAs) and a
compressed header (with possibly a Compressed Residue) SHOULD be obtained. Otherwise, the next Rule is tested.If no eligible Rule is found, then the header MUST be sent without compression, depending on the L2 PDU size, this is one
of the case that MAY require the use of the SCHC fragmentation process.Sending: If an eligible Rule is found, the Rule ID is sent to the other end followed by the Compression Residue (which
could be empty) and directly followed by the payload. The product of the Compression Residue is sent in the order expressed
in the Rule for all the fields.
The way the Rule ID is sent depends on the specific LPWAN layer two technology. For example, it can be either included in a
Layer 2 header or sent in the first byte of the L2 payload. (Cf. ). This process will be specified in the
LPWAN technology-specific document and is out of the scope of the present document. On LPWAN technologies that are byte-
oriented, the compressed header concatenated with the original packet payload is padded to a multiple of 8 bits, if needed.
See for details.Decompression: When doing decompression, in the network side the SCHC C/D needs to find the correct Rule based on the L2
address and in this way, it can use the Dev-ID and the Rule-ID. In the Dev side, only the Rule ID is needed to identify the
correct Rule since the Dev only holds Rules that apply to itself.
The receiver identifies the sender through its device-id (e.g. MAC address, if exists) and selects the appropriate Rule
from the Rule ID. If a source identifier is present in the L2 technology, it is used to select the Rule ID. This Rule
describes the compressed header format and associates the values to the header fields. The receiver applies the CDA action
to reconstruct the original header fields. The CDA application order can be different from the order given by the Rule. For
instance, Compute-* SHOULD be applied at the end, after all the other CDAs.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 indifferently 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:equal: The match result is True if a field value in a packet and the value in the TV are equal.ignore: No check is done between a field value in a packet and a TV
in the Rule. The result of the matching is always true.MSB(x): A match is obtained if the most significant x bits
of the field value in the header packet are equal to the TV in the Rule. The x parameter of the MSB Matching Operator
indicates how many bits are involved in the comparison.match-mapping: With match-mapping, the Target Value is a list of values. Each value of the list is identified by a short ID (or index). Compression is achieved by sending the index instead of the original header field value.
This operator matches if the header field value is equal to one of the values in the target list.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. summarizes the basic functions that can be used to compress and decompress
a field. The first column lists the actions name. The second and third
columns outline the reciprocal compression/decompression behavior for each action.Compression is done in order that Fields Descriptions appear in the Rule. The result of each Compression/Decompression Action is appended to the working 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.If the field is identified as being variable in the Field Description, then the size of the Compression Residue value in bytes MUST be sent first using the following coding:If the size is between 0 and 14 bytes, it is sent as a 4-bits integer.For values between 15 and 255, the first 4 bits sent are set to 1 and the size is sent using 8 bits integer.For higher values of size, the first 12 bits are set to 1 and the next two bytes contain the size value as a 16 bits integer.If a field does not exist in the packet but in the Rule and its FL is variable, the size zero MUST be used.The not-sent function is generally used when the field value is specified in the Rule and
therefore known by both the Compressor and the Decompressor. This action is generally used with the
“equal” MO. If MO is “ignore”, there is a risk to have a decompressed field
value different from the compressed field.The compressor does not send any value in the Compressed 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.The value-sent action is generally used when the field value is not known by both Compressor and Decompressor.
The value is sent 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 explicitly in the compression residue by indicating the length, as defined in . This function is generally
used with the “ignore” MO.The mapping-sent is used to send a smaller index (the index into
the Target Value list of values) instead of the original value. This function 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.The LSB(y) action is used together with the “MSB(x)” MO to avoid sending the higher part of the packet field if that part is
already known by the receiving end. A length can be specified in the rule to indicate
how many bits have to be sent. If the length is not specified, 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 combines the value received with the Target Value depending on the field type.If this action needs to be done on a variable length field, the size of the Compressed Residue in bytes MUST be sent as
described in .These functions 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 current LPWAN technologies
frames contain a single address, which is the Dev’s address.The IID value MAY be computed from the Device ID present in the Layer 2 header, or from some other stable identifier. The computation is specific for each LPWAN technology and MAY depend on the Device ID size.In the Downlink direction, these Deviid CDA is used to determine the L2 addresses used by the LPWAN.Some fields are elided during compression and reconstructed during decompression. This is the case for length and Checksum, so:compute-length: computes the length assigned to this field. This CDA MAY be used to compute IPv6 length or UDP
length.compute-checksum: computes a checksum from the information already received by the SCHC C/D.
This field MAY be used to compute UDP checksum.In LPWAN technologies, the L2 data unit size typically varies from tens to hundreds of bytes. The SCHC fragmentation MAY be
used either because after applying SCHC C/D or when SCHC C/D is not possible the entire SCHC packet still exceeds the L2 data
unit.The SCHC fragmentation functionality defined in this document has been designed under the assumption that data unit out-of-
sequence delivery will not happen between the entity performing fragmentation and the entity performing reassembly. This
assumption allows reducing the complexity and overhead of the SCHC fragmentation mechanism.To adapt the SCHC fragmentation to the capabilities of LPWAN technologies is required to enable optional SCHC fragment
retransmission and to allow a stepper delivery for the reliability of SCHC fragments. This document does not make any
decision with regard to which SCHC fragment delivery reliability mode will be used over a specific LPWAN technology. These
details will be defined in other technology-specific documents.This subsection describes the different tools that are used to enable the SCHC fragmentation functionality defined in this
document, such as fields in the SCHC fragmentation header frames (see the related formats in ), and the different
parameters supported in the reliability modes such as timers and parameters.Rule ID. The Rule ID is present in the SCHC fragment header and in the ACK header format. The Rule ID in a SCHC fragment
header is used to identify that a SCHC fragment is being carried, which SCHC fragmentation reliability mode is used and
which window size is used. The Rule ID in the SCHC fragmentation header also allows interleaving non-fragmented packets
and SCHC fragments that carry other SCHC packets. The Rule ID in an ACK identifies the message as an ACK.Fragment Compressed Number (FCN). The FCN is included in all SCHC fragments. This field can be understood as a truncated,
efficient representation of a larger-sized fragment number, and does not carry an absolute SCHC fragment number. There are
two FCN reserved values that are used for controlling the SCHC fragmentation process, as described next:
The FCN value with all the bits equal to 1 (All-1) denotes the last SCHC fragment of a packet. The last window of a
packet is called an All-1 window.The FCN value with all the bits equal to 0 (All-0) denotes the last SCHC fragment of a window that is not the last one of
the packet. Such a window is called an All-0 window.
The rest of the FCN values are assigned in a sequentially decreasing order, which has the purpose to avoid possible
ambiguity for the receiver that might arise under certain conditions. In the SCHC fragments, this field is an unsigned
integer, with a size of N bits. In the No-ACK mode, it is set to 1 bit (N=1), All-0 is used in all SCHC fragments and
All-1 for the last one.
For the other reliability modes, it is recommended to use a number of bits (N) equal to or greater than 3. Nevertheless,
the appropriate value of N MUST be defined in the corresponding technology-specific profile documents. For windows that are
not the last one from a SCHC fragmented packet, the FCN for the last SCHC fragment in such windows is an All-0. This
indicates that the window is finished and communication proceeds according to the reliability mode in use. The FCN for the
last SCHC fragment in the last window is an All-1, indicating the last SCHC fragment of the SCHC packet. It is also
important to note that, in the No-ACK mode or when N=1, the last SCHC fragment of the packet will carry a FCN equal to 1,
while all previous SCHC fragments will carry a FCN of 0. For further details see . The highest FCN in the
window, denoted MAX_WIND_FCN, MUST be a value equal to or smaller than 2^N-2. (Example for N=5, MAX_WIND_FCN MAY be set to
23, then subsequent FCNs are set sequentially and in decreasing order, and the FCN will wrap from 0 back to 23).Datagram Tag (DTag). The DTag field, if present, is set to the same value for all SCHC fragments carrying the same SCHC
packet, and to different values for different datagrams. Using this field, the sender can interleave fragments from
different SCHC packets, while the receiver can still tell them apart.
In the SCHC fragment formats, the size of the DTag field is T bits, which MAY be set to a value greater than or equal to 0
bits. For each new SCHC packet processed by the sender, DTag MUST be sequentially increased, from 0 to 2^T – 1 wrapping
back from 2^T - 1 to 0.
In the ACK format, DTag carries the same value as the DTag field in the SCHC fragments for which this ACK is intended.W (window): W is a 1-bit field. This field carries the same value for all SCHC fragments of a window, and it is
complemented for the next window. The initial value for this field is 0.
In the ACK format, this field also has a size of 1 bit. In all ACKs, the W bit carries the same value as the W bit carried
by the SCHC fragments whose reception is being positively or negatively acknowledged by the ACK.Message Integrity Check (MIC). This field, which has a size of M bits, is computed by the sender over the complete SCHC
packet before SCHC fragmentation. The MIC allows the receiver to check errors in the reassembled packet, while it also
enables compressing the UDP checksum by use of SCHC compression. The CRC32 as 0xEDB88320 (i.e. the reverse representation
of the polynomial used e.g. in the Ethernet standard ) is recommended as the default algorithm for computing the
MIC. Nevertheless, other algorithms MAY be required and are defined in the technology-specific documents.C (MIC checked): C is a 1-bit field. This field is used in the ACK packets to report the outcome of the MIC check, i.e.
whether the reassembled packet was correctly received or not. A value of 1 represents a positive MIC check at the receiver
side (i.e. the MIC computed by the receiver matches the received MIC).Retransmission Timer. A SCHC fragment sender uses it after the transmission of a window to detect a transmission error of
the ACK corresponding to this window. Depending on the reliability mode, it will lead to a request an ACK
retransmission (in ACK-Always mode) or it will trigger the transmission of the next window (in ACK-on-Error mode). The
duration of this timer is not defined in this document and MUST be defined in the corresponding technology documents.Inactivity Timer. A SCHC fragment receiver uses it to take action when there is a problem in the transmission of SCHC
fragments. Such a problem could be detected by the receiver not getting a single SCHC fragment during a given period of
time or not getting a given number of packets in a given period of time. When this happens, an Abort message will be sent
(see related text later in this section). Initially, and each time a SCHC fragment is received, the timer is reinitialized.
The duration of this timer is not defined in this document and MUST be defined in the specific technology document.Attempts. This counter counts the requests for a missing ACK. When it reaches the value MAX_ACK_REQUESTS,
the sender assume there are recurrent SCHC fragment transmission errors and determines that an Abort is needed. The default
value offered MAX_ACK_REQUESTS is not stated in this document, and it is expected to be defined in the specific technology
document. The Attempts counter is defined per window. It is initialized each time a new window is used.Bitmap. The Bitmap is a sequence of bits carried in an ACK. Each bit in the Bitmap corresponds to a SCHC
fragment of the current window, and provides feedback on whether the SCHC fragment has been received or not. The right-most
position on the Bitmap reports if the All-0 or All-1 fragment has been received or not. Feedback on the SCHC
fragment with the highest FCN value is provided by the bit in the left-most position of the Bitmap. In the Bitmap, a bit
set to 1 indicates that the SCHC fragment of FCN corresponding to that bit position has been correctly sent and received.
The text above describes the internal representation of the Bitmap. When inserted in the ACK for transmission from the
receiver to the sender, the Bitmap MAY be truncated for energy/bandwidth optimisation, see more details in .Abort. On expiration of the Inactivity timer, or when Attempts reached MAX_ACK_REQUESTS or upon an occurrence of some other
error, the sender or the receiver MUST use the Abort. When the receiver needs to abort the on-going SCHC fragmented
packet transmission, it sends the Receiver-Abort format. When the sender needs to abort the transmission, it sends the
Sender-Abort format. None of the Abort are acknowledged.Padding (P). If it is needed, the number of bits used for padding is not defined and depends on the size of the Rule ID,
DTag and FCN fields, and on the L2 payload size (see ). Some ACKs are byte-aligned and do not need padding (see
).This specification defines three reliability modes: No-ACK, ACK-Always and ACK-on-Error. ACK-Always and ACK-on-Error operate
on windows of SCHC fragments. A window of SCHC fragments is a subset of the full set of SCHC fragments needed to carry a
packet or an SCHC packet.No-ACK. No-ACK is the simplest SCHC fragment reliability mode. The receiver does not generate overhead in the form of
acknowledgments (ACKs). However, this mode does not enhance reliability beyond that offered by the underlying LPWAN
technology. In the No-ACK mode, the receiver MUST NOT issue ACKs. See further details in .ACK-Always. The ACK-Always mode provides flow control using a window scheme. This mode is also able to handle long bursts
of lost SCHC fragments since detection of such events can be done before the end of the SCHC packet transmission as long as
the window size is short enough. However, such benefit comes at the expense of ACK use. In ACK-Always the receiver sends an
ACK after a window of SCHC fragments has been received, where a window of SCHC fragments is a subset of the whole number of
SCHC fragments needed to carry a complete SCHC packet. The ACK is used to inform the sender if a SCHC fragment in the
actual window has been lost or well received. Upon an ACK reception, the sender retransmits the lost SCHC fragments. When
an ACK is lost and the sender has not received it before the expiration of the Inactivity Timer, the sender uses an ACK
request by sending the All-1 empty SCHC fragment. The maximum number of ACK requests is MAX_ACK_REQUESTS. If the
MAX_ACK_REQUEST is reached the transmission needs to be Aborted. See further details in .ACK-on-Error. The ACK-on-Error mode is suitable for links offering relatively low L2 data unit loss probability. In this
mode, the SCHC fragment receiver reduces the number of ACKs transmitted, which MAY be especially beneficial in asymmetric
scenarios. Because the SCHC fragments use the uplink of the underlying LPWAN technology, which has higher capacity than
downlink. The receiver transmits an ACK only after the complete window transmission and if at least one SCHC fragment of
this window has been lost. An exception to this behavior is in the last window, where the receiver MUST transmit an ACK,
including the C bit set based on the MIC checked result, even if all the SCHC fragments of the last window have been
correctly received.
The ACK gives the state of all the SCHC fragments (received or lost). Upon an ACK reception, the sender retransmits the
lost SCHC fragments. If an ACK is not transmitted back by the receiver at the end of a window, the sender assumes that all
SCHC fragments have been correctly received.
When the ACK is lost, the sender assumes that all SCHC fragments covered by the lost ACK have been successfully delivered,
so the sender continues transmitting the next window of SCHC fragments. If the next SCHC fragments received belong to the
next window, the receiver will abort the on-going fragmented packet transmission. See further details in {{ACK-on-Error-
subsection}}.The same reliability mode MUST be used for all SCHC fragments of an SCHC packet. The decision on which reliability mode will
be used and whether the same reliability mode applies to all SCHC packets is an implementation problem and is out of the
scope of this document.Note that the reliability mode choice is not necessarily tied to a particular characteristic of the underlying L2 LPWAN
technology, e.g. the No-ACK mode MAY be used on top of an L2 LPWAN technology with symmetric characteristics for uplink and
downlink. This document does not make any decision as to which SCHC fragment reliability mode(s) are supported by a specific
LPWAN technology.Examples of the different reliability modes described are provided in Appendix B.This section defines the SCHC fragment format, the All-0 and All-1 formats, the ACK format and the Abort formats.A SCHC fragment comprises a SCHC fragment header, a SCHC fragment payload and padding bits (if needed). A SCHC fragment
conforms to the general format shown in . The SCHC fragment payload carries a subset of SCHC packet.
A SCHC fragment is the payload of the L2 protocol data unit (PDU). Padding MAY be added in SCHC fragments and in ACKs if
necessary, therefore a padding field is optional (this is explicitly indicated in for the sake of
illustration clarity.In ACK-Always or ACK-on-Error, SCHC fragments except the last one SHALL conform the detailed format defined in {{Fig-
NotLastWin}}. The total size of the fragment header is R bits. Where is R is not a multiple of 8 bits.In the No-ACK mode, SCHC fragments except the last one SHALL conform to the detailed format defined in . The total size of the fragment header is R bits.In all these cases, R may not be a multiple of 8 bits.The All-0 format is used for sending the last SCHC fragment of a window that is not the last window of the packet.The All-0 empty fragment format is used by a sender to request the retransmission of an ACK by the receiver. It is only used in ACK-Always mode.In the No-ACK mode, the last SCHC fragment of an IPv6 datagram SHALL contain a SCHC fragment header that conforms to
the detaield format shown in . The total size of this SCHC fragment header is R+M bits.In any of the Window modes, the last fragment of an IPv6 datagram SHALL contain a SCHC fragment header that conforms to
the detailed format shown in . The total size of the SCHC fragment
header in this format is R+M bits.In either ACK-Always or ACK-on-Error, in order to request a retransmission of the ACK for the All-1 window, the fragment
sender uses the format shown in . The total size of the SCHC fragment header in this format is R+M bits.The values for R, N, T and M are not specified in this document, and SHOULD be determined in other documents (e.g. technology-specific profile documents).The format of an ACK that acknowledges a window that is not the last one (denoted as All-0 window) is shown in .To acknowledge the last window of a packet (denoted as All-1 window), a C bit (i.e. MIC checked) following the W bit is set
to 1 to indicate that the MIC check computed by the receiver matches the MIC present in the All-1 fragment. If the MIC check fails, the C bit is set to 0 and the Bitmap for the All-1 window follows.The Bitmap is transmitted by a receiver as part of the ACK format. An ACK message MAY include padding at the end to align its
number of transmitted bits to a multiple of 8 bits.Note that the ACK sent in response to an All-1 fragment includes the C bit. Therefore, the window size and thus the encoded
Bitmap size need to be determined taking into account the available space in the layer two frame payload, where there will be
1 bit less for an ACK sent in response to an All-1 fragment than in other ACKs. Note that the maximum number of SCHC
fragments of the last window is one unit smaller than that of the previous windows.When the receiver transmits an encoded Bitmap with a SCHC fragment that has not been sent during the transmission, the sender
will Abort the transmission.In order to reduce the resulting frame size, the encoded Bitmap is shortened by applying the following algorithm: all the
right-most contiguous bytes in the encoded Bitmap that have all their bits set to 1 MUST NOT be transmitted. Because the
SCHC fragment sender knows the actual Bitmap size, it can reconstruct the original Bitmap with the trailing 1 bit optimized
away. In the example shown in , the last 2 bytes of the Bitmap shown in
comprise bits that are all set to 1, therefore they are not sent. shows an example of an ACK with FCN ranging from 6 down to 0, where the Bitmap
indicates that the second and the fifth SCHC fragments have not been correctly received. shows an example of an ACK with FCN ranging from 6 down to 0, where the Bitmap indicates that the MIC check has failed but there are no missing SCHC fragments.Abort are coded as exceptions to the previous coding, a specific format is defined for each direction. When a SCHC fragment
sender needs to abort the transmission, it sends the Sender-Abort format , that is an All-1 fragment with no
MIC or payload. In regular cases All-1 fragment contains at least a MIC value. This absence of the MIC value indicates an
Abort.When a SCHC fragment receiver needs to abort the on-going SCHC fragmented packet transmission, it transmits the Receiver-
Abort format , creating an exception in the encoded Bitmap coding. Encoded Bitmap avoid sending the rigth
most bits of the Bitmap set to 1. Abort is coded as an ACK message with a Bitmap set to 1 until the byte boundary, followed
by an extra 0xFF byte. Such message never occurs in a regular acknowledgement and is view as an abort.None of these messages are not acknowledged nor retransmitted.The sender uses the Sender-Abort when the MAX_ACK_REQUEST is reached. The receiver uses the Receiver-Abort when the
Inactivity timer expires, or in the ACK-on-Error mode, ACK is lost and the sender transmits SCHC fragments of a new window.
Some other cases for Abort are explained in the or .If after applying SCHC header compression (or when SCHC header compression is not possible) the SCHC packet does not fit
within the payload of a single L2 data unit, the SCHC packet SHALL be broken into SCHC fragments and the fragments SHALL be
sent to the fragment receiver.
The fragment receiver needs to identify all the SCHC fragments that belong to a given SCHC packet. To this end, the receiver
SHALL use:The sender’s L2 source address (if present),The destination’s L2 address (if present),Rule ID,DTag (if present).Then, the fragment receiver MAY determine the SCHC fragment reliability mode that is used for this SCHC fragment based on the
Rule ID in that fragment.After a SCHC fragment reception, the receiver starts constructing the SCHC packet. It uses the FCN and the arrival order of
each SCHC fragment to determine the location of the individual fragments within the SCHC packet. For example, the receiver
MAY place the fragment payload within a payload datagram reassembly buffer at the location determined from the FCN, the
arrival order of the SCHC fragments, and the fragment payload sizes. In Window mode, the fragment receiver also uses the W
bit in the received SCHC fragments. Note that the size of the original, unfragmented packet cannot be determined from
fragmentation headers.Fragmentation functionality uses the FCN value to transmit the SCHC fragments. It has a length of N bits where the All-1 and
All-0 FCN values are used to control the fragmentation transmission. The rest of the FCN numbers MUST be assigned
sequentially in a decreasing order, the first FCN of a window is RECOMMENDED to be MAX_WIND_FCN, i.e. the highest possible
FCN value depending on the FCN number of bits.In all modes, the last SCHC fragment of a packet MUST contain a MIC which is used to check if there are errors or missing
SCHC fragments and MUST use the corresponding All-1 fragment format. Note that a SCHC fragment with an All-0 format is
considered the last SCHC fragment of the current window.If the receiver receives the last fragment of a datagram (All-1), it checks for the integrity of the reassembled datagram,
based on the MIC received. In No-ACK, if the integrity check indicates that the reassembled datagram does not match the
original datagram (prior to fragmentation), the reassembled datagram MUST be discarded. In Window mode, a MIC check is also
performed by the fragment receiver after reception of each subsequent SCHC fragment retransmitted after the first MIC check.There are three reliability modes: No-ACK, ACK-Always and ACK-on-Error. In ACK-Always and ACK-on-Error, a jumping window
protocol uses two windows alternatively, identified as 0 and 1. A SCHC fragment with all FCN bits set to 0 (i.e. an All-0
fragment) indicates that the window is over (i.e. the SCHC fragment is the last one of the window) and allows to switch from
one window to the next one. The All-1 FCN in a SCHC fragment indicates that it is the last fragment of the packet being
transmitted and therefore there will not be another window for this packet.In the No-ACK mode, there is no feedback communication from the fragment receiver. The sender will send all the SCHC
fragments of a packet without any possibility of knowing if errors or losses have occurred. As, in this mode, there is no
need to identify specific SCHC fragments, a one-bit FCN MAY be used. Consequently, the FCN All-0 value is used in all SCHC
fragments except the last one, which carries an All-1 FCN and the MIC.
The receiver will wait for SCHC fragments and will set the Inactivity timer. The receiver will use the MIC contained in the
last SCHC fragment to check for errors.
When the Inactivity Timer expires or if the MIC check indicates that the reassembled packet does not match the original one,
the receiver will release all resources allocated to reassembling this packet. The initial value of the Inactivity Timer will
be determined based on the characteristics of the underlying LPWAN technology and will be defined in other documents (e.g.
technology-specific profile documents).In ACK-Always, the sender transmits SCHC fragments by using the two-jumping-windows procedure. A delay between each SCHC
fragment can be added to respect local regulations or other constraints imposed by the applications. Each time a SCHC
fragment is sent, the FCN is decreased by one. When the FCN reaches value 0 and there are more SCHC fragments to be sent
after, the sender transmits the last SCHC fragment of this window using the All-0 fragment format, it starts the
Retransmission Timer and waits for an ACK. On the other hand, if the FCN has reached 0 and the SCHC fragment to be
transmitted is the last SCHC fragment of the SCHC packet, the sender uses the All-1 fragment format, which includes a MIC.
The sender sets the Retransmission Timer and waits for the ACK to know if transmission errors have occured.The Retransmission Timer is dimensioned based on the LPWAN technology in use. When the Retransmission Timer expires, the
sender sends an All-0 empty (resp. All-1 empty) fragment to request again the ACK for the window that ended with the All-0
(resp. All-1) fragment just sent. The window number is not changed.After receiving an All-0 or All-1 fragment, the receiver sends an ACK with an encoded Bitmap reporting whether any SCHC
fragments have been lost or not. When the sender receives an ACK, it checks the W bit carried by the ACK. Any ACK carrying an
unexpected W bit value is discarded. If the W bit value of the received ACK is correct, the sender analyzes the rest of the
ACK message, such as the encoded Bitmap and the MIC. If all the SCHC fragments sent for this window have been well received,
and if at least one more SCHC fragment needs to be sent, the sender advances its sending window to the next window value and
sends the next SCHC fragments. If no more SCHC fragments have to be sent, then the SCHC fragmented packet transmission is
finished.However, if one or more SCHC fragments have not been received as per the ACK (i.e. the corresponding bits are not set in the
encoded Bitmap) then the sender resends the missing SCHC fragments. When all missing SCHC fragments have been retransmitted,
the sender starts the Retransmission Timer, even if an All-0 or an All-1 has not been sent as part of this retransmission and
waits for an ACK. Upon receipt of the ACK, if one or more SCHC fragments have not yet been received, the counter Attempts is
increased and the sender resends the missing SCHC fragments again. When Attempts reaches MAX_ACK_REQUESTS, the sender aborts
the on-going SCHC fragmented packet transmission by sending an Abort message and releases any resources for transmission of
the packet. The sender also aborts an on-going SCHC fragmented packet transmission when a failed MIC check is reported by the
receiver or when a SCHC fragment that has not been sent is reported in the encoded Bitmap.On the other hand, at the beginning, the receiver side expects to receive window 0. Any SCHC fragment received but not
belonging to the current window is discarded. All SCHC fragments belonging to the correct window are accepted, and the
actual SCHC fragment number managed by the receiver is computed based on the FCN value. The receiver prepares the encoded
Bitmap to report the correctly received and the missing SCHC fragments for the current window. After each SCHC fragment is
received the receiver initializes the Inactivity timer, if the Inactivity Timer expires the transmission is aborted.When an All-0 fragment is received, it indicates that all the SCHC fragments have been sent in the current window. Since the
sender is not obliged to always send a full window, some SCHC fragment number not set in the receiver memory SHOULD not
correspond to losses. The receiver sends the corresponding ACK, the Inactivity Timer is set and the transmission of the next
window by the sender can start.If an All-0 fragment has been received and all SCHC fragments of the current window have also been received, the receiver
then expects a new Window and waits for the next SCHC fragment. Upon receipt of a SCHC fragment, if the window value has not
changed, the received SCHC fragments are part of a retransmission. A receiver that has already received a SCHC fragment
SHOULD discard it, otherwise, it updates the encoded Bitmap. If all the bits of the encoded Bitmap are set to one, the
receiver MUST send an ACK without waiting for an All-0 fragment and the Inactivity Timer is initialized.On the other hand, if the window value of the next received SCHC fragment is set to the next expected window value, this
means that the sender has received a correct encoded Bitmap reporting that all SCHC fragments have been received. The
receiver then updates the value of the next expected window.When an All-1 fragment is received, it indicates that the last SCHC fragment of the packet has been sent. Since the last
window is not always full, the MIC will be used to detect if all SCHC fragments of the packet have been received. A correct
MIC indicates the end of the transmission but the receiver MUST stay alive for an Inactivity Timer period to answer to any
empty All-1 fragments the sender MAY send if ACKs sent by the receiver are lost. If the MIC is incorrect, some SCHC fragments
have been lost. The receiver sends the ACK regardless of successful SCHC fragmented packet reception or not, the
Inactitivity Timer is set. In case of an incorrect MIC, the receiver waits for SCHC fragments belonging to the same window.
After MAX_ACK_REQUESTS, the receiver will abort the on-going SCHC fragmented packet transmission by transmitting a the
Receiver-Abort format. The receiver also aborts upon Inactivity Timer expiration.The senders behavior for ACK-on-Error and ACK-Always are similar. The main difference is that in ACK-on-Error the ACK with
the encoded Bitmap is not sent at the end of each window but only when at least one SCHC fragment of the current window has
been lost. Excepts for the last window where an ACK MUST be sent to finish the transmission.In ACK-on-Error, the Retransmission Timer expiration will be considered as a positive acknowledgment. This timer is set after
sending an All-0 or an All-1 fragment. When the All-1 fragment has been sent, then the on-going SCHC fragmentation process is
finished and the sender waits for the last ACK. If the Retransmission Timer expires while waiting for the ACK for the last
window, an All-1 empty MUST be sent to request the last ACK by the sender to complete the SCHC fragmented packet
transmission. When it expires the sender continue sending SCHC fragments of the next window.If the sender receives an ACK, it checks the window value. ACKs with an unexpected window number are discarded. If the
window number on the received encoded Bitmap is correct, the sender verifies if the receiver has received all SCHC fragments
of the current window. When at least one SCHC fragment has been lost, the counter Attempts is increased by one and the
sender resends the missing SCHC fragments again. When Attempts reaches MAX_ACK_REQUESTS, the sender sends an Abort message
and releases all resources for the on-going SCHC fragmented packet transmission. When the retransmission of the missing SCHC
fragments is finished, the sender starts listening for an ACK (even if an All-0 or an All-1 has not been sent during the
retransmission) and initializes the Retransmission Timer. After sending an All-1 fragment, the sender listens for an ACK,
initializes Attempts, and starts the Retransmission Timer. If the Retransmission Timer expires, Attempts is increased by one
and an empty All-1 fragment is sent to request the ACK for the last window. If Attempts reaches MAX_ACK_REQUESTS, the sender
aborts the on-going SCHC fragmented packet transmission by transmitting the Sender-Abort fragment.Unlike the sender, the receiver for ACK-on-Error has a larger amount of differences compared with ACK-Always. First, an ACK
is not sent unless there is a lost SCHC fragment or an unexpected behavior. With the exception of the last window, where an
ACK is always sent regardless of SCHC fragment losses or not.
The receiver starts by expecting SCHC fragments from window 0 and maintains the information regarding which SCHC fragments it
receives. After receiving an SCHC fragment, the Inactivity Timer is set. If no further SCHC fragment are received and the
Inactivity Timer expires, the SCHC fragment receiver aborts the on-going SCHC fragmented packet transmission by transmitting
the Receiver-Abort data unit.Any SCHC fragment not belonging to the current window is discarded. The actual SCHC fragment number is computed based on the
FCN value. When an All-0 fragment is received and all SCHC fragments have been received, the receiver updates the expected
window value and expects a new window and waits for the next SCHC fragment.
If the window value of the next SCHC fragment has not changed, the received SCHC fragment is a retransmission. A receiver
that has already received an SCHC fragment discard it. If all SCHC fragments of a window (that is not the last one) have been
received, the receiver does not send an ACK. While the receiver waits for the next window and if the window value is set to
the next value, and if an All-1 fragment with the next value window arrived the receiver knows that the last SCHC fragment of
the packet has been sent. Since the last window is not always full, the MIC will be used to detect if all SCHC fragments of
the window have been received. A correct MIC check indicates the end of the SCHC fragmented packet transmission. An ACK is
sent by the SCHC fragment receiver. In case of an incorrect MIC, the receiver waits for SCHC fragments belonging to the same
window or the expiration of the Inactivity Timer. The latter will lead the receiver to abort the on-going SCHC fragmented
packet transmission.If after receiving an All-0 fragment the receiver missed some SCHC fragments, the receiver uses an ACK with the encoded
Bitmap to ask the retransmission of the missing fragments and expect to receive SCHC fragments with the actual window. While
waiting the retransmission an All-0 empty fragment is received, the receiver sends again the ACK with the encoded Bitmap, if
the SCHC fragments received belongs to another window or an All-1 fragment is received, the transmission is aborted by
sending a Receiver-Abort fragment. Once it has received all the missing fragments it waits for the next window fragments.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.In some LPWAN technologies, as part of energy-saving techniques, downlink transmission is only possible immediately after an
uplink transmission. In order to avoid potentially high delay in the downlink transmission of a SCHC fragmented datagram, the
SCHC fragment receiver MAY perform an uplink transmission as soon as possible after reception of a SCHC fragment that is not
the last one. Such uplink transmission MAY be triggered by the L2 (e.g. an L2 ACK sent in response to a SCHC fragment
encapsulated in a L2 frame that requires an L2 ACK) or it MAY be triggered from an upper layer.For downlink transmission of a SCHC fragmented packet in ACK-Always mode, the SCHC fragment receiver MAY support timer-based
ACKretransmission. In this mechanism, the SCHC fragment receiver initializes and starts a timer (the Inactivity Timer is
used) after the transmission of an ACK, except when the 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 ACK.If, after transmission of an 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 ACK is stopped. However, if the Inactivity timer expires,
the 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 ACK, denoted
MAX_ACK_RETRIES, are not defined in this document, and need to be defined in other documents (e.g. technology-specific
profiles). 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 an ACK is received before expiration of this timer, the SCHC
fragment sender retransmits any lost SCHC fragments reported by the ACK, or if the ACK confirms successful reception of all
SCHC fragments of the last window, the transmission of the SCHC fragmented packet is considered complete. If the timer
expires, and no 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 ACK has been lost: this mechanism assumes that the retransmission
timer for the All-1 fragment is long enough to allow several 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).Default padding is defined for L2 frame with a variable length of bytes. Padding is done twice, after compression and in the
all-1 fragmentation.In compression, the rule and the compression residues are not aligned on a byte, but payload following the residue is always
a multiple of 8 bits. In that case, padding bits can be added after the payload to reach the first byte boundary. Since the
rule and the residue give the length of the SCHC header and payload is always a multiple of 8 bits, the receiver can without
ambiguity remove the padding bits which never excide 7 bits.SCHC fragmentation works on a byte aligned (i.e. padded SCHC packet). Fragmentation header may not be aligned on byte
boundary, but each fragment except the last one (All-1 fragment) must sent the maximum bits as possible. Only the last
fragment need to introduce padding to reach the next boundary limit. Since the SCHC is known to be a multiple of 8 bits, the
receiver can remove the extra bit to reach this limit.Default padding mechanism do not need to send the padding length and can lead to a maximum of 14 bits of padding.This section lists the different IPv6 and UDP header fields and how they can be compressed.This field always holds the same value. Therefore, in the rule, TV is set to 6, MO to “equal”
and CDA to “not-sent”.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, two possibilities can be considered depending on the variability of the value:One possibility is to not compress the field and send the original value. In the rule, TV is not set to any particular value, MO is set to “ignore” and CDA is set to “value-sent”.If some upper bits in the field are constant and known, a better option is to only send the LSBs. In the rule, TV is set to a value with the stable known upper part, MO is set to MSB(x) and CDA to LSB(y).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:One possibility is to not compress the field and send the original value. In the rule, TV is not set to any particular value, MO is set to “ignore” and CDA is set to “value-sent”.If some upper bits in the field are constant and known, a better option is to only send the LSBs. In the rule, TV is set to a value with the stable known upper part, MO is set to MSB(x) and CDA to LSB(y).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(s).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.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:in the Uplink, elide the field: the TV in the Field Descriptor is set to the known constant value, the MO
is set to “equal” and the CDA is set to “not-sent”.in the Downlink, send the value: TV is not set, MO is set to “ignore” and
CDA is set to “value-sent”.As in 6LoWPAN , 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 frame (source or destination).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 Otherwise, the TV contains the prefix, the MO is set to “equal” and the CDA is set to
“value-sent”.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 CDF is set to “not-sent”.
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”.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.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 frame (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”.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”.IPv6 mandates a checksum in the protocol above IP. Nevertheless, if a more efficient
mechanism such as L2 CRC or MIC is carried by or over the L2 (such as in the
LPWAN SCHC fragmentation process (see )), the UDP checksum transmission can be avoided.
In that case, the TV is not set, the MO is set to “ignore” and the CDA is set to
“compute-checksum”.In other cases, the checksum SHOULD be explicitly sent. The TV is not set, the MO is set to
“ignore” and the CDF is set to “value-sent”.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).This subsection describes potential attacks to LPWAN SCHC fragmentation
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 SCHC fragmented 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 splitted 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 Window mode – 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.Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier, Eduardo Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav,
Sergio Lopez Bernal, Antony Markovski, Alexander Pelov, Pascal Thubert, Juan Carlos Zuniga, Diego Dujovne, Edgar Ramos, and
Shoichi Sakane for useful design consideration and comments.Transmission of IPv6 Packets over IEEE 802.15.4 NetworksThis document describes the frame format for transmission of IPv6 packets and the method of forming IPv6 link-local addresses and statelessly autoconfigured addresses on IEEE 802.15.4 networks. Additional specifications include a simple header compression scheme using shared context and provisions for packet delivery in IEEE 802.15.4 meshes. [STANDARDS-TRACK]Internet Protocol, Version 6 (IPv6) SpecificationThis document specifies version 6 of the Internet Protocol (IPv6), also sometimes referred to as IP Next Generation or IPng. [STANDARDS-TRACK]Internet Protocol Small Computer System Interface (iSCSI) Cyclic Redundancy Check (CRC)/Checksum ConsiderationsSignificance of IPv6 Interface IdentifiersThe IPv6 addressing architecture includes a unicast interface identifier that is used in the creation of many IPv6 addresses. Interface identifiers are formed by a variety of methods. This document clarifies that the bits in an interface identifier have no meaning and that the entire identifier should be treated as an opaque value. In particular, RFC 4291 defines a method by which the Universal and Group bits of an IEEE link-layer address are mapped into an IPv6 unicast interface identifier. This document clarifies that those two bits are significant only in the process of deriving interface identifiers from an IEEE link-layer address, and it updates RFC 4291 accordingly.The RObust Header Compression (ROHC) FrameworkThe Robust Header Compression (ROHC) protocol provides an efficient, flexible, and future-proof header compression concept. It is designed to operate efficiently and robustly over various link technologies with different characteristics.The ROHC framework, along with a set of compression profiles, was initially defined in RFC 3095. To improve and simplify the ROHC specifications, this document explicitly defines the ROHC framework and the profile for uncompressed separately. More specifically, the definition of the framework does not modify or update the definition of the framework specified by RFC 3095.This specification obsoletes RFC 4995. It fixes one interoperability issue that was erroneously introduced in RFC 4995, and adds some minor clarifications. [STANDARDS-TRACK]A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)This document specifies a method for generating IPv6 Interface Identifiers to be used with IPv6 Stateless Address Autoconfiguration (SLAAC), such that an IPv6 address configured using this method is stable within each subnet, but the corresponding Interface Identifier changes when the host moves from one network to another. This method is meant to be an alternative to generating Interface Identifiers based on hardware addresses (e.g., IEEE LAN Media Access Control (MAC) addresses), such that the benefits of stable addresses can be achieved without sacrificing the security and privacy of users. The method specified in this document applies to all prefixes a host may be employing, including link-local, global, and unique-local prefixes (and their corresponding addresses).LPWAN OverviewLow Power Wide Area Networks (LPWAN) are wireless technologies with characteristics such as large coverage areas, low bandwidth, possibly very small packet and application layer data sizes and long battery life operation. This memo is an informational overview of the set of LPWAN technologies being considered in the IETF and of the gaps that exist between the needs of those technologies and the goal of running IP in LPWANs.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. presents the protocol stack for this Device. IPv6 and UDP are represented
with dotted lines since these protocols are compressed on the radio link.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.All the fields described in the three rules depicted on 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.This section provides examples for the different fragment reliability modes specified in this document. illustrates the transmission in No-ACK mode of an IPv6 packet that needs 11 fragments. FCN is 1 bit wide.In the following examples, N (i.e. the size if the FCN field) is 3 bits. Therefore, the All-1 FCN value is 7. illustrates the transmission in ACK-on-Error of an IPv6 packet that needs 11 fragments, with MAX_WIND_FCN=6 and no fragment loss. illustrates the transmission in ACK-on-Error mode of an IPv6 packet that needs 11
fragments, with MAX_WIND_FCN=6 and three lost fragments. illustrates the transmission in ACK-Always mode of an IPv6 packet that needs 11 fragments, with MAX_WIND_FCN=6 and no loss. illustrates the transmission in ACK-Always mode of an IPv6 packet that needs 11
fragments, with MAX_WIND_FCN=6 and three lost fragments. illustrates the transmission in ACK-Always mode of an IPv6 packet that needs 6
fragments, with MAX_WIND_FCN=6, three lost fragments and only one retry needed to recover each lost fragment. illustrates the transmission in ACK-Always mode of an IPv6 packet that needs 6
fragments, with MAX_WIND_FCN=6, three lost fragments, and the second ACK lost. illustrates the transmission in ACK-Always mode of an IPv6 packet that needs 6
fragments, with MAX_WIND_FCN=6, with three lost fragments, and one retransmitted fragment lost again. illustrates the transmission in ACK-Always mode of an IPv6 packet that needs 28 fragments, with
N=5, MAX_WIND_FCN=23 and two lost fragments. Note that MAX_WIND_FCN=23 may be useful when the maximum possible Bitmap size,
considering the maximum lower layer technology payload size and the value of R, is 3 bytes. Note also that the FCN of the
last fragment of the packet is the one with FCN=31 (i.e. FCN=2^N-1 for N=5, or equivalently, all FCN bits set to 1).The fragmentation state machines of the sender and the receiver, one for each of the different reliability modes, are
described in the following figures: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.