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 describes a header compression scheme and fragmentation functionality
for very low bandwidth networks. These techniques are specially tailored for Low Power Wide Area Network (LPWAN).The Static Context Header Compression (SCHC) offers a great level of flexibility
when processing the header fields. SCHC compression is based on a common static context stored in a LPWAN device and in the network. Static context means that the stored information does not change during packet transmission. The context describes the field values and keeps information that will not be transmitted through the constrained network.SCHC must be used for LPWAN networks because it avoids complex resynchronization mechanisms, which are incompatible
with LPWAN characteristics. And also, because with SCHC, in most cases IPv6/UDP headers can be reduced
to a small identifier called Rule ID. Even though, sometimes, a SCHC compressed packet will not fit in one L2 PDU, and the SCHC fragmentation protocol defined in this document may be used.This document describes the SCHC compression/decompression framework and applies it
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 L2 maximum payload size. Similar solutions for other protocols such as CoAP will be described in separate documents.Header compression is mandatory to efficiently bring Internet connectivity to the node
within a LPWAN network. Some LPWAN networks properties can be exploited to get an efficient header
compression:Topology is star-oriented; therefore, all the packets follow the same path.
For the needs of this draft, the architecture can be summarized to Devices (Dev)
exchanging information with LPWAN Application Server (App) through a Network Gateway (NGW).Traffic flows are mostly known in advance since devices embed built-in
applications. Contrary to computers or smartphones, new applications cannot
be easily installed.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) avoiding
complex resynchronization mechanisms, incompatible
with LPWAN characteristics. In most of the cases, IPv6/UDP headers are reduced
to a small context identifier.The SCHC header compression mechanism is independent of the specific LPWAN technology over which it will be used.LPWAN technologies are also characterized,
among others, by a very reduced data unit and/or payload size
. However, some of these technologies
do not support layer two fragmentation, therefore the only option for
them to support the IPv6 MTU requirement of 1280 bytes
is the use of a fragmentation protocol at the
adaptation layer below IPv6.
This draft defines also a fragmentation
functionality to support the IPv6 MTU requirement over LPWAN
technologies. Such functionality has been designed under the assumption that data unit reordering will not happen between the entity performing fragmentation and the entity performing reassembly.LPWAN technologies have similar 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 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.All-0. Fragmentation Packet format to send the last frame of a window.All-1. Fragmentation Packet format to send the last frame of a packet.All-0 empty. Fragmentation Packet format without payload to request the bitmap when the Retransmission Timer expires in a window.All-1 empty. Fragmentation Packet format without payload to request the bitmap when the Retransmission Timer expires in the last window.App: LPWAN Application. An application sending/receiving IPv6 packets to/from the Device.APP-IID: Application Interface Identifier. Second part of the IPv6 address to identify the application interfaceBi: Bidirectional, a rule entry that applies in both directions.C: Checked bit. Used in fragmentation header to determine when the MIC is correct (1) or not (0).CDA: Compression/Decompression Action. An action that is performed for both functionalities to compress a header field or to recover its original value in the decompression phase.Context: A set of rules used to compress/decompress headersDev: Device. A Node connected to the LPWAN. A Dev may implement SCHC.Dev-IID: Device Interface Identifier. Second part of the IPv6 address to identify the device interfaceDI: Direction Indicator is a differentiator for matching in order to be able to have different values for both sides.DTag: Datagram Tag is a fragmentation header field that is set to the same value for all fragments carrying the same IPv6 datagram.Dw: Down Link direction for compression, from SCHC C/D to DevFCN: Fragment Compressed Number is a fragmentation header field that carries an efficient representation of a larger-sized fragment number.FID: Field Identifier is an index to describe the header fields in the RuleFL: Field Length is a value to identify if the field is fixed or variable length.FP: Field Position is a value that is used to identify each instance a field appears in the header.IID: Interface Identifier. See the IPv6 addressing architecture Inactivity Timer. Timer to End the state machine when there is an error and there is no possibility to continue the transmission.MIC: Message Integrity Check. A 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. Timer used in the sender transmission to detect error in the link when waiting for an 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, and Dev share the same Rule ID for a specific flow. A set of Rule IDs are used to support fragmentation functionality.SCHC C/D: Static Context Header Compression Compressor/Decompressor. A process in the network to achieve compression/decompressing headers. SCHC C/D uses SCHC rules to perform compression and decompression.TV: Target value. A value contained in the Rule that will be matched with the value of a header field.Up: Up Link direction for compression, from Dev to SCHC C/D.W: Window bit. A fragmentation header field used in Window mode (see section 9), which carries the same value for all fragments of a window.Static Context Header Compression (SCHC) avoids context synchronization,
which is the most bandwidth-consuming operation in other header compression mechanisms
such as RoHC . Based on the fact
that the nature of data flows is highly predictable in LPWAN networks, some static
contexts may be stored on the Device (Dev). The contexts must be stored in both ends, and it can
either be learned by a provisioning protocol or by out of band means or it can be pre-provisioned, etc.
The way the context is learned on both sides are out of the scope of this document. represents the architecture for compression/decompression, it is based on
terminology. The Device is sending applications flows using IPv6 or IPv6/UDP protocols. These flows are compressed by a
Static Context Header Compression Compressor/Decompressor (SCHC C/D) to reduce headers size. The resulting
information is sent to a layer two (L2) frame to a LPWAN Radio Network (RG) which forwards
the frame to a Network Gateway (NGW).
The NGW sends the data to an SCHC C/D for decompression which shares the same rules with the Dev. The SCHC C/D can be
located on the Network Gateway (NGW) or in another place as long as a tunnel is established between the NGW and the SCHC C/D.
The SCHC C/D in both sides must share the same set of Rules.
After decompression, the packet can be sent on the Internet to one
or several LPWAN Application Servers (App).The SCHC C/D process is bidirectional, so the same principles can be applied in the other direction.The main idea of the SCHC compression scheme is to send the Rule id to the other end instead
of sending known field values. This Rule id identifies a rule that matches as much as possible the original
packet values. When a value is known by both
ends, it is not necessary to send it through the LPWAN network.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 the original packet format which
must be known from the compressor/decompressor. The rule just describes the
compression/decompression behavior for the header fields. In the rule, the description of the header field should be performed in the format packet order.The Rule also describes the compressed header fields which are transmitted regarding their position
in the rule which is used for data serialization on the compressor side and data deserialization on the decompressor side.The Context describes the header fields and its values with the following entries:A Field ID (FID) is a unique value to define the header field.A Field Length (FL) is the length of the field that can be of fixed length as in IPv6 or UDP headers or variable
length as in CoAP options. Fixed length fields shall be represented by its actual value in bits. Variable length fields shall be represented by a function or a variable.A Field Position (FP) indicating if several instances of the field exist in the
headers which one is targeted. The default position is 1A direction indicator (DI) indicating the packet direction. Three values are possible: UPLINK (Up) when the field or the value is only present in packets sent by the Dev to the App,DOWNLINK (Dw) when the field or the value is only present in packet sent from the App to the Dev andBIDIRECTIONAL (Bi) when the field or the value is present either upstream or downstream.A Target Value (TV) is the value used to make the comparison 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.A Matching Operator (MO) is the operator used to make the comparison between
the Field Value and the Target Value. The Matching Operator may require some
parameters. MO is only used during the compression phase.A Compression Decompression Action (CDA) is used to describe the compression
and the decompression process. The CDA may require some
parameters, CDA are used in both compression and decompression phases.Rule IDs are sent between both compression/decompression elements. The size
of the Rule ID is not specified in this document, it is implementation-specific and can vary regarding the
LPWAN technology, the number of flows, among others.Some values in the Rule ID space are reserved for other functionalities than header
compression as fragmentation. (See ).Rule IDs are specific to a Dev. Two Devs may use the same Rule ID for different
header compression. To identify the correct Rule ID, the SCHC C/D needs to combine the Rule ID with the Dev L2 identifier
to find the appropriate Rule.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 compression in the NGW side the SCHC C/D needs to find the
correct Rule to be used by identifying its Dev-ID and the Rule-ID. In the Dev, only the Rule-ID may be used.
The next step is to choose the fields by their direction, using the
direction indicator (DI), so the fields that do not correspond to the appropriated DI will be excluded.
Next, then the fields are identified according to their field identifier (FID) and field position (FP).
If the field position does not correspond, then the Rule is not used and the SCHC take next Rule.
Once the DI and the FP correspond to the header information, each field’s value 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 (MOs) of a Rule (i.e. all results are True),
the fields of the header are then processed according to the Compression/Decompression Actions (CDAs)
and a compressed header is obtained. Otherwise, the next rule is tested.
If no eligible rule is found, then the header must be sent without compression, in which case the fragmentation process
must be required.sending: The Rule ID is sent to the other end followed by the information resulting
from the compression of header fields, directly followed by the payload.
The product of field compression is sent in the order expressed in the Rule for the matching
fields. The way the Rule ID is sent depends on the specific LPWAN
layer two technology and will be specified in a specific document and is out of the scope of this document.
For example, it can be either included in a Layer 2 header or sent in the first byte of
the L2 payload. (Cf. ).decompression: In both directions, the receiver identifies the sender through its device-id
(e.g. MAC address) and selects the appropriate Rule through the Rule ID. This
Rule gives the compressed header format and associates these values to the header fields.
It 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-* may be applied at the end, after all the other CDAs.
If after using SCHC compression and adding the payload to the L2 frame the datagram is not multiple of 8 bits,
padding may be used.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 indifferently 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: A field value in a packet matches with a TV in a Rule if they 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(length): A matching is obtained if the most significant bits of the length field value bits
of the header are equal to the TV in the rule. The MSB Matching Operator needs a parameter,
indicating the number of bits, to proceed to the matching.match-mapping: The goal of mapping-sent is to reduce the size of a field by allocating
a shorter value. The Target Value contains a list of values. Each value is identified by a short ID (or index).
This operator matches if a field value is equal to one of those target values.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 basics functions defined to compress and decompress
a field. The first column gives the action’s name. The second and third
columns outline the compression/decompression behavior.Compression is done in the rule order and compressed values are sent in that order in the compressed
message. The receiver must be able to find 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, then its size must be sent first using the following coding:If the size is between 0 and 14 bytes it is sent using 4 bits.For values between 15 and 255, the first 4 bits sent are set to 1 and the size is sent using 8 bits.For higher value, the first 12 bits are set to 1 and the size is sent on 2 bytes.The not-sent function is generally used when the field value is specified in the rule and
therefore known by the both Compressor and 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 header for the field on which compression is applied.The decompressor restores the field value with the target value stored in the matched rule.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 compressed header
field by indicating the length. This function is generally used with the “ignore” MO.The mapping-sent is used to send a smaller index associated with the list of values
in the Target Value. This function is used together with the “match-mapping” MO.The compressor looks on the TV to find the field value and send the corresponding index.
The decompressor uses this index to restore the field value.The number of bits sent is the minimal size for coding all the possible indexes.LSB action is used to avoid sending the known part of the packet field header to the other end.
This action is used together with the “MSB” MO. 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
field length minus the bits’ length specified in the MSB MO.The compressor sends the “length” Least Significant Bits. The decompressor
combines the value received with the Target Value.If this action is made on a variable length field, the remaining size in byte has to be sent before.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.The IID value may be computed from the Device ID present in the Layer 2 header. The
computation is specific for each LPWAN technology and may depend on the Device ID size.In the downstream direction, these CDA may be used to determine the L2 addresses used by the LPWAN.These classes of functions are used by the decompressor to compute the compressed field value based on received information.
Compressed fields are elided during compression and reconstructed during decompression.compute-length: compute the length assigned to this field. For instance, regarding
the field ID, this CDA may be used to compute IPv6 length or UDP length.compute-checksum: compute 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. If the entire IPv6 datagram after applying SCHC header compression or when SCHC header compression is not possible, fits within a single L2 data unit, the fragmentation mechanism is not used and the packet is sent. Otherwise, the datagram SHALL be broken into fragments.LPWAN technologies impose some strict limitations on traffic, devices are sleeping most of the time and may receive data during a short period of time after transmission to preserve battery. To adapt the SCHC fragmentation to the capabilities of LPWAN technologies, it is desirable to enable optional fragment retransmission and to allow a gradation of fragment delivery reliability. This document does not make any decision with regard to which fragment delivery reliability option(s) will be used over a specific LPWAN technology.An important consideration is that LPWAN networks typically follow
the star topology, and therefore data unit reordering is not expected
in such networks. This specification assumes that reordering will
not happen between the entity performing fragmentation and the entity
performing reassembly. This assumption allows to reduce complexity
and overhead of the fragmentation mechanism.This subsection describes the different fields in the fragmentation header frames (see the fragmentation frames format in ) that are used to enable the fragmentation functionalities defined in this document, and the different reliability options supported.Rule ID. The Rule ID is present in the fragmentation header and in the ACK header format. The Rule ID is a fragmentation header is used to identify that a fragment is being carried, the fragmentation delivery reliability option used and it may indicate the window size in use (if any). The Rule ID in the fragmentation header also allows to interleave non-fragmented IPv6 datagrams with fragments that carry a larger IPv6 datagram. The Rule ID in an ACK allows to identify that the message is an ACK.Fragment Compressed Number (FCN). The FCN is included in all fragments. This field can be understood as a truncated,
efficient representation of a larger-sized fragment number, and does not carry an absolute fragment number. There are two FCN reserved values that are used for controlling the fragmentation process, as described next. The FCN value with all the bits equal to 1 (All-1) denotes the last
fragment of a packet. And the FCN value with all the bits equal to 0 (All-0) denotes the last
fragment of a window (when such window is not the last one of the packet) in any window mode or the fragments in No ACK mode. The rest of the FCN values are assigned in a sequential
and decreasing order, which has the purpose to avoid possible ambiguity for the receiver that might arise under certain
conditions.
In the 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). For the other reliability options, it is recommended to use a number of bits (N) equal to or greater than 3. Nevertheless, the apropriate value will be defined in the corresponding technology documents. The FCN MUST be set sequentially decreasing from the highest FCN in the window (which will be used for the first fragment), and MUST wrap from 0 back to the highest FCN in the window.
For windows that are not the last one from a fragmented packet, the FCN for the last fragment in such windows is an All-0. This indicates that the window is finished and communication proceeds according to the reliability option in use.
The FCN for the last fragment in the last window is an All-1. It is also
important to note that, for No ACK mode or N=1, the last fragment of the packet will carry a FCN equal to 1, while all previous fragments
will carry a FCN of 0.Datagram Tag (DTag). The DTag field, if present, is set to the same value for all fragments carrying the same IPv6 datagram. This field allows to interleave fragments that correspond to different IPv6 datagrams.
In the 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. DTag MUST be set sequentially increasing from 0 to 2^T - 1, and MUST wrap back from 2^T - 1 to 0.
In the ACK format, DTag carries the same value as the DTag field in the fragments for which this ACK is intended.W (window): W is a 1-bit field. This field carries the same value for all 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 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 packet (i.e. a SCHC compressed or an uncompressed IPv6 packet) before 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 is recommended as the default algorithm for computing the MIC. Nevertheless, other algorithm MAY be mandated in the corresponding technology documents (e.g. technology-specific profiles).C (MIC checked): C is a 1-bit field. This field is used in the ACK format packets to report the outcome of the MIC check, i.e. whether the reassembled packet was correctly received or not.Retransmission Timer. It is used by a fragment sender after the transmission of a window to detect a transmission error of the ACK corresponding to this window. Depending on the reliability option, it will lead to a request for an ACK retransmission on ACK-Always or it will trigger the next window on ACK-on-error. The dureation of this timer is not defined in this document and must be defined in the corresponding technology documents (e.g. technology-specific profiles).Inactivity Timer. This timer is used by a fragment receiver to detect when there is a problem in the transmission of fragments and the receiver does not get any fragment during a period of time or a number of packets in a period of time. When this happens, an Abort message needs to be sent. Initially, and each time a fragment is received the timer is reinitialized. The duration of this timer timer is not defined in this document and must be defined in the specific technology document (e.g. technology-specific profiles).Attempts. It is a counter used to request a missing ACK, and in consequence to determine when an Abort is needed, because there are recurrent fragment transmission errors, whose maximum value is MAX_ACK_REQUESTS. The default value of MAX_ACK_REQUESTS is not
stated in this document, and it is expected to be defined in other
documents (e.g. technology-specific profiles).Bitmap. The Bitmap is a sequence of bits carried in an ACK for a given window. Each bit in the Bitmap corresponds to a
fragment of the current window, and provides feedback on whether the fragment has been received or not. The right-most
position on the Bitmap is used to report whether the All-0 or All-1 fragments have been received or not. Feedback for a
fragment with the highest FCN value
is provided by the left-most position in the Bitmap. In the Bitmap, a bit set to 1 indicates that the corresponding FCN
fragment has been correctly sent and received. However, the sending format of the bitmap will be truncated until a byte
boundary where the last error is given. However, when all the Bitmap is transmitted, it may be truncated, see more details
in Abort. In case of error or when the Inactivity timer expires or the MAX_ACK_REQUESTS is reached the sender or the receiver may use the Abort frames. When the receiver needs to abort the on-going fragmented packet transmission, it uses the ACK Abort format packet with all the bits set to 1. The sender will use the All-1 Abort format to trigger the end of the transmission.Padding (P). Padding will be used to align the last byte of a fragment with a byte boundary. 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 layer two payload size.This specification defines the following three fragment delivery reliability options:No ACK.
No ACK is the simplest fragment delivery reliability option. The receiver does not generate overhead in the form of acknowledgments (ACKs). However, this option does not enhance delivery reliability beyond that offered by the underlying LPWAN technology. In the No ACK option, the receiver MUST NOT issue ACKs.Window mode - ACK always (ACK-always).
The ACK-always option provides flow control. In
addition, this option is able to handle long bursts of lost fragments, since
detection of such events can be done before the end of the IPv6 packet
transmission, as long as the window size is short enough. However,
such benefit comes at the expense of ACK use.
In ACK-always, an ACK is transmitted by the fragment
receiver after a window of fragments has been sent. A window of
fragments is a subset of the full set of fragments needed to carry an
IPv6 packet. In this mode, the ACK informs the sender about received
and/or missed fragments from the window of fragments. Upon receipt
of an ACK that informs about any lost fragments, the sender
retransmits the lost fragments. When an ACK is not received by the
fragment sender, the latter sends an ACK request using the All-1 empty fragment.
The maximum number of ACK requests is MAX_ACK_REQUESTS.Window mode - ACK-on-error (ACK-on-error).
The ACK-on-error option is suitable for links offering relatively low L2
data unit loss probability. This option reduces the number of ACKs
transmitted by the fragment receiver. This may be especially beneficial in asymmetric
scenarios, e.g. where fragmented data are sent uplink and the
underlying LPWAN technology downlink capacity or message rate is
lower than the uplink one.
In ACK-on-error, an ACK is transmitted by the fragment
receiver after a window of fragments have been sent, only if at least
one of the fragments in the window has been lost. In this mode, the
ACK informs the sender about received and/or missed fragments from
the window of fragments. Upon receipt of an ACK that informs about
any lost fragments, the sender retransmits the lost fragments. However, if an ACK is lost, the sender
assumes that all fragments covered by the ACK have been successfully
delivered. And the receiver will abort the on-going fragmented packet transmission. One exception to this behavior is in the last window, whete the receiver MUST transmit an ACK, even if all the fragments in the last window have been correctly received.The same reliability option MUST be used for all fragments of a
packet. It is up to implementers and/or representatives of the
underlying LPWAN technology to decide which reliability option to use
and whether the same reliability option applies to all IPv6 packets
or not. Note that the reliability option to be used is not
necessarily tied to the particular characteristics of the underlying
L2 LPWAN technology (e.g. the No ACK reliability option 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 fragment
delivery reliability option(s) are supported by a specific LPWAN
technology.Examples of the different reliability options described are provided
in Appendix B.This section defines the fragment format, the All-0 and All-1 frames format, the ACK format and the Abort frames format.A fragment comprises a fragmentation header, a fragment payload, and Padding bits (if any). A fragment conforms
to the format shown in . The fragment payload carries a subset of either a SCHC header
or an IPv6 header or the original IPv6 packet data payload.
A fragment is the payload in the L2 protocol data unit (PDU).In the No ACK option, fragments except the last one SHALL contain the format as defined in . The total size of the fragmentation header is R bits.In any of the Window mode options, fragments except the last one SHALL contain the fragmentation format as defined in . The total size of this fragmentation header is R bits.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 All-0 format is used for the last 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 an ACK in ACK-Always modeIn the No ACK option, the last fragment of an IPv6 datagram SHALL contain a fragmentation header that conforms to
the format shown in . The total size of this fragmentation
header is R+M bits.In any of the Window modes, the last fragment of an IPv6 datagram SHALL contain a fragmentation header that conforms to
the format shown in . The total size of the fragmentation
header in this format is R+M bits. It is used for request a retransmissionIn 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 fragmentation header in this format is R+M bits.The values for R, N, T and M are not specified in this document, and have to be determined in other documents (e.g. technology-specific profile documents).The All-1 Abort format and the ACK abort have the following formats.The fragment receiver needs to identify all the fragments that belong to a given IPv6 datagram. To this end, the receiver SHALL use:The sender’s L2 source address (if present),The destination’s L2 address (if present),Rule ID andDTag (the latter, if present).Then, the fragment receiver may determine the fragment delivery reliability option that is used for this fragment based on the Rule ID field in that fragment.Upon receipt of a link fragment, the receiver starts constructing the original unfragmented packet. It uses the FCN and the order of arrival of each fragment to determine the location of the individual fragments within the original unfragmented packet. A fragment payload may carry bytes from a SCHC compressed IPv6 header, an uncompressed IPv6 header or an IPv6 datagram data payload. An unfragmented packet could be a SCHC compressed or an uncompressed IPv6 packet (header and data). 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 fragments, and the fragment payload sizes. In Window mode, the fragment receiver also uses the W bit in the received fragments. Note that the size of the original, unfragmented packet cannot be determined from fragmentation headers.Fragmentation functionality uses the FCN value, which has a length of N bits. The All-1 and All-0 FCN values are used to control the fragmentation transmission. The FCN will be assigned sequentially in a decreasing order starting from 2^N-2, i.e. the highest possible FCN value depending on the FCN number of bits, but excluding the All-1 value. In all modes, the last fragment of a packet must contains a MIC which is used to check if there are errors or missing fragments, and must use the corresponding All-1 fragment format. Also note that, a fragment with an All-0 format is considered the last fragment of the current window.If the recipient 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 fragment retransmitted after the first MIC check.In the No ACK mode there is no feedback communication from the fragment receiver. The sender will send the fragments of a packet until the last one without any possibility to know if errors or a losses have occurred. As in this mode there is not a need to identify specific fragments a one-bit FCN is used, therefore FCN All-0 will be used in all fragments except the last one. The latter will carry an All-1 FCN and will also carry the MIC.
The receiver will wait for fragments and will set the Inactivity timer. The No ACK mode will use the MIC contained in the last fragment to check error.
When the Inactivity Timer expires or when the MIC check indicates that the reassembled packet does not match the originall one, the receiver will release all resources allocated to reassembly of the 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 Window modes, a jumping window protocol is using two windows alternatively, 0 and 1.
An FCN set to All-0 indicates that the window is over (i.e. the fragment is the last one of the window) and allows to switch from one window to another. The All-1 FCN in a fragment indicates that it is the last fragment of the packet and therefore there will not be another window for the packet.The Window mode offers two different reliability options modes: ACK-on-error and ACK-always.The sender starts sending fragments using the two windows procedure. A delay between each fragment can be added to respect
regulation rules or constraints imposed by the applications. Each time a fragment is sent the FCN is decreased by one and
the sending information is set locally. When the FCN reaches value 0 and there are more fragments to be sent, an All-0
fragment is sent and the retransmission timer is set. The sender waits for an ACK to know if there were some transmission
errors. If there are some errors the receiver sends an ACK with the corresponding errors in the Bitmap, otherwise, an ACK
without Bitmap will be sent and a new window should be sent. When the last fragment is sent, and All-1 fragment with MIC is
sent. The sender sets the retransmission timer to wait for the ACK corresponding to the last window.
During this period, the sender starts listening to the radio and starts an Inactivity timer, which is dimensioned based on
the received window available for the LPWAN technology in use. If the Inactivity timer expires an empty All-0 (or All-1 if
the last fragment has been sent) fragment is sent to ask the receiver to resend its ACK. The window number is not changed.When the sender receives an ACK, it checks the window value. The ACK fragments carrying an unexpected W bit are discarded.
If the window number of the received ACK is correct, the sender compares the sending information
with the received Bitmap. If the sending information is equal to the received Bitmap all the fragments sent during the
window have been well received. If at least one fragment needs to be sent, the sender moves its sending window to the next
value and sends the last fragment. If no more fragments have to be sent, then the fragmented packet transmission is
finished.If some fragments are missing (not set in the Bitmap) then the sender resends the missing fragments. When the
retransmission is finished, it starts the retransmission timer (even if an All-0 or an All-1 has not been sent during the
retransmission) and waits for ACK.If the sending information is different from the received Bitmap the counter Attempts is increased and the sender resends
the missing fragments again when a MAX_ACK_REQUESTS is reached, the sender sends an Abort and drops the fragments. The
sender Aborts the transmission when a corrupt MIC has been received or when MAX_ACK_REQUESTS has reached.At the beginning, the receiver side expects to receive window 0. Any fragment not belonging to the current window is
discarded. All Fragments belonging to the correct window are accepted, the fragment number is computed based on the FCN
value. The receiver updates the Bitmap with the correct received fragments.When All-0 fragment is received, it indicates that all the fragments have been sent in the current window. Since the sender
is not obliged to send a full window, some fragment number not set in the memory may not correspond to losses. It sends the
corresponding ACK and the next window can start.When All-1 fragment is received, it indicates that the transmission is finished. Since the last window is not full, the MIC
will be used to detect if all the fragments have been received. A correct MIC indicates the end of the transmission but the
receiver must stay alive an Inactivity timer period to answer to empty All-1 fragment the sender may send if the ACK is lost.If All-1 fragment has not been received, the receiver expects a new window. It waits for
the next fragment. If the window value has not changed, the received fragments are
part of a retransmission. A receiver that has already received a fragment should discard it, otherwise, it updates the
Bitmap. If all the bits of the Bitmap are set to one, the receiver may send an ACK without waiting for
an All-0 fragment.If the window value is set to the next value, this means that the sender has received
a correct bitmap, which means that all the fragments have been received. The receiver
changes the value of the expected window.If the receiver receives an All-0 fragment, the sender may send one or more fragments per window. Otherwise, some fragments
in the window have been lost.If the receiver receives an All-1 fragment this means that the transmission should be finished. If the MIC is incorrect
some fragments have been lost. It sends the ACK. In case of an incorrect MIC, the receivers wait for fragments belonging to the same window. After MAX_ACK_REQUESTS the
receiver will Abort the transmission. It can also Abort when the Inactivity timer has expired.The ACK-on-error is similar to the ACK-Always procedure, the difference is that in ACK-on-error the ACK is not sent at the end of each window but only when there is an error. In Ack-on-error mode, the retransmission timer expiration will be considered as a positive acknowledgment, it is set when receiving an All-0 or an All-1 fragment. If there are no more fragments then the fragmentation is finished.When the All-1 last fragment is sent and the correct MIC have been received an ACK is sent to confirms the end of the correct transmission. If the retransmission timer expires an All-1 empty request the last ACK that MUST be sent to complete the fragmentation transmission.If the sender receives an ACK, it checks the window value. ACKs with the non-expected window number are
discarded. If the window number on the received Bitmap is correct, the sender verifies if the receiver has all the
fragments. When all the fragments have been received the transmission of a new window should continue. Otherwise, when
there is an error the counter Attempts is increased and the sender resends the missing fragments again. When a
MAX_ACK_REQUESTS is reached, the sender sends an Abort. When the retransmission is finished, it starts
listening to the ACK (even if an All-0 or an All-1 has not been sent during the retransmission) and set the retransmission
timer. If the retransmission timer expires the transmission is aborted.Unlike the sender, the receiver for ACK-on-error has some differences. First, we are not sending ACK unless there is an error or an unexpected behavior. The receiver starts with the expected window and maintains the information indicating which fragments it has received (All-0 and All-1 occupy the same position). After receiving a fragment an Inactivity timer is set, if nothing has been received and the Inactivity timer expires the transmission is aborted.Any fragment not belonging to the current window is discarded. The Fragment Number is computed based on the FCN value. When an All-0 fragment is received and the Bitmap is full, the receiver changes the window value.An All-0 fragment and not a full bitmap indicate that all the fragments have been sent in the current window. Since the sender is not obligated to send a full window, some fragment number not used may not correspond to losses. As the receiver does not know if the missing fragments are lost or normal missing fragments, it sends an ACK.An All-1 fragment indicates that the transmission is finished. Since the last window is not full, the MIC will be used to
detect if all the fragments have been received. A correct MIC indicates the end of the transmission.If All-1 fragment has not been received, the receiver expects a new window. It waits for
the next fragment. If the window value has not changed, the received fragments are
part of a retransmission. A receiver that has already received a fragment should discard it. If all the bits of the Bitmap are set to one, the receiver waits for the next window without waiting for an All-0 fragment and it does not send an ACK either.
While the receiver waits for 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 goes to error and abort the transmission, it drops the fragments.If the receiver receives an All-1 fragment this means that the transmission should be finished. If the MIC is incorrect some fragments have been lost. It sends an ACK.In case of an incorrect MIC, the receivers wait for fragment belonging to the same window or the expiration of the Inactivity timer which will Abort the transmission.The Fragmentation Bitmap is the optmization of what has been received. It is transmitted in the ACK format fragment when there are some missing fragments. An ACK message may introduce padding at the end to align transmitted data to a byte boundary. The first byte boundary includes one or more complete bytes, depending on the size of Rule ID and DTag.The receiver generates the Bitmap which may have the size of a single downlink frame of the LPWAN technology used. To avoid this problem the FCN size should be set to the corresponding downlink size minus 1 bit for C bit. The C bit will be sent only in the ACK for the last frame of the packet (All-1).Bitmap transmitted MUST be optimized in size to reduce frame size. The right-most bytes with all Bitmap bit set to 1 MUST be removed from the transmission. As the receiver knows the Bitmap size, it can reconstruct the value. In the example the last 2 bytes of the bitmap are set to 1, therefore, they are not sent.In the last window, when checked bit C value is one, means that the MIC is corrected and the Bitmap is not sent. Otherwise, the Bitmap needs to be sent after the C bit. Note that the introduction of a C bit may force to reduce the number of fragments to allow the bitmap to fit in a frame. shows an example of an ACK (N=3), where the Bitmap
indicates that the second and the fifth fragments have not been correctly received. shows an example of an ACK (N=3), where the bitmap indicates that the MIC check has failed but there is no missing 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 may be used for packets that need to be carried by a large number of fragments. However, when the number of 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 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 fragments that belong to a 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 for fragmented datagram transmission in the downlink, the fragment receiver MAY perform an uplink transmission as soon as possible after reception of a 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 fragment encapsulated in a L2 frame that requires an L2 ACK) or it may be triggered from an upper layer.For fragmented packet transmission in the downlink, and when ACK Always
is used, the fragment receiver MAY support timer-based ACK
retransmission. In this mechanism, the 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 fragment of a
packet (All-1 fragment). In the latter case, the 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 fragment receiver receives a fragment that belongs to
the current window (e.g. a missing 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) fragment to be
retransmitted can find an opportunity for that transmission.When the fragment sender transmits the All-1 fragment, it
initializes and starts its retransmission timer to a
long value (e.g. several times the initial Inactivity timer). If an ACK
is received before expiration of this timer, the fragment sender
retransmits any lost fragments reported by the ACK, or if the ACK
confirms successful reception of all fragments of
the last window, transmission of the fragmented packet ends.
If the timer expires, and no ACK has been received since the
start of the timer, the 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 fragment receiver, and it also
assumes that it is unlikely that several ACKs become all lost).SCHC header, either for compression, fragmentation or acknowledgment does not preserve byte alignment. Since most of the LPWAN network technologies payload is expressed in an integer number of bytes; the sender will introduce at the end some padding bits while the receiver must be able to eliminate them.The algorithm for padding bit elimination for compressed or fragmented frames is simple. Based on the following principle:
* The SCHC header is not aligned on a byte boundary, but its size in bits is given by the rule.The data size is variable, but always a multiple of 8 bits.Padding bits MUST never exceed 7 bits.In that case, a receiver after decoding the SCHC header, must take the maximum multiple of 8 bits as data. The remaining bits are padding bits.This section lists the different IPv6 and UDP header fields and how they can be compressed.This field always holds the same value. Therefore, the TV is 6, the MO is “equal”
and the “CDA “not-sent””.If the DiffServ field identified by the rest of the rule do not vary and is known
by both sides, the TV should contain this well-known value, the MO should be “equal”
and the CDA must be “not-sent.If the DiffServ field identified by the rest of the rule varies over time or is not
known by both sides, then there are two possibilities depending on the variability of the value,
the first one is to do not compressed the field and sends the original value, or
the second where the values can be computed by sending only the LSB bits:TV is not set to any value, MO is set to “ignore” and CDA is set to “value-sent”TV contains a stable value, MO is MSB(X) and CDA is set to LSBIf the Flow Label field identified by the rest of the rule does not vary and is known
by both sides, the TV should contain this well-known value, the MO should be “equal”
and the CDA should be “not-sent”.If the Flow Label field identified by the rest of the rule varies during time or is not
known by both sides, there are two possibilities depending on the variability of the value,
the first one is without compression and then the value is sent
and the second where only part of the value is sent and the decompressor needs to compute the original value:TV is not set, MO is set to “ignore” and CDA is set to “value-sent”TV contains a stable value, MO is MSB(X) and CDA is set to LSBIf the LPWAN technology does not add padding, this field can be elided for the
transmission on the LPWAN network. The SCHC C/D recomputes the original payload length
value. The TV is not set, the MO is set to “ignore” and the 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)” and the
CDA to “LSB”. The ‘s’ parameter depends on the expected maximum packet length.On other cases, the payload length field must be sent and the CDA is replaced by “value-sent”.If the Next Header field identified by the rest of the rule does not vary and is known
by both sides, the TV should contain this Next Header value, the MO should be “equal”
and the CDA should be “not-sent”.If the Next header field identified by the rest of the rule varies during time or is not
known by both sides, then TV is not set, MO is set to “ignore” and CDA is set to
“value-sent”. A matching-list may also be used.The End System is generally a device and does not forward packets. Therefore, the
Hop Limit value is constant. So, the TV is set with a default value, the MO
is set to “equal” and the CDA is set to “not-sent”.Otherwise the value is sent on the LPWAN: TV is not set, MO is set to ignore and
CDA is set to “value-sent”.Note that the field behavior differs in upstream and downstream. In upstream, since there is
no IP forwarding between the Dev and the SCHC C/D, the value is relatively constant. On the
other hand, the downstream value depends of Internet routing and may change more frequently.
One solution could be to use the Direction Indicator (DI) to distinguish both directions to
elide the field in the upstream direction and send the value in the downstream direction.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 a single rule, these 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 (upstream, downstream) to select the appropriate
field.Both ends must be synchronized with the appropriate prefixes. For a specific flow,
the source and destination prefix 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”.In case the rule allows several prefixes, mapping-list must be used. The
different prefixes are listed in the TV associated with a short ID. The MO is set
to “match-mapping” and the CDA is set to “mapping-sent”.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 is generally carrying a single device identifier corresponding
to the DEV. The SCHC C/D may also not be aware of these values.If the DEV address has a static value that is not derived from an IEEE EUI-64,
then TV contains the actual Dev address value, the MO operator is set to
“equal” and the CDA is set to “not-sent”.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”.Otherwise the value variation of the IID may be reduced to 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 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 extension rules are currently defined. They can be based on the MOs and
CDAs described above.To allow a single rule, 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 (upstream, downstream) 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 on the LPWAN. The TV is not set, the MO is
set to “ignore” and the CDA is set to “value-sent”.If the LPWAN technology does not introduce padding, 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-UDP-length”.If the payload is small, the TV can be set to 0x0000, the MO set to “MSB” and the
CDA to “LSB”.On other cases, the length must 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 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-UDP-checksum”.In other cases, the checksum must 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 corruption
may be detected with end-to-end authentication and integrity mechanisms.
Denial of Service may be produced but its arise other security problems
that may be solved with or without header compression.This subsection describes potential attacks to LPWAN fragmentation
and suggests possible countermeasures.A node can perform a buffer reservation attack by sending a first
fragment to a target. Then, the receiver will reserve buffer space
for the IPv6 packet. Other incoming 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 fragments of multiple packets can be stored
in the reassembly buffer. To further increase the attack cost, the
reassembly buffer can be split into 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 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 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 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 fragments to be transmitted by a node,
by applying content-chaining to the different fragments, based on cryptographic
hash functionality. The aim of this technique is to allow a receiver to
identify illegitimate fragments.Further attacks may involve sending overlapped fragments (i.e.
comprising some overlapping parts of the original IPv6 datagram).
Implementers should make sure that correct operation is not affected
by such event.In Window mode – ACK on error, a malicious node may force a fragment sender to resend a fragment a number of times, with the aim to increase consumption of the fragment sender’s resources. To this end, the malicious node may repeatedly send a fake ACK to the fragment sender, with a bitmap that reports that one or more fragments have been lost. In order to mitigate this possible attack, MAX_FRAG_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_FRAG_RETRIES benefits fragment delivery reliability, therefore the trade-off needs to be carefully considered.Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier, Arunprabhu Kandasamy, Antony Markovski, Alexander
Pelov, Pascal Thubert, Juan Carlos Zuniga and Diego Dujovne 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]Significance 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]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 SCHC behavior.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 define statically 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 of different fragment delivery reliability options possible on the basis of this specification. illustrates the transmission of an IPv6 packet that needs 11 fragments in the No ACK option, FCN is always 1 bit. illustrates the transmission of an IPv6 packet that needs 11 fragments in ACK-on-error, for N=3, without losses. illustrates the transmission of an IPv6 packet that needs 11 fragments ACK-on-error, for N=3, with three losses. illustrates the transmission of an IPv6 packet that needs 11 fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, without losses. Note: in Window mode, an additional bit will be needed to number windows. illustrates the transmission of an IPv6 packet that needs 11 fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, with three losses. illustrates the transmission of an IPv6 packet that needs 6 fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, with three losses, and only one retry is needed for each lost fragment. Note that, since a single window is needed for transmission of the IPv6 packet in this case, the example illustrates behavior when losses happen in the last window. illustrates the transmission of an IPv6 packet that needs 6 fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, with three losses, and the second ACK is lost. Note that, since a single window is needed for transmission of the IPv6 packet in this case, the example illustrates behavior when losses happen in the last window. illustrates the transmission of an IPv6 packet that needs 6 fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, with three losses, and one retransmitted fragment is lost. Note that, since a single window is needed for transmission of the IPv6 packet in this case, the example illustrates behavior when losses happen in the last window. illustrates the transmission of an IPv6 packet that needs 28 fragments in ACK-Always, for N=5 and MAX_WIND_FCN=23, with two losses. 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 in the different reliability options are next in the following figures:A set of Rule IDs are allocated to support different aspects of fragmentation functionality as per this document. The allocation of IDs is to be defined in other documents. The set MAY include:one ID or a subset of IDs to identify a fragment as well as its reliability option and its window size, if multiple of these are supported.one ID to identify the ACK message.one ID to identify the Abort message as per Section 9.8.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.