• The network topology is star-oriented, which means that all packets between the same source-destination pair follow the same path. For the needs of this document, the architecture can simply be described as Devices (Dev) exchanging information with LPWAN Application Servers (Apps) through a Network Gateway (NGW).
• Because devices embed built-in applications, the traffic flows to be compressed are known in advance. Indeed, new applications are less frequently installed in an LPWAN device than they are in a general-purpose computer or smartphone.

• 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.
• The Radio Gateway (RGW) is the endpoint of the constrained link.
• The Network Gateway (NGW) is the interconnection node between the Radio Gateway and the Internet.
• The Application Server (App) is the endpoint of the application-level protocol on the Internet side.

 ()   ()   ()       |
  ()  () () ()     / \       +---------+
() () () () () () /   \======|    ^    |             +-----------+
 ()  ()   ()     |           | <--|--> |             |Application|
()  ()  ()  ()  / \==========|    v    |=============|   Server  |
  ()  ()  ()   /   \         +---------+             +-----------+
 Dev            RGWs             NGW                      App

App:

LPWAN Application Server, as defined by [RFC 8376]. It runs an application sending/receiving packets to/from the Dev.

AppIID:

Application Interface Identifier. The IID that identifies the App interface.

Compression Residue:

The bits that remain to be sent (beyond the RuleID itself) after applying the SCHC compression.

Context:

A set of Rules used to compress/decompress headers, or to fragment/reassemble a packet.

Dev:

Device, as defined by [RFC 8376].

DevIID:

Device Interface Identifier. The IID that identifies the Dev interface.

Downlink:

From the App to the Dev.

IID:

Interface Identifier. See the IPv6 addressing architecture [RFC 7136].

L2:

Layer 2. The immediate lower layer that SCHC interfaces with, for example an underlying LPWAN technology. It does not necessarily correspond to the OSI model definition of Layer 2.

L2 Word:

This is the minimum subdivision of payload data that the L2 will carry. In most L2 technologies, the L2 Word is an octet.In bit-oriented radio technologies, the L2 Word might be a single bit.The L2 Word size is assumed to be constant over time for each device.

Padding:

Extra bits that may be appended by SCHC to a data unit that it passes down to L2 for transmission.SCHC itself operates on bits, not bytes, and does not have any alignment prerequisite. See Section 9.

Profile:

SCHC offers variations in the way it is operated, with a number of parameters listed in Appendix D.A Profile indicates a particular setting of all these parameters.Both ends of a SCHC communication must be provisioned with the same Profile information and with the same set of Rules before the communication starts,so that there is no ambiguity in how they expect to communicate.

Rule:

Part of the Context that describes how a packet is compressed/decompressed or fragmented/reassembled.

RuleID:

Rule Identifier. An identifier for a Rule.

SCHC:

Static Context Header Compression and fragmentation (SCHC), a generic framework.

SCHC C/D:

SCHC Compressor/Decompressor, or SCHC Compression/Decompression. The SCHC entity or mechanism used on both sides, at the Dev and at the network, to achieve compression/decompression of headers.

SCHC F/R:

SCHC Fragmenter/Reassembler or SCHC Fragmentation/Reassembly. The SCHC entity or mechanism used on both sides, at the Dev and at the network, to achieve fragmentation/reassembly of SCHC Packets.

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 RuleID is used to indicate that the packet header has not been compressed). See Section 7 for more details.

Uplink:

From the Dev to the App.

• For SCHC C/D, to identify the Rule that is used to compress a packet header.
  • At least one RuleID MUST be allocated to tagging packets for which SCHC compression was not possible (i.e., no matching compression Rule was found).
• In SCHC F/R, to identify the specific mode and settings of fragmentation/reassembly for one direction of data traffic (Uplink or Downlink).
  • When SCHC F/R is used for both communication directions, at least two RuleID values are needed for fragmentation/reassembly: one per direction of data traffic. This is because fragmentation/reassembly may entail control messages flowing in the reverse direction compared to data traffic.

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

• Field Identifier (FID) designates a protocol and field (e.g., UDP Destination Port), unambiguously among all protocols that a SCHC compressor processes. In the presence of protocol nesting, the Field ID also identifies the nesting.
• Field Length (FL) represents the length of the original field. It can be either a fixed value (in bits) if the length is known when the Rule is created or a type if the length is variable. The length of a header field is defined by its own protocol specification (e.g., IPv6 or UDP). If the length is variable, the type defines the process to compute the length and its unit (bits, bytes...).
• Field Position (FP): most often, a field only occurs once in a packet header. However, some fields may occur multiple times. An example is the uri-path of CoAP. FP indicates which occurrence this Field Descriptor applies to. The default value is 1. The value 1 designates the first occurrence. The value 0 is special. It means "don't care", see Section 7.2.
• A Direction Indicator (DI) indicates the packet direction(s) this Field Descriptor applies to. It allows for asymmetric processing, using the same Rule. Three values are possible:

  Up:

  this Field Descriptor is only applicable to packets traveling Uplink.

  Dw:

  this Field Descriptor is only applicable to packets traveling Downlink.

  Bi:

  this Field Descriptor is applicable to packets traveling Uplink or Downlink.
• Target Value (TV) is the value used to match against the packet header field. The Target Value can be a scalar value of any type (integer, strings, etc.) or a more complex structure (array, list, etc.). The types and representations are out of scope for this document.
• Matching Operator (MO) is the operator used to match the field value and the Target Value. The Matching Operator may require some parameters. The set of MOs defined in this document can be found in Section 7.3.
• Compression/Decompression Action (CDA) describes the pair of actions that are performed at the compressor to compress a header field and at the decompressor to recover the original value of the header field. Some CDAs might use parameter values for their operation. The set of CDAs defined in this document can be found in Section 7.4.

Compression Rule selection:

the general idea is to browse the Rule set to find a Rule that has a matching Field Descriptor (given the DI and FP) for all and only those header fields that appear in the packet being compressed. The detailed algorithm is the following:

• The first step is to check the FIDs. If any header field of the packet being examined cannot be matched with a Field Descriptor with the correct FID, the Rule MUST be disregarded. If any Field Descriptor in the Rule has a FID that cannot be matched to one of the header fields of the packet being examined, the Rule MUST be disregarded.
• The next step is to match the Field Descriptors by their direction, using the DI. If any field of the packet header cannot be matched with a Field Descriptor with the correct FID and DI, the Rule MUST be disregarded.
• Then, the Field Descriptors are further selected according to FP. If any field of the packet header cannot be matched with a Field Descriptor with the correct FID, DI and FP, the Rule MUST be disregarded. The value 0 for FP means "don't care", i.e., the comparison of this Field Descriptor's FP with the position of the field of the packet header being compressed returns True, whatever that position. FP=0 can be useful to build compression Rules for protocol headers in which some fields order is irrelevant. An example could be uri-queries in CoAP. Care needs to be exercised when writing Rules containing FP=0 values. Indeed, it may result in decompressed packets having fields ordered differently compared to the original packet.
• Once each header field has been associated with a Field Descriptor with matching FID, DI, and FP, each packet field's value is then compared to the corresponding TV stored in the Rule for that specific field, using the MO. If every field in the packet header satisfies the corresponding MOs of a Rule (i.e., all MO results are True), that Rule is valid for use to compress the header. Otherwise, the Rule MUST be disregarded. This specification does not prevent multiple Rules from matching the above steps and, therefore, being valid for use. Which Rule to use among multiple valid Rules is left to the implementation. As long as the same Rule set is installed at both ends, this degree of freedom does not constitute an interoperability issue.
• If no valid compression Rule is found, then the packet MUST be sent uncompressed using the RuleID dedicated to this purpose (see Section 6). The entire packet header is the Compression Residue (see Figure 4). Sending an uncompressed header is likely to require SCHC F/R.

Compression:

if a valid Rule is found, each field of the header is compressed according to the CDAs of the Rule.The fields are compressed in the order that the Field Descriptors appear in the Rule.The compression of each field results in a residue, which may be empty.The Compression Residue for the packet header is the concatenation of the non-empty residues for each field of the header, in the order the Field Descriptors appear in the Rule.The order in which the Field Descriptors appear in the Rule is therefore semantically important.

    |------------------- Compression Residue -------------------|
    +-----------------+-----------------+-----+-----------------+
    | field 1 residue | field 2 residue | ... | field N residue |
    +-----------------+-----------------+-----+-----------------+

Sending:

The RuleID is sent to the other end jointly with the Compression Residue (which could be empty) or the uncompressed header, and directly followed by the payload (see Figure 4).The way the RuleID is sent will be specified in the Profile and is out of the scope of the present document.For example, it could be included in an L2 header or sent as part of the L2 payload.

Decompression:

when decompressing, on the Network Infrastructure side, the SCHC C/D needs to find the correct Rule based on the L2 address of the Dev. On the Dev side, only the RuleID is needed to identify the correct Rule since the Dev typically only holds Rules that apply to itself.

This Rule describes the compressed header format. From this, the decompressor determines the order of the residues, the fixed-size or variable-size nature of each residue (see Section 7.4.2), and the size of the fixed-size residues.

Therefore, from the received compressed header, it can retrieve all the residue values and associate them to the corresponding header fields.

For each field in the header, the receiver applies the CDA action associated with that field in order to reconstruct the original header field value. The CDA application order can be different from the order in which the fields are listed in the Rule. In particular, Compute-* MUST be applied after the application of the CDAs of all the fields it computes on.

equal:

The match result is True if the field value in the packet matches the TV.

ignore:

No matching is attempted between the field value in the packet and the TV in the Rule. The result is always True.

MSB(x):

A match is obtained if the most significant (leftmost) x bits of the packet header field value are equal to the TV in the Rule. The x parameter of the MSB MO indicates how many bits are involved in the comparison. If the FL is described as variable, the x parameter must be a multiple of the FL unit. For example, x must be multiple of 8 if the unit of the variable length is bytes.

match-mapping:

With match-mapping, TV is a list of values. Each value of the list is identified by an 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.

|--- field residue ---|
+-------+-------------+
|  size |    value    |
+-------+-------------+

• If the size is between 0 and 14, it is encoded as a 4-bit unsigned integer.
• Sizes between 15 and 254 are encoded as 0b1111 followed by the 8-bit unsigned integer.
• Larger sizes are encoded as 0xfff followed by the 16-bit unsigned integer.

SCHC Fragment:

A message that carries part of a SCHC Packet from the sender to the receiver.

SCHC ACK:

An acknowledgement for fragmentation, by the receiver to the sender.This message is used to indicate whether or not the reception of pieces of,or the whole of, the fragmented SCHC Packet was successful.

SCHC ACK REQ:

A request by the sender for a SCHC ACK from the receiver.

SCHC Sender-Abort:

A message by the sender telling the receiver that it has aborted the transmission of a fragmented SCHC Packet.

SCHC Receiver-Abort:

A message by the receiver to tell the sender to abort the transmission of a fragmented SCHC Packet.

• all the windows of a SCHC Packet, except the last one, MUST contain the same number of tiles. This number is WINDOW_SIZE.
• WINDOW_SIZE MUST be specified in a Profile.
• the windows are numbered.
• their numbers MUST increment by 1 from 0 upward, from the start of the SCHC Packet to its end.
• the last window MUST contain WINDOW_SIZE tiles or less.
• tiles are numbered within each window.
• the tile indices MUST decrement by 1 from WINDOW_SIZE - 1 downward, looking from the start of the SCHC Packet toward its end.
• therefore, each tile of a SCHC Packet is uniquely identified by a window number and a tile index within this window.

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

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

• Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to the sender in a SCHC ACK message.
• A Bitmap corresponds to exactly one Window.

• a bit set to 1 in the Bitmap indicates that a tile associated with that bit position has been correctly received for that window.
• a bit set to 0 in the Bitmap indicates that there has been no tile correctly received, associated with that bit position, for that window. Possible reasons include that the tile was not sent at all, not received, or received with errors.

Inactivity Timer:

a SCHC Fragment receiver uses this timer to abort waiting for a SCHC F/R message.

Retransmission Timer:

a SCHC Fragment sender uses this timer to abort waiting for an expected SCHC ACK.

Attempts:

this counter counts the requests for SCHC ACKs, up to MAX_ACK_REQUESTS.

RuleID:

this field is present in all the SCHC F/R messages. The Rule identifies:

• that a SCHC F/R message is being carried, as opposed to an unfragmented SCHC Packet,
• which SCHC F/R mode is used,
• in case this mode uses windows, what the value of WINDOW_SIZE is, and
• what other optional fields are present and what the field sizes are.

The Rule tells apart a non-fragmented SCHC Packet from SCHC Fragments. It will also tell apart SCHC Fragments of fragmented SCHC Packets that use different SCHC F/R modes or different parameters. Therefore, interleaved transmission of these is possible.

All SCHC F/R messages pertaining to the same SCHC Packet MUST bear the same RuleID.

Datagram Tag (DTag):

This field allows differentiating SCHC F/R messages belonging to different SCHC Packets that may be using the same RuleID simultaneously. Hence, it allows interleaving fragments of a new SCHC Packet with fragments of a previous SCHC Packet under the same RuleID.

The size of the DTag field (called "T", in bits) is defined by each Profile for each RuleID. When T is 0, the DTag field does not appear in the SCHC F/R messages and the DTag value is defined as 0.

When T is 0, there can be no more than one fragmented SCHC Packet in transit for each fragmentation RuleID.

If T is not 0, DTag:

• MUST be set to the same value for all the SCHC F/R messages related to the same fragmented SCHC Packet, and
• MUST be set to different values for SCHC F/R messages related to different SCHC Packets that are being fragmented under the same RuleID and whose transmission may overlap.

W:

The W field is optional. It is only present if windows are used. Its presence and size (called "M", in bits) is defined by each SCHC F/R mode and each Profile for each RuleID.

This field carries information pertaining to the window a SCHC F/R message relates to. If present, W MUST carry the same value for all the SCHC F/R messages related to the same window. Depending on the mode and Profile, W may carry the full window number, or just the LSB or any other partial representation of the window number.

Fragment Compressed Number (FCN):

The FCN field is present in the SCHC Fragment Header. Its size (called "N", in bits) is defined by each Profile for each RuleID.

This field conveys information about the progress in the sequence of tiles being transmitted by SCHC Fragment messages. For example, it can contain a partial, efficient representation of a larger-sized tile index. The description of the exact use of the FCN field is left to each SCHC F/R mode. However, two values are reserved for special purposes. They help control the SCHC F/R process:

• The FCN value with all the bits equal to 1 (called "All-1") signals that the very last tile of a SCHC Packet has been transmitted. By extension, if windows are used, the last window of a packet is called the "All-1" window.
• If windows are used, the FCN value with all the bits equal to 0 (called "All-0") signals the last tile of a window that is not the last one of the SCHC packet. By extension, such a window is called an "All-0 window".

Reassembly Check Sequence (RCS):

This field only appears in the All-1 SCHC Fragments. Its size (called "U", in bits) is defined by each Profile for each RuleID.

See Section 8.2.3 for the RCS default size, default polynomial and details on RCS computation.

C (integrity Check):

C is a 1-bit field. This field is used in the SCHC ACK message to report on the reassembled SCHC Packet integrity check (see Section 8.2.3).

A value of 1 tells that the integrity check was performed and is successful. A value of 0 tells that the integrity check was not performed or that it was a failure.

Compressed Bitmap:

The Compressed Bitmap is used together with windows and Bitmaps (see Section 8.2.2.3). Its presence and size is defined for each SCHC F/R mode for each RuleID.

This field appears in the SCHC ACK message to report on the receiver Bitmap (see Section 8.3.2.1).

• Build a temporary SCHC ACK message that contains the Header followed by the original Bitmap (see Section 8.2.2.3 for a description of Bitmaps).
• Position scissors at the end of the Bitmap, after its last bit.
• While the bit on the left of the scissors is 1 and belongs to the Bitmap, keep moving left, then stop.
• Then, while the scissors are not on an L2 Word boundary of the SCHC ACK message and there is a Bitmap bit on the right of the scissors, keep moving right, then stop.
• At this point, cut and drop off any bits to the right of the scissors.

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

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

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

+--------+-------+---+---+-------------+~~~~+
| RuleID |  DTag | W |C=0|1 0 1 0 1 1 1|pad.| transmitted SCHC ACK
+--------+-------+---+---+-------------+~~~~+
   next L2 Word boundary ->|<-- L2 Word --->|

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

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

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

• the fragment sender MUST set it to all ones. Other values are RESERVED.
• the fragment receiver MUST check its value. If the value is different from all ones, the message MUST be ignored.

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

• the fragment receiver MUST set it to all ones. Other values are RESERVED.
• if the value is different from all ones, the fragment sender MUST ignore the message.

• if the Header does not end at an L2 Word boundary, append bits set to 1 as needed to reach the next L2 Word boundary.
• append exactly one more L2 Word with bits all set to ones.

• allow for a range of reliability options, such as optional SCHC Fragment retransmission.
• support various LPWAN characteristics, such as links with variable MTU or unidirectional links.

• the size of the DTag field,
• the size and algorithm for the RCS field, and
• the expiration time of the Inactivity Timer.

• a value of N different from the recommended one, and
• the meaning of values sent in the FCN field, for values different from the All-1 value.

• the receiver MUST reset the Inactivity Timer.
• the receiver assembles the payloads of the SCHC Fragments.

• the receiver MUST append the All-1 SCHC Fragment Payload and the padding bits to the previously received SCHC Fragment Payloads for this SCHC Packet.
• the receiver MUST perform the integrity check.
• if integrity checking fails, the receiver MUST drop the reassembled SCHC Packet.
• the reassembly operation concludes.

• Data unit out-of-sequence delivery does not occur between the entity performing fragmentation and the entity performing reassembly,
• The L2 MTU value does not change while the fragments of a SCHC Packet are being transmitted, and
• There is a feedback path from the reassembler to the fragmenter. See Appendix F for a discussion on using ACK-Always mode on quasi-bidirectional links.

• the value of N,
• the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
• the size and algorithm for the RCS field,
• the value of T,
• the value of MAX_ACK_REQUESTS,
• the expiration time of the Retransmission Timer, and
• the expiration time of the Inactivity Timer.

• one Attempts counter
• one Retransmission Timer

• one Inactivity Timer, and
• one Attempts counter.

• the Fragment MUST be of the Regular type specified in Section 8.3.1.1.
• the FCN field MUST contain the tile index.
• each tile MUST be of a size that complements the SCHC Fragment Header so that the SCHC Fragment is a multiple of L2 Words without the need for padding bits.

• Then, upon receiving a SCHC ACK:
  • if the SCHC ACK indicates that some tiles are missing at the receiver, then the sender MUST transmit all the tiles that have been reported missing, it MUST increment Attempts, it MUST reset the Retransmission Timer, and MUST await the next SCHC ACK.
  • if the current window is not the last one and the SCHC ACK indicates that all tiles were correctly received, the sender MUST stop the Retransmission Timer, it MUST advance to the next fragmentation window, and it MUST start a blind transmission phase as described above.
  • if the current window is the last one and the SCHC ACK indicates that more tiles were received than the sender sent, the fragment sender MUST send a SCHC Sender-Abort, and it MAY exit with an error condition.
  • if the current window is the last one and the SCHC ACK indicates that all tiles were correctly received, yet the integrity check was a failure, the fragment sender MUST send a SCHC Sender-Abort, and it MAY exit with an error condition.
  • if the current window is the last one and the SCHC ACK indicates that integrity checking was successful, the sender exits successfully.
• on Retransmission Timer expiration:
  • if Attempts is strictly less that MAX_ACK_REQUESTS, the fragment sender MUST send a SCHC ACK REQ and MUST increment the Attempts counter.
  • otherwise, the fragment sender MUST send a SCHC Sender-Abort, and it MAY exit with an error condition.

• on receiving a SCHC Receiver-Abort, the fragment sender MAY exit with an error condition.
• on receiving a SCHC ACK that bears a W value different from the W value that it currently uses, the fragment sender MUST silently discard and ignore that SCHC ACK.

• the receiver SHOULD check if the DTag value has not recently been used for that RuleID value, thereby ensuring that the received SCHC Fragment is not a remnant of a prior fragmented SCHC Packet transmission. The initial value of the Inactivity Timer is the RECOMMENDED lifetime for the DTag value at the receiver. If the SCHC Fragment is determined to be such a remnant, the receiver MAY silently ignore it and discard it.
• the receiver MUST start a process to assemble a new SCHC Packet with that RuleID and DTag value pair.
• the receiver MUST start an Inactivity Timer for that RuleID and DTag pair. It MUST initialize an Attempts counter to 0 for that RuleID and DTag pair. It MUST initialize a window counter to 0. If the receiver is under-resourced to do this, it MUST respond to the sender with a SCHC Receiver-Abort.

• on receiving a SCHC Fragment or a SCHC ACK REQ, either one having the W bit different from the local W bit, the receiver MUST silently ignore and discard that message.
• on receiving a SCHC ACK REQ with the W bit equal to the local W bit, the receiver MUST send a SCHC ACK for this window.
• on receiving a SCHC Fragment with the W bit equal to the local W bit, the receiver MUST assemble the received tile based on the window counter and on the FCN field in the SCHC Fragment, and it MUST update the Bitmap.
  • if the SCHC Fragment received is an All-0 SCHC Fragment, the current window is determined to be a not-last window, the receiver MUST send a SCHC ACK for this window and it MUST enter the "retransmission phase" for this window.
  • if the SCHC Fragment received is an All-1 SCHC Fragment, the current window is determined to be the last window, the padding bits of the All-1 SCHC Fragment MUST be assembled after the received tile, the receiver MUST perform the integrity check and it MUST send a SCHC ACK for this window. Then:
    • If the integrity check indicates that the full SCHC Packet has been correctly reassembled, the receiver MUST enter the "clean-up phase" for this window.
    • If the integrity check indicates that the full SCHC Packet has not been correctly reassembled, the receiver enters the "retransmission phase" for this window.

• if the window is a not-last window:
  • on receiving a SCHC Fragment that is not All-0 or All-1 and that has a W bit different from the local W bit, the receiver MUST increment its window counter and allocate a fresh Bitmap, it MUST assemble the tile received and update the Bitmap, and it MUST enter the "acceptance phase" for that new window.
  • on receiving a SCHC ACK REQ with a W bit different from the local W bit, the receiver MUST increment its window counter and allocate a fresh Bitmap, it MUST send a SCHC ACK for that new window, and it MUST enter the "acceptance phase" for that new window.
  • on receiving a SCHC All-0 Fragment with a W bit different from the local W bit, the receiver MUST increment its window counter and allocate a fresh Bitmap, it MUST assemble the tile received and update the Bitmap, it MUST send a SCHC ACK for that new window, and it MUST stay in the "retransmission phase" for that new window.
  • on receiving a SCHC All-1 Fragment with a W bit different from the local W bit, the receiver MUST increment its window counter and allocate a fresh Bitmap; it MUST assemble the tile received, including the padding bits; it MUST update the Bitmap and perform the integrity check; it MUST send a SCHC ACK for the new window, which is determined to be the last window. Then:
    • If the integrity check indicates that the full SCHC Packet has been correctly reassembled, the receiver MUST enter the "clean-up phase" for that new window.
    • If the integrity check indicates that the full SCHC Packet has not been correctly reassembled, the receiver enters the "retransmission phase" for that new window.
  • on receiving a SCHC Fragment with a W bit equal to the local W bit:
    • if the SCHC Fragment received is an All-1 SCHC Fragment, the receiver MUST silently ignore it and discard it.
    • otherwise, the receiver MUST assemble the tile received and update the Bitmap. If the Bitmap becomes fully populated with 1's or if the SCHC Fragment is an All-0, the receiver MUST send a SCHC ACK for this window.
  • on receiving a SCHC ACK REQ with the W bit equal to the local W bit, the receiver MUST send a SCHC ACK for this window.
• if the window is the last window:
  • on receiving a SCHC Fragment or a SCHC ACK REQ, either one having a W bit different from the local W bit, the receiver MUST silently ignore and discard that message.
  • on receiving a SCHC ACK REQ with the W bit equal to the local W bit, the receiver MUST send a SCHC ACK for this window.
  • on receiving a SCHC Fragment with a W bit equal to the local W bit:
    • if the SCHC Fragment received is an All-0 SCHC Fragment, the receiver MUST silently ignore it and discard it.
    • otherwise, the receiver MUST update the Bitmap, and it MUST assemble the tile received. If the SCHC Fragment received is an All-1 SCHC Fragment, the receiver MUST assemble the padding bits of the All-1 SCHC Fragment after the received tile, it MUST perform the integrity check and:
      • if the integrity check indicates that the full SCHC Packet has been correctly reassembled, the receiver MUST send a SCHC ACK and it enters the "clean-up phase".
      • if the integrity check indicates that the full SCHC Packet has not been correctly reassembled:
        • if the SCHC Fragment received was an All-1 SCHC Fragment, the receiver MUST send a SCHC ACK for this window.

• On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, either one having the W bit equal to the local W bit, the receiver MUST send a SCHC ACK.
• Any other SCHC Fragment received MUST be silently ignored and discarded.

• The penultimate tile size MUST be the regular tile size, or
• the penultimate tile size MUST be either the regular tile size or the regular tile size minus one L2 Word.

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

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


SCHC Fragment msg   |-----------|

• integrity checking shows that the fragmented SCHC Packet has been correctly reassembled at the receive end, and this information has been conveyed back to the sender, or
• too many retransmission attempts were made, or
• the receiver determines that the transmission of this fragmented SCHC Packet has been inactive for too long.

• the tile size (a tile does not need to be multiple of an L2 Word, but it MUST be at least the size of an L2 Word),
• the value of M,
• the value of N,
• the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
• the size and algorithm for the RCS field,
• the value of T,
• the value of MAX_ACK_REQUESTS,
• the expiration time of the Retransmission Timer,
• the expiration time of the Inactivity Timer,
• if the last tile is carried in a Regular SCHC Fragment or an All-1 SCHC Fragment (see Section 8.4.3.1), and
• if the penultimate tile MAY be one L2 Word smaller than the regular tile size. In this case, the regular tile size MUST be at least twice the L2 Word size.

• one Attempts counter, and
• one Retransmission Timer.

• one Inactivity Timer, and
• one Attempts counter.

• the fragment sender MUST select a RuleID and DTag value pair for this SCHC Packet. A Rule MUST NOT be selected if the values of M and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot be fragmented in (2^M) * WINDOW_SIZE tiles or less.
• the fragment sender MUST initialize the Attempts counter to 0 for that RuleID and DTag value pair.

• the selected tiles MUST be contiguous in the original SCHC Packet, and
• they MUST be placed in the SCHC Fragment Payload adjacent to one another, in the order they appear in the SCHC Packet, from the start of the SCHC Packet toward its end.

• in a Regular SCHC Fragment, alone or as part of a multi-tiles Payload,
• alone in an All-1 SCHC Fragment, or
• with any of the above two methods.

• an All-1 SCHC Fragment, or
• a SCHC ACK REQ.

• it MUST increment the Attempts counter, and
• it MUST reset the Retransmission Timer.

• if the Attempts counter is strictly less than MAX_ACK_REQUESTS, the fragment sender MUST send either the All-1 SCHC Fragment or a SCHC ACK REQ with the W field corresponding to the last window,
• otherwise, the fragment sender MUST send a SCHC Sender-Abort, and it MAY exit with an error condition.

• if the W field in the SCHC ACK corresponds to the last window of the SCHC Packet:
  • if the C bit is set, the sender MAY exit successfully.
  • otherwise:
    • if the Profile mandates that the last tile be sent in an All-1 SCHC Fragment:
      • if the SCHC ACK shows no missing tile at the receiver, the sender:
        • MUST send a SCHC Sender-Abort, and
        • MAY exit with an error condition.
      • otherwise:
        • the fragment sender MUST send SCHC Fragment messages containing all the tiles that are reported missing in the SCHC ACK.
        • if the last of these SCHC Fragment messages is not an All-1 SCHC Fragment, then the fragment sender MUST in addition send after it a SCHC ACK REQ with the W field corresponding to the last window.
        • in doing the two items above, the sender MUST ascertain that the receiver will not receive the last tile through both a Regular SCHC Fragment and an All-1 SCHC Fragment.
    • otherwise:
      • if the SCHC ACK shows no missing tile at the receiver, the sender MUST send the All-1 SCHC Fragment
      • otherwise:
        • the fragment sender MUST send SCHC Fragment messages containing all the tiles that are reported missing in the SCHC ACK.
        • the fragment sender MUST then send either the All-1 SCHC Fragment or a SCHC ACK REQ with the W field corresponding to the last window.
• otherwise, the fragment sender:
  • MUST send SCHC Fragment messages containing the tiles that are reported missing in the SCHC ACK.
  • then, it MAY send a SCHC ACK REQ with the W field corresponding to the last window.

• the receiver SHOULD check if the DTag value has not recently been used for that RuleID value, thereby ensuring that the received SCHC Fragment is not a remnant of a prior fragmented SCHC Packet transmission. The initial value of the Inactivity Timer is the RECOMMENDED lifetime for the DTag value at the receiver. If the SCHC Fragment is determined to be such a remnant, the receiver MAY silently ignore it and discard it.
• the receiver MUST start a process to assemble a new SCHC Packet with that RuleID and DTag value pair. The receiver MUST start an Inactivity Timer for that RuleID and DTag value pair. It MUST initialize an Attempts counter to 0 for that RuleID and DTag value pair. If the receiver is under-resourced to do this, it MUST respond to the sender with a SCHC Receiver-Abort.

• if the FCN is All-1, if a Payload is present, the full SCHC Fragment Payload MUST be assembled including the padding bits. This is because the size of the last tile is not known by the receiver; therefore, padding bits are indistinguishable from the tile data bits, at this stage. They will be removed by the SCHC C/D sublayer. If the size of the SCHC Fragment Payload exceeds or equals the size of one regular tile plus the size of an L2 Word, this SHOULD raise an error flag.
• otherwise, tiles MUST be assembled based on the a priori known tile size.
  • If allowed by the Profile, the end of the payload MAY contain the last tile, which may be shorter. Padding bits are indistinguishable from the tile data bits, at this stage.
  • The payload may contain the penultimate tile that, if allowed by the Profile, MAY be exactly one L2 Word shorter than the regular tile size.
  • Otherwise, padding bits MUST be discarded. This is possible because:
    • the size of the tiles is known a priori,
    • tiles are larger than an L2 Word, and
    • padding bits are always strictly less than an L2 Word.

• if the receiver knows of any windows with missing tiles for the packet being reassembled, it MUST return a SCHC ACK for the lowest-numbered such window:
• otherwise:
  • if it has received at least one tile, it MUST return a SCHC ACK for the highest-numbered window it currently has tiles for,
  • otherwise, it MUST return a SCHC ACK for window numbered 0.

• a Sender-Abort has been received, or
• the Inactivity Timer has expired, or
• the Attempts counter has exceeded MAX_ACK_REQUESTS, or
• at least an All-1 SCHC Fragment has been received and integrity checking of the reassembled SCHC Packet is successful.

• 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. ECN functionality depends on both bits of the ECN field, which are the 2 LSBs of this field; hence, sending only a single LSB of this field is NOT RECOMMENDED.

• in an Up Field Descriptor, elide the field: the TV is set to the known constant value, the MO is set to "equal" and the CDA is set to "not-sent".
• in a Dw Field Descriptor, the Hop Limit is elided for transmission and forced to 1 at the receiver, by setting TV to 1, MO to "ignore" and CDA to "not-sent". This prevents any further forwarding.

[RFC2119]

S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC6936]

G. Fairhurst, and M. Westerlund, "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936, DOI 10.17487/RFC6936, April 2013,
<https://www.rfc-editor.org/info/rfc6936>.

[RFC8174]

B. Leiba, "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.

[RFC8200]

S. Deering, and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.

[RFC8376]

S. Farrell, "Low-Power Wide Area Network (LPWAN) Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.

[ETHERNET]

IEEE, "IEEE Standard for Ethernet", DOI 10.1109/IEEESTD.2012.6419735, IEEE Standard 802.3-2012, December 2012,
<https://ieeexplore.ieee.org/document/6419735>.

[RFC4944]

G. Montenegro, N. Kushalnagar, J. Hui, and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.

[RFC5795]

K. Sandlund, G. Pelletier, and L-E. Jonsson, "The RObust Header Compression (ROHC) Framework", RFC 5795, DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>.

[RFC6282]

J. Hui, and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.

[RFC6437]

S. Amante, B. Carpenter, S. Jiang, and J. Rajahalme, "IPv6 Flow Label Specification", RFC 6437, DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.

[RFC7136]

B. Carpenter, and S. Jiang, "Significance of IPv6 Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, February 2014,
<https://www.rfc-editor.org/info/rfc7136>.

[RFC8065]

D. Thaler, "Privacy Considerations for IPv6 Adaptation-Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065, February 2017,
<https://www.rfc-editor.org/info/rfc8065>.

• Most common uses cases, deployment scenarios.
• Mapping of the SCHC architectural elements onto the LPWAN architecture.
• Assessment of LPWAN integrity checking.
• Various potential channel conditions for the technology and the corresponding recommended use of SCHC C/D and SCHC F/R.

• RuleID numbering scheme, fixed-size or variable-size RuleIDs, number of Rules, the way the RuleID is transmitted.
• maximum packet size that should ever be reconstructed by SCHC decompression (MAX_PACKET_SIZE). See Section 12.
• Padding: size of the L2 Word (for most LPWAN technologies, this would be a byte; for some technologies, a bit).
• Decision to use SCHC fragmentation mechanism or not. If yes, the document must describe:
  • reliability mode(s) used, in which cases (e.g., based on link channel condition).
  • RuleID values assigned to each mode in use.
  • presence and number of bits for DTag (T) for each RuleID value, lifetime of DTag at the receiver.
  • support for interleaved packet transmission, to what extent.
  • WINDOW_SIZE, for modes that use windows.
  • number of bits for W (M) for each RuleID value, for modes that use windows.
  • number of bits for FCN (N) for each RuleID value, meaning of the FCN values.
  • what makes an All-0 SCHC Fragment and a SCHC ACK REQ distinguishable (see Section 8.3.1.1).
  • what makes an All-1 SCHC Fragment and a SCHC Sender-Abort distinguishable (see Section 8.3.1.2).
  • for RuleIDs that use ACK-on-Error mode: when the last tile of a SCHC Packet is to be sent in a Regular SCHC Fragment, alone in an All-1 SCHC Fragment or with any of these two methods.
  • for RuleIDs that use ACK-on-Error mode: if the penultimate tile of a SCHC Packet is of the regular size only or if it can also be one L2 Word shorter.
  • for RuleIDs that use ACK-on-Error mode: times at which the sender must listen for SCHC ACKs.
  • size of RCS and algorithm for its computation, for each RuleID, if different from the default CRC32. Byte fill-up with zeroes or other mechanism, to be specified. Support for UDP checksum elision.
  • Retransmission Timer duration for each RuleID value, if applicable to the SCHC F/R mode.
  • Inactivity Timer duration for each RuleID value, if applicable to the SCHC F/R mode.
  • MAX_ACK_REQUESTS value for each RuleID value, if applicable to the SCHC F/R mode.
• if L2 Word is wider than a bit and SCHC fragmentation is used, value of the padding bits (0 or 1).

• 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 fragmented SCHC Packet, 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 PDU that requires an L2 ACK) or it may be triggered from an upper layer. See Appendix F.
• the following parameters need to be addressed in documents other than this one but not necessarily in the LPWAN technology-specific documents:
  • The way the Contexts are provisioned.
  • The way the Rules are generated.

Ana Minaburo

Laurent Toutain

Carles Gomez

Dominique Barthel

Juan Carlos Zuniga