| lpwan Working Group | A. Minaburo |
| Internet-Draft | Acklio |
| Intended status: Standards Track | L. Toutain |
| Expires: June 7, 2020 | IMT-Atlantique |
| C. Gomez | |
| Universitat Politècnica de Catalunya | |
| D. Barthel | |
| Orange Labs | |
| JC. Zuniga | |
| SIGFOX | |
| December 05, 2019 |
Static Context Header Compression (SCHC) and fragmentation for LPWAN, application to UDP/IPv6
draft-ietf-lpwan-ipv6-static-context-hc-24
This document defines the Static Context Header Compression (SCHC) framework, which provides both a header compression mechanism and an optional fragmentation mechanism. SCHC has been designed for Low Power Wide Area Networks (LPWAN).
SCHC compression is based on a common static context stored both in the LPWAN device and in the network infrastructure side. This document defines a generic header compression mechanism and its application to compress IPv6/UDP headers.
This document also specifies an optional fragmentation and reassembly mechanism. It can be used to support the IPv6 MTU requirement over the LPWAN technologies. Fragmentation is needed for IPv6 datagrams that, after SCHC compression or when such compression was not possible, still exceed the layer-2 maximum payload size.
The SCHC header compression and fragmentation mechanisms are independent of the specific LPWAN technology over which they are used. This document defines generic functionalities and offers flexibility with regard to parameter settings and mechanism choices. This document standardizes the exchange over the LPWAN between two SCHC entities. Settings and choices specific to a technology or a product are expected to be grouped into profiles, which are specified in other documents. Data models for the context and profiles are out of scope.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 7, 2020.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
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This document defines the Static Context Header Compression (SCHC) framework, which provides both a header compression mechanism and an optional fragmentation mechanism. SCHC has been designed for Low Power Wide Area Networks (LPWAN).
LPWAN technologies impose some strict limitations on traffic. For instance, devices sleep most of the time and may only receive data during short periods of time after transmission, in order to preserve battery. LPWAN technologies are also characterized by a greatly reduced data unit and/or payload size (see [RFC8376]).
Header compression is needed for efficient Internet connectivity to a node within an LPWAN network. The following properties of LPWAN networks can be exploited to get an efficient header compression:
SCHC compression uses a Context (a set of Rules) in which information about header fields is stored. This Context is static: the values of the header fields and the actions to do compression/decompression do not change over time. This avoids the need for complex resynchronization mechanisms. Indeed, a return path may be more restricted/expensive, sometimes completely unavailable [RFC8376]. A compression protocol that relies on feedback is not compatible with the characteristics of such LPWANs.
In most cases, a small Rule identifier is enough to represent the full IPv6/UDP headers. The SCHC header compression mechanism is independent of the specific LPWAN technology over which it is used.
Furthermore, some LPWAN technologies do not provide a fragmentation functionality; to support the IPv6 MTU requirement of 1280 bytes [RFC8200], they require a fragmentation protocol at the adaptation layer below IPv6. Accordingly, this document defines an optional fragmentation/reassembly mechanism for LPWAN technologies to support the IPv6 MTU requirement.
This document defines generic functionality and offers flexibility with regard to parameters settings and mechanism choices. Technology-specific settings are expected to be grouped into Profiles specified in other documents.
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
LPWAN network architectures are similar among them, but each LPWAN technology names architecture elements differently. In this document, we use terminology from [RFC8376], which identifies the following entities in a typical LPWAN network (see Figure 1):
o Devices (Dev) are the end-devices or hosts (e.g., sensors, actuators, etc.). There can be a very high density of devices per radio gateway.
o The Radio Gateway (RGW) 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 Application Server (App) is the end point of the application level protocol on the Internet side.
() () () | () () () () / \ +---------+ () () () () () () / \======| ^ | +-----------+ () () () | | <--|--> | |Application| () () () () / \==========| v |=============| (App) | () () () / \ +---------+ +-----------+ Dev RGWs NGW
Figure 1: LPWAN Architecture, simplified from that shown in RFC8376
This section defines the terminology and acronyms used in this document. It extends the terminology of [RFC8376].
The SCHC acronym is pronounced like “sheek” in English (or “chic” in French). Therefore, this document writes “a SCHC Packet” instead of “an SCHC Packet”.
Additional terminology for the optional SCHC Fragmentation / Reassembly mechanism (SCHC F/R) is found in Section 8.2.
SCHC can be characterized as an adaptation layer between an upper layer (typically, IPv6) and an underlying layer (typically, an LPWAN technology). SCHC comprises two sublayers (i.e. the Compression sublayer and the Fragmentation sublayer), as shown in Figure 2.
+----------------+
| IPv6 |
+- +----------------+
| | Compression |
SCHC < +----------------+
| | Fragmentation |
+- +----------------+
|LPWAN technology|
+----------------+
Figure 2: Protocol stack comprising IPv6, SCHC and an LPWAN technology
Before an upper layer packet (e.g., an IPv6 packet) is transmitted to the underlying layer, header compression is first attempted. The resulting packet is called a SCHC Packet, whether or not any compression is performed. If needed by the underlying layer, the optional SCHC Fragmentation MAY be applied to the SCHC Packet. The inverse operations take place at the receiver. This process is illustrated in Figure 3.
A packet (e.g., an IPv6 packet)
| ^
v |
+------------------+ +--------------------+
| SCHC Compression | | SCHC Decompression |
+------------------+ +--------------------+
| ^
| If no fragmentation (*) |
+-------------- SCHC Packet -------------->|
| |
v |
+--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+
| ^ | ^
| | | |
| +---------- SCHC ACK (+) -------------+ |
| |
+-------------- SCHC Fragments -------------------+
Sender Receiver
*: the decision to not use SCHC Fragmentation is left to each Profile.
+: optional, depends on Fragmentation mode.
Figure 3: SCHC operations at the Sender and the Receiver
The SCHC Packet is composed of the Compressed Header followed by the payload from the original packet (see Figure 4). The Compressed Header itself is composed of the Rule ID and a Compression Residue, which is the output of compressing the packet header with that Rule (see Section 7). The Compression Residue may be empty. Both the Rule ID and the Compression Residue potentially have a variable size, and are not necessarily a multiple of bytes in size.
|------- Compressed Header -------| +---------------------------------+--------------------+ | Rule ID | Compression Residue | Payload | +---------------------------------+--------------------+
Figure 4: SCHC Packet
Figure 5 maps the functional elements of Figure 3 onto the LPWAN architecture elements of Figure 1.
Dev App
+----------------+ +----+ +----+ +----+
| App1 App2 App3 | |App1| |App2| |App3|
| | | | | | | |
| UDP | |UDP | |UDP | |UDP |
| IPv6 | |IPv6| |IPv6| |IPv6|
| | | | | | | |
|SCHC C/D and F/R| | | | | | |
+--------+-------+ +----+ +----+ +----+
| +---+ +---+ +----+ +----+ . . .
+~ |RGW| === |NGW| == |SCHC| == |SCHC|...... Internet ....
+---+ +---+ |F/R | |C/D |
+----+ +----+
Figure 5: Architecture
SCHC C/D and SCHC F/R are located on both sides of the LPWAN transmission, hereafter called “the Dev side” and “the Network infrastructure side”.
The operation in the Uplink direction is as follows. The Device application uses IPv6 or IPv6/UDP protocols. Before sending the packets, the Dev compresses their headers using SCHC C/D and, if the SCHC Packet resulting from the compression needs to be fragmented by SCHC, SCHC F/R is performed (see Section 8). The resulting SCHC Fragments are sent to an LPWAN Radio Gateway (RGW) which forwards them to a Network Gateway (NGW). The NGW sends the data to a SCHC F/R for re-assembly (if needed) and then to the SCHC C/D for decompression. After decompression, the packet can be sent over the Internet to one or several LPWAN Application Servers (App).
The SCHC F/R and C/D on the Network infrastructure side can be part of the NGW, or located in the Internet as long as a tunnel is established between them and the NGW. For some LPWAN technologies, it may be suitable to locate the SCHC F/R functionality nearer the NGW, in order to better deal with time constraints of such technologies.
The SCHC C/Ds on both sides MUST share the same set of Rules. So MUST the SCHC F/Rs on both sides.
The operation in the Downlink direction is similar to that in the Uplink direction, only reversing the order in which the architecture elements are traversed.
Rule IDs identify the Rules used for Compression/Decompression or for Fragmentation/Reassembly.
The scope of the Rule ID of a Compression/Decompression Rule is the link between the SCHC C/D in a given Dev and the corresponding SCHC C/D in the Network infrastructure side. The scope of the Rule ID of a Fragmentation/Reassembly Rule is the link between the SCHC F/R in a given Dev and the corresponding SCHC F/R in the Network infrastructure side. If such a link is bidirectional, the scope includes both directions.
Inside their scopes, Rules for Compression/Decompression and Rules for Fragmentation/Reassembly share the same Rule ID space.
The size of the Rule IDs is not specified in this document, as it is implementation-specific and can vary according to the LPWAN technology and the number of Rules, among others. It is defined in Profiles.
The Rule IDs are used:
Compression with SCHC is based on using a set of Rules, called the Context, to compress or decompress headers. SCHC avoids Context synchronization traffic, which consumes considerable bandwidth in other header compression mechanisms such as RoHC [RFC5795]. Since the content of packets is highly predictable in LPWAN networks, static Contexts can be stored beforehand. The Contexts MUST be stored at both ends, and they can be learned by a provisioning protocol or by out of band means, or they can be pre-provisioned. The way the Contexts are provisioned is out of the scope of this document.
The main idea of the SCHC compression scheme is to transmit the Rule ID to the other end instead of sending known field values. This Rule ID identifies a Rule that matches the original packet values. Hence, when a value is known by both ends, it is only necessary to send the corresponding Rule ID over the LPWAN network. The manner by which Rules are generated is out of the scope of this document. The Rules MAY be changed at run-time but the mechanism is out of scope of this document.
The Context is a set of Rules. See Figure 6 for a high level, abstract representation of the Context. The formal specification of the representation of the Rules is outside the scope of this document.
Each Rule itself contains a list of Field Descriptions composed of a Field Identifier (FID), a Field Length (FL), a Field Position (FP), a Direction Indicator (DI), a Target Value (TV), a Matching Operator (MO) and a Compression/Decompression Action (CDA).
/-----------------------------------------------------------------\ | Rule N | /-----------------------------------------------------------------\| | Rule i || /-----------------------------------------------------------------\|| | (FID) Rule 1 ||| |+-------+--+--+--+------------+-----------------+---------------+||| ||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||| |+-------+--+--+--+------------+-----------------+---------------+||| ||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||| |+-------+--+--+--+------------+-----------------+---------------+||| ||... |..|..|..| ... | ... | ... |||| |+-------+--+--+--+------------+-----------------+---------------+||/ ||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||| |+-------+--+--+--+------------+-----------------+---------------+|/ | | \-----------------------------------------------------------------/
Figure 6: A Compression/Decompression Context
A Rule does not describe how the compressor parses a packet header to find and identify each field (e.g., the IPv6 Source Address, the UDP Destination Port or a CoAP URI path option). It is assumed that there is a protocol parser alongside SCHC that is able to identify all the fields encountered in the headers to be compressed, and to label them with a Field ID. Rules only describe the compression/decompression behavior for each header field, after it has been identified.
In a Rule, the Field Descriptions are listed in the order in which the fields appear in the packet header. The Field Descriptions describe the header fields with the following entries:
Rule IDs are sent by the compression function in one side and are received for the decompression function in the other side. In SCHC C/D, the Rule IDs are specific to the Context related to one Dev. Hence, multiple Dev instances, which refer to different header compression Contexts, MAY reuse the same Rule ID for different Rules. On the Network infrastructure side, in order to identify the correct Rule to be applied, the SCHC Decompressor needs to associate the Rule ID with the Dev identifier. Similarly, the SCHC Compressor on the Network infrastructure side first identifies the destination Dev before looking for the appropriate compression Rule (and associated Rule ID) in the Context of that Dev.
The compression/decompression process follows several phases:
|------------------- Compression Residue -------------------|
+-----------------+-----------------+-----+-----------------+
| field 1 residue | field 2 residue | ... | field N residue |
+-----------------+-----------------+-----+-----------------+
Figure 7: Compression Residue structure
Matching Operators (MOs) are functions used by both SCHC C/D endpoints. They are not typed and can be applied to integer, string or any other data type. The result of the operation can either be True or False. MOs are defined as follows:
The Compression Decompression Action (CDA) describes the actions taken during the compression of header fields and the inverse action taken by the decompressor to restore the original value.
| Action | Compression | Decompression |
|---|---|---|
| not-sent | elided | use TV stored in Rule |
| value-sent | send | use received value |
| mapping-sent | send index | retrieve value from TV list |
| LSB | send LSB | concat. TV and received value |
| compute-* | elided | recompute at decompressor |
| DevIID | elided | build IID from L2 Dev addr |
| AppIID | elided | build IID from L2 App addr |
Table 1 summarizes the basic actions that can be used to compress and decompress a field. The first column shows the action’s name. The second and third columns show the compression and decompression behaviors for each action.
If the field is identified in the Field Description as being of fixed length, then applying the CDA to compress this field results in a fixed amount of bits. The residue for that field is simply the bits resulting from applying the CDA to the field. This value may be empty (e.g., not-sent CDA), in which case the field residue is absent from the Compression Residue.
|- field residue -| +-----------------+ | value | +-----------------+
Figure 8: fixed sized field residue structure
If the field is identified in the Field Description as being of variable length, then applying the CDA to compress this field may result in a value of fixed size (e.g., not-sent or mapping-sent) or of variable size (e.g., value-sent or LSB). In the latter case, the residue for that field is the bits that result from applying the CDA to the field, preceded with the size of the value. The most significant bit of the size is stored to the left (leftmost bit of the residue field).
|--- field residue ---| +-------+-------------+ | size | value | +-------+-------------+
Figure 9: variable sized field residue structure
The size (using the unit defined in the FL) is encoded on 4, 12 or 28 bits as follows:
If the field is identified in the Field Description as being of variable length and this field is not present in the packet header being compressed, size 0 MUST be sent to denote its absence.
The not-sent action can be used when the field value is specified in a Rule and therefore known by both the Compressor and the Decompressor. This action SHOULD be used with the “equal” MO. If MO is “ignore”, there is a risk to have a decompressed field value different from the original field that was compressed.
The compressor does not send any residue for a field on which not-sent compression is applied.
The decompressor restores the field value with the Target Value stored in the matched Rule identified by the received Rule ID.
The value-sent action can be used when the field value is not known by both the Compressor and the Decompressor. The field is sent in its entirety, using the same bit order as in the original packet header.
If this action is performed on a variable length field, the size of the residue value (using the units defined in FL) MUST be sent as described in Section 7.5.2.
This action is generally used with the “ignore” MO.
The mapping-sent action is used to send an index (the index into the Target Value list of values) instead of the original value. This action is used together with the “match-mapping” MO.
On the compressor side, the match-mapping Matching Operator searches the TV for a match with the header field value. The mapping-sent CDA then sends the corresponding index as the field residue. The most significant bit of the index is stored to the left (leftmost bit of the residue field).
On the decompressor side, the CDA uses the received index to restore the field value by looking up the list in the TV.
The number of bits sent is the minimal size for coding all the possible indices.
The first element in the list MUST be represented by index value 0, and successive elements in the list MUST have indices incremented by 1.
The LSB action is used together with the “MSB(x)” MO to avoid sending the most significant part of the packet field if that part is already known by the receiving end.
The compressor sends the Least Significant Bits as the field residue value. The number of bits sent is the original header field length minus the length specified in the MSB(x) MO. The bits appear in the residue in the same bit order as in the original packet header.
The decompressor concatenates the x most significant bits of Target Value and the received residue value.
If this action is performed on a variable length field, the size of the residue value (using the units defined in FL) MUST be sent as described in Section 7.5.2.
These actions are used to process respectively the Dev and the App Interface Identifiers (DevIID and AppIID) of the IPv6 addresses. AppIID CDA is less common since most current LPWAN technologies frames contain a single L2 address, which is the Dev’s address.
The IID value MAY be computed from the Device ID present in the L2 header, or from some other stable identifier. The computation is specific to each Profile and MAY depend on the Device ID size.
In the downlink direction (Dw), at the compressor, the DevIID CDA may be used to generate the L2 addresses on the LPWAN, based on the packet’s Destination Address.
Some fields can be elided at the compressor and recomputed locally at the decompressor.
Because the field is uniquely identified by its Field ID (e.g., UDP length), the relevant protocol specification unambiguously defines the algorithm for such computation.
Examples of fields that know how to recompute themselves are UDP length, IPv6 length and UDP checksum.
In LPWAN technologies, the L2 MTU typically ranges from tens to hundreds of bytes. Some of these technologies do not have an internal fragmentation/reassembly mechanism.
The optional SCHC Fragmentation/Reassembly (SCHC F/R) functionality enables such LPWAN technologies to comply with the IPv6 MTU requirement of 1280 bytes [RFC8200]. It is OPTIONAL to implement per this specification, but Profiles may specify that it is REQUIRED.
This specification includes several SCHC F/R modes, which allow for a range of reliability options such as optional SCHC Fragment retransmission. More modes may be defined in the future.
The same SCHC F/R mode MUST be used for all SCHC Fragments of a given SCHC Packet. This document does not specify which mode(s) must be implemented and used over a specific LPWAN technology. That information will be given in Profiles.
SCHC allows transmitting non-fragmented SCHC Packet concurrently with fragmented SCHC Packets. In addition, SCHC F/R provides protocol elements that allow transmitting several fragmented SCHC Packets concurrently, i.e. interleaving the transmission of fragments from different fragmented SCHC Packets. A Profile MAY restrict the latter behavior.
The L2 Word size (see Section 4) determines the encoding of some messages. SCHC F/R usually generates SCHC Fragments and SCHC ACKs that are multiples of L2 Words.
This subsection describes the different elements that are used to enable the SCHC F/R functionality defined in this document. These elements include the SCHC F/R messages, tiles, windows, bitmaps, counters, timers and header fields.
The elements are described here in a generic manner. Their application to each SCHC F/R mode is found in Section 8.4.
SCHC F/R defines the following messages:
The format of these messages is provided in Section 8.3.
The SCHC Packet is fragmented into pieces, hereafter called tiles. The tiles MUST be non-empty and pairwise disjoint. Their union MUST be equal to the SCHC Packet.
See Figure 10 for an example.
SCHC Packet
+----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Tiles | | | | | | | | | | | | | |
+----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Figure 10: a SCHC Packet fragmented in tiles
Modes (see Section 8.4) MAY place additional constraints on tile sizes.
Each SCHC Fragment message carries at least one tile in its Payload, if the Payload field is present.
Some SCHC F/R modes may handle successive tiles in groups, called windows.
If windows are used
See Figure 11 for an example.
+---------------------------------------------...-------------+
| SCHC Packet |
+---------------------------------------------...-------------+
Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 | 3 |
Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|-- 28 -|
Figure 11: a SCHC Packet fragmented in tiles grouped in 29 windows, with WINDOW_SIZE = 5
Appendix E discusses the benefits of selecting one among multiple window sizes depending on the size of the SCHC Packet to be fragmented.
When windows are used
Each bit in the Bitmap for a window corresponds to a tile in the window. Each Bitmap has therefore WINDOW_SIZE bits. The bit at the left-most position corresponds to the tile numbered WINDOW_SIZE - 1. Consecutive bits, going right, correspond to sequentially decreasing tile indices. In Bitmaps for windows that are not the last one of a SCHC Packet, the bit at the right-most position corresponds to the tile numbered 0. In the Bitmap for the last window, the bit at the right-most position corresponds either to the tile numbered 0 or to a tile that is sent/received as “the last one of the SCHC Packet” without explicitly stating its number (see Section 8.3.1.2).
At the receiver
Some SCHC F/R modes can use the following timers and counters
The integrity of the fragmentation-reassembly process of a SCHC Packet MUST be checked at the receive end. A Profile MUST specify how integrity checking is performed.
It is RECOMMENDED that integrity checking be performed by computing a Reassembly Check Sequence (RCS) based on the SCHC Packet at the sender side and transmitting it to the receiver for comparison with the RCS locally computed after reassembly.
The RCS supports UDP checksum elision by SCHC C/D (see Section 10.11).
The CRC32 polynomial 0xEDB88320 (i.e., the reversed polynomial representation, which is used in the Ethernet standard [ETHERNET]) is RECOMMENDED as the default algorithm for computing the RCS.
The RCS MUST be computed on the full SCHC Packet concatenated with the padding bits, if any, of the SCHC Fragment carrying the last tile. The rationale is that the SCHC reassembler has no way of knowing the boundary between the last tile and the padding bits. Indeed, this requires decompressing the SCHC Packet, which is out of the scope of the SCHC reassembler.
The concatenation of the complete SCHC Packet and any padding bits, if present, of the last SCHC Fragment does not generally constitute an integer number of bytes. CRC libraries are usually byte-oriented. It is RECOMMENDED that the concatenation of the complete SCHC Packet and any last fragment padding bits be zero-extended to the next byte boundary and that the RCS be computed on that byte array.
The SCHC F/R messages contain the following fields (see the formats in Section 8.3):
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. Interleaved transmission of these is therefore possible.
This section defines the SCHC Fragment formats, the SCHC ACK format, the SCHC ACK REQ format and the SCHC Abort formats.
A SCHC Fragment conforms to the general format shown in Figure 12. It comprises a SCHC Fragment Header and a SCHC Fragment Payload. The SCHC Fragment Payload carries one or several tile(s).
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~ | Fragment Header | Fragment Payload | padding (as needed) +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
Figure 12: SCHC Fragment general format
The Regular SCHC Fragment format is shown in Figure 13. Regular SCHC Fragments are generally used to carry tiles that are not the last one of a SCHC Packet. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile.
|--- SCHC Fragment Header ----|
|-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | FCN | Fragment Payload | padding (as needed)
+-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
Figure 13: Detailed Header Format for Regular SCHC Fragments
The FCN field MUST NOT contain all bits set to 1.
Profiles MUST ensure that a SCHC Fragment with FCN equal to 0 (called an All-0 SCHC Fragment) is distinguishable by size, even in the presence of padding, from a SCHC ACK REQ message (see Section 8.3.3) with the same Rule ID value and with the same T, M and N values. This condition is met if the Payload is at least the size of an L2 Word. This condition is also met if the SCHC Fragment Header is a multiple of L2 Words.
The All-1 SCHC Fragment format is shown in Figure 14. The sender uses the All-1 SCHC Fragment format for the message that completes the emission of a fragmented SCHC Packet. The DTag field, the W field, the RCS field and the Payload are OPTIONAL, their presence is specified by each mode and Profile. At least one of RCS field or Payload MUST be present. The FCN field is all ones.
|-------- SCHC Fragment Header -------|
|-- T --|-M-|-- N --|-- U --|
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 11..1 | RCS | Frag Payload | pad. (as needed)
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
(FCN)
Figure 14: Detailed Header Format for the All-1 SCHC Fragment
Profiles MUST ensure that an All-1 SCHC Fragment message is distinguishable by size, even in the presence of padding, from a SCHC Sender-Abort message (see Section 8.3.4) with the same Rule ID value and with the same T, M and N values. This condition is met if the RCS is present and is at least the size of an L2 Word, or if the Payload is present and at least the size an L2 Word. This condition is also met if the SCHC Sender-Abort Header is a multiple of L2 Words.
The SCHC ACK message is shown in Figure 15. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile. The Compressed Bitmap field MUST be present in SCHC F/R modes that use windows, and MUST NOT be present in other modes.
|---- SCHC ACK Header ----|
|-- T --|-M-| 1 |
+--- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W |C=1| padding as needed (success)
+--- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
+--- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
| Rule ID | DTag | W |C=0|Compressed Bitmap| pad. as needed (failure)
+--- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
Figure 15: Format of the SCHC ACK message
The SCHC ACK Header contains a C bit (see Section 8.2.4).
If the C bit is set to 1 (integrity check successful), no Bitmap is carried.
If the C bit is set to 0 (integrity check not performed or failed) and if windows are used, a Compressed Bitmap for the window referred to by the W field is transmitted as specified in Section 8.3.2.1.
For transmission, the Compressed Bitmap in the SCHC ACK message is defined by the following algorithm (see Figure 16 for a follow-along example):
When one or more bits have effectively been dropped off as a result of the above algorithm, the SCHC ACK message is a multiple of L2 Words, no padding bits will be appended.
Because the SCHC Fragment sender knows the size of the original Bitmap, it can reconstruct the original Bitmap from the Compressed Bitmap received in the SCH ACK message.
Figure 16 shows an example where L2 Words are actually bytes and where the original Bitmap contains 17 bits, the last 15 of which are all set to 1.
|---- SCHC ACK Header ----|-------- Bitmap --------|
|-- T --|-M-| 1 |
+--- ... -+- ... -+---+---+---------------------------------+
| Rule ID | DTag | W |C=0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+--- ... -+- ... -+---+---+---------------------------------+
next L2 Word boundary ->|
Figure 16: SCHC ACK Header plus uncompressed Bitmap
Figure 17 shows that the last 14 bits are not sent.
|---- SCHC ACK Header ----|CpBmp|
|-- T --|-M-| 1 |
+--- ... -+- ... -+---+---+-----+
| Rule ID | DTag | W |C=0|1 0 1|
+--- ... -+- ... -+---+---+-----+
next L2 Word boundary ->|
Figure 17: Resulting SCHC ACK message with Compressed Bitmap
Figure 18 shows an example of a SCHC ACK with tile indices ranging from 6 down to 0, where the Bitmap indicates that the second and the fourth tile of the window have not been correctly received.
|---- SCHC ACK Header ----|--- Bitmap --|
|-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
+---------+-------+---+---+-------------+
| Rule ID | DTag | W |C=0|1 0 1 0 1 1 1| uncompressed Bitmap
+---------+-------+---+---+-------------+
next L2 Word boundary ->|<-- L2 Word -->|
+---------+-------+---+---+-------------+~~~+
| Rule ID | DTag | W |C=0|1 0 1 0 1 1 1|Pad| transmitted SCHC ACK
+---------+-------+---+---+-------------+~~~+
next L2 Word boundary ->|<-- L2 Word -->|
Figure 18: Example of a SCHC ACK message, missing tiles
Figure 19 shows an example of a SCHC ACK with FCN ranging from 6 down to 0, where integrity check has not been performed or has failed and the Bitmap indicates that there is no missing tile in that window.
|---- SCHC ACK Header ----|--- Bitmap --|
|-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
+---------+-------+---+---+-------------+
| Rule ID | DTag | W |C=0|1 1 1 1 1 1 1| with uncompressed Bitmap
+---------+-------+---+---+-------------+
next L2 Word boundary ->|
+--- ... -+- ... -+---+---+-+
| Rule ID | DTag | W |C=0|1| transmitted SCHC ACK
+--- ... -+- ... -+---+---+-+
next L2 Word boundary ->|
Figure 19: Example of a SCHC ACK message, no missing tile
The SCHC ACK REQ is used by a sender to request a SCHC ACK from the receiver. Its format is shown in Figure 20. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile. The FCN field is all zero.
|---- SCHC ACK REQ Header ----|
|-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 0..0 | padding (as needed) (no payload)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 20: SCHC ACK REQ format
When a SCHC Fragment sender needs to abort an on-going fragmented SCHC Packet transmission, it sends a SCHC Sender-Abort message to the SCHC Fragment receiver.
The SCHC Sender-Abort format is shown in Figure 21. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile. The FCN field is all ones.
|---- Sender-Abort Header ----|
|-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 11..1 | padding (as needed)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 21: SCHC Sender-Abort format
If the W field is present,
The SCHC Sender-Abort MUST NOT be acknowledged.
When a SCHC Fragment receiver needs to abort an on-going fragmented SCHC Packet transmission, it transmits a SCHC Receiver-Abort message to the SCHC Fragment sender.
The SCHC Receiver-Abort format is shown in Figure 22. The DTag field and the W field are OPTIONAL, their presence is specified by each mode and Profile.
|--- Receiver-Abort Header ---|
|--- T ---|-M-| 1 |
+--- ... ---+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag | W |C=1| 1..1| 1..1 |
+--- ... ---+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
next L2 Word boundary ->|<-- L2 Word -->|
Figure 22: SCHC Receiver-Abort format
If the W field is present,
The SCHC Receiver-Abort has the same header as a SCHC ACK message. The bits that follow the SCHC Receiver-Abort Header MUST be as follows
Such a bit pattern never occurs in a legitimate SCHC ACK. This is how the fragment sender recognizes a SCHC Receiver-Abort.
The SCHC Receiver-Abort MUST NOT be acknowledged.
This specification includes several SCHC F/R modes, which
More modes may be defined in the future.
Appendix B provides examples of fragmentation sessions based on the modes described hereafter.
Appendix C provides examples of Finite State Machines implementing the SCHC F/R modes described hereafter.
The No-ACK mode has been designed under the assumption that data unit out-of-sequence delivery does not occur between the entity performing fragmentation and the entity performing reassembly. This mode supports LPWAN technologies that have a variable MTU.
In No-ACK mode, there is no communication from the fragment receiver to the fragment sender. The sender transmits all the SCHC Fragments without expecting any acknowledgement. Therefore, No-ACK does not require bidirectional links: unidirectional links are just fine.
In No-ACK mode, only the All-1 SCHC Fragment is padded as needed. The other SCHC Fragments are intrinsically aligned to L2 Words.
The tile sizes are not required to be uniform. Windows are not used. The Retransmission Timer is not used. The Attempts counter is not used.
Each Profile MUST specify which Rule ID value(s) correspond to SCHC F/R messages operating in this mode.
The W field MUST NOT be present in the SCHC F/R messages. SCHC ACK MUST NOT be sent. SCHC ACK REQ MUST NOT be sent. SCHC Sender-Abort MAY be sent. SCHC Receiver-Abort MUST NOT be sent.
The value of N (size of the FCN field) is RECOMMENDED to be 1.
Each Profile, for each Rule ID value, MUST define
Each Profile, for each Rule ID value, MAY define
For each active pair of Rule ID and DTag values, the receiver MUST maintain an Inactivity Timer. If the receiver is under-resourced to do this, it MUST silently drop the related messages.
At the beginning of the fragmentation of a new SCHC Packet, the fragment sender MUST select a Rule ID and DTag value pair for this SCHC Packet.
Each SCHC Fragment MUST contain exactly one tile in its Payload. The tile MUST be at least the size of an L2 Word. The sender MUST transmit the SCHC Fragments messages in the order that the tiles appear in the SCHC Packet. Except for the last tile of a SCHC Packet, each tile MUST be of a size that complements the SCHC Fragment Header so that the SCHC Fragment is a multiple of L2 Words without the need for padding bits. Except for the last one, the SCHC Fragments MUST use the Regular SCHC Fragment format specified in Section 8.3.1.1. The SCHC Fragment that carries the last tile MUST be an All-1 SCHC Fragment, described in Section 8.3.1.2.
The sender MAY transmit a SCHC Sender-Abort.
Figure 37 shows an example of a corresponding state machine.
Upon receiving each Regular SCHC Fragment,
On receiving an All-1 SCHC Fragment,
On expiration of the Inactivity Timer, the receiver MUST drop the SCHC Packet being reassembled.
On receiving a SCHC Sender-Abort, the receiver MAY drop the SCHC Packet being reassembled.
Figure 38 shows an example of a corresponding state machine.
The ACK-Always mode has been designed under the following assumptions
In ACK-Always mode, windows are used. An acknowledgement, positive or negative, is transmitted by the fragment receiver to the fragment sender at the end of the transmission of each window of SCHC Fragments.
The tiles are not required to be of uniform size. In ACK-Always mode, only the All-1 SCHC Fragment is padded as needed. The other SCHC Fragments are intrinsically aligned to L2 Words.
Briefly, the algorithm is as follows: after a first blind transmission of all the tiles of a window, the fragment sender iterates retransmitting the tiles that are reported missing until the fragment receiver reports that all the tiles belonging to the window have been correctly received, or until too many attempts were made. The fragment sender only advances to the next window of tiles when it has ascertained that all the tiles belonging to the current window have been fully and correctly received. This results in a per-window lock-step behavior between the sender and the receiver.
Each Profile MUST specify which Rule ID value(s) correspond to SCHC F/R messages operating in this mode.
The W field MUST be present and its size M MUST be 1 bit.
Each Profile, for each Rule ID value, MUST define
For each active pair of Rule ID and DTag values, the sender MUST maintain
For each active pair of Rule ID and DTag values, the receiver MUST maintain
At the beginning of the fragmentation of a new SCHC Packet, the fragment sender MUST select a Rule ID and DTag value pair for this SCHC Packet.
Each SCHC Fragment MUST contain exactly one tile in its Payload. All tiles with the index 0, as well as the last tile, MUST be at least the size of an L2 Word.
In all SCHC Fragment messages, the W field MUST be filled with the least significant bit of the window number that the sender is currently processing.
For a SCHC Fragment that carries a tile other than the last one of the SCHC Packet,
The SCHC Fragment that carries the last tile MUST be an All-1 SCHC Fragment, described in Section 8.3.1.2.
The fragment sender MUST start by transmitting the window numbered 0.
All message receptions being discussed in the rest of this section are to be understood as “matching the RuleID and DTag pair being processed”, even if not spelled out, for brevity.
The sender starts by a “blind transmission” phase, in which it MUST transmit all the tiles composing the window, in decreasing tile index order.
Then, it enters a “retransmission phase” in which it MUST initialize an Attempts counter to 0, it MUST start a Retransmission Timer and it MUST await a SCHC ACK. Then,
At any time,
Figure 39 shows an example of a corresponding state machine.
On receiving a SCHC Fragment with a Rule ID and DTag pair not being processed at that time
In the rest of this section, “local W bit” means the least significant bit of the window counter of the receiver.
On reception of any SCHC F/R message for the RuleID and DTag pair being processed, the receiver MUST reset the Inactivity Timer pertaining to that RuleID and DTag pair.
All message receptions being discussed in the rest of this section are to be understood as “matching the RuleID and DTag pair being processed”, even if not spelled out, for brevity.
The receiver MUST first initialize an empty Bitmap for the first window, then enter an “acceptance phase”, in which
In the “retransmission phase”:
In the “clean-up phase”:
At any time, on sending a SCHC ACK, the receiver MUST increment the Attempts counter.
At any time, on incrementing its window counter, the receiver MUST reset the Attempts counter.
At any time, on expiration of the Inactivity Timer, on receiving a SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the receiver MUST send a SCHC Receiver-Abort and it MAY exit the receive process for that SCHC Packet.
Figure 40 shows an example of a corresponding state machine.
The ACK-on-Error mode supports LPWAN technologies that have variable MTU and out-of-order delivery. It operates with links that provide a feedback path from the reassembler to the fragmenter. See Appendix F for a discussion on using ACK-on-Error mode on quasi-bidirectional links.
In ACK-on-Error mode, windows are used.
All tiles, but the last one and the penultimate one, MUST be of equal size, hereafter called “regular”. The size of the last tile MUST be smaller than or equal to the regular tile size. Regarding the penultimate tile, a Profile MUST pick one of the following two options:
A SCHC Fragment message carries one or several contiguous tiles, which may span multiple windows. A SCHC ACK reports on the reception of exactly one window of tiles.
See Figure 23 for an example.
+---------------------------------------------...-----------+
| SCHC Packet |
+---------------------------------------------...-----------+
Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 |3|
Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|- 28-|
SCHC Fragment msg |-----------|
Figure 23: a SCHC Packet fragmented in tiles, ACK-on-Error mode
The W field is wide enough that it unambiguously represents an absolute window number. The fragment receiver sends SCHC ACKs to the fragment sender about windows for which tiles are missing. No SCHC ACK is sent by the fragment receiver for windows that it knows have been fully received.
The fragment sender retransmits SCHC Fragments for tiles that are reported missing. It can advance to next windows even before it has ascertained that all tiles belonging to previous windows have been correctly received, and can still later retransmit SCHC Fragments with tiles belonging to previous windows. Therefore, the sender and the receiver may operate in a decoupled fashion. The fragmented SCHC Packet transmission concludes when
Each Profile MUST specify which Rule ID value(s) correspond to SCHC F/R messages operating in this mode.
The W field MUST be present in the SCHC F/R messages.
Each Profile, for each Rule ID value, MUST define
For each active pair of Rule ID and DTag values, the sender MUST maintain
For each active pair of Rule ID and DTag values, the receiver MUST maintain
At the beginning of the fragmentation of a new SCHC Packet,
A Regular SCHC Fragment message carries in its payload one or more tiles. If more than one tile is carried in one Regular SCHC Fragment
Tiles that are not the last one MUST be sent in Regular SCHC Fragments specified in Section 8.3.1.1. The FCN field MUST contain the tile index of the first tile sent in that SCHC Fragment.
In a Regular SCHC Fragment message, the sender MUST fill the W field with the window number of the first tile sent in that SCHC Fragment.
Depending on the Profile, the last tile of a SCHC Packet MUST be sent either
In an All-1 SCHC Fragment message, the sender MUST fill the W field with the window number of the last tile of the SCHC Packet.
The fragment sender MUST send SCHC Fragments such that, all together, they contain all the tiles of the fragmented SCHC Packet.
The fragment sender MUST send at least one All-1 SCHC Fragment.
The fragment sender MUST listen for SCHC ACK messages after having sent
A Profile MAY specify other times at which the fragment sender MUST listen for SCHC ACK messages. For example, this could be after sending a complete window of tiles.
Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC ACK REQ,
On Retransmission Timer expiration
All message receptions being discussed in the rest of this section are to be understood as “matching the RuleID and DTag pair being processed”, even if not spelled out, for brevity.
On receiving a SCHC ACK,
See Figure 41 for one among several possible examples of a Finite State Machine implementing a sender behavior obeying this specification.
On receiving a SCHC Fragment with a Rule ID and DTag pair not being processed at that time
On reception of any SCHC F/R message for the RuleID and DTag pair being processed, the receiver MUST reset the Inactivity Timer pertaining to that RuleID and DTag pair.
All message receptions being discussed in the rest of this section are to be understood as “matching the RuleID and DTag pair being processed”, even if not spelled out, for brevity.
On receiving a SCHC Fragment message, the receiver determines what tiles were received, based on the payload length and on the W and FCN fields of the SCHC Fragment.
On receiving a SCHC ACK REQ or an All-1 SCHC Fragment,
A Profile MAY specify other times and circumstances at which a receiver sends a SCHC ACK, and which window the SCHC ACK reports about in these circumstances.
Upon sending a SCHC ACK, the receiver MUST increase the Attempts counter.
After receiving an All-1 SCHC Fragment, a receiver MUST check the integrity of the reassembled SCHC Packet at least every time it prepares for sending a SCHC ACK for the last window.
Upon receiving a SCHC Sender-Abort, the receiver MAY exit with an error condition.
Upon expiration of the Inactivity Timer, the receiver MUST send a SCHC Receiver-Abort and it MAY exit with an error condition.
On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST send a SCHC Receiver-Abort and it MAY exit with an error condition.
Reassembly of the SCHC Packet concludes when
See Figure 42 for one among several possible examples of a Finite State Machine implementing a receiver behavior obeying this specification, and that is meant to match the sender Finite State Machine of Figure 41.
SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does not have any alignment prerequisite. The size of SCHC Packets can be any number of bits.
If the layer below SCHC constrains the payload to align to some boundary, called L2 Words (for example, bytes), the SCHC messages MUST be padded. When padding occurs, the number of appended bits MUST be strictly less than the L2 Word size.
If a SCHC Packet is sent unfragmented (see Figure 24), it is padded as needed for transmission.
If a SCHC Packet needs to be fragmented for transmission, it is not padded in itself. Only the SCHC F/R messages are padded as needed for transmission. Some SCHC F/R messages are intrinsically aligned to L2 Words.
A packet (e.g., an IPv6 packet)
| ^ (padding bits
v | dropped)
+------------------+ +--------------------+
| SCHC Compression | | SCHC Decompression |
+------------------+ +--------------------+
| ^
| If no fragmentation |
+---- SCHC Packet + padding as needed ----->|
| | (integrity
v | checked)
+--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+
| ^ | ^
| | | |
| +--- SCHC ACK + padding as needed --+ |
| |
+------- SCHC Fragments + padding as needed---------+
Sender Receiver
Figure 24: SCHC operations, including padding as needed
Each Profile MUST specify the size of the L2 Word. The L2 Word might actually be a single bit, in which case no padding will take place at all.
A Profile MAY define the value of the padding bits. The RECOMMENDED value is 0.
This section lists the IPv6 and UDP header fields and describes how they can be compressed. An example of a set of Rules for UDP/IPv6 header compression is provided in Appendix A.
The IPv6 version field is labeled by the protocol parser as being the “version” field of the IPv6 protocol. Therefore, it only exists for IPv6 packets. In the Rule, TV is set to 6, MO to “ignore” and CDA to “not-sent”.
If the DiffServ field does not vary and is known by both sides, the Field Descriptor in the Rule SHOULD contain a TV with this well-known value, an “equal” MO and a “not-sent” CDA.
Otherwise (e.g., ECN bits are to be transmitted), two possibilities can be considered depending on the variability of the value:
If the flow label is not set, i.e. its value is zero, the Field Descriptor in the Rule SHOULD contain a TV set to zero, an “equal” MO and a “not-sent” CDA.
If the flow label is set to a pseudo-random value according to [RFC6437], in the Rule, TV is not set to any particular value, MO is set to “ignore” and CDA is set to “value-sent”.
If the flow label is set according to some prior agreement, i.e. by a flow state establishment method as allowed by [RFC6437], the Field Descriptor in the Rule SHOULD contain a TV with this agreed-upon value, an “equal” MO and a “not-sent” CDA.
This field can be elided for the transmission on the LPWAN network. The SCHC C/D recomputes the original payload length value. In the Field Descriptor, TV is not set, MO is set to “ignore” and CDA is “compute-*”.
If the Next Header field does not vary and is known by both sides, the Field Descriptor in the Rule SHOULD contain a TV with this Next Header value, the MO SHOULD be “equal” and the CDA SHOULD be “not-sent”.
Otherwise, TV is not set in the Field Descriptor, MO is set to “ignore” and CDA is set to “value-sent”. Alternatively, a matching-list MAY also be used.
The field behavior for this field is different for uplink (Up) and downlink (Dw). In Up, since there is no IP forwarding between the Dev and the SCHC C/D, the value is relatively constant. On the other hand, the Dw value depends on Internet routing and can change more frequently. The Direction Indicator (DI) can be used to distinguish both directions:
As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit long fields; one for the prefix and one for the Interface Identifier (IID). These fields SHOULD be compressed. To allow for a single Rule being used for both directions, these values are identified by their role (Dev or App) and not by their position in the header (source or destination).
Both ends MUST be configured with the appropriate prefixes. For a specific flow, the source and destination prefixes can be unique and stored in the Context. In that case, the TV for the source and destination prefixes contain the values, the MO is set to “equal” and the CDA is set to “not-sent”.
If the Rule is intended to compress packets with different prefix values, match-mapping SHOULD be used. The different prefixes are listed in the TV, the MO is set to “match-mapping” and the CDA is set to “mapping-sent”. See Figure 26.
Otherwise, the TV is not set, the MO is set to “ignore” 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”. On LPWAN technologies where the frames carry a single identifier (corresponding to the Dev.), AppIID cannot be used.
As described in [RFC8065], it may be undesirable to build the Dev IPv6 IID out of the Dev address. Another static value is used instead. In that case, the TV contains the static 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”.
It may also happen that the IID variability only expresses itself on a few bytes. In that case, the TV is set to the stable part of the IID, the MO is set to “MSB” and the CDA is set to “LSB”.
Finally, the IID can be sent in its entirety 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”.
This document does not provide recommendations on how to compress IPv6 extension headers.
To allow for a single Rule being used for both directions, the UDP port values are identified by their role (Dev or App) and not by their position in the header (source or destination). The SCHC C/D MUST be aware of the traffic direction (Uplink, Downlink) to select the appropriate field. The following Rules apply for Dev and App port numbers.
If both ends know the port number, it can be elided. The TV contains the port number, the MO is set to “equal” and the CDA is set to “not-sent”.
If the port variation is on few bits, the TV contains the stable part of the port number, the MO is set to “MSB” and the CDA is set to “LSB”.
If some well-known values are used, the TV can contain the list of these values, the MO is set to “match-mapping” and the CDA is set to “mapping-sent”.
Otherwise the port numbers are sent over the LPWAN. The TV is not set, the MO is set to “ignore” and the CDA is set to “value-sent”.
The UDP length can be computed from the received data. The TV is not set, the MO is set to “ignore” and the CDA is set to “compute-*”.
The UDP checksum operation is mandatory with IPv6 for most packets but there are exceptions [RFC8200].
For instance, protocols that use UDP as a tunnel encapsulation may enable zero-checksum mode for a specific port (or set of ports) for sending and/or receiving. [RFC8200] requires any node implementing zero-checksum mode to follow the requirements specified in “Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums” [RFC6936].
6LoWPAN Header Compression [RFC6282] also specifies that a UDP checksum can be elided by the compressor and re-computed by the decompressor when an upper layer guarantees the integrity of the UDP payload and pseudo-header. A specific example of this is when a message integrity check protects the compressed message between the compressor that elides the UDP checksum and the decompressor that computes it, with a strength that is identical or better to the UDP checksum.
Similarly, a SCHC compressor MAY elide the UDP checksum when another layer guarantees at least equal integrity protection for the UDP payload and the pseudo-header. In this case, the TV is not set, the MO is set to “ignore” and the CDA is set to “compute-*”.
In particular, when SCHC fragmentation is used, a fragmentation RCS of 2 bytes or more provides equal or better protection than the UDP checksum; in that case, if the compressor is collocated with the fragmentation point and the decompressor is collocated with the packet reassembly point, and if the SCHC Packet is fragmented even when it would fit unfragmented in the L2 MTU, then the compressor MAY verify and then elide the UDP checksum. Whether and when the UDP Checksum is elided is to be specified in the Profile.
Since the compression happens before the fragmentation, implementers should understand the risks when dealing with unprotected data below the transport layer and take special care when manipulating that data.
In other cases, the checksum SHOULD be explicitly sent. The TV is not set, the MO is set to “ignore” and the CDA is set to “value-sent”.
This document has no request to IANA.
As explained in Section 5, SCHC is expected to be implemented on top of LPWAN technologies, which are expected to implement security measures.
In this section, we analyze the potential security threats that could be introduced into an LPWAN by adding the SCHC functionalities.
Let’s assume that an attacker is able to send a forged SCHC Packet to a SCHC Decompressor.
Let’s first consider the case where the Rule ID contained in that forged SCHC Packet does not correspond to a Rule allocated in the Rule table. An implementation should detect that the Rule ID is invalid and should silently drop the offending SCHC Packet.
Let’s now consider that the Rule ID corresponds to a Rule in the table. With the CDAs defined in this document, the reconstructed packet is at most a constant number of bits bigger than the SCHC Packet that was received. This assumes that the compute-* decompression actions produce a bounded number of bits, irrespective of the incoming SCHC Packet. This property is true for IPv6 Length, UDP Length and UDP Checksum, for which the compute-* CDA is recommended by this document.
As a consequence, SCHC Decompression does not amplify attacks, beyond adding a bounded number of bits to the SCHC Packet received. This bound is determined by the Rule stored in the receiving device.
As a general safety measure, a SCHC Decompressor should never re-construct a packet larger than MAX_PACKET_SIZE (defined in a Profile, with 1500 bytes as generic default).
Some packet compression methods are known to be susceptible to attacks, such as BREACH and CRIME. The attack involves injecting arbitrary data into the packet and observing the resulting compressed packet size. The observed size potentially reflects correlation between the arbitrary data and some content that was meant to remain secret, such as a security token, thereby allowing the attacker to get at the secret.
By contrast, SCHC Compression takes place header field by header field, with the SCHC Packet being a mere concatenation of the compression residues of each of the individual field. Any correlation between header fields does not result in a change in the SCHC Packet size compressed under the same Rule.
If SCHC C/D is used to compress packets that include a secret information field, such as a token, the Rule set should be designed so that the size of the compression residue for the field to remain secret is the same irrespective of the value of the secret information. This is achieved by e.g., sending this field in extenso with the “ignore” MO and the “value-sent” CDA. This recommendation is disputable if it is ascertained that the Rule set itself will remain secret.
As explained in Section 7.3, using FPs with value 0 in Field Descriptors in a Rule may result in header fields appearing in the decompressed packet in an order different from that in the original packet. Likewise, as stated in Section 7.5.3, using an “ignore” MO together with a “not-sent” CDA will result in the header field taking the TV value, which is likely to be different from the original value.
Depending on the protocol, the order of header fields in the packet may be functionally significant or not.
Furthermore, if the packet is protected by a checksum or a similar integrity protection mechanism, and if the checksum is transmitted instead of being recomputed as part of the decompression, these situations may result in the packet being considered corrupt and dropped.
Let’s assume that an attacker is able to send a forged SCHC Fragment to a SCHC Reassembler.
A node can perform a buffer reservation attack: the receiver will reserve buffer space for the SCHC Packet. If the implementation has only one buffer, other incoming fragmented SCHC Packets will be dropped while the reassembly buffer is occupied during the reassembly timeout. Once that timeout expires, the attacker can repeat the same procedure, and iterate, thus creating a denial of service attack. An implementation may have multiple reassembly buffers. The cost to mount this attack is linear with the number of buffers at the target node. Better, the cost for an attacker can be increased if individual fragments of multiple SCHC Packets can be stored in the reassembly buffer. The finer grained the reassembly buffer (downto the smallest tile size), the higher the cost of the attack. If buffer overload does occur, a smart receiver could selectively discard SCHC Packets being reassembled based on the sender behavior, which may help identify which SCHC Fragments have been sent by the attacker. Another mild counter-measure is for the target to abort the fragmentation/reassembly session as early as it detects a non-identical SCHC Fragment duplicate, anticipating for an eventual corrupt SCHC Packet, so as to save the sender the hassle of sending the rest of the fragments for this SCHC Packet.
Let’s assume that an attacker is able to send a forged SCHC Fragment to a SCHC Reassembler. The malicious node is additionally assumed to be able to hear an incoming communication destined to the target node.
It can then send a forged SCHC Fragment that looks like it belongs to a SCHC Packet already being reassembled at the target node. This can cause the SCHC Packet to be considered corrupt and be dropped by the receiver. The amplification happens here by a single spoofed SCHC Fragment rendering a full sequence of legit SCHC Fragments useless. If the target uses ACK-Always or ACK-on-Error mode, such a malicious node can also interfere with the acknowledgement and repetition algorithm of SCHC F/R. A single spoofed ACK, with all bitmap bits set to 0, will trigger the repetition of WINDOW_SIZE tiles. This protocol loop amplification depletes the energy source of the target node and consumes the channel bandwidth. Similarly, a spoofed ACK REQ will trigger the sending of a SCHC ACK, which may be much larger than the ACK REQ if WINDOW_SIZE is large. These consequences should be borne in mind when defining profiles for SCHC over specific LPWAN technologies.
Fragmentation is known for potentially allowing to force through a Network Inspection device (e.g., firewall) packets that would be rejected if unfragmented. This involves sending overlapping fragments to rewrite fields whose initial value led the Network Inspection device to allow the flow go through.
SCHC F/R is expected to be used over one LPWAN link, where no Network Inspection device is expected to sit. As described in Section 5.2, even if the SCHC F/R on the Network infrastructure side is located in the Internet, a tunnel is to be established between it and the NGW.
SCHC F/R allocates a DTag value to fragments belonging to the same SCHC Packet. Concerns were raised that, if DTag is a wide counter that is incremented in a predictable fashion for each new fragmented SCHC Packet, it might lead to a privacy issue, such as enabling tracking of a device across LPWANs.
However, SCHC F/R is expected to be used over exactly one LPWAN link. As described in Section 5.2, even if the SCHC F/R on the Network infrastructure side is located in the Internet, a tunnel is to be established between it and the NGW. Therefore, assuming the tunnel provides confidentiality, neither the DTag field nor any other SCHC-introduced field is visible over the Internet.
Thanks to (in alphabetical order) Sergio Aguilar Romero, David Black, Carsten Bormann, Deborah Brungard, Brian Carpenter, Philippe Clavier, Alissa Cooper, Roman Danyliw, Daniel Ducuara Beltran, Diego Dujovne, Eduardo Ingles Sanchez, Rahul Jadhav, Benjamin Kaduk, Arunprabhu Kandasamy, Suresh Krishnan, Mirja Kuehlewind, Barry Leiba, Sergio Lopez Bernal, Antoni Markovski, Alexey Melnikov, Georgios Papadopoulos, Alexander Pelov, Charles Perkins, Edgar Ramos, Alvaro Retana, Adam Roach, Shoichi Sakane, Joseph Salowey, Pascal Thubert, and Eric Vyncke for useful design considerations, reviews and comments.
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.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC6936] | Fairhurst, G. and M. Westerlund, "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936, DOI 10.17487/RFC6936, April 2013. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
| [RFC8200] | Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017. |
| [RFC8376] | Farrell, S., "Low-Power Wide Area Network (LPWAN) Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018. |
| [ETHERNET] | "IEEE Standard for Ethernet", IEEE standard, DOI 10.1109/ieeestd.2018.8457469, n.d.. |
| [RFC4944] | Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007. |
| [RFC5795] | Sandlund, K., Pelletier, G. and L-E. Jonsson, "The RObust Header Compression (ROHC) Framework", RFC 5795, DOI 10.17487/RFC5795, March 2010. |
| [RFC6282] | Hui, J. and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, September 2011. |
| [RFC6437] | Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme, "IPv6 Flow Label Specification", RFC 6437, DOI 10.17487/RFC6437, November 2011. |
| [RFC7136] | Carpenter, B. and S. Jiang, "Significance of IPv6 Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, February 2014. |
| [RFC8065] | Thaler, D., "Privacy Considerations for IPv6 Adaptation-Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065, February 2017. |
This section gives some scenarios of the compression mechanism for IPv6/UDP. The goal is to illustrate the behavior of SCHC.
The mechanisms defined in this document can be applied to a 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 Dev (using ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to beta::1/64. The last flow is for legacy applications using different ports numbers, the destination IPv6 address prefix is gamma::1/64.
Figure 25 presents the protocol stack. IPv6 and UDP are represented with dotted lines since these protocols are compressed on the radio link.
Management Data
+----------+---------+---------+
| CoAP | CoAP | legacy |
+----||----+---||----+---||----+
. UDP . UDP | UDP |
................................
. IPv6 . IPv6 . IPv6 .
+------------------------------+
| SCHC Header compression |
| and fragmentation |
+------------------------------+
| LPWAN L2 technologies |
+------------------------------+
Dev or NGW
Figure 25: Simplified Protocol Stack for LP-WAN
In some LPWAN technologies, only the Devs have a device ID. When such technologies are used, it is necessary to statically define an IID for the Link Local address for the SCHC C/D.
Rule 0 Special Rule ID used to tag an uncompressed UDP/IPV6 packet. Rule 1 +----------------+--+--+--+---------+--------+------------++------+ | Field |FL|FP|DI| Value | Match | Comp Decomp|| Sent | | | | | | | Opera. | Action ||[bits]| +----------------+--+--+--+---------+---------------------++------+ |IPv6 Version |4 |1 |Bi|6 | ignore | not-sent || | |IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || | |IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || | |IPv6 Length |16|1 |Bi| | ignore | compute-* || | |IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || | |IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || | |IPv6 DevPrefix |64|1 |Bi|FE80::/64| equal | not-sent || | |IPv6 DevIID |64|1 |Bi| | ignore | DevIID || | |IPv6 AppPrefix |64|1 |Bi|FE80::/64| equal | not-sent || | |IPv6 AppIID |64|1 |Bi|::1 | equal | not-sent || | +================+==+==+==+=========+========+============++======+ |UDP DevPort |16|1 |Bi|123 | equal | not-sent || | |UDP AppPort |16|1 |Bi|124 | equal | not-sent || | |UDP Length |16|1 |Bi| | ignore | compute-* || | |UDP checksum |16|1 |Bi| | ignore | compute-* || | +================+==+==+==+=========+========+============++======+ Rule 2 +----------------+--+--+--+---------+--------+------------++------+ | Field |FL|FP|DI| Value | Match | Action || Sent | | | | | | | Opera. | Action ||[bits]| +----------------+--+--+--+---------+--------+------------++------+ |IPv6 Version |4 |1 |Bi|6 | ignore | not-sent || | |IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || | |IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || | |IPv6 Length |16|1 |Bi| | ignore | compute-* || | |IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || | |IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || | |IPv6 DevPrefix |64|1 |Bi|[alpha/64, match- |mapping-sent|| 1 | | | | | |fe80::/64] mapping| || | |IPv6 DevIID |64|1 |Bi| | ignore | DevIID || | |IPv6 AppPrefix |64|1 |Bi|[beta/64,| match- |mapping-sent|| 2 | | | | | |alpha/64,| mapping| || | | | | | |fe80::64]| | || | |IPv6 AppIID |64|1 |Bi|::1000 | equal | not-sent || | +================+==+==+==+=========+========+============++======+ |UDP DevPort |16|1 |Bi|5683 | equal | not-sent || | |UDP AppPort |16|1 |Bi|5683 | equal | not-sent || | |UDP Length |16|1 |Bi| | ignore | compute-* || | |UDP checksum |16|1 |Bi| | ignore | compute-* || | +================+==+==+==+=========+========+============++======+ Rule 3 +----------------+--+--+--+---------+--------+------------++------+ | Field |FL|FP|DI| Value | Match | Action || Sent | | | | | | | Opera. | Action ||[bits]| +----------------+--+--+--+---------+--------+------------++------+ |IPv6 Version |4 |1 |Bi|6 | ignore | not-sent || | |IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || | |IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || | |IPv6 Length |16|1 |Bi| | ignore | compute-* || | |IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || | |IPv6 Hop Limit |8 |1 |Up|255 | ignore | not-sent || | |IPv6 Hop Limit |8 |1 |Dw| | ignore | value-sent || 8 | |IPv6 DevPrefix |64|1 |Bi|alpha/64 | equal | not-sent || | |IPv6 DevIID |64|1 |Bi| | ignore | DevIID || | |IPv6 AppPrefix |64|1 |Bi|gamma/64 | equal | not-sent || | |IPv6 AppIID |64|1 |Bi|::1000 | equal | not-sent || | +================+==+==+==+=========+========+============++======+ |UDP DevPort |16|1 |Bi|8720 | MSB(12)| LSB || 4 | |UDP AppPort |16|1 |Bi|8720 | MSB(12)| LSB || 4 | |UDP Length |16|1 |Bi| | ignore | compute-* || | |UDP checksum |16|1 |Bi| | ignore | compute-* || | +================+==+==+==+=========+========+============++======+
Figure 26: Context Rules
Figure 26 describes a example of a Rule set.
In this example, 0 was chosen as the special Rule ID that tags packets that cannot be compressed with any compression Rule.
All the fields described in Rules 1-3 are present in the IPv6 and UDP headers. The DevIID-DID value is found in the L2 header.
Rules 2-3 use global addresses. The way the Dev learns the prefix is not in the scope of the document.
Rule 3 compresses each port number to 4 bits.
This section provides examples for the various fragment reliability modes specified in this document. In the drawings, Bitmaps are shown in their uncompressed form.
Figure 27 illustrates the transmission in No-ACK mode of a SCHC Packet that needs 11 SCHC Fragments. FCN is 1 bit wide.
Sender Receiver
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-----FCN=1 + RCS --->| Integrity check: success
(End)
Figure 27: No-ACK mode, 11 SCHC Fragments
In the following examples, N (the size of the FCN field) is 3 bits. The All-1 FCN value is 7.
Figure 28 illustrates the transmission in ACK-on-Error mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, WINDOW_SIZE=7 and no lost SCHC Fragment.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4----->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2----->|
|-----W=0, FCN=1----->|
|-----W=0, FCN=0----->|
(no ACK)
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4----->|
|--W=1, FCN=7 + RCS-->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1
(End)
Figure 28: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment, no lost SCHC Fragment.
Figure 29 illustrates the transmission in ACK-on-Error mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, WINDOW_SIZE=7 and three lost SCHC Fragments.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->|
|-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| 6543210
|<-- ACK, W=0, C=0 ---| Bitmap:1101011
|-----W=0, FCN=4----->|
|-----W=0, FCN=2----->|
(no ACK)
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->|
|- W=1, FCN=7 + RCS ->| Integrity check: failure
|<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
|-----W=1, FCN=4----->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1
(End)
Figure 29: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment, lost SCHC Fragments.
Figure 30 shows an example of a transmission in ACK-on-Error mode of a SCHC Packet fragmented in 73 tiles, with N=5, WINDOW_SIZE=28, M=2 and 3 lost SCHC Fragments.
Sender Receiver
|-----W=0, FCN=27----->| 4 tiles sent
|-----W=0, FCN=23----->| 4 tiles sent
|-----W=0, FCN=19----->| 4 tiles sent
|-----W=0, FCN=15--X-->| 4 tiles sent (not received)
|-----W=0, FCN=11----->| 4 tiles sent
|-----W=0, FCN=7 ----->| 4 tiles sent
|-----W=0, FCN=3 ----->| 4 tiles sent
|-----W=1, FCN=27----->| 4 tiles sent
|-----W=1, FCN=23----->| 4 tiles sent
|-----W=1, FCN=19----->| 4 tiles sent
|-----W=1, FCN=15----->| 4 tiles sent
|-----W=1, FCN=11----->| 4 tiles sent
|-----W=1, FCN=7 ----->| 4 tiles sent
|-----W=1, FCN=3 --X-->| 4 tiles sent (not received)
|-----W=2, FCN=27----->| 4 tiles sent
|-----W=2, FCN=23----->| 4 tiles sent
^ |-----W=2, FCN=19----->| 1 tile sent
| |-----W=2, FCN=18----->| 1 tile sent
| |-----W=2, FCN=17----->| 1 tile sent
|-----W=2, FCN=16----->| 1 tile sent
s |-----W=2, FCN=15----->| 1 tile sent
m |-----W=2, FCN=14----->| 1 tile sent
a |-----W=2, FCN=13--X-->| 1 tile sent (not received)
l |-----W=2, FCN=12----->| 1 tile sent
l |---W=2, FCN=31 + RCS->| Integrity check: failure
e |<--- ACK, W=0, C=0 ---| C=0, Bitmap:1111111111110000111111111111
r |-----W=0, FCN=15----->| 1 tile sent
|-----W=0, FCN=14----->| 1 tile sent
L |-----W=0, FCN=13----->| 1 tile sent
2 |-----W=0, FCN=12----->| 1 tile sent
|<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000
M |-----W=1, FCN=3 ----->| 1 tile sent
T |-----W=1, FCN=2 ----->| 1 tile sent
U |-----W=1, FCN=1 ----->| 1 tile sent
|-----W=1, FCN=0 ----->| 1 tile sent
| |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001
| |-----W=2, FCN=13----->| Integrity check: success
V |<--- ACK, W=2, C=1 ---| C=1
(End)
Figure 30: ACK-on-Error mode, variable MTU.
In this example, the L2 MTU becomes reduced just before sending the “W=2, FCN=19” fragment, leaving space for only 1 tile in each forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are carried by a total of 25 SCHC Fragments, the last 9 being of smaller size.
Note: other sequences of events (e.g., regarding when ACKs are sent by the Receiver) are also allowed by this specification. Profiles may restrict this flexibility.
Figure 31 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with N=3, WINDOW_SIZE=7 and no loss.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4----->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2----->|
|-----W=0, FCN=1----->|
|-----W=0, FCN=0----->|
|<-- ACK, W=0, C=0 ---| Bitmap:1111111
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4----->|
|--W=1, FCN=7 + RCS-->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1
(End)
Figure 31: ACK-Always mode, 11 tiles, one tile per SCHC Fragment, no loss.
Figure 32 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3, WINDOW_SIZE=7 and three lost SCHC Fragments.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->|
|-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| 6543210
|<-- ACK, W=0, C=0 ---| Bitmap:1101011
|-----W=0, FCN=4----->|
|-----W=0, FCN=2----->|
|<-- ACK, W=0, C=0 ---| Bitmap:1111111
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->|
|--W=1, FCN=7 + RCS-->| Integrity check: failure
|<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
|-----W=1, FCN=4----->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1
(End)
Figure 32: ACK-Always mode, 11 tiles, one tile per SCHC Fragment, three lost SCHC Fragments.
Figure 33 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, WINDOW_SIZE=7, three lost SCHC Fragments and only one retry needed to recover each lost SCHC Fragment.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + RCS-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2----->| Integrity check: success
|<-- ACK, W=0, C=1 ---| C=1
(End)
Figure 33: ACK-Always mode, 6 tiles, one tile per SCHC Fragment, three lost SCHC Fragments.
Figure 34 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, WINDOW_SIZE=7, three lost SCHC Fragments, and the second SCHC ACK lost.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + RCS-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2----->| Integrity check: success
|<-X-ACK, W=0, C=1 ---| C=1
timeout | |
|--- W=0, ACK REQ --->| ACK REQ
|<-- ACK, W=0, C=1 ---| C=1
(End)
Figure 34: ACK-Always mode, 6 tiles, one tile per SCHC Fragment, SCHC ACK loss.
Figure 35 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 6 tiles, with N=3, WINDOW_SIZE=7, with three lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + RCS-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2--X-->|
timeout| |
|--- W=0, ACK REQ --->| ACK REQ
|<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101
|-----W=0, FCN=2----->| Integrity check: success
|<-- ACK, W=0, C=1 ---| C=1
(End)
Figure 35: ACK-Always mode, 6 tiles, retransmitted SCHC Fragment lost again.
Figure 36 illustrates the transmission in ACK-Always mode of a SCHC Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5, WINDOW_SIZE=24 and two lost SCHC Fragments.
Sender Receiver
|-----W=0, FCN=23----->|
|-----W=0, FCN=22----->|
|-----W=0, FCN=21--X-->|
|-----W=0, FCN=20----->|
|-----W=0, FCN=19----->|
|-----W=0, FCN=18----->|
|-----W=0, FCN=17----->|
|-----W=0, FCN=16----->|
|-----W=0, FCN=15----->|
|-----W=0, FCN=14----->|
|-----W=0, FCN=13----->|
|-----W=0, FCN=12----->|
|-----W=0, FCN=11----->|
|-----W=0, FCN=10--X-->|
|-----W=0, FCN=9 ----->|
|-----W=0, FCN=8 ----->|
|-----W=0, FCN=7 ----->|
|-----W=0, FCN=6 ----->|
|-----W=0, FCN=5 ----->|
|-----W=0, FCN=4 ----->|
|-----W=0, FCN=3 ----->|
|-----W=0, FCN=2 ----->|
|-----W=0, FCN=1 ----->|
|-----W=0, FCN=0 ----->|
| |
|<--- ACK, W=0, C=0 ---| Bitmap:110111111111101111111111
|-----W=0, FCN=21----->|
|-----W=0, FCN=10----->|
|<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
|-----W=1, FCN=23----->|
|-----W=1, FCN=22----->|
|-----W=1, FCN=21----->|
|--W=1, FCN=31 + RCS-->| Integrity check: success
|<--- ACK, W=1, C=1 ---| C=1
(End)
Figure 36: ACK-Always mode, 28 tiles, one tile per SCHC Fragment, lost SCHC Fragments.
The fragmentation state machines of the sender and the receiver, one for each of the different reliability modes, are described in the following figures:
+===========+
+------------+ Init |
| FCN=0 +===========+
| No Window
| No Bitmap
| +-------+
| +========+==+ | More Fragments
| | | <--+ ~~~~~~~~~~~~~~~~~~~~
+--------> | Send | send Fragment (FCN=0)
+===+=======+
| last fragment
| ~~~~~~~~~~~~
| FCN = 1
v send fragment+RCS
+============+
| END |
+============+
Figure 37: Sender State Machine for the No-ACK Mode
+------+ Not All-1
+==========+=+ | ~~~~~~~~~~~~~~~~~~~
| + <--+ set Inactivity Timer
| RCV Frag +-------+
+=+===+======+ |All-1 &
All-1 & | | |RCS correct
RCS wrong | |Inactivity |
| |Timer Exp. |
v | |
+==========++ | v
| Error |<-+ +========+==+
+===========+ | END |
+===========+
Figure 38: Receiver State Machine for the No-ACK Mode
+=======+
| INIT | FCN!=0 & more frags
| | ~~~~~~~~~~~~~~~~~~~~~~
+======++ +--+ send Window + frag(FCN)
W=0 | | | FCN-
Clear lcl_bm | | v set lcl_bm
FCN=max value | ++==+========+
+> | |
+---------------------> | SEND |
| +==+===+=====+
| FCN==0 & more frags | | last frag
| ~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~
| set lcl_bm | | set lcl_bm
| send wnd + frag(all-0) | | send wnd+frag(all-1)+RCS
| set Retrans_Timer | | set Retrans_Timer
| | |
|Recv_wnd == wnd & | |
|lcl_bm==recv_bm & | | +----------------------+
|more frag | | | lcl_bm!=rcv-bm |
|~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ |
|Stop Retrans_Timer | | | Attempt++ v
|clear lcl_bm v v | +=====+=+
|window=next_window +====+===+==+===+ |Resend |
+---------------------+ | |Missing|
+----+ Wait | |Frag |
not expected wnd | | Bitmap | +=======+
~~~~~~~~~~~~~~~~ +--->+ ++Retrans_Timer Exp |
discard frag +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
| | | ^ ^ |reSend(empty)All-* |
| | | | | |Set Retrans_Timer |
| | | | +--+Attempt++ |
C_bit==1 & | | | +-------------------------+
Recv_window==window & | | | all missing frags sent
no more frag| | | ~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~| | | Set Retrans_Timer
Stop Retrans_Timer| | |
+=============+ | | |
| END +<--------+ | |
+=============+ | | Attempt > MAX_ACK_REQUESTS
All-1 Window & | | ~~~~~~~~~~~~~~~~~~
C_bit ==0 & | v Send Abort
lcl_bm==recv_bm | +=+===========+
~~~~~~~~~~~~ +>| ERROR |
Send Abort +=============+
Figure 39: Sender State Machine for the ACK-Always Mode
Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set lcl_bm(FCN) | v v |discard
++===+===+===+=+
+---------------------+ Rcv +--->* ABORT
| +------------------+ Window |
| | +=====+==+=====+
| | All-0 & w=expect | ^ w =next & not-All
| | ~~~~~~~~~~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~
| | set lcl_bm(FCN) | |expected = next window
| | send lcl_bm | |Clear lcl_bm
| | | |
| | w=expected & not-All | |
| | ~~~~~~~~~~~~~~~~~~ | |
| | set lcl_bm(FCN)+-+ | | +--+ w=next & All-0
| | if lcl_bm full | | | | | | ~~~~~~~~~~~~~~~
| | send lcl_bm | | | | | | expected = nxt wnd
| | v | v | | | Clear lcl_bm
| |w=expected& All-1 +=+=+=+==+=++ | set lcl_bm(FCN)
| | ~~~~~~~~~~~ +->+ Wait +<+ send lcl_bm
| | discard +--| Next |
| | All-0 +---------+ Window +--->* ABORT
| | ~~~~~ +-------->+========+=++
| | snd lcl_bm All-1 & w=next| | All-1 & w=nxt
| | & RCS wrong| | & RCS right
| | ~~~~~~~~~~~~~~~~~| | ~~~~~~~~~~~~~~~~~~
| | set lcl_bm(FCN)| |set lcl_bm(FCN)
| | send lcl_bm| |send lcl_bm
| | | +----------------------+
| |All-1 & w=expected | |
| |& RCS wrong v +---+ w=expected & |
| |~~~~~~~~~~~~~~~~~~~~ +====+=====+ | RCS wrong |
| |set lcl_bm(FCN) | +<+ ~~~~~~~~~~~~~~ |
| |send lcl_bm | Wait End | set lcl_bm(FCN)|
| +--------------------->+ +--->* ABORT |
| +===+====+=+-+ All-1&RCS wrong|
| | ^ | ~~~~~~~~~~~~~~~|
| w=expected & RCS right | +---+ send lcl_bm |
| ~~~~~~~~~~~~~~~~~~~~~~ | |
| set lcl_bm(FCN) | +-+ Not All-1 |
| send lcl_bm | | | ~~~~~~~~~ |
| | | | discard |
|All-1&w=expected & RCS right | | | |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v +----+All-1 |
|set lcl_bm(FCN) +=+=+=+=+==+ |~~~~~~~~~ |
|send lcl_bm | +<+Send lcl_bm |
+-------------------------->+ END | |
+==========+<---------------+
--->* ABORT
In any state
on receiving a SCHC ACK REQ
Send a SCHC ACK for the current window
Figure 40: Receiver State Machine for the ACK-Always Mode
+=======+
| |
| INIT |
| | FCN!=0 & more frags
+======++ ~~~~~~~~~~~~~~~~~~~~~~
Frag RuleID trigger | +--+ Send cur_W + frag(FCN);
~~~~~~~~~~~~~~~~~~~ | | | FCN--;
cur_W=0; FCN=max_value;| | | set [cur_W, cur_Bmp]
clear [cur_W, Bmp_n];| | v
clear rcv_Bmp | ++==+==========+ **BACK_TO_SEND
+->+ | cur_W==rcv_W &
**BACK_TO_SEND | SEND | [cur_W,Bmp_n]==rcv_Bmp
+-------------------------->+ | & more frags
| +----------------------->+ | ~~~~~~~~~~~~
| | ++===+=========+ cur_W++;
| | FCN==0 & more frags| |last frag clear [cur_W, Bmp_n]
| | ~~~~~~~~~~~~~~~~~~~~~~~| |~~~~~~~~~
| | set cur_Bmp; | |set [cur_W, Bmp_n];
| |send cur_W + frag(All-0);| |send cur_W + frag(All-1)+RCS;
| | set Retrans_Timer| |set Retrans_Timer
| | | | +-----------------------------------+
| |Retrans_Timer expires & | | |cur_W==rcv_W&[cur_W,Bmp_n]!=rcv_Bmp|
| |more Frags | | | ~~~~~~~~~~~~~~~~~~~ |
| |~~~~~~~~~~~~~~~~~~~~ | | | Attempts++; W=cur_W |
| |stop Retrans_Timer; | | | +--------+ rcv_W==Wn &|
| |[cur_W,Bmp_n]==cur_Bmp; v v | | v [Wn,Bmp_n]!=rcv_Bmp|
| |cur_W++ +=====+===+=+=+==+ +=+=========+ ~~~~~~~~~~~|
| +-------------------+ | | Resend | Attempts++;|
+----------------------+ Wait x ACK | | Missing | W=Wn |
+--------------------->+ | | Frags(W) +<-------------+
| rcv_W==Wn &+-+ | +======+====+
| [Wn,Bmp_n]!=rcv_Bmp| ++=+===+===+==+==+ |
| ~~~~~~~~~~~~~~| ^ | | | ^ |
| send (cur_W,+--+ | | | +-------------+
| ALL-0-empty) | | | all missing frag sent(W)
| | | | ~~~~~~~~~~~~~~~~~
| Retrans_Timer expires &| | | set Retrans_Timer
| No more Frags| | |
| ~~~~~~~~~~~~~~| | |
| stop Retrans_Timer;| | |
|(re)send frag(All-1)+RCS | | |
+-------------------------+ | |
cur_W==rcv_W&| |
[cur_W,Bmp_n]==rcv_Bmp&| | Attempts > MAX_ACK_REQUESTS
No more Frags & RCS flag==OK| | ~~~~~~~~~~
~~~~~~~~~~~~~~~~~~| | send Abort
+=========+stop Retrans_Timer| | +===========+
| END +<-----------------+ +->+ ERROR |
+=========+ +===========+
Figure 41: Sender State Machine for the ACK-on-Error Mode
This is an example only. It is not normative. The specification in Section 8.4.3.1 allows for sequences of operations different from the one shown here.
+=======+ New frag RuleID received
| | ~~~~~~~~~~~~~
| INIT +-------+cur_W=0;clear([cur_W,Bmp_n]);
+=======+ |sync=0
|
Not All* & rcv_W==cur_W+---+ | +---+
~~~~~~~~~~~~~~~~~~~~ | | | | (E)
set[cur_W,Bmp_n(FCN)]| v v v |
++===+=+=+===+=+
+----------------------+ +--+ All-0&Full[cur_W,Bmp_n]
| ABORT *<---+ Rcv Window | | ~~~~~~~~~~
| +-------------------+ +<-+ cur_W++;set Inact_timer;
| | +->+=+=+=+=+=+====+ clear [cur_W,Bmp_n]
| | All-0 empty(Wn)| | | | ^ ^
| | ~~~~~~~~~~~~~~ +----+ | | | |rcv_W==cur_W & sync==0;
| | sendACK([Wn,Bmp_n]) | | | |& Full([cur_W,Bmp_n])
| | | | | |& All* || last_miss_frag
| | | | | |~~~~~~~~~~~~~~~~~~~~~~
| | All* & rcv_W==cur_W|(C)| |sendACK([cur_W,Bmp_n]);
| | & sync==0| | | |cur_W++; clear([cur_W,Bmp_n])
| |&no_full([cur_W,Bmp_n])| |(E)|
| | ~~~~~~~~~~~~~~~~ | | | | +========+
| | sendACK([cur_W,Bmp_n])| | | | | Error/ |
| | | | | | +----+ | Abort |
| | v v | | | | +===+====+
| | +===+=+=+=+===+=+ (D) ^
| | +--+ Wait x | | |
| | All-0 empty(Wn)+->| Missing Frags |<-+ |
| | ~~~~~~~~~~~~~~ +=============+=+ |
| | sendACK([Wn,Bmp_n]) +--------------+
| | *ABORT
v v
(A)(B)
(D) All* || last_miss_frag
(C) All* & sync>0 & rcv_W!=cur_W & sync>0
~~~~~~~~~~~~ & Full([rcv_W,Bmp_n])
Wn=oldest[not full(W)]; ~~~~~~~~~~~~~~~~~~~~
sendACK([Wn,Bmp_n]) Wn=oldest[not full(W)];
sendACK([Wn,Bmp_n]);sync--
ABORT-->* Uplink Only &
Inact_Timer expires
(E) Not All* & rcv_W!=cur_W || Attempts > MAX_ACK_REQUESTS
~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~
sync++; cur_W=rcv_W; send Abort
set[cur_W,Bmp_n(FCN)]
(A)(B)
| |
| | All-1 & rcv_W==cur_W & RCS!=OK All-0 empty(Wn)
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +-+ ~~~~~~~~~~
| | sendACK([cur_W,Bmp_n],C=0) | v sendACK([Wn,Bmp_n])
| | +===========+=++
| +--------------------->+ Wait End +-+
| +=====+=+====+=+ | All-1
| rcv_W==cur_W & RCS==OK | | ^ | & rcv_W==cur_W
| ~~~~~~~~~~~~~~~~~~~~~~ | | +---+ & RCS!=OK
| sendACK([cur_W,Bmp_n],C=1) | | ~~~~~~~~~~~~~~~~~~~
| | | sendACK([cur_W,Bmp_n],C=0);
| | | Attempts++
|All-1 & Full([cur_W,Bmp_n]) | |
|& RCS==OK & sync==0 | +-->* ABORT
|~~~~~~~~~~~~~~~~~~~ v
|sendACK([cur_W,Bmp_n],C=1) +=+=========+
+---------------------------->+ END |
+===========+
Figure 42: Receiver State Machine for the ACK-on-Error Mode
This section lists the information that needs to be provided in the LPWAN technology-specific documents.
This section lists the parameters that need to be defined in the Profile.
A Profile may define a delay to be added after each SCHC message transmission for compliance with local regulations or other constraints imposed by the applications.
For ACK-Always or ACK-on-Error, implementers may opt to support a single window size or multiple window sizes. The latter, when feasible, may provide performance optimizations. For example, a large window size should be used for packets that need to be split into a large number of tiles. However, when the number of tiles required to carry a packet is low, a smaller window size, and thus a shorter Bitmap, may be sufficient to provide reception status on all tiles. If multiple window sizes are supported, the Rule ID signals the window size in use for a specific packet transmission.
The ACK-Always and ACK-on-Error modes of SCHC F/R are bidirectional protocols: they require a feedback path from the reassembler to the fragmenter.
Some LPWAN technologies provide quasi-bidirectional connectivity, whereby a downlink transmission from the Network Infrastructure can only take place right after an uplink transmission by the Dev.
When using SCHC F/R to send fragmented SCHC Packets downlink over these quasi-bidirectional links, the following situation may arise: if an uplink SCHC ACK is lost, the SCHC ACK REQ message by the sender could be stuck indefinitely in the downlink queue at the Network Infrastructure, waiting for a transmission opportunity.
There are many ways by which this deadlock can be avoided. The Dev application might be sending recurring uplink messages such as keep-alive, or the Dev application stack might be sending other recurring uplink messages as part of its operation. However, these are out of the control of this generic SCHC specification.
In order to cope with quasi-bidirectional links, a SCHC-over-foo specification may want to amend the SCHC F/R specification to add a timer-based retransmission of the SCHC ACK. Below is an example of the suggested behavior for ACK-Always mode. Because it is an example, [RFC2119] language is deliberately not used here.
For downlink transmission of a fragmented SCHC Packet in ACK-Always mode, the SCHC Fragment receiver may support timer-based SCHC ACK retransmission. In this mechanism, the SCHC Fragment receiver initializes and starts a timer (the UplinkACK Timer) after the transmission of a SCHC ACK, except when the SCHC ACK is sent in response to the last SCHC Fragment of a packet (All-1 fragment). In the latter case, the SCHC Fragment receiver does not start a timer after transmission of the SCHC ACK.
If, after transmission of a SCHC ACK that is not an All-1 fragment, and before expiration of the corresponding UplinkACK timer, the SCHC Fragment receiver receives a SCHC Fragment that belongs to the current window (e.g., a missing SCHC Fragment from the current window) or to the next window, the UplinkACK timer for the SCHC ACK is stopped. However, if the UplinkACK timer expires, the SCHC ACK is resent and the UplinkACK timer is reinitialized and restarted.
The default initial value for the UplinkACK Timer, as well as the maximum number of retries for a specific SCHC ACK, denoted MAX_ACK_REQUESTS, is to be defined in a Profile. The initial value of the UplinkACK timer is expected to be greater than that of the Retransmission timer, in order to make sure that a (buffered) SCHC Fragment to be retransmitted finds an opportunity for that transmission. One exception to this recommendation is the special case of the All-1 SCHC Fragment transmission.
When the SCHC Fragment sender transmits the All-1 SCHC Fragment, it starts its Retransmission Timer with a large timeout value (e.g., several times that of the initial UplinkACK Timer). If a SCHC ACK is received before expiration of this timer, the SCHC Fragment sender retransmits any lost SCHC Fragments as reported by the SCHC ACK, or if the SCHC ACK confirms successful reception of all SCHC Fragments of the last window, the transmission of the fragmented SCHC Packet is considered complete. If the timer expires, and no SCHC ACK has been received since the start of the timer, the SCHC Fragment sender assumes that the All-1 SCHC Fragment has been successfully received (and possibly, the last SCHC ACK has been lost: this mechanism assumes that the Retransmission Timer for the All-1 SCHC Fragment is long enough to allow several SCHC ACK retries if the All-1 SCHC Fragment has not been received by the SCHC Fragment receiver, and it also assumes that it is unlikely that several ACKs become all lost).