Internet Engineering Task Force (IETF) A. Minaburo
Request for Comments: 8724 Acklio
Category: Standards Track L. Toutain
ISSN: 2070-1721 IMT Atlantique
C. Gomez
Universitat Politecnica de Catalunya
D. Barthel
Orange Labs
JC. Zuniga
SIGFOX
April 2020
SCHC: Generic Framework for Static Context Header Compression and
Fragmentation
Abstract
This document defines the Static Context Header Compression and
fragmentation (SCHC) framework, which provides both a header
compression mechanism and an optional fragmentation mechanism. SCHC
has been designed with Low-Power Wide Area Networks (LPWANs) in mind.
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8724.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Requirements Notation
3. LPWAN Architecture
4. Terminology
5. SCHC Overview
5.1. SCHC Packet Format
5.2. Functional Mapping
6. RuleID
7. Compression/Decompression
7.1. SCHC C/D Rules
7.2. Packet Processing
7.3. Matching Operators
7.4. Compression/Decompression Actions (CDA)
7.4.1. Processing Fixed-Length Fields
7.4.2. Processing Variable-Length Fields
7.4.3. Not-Sent CDA
7.4.4. Value-Sent CDA
7.4.5. Mapping-Sent CDA
7.4.6. LSB CDA
7.4.7. DevIID, AppIID CDA
7.4.8. Compute-*
8. Fragmentation/Reassembly
8.1. Overview
8.2. SCHC F/R Protocol Elements
8.2.1. Messages
8.2.2. Tiles, Windows, Bitmaps, Timers, Counters
8.2.3. Integrity Checking
8.2.4. Header Fields
8.3. SCHC F/R Message Formats
8.3.1. SCHC Fragment Format
8.3.2. SCHC ACK Format
8.3.3. SCHC ACK REQ Format
8.3.4. SCHC Sender-Abort Format
8.3.5. SCHC Receiver-Abort Format
8.4. SCHC F/R Modes
8.4.1. No-ACK Mode
8.4.2. ACK-Always Mode
8.4.3. ACK-on-Error Mode
9. Padding Management
10. SCHC Compression for IPv6 and UDP Headers
10.1. IPv6 Version Field
10.2. IPv6 Traffic Class Field
10.3. Flow Label Field
10.4. Payload Length Field
10.5. Next Header Field
10.6. Hop Limit Field
10.7. IPv6 Addresses Fields
10.7.1. IPv6 Source and Destination Prefixes
10.7.2. IPv6 Source and Destination IID
10.8. IPv6 Extension Headers
10.9. UDP Source and Destination Ports
10.10. UDP Length Field
10.11. UDP Checksum Field
11. IANA Considerations
12. Security Considerations
12.1. Security Considerations for SCHC Compression/Decompression
12.1.1. Forged SCHC Packet
12.1.2. Compressed Packet Size as a Side Channel to Guess a
Secret Token
12.1.3. Decompressed Packet Different from the Original Packet
12.2. Security Considerations for SCHC Fragmentation/Reassembly
12.2.1. Buffer Reservation Attack
12.2.2. Corrupt Fragment Attack
12.2.3. Fragmentation as a Way to Bypass Network Inspection
12.2.4. Privacy Issues Associated with SCHC Header Fields
13. References
13.1. Normative References
13.2. Informative References
Appendix A. Compression Examples
Appendix B. Fragmentation Examples
Appendix C. Fragmentation State Machines
Appendix D. SCHC Parameters
Appendix E. Supporting Multiple Window Sizes for Fragmentation
Appendix F. ACK-Always and ACK-on-Error on Quasi-Bidirectional
Links
Acknowledgements
Authors' Addresses
1. Introduction
This document defines the Static Context Header Compression and
fragmentation (SCHC) framework, which provides both a header
compression mechanism and an optional fragmentation mechanism. SCHC
has been designed with Low-Power Wide Area Networks (LPWANs) in mind.
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. The following properties of LPWANs can be
exploited to get an efficient header compression:
* The network topology is star-oriented, which means that all
packets between the same source-destination pair follow the same
path. For the needs of this document, the architecture can simply
be described as Devices (Dev) exchanging information with LPWAN
Application Servers (Apps) through a Network Gateway (NGW).
* Because devices embed built-in applications, the traffic flows to
be compressed are known in advance. Indeed, new applications are
less frequently installed in an LPWAN device than they are in a
general-purpose computer or smartphone.
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, or may sometimes be 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 to help LPWAN technologies support
the IPv6 MTU requirement.
This document defines generic functionality and offers flexibility
with regard to parameter settings and mechanism choices. Technology-
specific settings are expected to be grouped into Profiles specified
in other documents.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. LPWAN Architecture
LPWAN 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 (see Figure 1):
* Devices (Dev) are the end-devices or hosts (e.g., sensors,
actuators, etc.). There can be a very high density of devices per
Radio Gateway.
* The Radio Gateway (RGW) is the endpoint of the constrained link.
* The Network Gateway (NGW) is the interconnection node between the
Radio Gateway and the Internet.
* The Application Server (App) is the endpoint of the application-
level protocol on the Internet side.
() () () |
() () () () / \ +---------+
() () () () () () / \======| ^ | +-----------+
() () () | | <--|--> | |Application|
() () () () / \==========| v |=============| Server |
() () () / \ +---------+ +-----------+
Dev RGWs NGW App
Figure 1: LPWAN Architecture (Simplified from That Shown in RFC 8376)
4. Terminology
This section defines terminology and abbreviations 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".
App: LPWAN Application Server, as defined by [RFC8376]. It runs
an application sending/receiving packets to/from the Dev.
AppIID: Application Interface Identifier. The IID that identifies
the App interface.
Compression Residue: The bits that remain to be sent (beyond the
RuleID itself) after applying the SCHC compression.
Context: A set of Rules used to compress/decompress headers, or to
fragment/reassemble a packet.
Dev: Device, as defined by [RFC8376].
DevIID: Device Interface Identifier. The IID that identifies the
Dev interface.
Downlink: From the App to the Dev.
IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136].
L2: Layer 2. The immediate lower layer that SCHC interfaces
with, for example an underlying LPWAN technology. It does
not necessarily correspond to the OSI model definition of
Layer 2.
L2 Word: This is the minimum subdivision of payload data that the L2
will carry. In most L2 technologies, the L2 Word is an
octet. In bit-oriented radio technologies, the L2 Word
might be a single bit. The L2 Word size is assumed to be
constant over time for each device.
Padding: Extra bits that may be appended by SCHC to a data unit that
it passes down to L2 for transmission. SCHC itself operates
on bits, not bytes, and does not have any alignment
prerequisite. See Section 9.
Profile: SCHC offers variations in the way it is operated, with a
number of parameters listed in Appendix D. A Profile
indicates a particular setting of all these parameters.
Both ends of a SCHC communication must be provisioned with
the same Profile information and with the same set of Rules
before the communication starts, so that there is no
ambiguity in how they expect to communicate.
Rule: Part of the Context that describes how a packet is
compressed/decompressed or fragmented/reassembled.
RuleID: Rule Identifier. An identifier for a Rule.
SCHC: Static Context Header Compression and fragmentation (SCHC),
a generic framework.
SCHC C/D: SCHC Compressor/Decompressor, or SCHC Compression/
Decompression. The SCHC entity or mechanism used on both
sides, at the Dev and at the network, to achieve
compression/decompression of headers.
SCHC F/R: SCHC Fragmenter/Reassembler or SCHC Fragmentation/
Reassembly. The SCHC entity or mechanism used on both
sides, at the Dev and at the network, to achieve
fragmentation/reassembly of SCHC Packets.
SCHC Packet: A packet (e.g., an IPv6 packet) whose header has been
compressed as per the header compression mechanism defined
in this document. If the header compression process is
unable to actually compress the packet header, the packet
with the uncompressed header is still called a SCHC Packet
(in this case, a RuleID is used to indicate that the packet
header has not been compressed). See Section 7 for more
details.
Uplink: From the Dev to the App.
Additional terminology for the optional SCHC F/R is found in
Section 8.2.
Additional terminology for SCHC C/D is found in Section 7.1.
5. SCHC Overview
SCHC can be characterized as an adaptation layer between an upper
layer (for example, IPv6) and an underlying layer (for example, 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: Example of 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 not to use SCHC fragmentation is left to each Profile
+: optional, depends on Fragmentation mode
Figure 3: SCHC Operations at the Sender and the Receiver
5.1. SCHC Packet Format
The SCHC Packet is composed of the Compressed Header followed by the
payload from the original packet (see Figure 4). The Compressed
Header itself is composed of the RuleID and a Compression Residue,
which is the output of compressing the packet header with the Rule
identified by that RuleID (see Section 7). The Compression Residue
may be empty. Both the RuleID and the Compression Residue
potentially have a variable size, and are not necessarily a multiple
of bytes in size.
|------- Compressed Header -------|
+---------------------------------+--------------------+
| RuleID | Compression Residue | Payload |
+---------------------------------+--------------------+
Figure 4: SCHC Packet
5.2. Functional Mapping
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: Architectural Mapping
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; 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
reassembly (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 Apps.
The SCHC F/R and SCHC 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.
6. RuleID
RuleIDs identify the Rules used for compression/decompression or for
fragmentation/reassembly.
The scope of the RuleID 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 RuleID 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.
The RuleIDs are therefore specific to the Context related to one Dev.
Hence, multiple Dev instances, which refer to different Contexts, MAY
reuse the same RuleID for different Rules. On the Network
Infrastructure side, in order to identify the correct Rule to be
applied to Uplink traffic, the SCHC C/D or SCHC F/R needs to
associate the RuleID with the Dev identifier. Similarly, for
Downlink traffic, the SCHC C/D or SCHC F/R on the Network
Infrastructure side first needs to identify the destination Dev
before looking for the appropriate Rule (and associated RuleID) in
the Context of that Dev.
Inside their scopes, Rules for compression/decompression and Rules
for fragmentation/reassembly share the same RuleID space.
The size of the RuleIDs 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 other things. It is
defined in Profiles.
The RuleIDs are used:
* For SCHC C/D, to identify the Rule that is used to compress a
packet header.
- At least one RuleID MUST be allocated to tagging packets for
which SCHC compression was not possible (i.e., no matching
compression Rule was found).
* In SCHC F/R, to identify the specific mode and settings of
fragmentation/reassembly for one direction of data traffic (Uplink
or Downlink).
- When SCHC F/R is used for both communication directions, at
least two RuleID values are needed for fragmentation/
reassembly: one per direction of data traffic. This is because
fragmentation/reassembly may entail control messages flowing in
the reverse direction compared to data traffic.
7. Compression/Decompression
Compression with SCHC is based on using a set of Rules, which
constitutes the Context of SCHC C/D, to compress or decompress
headers. SCHC avoids Context synchronization traffic, which consumes
considerable bandwidth in other header compression mechanisms such as
RObust Header Compression (RoHC) [RFC5795]. Since the content of
packets is highly predictable in LPWANs, static Contexts can be
stored beforehand. The Contexts MUST be stored at both ends, and
they can be learned by a provisioning protocol, by out-of-band means,
or by pre-provisioning. The way the Contexts are provisioned is out
of the scope of this document.
7.1. SCHC C/D Rules
The main idea of the SCHC compression scheme is to transmit the
RuleID to the other end instead of sending known field values. This
RuleID 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 RuleID over the LPWAN. 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 SCHC C/D 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 Descriptors 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 SCHC C/D 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 Descriptors are listed in the order in which the
fields appear in the packet header. The Field Descriptors describe
the header fields with the following entries:
* Field Identifier (FID) designates a protocol and field (e.g., UDP
Destination Port), unambiguously among all protocols that a SCHC
compressor processes. In the presence of protocol nesting, the
Field ID also identifies the nesting.
* Field Length (FL) represents the length of the original field. It
can be either a fixed value (in bits) if the length is known when
the Rule is created or a type if the length is variable. The
length of a header field is defined by its own protocol
specification (e.g., IPv6 or UDP). If the length is variable, the
type defines the process to compute the length and its unit (bits,
bytes...).
* Field Position (FP): most often, a field only occurs once in a
packet header. However, some fields may occur multiple times. An
example is the uri-path of CoAP. FP indicates which occurrence
this Field Descriptor applies to. The default value is 1. The
value 1 designates the first occurrence. The value 0 is special.
It means "don't care", see Section 7.2.
* A Direction Indicator (DI) indicates the packet direction(s) this
Field Descriptor applies to. It allows for asymmetric processing,
using the same Rule. Three values are possible:
Up: this Field Descriptor is only applicable to packets traveling
Uplink.
Dw: this Field Descriptor is only applicable to packets traveling
Downlink.
Bi: this Field Descriptor is applicable to packets traveling
Uplink or Downlink.
* Target Value (TV) is the value used to match against the packet
header field. The Target Value can be a scalar value of any type
(integer, strings, etc.) or a more complex structure (array, list,
etc.). The types and representations are out of scope for this
document.
* Matching Operator (MO) is the operator used to match the field
value and the Target Value. The Matching Operator may require
some parameters. The set of MOs defined in this document can be
found in Section 7.3.
* Compression/Decompression Action (CDA) describes the pair of
actions that are performed at the compressor to compress a header
field and at the decompressor to recover the original value of the
header field. Some CDAs might use parameter values for their
operation. The set of CDAs defined in this document can be found
in Section 7.4.
7.2. Packet Processing
The compression/decompression process follows several phases:
Compression Rule selection: the general idea is to browse the Rule
set to find a Rule that has a matching Field Descriptor (given the
DI and FP) for all and only those header fields that appear in the
packet being compressed. The detailed algorithm is the following:
* The first step is to check the FIDs. If any header field of
the packet being examined cannot be matched with a Field
Descriptor with the correct FID, the Rule MUST be disregarded.
If any Field Descriptor in the Rule has a FID that cannot be
matched to one of the header fields of the packet being
examined, the Rule MUST be disregarded.
* The next step is to match the Field Descriptors by their
direction, using the DI. If any field of the packet header
cannot be matched with a Field Descriptor with the correct FID
and DI, the Rule MUST be disregarded.
* Then, the Field Descriptors are further selected according to
FP. If any field of the packet header cannot be matched with a
Field Descriptor with the correct FID, DI and FP, the Rule MUST
be disregarded.
The value 0 for FP means "don't care", i.e., the comparison of
this Field Descriptor's FP with the position of the field of
the packet header being compressed returns True, whatever that
position. FP=0 can be useful to build compression Rules for
protocol headers in which some fields order is irrelevant. An
example could be uri-queries in CoAP. Care needs to be
exercised when writing Rules containing FP=0 values. Indeed,
it may result in decompressed packets having fields ordered
differently compared to the original packet.
* Once each header field has been associated with a Field
Descriptor with matching FID, DI, and FP, each packet field's
value is then compared to the corresponding TV stored in the
Rule for that specific field, using the MO. If every field in
the packet header satisfies the corresponding MOs of a Rule
(i.e., all MO results are True), that Rule is valid for use to
compress the header. Otherwise, the Rule MUST be disregarded.
This specification does not prevent multiple Rules from
matching the above steps and, therefore, being valid for use.
Which Rule to use among multiple valid Rules is left to the
implementation. As long as the same Rule set is installed at
both ends, this degree of freedom does not constitute an
interoperability issue.
* If no valid compression Rule is found, then the packet MUST be
sent uncompressed using the RuleID dedicated to this purpose
(see Section 6). The entire packet header is the Compression
Residue (see Figure 4). Sending an uncompressed header is
likely to require SCHC F/R.
Compression: if a valid Rule is found, each field of the header is
compressed according to the CDAs of the Rule. The fields are
compressed in the order that the Field Descriptors appear in the
Rule. The compression of each field results in a residue, which
may be empty. The Compression Residue for the packet header is
the concatenation of the non-empty residues for each field of the
header, in the order the Field Descriptors appear in the Rule.
The order in which the Field Descriptors appear in the Rule is
therefore semantically important.
|------------------- Compression Residue -------------------|
+-----------------+-----------------+-----+-----------------+
| field 1 residue | field 2 residue | ... | field N residue |
+-----------------+-----------------+-----+-----------------+
Figure 7: Compression Residue Structure
Sending: The RuleID is sent to the other end jointly with the
Compression Residue (which could be empty) or the uncompressed
header, and directly followed by the payload (see Figure 4). The
way the RuleID is sent will be specified in the Profile and is out
of the scope of the present document. For example, it could be
included in an L2 header or sent as part of the L2 payload.
Decompression: when decompressing, on the Network Infrastructure
side, the SCHC C/D needs to find the correct Rule based on the L2
address of the Dev. On the Dev side, only the RuleID is needed to
identify the correct Rule since the Dev typically only holds Rules
that apply to itself.
This Rule describes the compressed header format. From this, the
decompressor determines the order of the residues, the fixed-size
or variable-size nature of each residue (see Section 7.4.2), and
the size of the fixed-size residues.
Therefore, from the received compressed header, it can retrieve
all the residue values and associate them to the corresponding
header fields.
For each field in the header, the receiver applies the CDA action
associated with that field in order to reconstruct the original
header field value. The CDA application order can be different
from the order in which the fields are listed in the Rule. In
particular, Compute-* MUST be applied after the application of the
CDAs of all the fields it computes on.
7.3. Matching Operators
MOs are functions used at the compression side of SCHC C/D. 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. The
following MOs are defined:
equal: The match result is True if the field value in the packet
matches the TV.
ignore: No matching is attempted between the field value in the
packet and the TV in the Rule. The result is always True.
MSB(x): A match is obtained if the most significant (leftmost) x
bits of the packet header field value are equal to the TV in the
Rule. The x parameter of the MSB MO indicates how many bits are
involved in the comparison. If the FL is described as variable,
the x parameter must be a multiple of the FL unit. For example, x
must be multiple of 8 if the unit of the variable length is bytes.
match-mapping: With match-mapping, TV is a list of values. Each
value of the list is identified by an index. Compression is
achieved by sending the index instead of the original header field
value. This operator matches if the header field value is equal
to one of the values in the target list.
7.4. Compression/Decompression Actions (CDA)
The CDA specifies the actions taken during the compression of header
fields and the inverse action taken by the decompressor to restore
the original value. The CDAs defined by this document are described
in detail in Section 7.4.3 to Section 7.4.8. They are summarized in
Table 1.
+--------------+------------------------+-----------------------+
| 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 least significant | concatenate TV and |
| | bits (LSB) | 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: Compression and Decompression Actions
The first column shows the action's name. The second and third
columns show the compression and decompression behaviors for each
action.
7.4.1. Processing Fixed-Length Fields
If the field is identified in the Field Descriptor 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-Size Field Residue Structure
7.4.2. Processing Variable-Length Fields
If the field is identified in the Field Descriptor 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-Size Field Residue Structure
The size (using the unit defined in the FL) is encoded on 4, 12, or
28 bits as follows:
* If the size is between 0 and 14, it is encoded as a 4-bit unsigned
integer.
* Sizes between 15 and 254 are encoded as 0b1111 followed by the
8-bit unsigned integer.
* Larger sizes are encoded as 0xfff followed by the 16-bit unsigned
integer.
If the field is identified in the Field Descriptor 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.
7.4.3. Not-Sent CDA
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 of having a decompressed field value
that is 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 TV stored in the
matched Rule identified by the received RuleID.
7.4.4. Value-Sent CDA
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 7.4.2.
This action is generally used with the "ignore" MO.
7.4.5. Mapping-Sent CDA
The mapping-sent action is used to send an index (the index into the
TV 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 MO 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.
7.4.6. LSB CDA
The LSB action is used together with the "MSB(x)" MO to avoid sending
the most significant part of the packet field if that part is already
known by the receiving end.
The compressor sends the LSBs 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 the TV
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 7.4.2.
7.4.7. DevIID, AppIID CDA
These actions are used to process the DevIID and AppIID of the IPv6
addresses, respectively. AppIID CDA is less common since most
current LPWAN technologies frames contain a single L2 address, which
is the Dev's address.
The DevIID value MAY be computed from the Dev ID present in the L2
header, or from some other stable identifier. The computation is
specific to each Profile and MAY depend on the Dev ID size.
In the Downlink direction, at the compressor, the DevIID CDA may be
used to generate the L2 addresses on the LPWAN, based on the packet's
Destination Address.
7.4.8. Compute-*
Some fields can be elided at the compressor and recomputed locally at
the decompressor.
Because the field is uniquely identified by its FID (e.g., IPv6
length), the relevant protocol specification unambiguously defines
the algorithm for such computation.
An example of a field that knows how to recompute itself is IPv6
length.
8. Fragmentation/Reassembly
8.1. Overview
In LPWAN technologies, the L2 MTU typically ranges from tens to
hundreds of bytes. Some of these technologies do not have an
internal fragmentation/reassembly mechanism.
The optional 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.
8.2. SCHC F/R Protocol Elements
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.
8.2.1. Messages
SCHC F/R defines the following messages:
SCHC Fragment: A message that carries part of a SCHC Packet from the
sender to the receiver.
SCHC ACK: An acknowledgement for fragmentation, by the receiver to
the sender. This message is used to indicate whether or not the
reception of pieces of, or the whole of, the fragmented SCHC
Packet was successful.
SCHC ACK REQ: A request by the sender for a SCHC ACK from the
receiver.
SCHC Sender-Abort: A message by the sender telling the receiver that
it has aborted the transmission of a fragmented SCHC Packet.
SCHC Receiver-Abort: A message by the receiver to tell the sender to
abort the transmission of a fragmented SCHC Packet.
The format of these messages is provided in Section 8.3.
8.2.2. Tiles, Windows, Bitmaps, Timers, Counters
8.2.2.1. Tiles
The SCHC Packet is fragmented into pieces, hereafter called "tiles".
The tiles MUST be 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: 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.
8.2.2.2. Windows
Some SCHC F/R modes may handle successive tiles in groups, called
windows.
If windows are used:
* all the windows of a SCHC Packet, except the last one, MUST
contain the same number of tiles. This number is WINDOW_SIZE.
* WINDOW_SIZE MUST be specified in a Profile.
* the windows are numbered.
* their numbers MUST increment by 1 from 0 upward, from the start of
the SCHC Packet to its end.
* the last window MUST contain WINDOW_SIZE tiles or less.
* tiles are numbered within each window.
* the tile indices MUST decrement by 1 from WINDOW_SIZE - 1
downward, looking from the start of the SCHC Packet toward its
end.
* therefore, each tile of a SCHC Packet is uniquely identified by a
window number and a tile index within this window.
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: 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:
* Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to
the sender in a SCHC ACK message.
* A Bitmap corresponds to exactly one Window.
8.2.2.3. Bitmaps
Each bit in the Bitmap for a window corresponds to a tile in the
window. Therefore, each Bitmap has WINDOW_SIZE bits. The bit at the
leftmost 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 rightmost position corresponds to the
tile numbered 0. In the Bitmap for the last window, the bit at the
rightmost 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:
* a bit set to 1 in the Bitmap indicates that a tile associated with
that bit position has been correctly received for that window.
* a bit set to 0 in the Bitmap indicates that there has been no tile
correctly received, associated with that bit position, for that
window. Possible reasons include that the tile was not sent at
all, not received, or received with errors.
8.2.2.4. Timers and Counters
Some SCHC F/R modes can use the following timers and counters:
Inactivity Timer: a SCHC Fragment receiver uses this timer to abort
waiting for a SCHC F/R message.
Retransmission Timer: a SCHC Fragment sender uses this timer to
abort waiting for an expected SCHC ACK.
Attempts: this counter counts the requests for SCHC ACKs, up to
MAX_ACK_REQUESTS.
8.2.3. Integrity Checking
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.
8.2.4. Header Fields
The SCHC F/R messages contain the following fields (see the formats
in Section 8.3):
RuleID: this field is present in all the SCHC F/R messages. The
Rule identifies:
* that a SCHC F/R message is being carried, as opposed to an
unfragmented SCHC Packet,
* which SCHC F/R mode is used,
* in case this mode uses windows, what the value of WINDOW_SIZE
is, and
* what other optional fields are present and what the field sizes
are.
The Rule tells apart a non-fragmented SCHC Packet from SCHC
Fragments. It will also tell apart SCHC Fragments of fragmented
SCHC Packets that use different SCHC F/R modes or different
parameters. Therefore, interleaved transmission of these is
possible.
All SCHC F/R messages pertaining to the same SCHC Packet MUST bear
the same RuleID.
Datagram Tag (DTag): This field allows differentiating SCHC F/R
messages belonging to different SCHC Packets that may be using the
same RuleID simultaneously. Hence, it allows interleaving
fragments of a new SCHC Packet with fragments of a previous SCHC
Packet under the same RuleID.
The size of the DTag field (called "T", in bits) is defined by
each Profile for each RuleID. When T is 0, the DTag field does
not appear in the SCHC F/R messages and the DTag value is defined
as 0.
When T is 0, there can be no more than one fragmented SCHC Packet
in transit for each fragmentation RuleID.
If T is not 0, DTag:
* MUST be set to the same value for all the SCHC F/R messages
related to the same fragmented SCHC Packet, and
* MUST be set to different values for SCHC F/R messages related
to different SCHC Packets that are being fragmented under the
same RuleID and whose transmission may overlap.
W: The W field is optional. It is only present if windows are used.
Its presence and size (called "M", in bits) is defined by each
SCHC F/R mode and each Profile for each RuleID.
This field carries information pertaining to the window a SCHC F/R
message relates to. If present, W MUST carry the same value for
all the SCHC F/R messages related to the same window. Depending
on the mode and Profile, W may carry the full window number, or
just the LSB or any other partial representation of the window
number.
Fragment Compressed Number (FCN): The FCN field is present in the
SCHC Fragment Header. Its size (called "N", in bits) is defined
by each Profile for each RuleID.
This field conveys information about the progress in the sequence
of tiles being transmitted by SCHC Fragment messages. For
example, it can contain a partial, efficient representation of a
larger-sized tile index. The description of the exact use of the
FCN field is left to each SCHC F/R mode. However, two values are
reserved for special purposes. They help control the SCHC F/R
process:
* The FCN value with all the bits equal to 1 (called "All-1")
signals that the very last tile of a SCHC Packet has been
transmitted. By extension, if windows are used, the last
window of a packet is called the "All-1" window.
* If windows are used, the FCN value with all the bits equal to 0
(called "All-0") signals the last tile of a window that is not
the last one of the SCHC packet. By extension, such a window
is called an "All-0 window".
Reassembly Check Sequence (RCS): This field only appears in the
All-1 SCHC Fragments. Its size (called "U", in bits) is defined
by each Profile for each RuleID.
See Section 8.2.3 for the RCS default size, default polynomial and
details on RCS computation.
C (integrity Check): C is a 1-bit field. This field is used in the
SCHC ACK message to report on the reassembled SCHC Packet
integrity check (see Section 8.2.3).
A value of 1 tells that the integrity check was performed and is
successful. A value of 0 tells that the integrity check was not
performed or that it was a failure.
Compressed Bitmap: The Compressed Bitmap is used together with
windows and Bitmaps (see Section 8.2.2.3). Its presence and size
is defined for each SCHC F/R mode for each RuleID.
This field appears in the SCHC ACK message to report on the
receiver Bitmap (see Section 8.3.2.1).
8.3. SCHC F/R Message Formats
This section defines the SCHC Fragment formats, the SCHC ACK format,
the SCHC ACK REQ format and the SCHC Abort formats.
8.3.1. SCHC Fragment Format
A SCHC Fragment conforms to the general format shown in Figure 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
8.3.1.1. Regular SCHC Fragment
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 --|
+-- ... -+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~
| RuleID | 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 RuleID 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.
8.3.1.2. All-1 SCHC Fragment
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
Fragment Payload MUST be present. The FCN field is all ones.
|------- SCHC Fragment Header -------|
|-- T --|-M-|-- N --|-- U --|
+-- ... -+- ... -+---+- ... -+- ... -+-----...-----+~~~~~~~~~~~~~~~~~
| RuleID | DTag | W | 11..1 | RCS | FragPayload | 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 RuleID 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 is at least the size an L2 Word. This
condition is also met if the SCHC Sender-Abort Header is a multiple
of L2 Words.
8.3.2. SCHC ACK Format
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 |
+-- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
| RuleID | DTag | W |C=1| padding as needed (success)
+-- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
+-- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
| RuleID | 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.
8.3.2.1. Bitmap Compression
For transmission, the Compressed Bitmap in the SCHC ACK message is
defined by the following algorithm (see Figure 16 for a follow-along
example):
* Build a temporary SCHC ACK message that contains the Header
followed by the original Bitmap (see Section 8.2.2.3 for a
description of Bitmaps).
* Position scissors at the end of the Bitmap, after its last bit.
* While the bit on the left of the scissors is 1 and belongs to the
Bitmap, keep moving left, then stop.
* Then, while the scissors are not on an L2 Word boundary of the
SCHC ACK message and there is a Bitmap bit on the right of the
scissors, keep moving right, then stop.
* At this point, cut and drop off any bits to the right of the
scissors.
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 SCHC 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 |
+-- ... -+- ... -+---+---+---------------------------------+
| RuleID | DTag | W |C=0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+-- ... -+- ... -+---+---+---------------------------------+
next L2 Word boundary ->|
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 |
+-- ... -+- ... -+---+---+-----+
| RuleID | 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 #)
+--------+-------+---+---+-------------+
| RuleID | DTag | W |C=0|1 0 1 0 1 1 1| uncompressed Bitmap
+--------+-------+---+---+-------------+
next L2 Word boundary ->|<-- L2 Word --->|
+--------+-------+---+---+-------------+~~~~+
| RuleID | DTag | W |C=0|1 0 1 0 1 1 1|pad.| transmitted SCHC ACK
+--------+-------+---+---+-------------+~~~~+
next L2 Word boundary ->|<-- L2 Word --->|
Figure 18: Example of a SCHC ACK Message, Missing Tiles
Figure 19 shows an example of a SCHC ACK with tile indices 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 #)
+--------+-------+---+---+-------------+
| RuleID | DTag | W |C=0|1 1 1 1 1 1 1| with uncompressed Bitmap
+--------+-------+---+---+-------------+
next L2 Word boundary ->|
+-- ... -+- ... -+---+---+-+
| RuleID | DTag | W |C=0|1| transmitted SCHC ACK
+-- ... -+- ... -+---+---+-+
next L2 Word boundary ->|
Figure 19: Example of a SCHC ACK Message, No Missing Tile
8.3.3. SCHC ACK REQ Format
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 --|
+-- ... -+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| RuleID | DTag | W | 0..0 | padding (as needed) (no payload)
+-- ... -+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 20: SCHC ACK REQ Format
8.3.4. SCHC Sender-Abort Format
When a SCHC Fragment sender needs to abort an ongoing 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 --|
+-- ... -+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| RuleID | DTag | W | 11..1 | padding (as needed)
+-- ... -+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 21: SCHC Sender-Abort Format
If the W field is present:
* the fragment sender MUST set it to all ones. Other values are
RESERVED.
* the fragment receiver MUST check its value. If the value is
different from all ones, the message MUST be ignored.
The SCHC Sender-Abort MUST NOT be acknowledged.
8.3.5. SCHC Receiver-Abort Format
When a SCHC Fragment receiver needs to abort an ongoing 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 |
+--- ... --+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
| RuleID | 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 fragment receiver MUST set it to all ones. Other values are
RESERVED.
* if the value is different from all ones, the fragment sender MUST
ignore the message.
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:
* if the Header does not end at an L2 Word boundary, append bits set
to 1 as needed to reach the next L2 Word boundary.
* append exactly one more L2 Word with bits all set to ones.
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.
8.4. SCHC F/R Modes
This specification includes several SCHC F/R modes that:
* allow for a range of reliability options, such as optional SCHC
Fragment retransmission.
* support various LPWAN characteristics, such as links with variable
MTU or unidirectional links.
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.
8.4.1. No-ACK Mode
The No-ACK mode has been designed under the assumption that data unit
out-of-sequence delivery does not occur between the entity performing
fragmentation and the entity performing reassembly. This mode
supports L2 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 RuleID value(s) corresponds 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 RuleID value, MUST define:
* the size of the DTag field,
* the size and algorithm for the RCS field, and
* the expiration time of the Inactivity Timer.
Each Profile, for each RuleID value, MAY define
* a value of N different from the recommended one, and
* the meaning of values sent in the FCN field, for values different
from the All-1 value.
For each active pair of RuleID 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.
8.4.1.1. Sender Behavior
At the beginning of the fragmentation of a new SCHC Packet, the
fragment sender MUST select a RuleID 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 39 shows an example of a corresponding state machine.
8.4.1.2. Receiver Behavior
Upon receiving each Regular SCHC Fragment:
* the receiver MUST reset the Inactivity Timer.
* the receiver assembles the payloads of the SCHC Fragments.
On receiving an All-1 SCHC Fragment:
* the receiver MUST append the All-1 SCHC Fragment Payload and the
padding bits to the previously received SCHC Fragment Payloads for
this SCHC Packet.
* the receiver MUST perform the integrity check.
* if integrity checking fails, the receiver MUST drop the
reassembled SCHC Packet.
* the reassembly operation concludes.
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 40 shows an example of a corresponding state machine.
8.4.2. ACK-Always Mode
The ACK-Always mode has been designed under the following
assumptions:
* Data unit out-of-sequence delivery does not occur between the
entity performing fragmentation and the entity performing
reassembly,
* The L2 MTU value does not change while the fragments of a SCHC
Packet are being transmitted, and
* There is a feedback path from the reassembler to the fragmenter.
See Appendix F for a discussion on using ACK-Always mode on quasi-
bidirectional links.
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 RuleID 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 RuleID value, MUST define:
* the value of N,
* the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
* the size and algorithm for the RCS field,
* the value of T,
* the value of MAX_ACK_REQUESTS,
* the expiration time of the Retransmission Timer, and
* the expiration time of the Inactivity Timer.
For each active pair of RuleID and DTag values, the sender MUST
maintain:
* one Attempts counter
* one Retransmission Timer
For each active pair of RuleID and DTag values, the receiver MUST
maintain
* one Inactivity Timer, and
* one Attempts counter.
8.4.2.1. Sender Behavior
At the beginning of the fragmentation of a new SCHC Packet, the
fragment sender MUST select a RuleID 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
LSB 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 Fragment MUST be of the Regular type specified in
Section 8.3.1.1.
* the FCN field MUST contain the tile index.
* each tile MUST be of a size that complements the SCHC Fragment
Header so that the SCHC Fragment is a multiple of L2 Words without
the need for padding bits.
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, upon receiving a SCHC ACK:
- if the SCHC ACK indicates that some tiles are missing at the
receiver, then the sender MUST transmit all the tiles that have
been reported missing, it MUST increment Attempts, it MUST
reset the Retransmission Timer, and MUST await the next SCHC
ACK.
- if the current window is not the last one and the SCHC ACK
indicates that all tiles were correctly received, the sender
MUST stop the Retransmission Timer, it MUST advance to the next
fragmentation window, and it MUST start a blind transmission
phase as described above.
- if the current window is the last one and the SCHC ACK
indicates that more tiles were received than the sender sent,
the fragment sender MUST send a SCHC Sender-Abort, and it MAY
exit with an error condition.
- if the current window is the last one and the SCHC ACK
indicates that all tiles were correctly received, yet the
integrity check was a failure, the fragment sender MUST send a
SCHC Sender-Abort, and it MAY exit with an error condition.
- if the current window is the last one and the SCHC ACK
indicates that integrity checking was successful, the sender
exits successfully.
* on Retransmission Timer expiration:
- if Attempts is strictly less that MAX_ACK_REQUESTS, the
fragment sender MUST send a SCHC ACK REQ and MUST increment the
Attempts counter.
- otherwise, the fragment sender MUST send a SCHC Sender-Abort,
and it MAY exit with an error condition.
At any time:
* on receiving a SCHC Receiver-Abort, the fragment sender MAY exit
with an error condition.
* on receiving a SCHC ACK that bears a W value different from the W
value that it currently uses, the fragment sender MUST silently
discard and ignore that SCHC ACK.
Figure 41 shows an example of a corresponding state machine.
8.4.2.2. Receiver Behavior
On receiving a SCHC Fragment with a RuleID and DTag pair not being
processed at that time:
* the receiver SHOULD check if the DTag value has not recently been
used for that RuleID value, thereby ensuring that the received
SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
transmission. The initial value of the Inactivity Timer is the
RECOMMENDED lifetime for the DTag value at the receiver. If the
SCHC Fragment is determined to be such a remnant, the receiver MAY
silently ignore it and discard it.
* the receiver MUST start a process to assemble a new SCHC Packet
with that RuleID and DTag value pair.
* the receiver MUST start an Inactivity Timer for that RuleID and
DTag pair. It MUST initialize an Attempts counter to 0 for that
RuleID and DTag pair. It MUST initialize a window counter to 0.
If the receiver is under-resourced to do this, it MUST respond to
the sender with a SCHC Receiver-Abort.
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:
* on receiving a SCHC Fragment or a SCHC ACK REQ, either one having
the W bit different from the local W bit, the receiver MUST
silently ignore and discard that message.
* on receiving a SCHC ACK REQ with the W bit equal to the local W
bit, the receiver MUST send a SCHC ACK for this window.
* on receiving a SCHC Fragment with the W bit equal to the local W
bit, the receiver MUST assemble the received tile based on the
window counter and on the FCN field in the SCHC Fragment, and it
MUST update the Bitmap.
- if the SCHC Fragment received is an All-0 SCHC Fragment, the
current window is determined to be a not-last window, the
receiver MUST send a SCHC ACK for this window and it MUST enter
the "retransmission phase" for this window.
- if the SCHC Fragment received is an All-1 SCHC Fragment, the
current window is determined to be the last window, the padding
bits of the All-1 SCHC Fragment MUST be assembled after the
received tile, the receiver MUST perform the integrity check
and it MUST send a SCHC ACK for this window. Then:
o If the integrity check indicates that the full SCHC Packet
has been correctly reassembled, the receiver MUST enter the
"clean-up phase" for this window.
o If the integrity check indicates that the full SCHC Packet
has not been correctly reassembled, the receiver enters the
"retransmission phase" for this window.
In the "retransmission phase":
* if the window is a not-last window:
- on receiving a SCHC Fragment that is not All-0 or All-1 and
that has a W bit different from the local W bit, the receiver
MUST increment its window counter and allocate a fresh Bitmap,
it MUST assemble the tile received and update the Bitmap, and
it MUST enter the "acceptance phase" for that new window.
- on receiving a SCHC ACK REQ with a W bit different from the
local W bit, the receiver MUST increment its window counter and
allocate a fresh Bitmap, it MUST send a SCHC ACK for that new
window, and it MUST enter the "acceptance phase" for that new
window.
- on receiving a SCHC All-0 Fragment with a W bit different from
the local W bit, the receiver MUST increment its window counter
and allocate a fresh Bitmap, it MUST assemble the tile received
and update the Bitmap, it MUST send a SCHC ACK for that new
window, and it MUST stay in the "retransmission phase" for that
new window.
- on receiving a SCHC All-1 Fragment with a W bit different from
the local W bit, the receiver MUST increment its window counter
and allocate a fresh Bitmap; it MUST assemble the tile
received, including the padding bits; it MUST update the Bitmap
and perform the integrity check; it MUST send a SCHC ACK for
the new window, which is determined to be the last window.
Then:
o If the integrity check indicates that the full SCHC Packet
has been correctly reassembled, the receiver MUST enter the
"clean-up phase" for that new window.
o If the integrity check indicates that the full SCHC Packet
has not been correctly reassembled, the receiver enters the
"retransmission phase" for that new window.
- on receiving a SCHC Fragment with a W bit equal to the local W
bit:
o if the SCHC Fragment received is an All-1 SCHC Fragment, the
receiver MUST silently ignore it and discard it.
o otherwise, the receiver MUST assemble the tile received and
update the Bitmap. If the Bitmap becomes fully populated
with 1's or if the SCHC Fragment is an All-0, the receiver
MUST send a SCHC ACK for this window.
- on receiving a SCHC ACK REQ with the W bit equal to the local W
bit, the receiver MUST send a SCHC ACK for this window.
* if the window is the last window:
- on receiving a SCHC Fragment or a SCHC ACK REQ, either one
having a W bit different from the local W bit, the receiver
MUST silently ignore and discard that message.
- on receiving a SCHC ACK REQ with the W bit equal to the local W
bit, the receiver MUST send a SCHC ACK for this window.
- on receiving a SCHC Fragment with a W bit equal to the local W
bit:
o if the SCHC Fragment received is an All-0 SCHC Fragment, the
receiver MUST silently ignore it and discard it.
o otherwise, the receiver MUST update the Bitmap, and it MUST
assemble the tile received. If the SCHC Fragment received
is an All-1 SCHC Fragment, the receiver MUST assemble the
padding bits of the All-1 SCHC Fragment after the received
tile, it MUST perform the integrity check and:
+ if the integrity check indicates that the full SCHC
Packet has been correctly reassembled, the receiver MUST
send a SCHC ACK and it enters the "clean-up phase".
+ if the integrity check indicates that the full SCHC
Packet has not been correctly reassembled:
* if the SCHC Fragment received was an All-1 SCHC
Fragment, the receiver MUST send a SCHC ACK for this
window.
In the "clean-up phase":
* On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, either one
having the W bit equal to the local W bit, the receiver MUST send
a SCHC ACK.
* Any other SCHC Fragment received MUST be silently ignored and
discarded.
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 42 shows an example of a corresponding state machine.
8.4.3. ACK-on-Error Mode
The ACK-on-Error mode supports L2 technologies that have variable MTU
and out-of-order delivery. It requires an L2 that provides 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 except 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:
* The penultimate tile size MUST be the regular tile size, or
* the penultimate tile size MUST be either the regular tile size or
the regular tile size minus one L2 Word.
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: 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 it 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:
* integrity checking shows that the fragmented SCHC Packet has been
correctly reassembled at the receive end, and this information has
been conveyed back to the sender, or
* too many retransmission attempts were made, or
* the receiver determines that the transmission of this fragmented
SCHC Packet has been inactive for too long.
Each Profile MUST specify which RuleID value(s) corresponds 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 RuleID value, MUST define:
* the tile size (a tile does not need to be multiple of an L2 Word,
but it MUST be at least the size of an L2 Word),
* the value of M,
* the value of N,
* the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
* the size and algorithm for the RCS field,
* the value of T,
* the value of MAX_ACK_REQUESTS,
* the expiration time of the Retransmission Timer,
* the expiration time of the Inactivity Timer,
* if the last tile is carried in a Regular SCHC Fragment or an All-1
SCHC Fragment (see Section 8.4.3.1), and
* if the penultimate tile MAY be one L2 Word smaller than the
regular tile size. In this case, the regular tile size MUST be at
least twice the L2 Word size.
For each active pair of RuleID and DTag values, the sender MUST
maintain:
* one Attempts counter, and
* one Retransmission Timer.
For each active pair of RuleID and DTag values, the receiver MUST
maintain:
* one Inactivity Timer, and
* one Attempts counter.
8.4.3.1. Sender Behavior
At the beginning of the fragmentation of a new SCHC Packet:
* the fragment sender MUST select a RuleID and DTag value pair for
this SCHC Packet. A Rule MUST NOT be selected if the values of M
and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot
be fragmented in (2^M) * WINDOW_SIZE tiles or less.
* the fragment sender MUST initialize the Attempts counter to 0 for
that RuleID and DTag value pair.
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:
* the selected tiles MUST be contiguous in the original SCHC Packet,
and
* they MUST be placed in the SCHC Fragment Payload adjacent to one
another, in the order they appear in the SCHC Packet, from the
start of the SCHC Packet toward its end.
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.
A Profile MUST define if the last tile of a SCHC Packet is sent:
* in a Regular SCHC Fragment, alone or as part of a multi-tiles
Payload,
* alone in an All-1 SCHC Fragment, or
* with any of the above two methods.
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.
In doing the two items above, the sender MUST ascertain that the
receiver will not receive the last tile through both a Regular SCHC
Fragment and an All-1 SCHC Fragment.
The fragment sender MUST listen for SCHC ACK messages after having
sent:
* an All-1 SCHC Fragment, or
* a SCHC ACK REQ.
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:
* it MUST increment the Attempts counter, and
* it MUST reset the Retransmission Timer.
On Retransmission Timer expiration:
* if the Attempts counter is strictly less than MAX_ACK_REQUESTS,
the fragment sender MUST send either the All-1 SCHC Fragment or a
SCHC ACK REQ with the W field corresponding to the last window,
* otherwise, the fragment sender MUST send a SCHC Sender-Abort, and
it MAY exit with an error condition.
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:
* if the W field in the SCHC ACK corresponds to the last window of
the SCHC Packet:
- if the C bit is set, the sender MAY exit successfully.
- otherwise:
o if the Profile mandates that the last tile be sent in an
All-1 SCHC Fragment:
+ if the SCHC ACK shows no missing tile at the receiver,
the sender:
* MUST send a SCHC Sender-Abort, and
* MAY exit with an error condition.
+ otherwise:
* the fragment sender MUST send SCHC Fragment messages
containing all the tiles that are reported missing in
the SCHC ACK.
* if the last of these SCHC Fragment messages is not an
All-1 SCHC Fragment, then the fragment sender MUST in
addition send after it a SCHC ACK REQ with the W field
corresponding to the last window.
* in doing the two items above, the sender MUST
ascertain that the receiver will not receive the last
tile through both a Regular SCHC Fragment and an All-1
SCHC Fragment.
o otherwise:
+ if the SCHC ACK shows no missing tile at the receiver,
the sender MUST send the All-1 SCHC Fragment
+ otherwise:
* the fragment sender MUST send SCHC Fragment messages
containing all the tiles that are reported missing in
the SCHC ACK.
* the fragment sender MUST then send either the All-1
SCHC Fragment or a SCHC ACK REQ with the W field
corresponding to the last window.
* otherwise, the fragment sender:
- MUST send SCHC Fragment messages containing the tiles that are
reported missing in the SCHC ACK.
- then, it MAY send a SCHC ACK REQ with the W field corresponding
to the last window.
See Figure 43 for one among several possible examples of a Finite
State Machine implementing a sender behavior obeying this
specification.
8.4.3.2. Receiver Behavior
On receiving a SCHC Fragment with a RuleID and DTag pair not being
processed at that time:
* the receiver SHOULD check if the DTag value has not recently been
used for that RuleID value, thereby ensuring that the received
SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
transmission. The initial value of the Inactivity Timer is the
RECOMMENDED lifetime for the DTag value at the receiver. If the
SCHC Fragment is determined to be such a remnant, the receiver MAY
silently ignore it and discard it.
* the receiver MUST start a process to assemble a new SCHC Packet
with that RuleID and DTag value pair. The receiver MUST start an
Inactivity Timer for that RuleID and DTag value pair. It MUST
initialize an Attempts counter to 0 for that RuleID and DTag value
pair. If the receiver is under-resourced to do this, it MUST
respond to the sender with a SCHC Receiver-Abort.
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.
* if the FCN is All-1, if a Payload is present, the full SCHC
Fragment Payload MUST be assembled including the padding bits.
This is because the size of the last tile is not known by the
receiver; therefore, padding bits are indistinguishable from the
tile data bits, at this stage. They will be removed by the SCHC
C/D sublayer. If the size of the SCHC Fragment Payload exceeds or
equals the size of one regular tile plus the size of an L2 Word,
this SHOULD raise an error flag.
* otherwise, tiles MUST be assembled based on the a priori known
tile size.
- If allowed by the Profile, the end of the payload MAY contain
the last tile, which may be shorter. Padding bits are
indistinguishable from the tile data bits, at this stage.
- The payload may contain the penultimate tile that, if allowed
by the Profile, MAY be exactly one L2 Word shorter than the
regular tile size.
- Otherwise, padding bits MUST be discarded. This is possible
because:
o the size of the tiles is known a priori,
o tiles are larger than an L2 Word, and
o padding bits are always strictly less than an L2 Word.
On receiving a SCHC ACK REQ or an All-1 SCHC Fragment:
* if the receiver knows of any windows with missing tiles for the
packet being reassembled, it MUST return a SCHC ACK for the
lowest-numbered such window:
* otherwise:
- if it has received at least one tile, it MUST return a SCHC ACK
for the highest-numbered window it currently has tiles for,
- otherwise, it MUST return a SCHC ACK for window numbered 0.
A 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:
* a Sender-Abort has been received, or
* the Inactivity Timer has expired, or
* the Attempts counter has exceeded MAX_ACK_REQUESTS, or
* at least an All-1 SCHC Fragment has been received and integrity
checking of the reassembled SCHC Packet is successful.
See Figure 44 for one among several possible examples of a Finite
State Machine implementing a receiver behavior obeying this
specification. The example provided is meant to match the sender
Finite State Machine of Figure 43.
9. Padding Management
SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does
not have any alignment prerequisite. The size of SCHC Packets can be
any number of bits.
If the L2 constrains the payload to align to coarser boundaries (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 MUST define the value of the padding bits if the L2 Word is
wider than a single bit. The RECOMMENDED value is 0.
10. SCHC Compression for IPv6 and UDP Headers
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.
10.1. IPv6 Version Field
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".
10.2. IPv6 Traffic Class Field
If the Diffserv field does not vary and is known by both sides, the
Field Descriptor in the Rule SHOULD contain a TV with this well-known
value, an "equal" MO, and a "not-sent" CDA.
Otherwise (e.g., ECN bits are to be transmitted), two possibilities
can be considered depending on the variability of the value:
* One possibility is to not compress the field and send the original
value. In the Rule, TV is not set to any particular value, MO is
set to "ignore", and CDA is set to "value-sent".
* If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the Rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x), and
CDA to LSB.
ECN functionality depends on both bits of the ECN field, which are
the 2 LSBs of this field; hence, sending only a single LSB of this
field is NOT RECOMMENDED.
10.3. Flow Label Field
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 pseudorandom 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.
10.4. Payload Length Field
This field can be elided for the transmission on the LPWAN. 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-*".
10.5. Next Header Field
If the Next Header field does not vary and is known by both sides,
the Field Descriptor in the Rule SHOULD contain a TV with this Next
Header value, the MO SHOULD be "equal", and the CDA SHOULD be "not-
sent".
Otherwise, TV is not set in the Field Descriptor, MO is set to
"ignore", and CDA is set to "value-sent". Alternatively, a matching-
list MAY also be used.
10.6. Hop Limit Field
The field behavior for this field is different for Uplink and
Downlink. In Uplink, since there is no IP forwarding between the Dev
and the SCHC C/D, the value is relatively constant. On the other
hand, the Downlink value depends on Internet routing and can change
more frequently. The DI can be used to distinguish both directions:
* in an Up Field Descriptor, elide the field: the TV is set to the
known constant value, the MO is set to "equal" and the CDA is set
to "not-sent".
* in a Dw Field Descriptor, the Hop Limit is elided for transmission
and forced to 1 at the receiver, by setting TV to 1, MO to
"ignore" and CDA to "not-sent". This prevents any further
forwarding.
10.7. IPv6 Addresses Fields
As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit-
long fields; one for the prefix and one for the Interface Identifier
(IID). These fields SHOULD be compressed. To allow for a single
Rule being used for both directions, these values are identified by
their role (Dev or App) and not by their position in the header
(source or destination).
10.7.1. IPv6 Source and Destination Prefixes
Both ends MUST be 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".
10.7.2. IPv6 Source and Destination IID
If the Dev or App IID are based on an L2 address, then the IID can be
reconstructed with information coming from the L2 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 L2. In that
case, the TV is not set, the MO is set to "ignore", and the CDA is
set to "value-sent".
10.8. IPv6 Extension Headers
This document does not provide recommendations on how to compress
IPv6 extension headers.
10.9. UDP Source and Destination Ports
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 L2. The TV is not set,
the MO is set to "ignore" and the CDA is set to "value-sent".
10.10. UDP Length Field
The parser MUST NOT label this field unless the UDP Length value
matches the Payload Length value from the IPv6 header. The TV is not
set, the MO is set to "ignore", and the CDA is set to "compute-*".
10.11. UDP Checksum Field
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 recomputed 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".
11. IANA Considerations
This document has no IANA actions.
12. Security Considerations
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.
12.1. Security Considerations for SCHC Compression/Decompression
12.1.1. Forged SCHC Packet
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 RuleID contained in that
forged SCHC Packet does not correspond to a Rule allocated in the
Rule table. An implementation should detect that the RuleID is
invalid and should silently drop the offending SCHC Packet.
Let's now consider that the RuleID 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
reconstruct a packet larger than MAX_PACKET_SIZE (defined in a
Profile, with 1500 bytes as generic default).
12.1.2. Compressed Packet Size as a Side Channel to Guess a Secret
Token
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.
12.1.3. Decompressed Packet Different from the Original Packet
As explained in Section 7.2, 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 7.4.3, 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 or may not be functionally significant.
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.
12.2. Security Considerations for SCHC Fragmentation/Reassembly
12.2.1. Buffer Reservation Attack
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 (down to
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 countermeasure 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.
12.2.2. Corrupt Fragment Attack
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 to be dropped by
the receiver. The amplification happens here by a single spoofed
SCHC Fragment rendering a full sequence of legitimate 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.
12.2.3. Fragmentation as a Way to Bypass Network Inspection
Fragmentation is known for potentially allowing one 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 to 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.
12.2.4. Privacy Issues Associated with SCHC Header Fields
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.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc6936>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.
13.2. Informative References
[ETHERNET] IEEE, "IEEE Standard for Ethernet",
DOI 10.1109/IEEESTD.2012.6419735, IEEE
Standard 802.3-2012, December 2012,
<https://ieeexplore.ieee.org/document/6419735>.
[RFC4944] 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,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <https://www.rfc-editor.org/info/rfc7136>.
[RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation-
Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
February 2017, <https://www.rfc-editor.org/info/rfc8065>.
Appendix A. Compression Examples
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 is 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
Rule 0
Special RuleID used to tag an uncompressed UDP/IPV6 packet.
Rule 1
+----------------+--+--+--+---------+--------+------------++------+
| FID |FL|FP|DI| TV | MO | CDA || Sent |
| | | | | | | ||[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-* || |
+================+==+==+==+=========+========+============++======+
Figure 26: Context Rules - Rule 0 and Rule 1
Rule 2
+----------------+--+--+--+---------+--------+------------++------+
| FID |FL|FP|DI| TV | MO | CDA || Sent |
| | | | | | | ||[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-* || |
+================+==+==+==+=========+========+============++======+
Figure 27: Context Rules - Rule 2
Rule 3
+----------------+--+--+--+---------+--------+------------++------+
| FID |FL|FP|DI| TV | MO | CDA || Sent |
| | | | | | | ||[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 28: Context Rules - Rule 3
Figures 26 to 28 describe an example of a Rule set.
In this example, 0 was chosen as the special RuleID 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 value is inferred from 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.
Appendix B. Fragmentation Examples
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 29 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 29: 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 therefore 7.
Figure 30 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 30: ACK-on-Error Mode, 11 Tiles, One Tile per SCHC
Fragment, No Lost SCHC Fragment
Figure 31 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 31: ACK-on-Error Mode, 11 Tiles, One Tile per SCHC
Fragment, Lost SCHC Fragments
Figure 32 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 three 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 32: 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 one tile in each
forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are
carried by a total of 25 SCHC Fragments, the last nine 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 33 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 33: ACK-Always Mode, 11 Tiles, One Tile per SCHC Fragment,
No Loss
Figure 34 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 34: ACK-Always Mode, 11 Tiles, One Tile per SCHC Fragment,
Three Lost SCHC Fragments
Figure 35 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in six 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 35: ACK-Always Mode, Six Tiles, One Tile per SCHC
Fragment, Three Lost SCHC Fragments
Figure 36 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in six 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 36: ACK-Always Mode, Six Tiles, One Tile per SCHC
Fragment, SCHC ACK Loss
Figure 37 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in six 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 37: ACK-Always Mode, Six Tiles, Retransmitted SCHC
Fragment Lost Again
Figure 38 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 38: ACK-Always Mode, 28 Tiles, One Tile per SCHC Fragment,
Lost SCHC Fragments
Appendix C. Fragmentation State Machines
The fragmentation state machines of the sender and the receiver, one
for each of the different reliability modes, are described in the
following figures:
+===========+
+------------+ Init |
| FCN=0 +===========+
| No Window
| No Bitmap
| +-------+
| +========+==+ | More Fragments
| | | <--+ ~~~~~~~~~~~~~~~~~~~~
+--------> | Send | send Fragment (FCN=0)
+===+=======+
| last fragment
| ~~~~~~~~~~~~
| FCN = 1
v send fragment+RCS
+============+
| END |
+============+
Figure 39: 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 40: 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 41: 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 42: 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
| | | | +---------------------------------+
| | | | |cur_W == |
| |Retrans_Timer expires & | | | 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 43: 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 44: Receiver State Machine for the ACK-on-Error Mode
Appendix D. SCHC Parameters
This section lists the information that needs to be provided in the
LPWAN technology-specific documents.
* Most common uses cases, deployment scenarios.
* Mapping of the SCHC architectural elements onto the LPWAN
architecture.
* Assessment of LPWAN integrity checking.
* Various potential channel conditions for the technology and the
corresponding recommended use of SCHC C/D and SCHC F/R.
This section lists the parameters that need to be defined in the
Profile.
* RuleID numbering scheme, fixed-size or variable-size RuleIDs,
number of Rules, the way the RuleID is transmitted.
* maximum packet size that should ever be reconstructed by SCHC
decompression (MAX_PACKET_SIZE). See Section 12.
* Padding: size of the L2 Word (for most LPWAN technologies, this
would be a byte; for some technologies, a bit).
* Decision to use SCHC fragmentation mechanism or not. If yes, the
document must describe:
- reliability mode(s) used, in which cases (e.g., based on link
channel condition).
- RuleID values assigned to each mode in use.
- presence and number of bits for DTag (T) for each RuleID value,
lifetime of DTag at the receiver.
- support for interleaved packet transmission, to what extent.
- WINDOW_SIZE, for modes that use windows.
- number of bits for W (M) for each RuleID value, for modes that
use windows.
- number of bits for FCN (N) for each RuleID value, meaning of
the FCN values.
- what makes an All-0 SCHC Fragment and a SCHC ACK REQ
distinguishable (see Section 8.3.1.1).
- what makes an All-1 SCHC Fragment and a SCHC Sender-Abort
distinguishable (see Section 8.3.1.2).
- for RuleIDs that use ACK-on-Error mode: when the last tile of a
SCHC Packet is to be sent in a Regular SCHC Fragment, alone in
an All-1 SCHC Fragment or with any of these two methods.
- for RuleIDs that use ACK-on-Error mode: if the penultimate tile
of a SCHC Packet is of the regular size only or if it can also
be one L2 Word shorter.
- for RuleIDs that use ACK-on-Error mode: times at which the
sender must listen for SCHC ACKs.
- size of RCS and algorithm for its computation, for each RuleID,
if different from the default CRC32. Byte fill-up with zeroes
or other mechanism, to be specified. Support for UDP checksum
elision.
- Retransmission Timer duration for each RuleID value, if
applicable to the SCHC F/R mode.
- Inactivity Timer duration for each RuleID value, if applicable
to the SCHC F/R mode.
- MAX_ACK_REQUESTS value for each RuleID value, if applicable to
the SCHC F/R mode.
* if L2 Word is wider than a bit and SCHC fragmentation is used,
value of the padding bits (0 or 1).
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.
* In some LPWAN technologies, as part of energy-saving techniques,
Downlink transmission is only possible immediately after an Uplink
transmission. In order to avoid potentially high delay in the
Downlink transmission of a fragmented SCHC Packet, the SCHC
Fragment receiver may perform an Uplink transmission as soon as
possible after reception of a SCHC Fragment that is not the last
one. Such Uplink transmission may be triggered by the L2 (e.g.,
an L2 ACK sent in response to a SCHC Fragment encapsulated in a L2
PDU that requires an L2 ACK) or it may be triggered from an upper
layer. See Appendix F.
* the following parameters need to be addressed in documents other
than this one but not necessarily in the LPWAN technology-specific
documents:
- The way the Contexts are provisioned.
- The way the Rules are generated.
Appendix E. Supporting Multiple Window Sizes for Fragmentation
For ACK-Always or ACK-on-Error, implementers may opt to support a
single window size or multiple window sizes. The latter, when
feasible, may provide performance optimizations. For example, a
large WINDOW_SIZE should be used for packets that need to be 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 RuleID signals
what WINDOW_SIZE is in use for a specific packet transmission.
Appendix F. ACK-Always and ACK-on-Error on Quasi-Bidirectional Links
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).
Acknowledgements
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.
Authors' Addresses
Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Laurent Toutain
IMT Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
Carles Gomez
Universitat Politecnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: carlesgo@entel.upc.edu
Dominique Barthel
Orange Labs
28 chemin du Vieux Chene
38243 Meylan
France
Email: dominique.barthel@orange.com
Juan Carlos Zuniga
SIGFOX
425 rue Jean Rostand
31670 Labege
France
Email: JuanCarlos.Zuniga@sigfox.com
