LPWAN Static Context Header Compression (SCHC) and fragmentation for IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-12
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (lpwan WG) | |
|---|---|---|---|
| Authors | Ana Minaburo , Laurent Toutain , Carles Gomez | ||
| Last updated | 2018-05-18 (Latest revision 2018-04-13) | ||
| Replaces | draft-toutain-lpwan-ipv6-static-context-hc | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
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| Document shepherd | Dominique Barthel | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Dominique Barthel <dominique.barthel@orange.com> |
draft-ietf-lpwan-ipv6-static-context-hc-12
lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational L. Toutain
Expires: November 19, 2018 IMT-Atlantique
C. Gomez
Universitat Politecnica de Catalunya
May 18, 2018
LPWAN Static Context Header Compression (SCHC) and fragmentation for
IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-12
Abstract
This document defines the Static Context Header Compression (SCHC)
framework, which provides header compression and fragmentation
functionality. SCHC has been tailored for Low Power Wide Area
Networks (LPWAN).
SCHC compression is based on a common static context stored in both
LPWAN devices and in the network sides. This document defines SCHC
header compression mechanism and its deployment for IPv6/UDP headers.
This document also specifies a fragmentation and reassembly mechanism
that is used to support the IPv6 MTU requirement over the LPWAN
technologies. The Fragmentation is needed for IPv6 datagrams that,
after SCHC compression or when it has not been possible to apply such
compression, still exceed the layer two maximum payload size.
The SCHC header compression mechanism is independent of the specific
LPWAN technology over which it will be used. Note that this document
defines generic functionalities and advisedly offers flexibility with
regard to parameters settings and mechanism choices, that are
expected to be made in other technology-specific documents.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 19, 2018.
Copyright Notice
Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. SCHC overview . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Static Context Header Compression . . . . . . . . . . . . . . 11
6.1. SCHC C/D Rules . . . . . . . . . . . . . . . . . . . . . 12
6.2. Rule ID for SCHC C/D . . . . . . . . . . . . . . . . . . 14
6.3. Packet processing . . . . . . . . . . . . . . . . . . . . 14
6.4. Matching operators . . . . . . . . . . . . . . . . . . . 16
6.5. Compression Decompression Actions (CDA) . . . . . . . . . 17
6.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 18
6.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 18
6.5.3. mapping-sent CDA . . . . . . . . . . . . . . . . . . 18
6.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19
6.5.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . 19
6.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 19
7. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Fragmentation Tools . . . . . . . . . . . . . . . . . . . 20
7.3. Reliability modes . . . . . . . . . . . . . . . . . . . . 23
7.4. Fragmentation Formats . . . . . . . . . . . . . . . . . . 25
7.4.1. Fragment format . . . . . . . . . . . . . . . . . . . 25
7.4.2. All-1 and All-0 formats . . . . . . . . . . . . . . . 26
7.4.3. SCHC ACK format . . . . . . . . . . . . . . . . . . . 28
7.4.4. Abort formats . . . . . . . . . . . . . . . . . . . . 30
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7.5. Baseline mechanism . . . . . . . . . . . . . . . . . . . 31
7.5.1. No-ACK . . . . . . . . . . . . . . . . . . . . . . . 33
7.5.2. ACK-Always . . . . . . . . . . . . . . . . . . . . . 33
7.5.3. ACK-on-Error . . . . . . . . . . . . . . . . . . . . 35
7.6. Supporting multiple window sizes . . . . . . . . . . . . 37
7.7. Downlink SCHC Fragment transmission . . . . . . . . . . . 37
8. Padding management . . . . . . . . . . . . . . . . . . . . . 38
9. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 39
9.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 39
9.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 39
9.3. Flow label field . . . . . . . . . . . . . . . . . . . . 40
9.4. Payload Length field . . . . . . . . . . . . . . . . . . 40
9.5. Next Header field . . . . . . . . . . . . . . . . . . . . 40
9.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 40
9.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 41
9.7.1. IPv6 source and destination prefixes . . . . . . . . 41
9.7.2. IPv6 source and destination IID . . . . . . . . . . . 41
9.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 42
9.9. UDP source and destination port . . . . . . . . . . . . . 42
9.10. UDP length field . . . . . . . . . . . . . . . . . . . . 42
9.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 43
10. Security considerations . . . . . . . . . . . . . . . . . . . 43
10.1. Security considerations for header compression . . . . . 43
10.2. Security considerations for SCHC
Fragmentation/Reassembly . . . . . . . . . . . . . . . . 43
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 44
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 44
12.1. Normative References . . . . . . . . . . . . . . . . . . 45
12.2. Informative References . . . . . . . . . . . . . . . . . 45
Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 45
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 48
Appendix C. Fragmentation State Machines . . . . . . . . . . . . 54
Appendix D. SCHC Parameters - Ticket #15 . . . . . . . . . . . . 61
Appendix E. Note . . . . . . . . . . . . . . . . . . . . . . . . 62
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62
1. Introduction
This document defines a header compression scheme and fragmentation
functionality, both specially tailored for Low Power Wide Area
Networks (LPWAN).
Header compression is needed to efficiently bring Internet
connectivity to the node within an LPWAN network. Some LPWAN
networks properties can be exploited to get an efficient header
compression:
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o The topology is star-oriented which means that all packets follow
the same path. For the necessity of this draft, the architecture
is simple and is described as Devices (Dev) exchanging information
with LPWAN Application Servers (App) through Network Gateways
(NGW).
o The traffic flows can be known in advance since devices embed
built-in applications. New applications cannot be easily
installed in LPWAN devices, as they would in computers or
smartphones.
The Static Context Header Compression (SCHC) is defined for this
environment. SCHC uses a context, where header information is kept
in the header format order. This context is static: the values of
the header fields do not change over time. This avoids complex
resynchronization mechanisms, that would be incompatible with LPWAN
characteristics. In most cases, a small context identifier is enough
to represent the full IPv6/UDP headers. The SCHC header compression
mechanism is independent of the specific LPWAN technology over which
it is used.
LPWAN technologies impose some strict limitations on traffic. For
instance, devices are sleeping most of the time and MAY receive data
during short periods of time after transmission to preserve battery.
LPWAN technologies are also characterized, among others, by a very
reduced data unit and/or payload size [I-D.ietf-lpwan-overview].
However, some of these technologies do not provide fragmentation
functionality, therefore the only option for them to support the IPv6
MTU requirement of 1280 bytes [RFC2460] is to use a fragmentation
protocol at the adaptation layer, below IPv6. In response to this
need, this document also defines a fragmentation/reassembly
mechanism, which supports the IPv6 MTU requirement over LPWAN
technologies. Such functionality has been designed under the
assumption that data unit out-of-sequence delivery will not happen
between the entity performing fragmentation and the entity performing
reassembly.
Note that this document defines generic functionality and
purposefully offers flexibility with regard to parameter settings and
mechanism choices, that are expected to be made in other, technology-
specific documents.
2. LPWAN Architecture
LPWAN technologies have similar network architectures but different
terminology. We can identify different types of entities in a
typical LPWAN network, see Figure 1:
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o Devices (Dev) are the end-devices or hosts (e.g. sensors,
actuators, etc.). There can be a very high density of devices per
radio gateway.
o The Radio Gateway (RGW), which is the end point of the constrained
link.
o The Network Gateway (NGW) is the interconnection node between the
Radio Gateway and the Internet.
o LPWAN-AAA Server, which controls the user authentication and the
applications.
o Application Server (App)
+------+
() () () | |LPWAN-|
() () () () / \ +---------+ | AAA |
() () () () () () / \======| ^ |===|Server| +-----------+
() () () | | <--|--> | +------+ |APPLICATION|
() () () () / \==========| v |=============| (App) |
() () () / \ +---------+ +-----------+
Dev Radio Gateways NGW
Figure 1: LPWAN Architecture
3. Terminology
This section defines the terminology and acronyms used in this
document.
o Abort. A SCHC Fragment format to signal the other end-point that
the on-going fragment transmission is stopped and finished.
o All-0. The SCHC Fragment format for the last frame of a window
that is not the last one of a packet (see Window in this
glossary).
o All-1. The SCHC Fragment format for the last frame of the packet.
o All-0 empty. An All-0 SCHC Fragment without payload. It is used
to request the SCHC ACK with the encoded Bitmap when the
Retransmission Timer expires, in a window that is not the last one
of a packet.
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o All-1 empty. An All-1 SCHC Fragment without payload. It is used
to request the SCHC ACK with the encoded Bitmap when the
Retransmission Timer expires in the last window of a packet.
o App: LPWAN Application. An application sending/receiving IPv6
packets to/from the Device.
o APP-IID: Application Interface Identifier. Second part of the
IPv6 address that identifies the application server interface.
o Bi: Bidirectional, a rule entry that applies to headers of packets
travelling in both directions (Up and Dw).
o Bitmap: a field of bits in an acknowledgment message that tells
the sender which SCHC Fragments of a window were correctly
received.
o C: Checked bit. Used in an acknowledgment (SCHC ACK) header to
determine if the MIC locally computed by the receiver matches (1)
the received MIC or not (0).
o CDA: Compression/Decompression Action. Describes the reciprocal
pair of actions that are performed at the compressor to compress a
header field and at the decompressor to recover the original
header field value.
o Compression Residue. The bits that need to be sent after applying
the SCHC compression over each header field
o Context: A set of rules used to compress/decompress headers.
o Dev: Device. A node connected to the LPWAN. A Dev SHOULD
implement SCHC.
o Dev-IID: Device Interface Identifier. Second part of the IPv6
address that identifies the device interface.
o DI: Direction Indicator. This field tells which direction of
packet travel (Up, Dw or Bi) a rule applies to. This allows for
assymmetric processing.
o DTag: Datagram Tag. This SCHC F/R header field is set to the same
value for all SCHC Fragments carrying the same IPv6 datagram.
o Dw: Downlink direction for compression/decompression in both
sides, from SCHC C/D in the network to SCHC C/D in the Dev.
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o FCN: Fragment Compressed Number. This SCHC F/R header field
carries an efficient representation of a larger-sized fragment
number.
o Field Description. A line in the Rule Table.
o FID: Field Identifier. This is an index to describe the header
fields in a Rule.
o FL: Field Length is the length of the field in bits for fixed
values or a type (variable, token length, ...) for length unknown
at the rule creation. The length of a header field is defined in
the specific protocol standard.
o FP: Field Position is a value that is used to identify the
position where each instance of a field appears in the header.
o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136]
o Inactivity Timer. A timer used after receiving a SCHC Fragment to
detect when there is an error and there is no possibility to
continue an on-going SCHC Fragmented packet transmission.
o L2: Layer two. The immediate lower layer SCHC interfaces with.
It is provided by an underlying LPWAN technology.
o MIC: Message Integrity Check. A SCHC F/R header field computed
over an IPv6 packet before fragmentation, used for error detection
after IPv6 packet reassembly.
o MO: Matching Operator. An operator used to match a value
contained in a header field with a value contained in a Rule.
o Retransmission Timer. A timer used by the SCHC Fragment sender
during an on-going SCHC Fragmented packet transmission to detect
possible link errors when waiting for a possible incoming SCHC
ACK.
o Rule: A set of header field values.
o Rule entry: A column in the rule that describes a parameter of the
header field.
o Rule ID: An identifier for a rule, SCHC C/D in both sides share
the same Rule ID for a specific packet. A set of Rule IDs are
used to support SCHC F/R functionality.
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o SCHC ACK: A SCHC acknowledgement for fragmentation, this format
used to report the success or unsuccess reception of a set of SCHC
Fragments. See Section 7 for more details.
o SCHC C/D: Static Context Header Compression Compressor/
Decompressor. A mechanism used in both sides, at the Dev and at
the network to achieve Compression/Decompression of headers. SCHC
C/D uses SCHC rules to perform compression and decompression.
o SCHC F/R: Static Context Header Compression Fragmentation/
Reassembly. A protocol used in both sides, at the Dev and at the
network to achieve Fragmentation/Reassembly of fragments. SCHC F/
R has three reliability modes.
o SCHC Fragment: A data unit that carries a subset of a SCHC Packet.
SCHC F/R is needed when the size of a SCHC packet exceeds the
available payload size of the underlying L2 technology data unit.
See Section 7.
o SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been
compressed as per the header compression mechanism defined in this
document. If the header compression process is unable to actually
compress the packet header, the packet with the uncompressed
header is still called a SCHC Packet (in this case, a Rule ID is
used to indicate that the packet header has not been compressed).
See Section 6 for more details.
o TV: Target value. A value contained in the Rule that will be
matched with the value of a header field.
o Up: Uplink direction for compression/decompression in both sides,
from the Dev SCHC C/D to the network SCHC C/D.
o W: Window bit. A SCHC Fragment header field used in Window mode
Section 7, which carries the same value for all SCHC Fragments of
a window.
o Window: A subset of the SCHC Fragments needed to carry a packet
Section 7.
4. SCHC overview
SCHC can be abstracted as an adaptation layer between IPv6 and the
underlying LPWAN technology. SCHC comprises two sublayers (i.e. the
Compression sublayer and the Fragmentation sublayer), as shown in
Figure 2.
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+----------------+
| IPv6 |
+- +----------------+
| | Compression |
SCHC < +----------------+
| | Fragmentation |
+- +----------------+
|LPWAN technology|
+----------------+
Figure 2: Protocol stack comprising IPv6, SCHC and an LPWAN
technology
As per this document, when a packet (e.g. an IPv6 packet) needs to be
transmitted, header compression is first applied to the packet. The
resulting packet after header compression (whose header may or may
not actually be smaller than that of the original packet) is called a
SCHC Packet. If the SCHC Packet size exceeds the layer 2 (L2) MTU,
fragmentation is then applied to the SCHC Packet. The SCHC Packet or
the SCHC Fragments are then transmitted over the LPWAN. The
reciprocal 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 ------------>|
| |
+--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+
^ | ^ |
| | | |
| +---------- SCHC Fragments ----------+ |
+-------------- SCHC ACK ------------------------+
SENDER RECEIVER
*: see Section 7 to define the use of Fragmentation and the
technology-specific documents for the L2 decision.
Figure 3: SCHC operations taking place at the sender and the receiver
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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 a Rule ID and a Compression Residue.
The Compression Residue may be absent, see Section 6. Both the Rule
ID and the Compression Residue potentially have a variable size, and
generally are not a mutiple of bytes in size.
| Rule ID + Compression Residue |
+---------------------------------+--------------------+
| Compressed Header | Payload |
+---------------------------------+--------------------+
Figure 4: SCHC Packet
The Fragment Header size is variable and depends on the Fragmentation
parameters. The Fragment payload may contain: part of the SCHC
Packet or Payload or both and its size depends on the L2 data unit,
see Section 7. The SCHC Fragment has the following format:
| Rule ID + DTAG + W + FCN [+ MIC ] | Partial SCHC Packet |
+-----------------------------------+-------------------------+
| Fragment Header | Fragment Payload |
+-----------------------------------+-------------------------+
Figure 5: SCHC Fragment
The SCHC ACK is byte aligned and the ACK Header and the encoded
Bitmap both have variable size. The SCHC ACK is used only in
Fragmentation and has the following format:
|Rule ID + DTag + W|
+------------------+-------- ... ---------+
| ACK Header | encoded Bitmap |
+------------------+-------- ... ---------+
Figure 6: SCHC ACK
5. Rule ID
Rule ID are identifiers used to select either the correct context to
be used for Compression/Decompression functionalities or for
Fragmentation/Reassembly or after trying to do SCHC C/D and SCHC F/R
the packet is sent as is. The size of the Rule ID is not specified
in this document, as it is implementation-specific and can vary
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according to the LPWAN technology and the number of Rules, among
others.
The Rule IDs identifiers are used:
o In the SCHC C/D context to keep the Field Description of the
header packet.
o In SCHC F/R to identify the specific modes and settings. In
bidirectional SCHC F/R at least two Rules
ID are needed.
o To identify the SCHC ACK in SCHC F/R
o And at least one Rule ID MAY be reserved to the case where no SCHC
C/D nor SCHC F/R were possible.
6. Static Context Header Compression
In order to perform header compression, this document defines a
mechanism called Static Context Header Compression (SCHC), which is
based on using context, i.e. a set of rules to compress or decompress
headers. SCHC avoids context synchronization, which is the most
bandwidth-consuming operation in other header compression mechanisms
such as RoHC [RFC5795]. Since the nature of packets are highly
predictable in LPWAN networks, static contexts MAY be stored
beforehand to omit transmitting some information over the air. The
contexts MUST be stored at both ends, and they can either be learned
by a provisioning protocol, by out of band means, or they can be pre-
provisioned. The way the contexts are provisioned on both ends is
out of the scope of this document.
Dev App
+----------------+ +--------------+
| APP1 APP2 APP3 | |APP1 APP2 APP3|
| | | |
| UDP | | UDP |
| IPv6 | | IPv6 |
| | | |
|SCHC Comp / Frag| | |
+--------+-------+ +-------+------+
| +--+ +----+ +-----------+ .
+~~ |RG| === |NGW | === | SCHC |... Internet ..
+--+ +----+ |Comp / Frag|
+-----------+
Figure 7: Architecture
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Figure 7 The figure represents the architecture for SCHC (Static
Context Header Compression) Compression/Fragmentation where SCHC C/D
(Compressor/Decompressor) and SCHC F/R (Fragmentation/Reassembly) are
performed. It is based on {{I-D.ietf- lpwan-overview}} terminology.
SCHC Compression/Fragmentation is located on both sides of the
transmission in the Dev and in the Network side. In the Uplink
direction, the Device application packets use IPv6 or IPv6/UDP
protocols. Before sending these packets, the Dev compresses their
headers using SCHC C/D and if the SCHC Packet resulting from the
compression exceeds the maximum payload size of the underlying LPWAN
technology, SCHC F/R is performed, see Section 7. The resulting SCHC
Fragments are sent as one or more L2 frames to an LPWAN Radio Gateway
(RG) which forwards the frame(s) to a Network Gateway (NGW).
The NGW sends the data to a SCHC F/R and then to the SCHC C/D for
decompression. The SCHC C/D in the Network side can be located in
the Network Gateway (NGW) or somewhere else as long as a tunnel is
established between the NGW and the SCHC Compression/Fragmentation.
Note that, for some LPWAN technologies, it MAY be suitable to locate
SCHC Fragmentation/Reassembly functionality nearer the NGW, in order
to better deal with time constraints of such technologies. The SCHC
C/Ds on both sides MUST share the same set of Rules. After
decompression, the packet can be sent over the Internet to one or
several LPWAN Application Servers (App).
The SCHC Compression/Fragmentation process is symmetrical, therefore
the same description applies to the reverse direction.
6.1. SCHC C/D Rules
The main idea of the SCHC compression scheme is to transmit the Rule
ID to the other end instead of sending known field values. This Rule
ID identifies a rule that provides the closest match to the original
packet values. Hence, when a value is known by both ends, it is only
necessary to send the corresponding Rule ID over the LPWAN network.
How Rules are generated is out of the scope of this document. The
rule MAY be changed but it will be specified in another document.
The context contains a list of rules (cf. Figure 8). Each Rule
contains itself a list of Fields Descriptions composed of a field
identifier (FID), a field length (FL), a field position (FP), a
direction indicator (DI), a target value (TV), a matching operator
(MO) and a Compression/Decompression Action (CDA).
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/-----------------------------------------------------------------\
| 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 8: Compression/Decompression Context
The Rule does not describe how to delineate each field in the
original packet header. This MUST be known from the compressor/
decompressor. The rule only describes the compression/decompression
behavior for each header field. In the rule, the Fields Descriptions
are listed in the order in which the fields appear in the packet
header.
The Rule also describes the Compression Residue sent regarding the
order of the Fields Descriptions in the Rule.
The Context describes the header fields and its values with the
following entries:
o Field ID (FID) is a unique value to define the header field.
o Field Length (FL) represents the length of the field in bits for
fixed values or a type (variable, token length, ...) for Field
Description length unknown at the rule creation. The length of a
header field is defined in the specific protocol standard.
o Field Position (FP): indicating if several instances of a field
exist in the headers which one is targeted. The default position
is 1.
o A direction indicator (DI) indicates the packet direction(s) this
Field Description applies to. Three values are possible:
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* UPLINK (Up): this Field Description is only applicable to
packets sent by the Dev to the App,
* DOWNLINK (Dw): this Field Description is only applicable to
packets sent from the App to the Dev,
* BIDIRECTIONAL (Bi): this Field Description is applicable to
packets travelling both Up and Dw.
o Target Value (TV) is the value used to make the match with the
packet header field. The Target Value can be of any type
(integer, strings, etc.). For instance, it can be a single value
or a more complex structure (array, list, etc.), such as a JSON or
a CBOR structure.
o Matching Operator (MO) is the operator used to match the Field
Value and the Target Value. The Matching Operator may require
some parameters. MO is only used during the compression phase.
The set of MOs defined in this document can be found in
Section 6.4.
o Compression Decompression Action (CDA) describes the compression
and decompression processes to be performed after the MO
is applied. The CDA MAY require some parameters to be processed.
CDAs are used in both the compression and the decompression
functions. The set of CDAs defined in this document can be found
in Section 6.5.
6.2. Rule ID for SCHC C/D
Rule IDs are sent by the compression function in one side and are
received for the decompression function in the other side. In SCHC
C/D, the Rule IDs are specific to a Dev. Hence, multiple Dev
instances MAY use the same Rule ID to define different header
compression contexts. To identify the correct Rule ID, the SCHC C/D
needs to correlate the Rule ID with the Dev identifier to find the
appropriate Rule to be applied.
6.3. Packet processing
The compression/decompression process follows several steps:
o Compression Rule selection: The goal is to identify which Rule(s)
will be used to compress the packet's headers. When
doing decompression, in the network side the SCHC C/D needs to
find the correct Rule based on the L2 address and in this way, it
can use the Dev-ID and the Rule-ID. In the Dev side, only the
Rule ID is needed to identify the correct Rule since the Dev only
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holds Rules that apply to itself. The Rule will be selected by
matching the Fields Descriptions to the packet header as described
below. When the selection of a Rule is done, this Rule is used to
compress the header. The detailed steps for compression Rule
selection are the following:
* The first step is to choose the Fields Descriptions by their
direction, using the direction indicator (DI). A Field
Description that does not correspond to the appropriate DI will
be ignored, if all the fields of the packet do not have a Field
Description with the correct DI the Rule is discarded and SCHC
C/D proceeds to explore the next Rule.
* When the DI has matched, then the next step is to identify the
fields according to Field Position (FP). If the Field Position
does not correspond, the Rule is not used and the SCHC C/D
proceeds to consider the next Rule.
* Once the DI and the FP correspond to the header information,
each field's value of the packet is then compared to the
corresponding Target Value (TV) stored in the Rule for that
specific field using the matching operator (MO).
If all the fields in the packet's header satisfy all the
matching operators (MO) of a Rule (i.e. all MO results are
True), the fields of the header are then compressed according
to the Compression/Decompression Actions (CDAs) and a
compressed header (with possibly a Compression Residue) SHOULD
be obtained. Otherwise, the next Rule is tested.
* If no eligible Rule is found, then the header MUST be sent
without compression, depending on the L2 PDU size, this is one
of the case that MAY require the use of the SCHC F/R process.
o Sending: If an eligible Rule is found, the Rule ID is sent to the
other end followed by the Compression Residue (which could be
empty) and directly followed by the payload. The Compression
Residue is the concatenation of the Compression Residues for each
field according to the CDAs for that rule. The way the Rule ID is
sent depends on the specific LPWAN layer two technology. For
example, it can be either included in a Layer 2 header or sent in
the first byte of the L2 payload. (Cf. Figure 9). This process
will be specified in the LPWAN technology-specific document and is
out of the scope of the present document. On LPWAN technologies
that are byte- oriented, the compressed header concatenated with
the original packet payload is padded to a multiple of 8 bits, if
needed. See Section 8 for details.
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o Decompression: When doing decompression, in the network side the
SCHC C/D needs to find the correct Rule based on the L2 address
and in this way, it can use the Dev-ID and the Rule-ID. In the
Dev side, only the Rule ID is needed to identify the correct Rule
since the Dev only holds Rules that apply to itself.
The receiver identifies the sender through its device-id (e.g.
MAC address, if exists) and selects the appropriate Rule
from the Rule ID. If a source identifier is present in the L2
technology, it is used to select the Rule ID. This Rule describes
the compressed header format and associates the values to the
header fields. The receiver applies the CDA action to reconstruct
the original header fields. The CDA application order can be
different from the order given by the Rule. For instance,
Compute-* SHOULD be applied at the end, after all the other CDAs.
+--- ... --+------- ... -------+------------------+~~~~~~~
| Rule ID |Compression Residue| packet payload |padding
+--- ... --+------- ... -------+------------------+~~~~~~~
(optional)
|----- compressed header ------|
Figure 9: SCHC C/D Packet Format
6.4. Matching operators
Matching Operators (MOs) are functions used by both SCHC C/D
endpoints involved in the header compression/decompression. They are
not typed and can be indifferently applied to integer, string or any
other data type. The result of the operation can either be True or
False. MOs are defined as follows:
o equal: The match result is True if a field value in a packet and
the value in the TV are equal.
o ignore: No check is done between a field value in a packet and a
TV in the Rule. The result of the matching is always true.
o MSB(x): A match is obtained if the most significant x bits of the
field value in the header packet are equal to the TV in the Rule.
The x parameter of the MSB Matching Operator indicates how many
bits are involved in the comparison.
o match-mapping: With match-mapping, the Target Value is a list of
values. Each value of the list is identified by a short ID (or
index). Compression is achieved by sending the index instead of
the original header field value. This operator matches if the
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header field value is equal to one of the values in the target
list.
6.5. Compression Decompression Actions (CDA)
The Compression Decompression Action (CDA) describes the actions
taken during the compression of headers fields, and inversely, the
action taken by the decompressor to restore the original value.
/--------------------+-------------+----------------------------\
| Action | Compression | Decompression |
| | | |
+--------------------+-------------+----------------------------+
|not-sent |elided |use value stored in ctxt |
|value-sent |send |build from received value |
|mapping-sent |send index |value from index on a table |
|LSB |send LSB |TV, received value |
|compute-length |elided |compute length |
|compute-checksum |elided |compute UDP checksum |
|Deviid |elided |build IID from L2 Dev addr |
|Appiid |elided |build IID from L2 App addr |
\--------------------+-------------+----------------------------/
y=size of the transmitted bits
Figure 10: Compression and Decompression Functions
Figure 10 summarizes the basic functions that can be used to compress
and decompress a field. The first column lists the actions name.
The second and third columns outline the reciprocal compression/
decompression behavior for each action.
Compression is done in order that Fields Descriptions appear in the
Rule. The result of each Compression/Decompression Action is
appended to the working Compression Residue in that same order. The
receiver knows the size of each compressed field which can be given
by the rule or MAY be sent with the compressed header.
If the field is identified as being variable in the Field
Description, then the size of the Compression Residue value in bytes
MUST be sent first using the following coding:
o If the size is between 0 and 14 bytes, it is sent as a 4-bits
integer.
o For values between 15 and 254, the first 4 bits sent are set to 1
and the size is sent using 8 bits integer.
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o For higher values of size, the first 12 bits are set to 1 and the
next two bytes contain the size value as a 16 bits integer.
o If a field does not exist in the packet but in the Rule and its FL
is variable, the size zero MUST be used.
6.5.1. not-sent CDA
The not-sent function is generally used when the field value is
specified in the Rule and therefore known by both the Compressor and
the Decompressor. This action is generally used with the "equal" MO.
If MO is "ignore", there is a risk to have a decompressed field value
different from the compressed field.
The compressor does not send any Compression Residue for a field on
which not-sent compression is applied.
The decompressor restores the field value with the Target Value
stored in the matched Rule identified by the received Rule ID.
6.5.2. value-sent CDA
The value-sent action is generally used when the field value is not
known by both Compressor and Decompressor. The value is sent in the
compressed message header. Both Compressor and Decompressor MUST
know the size of the field, either implicitly (the size is known by
both sides) or by explicitly indicating the length in the Compression
Residue, as defined in Section 6.5. This function is generally used
with the "ignore" MO.
6.5.3. mapping-sent CDA
The mapping-sent is used to send a smaller index (the index into the
Target Value list of values) instead of the original value. This
function is used together with the "match-mapping" MO.
On the compressor side, the match-mapping Matching Operator searches
the TV for a match with the header field value and the mapping-sent
CDA appends the corresponding index to the Compression Residue to be
sent. On the decompressor side, the CDA uses the received index to
restore the field value by looking up the list in the TV.
The number of bits sent is the minimal size for coding all the
possible indices.
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6.5.4. LSB CDA
The LSB action is used together with the "MSB(x)" MO to avoid sending
the higher part of the packet field if that part is already known by
the receiving end. A length can be specified in the rule to indicate
how many bits have to be sent. If the length is not specified, the
number of bits sent is the original header field length minus the
length specified in the MSB(x) MO.
The compressor sends the Least Significant Bits (e.g. LSB of the
length field). The decompressor combines the value received with the
Target Value depending on the field type.
If this action needs to be done on a variable length field, the size
of the Compression Residue in bytes MUST be sent as described in
Section 6.5.
6.5.5. DEViid, APPiid CDA
These functions are used to process respectively the Dev and the App
Interface Identifiers (Deviid and Appiid) of the IPv6 addresses.
Appiid CDA is less common since current LPWAN technologies frames
contain a single address, which is the Dev's address.
The IID value MAY be computed from the Device ID present in the Layer
2 header, or from some other stable identifier. The computation is
specific for each LPWAN technology and MAY depend on the Device ID
size.
In the Downlink direction, these Deviid CDA is used to determine the
L2 addresses used by the LPWAN.
6.5.6. Compute-*
Some fields are elided during compression and reconstructed during
decompression. This is the case for length and Checksum, so:
o compute-length: computes the length assigned to this field. This
CDA MAY be used to compute IPv6 length or UDP length.
o compute-checksum: computes a checksum from the information already
received by the SCHC C/D. This field MAY be used to compute UDP
checksum.
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7. Fragmentation
7.1. Overview
In LPWAN technologies, the L2 data unit size typically varies from
tens to hundreds of bytes. The SCHC Fragmentation /Reassembly MAY be
used either because after applying SCHC C/D or when SCHC C/D is not
possible the entire SCHC Packet still exceeds the L2 data unit.
The SCHC F/R functionality defined in this document has been designed
under the assumption that data unit out-of- sequence delivery will
not happen between the entity performing fragmentation and the entity
performing reassembly. This assumption allows reducing the
complexity and overhead of the SCHC F/R mechanism.
To adapt the SCHC F/R to the capabilities of LPWAN technologies is
required to enable optional SCHC Fragment retransmission and to allow
a stepper delivery for the reliability of SCHC Fragments. This
document does not make any decision with regard to which SCHC
Fragment delivery reliability mode will be used over a specific LPWAN
technology. These details will be defined in other technology-
specific documents.
7.2. Fragmentation Tools
This subsection describes the different tools that are used to enable
the SCHC F/R functionality defined in this document, such as fields
in the SCHC F/R header frames (see the related formats in
Section 7.4), and the different parameters supported in the
reliability modes such as timers and parameters.
o Rule ID. The Rule ID is present in the SCHC Fragment header and
in the SCHC ACK header format. The Rule ID in a SCHC fragment
header is used to identify that a SCHC Fragment is being carried,
which SCHC F/R reliability mode is used and which window size is
used. The Rule ID in the SCHC F/R header also allows interleaving
non-fragmented
packets and SCHC Fragments that carry other SCHC Packets. The
Rule ID in an SCHC ACK identifies the message as an SCHC ACK.
o Fragment Compressed Number (FCN). The FCN is included in all SCHC
Fragments. This field can be understood as a truncated,
efficient representation of a larger-sized fragment number, and
does not carry an absolute SCHC Fragment number. There are two
FCN reserved values that are used for controlling the SCHC F/R
process, as described next:
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* The FCN value with all the bits equal to 1 (All-1) denotes the
last SCHC Fragment of a packet. The last window of a packet is
called an All-1 window.
* The FCN value with all the bits equal to 0 (All-0) denotes the
last SCHC Fragment of a window that is not the last one of the
packet. Such a window is called an All-0 window.
The rest of the FCN values are assigned in a sequentially
decreasing order, which has the purpose to avoid possible
ambiguity for the receiver that might arise under certain
conditions. In the SCHC Fragments, this field is an unsigned
integer, with a size of N bits. In the No-ACK mode, it is set to
1 bit (N=1), All-0 is used in all SCHC Fragments and All-1 for the
last one. For the other reliability modes, it is recommended to
use a number of bits (N) equal to or greater than 3.
Nevertheless, the appropriate value of N MUST be defined in the
corresponding technology-specific profile documents. For windows
that are not the last one from a SCHC Fragmented packet, the FCN
for the last SCHC Fragment in such windows is an All-0. This
indicates that the window is finished and communication proceeds
according to the reliability mode in use. The FCN for the last
SCHC Fragment in the last window is an All-1, indicating the last
SCHC Fragment of the SCHC Packet. It is also important to note
that, in the No-ACK mode or when N=1, the last SCHC Fragment of
the packet will carry a FCN equal to 1, while all previous SCHC
Fragments will carry a FCN to 0. For further details see
Section 7.5. The highest FCN in the window, denoted MAX_WIND_FCN,
MUST be a value equal to or smaller than 2^N-2. (Example for N=5,
MAX_WIND_FCN MAY be set to 23, then subsequent FCNs are set
sequentially and in decreasing order, and the FCN will wrap from 0
back to 23).
o Datagram Tag (DTag). The DTag field, if present, is set to the
same value for all SCHC Fragments carrying the same SCHC
packet, and to different values for different SCHC Packets. Using
this field, the sender can interleave fragments from different
SCHC Packets, while the receiver can still tell them apart. In
the SCHC Fragment formats, the size of the DTag field is T bits,
which MAY be set to a value greater than or equal to 0 bits. For
each new SCHC Packet processed by the sender, DTag MUST be
sequentially increased, from 0 to 2^T - 1 wrapping back from 2^T -
1 to 0. In the SCHC ACK format, DTag carries the same value as
the DTag field in the SCHC Fragments for which this SCHC ACK is
intended. When there is no Dtag, there can be only 1 SCHC Packet
in transist. And only after all its fragments have been
transmitted another SCHC Packet could be sent. The length of
DTag, denoted T is not given in this document because is technolgy
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dependant, and will be defined in the corresponding technology-
documents. DTag is based on the number of simultaneous packets
supported.
o W (window): W is a 1-bit field. This field carries the same value
for all SCHC Fragments of a window, and it is complemented for the
next window. The initial value for this field is 0. In the SCHC
ACK format, this field also has a size of 1 bit. In all SCHC
ACKs, the W bit carries the same value as the W bit carried by the
SCHC Fragments whose reception is being positively or negatively
acknowledged by the SCHC ACK.
o Message Integrity Check (MIC). This field is computed by the
sender over the complete SCHC Packet and before SCHC
fragmentation. The MIC allows the receiver to check errors in the
reassembled packet, while it also enables compressing the UDP
checksum by use of SCHC compression. The CRC32 as 0xEDB88320
(i.e. the reverse representation of the polynomial used e.g. in
the Ethernet standard [RFC3385]) is recommended as the default
algorithm for computing the MIC. Nevertheless, other algorithms
MAY be required and are defined in the technology-specific
documents as well as the length in bits of the MIC used.
o C (MIC checked): C is a 1-bit field. This field is used in the
SCHC ACK packets to report the outcome of the MIC check, i.e.
whether the reassembled packet was correctly received or not. A
value of 1 represents a positive MIC check at the receiver side
(i.e. the MIC computed by the receiver matches the received MIC).
o Retransmission Timer. A SCHC Fragment sender uses it after the
transmission of a window to detect a transmission error of the
SCHC ACK corresponding to this window. Depending on the
reliability mode, it will lead to a request an SCHC ACK
retransmission (in ACK-Always mode) or it will trigger the
transmission of the next window (in ACK-on-Error mode). The
duration of this timer is not defined in this document and MUST be
defined in the corresponding technology documents.
o Inactivity Timer. A SCHC Fragment receiver uses it to take action
when there is a problem in the transmission of SCHC fragments.
Such a problem could be detected by the receiver not getting a
single SCHC Fragment during a given period of time or not getting
a given number of packets in a given period of time. When this
happens, an Abort message will be sent (see related text later in
this section). Initially, and each time a SCHC Fragment is
received, the timer is reinitialized. The duration of this timer
is not defined in this document and MUST be defined in the
specific technology document.
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o Attempts. This counter counts the requests for a missing SCHC
ACK. When it reaches the value MAX_ACK_REQUESTS, the sender
assume there are recurrent SCHC Fragment transmission errors and
determines that an Abort is needed. The default value offered
MAX_ACK_REQUESTS is not stated in this document, and it is
expected to be defined in the specific technology document. The
Attempts counter is defined per window. It is initialized each
time a new window is used.
o Bitmap. The Bitmap is a sequence of bits carried in an SCHC ACK.
Each bit in the Bitmap corresponds to a SCHC fragment of the
current window, and provides feedback on whether the SCHC Fragment
has been received or not. The right-most position on the Bitmap
reports if the All-0 or All-1 fragment has been received or not.
Feedback on the SCHC fragment with the highest FCN value is
provided by the bit in the left-most position of the Bitmap. In
the Bitmap, a bit set to 1 indicates that the SCHC Fragment of FCN
corresponding to that bit position has been correctly sent and
received. The text above describes the internal representation of
the Bitmap. When inserted in the SCHC ACK for transmission from
the receiver to the sender, the Bitmap MAY be truncated for
energy/bandwidth optimisation, see more details in
Section 7.4.3.1.
o Abort. On expiration of the Inactivity timer, or when Attempts
reached MAX_ACK_REQUESTS or upon an occurrence of some other
error, the sender or the receiver MUST use the Abort. When the
receiver needs to abort the on-going SCHC Fragmented packet
transmission, it sends the Receiver-Abort format. When the sender
needs to abort the transmission, it sends the Sender-Abort format.
None of the Abort are acknowledged.
o Padding (P). If it is needed, the number of bits used for padding
is not defined and depends on the size of the Rule ID, DTag and
FCN fields, and on the L2 payload size (see Section 8). Some SCHC
ACKs are byte-aligned and do not need padding (see
Section 7.4.3.1).
7.3. Reliability modes
This specification defines three reliability modes: No-ACK, ACK-
Always and ACK-on-Error. ACK-Always and ACK-on-Error operate on
windows of SCHC Fragments. A window of SCHC Fragments is a subset of
the full set of SCHC Fragments needed to carry a packet or an SCHC
Packet.
o No-ACK. No-ACK is the simplest SCHC Fragment reliability mode.
The receiver does not generate overhead in the form of
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acknowledgments (ACKs). However, this mode does not enhance
reliability beyond that offered by the underlying LPWAN
technology. In the No-ACK mode, the receiver MUST NOT issue SCHC
ACKs. See further details in Section 7.5.1.
o ACK-Always. The ACK-Always mode provides flow control using a
window scheme. This mode is also able to handle long bursts of
lost SCHC Fragments since detection of such events can be done
before the end of the SCHC Packet transmission as long as the
window size is short enough. However, such benefit comes at the
expense of SCHC ACK use. In ACK-Always the receiver sends an SCHC
ACK after a window of SCHC Fragments has been received, where a
window of SCHC Fragments is a subset of the whole number of SCHC
Fragments needed to carry a complete SCHC Packet. The SCHC ACK is
used to inform the sender if a SCHC fragment in the actual window
has been lost or well received. Upon an SCHC ACK reception, the
sender retransmits the lost SCHC Fragments. When an SCHC ACK is
lost and the sender has not received it before the expiration of
the Retransmission Timer, the sender uses an SCHC ACK request by
sending the All-0 empty SCHC Fragment when it is not the last
windown and the ALL-1 empty Fragment when it is the last window.
The maximum number of SCHC ACK requests is MAX_ACK_REQUESTS. If
the MAX_ACK_REQUEST is reached the transmission needs to be
Aborted. See further details in Section 7.5.2.
o ACK-on-Error. The ACK-on-Error mode is suitable for links
offering relatively low L2 data unit loss probability. In this
mode, the SCHC Fragment receiver reduces the number of SCHC ACKs
transmitted, which MAY be especially beneficial in asymmetric
scenarios. Because the SCHC Fragments use the uplink of the
underlying LPWAN technology, which has higher capacity than
downlink. The receiver transmits an SCHC ACK only after the
complete window transmission and if at least one SCHC Fragment of
this window has been lost. An exception to this behavior is in
the last window, where the receiver MUST transmit an SCHC ACK,
including the C bit set based on the MIC checked result, even if
all the SCHC Fragments of the last window have been correctly
received. The SCHC ACK gives the state of all the SCHC Fragments
(received or lost). Upon an SCHC ACK reception, the sender
retransmits the lost SCHC Fragments. If an SCHC ACK is not
transmitted back by the receiver at the end of a window, the
sender assumes that all SCHC Fragments have been correctly
received. When the SCHC ACK is lost, the sender assumes that all
SCHC Fragments covered by the lost SCHC ACK have been successfully
delivered, so the sender continues transmitting the next window of
SCHC Fragments. If the next SCHC Fragments received belong to the
next window, the receiver will abort the on-going fragmented
packet transmission. See further details in Section 7.5.3.
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The same reliability mode MUST be used for all SCHC Fragments of an
SCHC Packet. The decision on which reliability mode will be used and
whether the same reliability mode applies to all SCHC Packets is an
implementation problem and is out of the scope of this document.
Note that the reliability mode choice is not necessarily tied to a
particular characteristic of the underlying L2 LPWAN technology, e.g.
the No-ACK mode MAY be used on top of an L2 LPWAN technology with
symmetric characteristics for uplink and downlink. This document
does not make any decision as to which SCHC Fragment reliability
mode(s) are supported by a specific LPWAN technology.
Examples of the different reliability modes described are provided in
Appendix B.
7.4. Fragmentation Formats
This section defines the SCHC Fragment format, the All-0 and All-1
formats, the SCHC ACK format and the Abort formats.
7.4.1. Fragment format
A SCHC Fragment comprises a SCHC Fragment header, a SCHC Fragment
payload and padding bits (if needed). A SCHC Fragment conforms to
the general format shown in Figure 11. The SCHC Fragment payload
carries a subset of SCHC Packet. A SCHC Fragment is the payload of
the L2 protocol data unit (PDU). Padding MAY be added in SCHC
Fragments and in SCHC ACKs if necessary, therefore a padding field is
optional (this is explicitly indicated in Figure 11 for the sake of
illustration clarity.
+-----------------+-----------------------+~~~~~~~~~~~~~~~
| Fragment Header | Fragment payload | padding (opt.)
+-----------------+-----------------------+~~~~~~~~~~~~~~~
Figure 11: Fragment general format. Presence of a padding field is
optional
In ACK-Always or ACK-on-Error, SCHC Fragments except the last one
SHALL conform the detailed format defined in Figure 12. The total
size of the fragment header is not byte aligned.
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|---Fragmentation Header----|
|-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+--------...-------+
| Rule ID | DTag |W| FCN | Fragment payload |
+-- ... --+- ... -+-+- ... -+--------...-------+
Figure 12: Fragment Detailed Format for Fragments except the Last
One, ACK-Always and ACK-on-Error
In the No-ACK mode, SCHC Fragments except the last one SHALL conform
to the detailed format defined in Figure 13. The total size of the
fragment header is not byte aligned.
|---Fragmentation Header---|
|-- T --|-- N --|
+-- ... --+- ... -+- ... -+--------...-------+
| Rule ID | DTag | FCN | Fragment payload |
+-- ... --+- ... -+- ... -+--------...-------+
Figure 13: Fragment Detailed Format for Fragments except the Last
One, No-ACK mode
In all these cases, the total size of the fragment header is not byte
aligned.
7.4.2. All-1 and All-0 formats
The All-0 format is used for sending the last SCHC Fragment of a
window that is not the last window of the packet.
|-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+--- ... ---+
| Rule ID | DTag |W| 0..0 | payload |
+-- ... --+- ... -+-+- ... -+--- ... ---+
Figure 14: All-0 fragment detailed format
The All-0 empty fragment format is used by a sender to request the
retransmission of an SCHC ACK by the receiver. It is only used in
ACK-Always mode.
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|-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+
| Rule ID | DTag |W| 0..0 | (no payload)
+-- ... --+- ... -+-+- ... -+
Figure 15: All-0 empty fragment detailed format
In the No-ACK mode, the last SCHC Fragment of an IPv6 datagram SHALL
contain a SCHC Fragment header that conforms to the detaield format
shown in Figure 16.
|-- T --|-N=1-|
+---- ... ---+- ... -+-----+---- ... ----+---...---+
| Rule ID | DTag | 1 | MIC | payload |
+---- ... ---+- ... -+-----+---- ... ----+---...---+
Figure 16: All-1 Fragment Detailed Format for the Last Fragment, No-
ACK mode
In any of the Window modes, the last fragment of an IPv6 datagram
SHALL contain a SCHC Fragment header that conforms to the detailed
format shown in Figure 17. The total size of the SCHC Fragment
header in this format is not byte aligned.
|-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+---- ... ----+---...---+
| Rule ID | DTag |W| 11..1 | MIC | payload |
+-- ... --+- ... -+-+- ... -+---- ... ----+---...---+
(FCN)
Figure 17: All-1 Fragment Detailed Format for the Last Fragment, ACK-
Always or ACK-on-Error
In either ACK-Always or ACK-on-Error, in order to request a
retransmission of the SCHC ACK for the All-1 window, the fragment
sender uses the format shown in Figure 18. The total size of the
SCHC Fragment header in not byte aligned.
|-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+---- ... ----+
| Rule ID | DTag |W| 1..1 | MIC | (no payload)
+-- ... --+- ... -+-+- ... -+---- ... ----+
Figure 18: All-1 for Retries format, also called All-1 empty
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The values for Fragmentation Header, N, T and the length of MIC are
not specified in this document, and SHOULD be determined in other
documents (e.g. technology-specific profile documents).
7.4.3. SCHC ACK format
The format of an SCHC ACK that acknowledges a window that is not the
last one (denoted as All-0 window) is shown in Figure 19.
|-- T --|1|
+---- ... --+- ... -+-+---- ... -----+
| Rule ID | DTag |W|encoded Bitmap| (no payload)
+---- ... --+- ... -+-+---- ... -----+
Figure 19: ACK format for All-0 windows
To acknowledge the last window of a packet (denoted as All-1 window),
a C bit (i.e. MIC checked) following the W bit is set to 1 to
indicate that the MIC check computed by the receiver matches the MIC
present in the All-1 fragment. If the MIC check fails, the C bit is
set to 0 and the Bitmap for the All-1 window follows.
|-- T --|1|1|
+---- ... --+- ... -+-+-+
| Rule ID | DTag |W|1| (MIC correct)
+---- ... --+- ... -+-+-+
+---- ... --+- ... -+-+-+----- ... -----+
| Rule ID | DTag |W|0|encoded Bitmap |(MIC Incorrect)
+---- ... --+- ... -+-+-+----- ... -----+
C
Figure 20: Format of an SCHC ACK for All-1 windows
7.4.3.1. Bitmap Encoding
The Bitmap is transmitted by a receiver as part of the SCHC ACK
format. An SCHC ACK message MAY include padding at the end to align
its number of transmitted bits to a multiple of 8 bits.
Note that the SCHC ACK sent in response to an All-1 fragment includes
the C bit. Therefore, the window size and thus the encoded Bitmap
size need to be determined taking into account the available space in
the layer two frame payload, where there will be 1 bit less for an
SCHC ACK sent in response to an All-1 fragment than in other SCHC
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ACKs. Note that the maximum number of SCHC Fragments of the last
window is one unit smaller than that of the previous windows.
When the receiver transmits an encoded Bitmap with a SCHC Fragment
that has not been sent during the transmission, the sender will Abort
the transmission.
|---- Bitmap bits ----|
| Rule ID | DTag |W|1|0|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|
|--- byte boundary ----| 1 byte next | 1 byte next |
Figure 21: A non-encoded Bitmap
In order to reduce the resulting frame size, the encoded Bitmap is
shortened by applying the following algorithm: all the right-most
contiguous bytes in the encoded Bitmap that have all their bits set
to 1 MUST NOT be transmitted. Because the SCHC Fragment sender knows
the actual Bitmap size, it can reconstruct the original Bitmap with
the trailing 1 bit optimized away. In the example shown in
Figure 22, the last 2 bytes of the Bitmap shown in Figure 21 comprise
bits that are all set to 1, therefore they are not sent.
|-- T --|1|
+---- ... --+- ... -+-+-+-+
| Rule ID | DTag |W|1|0|
+---- ... --+- ... -+-+-+-+
|---- byte boundary -----|
Figure 22: Optimized Bitmap format
Figure 23 shows an example of an SCHC ACK with FCN ranging from 6
down to 0, where the Bitmap indicates that the second and the fifth
SCHC Fragments have not been correctly received.
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6 5 4 3 2 1 0 (*)
|-- T --|1|
+---------+-------+-+-+-+-+-+-+-+-----+
| Rule ID | DTag |W|1|0|1|1|0|1|all-0| Bitmap(before tx)
+---------+-------+-+-+-+-+-+-+-+-----+
|<-- byte boundary ->|<---- 1 byte---->|
(*)=(FCN values)
+---------+------+-+-+-+-+-+-+-+-----+~~
| Rule ID | DTag |W|1|0|1|1|0|1|all-0|Padding(opt.) encoded Bitmap
+---------+------+-+-+-+-+-+-+-+-----+~~
|<-- byte boundary ->|<---- 1 byte---->|
Figure 23: Example of a Bitmap before transmission, and the
transmitted one, in any window except the last one
Figure 24 shows an example of an SCHC ACK with FCN ranging from 6
down to 0, where the Bitmap indicates that the MIC check has failed
but there are no missing SCHC Fragments.
|-Fragmentation Header-|6 5 4 3 2 1 7 (*)
|-- T --|1|
| Rule ID | DTag |W|0|1|1|1|1|1|1|1|padding| Bitmap (before tx)
|---- byte boundary -----| 1 byte next |
C
+---- ... --+-... -+-+-+-+
| Rule ID | DTag |W|0|1| encoded Bitmap
+---- ... --+-... -+-+-+-+
|---- byte boundary -----|
(*) = (FCN values indicating the order)
Figure 24: Example of the Bitmap in ACK-Always or ACK-on-Error for
the last window, for N=3)
7.4.4. Abort formats
Abort are coded as exceptions to the previous coding, a specific
format is defined for each direction. When a SCHC Fragment sender
needs to abort the transmission, it sends the Sender-Abort format
Figure 25, that is an All-1 fragment with no MIC or payload. In
regular cases All-1 fragment contains at least a MIC value. This
absence of the MIC value indicates an Abort.
When a SCHC Fragment receiver needs to abort the on-going SCHC
Fragmented packet transmission, it transmits the Receiver- Abort
format Figure 26, creating an exception in the encoded Bitmap coding.
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Encoded Bitmap avoid sending the rigth most bits of the Bitmap set to
1. Abort is coded as an SCHC ACK message with a Bitmap set to 1
until the byte boundary, followed by an extra 0xFF byte. Such
message never occurs in a regular acknowledgement and is view as an
abort.
None of these messages are not acknowledged nor retransmitted.
The sender uses the Sender-Abort when the MAX_ACK_REQUEST is reached.
The receiver uses the Receiver-Abort when the Inactivity timer
expires, or in the ACK-on-Error mode, SCHC ACK is lost and the sender
transmits SCHC Fragments of a new window. Some other cases for Abort
are explained in the Section 7.5 or Appendix C.
|-- Fragmentation Header ---|--- 1 byte ----|
+--- ... ---+- ... -+-+-...-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |W| FCN | FF | (no MIC & no payload)
+--- ... ---+- ... -+-+-...-+-+-+-+-+-+-+-+-+
Figure 25: Sender-Abort format. All FCN fields in this format are
set to 1
|----- byte boundary ------|---- 1 byte ---|
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |W| 1..1| FF |
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: Receiver-Abort format
7.5. Baseline mechanism
If after applying SCHC header compression (or when SCHC header
compression is not possible) the SCHC Packet does not fit within the
payload of a single L2 data unit, the SCHC Packet SHALL be broken
into SCHC Fragments and the fragments SHALL be sent to the fragment
receiver. The fragment receiver needs to identify all the SCHC
Fragments that belong to a given SCHC Packet. To this end, the
receiver SHALL use:
o The sender's L2 source address (if present),
o The destination's L2 address (if present),
o Rule ID,
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o DTag (if present).
Then, the fragment receiver MAY determine the SCHC Fragment
reliability mode that is used for this SCHC Fragment based on the
Rule ID in that fragment.
After a SCHC Fragment reception, the receiver starts constructing the
SCHC Packet. It uses the FCN and the arrival order of each SCHC
Fragment to determine the location of the individual fragments within
the SCHC Packet. For example, the receiver MAY place the fragment
payload within a payload datagram reassembly buffer at the location
determined from the FCN, the arrival order of the SCHC Fragments, and
the fragment payload sizes. In Window mode, the fragment receiver
also uses the W bit in the received SCHC Fragments. Note that the
size of the original, unfragmented packet cannot be determined from
fragmentation headers.
Fragmentation functionality uses the FCN value to transmit the SCHC
Fragments. It has a length of N bits where the All-1 and All-0 FCN
values are used to control the fragmentation transmission. The rest
of the FCN numbers MUST be assigned sequentially in a decreasing
order, the first FCN of a window is RECOMMENDED to be MAX_WIND_FCN,
i.e. the highest possible FCN value depending on the FCN number of
bits.
In all modes, the last SCHC Fragment of a packet MUST contain a MIC
which is used to check if there are errors or missing SCHC Fragments
and MUST use the corresponding All-1 fragment format. Note that a
SCHC Fragment with an All-0 format is considered the last SCHC
Fragment of the current window.
If the receiver receives the last fragment of a datagram (All-1), it
checks for the integrity of the reassembled datagram, based on the
MIC received. In No-ACK, if the integrity check indicates that the
reassembled datagram does not match the original datagram (prior to
fragmentation), the reassembled datagram MUST be discarded. In
Window mode, a MIC check is also performed by the fragment receiver
after reception of each subsequent SCHC Fragment retransmitted after
the first MIC check.
There are three reliability modes: No-ACK, ACK-Always and ACK-on-
Error. In ACK-Always and ACK-on-Error, a jumping window protocol
uses two windows alternatively, identified as 0 and 1. A SCHC
Fragment with all FCN bits set to 0 (i.e. an All-0 fragment)
indicates that the window is over (i.e. the SCHC Fragment is the last
one of the window) and allows to switch from one window to the next
one. The All-1 FCN in a SCHC Fragment indicates that it is the last
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fragment of the packet being transmitted and therefore there will not
be another window for this packet.
7.5.1. No-ACK
In the No-ACK mode, there is no feedback communication from the
fragment receiver. The sender will send all the SCHC fragments of a
packet without any possibility of knowing if errors or losses have
occurred. As, in this mode, there is no need to identify specific
SCHC Fragments, a one-bit FCN MAY be used. Consequently, the FCN
All-0 value is used in all SCHC fragments except the last one, which
carries an All-1 FCN and the MIC. The receiver will wait for SCHC
Fragments and will set the Inactivity timer. The receiver will use
the MIC contained in the last SCHC Fragment to check for errors.
When the Inactivity Timer expires or if the MIC check indicates that
the reassembled packet does not match the original one, the receiver
will release all resources allocated to reassembling this packet.
The initial value of the Inactivity Timer will be determined based on
the characteristics of the underlying LPWAN technology and will be
defined in other documents (e.g. technology-specific profile
documents).
7.5.2. ACK-Always
In ACK-Always, the sender transmits SCHC Fragments by using the two-
jumping-windows procedure. A delay between each SCHC fragment can be
added to respect local regulations or other constraints imposed by
the applications. Each time a SCHC fragment is sent, the FCN is
decreased by one. When the FCN reaches value 0 and there are more
SCHC Fragments to be sent after, the sender transmits the last SCHC
Fragment of this window using the All-0 fragment format, it starts
the transmitted is the last SCHC Fragment of the SCHC Packet, the
sender uses the All-1 fragment format, which includes a MIC. The
sender sets the Retransmission Timer and waits for the SCHC ACK to
know if transmission errors have occured.
The Retransmission Timer is dimensioned based on the LPWAN technology
in use. When the Retransmission Timer expires, the sender sends an
All-0 empty (resp. All-1 empty) fragment to request again the SCHC
ACK for the window that ended with the All-0 (resp. All-1) fragment
just sent. The window number is not changed.
After receiving an All-0 or All-1 fragment, the receiver sends an
SCHC ACK with an encoded Bitmap reporting whether any SCHC fragments
have been lost or not. When the sender receives an SCHC ACK, it
checks the W bit carried by the SCHC ACK. Any SCHC ACK carrying an
unexpected W bit value is discarded. If the W bit value of the
received SCHC ACK is correct, the sender analyzes the rest of the
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SCHC ACK message, such as the encoded Bitmap and the MIC. If all the
SCHC Fragments sent for this window have been well received, and if
at least one more SCHC Fragment needs to be sent, the sender advances
its sending window to the next window value and sends the next SCHC
Fragments. If no more SCHC Fragments have to be sent, then the SCHC
fragmented packet transmission is finished.
However, if one or more SCHC Fragments have not been received as per
the SCHC ACK (i.e. the corresponding bits are not set in the encoded
Bitmap) then the sender resends the missing SCHC Fragments. When all
missing SCHC Fragments have been retransmitted, the sender starts the
Retransmission Timer, even if an All-0 or an All-1 has not been sent
as part of this retransmission and waits for an SCHC ACK. Upon
receipt of the SCHC ACK, if one or more SCHC Fragments have not yet
been received, the counter Attempts is increased and the sender
resends the missing SCHC Fragments again. When Attempts reaches
MAX_ACK_REQUESTS, the sender aborts the on-going SCHC Fragmented
packet transmission by sending an Abort message and releases any
resources for transmission of the packet. The sender also aborts an
on-going SCHC Fragmented packet transmission when a failed MIC check
is reported by the receiver or when a SCHC Fragment that has not been
sent is reported in the encoded Bitmap.
On the other hand, at the beginning, the receiver side expects to
receive window 0. Any SCHC Fragment received but not belonging to
the current window is discarded. All SCHC Fragments belonging to the
correct window are accepted, and the actual SCHC Fragment number
managed by the receiver is computed based on the FCN value. The
receiver prepares the encoded Bitmap to report the correctly received
and the missing SCHC Fragments for the current window. After each
SCHC Fragment is received the receiver initializes the Inactivity
timer, if the Inactivity Timer expires the transmission is aborted.
When an All-0 fragment is received, it indicates that all the SCHC
Fragments have been sent in the current window. Since the sender is
not obliged to always send a full window, some SCHC Fragment number
not set in the receiver memory SHOULD not correspond to losses. The
receiver sends the corresponding SCHC ACK, the Inactivity Timer is
set and the transmission of the next window by the sender can start.
If an All-0 fragment has been received and all SCHC Fragments of the
current window have also been received, the receiver then expects a
new Window and waits for the next SCHC Fragment. Upon receipt of a
SCHC Fragment, if the window value has not changed, the received SCHC
Fragments are part of a retransmission. A receiver that has already
received a SCHC Fragment SHOULD discard it, otherwise, it updates the
encoded Bitmap. If all the bits of the encoded Bitmap are set to
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one, the receiver MUST send an SCHC ACK without waiting for an All-0
fragment and the Inactivity Timer is initialized.
On the other hand, if the window value of the next received SCHC
Fragment is set to the next expected window value, this means that
the sender has received a correct encoded Bitmap reporting that all
SCHC Fragments have been received. The receiver then updates the
value of the next expected window.
When an All-1 fragment is received, it indicates that the last SCHC
Fragment of the packet has been sent. Since the last window is not
always full, the MIC will be used to detect if all SCHC Fragments of
the packet have been received. A correct MIC indicates the end of
the transmission but the receiver MUST stay alive for an Inactivity
Timer period to answer to any empty All-1 fragments the sender MAY
send if SCHC ACKs sent by the receiver are lost. If the MIC is
incorrect, some SCHC Fragments have been lost. The receiver sends
the SCHC ACK regardless of successful SCHC Fragmented packet
reception or not, the Inactitivity Timer is set. In case of an
incorrect MIC, the receiver waits for SCHC Fragments belonging to the
same window. After MAX_ACK_REQUESTS, the receiver will abort the on-
going SCHC Fragmented packet transmission by transmitting a the
Receiver-Abort format. The receiver also aborts upon Inactivity
Timer expiration.
7.5.3. ACK-on-Error
The senders behavior for ACK-on-Error and ACK-Always are similar.
The main difference is that in ACK-on-Error the SCHC ACK with the
encoded Bitmap is not sent at the end of each window but only when at
least one SCHC Fragment of the current window has been lost. Excepts
for the last window where an SCHC ACK MUST be sent to finish the
transmission.
In ACK-on-Error, the Retransmission Timer expiration will be
considered as a positive acknowledgment. This timer is set after
sending an All-0 or an All-1 fragment. When the All-1 fragment has
been sent, then the on-going SCHC F/R process is finished and the
sender waits for the last SCHC ACK. If the Retransmission Timer
expires while waiting for the SCHC ACK for the last window, an All-1
empty MUST be sent to request the last SCHC ACK by the sender to
complete the SCHC Fragmented packet transmission. When it expires
the sender continue sending SCHC Fragments of the next window.
If the sender receives an SCHC ACK, it checks the window value. SCHC
ACKs with an unexpected window number are discarded. If the window
number on the received encoded Bitmap is correct, the sender verifies
if the receiver has received all SCHC fragments of the current
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window. When at least one SCHC Fragment has been lost, the counter
Attempts is increased by one and the sender resends the missing SCHC
Fragments again. When Attempts reaches MAX_ACK_REQUESTS, the sender
sends an Abort message and releases all resources for the on-going
SCHC Fragmented packet transmission. When the retransmission of the
missing SCHC Fragments is finished, the sender starts listening for
an SCHC ACK (even if an All-0 or an All-1 has not been sent during
the retransmission) and initializes the Retransmission Timer. After
sending an All-1 fragment, the sender listens for an SCHC ACK,
initializes Attempts, and starts the Retransmission Timer. If the
Retransmission Timer expires, Attempts is increased by one and an
empty All-1 fragment is sent to request the SCHC ACK for the last
window. If Attempts reaches MAX_ACK_REQUESTS, the sender aborts the
on-going SCHC Fragmented packet transmission by transmitting the
Sender-Abort fragment.
Unlike the sender, the receiver for ACK-on-Error has a larger amount
of differences compared with ACK-Always. First, an SCHC ACK is not
sent unless there is a lost SCHC Fragment or an unexpected behavior.
With the exception of the last window, where an SCHC ACK is always
sent regardless of SCHC Fragment losses or not. The receiver starts
by expecting SCHC Fragments from window 0 and maintains the
information regarding which SCHC Fragments it receives. After
receiving an SCHC Fragment, the Inactivity Timer is set. If no
further SCHC Fragment are received and the Inactivity Timer expires,
the SCHC Fragment receiver aborts the on-going SCHC Fragmented packet
transmission by transmitting the Receiver-Abort data unit.
Any SCHC Fragment not belonging to the current window is discarded.
The actual SCHC Fragment number is computed based on the FCN value.
When an All-0 fragment is received and all SCHC Fragments have been
received, the receiver updates the expected window value and expects
a new window and waits for the next SCHC Fragment.
If the window value of the next SCHC Fragment has not changed, the
received SCHC Fragment is a retransmission. A receiver that has
already received an SCHC Fragment discard it. If all SCHC Fragments
of a window (that is not the last one) have been received, the
receiver does not send an SCHC ACK. While the receiver waits for the
next window and if the window value is set to the next value, and if
an All-1 fragment with the next value window arrived the receiver
knows that the last SCHC Fragment of the packet has been sent. Since
the last window is not always full, the MIC will be used to detect if
all SCHC Fragments of the window have been received. A correct MIC
check indicates the end of the SCHC Fragmented packet transmission.
An ACK is sent by the SCHC Fragment receiver. In case of an
incorrect MIC, the receiver waits for SCHC Fragments belonging to the
same window or the expiration of the Inactivity Timer. The latter
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will lead the receiver to abort the on-going SCHC fragmented packet
transmission.
If after receiving an All-0 fragment the receiver missed some SCHC
Fragments, the receiver uses an SCHC ACK with the encoded Bitmap to
ask the retransmission of the missing fragments and expect to receive
SCHC Fragments with the actual window. While waiting the
retransmission an All-0 empty fragment is received, the receiver
sends again the SCHC ACK with the encoded Bitmap, if the SCHC
Fragments received belongs to another window or an All-1 fragment is
received, the transmission is aborted by sending a Receiver-Abort
fragment. Once it has received all the missing fragments it waits
for the next window fragments.
7.6. Supporting multiple window sizes
For ACK-Always or ACK-on-Error, implementers MAY opt to support a
single window size or multiple window sizes. The latter, when
feasible, may provide performance optimizations. For example, a
large window size SHOULD be used for packets that need to be carried
by a large number of SCHC Fragments. However, when the number of
SCHC Fragments required to carry a packet is low, a smaller window
size, and thus a shorter Bitmap, MAY be sufficient to provide
feedback on all SCHC Fragments. If multiple window sizes are
supported, the Rule ID MAY be used to signal the window size in use
for a specific packet transmission.
Note that the same window size MUST be used for the transmission of
all SCHC Fragments that belong to the same SCHC Packet.
7.7. Downlink SCHC Fragment transmission
In some LPWAN technologies, as part of energy-saving techniques,
downlink transmission is only possible immediately after an uplink
transmission. In order to avoid potentially high delay in the
downlink transmission of a SCHC Fragmented datagram, the SCHC
Fragment receiver MAY perform an uplink transmission as soon as
possible after reception of a SCHC Fragment that is not the last one.
Such uplink transmission MAY be triggered by the L2 (e.g. an L2 ACK
sent in response to a SCHC Fragment encapsulated in a L2 frame that
requires an L2 ACK) or it MAY be triggered from an upper layer.
For downlink transmission of a SCHC Fragmented packet in ACK-Always
mode, the SCHC Fragment receiver MAY support timer-based SCHC ACK
retransmission. In this mechanism, the SCHC Fragment receiver
initializes and starts a timer (the Inactivity Timer is used) after
the transmission of an SCHC ACK, except when the SCHC ACK is sent in
response to the last SCHC Fragment of a packet (All-1 fragment). In
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the latter case, the SCHC Fragment receiver does not start a timer
after transmission of the SCHC ACK.
If, after transmission of an SCHC ACK that is not an All-1 fragment,
and before expiration of the corresponding Inactivity timer, the SCHC
Fragment receiver receives a SCHC Fragment that belongs to the
current window (e.g. a missing SCHC Fragment from the current window)
or to the next window, the Inactivity timer for the SCHC ACK is
stopped. However, if the Inactivity timer expires, the SCHC ACK is
resent and the Inactivity timer is reinitialized and restarted.
The default initial value for the Inactivity timer, as well as the
maximum number of retries for a specific SCHC ACK, denoted
MAX_ACK_RETRIES, are not defined in this document, and need to be
defined in other documents (e.g. technology-specific profiles). The
initial value of the Inactivity timer is expected to be greater than
that of the Retransmission timer, in order to make sure that a
(buffered) SCHC Fragment to be retransmitted can find an opportunity
for that transmission.
When the SCHC Fragment sender transmits the All-1 fragment, it starts
its Retransmission Timer with a large timeout value (e.g. several
times that of the initial Inactivity timer). If an SCHC ACK is
received before expiration of this timer, the SCHC Fragment sender
retransmits any lost SCHC Fragments 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 SCHC Fragmented 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 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 fragment is long enough
to allow several SCHC ACK retries if the All-1 fragment has not been
received by the SCHC Fragment receiver, and it also assumes that it
is unlikely that several ACKs become all lost).
8. Padding management
Default padding is defined for L2 frame with a variable length of
bytes. Padding is done twice, after compression and in the all-1
fragmentation.
In compression, the Compressed Header is generally not a multiple of
bytes in size, but the payload following the Compressed Header is
always a multiple of 8 bits (see Figure 4). If needed, padding bits
can be added after the payload to reach the next byte boundary.
Since the Compressed Header (through the Rule ID and the Compression
Residue) tells its length and the payload is always a multiple of 8
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bits, the receiver can without ambiguity remove the padding bits,
which never exceed 7 bits.
SCHC F/R works on a byte aligned (i.e. padded SCHC Packet).
Fragmentation header may not be aligned on byte boundary, but each
fragment except the last one (All-1 fragment) must sent the maximum
bits as possible. Only the last fragment need to introduce padding
to reach the next boundary limit. Since the SCHC is known to be a
multiple of 8 bits, the receiver can remove the extra bit to reach
this limit.
Default padding mechanism do not need to send the padding length and
can lead to a maximum of 14 bits of padding.
The padding is not mandatory and is optional to the technology-
specific document to give a different solution. In this docuement
there are some inputs on how to manage the padding.
9. SCHC Compression for IPv6 and UDP headers
This section lists the different IPv6 and UDP header fields and how
they can be compressed.
9.1. IPv6 version field
This field always holds the same value. Therefore, in the rule, TV
is set to 6, MO to "equal" and CDA to "not-sent".
9.2. 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, two possibilities can be considered depending on the
variability of the value:
o 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".
o If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x) and
CDA to LSB(y).
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9.3. Flow label field
If the Flow Label field does not vary and is known by both sides, the
Field Descriptor in the rule SHOULD contain a TV with this well-known
value, an "equal" MO and a "not-sent" CDA.
Otherwise, two possibilities can be considered:
o 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".
o If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x) and
CDA to LSB(y).
9.4. Payload Length field
This field can be elided for the transmission on the LPWAN network.
The SCHC C/D recomputes the original payload length value. In the
Field Descriptor, TV is not set, MO is set to "ignore" and CDA is
"compute-IPv6-length".
If the payload length needs to be sent and does not need to be coded
in 16 bits, the TV can be set to 0x0000, the MO set to MSB(16-s)
where 's' is the number of bits to code the maximum length, and CDA
is set to LSB(s).
9.5. 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.
9.6. 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 of Internet routing and MAY change
more frequently. One neat way of processing this field is to use the
Direction Indicator (DI) to distinguish both directions:
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o in the Uplink, elide the field: the TV in the Field Descriptor is
set to the known constant value, the MO is set to "equal" and the
CDA is set to "not-sent".
o in the Downlink, send the value: TV is not set, MO is set to
"ignore" and CDA is set to "value-sent".
9.7. 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 frame
(source or destination).
9.7.1. IPv6 source and destination prefixes
Both ends MUST be synchronized with the appropriate prefixes. For a
specific flow, the source and destination prefixes can be unique and
stored in the context. It can be either a link-local prefix or a
global prefix. In that case, the TV for the source and destination
prefixes contain the values, the MO is set to "equal" and the CDA is
set to "not-sent".
If the rule is intended to compress packets with different prefix
values, match-mapping SHOULD be used. The different prefixes are
listed in the TV, the MO is set to "match-mapping" and the CDA is set
to "mapping-sent". See Figure 28
Otherwise, the TV contains the prefix, the MO is set to "equal" and
the CDA is set to "value-sent".
9.7.2. IPv6 source and destination IID
If the DEV or APP IID are based on an LPWAN address, then the IID can
be reconstructed with information coming from the LPWAN header. In
that case, the TV is not set, the MO is set to "ignore" and the CDA
is set to "DEViid" or "APPiid". Note that the LPWAN technology
generally carries a single identifier corresponding to the DEV.
Therefore Appiid cannot be used.
For privacy reasons or if the DEV address is changing over time, a
static value that is not equal to the DEV address SHOULD be used. In
that case, the TV contains the static value, the MO operator is set
to "equal" and the CDF is set to "not-sent". [RFC7217] provides some
methods that MAY be used to derive this static identifier.
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If several IIDs are possible, then the TV contains the list of
possible IIDs, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent".
It MAY also happen that the IID variability only expresses itself on
a few bytes. In that case, the TV is set to the stable part of the
IID, the MO is set to "MSB" and the CDA is set to "LSB".
Finally, the IID can be sent in extenso on the LPWAN. In that case,
the TV is not set, the MO is set to "ignore" and the CDA is set to
"value-sent".
9.8. IPv6 extensions
No rule is currently defined that processes IPv6 extensions. If such
extensions are needed, their compression/decompression rules can be
based on the MOs and CDAs described above.
9.9. UDP source and destination port
To allow for a single rule being used for both directions, the UDP
port values are identified by their role (DEV or APP) and not by
their position in the frame (source or destination). The SCHC C/D
MUST be aware of the traffic direction (Uplink, Downlink) to select
the appropriate field. The following rules apply for DEV and APP
port numbers.
If both ends know the port number, it can be elided. The TV contains
the port number, the MO is set to "equal" and the CDA is set to "not-
sent".
If the port variation is on few bits, the TV contains the stable part
of the port number, the MO is set to "MSB" and the CDA is set to
"LSB".
If some well-known values are used, the TV can contain the list of
these values, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent".
Otherwise the port numbers are sent over the LPWAN. The TV is not
set, the MO is set to "ignore" and the CDA is set to "value-sent".
9.10. UDP length field
The UDP length can be computed from the received data. In that case,
the TV is not set, the MO is set to "ignore" and the CDA is set to
"compute-length".
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If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB".
In other cases, the length SHOULD be sent and the CDA is replaced by
"value-sent".
9.11. UDP Checksum field
IPv6 mandates a checksum in the protocol above IP. Nevertheless, if
a more efficient mechanism such as L2 CRC or MIC is carried by or
over the L2 (such as in the LPWAN SCHC F/R process (see Section 7)),
the UDP checksum transmission can be avoided. In that case, the TV
is not set, the MO is set to "ignore" and the CDA is set to "compute-
checksum".
In other cases, the checksum SHOULD be explicitly sent. The TV is
not set, the MO is set to "ignore" and the CDF is set to "value-
sent".
10. Security considerations
10.1. Security considerations for header compression
A malicious header compression could cause the reconstruction of a
wrong packet that does not match with the original one. Such a
corruption MAY be detected with end-to-end authentication and
integrity mechanisms. Header Compression does not add more security
problem than what is already needed in a transmission. For instance,
to avoid an attack, never re-construct a packet bigger than some
configured size (with 1500 bytes as generic default).
10.2. Security considerations for SCHC Fragmentation/Reassembly
This subsection describes potential attacks to LPWAN SCHC F/R and
suggests possible countermeasures.
A node can perform a buffer reservation attack by sending a first
SCHC Fragment to a target. Then, the receiver will reserve buffer
space for the IPv6 packet. Other incoming SCHC Fragmented packets
will be dropped while the reassembly buffer is occupied during the
reassembly timeout. Once that timeout expires, the attacker can
repeat the same procedure, and iterate, thus creating a denial of
service attack. The (low) cost to mount this attack is linear with
the number of buffers at the target node. However, the cost for an
attacker can be increased if individual SCHC Fragments of multiple
packets can be stored in the reassembly buffer. To further increase
the attack cost, the reassembly buffer can be split into SCHC
Fragment-sized buffer slots. Once a packet is complete, it is
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processed normally. If buffer overload occurs, a receiver can
discard packets based on the sender behavior, which MAY help identify
which SCHC Fragments have been sent by an attacker.
In another type of attack, the malicious node is required to have
overhearing capabilities. If an attacker can overhear a SCHC
Fragment, it can send a spoofed duplicate (e.g. with random payload)
to the destination. If the LPWAN technology does not support
suitable protection (e.g. source authentication and frame counters to
prevent replay attacks), a receiver cannot distinguish legitimate
from spoofed SCHC Fragments. Therefore, the original IPv6 packet
will be considered corrupt and will be dropped. To protect resource-
constrained nodes from this attack, it has been proposed to establish
a binding among the SCHC Fragments to be transmitted by a node, by
applying content-chaining to the different SCHC Fragments, based on
cryptographic hash functionality. The aim of this technique is to
allow a receiver to identify illegitimate SCHC Fragments.
Further attacks MAY involve sending overlapped fragments (i.e.
comprising some overlapping parts of the original IPv6 datagram).
Implementers SHOULD make sure that the correct operation is not
affected by such event.
In Window mode - ACK on error, a malicious node MAY force a SCHC
Fragment sender to resend a SCHC Fragment a number of times, with the
aim to increase consumption of the SCHC Fragment sender's resources.
To this end, the malicious node MAY repeatedly send a fake ACK to the
SCHC Fragment sender, with a Bitmap that reports that one or more
SCHC Fragments have been lost. In order to mitigate this possible
attack, MAX_ACK_RETRIES MAY be set to a safe value which allows to
limit the maximum damage of the attack to an acceptable extent.
However, note that a high setting for MAX_ACK_RETRIES benefits SCHC
Fragment reliability modes, therefore the trade-off needs to be
carefully considered.
11. Acknowledgements
Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier,
Eduardo Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav, Sergio
Lopez Bernal, Antony Markovski, Alexander Pelov, Pascal Thubert, Juan
Carlos Zuniga, Diego Dujovne, Edgar Ramos, and Shoichi Sakane for
useful design consideration and comments.
12. References
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12.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC3385] Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna,
"Internet Protocol Small Computer System Interface (iSCSI)
Cyclic Redundancy Check (CRC)/Checksum Considerations",
RFC 3385, DOI 10.17487/RFC3385, September 2002,
<https://www.rfc-editor.org/info/rfc3385>.
[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>.
[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>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
12.2. Informative References
[I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-10 (work in progress), February 2018.
Appendix A. SCHC Compression Examples
This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the behavior of SCHC.
The most common case using the mechanisms defined in this document
will be a LPWAN Dev that embeds some applications running over CoAP.
In this example, three flows are considered. The first flow is for
the device management based on CoAP using Link Local IPv6 addresses
and UDP ports 123 and 124 for Dev and App, respectively. The second
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flow will be a CoAP server for measurements done by the Device (using
ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to
beta::1/64. The last flow is for legacy applications using different
ports numbers, the destination IPv6 address prefix is gamma::1/64.
Figure 27 presents the protocol stack for this Device. 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 27: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the Devs have a device ID.
Therefore, when such technologies are used, it is necessary to
statically define an IID for the Link Local address for the SCHC C/D.
Rule 0
+----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Comp Decomp|| Sent |
| | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+---------------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | 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 | comp-length|| |
|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 || |
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|UDP APPport |16|1 |Bi|124 | equal | not-sent || |
|UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Rule 1
+----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Action || Sent |
| | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+--------+------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | 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 | comp-length|| |
|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 | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Rule 2
+----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Action || Sent |
| | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+--------+------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | 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 | comp-length|| |
|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] |
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|UDP APPport |16|1 |Bi|8720 | MSB(12)| LSB || [4] |
|UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Figure 28: Context rules
All the fields described in the three rules depicted on Figure 28 are
present in the IPv6 and UDP headers. The DEViid-DID value is found
in the L2 header.
The second and third rules use global addresses. The way the Dev
learns the prefix is not in the scope of the document.
The third rule compresses port numbers to 4 bits.
Appendix B. Fragmentation Examples
This section provides examples for the different fragment reliability
modes specified in this document.
Figure 29 illustrates the transmission in No-ACK mode of an IPv6
packet that needs 11 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 + MIC --->|MIC checked: success =>
Figure 29: Transmission in No-ACK mode of an IPv6 packet carried by
11 fragments
In the following examples, N (i.e. the size if the FCN field) is 3
bits. Therefore, the All-1 FCN value is 7.
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Figure 30 illustrates the transmission in ACK-on-Error of an IPv6
packet that needs 11 fragments, with MAX_WIND_FCN=6 and no fragment
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----->|
(no ACK)
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4----->|
|--W=1, FCN=7 + MIC-->|MIC checked: success =>
|<---- ACK, W=1 ------|
Figure 30: Transmission in ACK-on-Error mode of an IPv6 packet
carried by 11 fragments, with MAX_WIND_FCN=6 and no loss.
Figure 31 illustrates the transmission in ACK-on-Error mode of an
IPv6 packet that needs 11 fragments, with MAX_WIND_FCN=6 and three
lost fragments.
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Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->| 7
|-----W=0, FCN=1----->| /
|-----W=0, FCN=0----->| 6543210
|<-----ACK, W=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 + MIC ->|MIC checked: failed
|<-----ACK, W=1-------|C=0 Bitmap:1100001
|-----W=1, FCN=4----->|MIC checked: success =>
|<---- ACK, W=1 ------|C=1, no Bitmap
Figure 31: Transmission in ACK-on-Error mode of an IPv6 packet
carried by 11 fragments, with MAX_WIND_FCN=6 and three lost
fragments.
Figure 32 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 11 fragments, with MAX_WIND_FCN=6 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-------| Bitmap:1111111
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4----->|
|--W=1, FCN=7 + MIC-->|MIC checked: success =>
|<-----ACK, W=1-------| C=1 no Bitmap
(End)
Figure 32: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6 and no lost fragment.
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Figure 33 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 11 fragments, with MAX_WIND_FCN=6 and three lost
fragments.
Sender Receiver
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->|
|-----W=1, FCN=3----->|
|-----W=1, FCN=2--X-->| 7
|-----W=1, FCN=1----->| /
|-----W=1, FCN=0----->| 6543210
|<-----ACK, W=1-------|Bitmap:1101011
|-----W=1, FCN=4----->|
|-----W=1, FCN=2----->|
|<-----ACK, W=1-------|Bitmap:
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|--W=0, FCN=7 + MIC-->|MIC checked: failed
|<-----ACK, W=0-------| C= 0 Bitmap:11000001
|-----W=0, FCN=4----->|MIC checked: success =>
|<-----ACK, W=0-------| C= 1 no Bitmap
(End)
Figure 33: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6 and three lost fragments.
Figure 34 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost
fragments and only one retry needed to recover each lost fragment.
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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 + MIC-->|MIC checked: failed
|<-----ACK, W=0-------|C= 0 Bitmap:1100001
|-----W=0, FCN=4----->|MIC checked: failed
|-----W=0, FCN=3----->|MIC checked: failed
|-----W=0, FCN=2----->|MIC checked: success
|<-----ACK, W=0-------|C=1 no Bitmap
(End)
Figure 34: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6, three lost framents and only
one retry needed for each lost fragment.
Figure 35 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost
fragments, and the second ACK lost.
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 + MIC-->|MIC checked: failed
|<-----ACK, W=0-------|C=0 Bitmap:1100001
|-----W=0, FCN=4----->|MIC checked: failed
|-----W=0, FCN=3----->|MIC checked: failed
|-----W=0, FCN=2----->|MIC checked: success
| X---ACK, W=0-------|C= 1 no Bitmap
timeout | |
|--W=0, FCN=7 + MIC-->|
|<-----ACK, W=0-------|C= 1 no Bitmap
(End)
Figure 35: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6, three lost fragments, and the
second ACK lost.
Figure 36 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 6 fragments, with MAX_WIND_FCN=6, with three lost
fragments, and one retransmitted fragment lost again.
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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 + MIC-->|MIC checked: failed
|<-----ACK, W=0-------|C=0 Bitmap:1100001
|-----W=0, FCN=4----->|MIC checked: failed
|-----W=0, FCN=3----->|MIC checked: failed
|-----W=0, FCN=2--X-->|
timeout| |
|--W=0, FCN=7 + MIC-->|All-0 empty
|<-----ACK, W=0-------|C=0 Bitmap: 1111101
|-----W=0, FCN=2----->|MIC checked: success
|<-----ACK, W=0-------|C=1 no Bitmap
(End)
Figure 36: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6, with three lost fragments, and
one retransmitted fragment lost again.
Figure 37 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 28 fragments, with N=5, MAX_WIND_FCN=23 and two
lost fragments. Note that MAX_WIND_FCN=23 may be useful when the
maximum possible Bitmap size, considering the maximum lower layer
technology payload size and the value of R, is 3 bytes. Note also
that the FCN of the last fragment of the packet is the one with
FCN=31 (i.e. FCN=2^N-1 for N=5, or equivalently, all FCN bits set to
1).
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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 ----->|
| |lcl-Bitmap:110111111111101111111111
|<------ACK, W=0-------|encoded Bitmap:1101111111111011
|-----W=0, FCN=21----->|
|-----W=0, FCN=10----->|
|<------ACK, W=0-------|no Bitmap
|-----W=1, FCN=23----->|
|-----W=1, FCN=22----->|
|-----W=1, FCN=21----->|
|--W=1, FCN=31 + MIC-->|MIC checked: sucess =>
|<------ACK, W=1-------|no Bitmap
(End)
Figure 37: Transmission in ACK-Always mode of an IPv6 packet carried
by 28 fragments, with N=5, MAX_WIND_FCN=23 and two lost 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:
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+===========+
+------------+ Init |
| FCN=0 +===========+
| No Window
| No Bitmap
| +-------+
| +========+==+ | More Fragments
| | | <--+ ~~~~~~~~~~~~~~~~~~~~
+--------> | Send | send Fragment (FCN=0)
+===+=======+
| last fragment
| ~~~~~~~~~~~~
| FCN = 1
v send fragment+MIC
+============+
| END |
+============+
Figure 38: Sender State Machine for the No-ACK Mode
+------+ Not All-1
+==========+=+ | ~~~~~~~~~~~~~~~~~~~
| + <--+ set Inactivity Timer
| RCV Frag +-------+
+=+===+======+ |All-1 &
All-1 & | | |MIC correct
MIC wrong | |Inactivity |
| |Timer Exp. |
v | |
+==========++ | v
| Error |<-+ +========+==+
+===========+ | END |
+===========+
Figure 39: Receiver State Machine for the No-ACK Mode
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+=======+
| INIT | FCN!=0 & more frags
| | ~~~~~~~~~~~~~~~~~~~~~~
+======++ +--+ send Window + frag(FCN)
W=0 | | | FCN-
Clear local Bitmap | | v set local Bitmap
FCN=max value | ++==+========+
+> | |
+---------------------> | SEND |
| +==+===+=====+
| FCN==0 & more frags | | last frag
| ~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~
| set local-Bitmap | | set local-Bitmap
| send wnd + frag(all-0) | | send wnd+frag(all-1)+MIC
| set Retrans_Timer | | set Retrans_Timer
| | |
|Recv_wnd == wnd & | |
|Lcl_Bitmap==recv_Bitmap& | | +----------------------+
|more frag | | |lcl-Bitmap!=rcv-Bitmap|
|~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ |
|Stop Retrans_Timer | | | Attemp++ v
|clear local_Bitmap v v | +=====+=+
|window=next_window +====+===+==+===+ |Resend |
+---------------------+ | |Missing|
+----+ Wait | |Frag |
not expected wnd | | Bitmap | +=======+
~~~~~~~~~~~~~~~~ +--->+ ++Retrans_Timer Exp |
discard frag +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
| | | ^ ^ |reSend(empty)All-* |
| | | | | |Set Retrans_Timer |
MIC_bit==1 & | | | | +--+Attemp++ |
Recv_window==window & | | | +-------------------------+
Lcl_Bitmap==recv_Bitmap &| | | all missing frag sent
no more frag| | | ~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~| | | Set Retrans_Timer
Stop Retrans_Timer| | |
+=============+ | | |
| END +<--------+ | | Attemp > MAX_ACK_REQUESTS
+=============+ | | ~~~~~~~~~~~~~~~~~~
All-1 Window | v Send Abort
~~~~~~~~~~~~ | +=+===========+
MIC_bit ==0 & +>| ERROR |
Lcl_Bitmap==recv_Bitmap +=============+
Figure 40: Sender State Machine for the ACK-Always Mode
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Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set local_Bitmap(FCN) | v v |discard
++===+===+===+=+
+---------------------+ Rcv +--->* ABORT
| +------------------+ Window |
| | +=====+==+=====+
| | All-0 & w=expect | ^ w =next & not-All
| | ~~~~~~~~~~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~
| | set lcl_Bitmap(FCN)| |expected = next window
| | send local_Bitmap | |Clear local_Bitmap
| | | |
| | w=expct & not-All | |
| | ~~~~~~~~~~~~~~~~~~ | |
| | set lcl_Bitmap(FCN)+-+ | | +--+ w=next & All-0
| | if lcl_Bitmap full | | | | | | ~~~~~~~~~~~~~~~
| | send lcl_Bitmap | | | | | | expct = nxt wnd
| | v | v | | | Clear lcl_Bitmap
| | w=expct & All-1 +=+=+=+==+=++ | set lcl_Bitmap(FCN)
| | ~~~~~~~~~~~ +->+ Wait +<+ send lcl_Bitmap
| | discard +--| Next |
| | All-0 +---------+ Window +--->* ABORT
| | ~~~~~ +-------->+========+=++
| | snd lcl_bm All-1 & w=next| | All-1 & w=nxt
| | & MIC wrong| | & MIC right
| | ~~~~~~~~~~~~~~~~~| | ~~~~~~~~~~~~~~~~~~
| | set local_Bitmap(FCN)| |set lcl_Bitmap(FCN)
| | send local_Bitmap| |send local_Bitmap
| | | +----------------------+
| |All-1 & w=expct | |
| |& MIC wrong v +---+ w=expctd & |
| |~~~~~~~~~~~~~~~~~~~~ +====+=====+ | MIC wrong |
| |set local_Bitmap(FCN) | +<+ ~~~~~~~~~~~~~~ |
| |send local_Bitmap | Wait End | set lcl_btmp(FCN)|
| +--------------------->+ +--->* ABORT |
| +===+====+=+-+ All-1&MIC wrong|
| | ^ | ~~~~~~~~~~~~~~~|
| w=expected & MIC right | +---+ send lcl_btmp |
| ~~~~~~~~~~~~~~~~~~~~~~ | |
| set local_Bitmap(FCN) | +-+ Not All-1 |
| send local_Bitmap | | | ~~~~~~~~~ |
| | | | discard |
|All-1 & w=expctd & MIC right | | | |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v +----+All-1 |
|set local_Bitmap(FCN) +=+=+=+=+==+ |~~~~~~~~~ |
|send local_Bitmap | +<+Send lcl_btmp |
+-------------------------->+ END | |
+==========+<---------------+
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--->* ABORT
~~~~~~~
Inactivity_Timer = expires
When DWN_Link
IF Inactivity_Timer expires
Send DWL Request
Attemp++
Figure 41: Receiver State Machine for the ACK-Always Mode
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+=======+
| |
| INIT |
| | FCN!=0 & more frags
+======++ +--+ ~~~~~~~~~~~~~~~~~~~~~~
W=0 | | | send Window + frag(FCN)
~~~~~~~~~~~~~~~~~~ | | | FCN-
Clear local Bitmap | | v set local Bitmap
FCN=max value | ++=============+
+> | |
| SEND |
+-------------------------> | |
| ++=====+=======+
| FCN==0 & more frags| |last frag
| ~~~~~~~~~~~~~~~~~~~~~~~| |~~~~~~~~~~~~~~~~~
| set local-Bitmap| |set local-Bitmap
| send wnd + frag(all-0)| |send wnd+frag(all-1)+MIC
| set Retrans_Timer| |set Retrans_Timer
| | |
|Retrans_Timer expires & | | lcl-Bitmap!=rcv-Bitmap
|more fragments | | ~~~~~~~~~~~~~~~~~~~~~~
|~~~~~~~~~~~~~~~~~~~~ | | Attemp++
|stop Retrans_Timer | | +-----------------+
|clear local-Bitmap v v | v
|window = next window +=====+=====+==+==+ +====+====+
+----------------------+ + | Resend |
+--------------------->+ Wait Bitmap | | Missing |
| +-- + | | Frag |
| not expected wnd | ++=+===+===+===+==+ +======+==+
| ~~~~~~~~~~~~~~~~ | ^ | | | ^ |
| discard frag +----+ | | | +-------------------+
| | | | all missing frag sent
|Retrans_Timer expires & | | | ~~~~~~~~~~~~~~~~~~~~~
| No more Frag | | | Set Retrans_Timer
| ~~~~~~~~~~~~~~~~~~~~~~~ | | |
| Stop Retrans_Timer | | |
| Send ALL-1-empty | | |
+-------------------------+ | |
| |
Local_Bitmap==Recv_Bitmap| |
~~~~~~~~~~~~~~~~~~~~~~~~~| |Attemp > MAX_ACK_REQUESTS
+=========+Stop Retrans_Timer | |~~~~~~~~~~~~~~~~~~~~~~~
| END +<------------------+ v Send Abort
+=========+ +=+=========+
| ERROR |
+===========+
Figure 42: Sender State Machine for the ACK-on-Error Mode
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Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set local_Bitmap(FCN) | v v |discard
++===+===+===+=+
+-----------------------+ +--+ All-0 & full
| ABORT *<---+ Rcv Window | | ~~~~~~~~~~~~
| +--------------------+ +<-+ w =next
| | All-0 empty +->+=+=+===+======+ clear lcl_Bitmap
| | ~~~~~~~~~~~ | | | ^
| | send bitmap +----+ | |w=expct & not-All & full
| | | |~~~~~~~~~~~~~~~~~~~~~~~~
| | | |set lcl_Bitmap; w =nxt
| | | |
| | All-0 & w=expect | | w=next
| | & no_full Bitmap | | ~~~~~~~~ +========+
| | ~~~~~~~~~~~~~~~~~ | | Send abort| Error/ |
| | send local_Bitmap | | +---------->+ Abort |
| | | | | +-------->+========+
| | v | | | all-1 ^
| | All-0 empty +====+===+==+=+=+ ~~~~~~~ |
| | ~~~~~~~~~~~~~ +--+ Wait | Send abort |
| | send lcl_btmp +->| Missing Fragm.| |
| | +==============++ |
| | +--------------+
| | Uplink Only &
| | Inactivity_Timer = expires
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~
| | Send Abort
| |All-1 & w=expect & MIC wrong
| |~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +-+ All-1
| |set local_Bitmap(FCN) | v ~~~~~~~~~~
| |send local_Bitmap +===========+==+ snd lcl_btmp
| +--------------------->+ Wait End +-+
| +=====+=+====+=+ | w=expct &
| w=expected & MIC right | | ^ | MIC wrong
| ~~~~~~~~~~~~~~~~~~~~~~ | | +---+ ~~~~~~~~~
| set & send local_Bitmap(FCN) | | set lcl_Bitmap(FCN)
| | |
|All-1 & w=expected & MIC right | +-->* ABORT
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v
|set & send local_Bitmap(FCN) +=+==========+
+---------------------------->+ END |
+============+
--->* ABORT
Only Uplink
Inactivity_Timer = expires
~~~~~~~~~~~~~~~~~~~~~~~~~~
Send Abort
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Figure 43: Receiver State Machine for the ACK-on-Error Mode
Appendix D. SCHC Parameters - Ticket #15
This section gives the list of parameters that need to be defined in
the technology-specific documents, technology developers must
evaluate that L2 has strong enough integrity checking to match SCHC's
assumption:
o LPWAN Architecture. Explain the SCHC entities (Compression and
Fragmentation), how/where are they be represented in the
corresponding technology architecture.
o L2 fragmentation decision
o Rule ID number of rules
o Size of the Rule ID
o The way the Rule ID is sent (L2 or L3) and how (describe)
o Fragmentation delivery reliability mode used in which cases
o Define the number of bits FCN (N) and DTag (T)
o The MIC algorithm to be used and the size if different from the
default CRC32
o Retransmission Timer duration
o Inactivity Timer duration
o Define the MAX_ACK_REQUEST (number of attempts)
o Use of padding or not and how and when to use it
o Take into account that the length of rule-id + N + T + W when
possible is good to have a multiple of 8 bits to complete a byte
and avoid padding
o In the ACK format to have a length for Rule-ID + T + W bit into a
complete number of byte to do optimization more easily
o The technology documents will describe if Rule ID is constrained
by any alignment
And the following parameters need to be addressed in another document
but not forcely in the technology-specific one:
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o The way the contexts are provisioning
o The way the Rules as generated
Appendix E. Note
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
2bis rue de la Chataigneraie
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
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