LPWAN Static Context Header Compression (SCHC) and fragmentation for IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-07
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8724.
|
|
|---|---|---|---|
| Authors | Ana Minaburo , Laurent Toutain , Carles Gomez | ||
| Last updated | 2017-10-20 | ||
| Replaces | draft-toutain-lpwan-ipv6-static-context-hc | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-21)
by Pete Resnick
Ready w/issues
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Dominique Barthel | ||
| IESG | IESG state | Became RFC 8724 (Proposed Standard) | |
| 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-07
lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational L. Toutain
Expires: April 23, 2018 IMT-Atlantique
C. Gomez
Universitat Politecnica de Catalunya
October 20, 2017
LPWAN Static Context Header Compression (SCHC) and fragmentation for
IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-07
Abstract
This document describes a header compression scheme and fragmentation
functionality for very low bandwidth networks. These techniques are
specially tailored for LPWAN (Low Power Wide Area Network) networks.
The Static Context Header Compression (SCHC) offers a great level of
flexibility when processing the header fields. SCHC compression is
based on a common static context stored in a LPWAN device and in the
network. Static context means that the stored information does not
change during the packet transmission. The context describes the
field values and keeps information that will not be transmitted
through the constrained network.
SCHC must be used for LPWAN networks because it avoids complex
resynchronization mechanisms, which are incompatible with LPWAN
characteristics. And also because in most cases, IPv6/UDP headers
are reduced to a small identifier called Rule ID. Eventhough
sometimes, a SCHC compressed packet will not fit in one L2 PDU, and
the SCHC fragmentation protocol will be used. The SCHC fragmentation
and reassembly mechanism is used in two situations: for SCHC-
compressed packets that still exceed the L2 PDU size; and for the
case where the SCHC compression cannot be performed.
This document describes the SCHC compression/decompression framework
and applies it to IPv6/UDP headers. This document also specifies a
fragmentation and reassembly mechanism that is used to support the
IPv6 MTU requirement over LPWAN technologies. Fragmentation is
mandatory for IPv6 datagrams that, after SCHC compression or when it
has not been possible to apply such compression, still exceed the L2
maximum payload size. Similar solutions for other protocols such as
CoAP will be described in separate documents.
Minaburo, et al. Expires April 23, 2018 [Page 1]
Internet-Draft LPWAN SCHC October 2017
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
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 April 23, 2018.
Copyright Notice
Copyright (c) 2017 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Static Context Header Compression . . . . . . . . . . . . . . 7
4.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Packet processing . . . . . . . . . . . . . . . . . . . . 10
4.4. Matching operators . . . . . . . . . . . . . . . . . . . 11
4.5. Compression Decompression Actions (CDA) . . . . . . . . . 12
4.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 13
4.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 13
4.5.3. mapping-sent . . . . . . . . . . . . . . . . . . . . 13
4.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 13
4.5.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . 14
Minaburo, et al. Expires April 23, 2018 [Page 2]
Internet-Draft LPWAN SCHC October 2017
4.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 14
5. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. Reliability options . . . . . . . . . . . . . . . . . . . 15
5.3. Functionalities . . . . . . . . . . . . . . . . . . . . . 16
5.4. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.4.1. Fragment format . . . . . . . . . . . . . . . . . . . 18
5.4.2. Fragmentation header formats . . . . . . . . . . . . 18
5.4.3. ACK format . . . . . . . . . . . . . . . . . . . . . 19
5.4.4. All-1 and All-0 formats . . . . . . . . . . . . . . . 20
5.5. Baseline mechanism . . . . . . . . . . . . . . . . . . . 21
5.6. Supporting multiple window sizes . . . . . . . . . . . . 22
5.7. Aborting fragmented datagram transmissions . . . . . . . 23
5.8. Downlink fragment transmission . . . . . . . . . . . . . 23
5.9. Fragmentation Mode of Operation Description . . . . . . . 23
5.9.1. No ACK Mode . . . . . . . . . . . . . . . . . . . . . 23
5.9.2. The Window modes . . . . . . . . . . . . . . . . . . 25
5.9.3. ACK Always . . . . . . . . . . . . . . . . . . . . . 25
5.9.4. ACK on error . . . . . . . . . . . . . . . . . . . . 30
6. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 35
6.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 35
6.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 35
6.3. Flow label field . . . . . . . . . . . . . . . . . . . . 35
6.4. Payload Length field . . . . . . . . . . . . . . . . . . 36
6.5. Next Header field . . . . . . . . . . . . . . . . . . . . 36
6.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 36
6.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 37
6.7.1. IPv6 source and destination prefixes . . . . . . . . 37
6.7.2. IPv6 source and destination IID . . . . . . . . . . . 37
6.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 38
6.9. UDP source and destination port . . . . . . . . . . . . . 38
6.10. UDP length field . . . . . . . . . . . . . . . . . . . . 38
6.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 39
7. Security considerations . . . . . . . . . . . . . . . . . . . 39
7.1. Security considerations for header compression . . . . . 39
7.2. Security considerations for fragmentation . . . . . . . . 39
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 40
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
9.1. Normative References . . . . . . . . . . . . . . . . . . 40
9.2. Informative References . . . . . . . . . . . . . . . . . 41
Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 41
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 44
Appendix C. Allocation of Rule IDs for fragmentation . . . . . . 50
Appendix D. Note . . . . . . . . . . . . . . . . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 51
Minaburo, et al. Expires April 23, 2018 [Page 3]
Internet-Draft LPWAN SCHC October 2017
1. Introduction
Header compression is mandatory to efficiently bring Internet
connectivity to the node within a LPWAN network. Some LPWAN networks
properties can be exploited to get an efficient header compression:
o Topology is star-oriented, therefore all the packets follow the
same path. For the needs of this draft, the architecture can be
summarized to Devices (Dev) exchanging information with LPWAN
Application Server (App) through a Network Gateway (NGW).
o Traffic flows are mostly known in advance since devices embed
built-in applications. Contrary to computers or smartphones, new
applications cannot be easily installed.
The Static Context Header Compression (SCHC) is defined for this
environment. SCHC uses a context where header information is kept in
the header format order. This context is static (the values of the
header fields do not change over time) avoiding complex
resynchronization mechanisms, incompatible with LPWAN
characteristics. In most of the cases, IPv6/UDP headers are reduced
to a small context identifier.
The SCHC header compression mechanism is independent of the specific
LPWAN technology over which it will be used.
LPWAN technologies are also characterized, among others, by a very
reduced data unit and/or payload size [I-D.ietf-lpwan-overview].
However, some of these technologies do not support layer two
fragmentation, therefore the only option for them to support the IPv6
MTU requirement of 1280 bytes [RFC2460] is the use of a fragmentation
protocol at the adaptation layer below IPv6. This draft defines also
a fragmentation functionality to support the IPv6 MTU requirement
over LPWAN technologies. Such functionality has been designed under
the assumption that data unit reordering will not happen between the
entity performing fragmentation and the entity performing reassembly.
2. LPWAN Architecture
LPWAN technologies have similar architectures but different
terminology. We can identify different types of entities in a
typical LPWAN network, see Figure 1:
o Devices (Dev) are the end-devices or hosts (e.g. sensors,
actuators, etc.). There can be a high density of devices per radio
gateway.
Minaburo, et al. Expires April 23, 2018 [Page 4]
Internet-Draft LPWAN SCHC October 2017
o The Radio Gateway (RG), 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. We use the term LPWAN-AAA server because we are not
assuming that this entity speaks RADIUS or Diameter as many/most AAA
servers do, but equally we don't want to rule that out, as the
functionality will be similar.
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 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 to identify the application interface
o Bi: Bidirectional, it can be used in both senses
o CDA: Compression/Decompression Action. An action that is
performed for both functionalities to compress a header field or
to recover its original value in the decompression phase.
o Context: A set of rules used to compress/decompress headers
o Dev: Device. A Node connected to the LPWAN. A Dev may implement
SCHC.
Minaburo, et al. Expires April 23, 2018 [Page 5]
Internet-Draft LPWAN SCHC October 2017
o Dev-IID: Device Interface Identifier. Second part of the IPv6
address to identify the device interface
o DI: Direction Indicator is a differentiator for matching in order
to be able to have different values for both sides.
o DTag: Datagram Tag is a fragmentation header field that is set to
the same value for all fragments carrying the same IPv6 datagram.
o Dw: Down Link direction for compression, from SCHC C/D to Dev
o FCN: Fragment Compressed Number is a fragmentation header field
that carries an efficient representation of a larger-sized
fragment number.
o FID: Field Identifier is an index to describe the header fields in
the Rule
o FL: Field Length is a value to identify if the field is fixed or
variable length.
o FP: Field Position is a value that is used to identify each
instance a field apears in the header.
o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136]
o MIC: Message Integrity Check. A fragmentation 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 Rule: A set of header field values.
o Rule ID: An identifier for a rule, SCHC C/D, and Dev share the
same Rule ID for a specific flow. A set of Rule IDs are used to
support fragmentation functionality.
o SCHC C/D: Static Context Header Compression Compressor/
Decompressor. A process in the network to achieve compression/
decompressing headers. SCHC C/D uses SCHC rules to perform
compression and decompression.
o TV: Target value. A value contained in the Rule that will be
matched with the value of a header field.
Minaburo, et al. Expires April 23, 2018 [Page 6]
Internet-Draft LPWAN SCHC October 2017
o Up: Up Link direction for compression, from Dev to SCHC C/D.
o W: Window bit. A fragmentation header field used in Window mode
(see section 9), which carries the same value for all fragments of
a window.
4. Static Context Header Compression
Static Context Header Compression (SCHC) avoids context
synchronization, which is the most bandwidth-consuming operation in
other header compression mechanisms such as RoHC [RFC5795]. Based on
the fact that the nature of data flows is highly predictable in LPWAN
networks, some static contexts may be stored on the Device (Dev).
The contexts must be stored in both ends, and it can either be
learned by a provisioning protocol or by out of band means or it can
be pre-provisioned, etc. The way the context is learned on both
sides is out of the scope of this document.
Dev App
+--------------+ +--------------+
|APP1 APP2 APP3| |APP1 APP2 APP3|
| | | |
| UDP | | UDP |
| IPv6 | | IPv6 |
| | | |
| SCHC C/D | | |
| (context) | | |
+-------+------+ +-------+------+
| +--+ +----+ +---------+ .
+~~ |RG| === |NGW | === |SCHC C/D |... Internet ..
+--+ +----+ |(context)|
+---------+
Figure 2: Architecture
Figure 2 represents the architecture for compression/decompression,
it is based on [I-D.ietf-lpwan-overview] terminology. The Device is
sending applications flows using IPv6 or IPv6/UDP protocols. These
flows are compressed by an Static Context Header Compression
Compressor/Decompressor (SCHC C/D) to reduce headers size. The
resulting information is sent to a layer two (L2) frame to a LPWAN
Radio Network (RG) which forwards the frame to a Network Gateway
(NGW). The NGW sends the data to an SCHC C/D for decompression which
shares the same rules with the Dev. The SCHC C/D can be located on
the Network Gateway (NGW) or in another place as long as a tunnel is
established between the NGW and the SCHC C/D. The SCHC C/D in both
sides must share the same set of Rules. After decompression, the
Minaburo, et al. Expires April 23, 2018 [Page 7]
Internet-Draft LPWAN SCHC October 2017
packet can be sent on the Internet to one or several LPWAN
Application Servers (App).
The SCHC C/D process is bidirectional, so the same principles can be
applied in the other direction.
4.1. SCHC Rules
The main idea of the SCHC compression scheme is to send the Rule id
to the other end instead of sending known field values. This Rule id
identifies a rule that matches as much as possible the original
packet values. When a value is known by both ends, it is not
necessary to send it through the LPWAN network.
The context contains a list of rules (cf. Figure 3). 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).
/-----------------------------------------------------------------\
| 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 3: Compression/Decompression Context
The Rule does not describe the original packet format which must be
known from the compressor/decompressor. The rule just describes the
compression/decompression behavior for the header fields. In the
rule, the description of the header field must be performed in the
format packet order.
The Rule also describes the compressed header fields which are
transmitted regarding their position in the rule which is used for
Minaburo, et al. Expires April 23, 2018 [Page 8]
Internet-Draft LPWAN SCHC October 2017
data serialization on the compressor side and data deserialization on
the decompressor side.
The Context describes the header fields and its values with the
following entries:
o A Field ID (FID) is a unique value to define the header field.
o A Field Length (FL) is the length of the field that can be of
fixed length as in IPv6 or UDP headers or variable length as in
CoAP options. Fixed length fields shall be represented by its
actual value in bits. Variable length fields shall be represented
by a function or a variable.
o A Field Position (FP) indicating if several instances of the field
exist in the headers which one is targeted. The default position
is 1
o A direction indicator (DI) indicating the packet direction. Three
values are possible:
* UPLINK (Up) when the field or the value is only present in
packets sent by the Dev to the App,
* DOWNLINK (Dw) when the field or the value is only present in
packet sent from the App to the Dev and
* BIDIRECTIONAL (Bi) when the field or the value is present
either upstream or downstream.
o A Target Value (TV) is the value used to make the comparison with
the packet header field. The Target Value can be of any type
(integer, strings,...). For instance, it can be a single value or
a more complex structure (array, list,...), such as a JSON or a
CBOR structure.
o A Matching Operator (MO) is the operator used to make the
comparison between the Field Value and the Target Value. The
Matching Operator may require some parameters. MO is only used
during the compression phase.
o A Compression Decompression Action (CDA) is used to describe the
compression and the decompression process. The CDA may require
some parameters, CDA are used in both compression and
decompression phases.
Minaburo, et al. Expires April 23, 2018 [Page 9]
Internet-Draft LPWAN SCHC October 2017
4.2. Rule ID
Rule IDs are sent between both compression/decompression elements.
The size of the Rule ID is not specified in this document, it is
implementation-specific and can vary regarding the LPWAN technology,
the number of flows, among others.
Some values in the Rule ID space are reserved for other
functionalities than header compression as fragmentation. (See
Section 5).
Rule IDs are specific to a Dev. Two Devs may use the same Rule ID for
different header compression. To identify the correct Rule ID, the
SCHC C/D needs to combine the Rule ID with the Dev L2 identifier to
find the appropriate Rule.
4.3. 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
compression from Dw to Up the SCHC C/D needs to find the correct
Rule to be used by identifying its Dev-ID and the Rule-ID. In the
Up situation, only the Rule-ID is used. The next step is to
choose the fields by their direction, using the direction
indicator (DI), so the fields that do not correspond to the
appropriated DI will be excluded. Next, then the fields are
identified according to their field identifier (FID) and field
position (FP). If the field position does not correspond, then
the Rule is not used and the SCHC take next Rule. Once the DI and
the FP correspond to the header information, each field's value is
then compared to the corresponding target value (TV) stored in the
Rule for that specific field using the matching operator (MO). If
all the fields in the packet's header satisfy all the matching
operators (MOs) of a Rule (i.e. all results are True), the fields
of the header are then processed according to the Compression/
Decompression Actions (CDAs) and a compressed header is obtained.
Otherwise, the next rule is tested. If no eligible rule is found,
then the header must be sent without compression, in which case
the fragmentation process must be required.
o sending: The Rule ID is sent to the other end followed by the
information resulting from the compression of header fields,
directly followed by the payload. The product of field
compression is sent in the order expressed in the Rule for the
matching fields. The way the Rule ID is sent depends on the
specific LPWAN layer two technology and will be specified in a
Minaburo, et al. Expires April 23, 2018 [Page 10]
Internet-Draft LPWAN SCHC October 2017
specific document and is out of the scope of this document. For
example, it can be either included in a Layer 2 header or sent in
the first byte of the L2 payload. (Cf. Figure 4).
o decompression: In both directions, The receiver identifies the
sender through its device-id (e.g. MAC address) and selects the
appropriate Rule through the Rule ID. This Rule gives the
compressed header format and associates these values to the header
fields. It applies the CDA action to reconstruct the original
header fields. The CDA application order can be different from
the order given by the Rule. For instance Compute-* may be
applied at the end, after all the other CDAs.
If after using SCHC compression and adding the payload to the L2
frame the datagram is not multiple of 8 bits, padding may be used.
+--- ... --+-------------- ... --------------+-----------+--...--+
| Rule ID |Compressed Hdr Fields information| payload |padding|
+--- ... --+-------------- ... --------------+-----------+--...--+
Figure 4: LPWAN Compressed Format Packet
4.4. 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 applied indifferently to integer, string or any
other data type. The result of the operation can either be True or
False. MOs are defined as follows:
o equal: A field value in a packet matches with a TV in a Rule if
they 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(length): A matching is obtained if the most significant bits
of the length field value bits of the header are equal to the TV
in the rule. The MSB Matching Operator needs a parameter,
indicating the number of bits, to proceed to the matching.
o match-mapping: The goal of mapping-sent is to reduce the size of a
field by allocating a shorter value. The Target Value contains a
list of values. Each value is identified by a short ID (or
index). This operator matches if a field value is equal to one of
those target values.
Minaburo, et al. Expires April 23, 2018 [Page 11]
Internet-Draft LPWAN SCHC October 2017
4.5. 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(length) |send LSB |TV OR 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 |
\--------------------+-------------+----------------------------/
Figure 5: Compression and Decompression Functions
Figure 5 summarizes the basics functions defined to compress and
decompress a field. The first column gives the action's name. The
second and third columns outline the compression/decompression
behavior.
Compression is done in the rule order and compressed values are sent
in that order in the compressed message. The receiver must be able
to find the size of each compressed field which can be given by the
rule or may be sent with the compressed header.
If the field is identified as being variable, then its size must be
sent first using the following coding:
o If the size is between 0 and 14 bytes it is sent using 4 bits.
o For values between 15 and 255, the first 4 bits sent are set to 1
and the size is sent using 8 bits.
o For higher value, the first 12 bits are set to 1 and the size is
sent on 2 bytes.
Minaburo, et al. Expires April 23, 2018 [Page 12]
Internet-Draft LPWAN SCHC October 2017
4.5.1. not-sent CDA
The not-sent function is generally used when the field value is
specified in the rule and therefore known by the both Compressor and
Decompressor. This action is generally used with the "equal" MO. If
MO is "ignore", there is a risk to have a decompressed field value
different from the compressed field.
The compressor does not send any value in the compressed header for
the field on which compression is applied.
The decompressor restores the field value with the target value
stored in the matched rule.
4.5.2. 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 explicitly in the compressed header field by
indicating the length. This function is generally used with the
"ignore" MO.
4.5.3. mapping-sent
mapping-sent is used to send a smaller index associated with the list
of values in the Target Value. This function is used together with
the "match-mapping" MO.
The compressor looks on the TV to find the field value and send the
corresponding index. The decompressor uses this index to restore the
field value.
The number of bits sent is the minimal size for coding all the
possible indexes.
4.5.4. LSB CDA
LSB action is used to avoid sending the known part of the packet
field header to the other end. This action is used together with the
"MSB" MO. A length can be specified in the rule to indicate how many
bits have to be sent. If the length is not specified, the number of
bits sent is the field length minus the bits length specified in the
MSB MO.
The compressor sends the "length" Least Significant Bits. The
decompressor combines the value received with the Target Value.
Minaburo, et al. Expires April 23, 2018 [Page 13]
Internet-Draft LPWAN SCHC October 2017
If this action is made on a variable length field, the remaining size
in byte has to be sent before.
4.5.5. 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.
The IID value may be computed from the Device ID present in the Layer
2 header. The computation is specific for each LPWAN technology and
may depend on the Device ID size.
In the downstream direction, these CDA may be used to determine the
L2 addresses used by the LPWAN.
4.5.6. Compute-*
These classes of functions are used by the decompressor to compute
the compressed field value based on received information. Compressed
fields are elided during compression and reconstructed during
decompression.
o compute-length: compute the length assigned to this field. For
instance, regarding the field ID, this CDA may be used to compute
IPv6 length or UDP length.
o compute-checksum: compute a checksum from the information already
received by the SCHC C/D. This field may be used to compute UDP
checksum.
5. Fragmentation
5.1. Overview
In LPWAN technologies, the L2 data unit size typically varies from
tens to hundreds of bytes. If the entire IPv6 datagram after
applying SCHC header compression or when SCHC is not possible, fits
within a single L2 data unit, the fragmentation mechanism is not used
and the packet is sent. Otherwise, the datagram SHALL be broken into
fragments.
LPWAN technologies impose some strict limitations on traffic, devices
are sleeping most of the time and may receive data during a short
period of time after transmission to preserve battery. To adapt the
SCHC fragmentation to the capabilities of LPWAN technologies, it is
desirable to enable optional fragment retransmission and to allow a
Minaburo, et al. Expires April 23, 2018 [Page 14]
Internet-Draft LPWAN SCHC October 2017
gradation of fragment delivery reliability. This document does not
make any decision with regard to which fragment delivery reliability
option may be used over a specific LPWAN technology.
An important consideration is that LPWAN networks typically follow
the star topology, and therefore data unit reordering is not expected
in such networks. This specification assumes that reordering will
not happen between the entity performing fragmentation and the entity
performing reassembly. This assumption allows to reduce complexity
and overhead of the fragmentation mechanism.
5.2. Reliability options
This specification defines the following three fragment delivery
reliability options:
o No ACK. No ACK is the simplest fragment delivery reliability
option. The receiver does not generate overhead in the form of
acknowledgments (ACKs). However, this option does not enhance
delivery reliability beyond that offered by the underlying LPWAN
technology. In the No ACK option, the receiver MUST NOT issue ACKs.
o Window mode - ACK always (ACK-always).
The ACK-always option provides flow control. In addition, it is able
to handle long bursts of lost fragments, since detection of such
events can be done before the end of the IPv6 packet transmission, as
long as the window size is short enough. However, such benefit comes
at the expense of ACK use. In ACK-always, an ACK is transmitted by
the fragment receiver after a window of fragments have been sent. A
window of fragments is a subset of the full set of fragments needed
to carry an IPv6 packet. In this mode, the ACK informs the sender
about received and/or missed fragments from the window of fragments.
Upon receipt of an ACK that informs about any lost fragments, the
sender retransmits the lost fragments. When an ACK is not received
by the fragment sender, the latter retransmits an all-1 empty
fragment, which serves as an ACK request. The maximum number of ACK
requests is MAX_ACK_REQUESTS. The default value of MAX_ACK_REQUESTS
is not stated in this document, and it is expected to be defined in
other documents (e.g. technology- specific profiles).
o Window mode - ACK-on-error. The ACK-on-error option is suitable
for links offering relatively low L2 data unit loss probability.
This option reduces the number of ACKs transmitted by the fragment
receiver. This may be especially beneficial in asymmetric scenarios,
e.g. where fragmented data are sent uplink and the underlying LPWAN
technology downlink capacity or message rate is lower than the uplink
one. However, if an ACK is lost, the sender assumes that all
Minaburo, et al. Expires April 23, 2018 [Page 15]
Internet-Draft LPWAN SCHC October 2017
fragments covered by the ACK have been successfully delivered. And
the receiver will abort the fragmentation.
In ACK-on-error, an ACK is transmitted by the fragment receiver after
a window of fragments has been sent, only if at least one of the
fragments in the window has been lost. In this mode, the ACK informs
the sender about received and/or missed fragments from the window of
fragments. Upon receipt of an ACK that informs about any lost
fragments, the sender retransmits the lost fragments.
The same reliability option MUST be used for all fragments of a
packet. It is up to implementers and/or representatives of the
underlying LPWAN technology to decide which reliability option to use
and whether the same reliability option applies to all IPv6 packets
or not. Note that the reliability option to be used is not
necessarily tied to the particular characteristics of the underlying
L2 LPWAN technology (e.g. the No ACK reliability option may be used
on top of an L2 LPWAN technology with symmetric characteristics for
uplink and downlink).
This document does not make any decision as to which fragment
delivery reliability option(s) are supported by a specific LPWAN
technology.
Examples of the different reliability options described are provided
in Appendix A.
5.3. Functionalities
This subsection describes the different fields in the fragmentation
header that are used to enable the described fragmentation
functionalities and the different reliability options supported.
o Rule ID. The Rule ID in the fragmentation part is used to identify
the fragmentation mode used, also to idenitfy fragments from ACK and
Abort frames. The also allows to interleave non-fragmented IPv6
datagrams with fragments that carry a larger IPv6 datagram. In the
fragments format this field has a size of R - T - N - 1 bits when
Window mode is used. In No ACK mode, the Rule ID field has a size of
R - T - N bits see format section.
o Fragment Compressed Number (FCN). The FCN is included in all
fragments. This field can be understood as a truncated, efficient
representation of a larger-sized fragment number, and does not carry
an absolute fragment number. There are two reserved values used for
the control of the fragmentation. The FCN value when all the bits
equals 1 (all-1) denotes the last fragment of a packet. And the FCN
value when all the bits equals 0 (all-0) denotes the last fragment of
the windonw in any window mode or the fragments in No ACK mode. The
rest of the FCN values are used in a sequential and decreasing order,
Minaburo, et al. Expires April 23, 2018 [Page 16]
Internet-Draft LPWAN SCHC October 2017
which has the purpose to avoid possible ambiguity for the receiver
that might arise under certain conditions. In the fragments, this
field is an unsigned integer, with a size of N bits. In the No ACK
mode it is set to 1 bit (N=1). For the other modes it is recommended
to use a number of bits (N) equal to or greater than 3. The FCN MUST
be set sequentially
decreasing from the highest FCN in the window (which will be used for
the first fragment), and MUST wrap from 0 back to the highest FCN in
the window.
The FCN for the last fragment in a window is an all-0, which
indicates that the window is finished and it proceeds according to
the mode in use: either an ack is sent or the next window fragments
are expected when there is no error. The FCN for the last fragment
is an all-1. It is also important to note that, for No ACK mode or
N=1, the last fragment of the packet will carry a FCN equal to 1,
while all previous fragments will carry a FCN of 0.
o Datagram Tag (DTag). The DTag field, if present, is set to the
same value for all fragments carrying the same IPv6 datagram, allows
to interleave fragments that correspond to different IPv6 datagrams.
In the fragment formats the size of the DTag field is T bits, which
may be set to a value greater than or equal to 0 bits. DTag MUST be
set sequentially increasing from 0 to 2^T - 1, and MUST wrap back
from 2^T - 1 to 0. In the ACK format, DTag carries the same value as
the DTag field in the fragments for which this ACK is intended.
o W (window): W is a 1-bit field. This field carries the same value
for all fragments of a window, and it is complemented for the next
window. The initial value for this field is 0. In the ACK format,
this field has a size of 1 bit. In all ACKs, the W bit carries the
same value as the W bit carried by the fragments whose reception is
being positively or negatively acknowledged by the ACK.
o Message Integrity Check (MIC). This field, which has a size of M
bits. It is computed by the sender over the complete packet (i.e. a
SCHC compressed or an uncompressed IPv6 packet) before fragmentation.
The algorithm to be used to compute the MIC is not defined in this
document, and needs to be defined in other documents (e.g.
technology-specific profiles). 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.
o Bitmap. The bitmap is a sequence of bits included in the ACK for a
given window, each bit in the Bitmap identifies a fragment. It
provides feedback on whether each fragment of the current window has
been received or not. FCN set to All-0 and All-1 fragments are set
to the right-most position on the bitmap in this order. Highest FCN
is set to the left-most position. A bit set to 1 indicates that the
Minaburo, et al. Expires April 23, 2018 [Page 17]
Internet-Draft LPWAN SCHC October 2017
corresponding FCN fragment has been correctly sent and received.
TODO (it is missing to explain the optimization of bitmap in order to
have a way to send an abort)
5.4. Formats
This section defines the fragment format, the fragmentation header
formats, and the ACK format.
5.4.1. Fragment format
A fragment comprises a fragmentation header and a fragment payload,
and conforms to the format shown in Figure 6. The fragment payload
carries a subset of either a SCHC header or an IPv6 header or the
original IPv6 packet payload which could not be compressed. A
fragment is the payload in the L2 protocol data unit (PDU).
+---------------+-----------------------+
| Fragm. Header | Fragment payload |
+---------------+-----------------------+
Figure 6: Fragment format.
5.4.2. Fragmentation header formats
In the No ACK option, fragments except the last one SHALL contain the
fragmentation header as defined in Figure 7. The total size of this
fragmentation header is R bits.
<------------ R ---------->
<--T--> <--N-->
+-- ... --+- ... -+- ... -+---...---+
| Rule ID | DTag | FCN | payload |
+-- ... --+- ... -+- ... -+---...---+
Figure 7: Fragmentation Header for Fragments except the Last One, No
ACK option
In any of the Window mode options, fragments except the last one
SHALL contain the fragmentation header as defined in Figure 8. The
total size of this fragmentation header is R bits.
Minaburo, et al. Expires April 23, 2018 [Page 18]
Internet-Draft LPWAN SCHC October 2017
<------------ R ---------->
<--T--> 1 <--N-->
+-- ... --+- ... -+-+- ... -+---...---+
| Rule ID | DTag |W| FCN | payload |
+-- ... --+- ... -+-+- ... -+---...---+
Figure 8: Fragmentation Header for Fragments except the Last One,
Window mode
5.4.3. ACK format
The format of an ACK is shown in Figure 9:
<-------- R ------->
<- T -> 1
+---- ... --+-... -+-+----- ... ---+
| Rule ID | DTag |W| bitmap |
+---- ... --+-... -+-+----- ... ---+
Figure 9: Format of an ACK
Figure 10 shows an example of an ACK (N=3), where the bitmap
indicates that the second and the fifth fragments have not been
correctly received.
<------- R ------->
<- T -> 1 6 5 4 3 2 1 0
+---- ... --+-... -+-+-+-+-+-+-+-+-----+
| Rule ID | DTag |W|1|0|1|1|0|1|all-0| TODO
+---- ... --+-... -+-+-+-+-+-+-+-+-----+
Figure 10: Example of the bitmap in Window mode, in any window unless
the last one, for N=3)
<------- R ------->
<- T -> 1 6 5 4 3 2 1 7
+---- ... --+-... -+-+-+-+-+-+-+-+-----+
| Rule ID | DTag |W|1|0|1|1|0|1|all-1| TODO
+---- ... --+-... -+-+-+-+-+-+-+-+-----+
Figure 11: Example of the bitmap in Window mode for the last window,
for N=3)
Minaburo, et al. Expires April 23, 2018 [Page 19]
Internet-Draft LPWAN SCHC October 2017
5.4.4. All-1 and All-0 formats
<------------ R ------------>
<- T -> 1 <- N ->
+-- ... --+- ... -+-+- ... -+--- ... ---+
| Rule ID | DTag |W| 0..0 | payload | TODO
+-- ... --+- ... -+-+- ... -+--- ... ---+
Figure 12: All-0 format fragment
In the No ACK option, the last fragment of an IPv6 datagram SHALL
contain a fragmentation header that conforms to the format shown in
Figure 14. The total size of this fragmentation header is R+M bits.
<------------ R ------------>
<- T -> 1 <- N ->
+-- ... --+- ... -+-+- ... -+
| Rule ID | DTag |W| 0..0 | TODO
+-- ... --+- ... -+-+- ... -+
Figure 13: All-0 empty format fragment
<------------- R ---------->
<- T -> <-N-><----- M ----->
+---- ... ---+- ... -+-----+---- ... ----+---...---+
| Rule ID | DTag | 1 | MIC | payload |
+---- ... ---+- ... -+-----+---- ... ----+---...---+
Figure 14: All-1 Fragmentation Header for the Last Fragment, No ACK
option
In any of the Window modes, the last fragment of an IPv6 datagram
SHALL contain a fragmentation header that conforms to the format
shown in Figure 15. The total size of this fragmentation header is
R+M bits. It is used for retransmissions
<------------ R ------------>
<- T -> 1 <- N -> <---- M ----->
+-- ... --+- ... -+-+- ... -+---- ... ----+---...---+
| Rule ID | DTag |W| 11..1 | MIC | payload |
+-- ... --+- ... -+-+- ... -+---- ... ----+---...---+
(FCN)
Figure 15: All-1 Fragmentation Header for the Last Fragment, Window
mode
Minaburo, et al. Expires April 23, 2018 [Page 20]
Internet-Draft LPWAN SCHC October 2017
The values for R, N, T and M are not specified in this document, and
have to be determined in other documents (e.g. technology-specific
profile documents).
<------------ R ------------>
<- T -> 1 <- N -> <---- M ----->
+-- ... --+- ... -+-+- ... -+---- ... ----+
| Rule ID | DTag |W| 1..1 | MIC | (no payload) TODO
+-- ... --+- ... -+-+- ... -+---- ... ----+
Figure 16: All-1 for Retries format fragment also called All-1 empty
<------------ R ------------>
<- T -> 1 <- N ->
+-- ... --+- ... -+-+- ... -+
| Rule ID | DTag |W| 11..1 | (no MIC and no payload) TODO
+-- ... --+- ... -+-+- ... -+
Figure 17: All-1 for Abort format fragment
<----- Complete Byte ------><--- 1 byte --->
<------- R ------->
<- T -> 1
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |W| 1..1| FF | TODO
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18: ACK Abort format fragment
5.5. Baseline mechanism
The receiver needs to identify all the fragments that belong to a
given IPv6 datagram. To this end, the receiver SHALL use: * The
sender's L2 source address (if present), * The destination's L2
address (if present), * Rule ID and * DTag (the latter, if present).
Then, the fragment receiver may determine the fragment delivery
reliability option that is used for this fragment based on the Rule
ID field in that fragment.
Upon receipt of a link fragment, the receiver starts constructing the
original unfragmented packet. It uses the FCN and the order of
arrival of each fragment to determine the location of the individual
fragments within the original unfragmented packet. A fragment
payload may carry bytes from a SCHC compressed IPv6 header, an
uncompressed IPv6 header or an IPv6 datagram data payload. An
unfragmented packet could be a SCHC compressed or an uncompressed
IPv6 packet (header and data). For example, the receiver may place
the fragment payload within a payload datagram reassembly buffer at
Minaburo, et al. Expires April 23, 2018 [Page 21]
Internet-Draft LPWAN SCHC October 2017
the location determined from: the FCN, the arrival order of the
fragments, and the fragment payload sizes. In Window mode, the
fragment receiver also uses the W bit in the received fragments.
Note that the size of the original, unfragmented packet cannot be
determined from fragmentation headers.
Note that, in Window mode, the first fragment of the window is the
one with FCN set to MAX_WIND_FCN. Also note that, in Window mode,
the fragment with all-0 is considered the last fragment of its
window, except for the last fragment of the whole packet (all-1),
which is also the last fragment of the last window.
If the recipient receives the last fragment of a datagram (all-1), it
checks for the integrity of the reassembled datagram, based on the
MIC received. In No ACK, if the integrity check indicates that the
reassembled datagram does not match the original datagram (prior to
fragmentation), the reassembled datagram MUST be discarded. In
Window mode, a MIC check is also performed by the fragment receiver
after reception of each subsequent fragment retransmitted after the
first MIC check. In ACK always, if a MIC check indicates that the
datagram has been successfully reassembled, the fragment receiver
sends an ACK without a bitmap to the fragment sender.
If a fragment recipient disassociates from its L2 network, the
recipient MUST discard all link fragments of all partially
reassembled payload datagrams, and fragment senders MUST discard all
not yet transmitted link fragments of all partially transmitted
payload (e.g., IPv6) datagrams. Similarly, when either end of the
LPWAN link first receives a fragment of a packet, it starts a
reassembly timer. When this time expires, if the entire packet has
not been reassembled, the existing fragments MUST be discarded and
the reassembly state MUST be flushed. The value for this timer is
not provided by this specification, and is expected to be defined in
technology-specific profile documents.
TODO (explain the Bitmap optimization)
5.6. Supporting multiple window sizes
For Window mode operation, implementers may opt to support a single
window size or multiple window sizes. The latter, when feasible, may
provide performance optimizations. For example, a large window size
may be used for packets that need to be carried by a large number of
fragments. However, when the number of fragments required to carry
an packet is low, a smaller window size, and thus a shorter bitmap,
may be sufficient to provide feedback on all fragments. If multiple
window sizes are supported, the Rule ID may be used to signal the
window size in use for a specific packet transmission.
Minaburo, et al. Expires April 23, 2018 [Page 22]
Internet-Draft LPWAN SCHC October 2017
TODO (does it works for ACK-on-error?)
5.7. Aborting fragmented datagram transmissions
For several reasons, a fragment sender or a fragment receiver may
want to abort the on-going transmission of one or several fragmented
IPv6 datagrams.
TODO (explain the abort format packets)
Upon transmission or reception of the abortion signal, both entities
MUST release any resources allocated for the fragmented datagram
transmissions being aborted.
5.8. Downlink 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 for
fragmented datagram transmission in the downlink, the fragment
receiver MAY perform an uplink transmission as soon as possible after
reception of a fragment that is not the last one. Such uplink
transmission may be triggered by the L2 (e.g. an L2 ACK sent in
response to a fragment encapsulated in a L2 frame that requires an L2
ACK) or it may be triggered from an upper layer.
5.9. Fragmentation Mode of Operation Description
The fragmentation is based on the FCN value, which has a length of N
bits. The All-1 and All-0 values are reserved, and are used to
control the fragmentation transmission. The FCN will be sent in
downwards position this means from larger to smaller and the number
of bits depends on the implementation. The last fragment in all
modes must contains a MIC which is used to check if there are error
or missing fragments.
5.9.1. No ACK Mode
In the No ACK mode there is no feedback communication. The sender
will send the fragments until the last one whithout any possibility
to know if there were an error or lost. As there is not any control
one bit FCN is used, where FCN all-0 will be sent for all the
fragments except the last one which will use FCN to all-1 and will
send the MIC. Figure 19 shows the state machine for the sender.
Minaburo, et al. Expires April 23, 2018 [Page 23]
Internet-Draft LPWAN SCHC October 2017
+-----------+
+------------+ Init |
| FCN=0 +-----------+
| No Window
| No Bitmap
| +-------+
| +--------+--+ | More Fragments
| | | <--+ ~~~~~~~~~~~~~~~~~~~~
+--------> | Send | send Fragment (FCN=0)
+---+-------+
| last fragment
| ~~~~~~~~~~~~
| FCN = 1
v send fragment+MIC
+------------+
| END |
+------------+
Figure 19: Sender State Machine for the No ACK Mode
The receiver waits for fragments and will set a timer in order to see
if there is no missing fragments. The No ACK mode will use the MIC
contained in the last fragment to check error. The FCN is set to
all-1 for the last fragment. Figure 20 shows the state machine for
the receiver. When the Timer expires or when the check of MIC gives
an error it will abort the communication and go to error state, all
the fragments will be dropped. The Inactivity Timer will be based on
the LPWAN technology and will be defined in the specific technology
document.
+------+ Not All-1
+----------+-+ | ~~~~~~~~~~~~~~~~~~~
| + <--+ set Inactivity Timer
| RCV Frag +-------+
+-+---+------+ |All-1 &
All-1 & | | |MIC correct
MIC wrong | |Inactivity |
| |Timer Exp. |
v | |
+----------++ | v
| Error |<-+ +--------+--+
+-----------+ | END |
+-----------+
Figure 20: Receiver State Machine for the No ACK Mode
Minaburo, et al. Expires April 23, 2018 [Page 24]
Internet-Draft LPWAN SCHC October 2017
5.9.2. The Window modes
The jumping window protocol is using two windows alternatively 0 and
1. The FCN to all-0 fragment means that the window is over and
allows to switch from one window to another. The FCN to all-1
fragment indicates that it is the last fragment and there will not be
another window.
In all the cases, the sender may not have to send all the fragments
contained in the window. To ease FN (fragment number) reconstruction
from FCN, it is recommended to send sequentially all the fragments on
a window and for all non-terminating window to fill entirely the
window.
The receiver generates the bitmap which may have the size of a single
frame based on the size of downlink frame of the LPWAN technology
used. When the bitmap cannot be sent in one frame or for the last
window,
, then first the FCN should be set to the lowest possible value.
The Window mode has two different mode of operation: The ACK on error
and the ACK always.
5.9.3. ACK Always
The Figure 21 finite state machine describes the sender behavior.
Intially, when a fragmented packet need to be sent, the window is set
to 0, a local_bit map is set to 0, and FCN is set the the highest
possible value depending on the number of fragment that will be sent
in the window (INIT STATE).
The sender starts sending fragments (SEND STATE), the sender will
indicate in the fragmentation header: the current window and the FCN
number. A delay between each fragment can be added to respect
regulation rules or constraints imposed by the applications. Each
time a fragment is sent the FCN is decreased of one value and the
bitmap is set. The send state can be leaved for different reasons
(for both reasons it goes to WAIT BITMAP STATE):
o The FCN reaches value 0 and there are more fragments. In that
case an all-0 fragmet is sent and the timer is set. The sender
will wait for the bitmap acknowledged by the receiver.
o The last fragment is sent. In that case an all-1 fragment with
the MIC is sent and the sender will wait for the bitmap
acknowledged by the receiver. The sender set a timer to wait for
the ack.
Minaburo, et al. Expires April 23, 2018 [Page 25]
Internet-Draft LPWAN SCHC October 2017
During the transition between the SEND state of the current window
and the WAIT BITMAP, the sender start listening to the radio and
start a timer. This timer is dimensioned to the receiving window
depending on the LPWAN technology.
In ACK Always, if the timer expire, an empty All-0 (or All-1 if the
last fragment has been sent) fragment is sent to ask the receiver to
resent its bitmap. The window number is not changed.
The sender receives a bitmap, it checks the window value.
Acknowledgment with the non expected window are discarded.
If the window number on the received bitmap is correct, the sender
compares the local bitmap with the received bitmap. If they are
equal all the fragments sent during the window have been well
received. If at least one fragment need to be sent, the sender clear
the bitmap, stop the timer and move its sending window to the next
value. If no more fragments have to be sent, then the fragmented
packet transmission is terminated.
If some fragments are missing (not set in the bit map) then the
sender resend the missing fragments. When the retransmission is
finished, it start listening to the bitmap (even if a All-0 or All-1
has not been sent during the retransmission) and returns to the
waiting bitmap state.
If the local-bitmap is different from the received bitmap the counter
Attemps is increased and the sender resend the missing fragments
again, when a MAX_ATTEMPS is reached the sender sends an Abort and
goes to error.
Minaburo, et al. Expires April 23, 2018 [Page 26]
Internet-Draft LPWAN SCHC October 2017
+-------+
| 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 Timer | | set Timer
| | |
|Recv_wnd == wnd & | |
|Lcl_bitmap==recv_bitmap& | | +-------------------------+
|more frag | | |local-bitmap!=rcv-bitmap |
|~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ |
|Stop Timer | | | Attemp++ v
|clear local_bitmap v v | +------++
|window=next_window +----+-----+--+--+ |Resend |
+---------------------+ | |Missing|
+----+ Wait | |Frag |
not expected wnd | | bitmap | +------++
~~~~~~~~~~~~~~~~ +--->+ +---+ Timer Exp |
discard frag +--+---+---+---+-+ |~~~~~~~~~~~~~~~~~ |
| | ^ ^ |Snd(empty)frag(0) |
| | | | |Set Timer |
| | | +-----+ |
Recv_window==window & | | +----------------------------+
Lcl_bitmap==recv_bitmap &| | all missing frag sent
no more frag| | ~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~| | Set Timer
Stop Timer| |
+-------------+ | |
| +<----+ | MAX_ATTEMPS > limit
| END | | ~~~~~~~~~~~~~~~~~~
| | v Send Abort
+-------------+ +-+-----------+
| ERROR |
+-------------+
Figure 21: Sender State Machine for the ACK Always Mode
The Figure 22 finite state machine describes the receiver behavior.
The receiver starts with window 0 as the expecting window and
Minaburo, et al. Expires April 23, 2018 [Page 27]
Internet-Draft LPWAN SCHC October 2017
maintain a local_bitmap indicating which fragments it has received
(all-0 and all-1 occupy the same position).
Any fragment not belonging to the current window is discarded.
Fragment belonging to the correct window are accepted, FN is computed
based on the FCN value. The receiver leaves this state when
receiving a:
o All-0 fragment which indicates that all the fragments have been
sent in the current window. Since the sender is not obliged to
send a full window, some fragment number not set in the
local_bitmap may not correspond to losses.
o All-1 fragment which indicated that the transmission is finished.
Since the last window is not full, the MIC will be used to detect
if all the fragments have been received.
A correct MIC indicates the end of the transmission. The receiver
must stay in this state during a period of time to answer to empty
all-1 frag the sender may send if the bitmap is lost.
If All-1 frag has not been received, the receiver expect a new
window. It waits for the next fragment. If the window value has not
changed, the received fragments are part of a retransmission. A
receiver that has already received a frag should discard it (not
represented in the state machine), otherwise it completes its bitmap.
If all the bit of the bitmap are set to one, the receiver may send a
bitmap without waiting for a all-0 frag.
If the window value is set to the next value, this means that the
sender has received a correct bitmap, which means that all the
fragments have been received. The receiver change the value of the
expected window.
If the receiver receives an all-0 fragment, it stays in the same
state. Sender may send more one fragment per window or more.
Otherwise some fragments in the window have been lost.
If the receiver receives an all-1 fragment this means that the
transmission should be finished. If the MIC is incorrect some
fragments have been lost. It sends its bitmap.
In case of an incorrect MIC, the receivers wait for fragment
belonging to the same window.
Minaburo, et al. Expires April 23, 2018 [Page 28]
Internet-Draft LPWAN SCHC October 2017
Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set local_bitmap(FCN) | v v |discard
++---+---+---+-+
+---------------------+ Rcv |
| +------------------+ 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 v | v | | | expct = nxt wnd
| | +-+-+-+--+-++ | Clear lcl_bitmap
| | w=expected & +->+ Wait +<+ set lcl_bitmap(FCN)
| | All-1 | | Next | send lcl_bitmap
| | ~~~~~~~~~~~~ +--+ Window |
| | discard +--------+-++
| | 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)|
| +--------------------->+ | |
| +---+----+-+ |
| w=expected & MIC right| |
| ~~~~~~~~~~~~~~~~~~~~~~| +-+ Not All-1 |
| set local_bitmap(FCN)| | | ~~~~~~~~~ |
| send local_bitmap| | | discard |
| | | | |
|All-1 & w=expctd & MIC right | | | +-+ All-1 |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v | v ~~~~~~~~~ |
|set local_bitmap(FCN) +-+-+-+-+-++Send lcl_btmp |
|send local_bitmap | | |
+-------------------------->+ END +<---------------+
++--+------+
Figure 22: Receiver State Machine for the ACK Always Mode
Minaburo, et al. Expires April 23, 2018 [Page 29]
Internet-Draft LPWAN SCHC October 2017
5.9.4. ACK on error
The ACK on error sender is very similar to the ACK always sender,
Intially, when a fragmented packet is sent, the window is set to 0, a
local_bit map is set to 0, and FCN is set the highest possible value
depending on the number of fragment that will be sent in the window.
See Figure 23
The sender starts sending fragments indicating in the fragmentation
header with the current window and the FCN number. A delay between
each fragment can be added to respect regulation rules or constraints
imposed by the applications. This state can be leaved for different
reasons:
o The FCN reaches value 0. In that case a all-0 fragmet is sent and
the sender will wait for the bitmap acknowledged by the receiver.
o The last fragment is sent. In that case a all-1 fragment is sent
and the sender will wait for the bitmap acknowledged by the
receiver.
During the transition between the sending the fragment of the current
window and waiting for bitmap, the sender start listening to the
radio and start a timer. This timer is dimensioned to the receiving
window depending on the LPWAN technology.
In Ack on error mode, the timer expiration will be considered as a
positive acknowledgment. If there are no more fragments then the
fragmentation is finished.
If the sender receives a bitmap, it checks the window value.
Acknowledgment with the non expected window are discarded.
If the window number on the received bitmap is correct, the sender
compare the local bitmap with the received bitmap. If they are equal
all the fragments sent during the window have been well received. If
at least one fragment need to be sent, the sender clear the bitmap,
stop the timer and move its sending window to the next value. If no
more fragments have to be sent, then the fragmented packet
transmission is terminated.
If some fragments are missing (not set in the bit map) then the
sender resend the missing fragments. When the retransmission is
finished, it start listening to the bitmap (even if a All-0 or All-1
has not been sent during the retransmission) and returns to the
waiting bitmap state.
Minaburo, et al. Expires April 23, 2018 [Page 30]
Internet-Draft LPWAN SCHC October 2017
If the local-bitmap is different from the received bitmap the counter
Attemps is increased and the sender resend the missing fragments
again, when a MAX_ATTEMPS is reached the sender sends an Abort and
goes to error.
Minaburo, et al. Expires April 23, 2018 [Page 31]
Internet-Draft LPWAN SCHC October 2017
+-------+
| |
| 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 Timer| |set Timer
| | |
|Timer expires & | | local-bitmap!=rcv-bitmap
|more fragments | | +-----------------+
|~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~~~~~ |
|stop Timer | | | Attemp++ |
|clear local.bitmap v v | v
|window = next window +-----+-----+--+--+ +----+----+
+---------------------->+ + | Resend |
| Wait bitmap | | Missing |
+-- + | | Frag |
not expected wnd | ++-+-------+---+--+ +------+--+
~~~~~~~~~~~~~~~~ | ^ | | ^ |
discard frag +----+ | | +-------------------+
| | all missing frag sent
| | ~~~~~~~~~~~~~~~~~~~~~
Timer expires & | | Set Timer
No more Frag | |
~~~~~~~~~~~~~~~~ | |
Stop Timer | | MAX_ATTEMPS > limit
+-----------+ | | ~~~~~~~~~~~~~~~~~~
| +<--------+ | Send Abort
| END | v
+-----------+ +-+----------+
| ERROR |
+------------+
Figure 23: Sender State Machine for the ACK on error Mode
Unlike the sender, the receiver for ACK on error has some
differences. First we are not sending the bitmap unless there is an
Minaburo, et al. Expires April 23, 2018 [Page 32]
Internet-Draft LPWAN SCHC October 2017
error or an unexpected behavior. The Figure 24 finite state machine
describes the receiver behavior. The receiver starts with an the
expecting window and maintain a local_bitmap indicating which
fragments it has received (all-0 and all-1 occupy the same position).
Any fragment not belonging to the current window is discarded.
Fragment belonging to the correct window are accepted, FN is computed
based on the FCN value. When an All-0 fragment is received and the
bitmap is full the receiver changes the window value and clear the
bitmap. The receiver leaves this state when receiving a:
o All-0 fragment and not a full bitmap indicate that all the
fragments have been sent in the current window. Since the sender
is not obliged to send a full window, some fragment number not set
in the local_bitmap may not correspond to losses. As the receiver
does not know if the missing fragments are looses or normal
missing fragments it sned s a local bitmap.
o All-1 fragment which indicates that the transmission is finished.
Since the last window is not full, the MIC will be used to detect
if all the fragments have been received. A correct MIC indicates
the end of the transmission.
If All-1 frag has not been received, the receiver expect a new
window. It waits for the next fragment. If the window value has not
changed, the received fragments are part of a retransmission. A
receiver that has already received a frag should discard it (not
represented in the state machine), otherwise it completes its bitmap.
If all the bits of the bitmap are set to one, the receiver clear the
bitmap and wait for the next window without waiting for a all-0 frag.
While the receiver waits for next window and if the window value is
set to the next value, and all-1 fragment with the next value window
arrived the receiver goes to error and abort the transmission, it
drops the fragments.
If the receiver receives an all-0 fragment, it stays in the same
state. Sender may send more one fragment per window or more.
Otherwise some fragments in the window have been lost.
If the receiver receives an all-1 fragment this means that the
transmission should be finished. If the MIC is incorrect some
fragments have been lost. It sends its bitmap.
In case of an incorrect MIC, the receivers wait for fragment
belonging to the same window.
Minaburo, et al. Expires April 23, 2018 [Page 33]
Internet-Draft LPWAN SCHC October 2017
Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set local_bitmap(FCN) | v v |discard
++---+---+---+-+
+-----------------------+ +--+ All-0 & full
| | Rcv Window | | ~~~~~~~~~~~~
| +--------------------+ +<-+ w =next
| | +---+---+------+ clear lcl_bitmap
| | | ^
| | All-0 & w=expect| |w=expct & not-All & full
| | & no_full bitmap| |~~~~~~~~~~~~~~~~~~~~~~~~
| | ~~~~~~~~~~~~~~~~~| |clear lcl_bitmap; w =nxt
| | send local_bitmap| |
| | | | +--------+
| | | | +---------->+ |
| | | | |w=next | Error/ |
| | | | |~~~~~~~~ | Abort |
| | | | |Send abort ++-------+
| | v | | ^ w=expct
| | +-+---+--+------+ | & all-1
| | | Wait +------+ ~~~~~~~
| | | Next Window | Send abort
| | +-------+---+---+
| | All-1 & w=next & MIC wrong | |
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~ | +----------------+
| | set local_bitmap(FCN) | All-1 & w=next|
| | send local_bitmap | & MIC right|
| | | ~~~~~~~~~~~~~~~~~~|
| | | set lcl_bitmap(FCN)|
| |All-1 & w=expect & MIC wrong | |
| |~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
| |set local_bitmap(FCN) v |
| |send local_bitmap +-------+------+ |
| +--------------------->+ Wait End +-+ |
| +-----+------+-+ | w=expct & |
| w=expected & MIC right | ^ | MIC wrong |
| ~~~~~~~~~~~~~~~~~~~~~~ | +---+ ~~~~~~~~~ |
| set local_bitmap(FCN) | set lcl_bitmap(FCN)|
| | |
|All-1 & w=expected & MIC right | |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v |
|set local_bitmap(FCN) +-+----------+ |
+---------------------------->+ END +<----------+
+------------+
Figure 24: Receiver State Machine for the ACK on error Mode
Minaburo, et al. Expires April 23, 2018 [Page 34]
Internet-Draft LPWAN SCHC October 2017
6. SCHC Compression for IPv6 and UDP headers
This section lists the different IPv6 and UDP header fields and how
they can be compressed.
6.1. IPv6 version field
This field always holds the same value, therefore the TV is 6, the MO
is "equal" and the "CDA "not-sent"".
6.2. IPv6 Traffic class field
If the DiffServ field identified by the rest of the rule do not vary
and is known by both sides, the TV should contain this well-known
value, the MO should be "equal" and the CDA must be "not-sent.
If the DiffServ field identified by the rest of the rule varies over
time or is not known by both sides, then there are two possibilities
depending on the variability of the value, the first one is to do not
compressed the field and sends the original value, or the second
where the values can be computed by sending only the LSB bits:
o TV is not set to any value, MO is set to "ignore" and CDA is set
to "value-sent"
o TV contains a stable value, MO is MSB(X) and CDA is set to LSB
6.3. Flow label field
If the Flow Label field identified by the rest of the rule does not
vary and is known by both sides, the TV should contain this well-
known value, the MO should be "equal" and the CDA should be "not-
sent".
If the Flow Label field identified by the rest of the rule varies
during time or is not known by both sides, there are two
possibilities depending on the variability of the value, the first
one is without compression and then the value is sent and the second
where only part of the value is sent and the decompressor needs to
compute the original value:
o TV is not set, MO is set to "ignore" and CDA is set to "value-
sent"
o TV contains a stable value, MO is MSB(X) and CDA is set to LSB
Minaburo, et al. Expires April 23, 2018 [Page 35]
Internet-Draft LPWAN SCHC October 2017
6.4. Payload Length field
If the LPWAN technology does not add padding, this field can be
elided for the transmission on the LPWAN network. The SCHC C/D
recomputes the original payload length value. The TV is not set, the
MO is set to "ignore" and the CDA is "compute-IPv6-length".
If the payload length needs to be sent and does not need to be coded
in 16 bits, the TV can be set to 0x0000, the MO set to "MSB (16-s)"
and the CDA to "LSB". The 's' parameter depends on the expected
maximum packet length.
On other cases, the payload length field must be sent and the CDA is
replaced by "value-sent".
6.5. Next Header field
If the Next Header field identified by the rest of the rule does not
vary and is known by both sides, the TV should contain this Next
Header value, the MO should be "equal" and the CDA should be "not-
sent".
If the Next header field identified by the rest of the rule varies
during time or is not known by both sides, then TV is not set, MO is
set to "ignore" and CDA is set to "value-sent". A matching-list may
also be used.
6.6. Hop Limit field
The End System is generally a device and does not forward packets,
therefore the Hop Limit value is constant. So the TV is set with a
default value, the MO is set to "equal" and the CDA is set to "not-
sent".
Otherwise the value is sent on the LPWAN: TV is not set, MO is set to
ignore and CDA is set to "value-sent".
Note that the field behavior differs in upstream and downstream. In
upstream, since there is no IP forwarding between the Dev and the
SCHC C/D, the value is relatively constant. On the other hand, the
downstream value depends of Internet routing and may change more
frequently. One solution could be to use the Direction Indicator
(DI) to distinguish both directions to elide the field in the
upstream direction and send the value in the downstream direction.
Minaburo, et al. Expires April 23, 2018 [Page 36]
Internet-Draft LPWAN SCHC October 2017
6.7. 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 a single rule,
these values are identified by their role (DEV or APP) and not by
their position in the frame (source or destination). The SCHC C/D
must be aware of the traffic direction (upstream, downstream) to
select the appropriate field.
6.7.1. IPv6 source and destination prefixes
Both ends must be synchronized with the appropriate prefixes. For a
specific flow, the source and destination prefix can be unique and
stored in the context. It can be either a link-local prefix or a
global prefix. In that case, the TV for the source and destination
prefixes contains the values, the MO is set to "equal" and the CDA is
set to "not-sent".
In case the rule allows several prefixes, mapping-list must be used.
The different prefixes are listed in the TV associated with a short
ID. The MO is set to "match-mapping" and the CDA is set to "mapping-
sent".
Otherwise the TV contains the prefix, the MO is set to "equal" and
the CDA is set to value-sent.
6.7.2. 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 is
generally carrying a single device identifier corresponding to the
DEV. The SCHC C/D may also not be aware of these values.
If the DEV address has a static value that is not derived from an
IEEE EUI-64, then TV contains the actual Dev address value, the MO
operator is set to "equal" and the CDA is set to "not-sent".
If several IIDs are possible, then the TV contains the list of
possible IIDs, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent".
Otherwise the value variation of the IID may be reduced to few bytes.
In that case, the TV is set to the stable part of the IID, the MO is
set to MSB and the CDA is set to LSB.
Minaburo, et al. Expires April 23, 2018 [Page 37]
Internet-Draft LPWAN SCHC October 2017
Finally, the IID can be sent on the LPWAN. In that case, the TV is
not set, the MO is set to "ignore" and the CDA is set to "value-
sent".
6.8. IPv6 extensions
No extension rules are currently defined. They can be based on the
MOs and CDAs described above.
6.9. UDP source and destination port
To allow a single rule, the UDP port values are identified by their
role (DEV or APP) and not by their position in the frame (source or
destination). The SCHC C/D must be aware of the traffic direction
(upstream, downstream) to select the appropriate field. The
following rules apply for DEV and APP port numbers.
If both ends know the port number, it can be elided. The TV contains
the port number, the MO is set to "equal" and the CDA is set to "not-
sent".
If the port variation is on few bits, the TV contains the stable part
of the port number, the MO is set to "MSB" and the CDA is set to
"LSB".
If some well-known values are used, the TV can contain the list of
this values, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent".
Otherwise the port numbers are sent on the LPWAN. The TV is not set,
the MO is set to "ignore" and the CDA is set to "value-sent".
6.10. UDP length field
If the LPWAN technology does not introduce padding, the UDP length
can be computed from the received data. In that case the TV is not
set, the MO is set to "ignore" and the CDA is set to "compute-UDP-
length".
If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB".
On other cases, the length must be sent and the CDA is replaced by
"value-sent".
Minaburo, et al. Expires April 23, 2018 [Page 38]
Internet-Draft LPWAN SCHC October 2017
6.11. 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 fragmentation process (see section
Section 5)), the UDP checksum transmission can be avoided. In that
case, the TV is not set, the MO is set to "ignore" and the CDA is set
to "compute-UDP-checksum".
In other cases the checksum must be explicitly sent. The TV is not
set, the MO is set to "ignore" and the CDF is set to "value-sent".
7. Security considerations
7.1. 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
corruption may be detected with end-to-end authentication and
integrity mechanisms. Denial of Service may be produced but its
arise other security problems that may be solved with or without
header compression.
7.2. Security considerations for fragmentation
This subsection describes potential attacks to LPWAN fragmentation
and suggests possible countermeasures.
A node can perform a buffer reservation attack by sending a first
fragment to a target. Then, the receiver will reserve buffer space
for the IPv6 packet. Other incoming fragmented packets will be
dropped while the reassembly buffer is occupied during the reassembly
timeout. Once that timeout expires, the attacker can repeat the same
procedure, and iterate, thus creating a denial of service attack.
The (low) cost to mount this attack is linear with the number of
buffers at the target node. However, the cost for an attacker can be
increased if individual fragments of multiple packets can be stored
in the reassembly buffer. To further increase the attack cost, the
reassembly buffer can be split into fragment-sized buffer slots.
Once a packet is complete, it is processed normally. If buffer
overload occurs, a receiver can discard packets based on the sender
behavior, which may help identify which fragments have been sent by
an attacker.
In another type of attack, the malicious node is required to have
overhearing capabilities. If an attacker can overhear a fragment, it
can send a spoofed duplicate (e.g. with random payload) to the
destination. If the LPWAN technology does not support suitable
Minaburo, et al. Expires April 23, 2018 [Page 39]
Internet-Draft LPWAN SCHC October 2017
protection (e.g. source authentication and frame counters to prevent
replay attacks), a receiver cannot distinguish legitimate from
spoofed fragments. Therefore, the original IPv6 packet will be
considered corrupt and will be dropped. To protect resource-
constrained nodes from this attack, it has been proposed to establish
a binding among the fragments to be transmitted by a node, by
applying content-chaining to the different fragments, based on
cryptographic hash functionality. The aim of this technique is to
allow a receiver to identify illegitimate fragments.
Further attacks may involve sending overlapped fragments (i.e.
comprising some overlapping parts of the original IPv6 datagram).
Implementers should make sure that correct operation is not affected
by such event.
In Window mode - ACK on error, a malicious node may force a fragment
sender to resend a fragment a number of times, with the aim to
increase consumption of the fragment sender's resources. To this
end, the malicious node may repeatedly send a fake ACK to the
fragment sender, with a bitmap that reports that one or more
fragments have been lost. In order to mitigate this possible attack,
MAX_FRAG_RETRIES may be set to a safe value which allows to limit the
maximum damage of the attack to an acceptable extent. However, note
that a high setting for MAX_FRAG_RETRIES benefits fragment delivery
reliability, therefore the trade-off needs to be carefully
considered.
8. Acknowledgements
Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier,
Arunprabhu Kandasamy, Antony Markovski, Alexander Pelov, Pascal
Thubert, Juan Carlos Zuniga and Diego Dujovne for useful design
consideration and comments.
9. References
9.1. 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>.
[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>.
Minaburo, et al. Expires April 23, 2018 [Page 40]
Internet-Draft LPWAN SCHC October 2017
[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>.
9.2. Informative References
[I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-07 (work in progress), October 2017.
Appendix A. SCHC Compression Examples
This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the SCHC behavior.
The most common case using the mechanisms defined in this document
will be a LPWAN Dev that embeds some applications running over CoAP.
In this example, three flows are considered. The first flow is for
the device management based on CoAP using Link Local IPv6 addresses
and UDP ports 123 and 124 for Dev and App, respectively. The second
flow will be a CoAP server for measurements done by the Device (using
ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to
beta::1/64. The last flow is for legacy applications using different
ports numbers, the destination IPv6 address prefix is gamma::1/64.
Figure 25 presents the protocol stack for this Device. IPv6 and UDP
are represented with dotted lines since these protocols are
compressed on the radio link.
Minaburo, et al. Expires April 23, 2018 [Page 41]
Internet-Draft LPWAN SCHC October 2017
Management Data
+----------+---------+---------+
| CoAP | CoAP | legacy |
+----||----+---||----+---||----+
. UDP . UDP | UDP |
................................
. IPv6 . IPv6 . IPv6 .
+------------------------------+
| SCHC Header compression |
| and fragmentation |
+------------------------------+
| LPWAN L2 technologies |
+------------------------------+
DEV or NGW
Figure 25: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the Devs have a device ID.
Therefore, when such technologies are used, it is necessary to define
statically an IID for the Link Local address for the SCHC C/D.
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 || |
|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]|
Minaburo, et al. Expires April 23, 2018 [Page 42]
Internet-Draft LPWAN SCHC October 2017
+----------------+--+--+--+---------+--------+------------++------+
|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) || [4] |
|UDP APPport |16|1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
|UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Figure 26: Context rules
Minaburo, et al. Expires April 23, 2018 [Page 43]
Internet-Draft LPWAN SCHC October 2017
All the fields described in the three rules depicted on Figure 26 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 of different fragment delivery
reliability options possible on the basis of this specification.
Figure 27 illustrates the transmission of an IPv6 packet that needs
11 fragments in the No ACK option, FCN is always 1 bit.
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 checked =>
Figure 27: Transmission of an IPv6 packet carried by 11 fragments in
the No ACK option
Figure 28 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK on error, for N=3, without losses.
Minaburo, et al. Expires April 23, 2018 [Page 44]
Internet-Draft LPWAN SCHC October 2017
Sender Receiver
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4----->|
|-----W=1, FCN=3----->|
|-----W=1, FCN=2----->|
|-----W=1, FCN=1----->|
|-----W=1, FCN=0----->|
(no ACK)
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4----->|
|-----W=0, FCN=7----->|MIC checked =>
(no ACK)
Figure 28: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3 and MAX_WIND_FCN=6, without
losses.
Figure 29 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK on error, for N=3, with three
losses.
Sender Receiver
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->|
|-----W=1, FCN=3----->|
|-----W=1, FCN=2--X-->|
|-----W=1, FCN=1----->|
|-----W=1, FCN=0----->|
|<-----ACK, W=1-------|Bitmap:11010111
|-----W=1, FCN=4----->|
|-----W=1, FCN=2----->|
(no ACK)
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=7----->|MIC checked
|<-----ACK, W=0-------|Bitmap:11000001
|-----W=0, FCN=4----->|MIC checked =>
(no ACK)
Figure 29: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3 and MAX_WIND_FCN=6, three losses.
Minaburo, et al. Expires April 23, 2018 [Page 45]
Internet-Draft LPWAN SCHC October 2017
Figure 30 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK "always", for N=3 and
MAX_WIND_FCN=6, without losses. Note: in Window mode, an additional
bit will be needed to number windows.
Sender Receiver
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4----->|
|-----W=1, FCN=3----->|
|-----W=1, FCN=2----->|
|-----W=1, FCN=1----->|
|-----W=1, FCN=0----->|
|<-----ACK, W=1-------|no bitmap
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4----->|
|-----W=0, FCN=7----->|MIC checked =>
|<-----ACK, W=0-------|no bitmap
(End)
Figure 30: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6, no losses.
Figure 31 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK "always", for N=3 and
MAX_WIND_FCN=6, with three losses.
Minaburo, et al. Expires April 23, 2018 [Page 46]
Internet-Draft LPWAN SCHC October 2017
Sender Receiver
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->|
|-----W=1, FCN=3----->|
|-----W=1, FCN=2--X-->|
|-----W=1, FCN=1----->|
|-----W=1, FCN=0----->|
|<-----ACK, W=1-------|bitmap:11010111
|-----W=1, FCN=4----->|
|-----W=1, FCN=2----->|
|<-----ACK, W=1-------|no bitmap
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001
|-----W=0, FCN=4----->|MIC checked =>
|<-----ACK, W=0-------|no bitmap
(End)
Figure 31: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
losses.
Figure 32 illustrates the transmission of an IPv6 packet that needs 6
fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
with three losses, and only one retry is needed for each lost
fragment. Note that, since a single window is needed for
transmission of the IPv6 packet in this case, the example illustrates
behavior when losses happen in the last window.
Minaburo, et al. Expires April 23, 2018 [Page 47]
Internet-Draft LPWAN SCHC October 2017
Sender Receiver
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=3--X-->|
|-----W=0, CFN=2--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked: failed
|-----W=0, CFN=3----->|MIC checked: failed
|-----W=0, CFN=2----->|MIC checked: success
|<-----ACK, W=0-------|no bitmap
(End)
Figure 32: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
losses, and only one retry is needed for each lost fragment.
Figure 33 illustrates the transmission of an IPv6 packet that needs 6
fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
with three losses, and the second ACK is lost. Note that, since a
single window is needed for transmission of the IPv6 packet in this
case, the example illustrates behavior when losses happen in the last
window.
Sender Receiver
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=3--X-->|
|-----W=0, CFN=2--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked: wrong
|-----W=0, CFN=3----->|MIC checked: wrong
|-----W=0, CFN=2----->|MIC checked: right
| X---ACK, W=0-------|no bitmap
timeout | |
|-----W=0, CFN=7----->|
|<-----ACK, W=0-------|no bitmap
(End)
Figure 33: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
losses, and the second ACK is lost.
Minaburo, et al. Expires April 23, 2018 [Page 48]
Internet-Draft LPWAN SCHC October 2017
Figure 34 illustrates the transmission of an IPv6 packet that needs 6
fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
with three losses, and one retransmitted fragment is lost. Note
that, since a single window is needed for transmission of the IPv6
packet in this case, the example illustrates behavior when losses
happen in the last window.
Sender Receiver
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=3--X-->|
|-----W=0, CFN=2--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked: wrong
|-----W=0, CFN=3----->|MIC checked: wrong
|-----W=0, CFN=2--X-->|
timeout| |
|-----W=0, CFN=7----->|
|<-----ACK, W=0-------|bitmap:11110001
|-----W=0, CFN=2----->|MIC checked: right
|<-----ACK, W=0-------|no bitmap
(End)
Figure 34: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
losses, and one retransmitted fragment is lost.
Appendix C illustrates the transmission of an IPv6 packet that needs
28 fragments in Window mode - ACK "always", for N=5 and
MAX_WIND_FCN=23, with two losses. Note that MAX_WIND_FCN=23 may be
useful when the maximum possible bitmap size, considering the maximum
lower layer technology payload size and the value of R, is 3 bytes.
Note also that the FCN of the last fragment of the packet is the one
with FCN=31 (i.e. FCN=2^N-1 for N=5, or equivalently, all FCN bits
set to 1).
Minaburo, et al. Expires April 23, 2018 [Page 49]
Internet-Draft LPWAN SCHC October 2017
Sender Receiver
|-----W=1, CFN=23----->|
|-----W=1, CFN=22----->|
|-----W=1, CFN=21--X-->|
|-----W=1, CFN=20----->|
|-----W=1, CFN=19----->|
|-----W=1, CFN=18----->|
|-----W=1, CFN=17----->|
|-----W=1, CFN=16----->|
|-----W=1, CFN=15----->|
|-----W=1, CFN=14----->|
|-----W=1, CFN=13----->|
|-----W=1, CFN=12----->|
|-----W=1, CFN=11----->|
|-----W=1, CFN=10--X-->|
|-----W=1, CFN=9 ----->|
|-----W=1, CFN=8 ----->|
|-----W=1, CFN=7 ----->|
|-----W=1, CFN=6 ----->|
|-----W=1, CFN=5 ----->|
|-----W=1, CFN=4 ----->|
|-----W=1, CFN=3 ----->|
|-----W=1, CFN=2 ----->|
|-----W=1, CFN=1 ----->|
|-----W=1, CFN=0 ----->|
|<------ACK, W=1-------|bitmap:110111111111101111111111
|-----W=1, CFN=21----->|
|-----W=1, CFN=10----->|
|<------ACK, W=1-------|no bitmap
|-----W=0, CFN=23----->|
|-----W=0, CFN=22----->|
|-----W=0, CFN=21----->|
|-----W=0, CFN=31----->|MIC checked =>
|<------ACK, W=0-------|no bitmap
(End)
Appendix C. Allocation of Rule IDs for fragmentation
A set of Rule IDs are allocated to support different aspects of
fragmentation functionality as per this document. The allocation of
IDs is to be defined in other documents. The set MAY include:
o one ID or a subset of IDs to identify a fragment as well as its
reliability option and its window size, if multiple of these are
supported.
o one ID to identify the ACK message.
Minaburo, et al. Expires April 23, 2018 [Page 50]
Internet-Draft LPWAN SCHC October 2017
o one ID to identify the Abort message as per Section 9.8.
Appendix D. 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
Minaburo, et al. Expires April 23, 2018 [Page 51]