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24 RFC 8724
lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational L. Toutain
Expires: December 18, 2017 IMT-Atlantique
C. Gomez
Universitat Politecnica de Catalunya
June 16, 2017
LPWAN Static Context Header Compression (SCHC) and fragmentation for
IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-04
Abstract
This document describes a header compression scheme and fragmentation
functionality for very low bandwidth networks. These techniques are
especially 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 and must be used for
this kind of networks. A common context stored in a LPWAN device and
in the network is used. This context stores information that will
not be transmitted in the constrained network. Static context means
that information stored in the context which describes field values,
does not change during packet transmission, avoiding complex
resynchronization mechanisms, incompatible with LPWAN
characteristics. In most of the cases, IPv6/UDP headers are reduced
to a small identifier called Rule ID. But sometimes the SCHC header
compression will not be enough to send the packet in one L2 PDU, so
this document also describes a Fragmentation protocol that must be
used when needed.
This document describes the generic compression/decompression
mechanism and applies it to IPv6/UDP headers. Similar mechanisms for
other protocols such as CoAP will be described in separate documents.
Moreover, this document specifies fragmentation and reassembly
mechanims for SCHC compressed packets exceeding the L2 PDU size and
for the case where the SCHC compression is not possible then the
IPv6/UDP packet is sent using the fragmentation protocol.
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
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working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://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 December 18, 2017.
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
(http://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Static Context Header Compression . . . . . . . . . . . . . . 6
4.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Packet processing . . . . . . . . . . . . . . . . . . . . 9
5. Matching operators . . . . . . . . . . . . . . . . . . . . . 10
6. Compression Decompression Actions (CDA) . . . . . . . . . . . 11
6.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . . . 12
6.2. value-sent CDA . . . . . . . . . . . . . . . . . . . . . 12
6.3. mapping-sent . . . . . . . . . . . . . . . . . . . . . . 12
6.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . . . 13
6.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Application to IPv6 and UDP headers . . . . . . . . . . . . . 14
7.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 14
7.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 14
7.3. Flow label field . . . . . . . . . . . . . . . . . . . . 14
7.4. Payload Length field . . . . . . . . . . . . . . . . . . 15
7.5. Next Header field . . . . . . . . . . . . . . . . . . . . 15
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7.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 15
7.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 16
7.7.1. IPv6 source and destination prefixes . . . . . . . . 16
7.7.2. IPv6 source and destination IID . . . . . . . . . . . 16
7.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 17
7.9. UDP source and destination port . . . . . . . . . . . . . 17
7.10. UDP length field . . . . . . . . . . . . . . . . . . . . 17
7.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 18
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. IPv6/UDP compression . . . . . . . . . . . . . . . . . . 18
9. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 21
9.2. Reliability options: definition . . . . . . . . . . . . . 22
9.3. Reliability options: discussion . . . . . . . . . . . . . 23
9.4. Tools . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.5. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.5.1. Fragment format . . . . . . . . . . . . . . . . . . . 24
9.5.2. Fragmentation header formats . . . . . . . . . . . . 24
9.5.3. ACK format . . . . . . . . . . . . . . . . . . . . . 26
9.6. Baseline mechanism . . . . . . . . . . . . . . . . . . . 28
9.7. Supporting multiple window sizes . . . . . . . . . . . . 29
9.8. Aborting fragmented IPv6 datagram transmissions . . . . . 30
9.9. Downlink fragment transmission . . . . . . . . . . . . . 30
10. Security considerations . . . . . . . . . . . . . . . . . . . 30
10.1. Security considerations for header compression . . . . . 30
10.2. Security considerations for fragmentation . . . . . . . 31
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
12.1. Normative References . . . . . . . . . . . . . . . . . . 32
12.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Fragmentation examples . . . . . . . . . . . . . . . 32
Appendix B. Rule IDs for fragmentation . . . . . . . . . . . . . 35
Appendix C. Note . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
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 for 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).
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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 on 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 these 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 requirements 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.
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, 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.
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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 CDA: Compression/Decompression Action. An action that is perfomed
for both functionnalities 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. Node connected to the LPWAN. A Dev may implement
SCHC.
o App: LPWAN Application. An application sending/receiving IPv6
packets to/from the Device.
o SCHC C/D: LPWAN Compressor/Decompressor. A process in the network
to achieve compression/decompressing headers. SCHC C/D uses SCHC
rules to perform compression and decompression.
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.
o TV: Target value. A value contained in the Rule that will be
matched with the value of a header field.
o FID: Field Indentifier is an index to describe the header fields
in the Rule
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o FP: Field Position is a list of possible correct values that a
field may use
o DI: Direction Indicator is a differentiator for matching in order
to be able to have different values for both sides.
o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136]
o Dev-IID: Device Interface Identifier. Second part of the IPv6
address to identify the device interface
o APP-IID: Application Interface Identifier. Second part of the
IPv6 address to identify the application interface
o Dw: Down Link direction for compression, from SCHC C/D to Dev
o Up: Up Link direction for compression, from Dev to SCHC C/D
o Bi: Bidirectional, it can be used in both senses
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, a static context may be stored on the Device (Dev). The
context 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-
provosioned, etc. The way the context is learned on both sides is
out of the scope of this document.
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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 based on [I-D.ietf-lpwan-overview] terminology represents
the architecture for compression/decompression. 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. Resulting
information is sent on a layer two (L2) frame to the LPWAN Radio
Network to a Radio Gateway (RG) which forwards the frame to a Network
Gateway (NGW). The NGW sends the data to a 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.
SCHC C/D in both sides must share the same set of Rules. After
decompression, the 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 that match as much as possible the original packet
values instead of sending known field values. When a value is known
by both ends, it is not necessary sent 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 position (FP), a direction indicator (DI),
a target value (TV), a matching operator (MO) and a Compression/
Decompression Action (CDA).
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/--------------------------------------------------------------\
| Rule N |
/--------------------------------------------------------------\|
| Rule i ||
/--------------------------------------------------------------\||
| (FID) Rule 1 |||
|+-------+--+--+------------+-----------------+---------------+|||
||Field 1|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+------------+-----------------+---------------+|||
||Field 2|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+------------+-----------------+---------------+|||
||... |..|..| ... | ... | ... ||||
|+-------+--+--+------------+-----------------+---------------+||/
||Field N|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 describes also the compressed header fields which are
transmitted regarding their position in the rule which is used for
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 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:
* UP LINK (Up) when the field or the value is only present in
packets sent by the Dev to the App,
* DOWN LINK (Dw) when the field or the value is only present in
packet sent from the App to the Dev and
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* 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.
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 may be reserved for goals other than
header compression as fragmentation. (See Section 9).
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 use 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 does not correspond to the appropiated DI will
be excluded. Next, then 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 use and the SCHC
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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/
Decompession 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
information resulting from the compression of header fields. This
information 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
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 header
fields. It applies the CDA action to reconstruct the original
header fields. The CDA application order can be different of the
order given by the Rule. For instance Compute-* may be applied at
end, after the other CDAs.
+--- ... ---+-------------- ... --------------+
| Rule ID |Compressed Hdr Fields information|
+--- ... ---+-------------- ... --------------+
Figure 4: LPWAN Compressed Format Packet
5. 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.
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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 idenfied by a short ID (or index).
This operator matches if a field value is equal to one of those
target values.
6. Compression Decompression Actions (CDA)
The Compression Decompression Actions (CDA) describes the action
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 sumarizes the basics functions defined to compress and
decompress a field. The first column gives the action's name. The
second and third columns outlines 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.
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If the field is identified as 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 bit sent are set to 1
and the size is sent using 8 bits.
o For higher value, the first 12 bytes are set to 1 and the size is
sent on 2 bytes.
6.1. not-sent CDA
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 on 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.
6.2. 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.
The compressor sends the Target Value stored on the rule in the
compressed header message. The decompressor restores the field value
with the one received from the LPWAN
6.3. mapping-sent
mapping-sent is used to send a smaller index associated to the list
of values in the Target Value. This function is used together with
the "match-mapping" MO.
The compressor looks in the TV to find the field value and send the
corresponding index. The decompressor uses this index to restore the
field value.
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The number of bits sent is the minimal size to code all the possible
indexes.
6.4. LSB CDA
LSB action is used to avoid sendind 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 not length is specified, the number of bits
sent are 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.
If this action is made on a variable length field, the remaning size
in byte has to be sent before.
6.5. 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 can be computed from the Device ID present in the Layer
2 header. The computation is specific for each LPWAN technology and
depends on the Device ID size.
In the downstream direction, these CDA are used to determine the L2
addresses used by the LPWAN.
6.6. Compute-*
Thes 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.
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7. Application to IPv6 and UDP headers
This section lists the different IPv6 and UDP header fields and how
they can be compressed.
7.1. IPv6 version field
This field always holds the same value, therefore the TV is 6, the MO
is "equal" and the "CDA "not-sent"".
7.2. 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
7.3. 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
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7.4. 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".
7.5. 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.
7.6. 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.
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7.7. 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.
7.7.1. 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.
7.7.2. 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 derivated from the
EUI-64, then TV contains the 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.
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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".
7.8. IPv6 extensions
No extension rules are currently defined. They can be based on the
MOs and CDAs described above.
7.9. 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".
7.10. 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".
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7.11. 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 9)), 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".
8. Examples
This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the SCHC behavior.
8.1. IPv6/UDP compression
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 6 presents the protocol stack for this Device. IPv6 and UDP
are represented with dotted lines since these protocols are
compressed on the radio link.
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Managment Data
+----------+---------+---------+
| CoAP | CoAP | legacy |
+----||----+---||----+---||----+
. UDP . UDP | UDP |
................................
. IPv6 . IPv6 . IPv6 .
+------------------------------+
| SCHC Header compression |
| and fragmentation |
+------------------------------+
| LPWAN L2 technologies |
+------------------------------+
DEV or NGW
Figure 6: 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 |FP|DI| Value | Match | Comp Decomp || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+----------------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 APPiid |1 |Bi|::1 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|123 | equal | not-sent || |
|UDP APPport |1 |Bi|124 | equal | not-sent || |
|UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Rule 1
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Action || Sent |
| | | | | Opera. | Action ||[bits]|
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+----------------+--+--+---------+--------+-------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |1 |Bi|[alpha/64, match- | mapping-sent|| [1] |
| |1 |Bi|fe80::/64] mapping| || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|[beta/64,| match- | mapping-sent|| [2] |
| | | |alpha/64,| mapping| || |
| | | |fe80::64]| | || |
|IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|5683 | equal | not-sent || |
|UDP APPport |1 |Bi|5683 | equal | not-sent || |
|UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Rule 2
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Action || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+--------+-------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Up|255 | ignore | not-sent || |
|IPv6 Hop Limit |1 |Dw| | ignore | value-sent || [8] |
|IPv6 DEVprefix |1 |Bi|alpha/64 | equal | not-sent || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|gamma/64 | equal | not-sent || |
|IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
|UDP APPport |1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
|UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Figure 7: Context rules
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All the fields described in the three rules depicted on Figure 7 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.
9. Fragmentation
9.1. Overview
Fragmentation support in LPWAN is mandatory when the underlying LPWAN
technology is not capable of fulfilling the IPv6 MTU requirement.
Fragmentation is used if, after SCHC header compression, the size of
the resulting IPv6 packet is larger than the L2 data unit maximum
payload. Fragmentation is also used if SCHC header compression has
not been able to compress an IPv6 packet that is larger than the L2
data unit maximum payload. In LPWAN technologies, the L2 data unit
size typically varies from tens to hundreds of bytes. If the entire
IPv6 datagram fits within a single L2 data unit, the fragmentation
mechanism is not used and the packet is sent unfragmented.
If the datagram does not fit within a single L2 data unit, it SHALL
be broken into fragments.
Moreover, LPWAN technologies impose some strict limitations on
traffic; therefore it is desirable to enable optional fragment
retransmission, while a single fragment loss should not lead to
retransmitting the full IPv6 datagram. On the other hand, in order
to preserve energy, Devices are sleeping most of the time and may
receive data during a short period of time after transmission. In
order to adapt to the capabilities of various LPWAN technologies,
this specification allows for a gradation of fragment delivery
reliability. This document does not make any decision with regard to
which fragment delivery reliability option is 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.
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9.2. Reliability options: definition
This specification defines the following three fragment delivery
reliability options:
o No ACK
o Window mode - ACK "always"
o Window mode - ACK on error
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).
In the No ACK option, the receiver MUST NOT issue acknowledgments
(ACK).
In Window mode - 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 missing fragments from the window of fragments.
In Window mode - ACK on error, an ACK is transmitted by the fragment
receiver after a window of fragments have 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 missing fragments from
the window of fragments.
In Window mode, upon receipt of an ACK that informs about any lost
fragments, the sender retransmits the lost fragments. The maximum
number of ACKs to be sent by the receiver for a specific window,
denoted MAX_ACKS_PER_WINDOW, is not stated in this document, and it
is expected to be defined in other documents (e.g. technology-
specific profiles).
This document does not make any decision as to which fragment
delivery reliability option(s) need to be supported over a specific
LPWAN technology.
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Examples of the different reliability options described are provided
in Appendix A.
9.3. Reliability options: discussion
This section discusses the properties of each fragment delivery
reliability option defined in the previous section.
No ACK is the most simple fragment delivery reliability option. With
this option, the receiver does not generate overhead in the form of
ACKs. However, this option does not enhance delivery reliability
beyond that offered by the underlying LPWAN technology.
The Window mode - ACK on error option is based on the optimistic
expectation that the underlying links will offer relatively low L2
data unit loss probability. This option reduces the number of ACKs
transmitted by the fragment receiver compared to the Window mode -
ACK "always" option. 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 fragments covered by the ACK have been successfully
delivered. In contrast, the Window mode - ACK "always" option does
not suffer that issue, at the expense of an ACK overhead increase.
The Window mode - 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 end of the IPv6 packet
transmission, as long as the window size is short enough. However,
such benefit comes at the expense of higher ACK overhead.
9.4. Tools
This subsection describes the different tools that are used to enable
the described fragmentation functionality and the different
reliability options supported. Each tool has a corresponding header
field format that is defined in the next subsection. The list of
tools follows:
o Rule ID. The Rule ID is used in fragments and in ACKs. The Rule
ID in a fragment is set to a value that indicates that the data unit
being carried is a fragment. This also allows to interleave non-
fragmented IPv6 datagrams with fragments that carry a larger IPv6
datagram. Rule ID may also be used to signal which reliability
option is in use for the IPv6 packet being carried. In an ACK, the
Rule ID signals that the message this Rule ID is prepended to is an
ACK.
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o Compressed Fragment Number (CFN). The CFN is included in all
fragments. This field can be understood as a truncated, efficient
representation of a larger-sized fragment number, and does not
necessarily carry an absolute fragment number. A special CFN value
signals the last fragment that carries a fragmented IPv6 packet. In
Window mode, the CFN is augmented with the W bit, which has the
purpose of avoiding possible ambiguity for the receiver that might
arise under certain conditions
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.
o Message Integrity Check (MIC). It is computed by the sender over
the complete IPv6 packet before fragmentation by using the TBD
algorithm. The MIC allows the receiver to check for errors in the
reassembled IPv6 packet, while it also enables compressing the UDP
checksum by use of SCHC.
o Bitmap. The bitmap is a sequence of bits included in the ACK for a
given window, that provides feedback on whether each fragment of the
current window has been received or not.
9.5. Formats
This section defines the fragment format, the fragmentation header
formats, and the ACK format.
9.5.1. Fragment format
A fragment comprises a fragmentation header and a fragment payload,
and conforms to the format shown in Figure 8. The fragment payload
carries a subset of either the IPv6 packet after header compression
or an IPv6 packet which could not be compressed. A fragment is the
payload in the L2 protocol data unit (PDU).
+---------------+-----------------------+
| Fragm. Header | Fragment payload |
+---------------+-----------------------+
Figure 8: Fragment format.
9.5.2. Fragmentation header formats
In the No ACK option, fragments except the last one SHALL contain the
fragmentation header as defined in Figure 9. The total size of this
fragmentation header is R bits.
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<------------ R ---------->
<--T--> <--N-->
+-- ... --+- ... -+- ... -+
| Rule ID | DTag | CFN |
+-- ... --+- ... -+- ... -+
Figure 9: 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 10. The total
size of this fragmentation header is R bits.
<------------ R ---------->
<--T--> 1 <--N-->
+-- ... --+- ... -+-+- ... -+
| Rule ID | DTag |W| CFN |
+-- ... --+- ... -+-+- ... -+
Figure 10: Fragmentation Header for Fragments except the Last One,
Window mode
The last fragment of an IPv6 datagram SHALL contain a fragmentation
header that conforms to the format shown in Figure 11. The total
size of this fragmentation header is R+M bits.
<------------- R ------------>
<- T -> <- N -> <---- M ----->
+---- ... ---+- ... -+- ... -+---- ... ----+
| Rule ID | DTag | 11..1 | MIC |
+---- ... ---+- ... -+- ... -+---- ... ----+
Figure 11: Fragmentation Header for the Last Fragment
o Rule ID: This field has a size of R - T - N - 1 bits in all
fragments that are not the last one, when Window mode is used. In
all other fragments, the Rule ID field has a size of R - T - N
bits.
o DTag: The size of the DTag field is T bits, which may be set to a
value greater than or equal to 0 bits. The DTag field in all
fragments that carry the same IPv6 datagram MUST be set to the
same value. DTag MUST be set sequentially increasing from 0 to
2^T - 1, and MUST wrap back from 2^T - 1 to 0.
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o CFN: This field is an unsigned integer, with a size of N bits,
that carries the CFN of the fragment. In the No ACK option, N=1.
For the rest of options, N equal to or greater than 3 is
recommended. The CFN MUST be set sequentially decreasing from the
highest CFN in the window (which will be used for the first
fragment), and MUST wrap from 0 back to the highest CFN in the
window. The highest CFN in the window MUST be a value equal to or
smaller than 2^N-2. (Example 1: for N=5, the highest CFN value
may be configured to be 30, then subsequent CFNs are set
sequentially and in decreasing order, and CFN will wrap from 0
back to 30. Example 2: for N=5, the highest CFN value may be set
to 23, then subsequent CFNs are set sequentially and in decreasing
order, and the CFN will wrap from 0 back to 23). The CFN for the
last fragment has all bits set to 1. Note that, by this
definition, the CFN value of 2^N - 1 is only used to identify a
fragment as the last fragment carrying a subset of the IPv6 packet
being transported, and thus the CFN does not strictly correspond
to the N least significant bits of the actual absolute fragment
number. It is also important to note that, for N=1, the last
fragment of the packet will carry a CFN equal to 1, while all
previous fragments will carry a CFN of 0.
o W: 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 1.
o MIC: This field, which has a size of M bits, carries the MIC for
the IPv6 packet.
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).
9.5.3. ACK format
The format of an ACK is shown in Figure 12:
<-------- R ------->
<- T -> 1
+---- ... --+-... -+-+----- ... ---+
| Rule ID | DTag |W| bitmap |
+---- ... --+-... -+-+----- ... ---+
Figure 12: Format of an ACK
Rule ID: In all ACKs, Rule ID has a size of R - T - 1 bits.
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DTag: DTag has a size of T bits. DTag carries the same value as the
DTag field in the fragments carrying the IPv6 datagram for which this
ACK is intended.
W: 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.
bitmap: This field carries the bitmap sent by the receiver to inform
the sender about whether fragments in the current window have been
received or not. Size of the bitmap field of an ACK can be equal to
0 or Ceiling(Number_of_Fragments/8) octets, where Number_of_Fragments
denotes the number of fragments of a window. The bitmap is a
sequence of bits, where the n-th bit signals whether the n-th
fragment transmitted in the current window has been correctly
received (n-th bit set to 1) or not (n-th bit set to 0). Remaining
bits with bit order greater than the number of fragments sent (as
determined by the receiver) are set to 0, except for the last bit in
the bitmap, which is set to 1 if the last fragment of the window has
been correctly received, and 0 otherwise. Feedback on reception of
the fragment with CFN = 2^N - 1 (last fragment carrying an IPv6
packet) is only given by the last bit of the corresponding window.
Absence of the bitmap in an ACK confirms correct reception of all
fragments to be acknowledged by means of the ACK.
Figure 13 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 -> 0 1 2 3 4 5 6 7
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |W|1|0|1|1|0|1|1|1|
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+
Figure 13: Example of the bitmap in an ACK (in Window mode, for N=3)
Figure 14 illustrates an ACK without a bitmap.
<------- R ------->
<- T ->
+---- ... --+-... -+-+
| Rule ID | DTag |W|
+---- ... --+-... -+-+
Figure 14: Example of an ACK without a bitmap
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Note that, in order to exploit the available L2 payload space to the
fullest, a bitmap may have a size smaller than 2^N bits. In that
case, the window in use will have a size lower than 2^N-1 fragments.
For example, if the maximum available space for a bitmap is 56 bits,
N can be set to 6, and the window size can be set to a maximum of 56
fragments.
9.6. Baseline mechanism
The receiver of link fragments SHALL use (1) the sender's L2 source
address (if present), (2) the destination's L2 address (if present),
(3) Rule ID and (4) DTag to identify all the fragments that belong to
a given IPv6 datagram. The fragment receiver may determine the
fragment delivery reliability option in use for the 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 CFN and the order of
arrival of each fragment to determine the location of the individual
fragments within the original unfragmented packet. For example, it
may place the data payload of the fragments within a payload datagram
reassembly buffer at the location determined from the CFN and order
of arrival 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
IPv6 packet cannot be determined from fragmentation headers.
When Window mode - ACK on error is used, the fragment receiver starts
a timer (denoted "ACK on Error Timer") upon reception of the first
fragment for an IPv6 datagram. The initial value for this timer is
not provided by this specification, and is expected to be defined in
additional documents. This timer is reset every time that a new
fragment carrying data from the same IPv6 datagram is received. In
Window mode - ACK on error, upon timer expiration, if neither the
last fragment of the IPv6 datagram nor the last fragment of the
current window (i.e. with CFN=0) have been received, an ACK MUST be
transmitted by the fragment receiver to indicate received and not
received fragments for the current window.
Note that, in Window mode, the first fragment of the window is the
one sent with CFN=2^N-2. Also note that, in Window mode, the
fragment with CFN=0 is considered the last fragment of its window,
except for the last fragment of the whole packet (with all CFN bits
set to 1), which is also the last fragment of the last window. Upon
receipt of the last fragment of a window, if Window mode - ACK
"Always" is used, the fragment receiver MUST send an ACK to the
fragment sender. The ACK provides feedback on the fragments received
and lost that correspond to the last window.
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If the recipient receives the last fragment of an IPv6 datagram, it
checks for the integrity of the reassembled IPv6 datagram, based on
the MIC received. In No ACK mode, if the integrity check indicates
that the reassembled IPv6 datagram does not match the original IPv6
datagram (prior to fragmentation), the reassembled IPv6 datagram MUST
be discarded. If Window mode - ACK "Always" is used, the recipient
MUST transmit an ACK to the fragment sender. The ACK provides
feedback on the fragments that correspond to the last window. If
Window mode - ACK on error is used, the recipient MUST NOT transmit
an ACK to the sender if no losses have been detected for the last
window. If losses have been detected, the recipient MUST then
transmit an ACK to the sender to provide feedback on the last window.
When Window mode - ACK "Always" is used, the fragment sender starts a
timer (denoted "ACK Always Timer") after transmitting the last
fragment of a fragmented IPv6 datagram. The fragment sender also
starts the ACK Always Timer after transmitting the last fragment of a
window. The initial value for this timer is not provided by this
specification, and is expected to be defined in additional documents.
Upon expiration of the timer, if no ACK has been received for the
current window, the sender retransmits the last fragment, and it
reinitializes and restarts the timer. Note that retransmitting the
last fragment of a window as described serves as an ACK request. The
maximum number of requests for a specific ACK, denoted
MAX_ACK_REQUESTS, is not stated in this document, and it is expected
to be defined in other documents (e.g. technology-specific profiles).
In all Window mode options, the fragment sender retransmits any lost
fragments reported in an ACK.
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 a node 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.
9.7. 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 IPv6 packets that need to be carried by a large
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number of fragments. However, when the number of fragments required
to carry an IPv6 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 IPv6 packet
transmission.
9.8. Aborting fragmented IPv6 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. The entity (either the fragment sender or the
fragment receiver) that triggers abortion transmits to the other
endpoint a format that only comprises a Rule ID (of size R bits),
which signals abortion of all on-going fragmented IPv6 packet
transmissions. The specific value to be used for the Rule ID of this
abortion signal is not defined in this document, and is expected to
be defined in future documents.
Upon transmission or reception of the abortion signal, both entities
MUST release any resources allocated for the fragmented IPv6 datagram
transmissions being aborted.
9.9. 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 IPv6 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.
10. Security considerations
10.1. Security considerations for header compression
A malicious header compression could cause the reconstruction of a
wrong packet that does not match with the original one, such
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.
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10.2. 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. 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.
11. 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.
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12. References
12.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://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,
<http://www.rfc-editor.org/info/rfc4944>.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
<http://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, <http://www.rfc-editor.org/info/rfc7136>.
12.2. Informative References
[I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-04 (work in progress), June 2017.
Appendix A. Fragmentation examples
This section provides examples of different fragment delivery
reliability options possible on the basis of this specification.
Figure 15 illustrates the transmission of an IPv6 packet that needs
11 fragments in the No ACK option.
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Sender Receiver
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=0-------->|
|-------CFN=1-------->|MIC checked =>
Figure 15: Transmission of an IPv6 packet carried by 11 fragments in
the No ACK option
Figure 16 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK on error, for N=3, without losses.
Sender Receiver
|-----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----->|
(no ACK)
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4----->|
|-----W=0, CFN=7----->|MIC checked =>
(no ACK)
Figure 16: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3, without losses.
Figure 17 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK on error, for N=3, with three
losses.
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Sender Receiver
|-----W=1, CFN=6----->|
|-----W=1, CFN=5----->|
|-----W=1, CFN=4--X-->|
|-----W=1, CFN=3----->|
|-----W=1, CFN=2--X-->|
|-----W=1, CFN=1----->|
|-----W=1, CFN=0----->|
|<-----ACK, W=1-------|Bitmap:11010111
|-----W=1, CFN=4----->|
|-----W=1, CFN=2----->|
(no ACK)
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-----ACK, W=0-------|Bitmap:11000001
|-----W=0, CFN=4----->|MIC checked =>
(no ACK)
Figure 17: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3, three losses.
Figure 18 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK "always", for N=3, without losses.
Note: in Window mode, an additional bit will be needed to number
windows.
Sender Receiver
|-----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-------|no bitmap
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4----->|
|-----W=0, CFN=7----->|MIC checked =>
|<-----ACK, W=0-------|no bitmap
(End)
Figure 18: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "always", for N=3, no losses.
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Figure 19 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK "always", for N=3, with three
losses.
Sender Receiver
|-----W=1, CFN=6----->|
|-----W=1, CFN=5----->|
|-----W=1, CFN=4--X-->|
|-----W=1, CFN=3----->|
|-----W=1, CFN=2--X-->|
|-----W=1, CFN=1----->|
|-----W=1, CFN=0----->|
|<-----ACK, W=1-------|bitmap:11010111
|-----W=1, CFN=4----->|
|-----W=1, CFN=2----->|
|<-----ACK, W=1-------|no bitmap
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked =>
|<-----ACK, W=0-------|no bitmap
(End)
Figure 19: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, with three losses.
Appendix B. Rule IDs for fragmentation
Different Rule IDs may be used for different aspects of fragmentation
functionality as per this document. A summary of such Rule IDs
follows:
o A fragment, and the reliability option in use for the IPv6
datagram being carried: i) No ACK, ii) Window mode - ACK on error,
iii) Window mode - ACK "always". In Window mode, a specific Rule
ID may be used for each supported window size.
o An ACK message.
o A message to abort all on-going transmissions.
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Appendix C. Note
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336, and by the ERDF and the Spanish
Government through project TEC2016-79988-P. Part of his contribution
to this work has been carried out during his stay as a visiting
scholar at the Computer Laboratory of the University of Cambridge.
Authors' Addresses
Ana Minaburo
Acklio
2bis rue de la Chataigneraie
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Laurent Toutain
IMT-Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
Carles Gomez
Universitat Politecnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: carlesgo@entel.upc.edu
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