LPWAN Static Context Header Compression (SCHC) and fragmentation for IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-03
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 | 2017-05-05 (Latest revision 2017-03-10) | ||
| Replaces | draft-toutain-lpwan-ipv6-static-context-hc | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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by Pete Resnick
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||
| 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-03
lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational L. Toutain
Expires: November 6, 2017 IMT-Atlantique
C. Gomez
Universitat Politecnica de Catalunya
May 05, 2017
LPWAN Static Context Header Compression (SCHC) and fragmentation for
IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-03
Abstract
This document describes a header compression scheme and fragmentation
functionality for IPv6/UDP protocols. These techniques are
especially tailored for LPWAN (Low Power Wide Area Network) networks
and could be extended to other protocol stacks.
The Static Context Header Compression (SCHC) offers a great level of
flexibility when processing the header fields. Static context means
that information stored in the context which, describes field values,
does not change during the packet transmission, avoiding complex
resynchronization mechanisms, incompatible with LPWAN
characteristics. In most of the cases, IPv6/UDP headers are reduced
to a small identifier.
This document describes the generic compression/decompression process
and applies it to IPv6/UDP headers. Similar mechanisms for other
protocols such as CoAP will be described in a separate document.
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.
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 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
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 6, 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
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Vocabulary . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Static Context Header Compression . . . . . . . . . . . . . . 5
3.1. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Packet processing . . . . . . . . . . . . . . . . . . . . 7
4. Matching operators . . . . . . . . . . . . . . . . . . . . . 8
5. Compression Decompression Actions (CDA) . . . . . . . . . . . 9
5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . . . 10
5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . . . 10
5.3. mapping-sent . . . . . . . . . . . . . . . . . . . . . . 10
5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5. DEViid-DID, APPiid-DID CDA . . . . . . . . . . . . . . . 11
5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Application to IPv6 and UDP headers . . . . . . . . . . . . . 11
6.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 11
6.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 12
6.3. Flow label field . . . . . . . . . . . . . . . . . . . . 12
6.4. Payload Length field . . . . . . . . . . . . . . . . . . 12
6.5. Next Header field . . . . . . . . . . . . . . . . . . . . 13
6.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 13
6.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 13
6.7.1. IPv6 source and destination prefixes . . . . . . . . 13
6.7.2. IPv6 source and destination IID . . . . . . . . . . . 14
6.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 14
6.9. UDP source and destination port . . . . . . . . . . . . . 15
6.10. UDP length field . . . . . . . . . . . . . . . . . . . . 15
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6.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 15
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. IPv6/UDP compression . . . . . . . . . . . . . . . . . . 16
8. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 18
8.2. Reliability options: definition . . . . . . . . . . . . . 19
8.3. Reliability options: discussion . . . . . . . . . . . . . 20
8.4. Fragment format . . . . . . . . . . . . . . . . . . . . . 21
8.5. Fragmentation header formats . . . . . . . . . . . . . . 22
8.6. ACK format . . . . . . . . . . . . . . . . . . . . . . . 24
8.7. Baseline mechanism . . . . . . . . . . . . . . . . . . . 25
8.8. Aborting a fragmented IPv6 datagram transmission . . . . 28
8.9. Downlink fragment transmission . . . . . . . . . . . . . 28
9. Security considerations . . . . . . . . . . . . . . . . . . . 29
9.1. Security considerations for header compression . . . . . 29
9.2. Security considerations for fragmentation . . . . . . . . 29
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Normative References . . . . . . . . . . . . . . . . . . 30
11.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Fragmentation examples . . . . . . . . . . . . . . . 30
Appendix B. Note . . . . . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
Header compression is mandatory to efficiently bring Internet
connectivity to the node within a LPWAN network
[I-D.minaburo-lp-wan-gap-analysis].
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).
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
order. This context is static (the values on the header fields do
not change during time) avoiding complex resynchronization
mechanisms, incompatible with LPWAN characteristics. In most of the
cases, IPv6/UDP headers are reduced to a small context identifier.
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The SCHC header compression is indedependent of the specific LPWAN
technology over which it will be used.
On the other hand, LPWAN technologies are 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 mechanism at the adaptation layer below
IPv6. This specification defines 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. Vocabulary
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/consuming 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 compare 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. Rule ID is sent on the LPWAN.
o TV: Target value. A value contained in the rule that will be
matched with the value of a header field.
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3. 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. 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. It can also be learned by using
a provisionning protocol that is out of the scope of this draft.
DEVICE Appl Servers
+---------------+ +---------------+
| APP1 APP2 APP3| |APP1 APP2 APP3|
| | | |
| UDP | | UDP |
| IPv6 | | IPv6 |
| | | |
| SCHC C/D | | |
| (context) | | |
+--------+------+ +-------+-------+
| +--+ +----+ +---------+ .
+~~ |RG| === |NGW | === |SCHC C/D |... Internet ...
+--+ +----+ |(context)|
+---------+
Figure 1: Architecture
Figure 1 based on [I-D.ietf-lpwan-overview] terminology represents
the architecture for compression/decompression. The Device is
running applications which produce IPv6 or IPv6/UDP flows. These
flows are compressed by an Static Context Header Compression
Compressor/Decompressor (SCHC C/D) to reduce the 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 places if a
tunnel is established between the NGW and the SCHC C/D. This
architecture forms a star topology. After decompression, the packet
can be sent on the Internet to one or several LPWAN Application
Servers (APP).
The principle is exactly the same in the other direction.
The context contains a list of rules (cf. Figure 2). Each rule
contains itself a list of fields descriptions composed of a field
identifier (FID), a field position (FP), a direction indicator (DI),
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a target value (TV), a matching operator (MO) and a Compression/
Decompression Action (CDA).
/----------------------------------------------------------------\
| Rule N |
/----------------------------------------------------------------\|
| Rule i ||
/----------------------------------------------------------------\||
| (FID) Rule 1 |||
|+-------+---+---+------------+-----------------+---------------+|||
||Field 1|Pos|Dir|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+---+---+------------+-----------------+---------------+|||
||Field 2|Pos|Dir|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+---+---+------------+-----------------+---------------+|||
||... |...|...| ... | ... | ... ||||
|+-------+---+---+------------+-----------------+---------------+||/
||Field N|Pos|Dir|Target Value|Matching Operator|Comp/Decomp Act|||
|+-------+---+---+------------+-----------------+---------------+|/
| |
\----------------------------------------------------------------/
Figure 2: 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 done in the same
order they appear in the packet.
On the other hand, the rule describes the compressed header 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 main idea of the compression scheme is to send the rule id to the
other end instead of known field values. When a value is known by
both ends, it is not necessary to send it on the LPWAN network.
The field description is composed of different entries:
o A Field ID (FID) is a unique value to define the field.
o A Field Position (FP) indicating if several instances of the field
exist in the headers which one is targeted.
o A direction indicator (DI) indicating the packet direction. Three
values are possible:
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* upstream when the field or the value is only present in packets
sent by the DEV to the APP,
* downstream when the field or the value is only present in
packet sent from the APP to the DEV and
* bi-directional 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,...). It can be a single value or a more
complex structure (array, list,...). It can be considered as 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, which can be
considered as a CBOR structure. 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, which can be considered as a CBOR structure.
3.1. Rule ID
Rule IDs are sent between both compression/decompression elements.
The size of the rule ID is not specified in this document and can
vary regarding the LPWAN technology, the number of flows,...
Some values in the rule ID space may be reserved for goals other than
header compression, for example fragmentation.
Rule IDs are specific to a DEV. Two DEVs may use the same rule ID
for different header compression. The SCHC C/D needs to combine the
rule ID with the DEV L2 address to find the appropriate rule.
3.2. 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 headers. Each field is associated to
a matching operator for compression. Each header field's value is
compared to the corresponding target value stored in the rule for
that field using the matching operator. This comparison includes
the direction indicator and the field position in the header. If
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all the fields in the packet's header satisfy all the matching
operators (excluding unappropriate direction or position) of a
rule, the packet is processed using Compression Decompression
Function associated with the fields. Otherwise the next rule is
tested. If no eligible rule is found, then the packet is sent
without compression, which may require using the fragmentation
procedure.
In the downstrean direction, the rule is also used to find the
device ID.
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 layer
two technology and will be specified in a specific document. For
example, it can either be included in a Layer 2 header or sent in
the first byte of the L2 payload. (cf. Figure 3)
o decompression: 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 order
can be different of the order given by the rule. For instance
Compute-* may be applied after the other CDAs.
+--- ... ---+-------------- ... --------------+
| Rule ID |Compressed Hdr Fields information|
+--- ... ---+-------------- ... --------------+
Figure 3: LPWAN Compressed Format Packet
4. Matching operators
This document describes basic matching operators (MO)s which must be
known 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 type. The MOs and their definitions
are provided next:
o equal: a field value in a packet matches with a field value 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
field value in the rule. The result of the matching is always
true.
o MSB(length): a field value of a size equal to "length" bits in a
packet matches with a field value in a rule if the most
significant "length" bits are equal.
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 pairs. Each pair is composed of a value and a short ID
(or index). This operator matches if a field value is equal to
one of the pairs' values.
Matching Operators and match-mapping needs a parameter to proceed to
the matching. Match-mapping requires a list of values associated to
an index and MSB requires an integer indicating the number of bits to
test.
5. 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 |ctxt value OR rcvd value |
|compute-length |elided |compute length |
|compute-checksum |elided |compute UDP checksum |
|DEViid-DID |elided |build IID from L2 DEV addr |
|APPiid-DID |elided |build IID from L2 APP addr |
\--------------------+-------------+----------------------------/
Figure 4: Compression and Decompression Functions
Figure 4 sumarizes the 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
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to find the size of each compressed field which can be given by the
rule or may be sent with the compressed header.
5.1. 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
that field on which compression is applied.
The decompressor restores the field value with the target value
stored in the matched rule.
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.
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
5.3. mapping-sent
mapping-sent is used to send a smaller index associated to the field
value 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.
The number of bit sent is the minimal number to code all the indexes.
5.4. LSB CDA
LSB action is used to send a fixed part of the packet field header to
the other end. This action is used together with the "MSB" MO. A
length can be specified to indicate how many bits have to be sent.
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If not length is specified, the number of bit sent are the field
length minus the bit length specified in the MSB MO.
The compressor sends the "length" Least Significant Bits. The
decompressor combines with an OR operator the value received with the
Target Value.
5.5. DEViid-DID, APPiid-DID CDA
These functions are used to process respectively the Device and the
Application Device Identifier (DID). APPiid-DID CDA is less common,
since current LPWAN technologies frames contain a single address.
The IID value is computed from the Device ID present in the Layer 2
header. The computation depends on the technology and the Device ID
size.
In the downstream direction, these CDA are used to determine the L2
addresses used by the LPWAN.
5.6. Compute-*
These functions are used by the decompressor to compute the
compressed field value based on received information. Compressed
fields are elided during the compression and reconstructed during the
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.
6. Application to 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".
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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 wellknown
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 there is
without compression and the original value is sent, or the sencond
where the values can be computed by sending only the LSB bits:
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(8-X)
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 dpending 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(20-X)
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
recompute 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 is small, the TV can be set to 0x0000, the MO set to
"MSB (16-s)" and the CDA to "LSB (s)". The 's' parameter depends on
the maximum packet length.
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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 host 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.
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
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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-DID" or "APPiid-DID". 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.
For privacy reasons or if the DEV address is changing over time, it
maybe better to use a static value. In that case, the 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 IID, 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 CDF is set to LSB.
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.
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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 knows 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".
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 XXXX)),
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".
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7. Examples
This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the SCHC behavior.
7.1. 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 5 presents the protocol stack for this Device. IPv6 and UDP
are represented with dotted lines since these protocols are
compressed on the radio link.
Managment Data
+----------+---------+---------+
| CoAP | CoAP | legacy |
+----||----+---||----+---||----+
. UDP . UDP | UDP |
................................
. IPv6 . IPv6 . IPv6 .
+------------------------------+
| SCHC Header compression |
| and fragmentation |
+------------------------------+
| LPWAN L2 technologies |
+------------------------------+
DEV or NGW
Figure 5: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the DEVs have a device ID.
Therefore, when such technologie are used, it is necessary to define
statically an IID for the Link Local address for the SCHC C/D.
Rule 0
+----------------+---------+--------+-------------++------+
| Field | Value | Match | Function || Sent |
+----------------+---------+----------------------++------+
|IPv6 version |6 | equal | not-sent || |
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|IPv6 DiffServ |0 | equal | not-sent || |
|IPv6 Flow Label |0 | equal | not-sent || |
|IPv6 Length | | ignore | comp-length || |
|IPv6 Next Header|17 | equal | not-sent || |
|IPv6 Hop Limit |255 | ignore | not-sent || |
|IPv6 DEVprefix |FE80::/64| equal | not-sent || |
|IPv6 DEViid | | ignore | DEViid-DID || |
|IPv6 APPprefix |FE80::/64| equal | not-sent || |
|IPv6 APPiid |::1 | equal | not-sent || |
+================+=========+========+=============++======+
|UDP DEVport |123 | equal | not-sent || |
|UDP APPport |124 | equal | not-sent || |
|UDP Length | | ignore | comp-length || |
|UDP checksum | | ignore | comp-chk || |
+================+=========+========+=============++======+
Rule 1
+----------------+---------+--------+-------------++------+
| Field | Value | Match | Function || Sent |
+----------------+---------+--------+-------------++------+
|IPv6 version |6 | equal | not-sent || |
|IPv6 DiffServ |0 | equal | not-sent || |
|IPv6 Flow Label |0 | equal | not-sent || |
|IPv6 Length | | ignore | comp-length || |
|IPv6 Next Header|17 | equal | not-sent || |
|IPv6 Hop Limit |255 | ignore | not-sent || |
|IPv6 DEVprefix |alpha/64 | equal | not-sent || |
|IPv6 DEViid | | ignore | DEViid-DID || |
|IPv6 APPprefix |beta/64 | equal | not-sent || |
|IPv6 APPiid |::1000 | equal | not-sent || |
+================+=========+========+=============++======+
|UDP DEVport |5683 | equal | not-sent || |
|UDP APPport |5683 | equal | not-sent || |
|UDP Length | | ignore | comp-length || |
|UDP checksum | | ignore | comp-chk || |
+================+=========+========+=============++======+
Rule 2
+----------------+---------+--------+-------------++------+
| Field | Value | Match | Function || Sent |
+----------------+---------+--------+-------------++------+
|IPv6 version |6 | equal | not-sent || |
|IPv6 DiffServ |0 | equal | not-sent || |
|IPv6 Flow Label |0 | equal | not-sent || |
|IPv6 Length | | ignore | comp-length || |
|IPv6 Next Header|17 | equal | not-sent || |
|IPv6 Hop Limit |255 | ignore | not-sent || |
|IPv6 DEVprefix |alpha/64 | equal | not-sent || |
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|IPv6 DEViid | | ignore | DEViid-DID || |
|IPv6 APPprefix |gamma/64 | equal | not-sent || |
|IPv6 APPiid |::1000 | equal | not-sent || |
+================+=========+========+=============++======+
|UDP DEVport |8720 | MSB(12)| LSB(4) || lsb |
|UDP APPport |8720 | MSB(12)| LSB(4) || lsb |
|UDP Length | | ignore | comp-length || |
|UDP checksum | | ignore | comp-chk || |
+================+=========+========+=============++======+
Figure 6: Context rules
All the fields described in the three rules Figure 6 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.
8. Fragmentation
8.1. 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, Things (End Systems) 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
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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.
8.2. Reliability options: definition
This specification defines the following five fragment delivery
reliability options:
o No ACK
o Packet mode - ACK "always"
o Packet mode - ACK on error
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 the underlying LPWAN technology to decide which
reliability option to use and whether the same reliability option
applies to all IPv6 packets. Note that the reliability option to be
used is not necessarily tied to the particular characteristics of the
underlying L2 LPWAN technology (e.g. a reliability option without
receiver feedback 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 Packet mode - ACK "always", the receiver transmits one ACK after
all fragments carrying an IPv6 packet have been transmitted. The ACK
informs the sender about received and/or missing fragments from the
IPv6 packet.
In Packet mode - ACK on error, the receiver transmits one ACK after
all fragments carrying an IPv6 packet have been transmitted, only if
at least one of those fragments has been lost. The ACK informs the
sender about received and/or missing fragments from the IPv6 packet.
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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 Packet or Window mode, upon receipt of an ACK that informs about
any lost fragments, the sender retransmits the lost fragments, up to
a maximum number of ACK and retransmission rounds that is TBD.
This document does not make any decision as to which fragment
delivery reliability option(s) need to be supported over a specific
LPWAN technology.
Examples of the different reliability options described are provided
in Appendix A.
8.3. Reliability options: discussion
This section discusses the properties of each fragment delivery
reliability option defined in the previous section. Figure Figure 7
summarizes advantages and disadvantages of the reliability options
that provide receiver feedback.
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.
ACK on error options are based on the optimistic expectation that the
underlying links will offer relatively low L2 data unit loss
probability. ACK on error reduces the number of ACKs transmitted by
the fragment receiver compared to ACK "always" options. 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.
The Packet mode - ACK on error option provides reliability with low
ACK overhead. However, if an ACK is lost, the sender assumes that
all fragments carrying the IPv6 datagram have been successfully
delivered. In contrast, the Packet mode - ACK "always" option does
not suffer that issue, at the expense of a moderate ACK overhead. An
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issue with any of the Packet modes is that detection of a long burst
of lost frames is only possible after relatively long time (i.e. at
the end of the transmission of all fragments carrying an IPv6
datagram).
In contrast with Packet modes, the Window mode - ACK "always" option
provides flow control. In addition, it is able to better handle long
bursts of lost fragments, since detection of such events can be done
earlier than with any of the Packet modes. However, the benefits of
Window mode - ACK "always" come at the expense of higher ACK
overhead.
With regard to the Window mode - ACK on error option, there is no
known use case for it at the time of the writing.
+-----------------------+------------------------+
| Packet mode | Window mode |
+-----------------+-----------------------+------------------------+
| | + Low ACK overhead | |
| ACK on error | - Long loss burst | (Use case unknown) |
| | - No flow control | |
+-----------------+-----------------------+------------------------+
| | + Moderate ACK overh. | + Flow control |
| ACK "always" | - Long loss burst | + Long loss burst |
| | - No flow control | - Higher ACK overhead |
+-----------------+-----------------------+------------------------+
Figure 7: Summary of fragment delivery options that provide receiver
feedback, and their main advantages (+) and disadvantages (-).
8.4. 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.
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8.5. Fragmentation header formats
In any of the Window modes, 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.
<------------ R ---------->
<--T--> 1 <--N-->
+-- ... --+- ... -+-+- ... -+
| Rule ID | DTag |W| CFN |
+-- ... --+- ... -+-+- ... -+
Figure 9: Fragmentation Header for Fragments except the Last One,
Window mode
In any of the Packet modes, fragments (except the last one) that are
transmitted for the first time SHALL contain the fragmentation header
shown in Figure 10. The total size of this fragmentation header is R
bits.
<------------- R ------------>
<- T -> <- N ->
+---- ... ---+- ... -+- ... -+
| Rule ID | DTag | CFN |
+---- ... ---+- ... -+- ... -+
Figure 10: Fragmentation Header for Fragments except the Last One, in
a Packet mode; first transmission attempt
In any of the Packet modes, fragments (except the last one) that are
retransmitted SHALL
contain the fragmentation header as defined in Figure 11.
<------------- R ------------>
<- T -> <----- A ---->
+---- ... ---+- ... -+----- ... ----+
| Rule ID | DTag | AFN |
+---- ... ---+- ... -+----- ... ----+
Figure 11: Fragmentation Header for Retransmitted Fragments (Except
the Last One) in a Packet mode
The last fragment of an IPv6 datagram, regardless of whether a Packet
mode or Window mode is in use, SHALL contain a fragmentation header
that conforms to the format shown in Figure 12. The total size of
this fragmentation header is R+M bits.
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<------------- R ------------>
<- T -> <- N -> <---- M ----->
+---- ... ---+- ... -+- ... -+---- ... ----+
| Rule ID | DTag | 11..1 | MIC |
+---- ... ---+- ... -+- ... -+---- ... ----+
Figure 12: 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. 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 be used to signal which
reliability option is in use. In any of the Packet modes, Rule ID
is also used to indicate whether the fragment is a first
transmission or a retransmission.
o DTag: DTag stands for Datagram Tag. 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. The DTag field allows to
interleave fragments that correspond to different IPv6 datagrams.
DTag MUST be set sequentially increasing from 0 to 2^T - 1, and
MUST wrap back from 2^T - 1 to 0.
o CFN: CFN stands for Compressed Fragment Number. The size of the
CFN field is N bits. In the No ACK option, N=1. For the rest of
options,
N equal to or greater than 3 is recommended. This field is an
unsigned integer that carries a non-absolute fragment number. The
CFN MUST be set sequentially decreasing from 2^N - 2 for the first
fragment, and MUST wrap from 0 back to 2^N - 2 (e.g. for N=3, the
first fragment has CFN=6, subsequent CFNs are set sequentially and
in decreasing order, and CFN will wrap from 0 back to 6). 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 flag that is used in Window mode. Its purpose is
avoiding possible ambiguity for the receiver that might arise
under certain conditions. This flag carries the same value for
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all fragments of a window, and it is set to the other value for
the next window. The initial value for this flag is 1.
o AFN: AFN stands for Absolute Fragment Number. This field has a
size of A bits. 'A' may be greater than N. The AFN is an
unsigned integer that carries the absolute fragment number that
corresponds to a fragment from an IPv6 packet. The AFN MUST be
set sequentially and in increasing order, starting from 0.
o MIC: MIC stands for Message Integrity Check. This field has a
size of M bits. It is computed by the sender over the complete
IPv6 packet before fragmentation by using the TBD algorithm. The
MIC allows to check for errors in the reassembled IPv6 packet,
while it also enables compressing the UDP checksum by use of SCHC.
The values for R, N, A and M are not specified in this document,
and have to be determined by the underlying LPWAN technology.
8.6. ACK format
The format of an ACK is shown in Figure 13:
<------- R ------>
<- T ->
+---- ... --+-... -+----- ... ---+
| Rule ID | DTag | bitmap |
+---- ... --+-... -+----- ... ---+
Figure 13: Format of an ACK
Rule ID: In all ACKs, Rule ID has a size of R bits and SHALL be set
to TBD_ACK to signal that the message is an ACK.
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.
bitmap: 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 (in Window mode) or the
number of fragments that carry the IPv6 packet (in Packet mode). The
bitmap is a sequence of bits, where the n-th bit signals whether the
n-th fragment transmitted 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 (carrying the MIC) has been
correctly received, and 0 otherwise. Absence of the bitmap in an ACK
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confirms correct reception of all fragments to be acknowledged by
means of the ACK.
Figure 14 shows an example of an ACK in Packet mode, where the bitmap
indicates that the second and the ninth fragments have not been
correctly received. In this example, the IPv6 packet is carried by
eleven fragments in total, therefore the bitmap has a size of two
bytes.
<------ R ------> 1
<- T -> 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |1|0|1|1|1|1|1|1|0|1|1|0|0|0|0|1|
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Example of the Bitmap in an ACK
Figure 15 shows an example of an ACK in Window mode (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 |1|0|1|1|0|1|1|1|
+---- ... --+-... -+-+-+-+-+-+-+-+-+
Figure 15: Example of the bitmap in an ACK (in Window mode, for N=3)
Figure 16 illustrates an ACK without bitmap.
<------ R ------>
<- T ->
+---- ... --+-... -+
| Rule ID | DTag |
+---- ... --+-... -+
Figure 16: Example of an ACK without bitmap
8.7. 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
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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 flag in the
received fragments. Note that the size of the original, unfragmented
IPv6 packet cannot be determined from fragmentation headers.
When ACK on error is used (for either Packet mode or Window mode),
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 Packet mode - ACK on error, upon timer
expiration, if the last fragment of the IPv6 datagram (i.e. carrying
all CFN bits set to 1) has not been received, an ACK MUST be
transmitted by the fragment receiver to indicate received and not
received fragments for that IPv6 datagram.
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 (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.
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 ACK "Always" is used, the recipient MUST transmit
an ACK to the fragment sender. The ACK
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provides feedback on the whole set of fragments sent that carry the
complete IPv6 packet (Packet mode) or on the fragments that
correspond to the last window (Window mode). If ACK on error is
used, the recipient MUST NOT transmit an ACK to the sender if no
losses have been detected for the whole IPv6 packet (Packet mode) or
in the last window (Window mode). If losses have been detected, the
recipient MUST then transmit an ACK to the sender to provide feedback
on the whole IPv6 packet (Packet mode) or in the last window (Window
mode).
When ACK "Always" is used (in either Packet mode or Window mode), the
fragment sender starts a timer (denoted "ACK Always Timer") after
transmitting the last fragment of a fragmented IPv6 datagram. 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 this IPv6
datagram, the sender retransmits the last fragment, and it
reinitializes and restarts the timer. In Window mode - ACK "Always",
the fragment sender also starts the ACK Always Timer after
transmitting the last fragment of a window. Upon expiration of the
timer, if no ACK has been received for this window, the sender
retransmits the last fragment, and it reinitializes and restarts the
timer. Note that retransmitting the last fragment of a packet or a
window as described serves as an ACK request. The maximum number of
ACK requests in Packet mode or in Window mode is TBD.
In all reliability options, except for the No ACK option, the
fragment sender retransmits any lost fragments reported in an ACK.
In Packet modes, in order to minimize the probability of ambiguity
with the CFN of different retransmitted fragments, the fragment
sender
renumbers the CFNs of the fragments to be retransmitted by following
the same approach as for a sequence of new fragments: the CFN for
retransmitted fragments is set sequentially decreasing from 2^N - 2
for the first fragment, and MUST wrap from 0 back to 2^N - 2.
However, the last fragment of the set of retransmitted fragments only
carries a CFN with all bits set to 1 if it is actually a
retransmission of the last fragment of the packet (i.e. the last
fragment had been lost in the first place). Examples of fragment
renumbering for retransmitted fragments in Packet modes can be found
in Appendix A.
A maximum of TBD iterations of ACK and fragment retransmission rounds
are allowed per-window or per-IPv6-packet in Window mode or in Packet
mode, respectively.
If a fragment recipient disassociates from its L2 network, the
recipient MUST discard all link fragments of all partially
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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.
8.8. Aborting a fragmented IPv6 datagram transmission
For several reasons, a fragment sender or a fragment receiver may
want to abort the transmission of a fragmented IPv6 datagram.
If the fragment sender triggers abortion, it transmits to the
receiver a format equivalent to a fragmentation header (with the
format for a fragment that is not the last one), with the Rule ID
field (of size R - T - N bits) set to TBD_ABORT_TX and all CFN bits
set to 1. No data is carried along with this fragmentation header.
If the fragment receiver triggers abortion, it transmits to the
fragment sender a Rule ID (of size R bits) set to TBD_ABORT_RX. The
entity that triggers abortion (either a fragment sender or a fragment
receiver) MUST release any resources allocated for the fragmented
IPv6 datagram transmission being aborted.
When a fragment receiver receives an L2 frame containing a Rule ID
set to TBD ABORT_TX and a CFN field with all bits set to 1, the
receiver MUST release any resources allocated for the fragmented IPv6
datagram transmission being aborted.
When a fragment sender receives an L2 frame containing a Rule ID set
to TBD_ABORT_RX, the fragment sender MUST abort transmission of the
fragmented IPv6 datagram being transmitted, and MUST release any
resources allocated for the fragmented IPv6 datagram transmission
being aborted.
A further Rule ID value may be used by an entity to signal abortion
of all on- going, possibly interleaved, fragmented IPv6 datagram
transmissions.
8.9. 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
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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.
9. Security considerations
9.1. Security considerations for header compression
TBD
9.2. Security considerations for fragmentation
This subsection describes potential attacks to LPWAN fragmentation
and proposes countermeasures, based on existing analysis of attacks
to 6LoWPAN fragmentation {HHWH}.
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 whole packet on the basis of the datagram size announced in
that first fragment. 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.
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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.
10. Acknowledgements
Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier,
Arunprabhu Kandasamy, Antony Markovski, Alexander Pelov, Pascal
Thubert, Juan Carlos Zuniga for useful design consideration.
11. References
11.1. 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>.
11.2. Informative References
[I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-01 (work in progress), February 2017.
[I-D.minaburo-lp-wan-gap-analysis]
Minaburo, A., Pelov, A., and L. Toutain, "LP-WAN GAP
Analysis", draft-minaburo-lp-wan-gap-analysis-01 (work in
progress), February 2016.
Appendix A. Fragmentation examples
This section provides examples of different fragment delivery
reliability options possible on the basis of this specification.
Figure 17 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 17: Transmission of an IPv6 packet carried by 11 fragments in
the No ACK option
Figure 18 illustrates the transmission of an IPv6 packet that needs
11 fragments in Packet mode - ACK on error, for N=3, without losses.
Sender Receiver
|-------CFN=6-------->|
|-------CFN=5-------->|
|-------CFN=4-------->|
|-------CFN=3-------->|
|-------CFN=2-------->|
|-------CFN=1-------->|
|-------CFN=0-------->|
|-------CFN=6-------->|
|-------CFN=5-------->|
|-------CFN=4-------->|
|-------CFN=7-------->|MIC checked =>
(no ACK)
Figure 18: Transmission of an IPv6 packet carried by 11 fragments in
Packet mode - ACK on error, for N=3, no losses.
Figure 19 illustrates the transmission of an IPv6 packet that needs
11 fragments in Packet mode - ACK on error, for N=3, with three
losses.
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Sender Receiver
(AFN=0) |-------CFN=6-------->|
(AFN=1) |-------CFN=5-------->|
(AFN=2) |-------CFN=4---X---->|
(AFN=3) |-------CFN=3-------->|
(AFN=4) |-------CFN=2---X---->|
(AFN=5) |-------CFN=1-------->|
(AFN=6) |-------CFN=0-------->|
(AFN=7) |-------CFN=6-------->|
(AFN=8) |-------CFN=5-------->|
(AFN=9) |-------CFN=4---X---->|
|-------CFN=7-------->|MIC checked
|<-------ACK----------|Bitmap:1101011110100001
|-------AFN=2-------->|
|-------AFN=4-------->|
|-------AFN=9-------->|MIC checked =>
(no ACK)
Figure 19: Transmission of an IPv6 packet carried by 11 fragments in
Packet mode - ACK on error, for N=3, three losses. In the figure,
(AFN=x) indicates the AFN value computed by the sender for each
fragment.
Figure 20 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 20: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3, without losses.
Figure 21 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----------|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----------|Bitmap:11010001
|-----W=0, CFN=4----->|MIC checked =>
(no ACK)
Figure 21: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3, three losses.
Figure 22 illustrates the transmission of an IPv6 packet that needs
11 fragments in Packet mode - ACK "Always", for N=3, without losses.
Sender Receiver
|-------CFN=6-------->|
|-------CFN=5-------->|
|-------CFN=4-------->|
|-------CFN=3-------->|
|-------CFN=2-------->|
|-------CFN=1-------->|
|-------CFN=0-------->|
|-------CFN=6-------->|
|-------CFN=5-------->|
|-------CFN=4-------->|
|-------CFN=7-------->|MIC checked =>
|<-------ACK----------|no bitmap
(End)
Figure 22: Transmission of an IPv6 packet carried by 11 fragments in
Packet mode - ACK "Always", for N=3, no losses.
Figure 23 illustrates the transmission of an IPv6 packet that needs
11 fragments in Packet mode - ACK "Always", for N=3, with three
losses.
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Sender Receiver
(AFN=0) |-------CFN=6-------->|
(AFN=1) |-------CFN=5-------->|
(AFN=2) |-------CFN=4---X---->|
(AFN=3) |-------CFN=3-------->|
(AFN=4) |-------CFN=2---X---->|
(AFN=5) |-------CFN=1-------->|
(AFN=6) |-------CFN=0-------->|
(AFN=7) |-------CFN=6-------->|
(AFN=8) |-------CFN=5-------->|
(AFN=9) |-------CFN=4---X---->|
|-------CFN=7-------->|MIC checked
|<-------ACK----------|bitmap:1101011110100001
|-------AFN=2-------->|
|-------AFN=4-------->|
|-------AFN=9-------->|MIC checked =>
|<-------ACK----------|no bitmap
(End)
Figure 23: Transmission of an IPv6 packet carried by 11 fragments in
Packet mode - ACK "Always", for N=3, with three losses.
Figure 24 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----------|no bitmap
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4----->|
|-----W=0, CFN=7----->|MIC checked =>
|<-------ACK----------|no bitmap
(End)
Figure 24: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, no losses.
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Figure 25 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----------|bitmap:11010111
|-----W=1, CFN=4----->|
|-----W=1, CFN=2----->|
|<-------ACK----------|no bitmap
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-------ACK----------|bitmap:11010001
|-----W=0, CFN=4----->|MIC checked =>
|<-------ACK----------|no bitmap
(End)
Figure 25: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, with three losses.
Appendix B. 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
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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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