lpwan Working Group
Internet Engineering Task Force (IETF) A. Minaburo
Internet-Draft
Request for Comments: 8824 Acklio
Intended status:
Category: Standards Track L. Toutain
Expires: September 9, 2021 Institut MINES TELECOM;
ISSN: 2070-1721 IMT Atlantique
R. Andreasen
Universidad de Buenos Aires
March 08,
June 2021
LPWAN
Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-19 the
Constrained Application Protocol (CoAP)
Abstract
This draft document defines how to compress the Constrained Application
Protocol (CoAP) headers using the Static Context Header Compression (SCHC).
and fragmentation (SCHC) framework. SCHC is defines a header
compression mechanism adapted for Constrained Devices. SCHC uses a
static description of the header to reduce the header's redundancy
and size. While RFC 8724 describes the SCHC compression and
fragmentation framework, and its application for IPv6/UDP headers,
this document applies SCHC for to CoAP headers. The CoAP header
structure differs from IPv6 and UDP UDP, since CoAP uses a flexible
header with a variable number of options, themselves of variable
length. The CoAP protocol messages message format is asymmetric: the request messages
have a header format different from the one format in the response
messages. This specification gives guidance on applying SCHC to
flexible headers and how to leverage the asymmetry for more efficient
compression Rules.
Status of This Memo
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This Internet-Draft will expire on September 9, 2021.
https://www.rfc-editor.org/info/rfc8824.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. SCHC Applicability to CoAP . . . . . . . . . . . . . . . . . 4
3. CoAP Headers compressed Compressed with SCHC . . . . . . . . . . . . . . 7
3.1. Differences between CoAP and UDP/IP Compression . . . . . 8
4. Compression of CoAP header fields . . . . . . . . . . . . . . 9 Header Fields
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 9 Version Field
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 9 Type Field
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 9 Code Field
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 10 Field
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 10 Fields
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 10 Options
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 11 Options
5.2. CoAP option Option Max-Age, Uri-Host, and Uri-Port fields . . . 11 Fields
5.3. CoAP option Option Uri-Path and Uri-Query fields . . . . . . . . 11 Fields
5.3.1. Variable number Number of Path or Query elements . . . . . . 13 Elements
5.4. CoAP option Option Size1, Size2, Proxy-URI Proxy-URI, and Proxy-Scheme
fields . . . . . . . . . . . . . . . . . . . . . . . . . 13
Fields
5.5. CoAP option Option ETag, If-Match, If-None-Match, Location-Path,
and Location-Query fields . . . . . . . . . . . . . . . . 13 Fields
6. SCHC compression Compression of CoAP extension RFCs . . . . . . . . . . . 13 Extensions
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 14
6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Examples of CoAP header compression . . . . . . . . . . . . . 15 Header Compression
7.1. Mandatory header Header with CON message . . . . . . . . . . . . 15 Message
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 16
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 Example OSCORE Compression
8. IANA Considerations
9. Security considerations . . . . . . . . . . . . . . . . . . . 31 Considerations
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
11. Normative References . . . . . . . . . . . . . . . . . . . . 32
Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
CoAP
The Constrained Application Protocol (CoAP) [RFC7252] is a command/response command/
response protocol designed for micro-
controllers microcontrollers with a small RAM and
ROM and optimized for REST-based
(Representative state transfer) services. services based on REST (Representational State
Transfer). Although the Constrained Devices leads are a leading factor in
the CoAP design, design of CoAP, a CoAP header's size is still too large for LPWAN (Low Power Wide Area
LPWANs (Low-Power Wide-Area Networks). SCHC header
compression Static Context Header
Compression and fragmentation (SCHC) over CoAP header headers is required to
increase performance or to use CoAP over LPWAN technologies.
The
[RFC8724] defines SCHC, the SCHC framework, which includes a header
compression mechanism for the
LPWAN network LPWANs that is based on a static context.
Section 5 of the [RFC8724] explains where compression and decompression
occur in the architecture. The SCHC compression scheme assumes as a
prerequisite that both end-points endpoints know the static context before
transmission. The way the context is configured, provisioned, or
exchanged is out of this document's scope.
CoAP is an application protocol, so CoAP compression requires
installing common Rules between the two SCHC instances. SCHC
compression may apply at two different levels: at IP and UDP in the
LPWAN network and another at the application level for CoAP. These two compressions
compression techniques may be independent. Both follow the same
principle as that described in [RFC8724]. As different entities
manage the CoAP compression process at different levels, the SCHC
Rules driving the compression/decompression are also different. The
[RFC8724] describes how to use SCHC for IP and UDP headers. This
document specifies how to apply SCHC compression to CoAP headers.
SCHC compresses and decompresses headers based on common contexts
between Devices. The SCHC context includes multiple Rules. Each
Rule can match the header fields to specific values or ranges of
values. If a Rule matches, the matched header fields are replaced by
the RuleID and the Compression Residue that contains the residual
bits of the compression. Thus, different Rules may correspond to
different protocol headers in the packet that a Device expects to
send or receive.
A Rule describes the packets' entire header with an ordered list of
fields descriptions;
Field Descriptors; see section Section 7 of [RFC8724]. Thereby Thereby, each
description contains the field Field ID (FID), its length Field Length (FL), and
its position Field
Position (FP), as well as a direction indicator Direction Indicator (DI) (upstream,
downstream, and bidirectional), bidirectional) and some associated Target Values (TV).
(TVs). The
direction indicator DI is used for compression to give the best TV to the FID
when these values differ in the their transmission direction. So So, a
field may be described several times.
A Matching Operator (MO) is associated with each header field
description. Field
Descriptor. The Rule is selected if all the MOs fit the TVs for all
fields of the incoming header. A Rule cannot be selected if the
message contains an unknown a field that is unknown to the SCHC compressor.
In that case, a Compression/Decompression Action (CDA) associated
with each field gives the method to compress and decompress each
field. Compression mainly results in one of 4 four actions:
o
* send the field value (value-sent),
o
* send nothing (not-sent),
o
* send some least significant bits Least Significant Bits (LSBs) of the field (LSB) or,
o field, or
* send an index (mapping-sent).
After applying the compression, there may be some bits to be sent.
These values are called Compression Residue. "Compression Residue".
SCHC is a general mechanism applied to different protocols, with the
exact Rules to be used depending on the protocol and the Application. application.
Section 10 of the [RFC8724] describes the compression scheme for IPv6 and
UDP headers. This document targets the CoAP header compression using
SCHC.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119][RFC8174] [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. SCHC Applicability to CoAP
SCHC Compression compression for CoAP header headers MAY be done in conjunction with the
lower layers (IPv6/UDP) or independently. The SCHC adaptation
layers, described in Section 5 of [RFC8724], may be used as shown in
Figure
Figures 1, Figure 2, and Figure 3.
In the first example, Figure 1, a Rule compresses the complete header
stack from IPv6 to CoAP. In this case, the Device and the NGW Network
Gateway (NGW) perform SCHC C/D (Static Context Header Compression Compressor/
Decompressor). (SCHC Compression/Decompression; see
[RFC8724]). The Application application communicating with the Device does not
implement SCHC C/D.
(Device) (NGW) (App)
+--------+ +--------+
| CoAP | | CoAP |
+--------+ +--------+
| UDP | | UDP |
+--------+ +----------------+ +--------+
| IPv6 | | IPv6 | | IPv6 |
+--------+ +--------+-------+ +--------+
| SCHC | | SCHC | | | |
+--------+ +--------+ + + +
| LPWAN | | LPWAN | | | |
+--------+ +--------+-------+ +--------+
((((LPWAN)))) ------ Internet ------
Figure 1: Compression/Decompression at the LPWAN boundary. Boundary
Figure 1 shows the use of SCHC header compression above layer Layer 2 in
the Device and the NGW. The SCHC layer receives non-encrypted
packets and can apply compression Rules to all the headers in the
stack. On the other end, the NGW receives the SCHC packet and
reconstructs the headers using the Rule and the Compression Residue.
After the decompression, the NGW forwards the IPv6 packet toward the
destination. The same process applies in the other direction when a
non-encrypted packet arrives at the NGW. Thanks to the IP forwarding
based on the IPv6 prefix, the NGW identifies the Device and
compresses headers using the Device's Rules.
In the second example, Figure 2, the SCHC compression is applied in the
CoAP layer, compressing the CoAP header independently of the other
layers. The RuleID, the Compression Residue, and CoAP payload are
encrypted using a mechanism such as DTLS. Only the other end (App)
can decipher the information. If needed, layers below use SCHC to
compress the header as defined in [RFC8724] (represented in by dotted
lines).
lines in the figure).
This use case needs an end-to-end context initialization between the
Device and the Application. application. The context initialization is out of the
scope of for this document.
(Device) (NGW) (App)
+--------+ +--------+
| CoAP | | CoAP |
+--------+ +--------+
| SCHC | | SCHC |
+--------+ +--------+
| DTLS | | DTLS |
+--------+ +--------+
. udp . . udp .
.......... .................. ..........
. ipv6 . . ipv6 . . ipv6 .
.......... .................. ..........
. schc . . schc . . . .
.......... .......... . . .
. lpwan . . lpwan . . . .
.......... .................. ..........
((((LPWAN)))) ------ Internet ------
Figure 2: Standalone CoAP end-to-end End-to-End Compression/Decompression
The third example, Figure 3, shows the use of Object Security for
Constrained RESTful Environments (OSCORE) [RFC8613]. In this case,
SCHC needs two Rules to compress the CoAP header. A first Rule
focused
focuses on the inner Inner header. The result of this first compression is
encrypted using the OSCORE mechanism. Then Then, a second Rule compresses
the outer Outer header, including the OSCORE Options. options.
(Device) (NGW) (App)
+--------+ +--------+
| CoAP | | CoAP |
| inner Inner | | inner Inner |
+--------+ +--------+
| SCHC | | SCHC |
| inner Inner | | inner Inner |
+--------+ +--------+
| CoAP | | CoAP |
| outer Outer | | outer Outer |
+--------+ +--------+
| SCHC | | SCHC |
| outer Outer | | outer Outer |
+--------+ +--------+
. udp . . udp .
.......... .................. ..........
. ipv6 . . ipv6 . . ipv6 .
.......... .................. ..........
. schc . . schc . . . .
.......... .......... . . .
. lpwan . . lpwan . . . .
.......... .................. ..........
((((LPWAN)))) ------ Internet ------
Figure 3: OSCORE compression/decompression. Compression/Decompression
In the case of several SCHC instances, as shown in Figure Figures 2 and
Figure 3,
the Rules may come from different provisioning domains.
This document focuses on CoAP compression compression, as represented in by the
dashed boxes in the previous figures.
3. CoAP Headers compressed Compressed with SCHC
The use of SCHC over the CoAP header uses applies the same description, description and
compression/decompression techniques like as the one technique used for IP and UDP
UDP, as explained in the [RFC8724]. For CoAP, the SCHC Rules description
uses the direction information to optimize the compression by
reducing the number of Rules needed to compress headers. The field
description Field
Descriptor MAY define both request/response headers and target
values TVs in the
same Rule, using the DI (direction indicator) to make indicate the difference. header type.
As for other header compression protocols, when the compressor does
not find a correct Rule to compress the header, the packet MUST be
sent uncompressed using the RuleID dedicated to this purpose. Where purpose, and
where the Compression Residue is the complete header of the packet.
See
section Section 6 of [RFC8724].
3.1. Differences between CoAP and UDP/IP Compression
CoAP compression differs from IPv6 and UDP compression in the
following aspects:
o
* The CoAP protocol message format is asymmetric; the headers are different
for a request or a response. For example, the URI-Path Uri-Path option is
mandatory in the request, and it might not be present in the
response. A request might contain an Accept option, and the
response might include a Content-Format option. In comparison,
the IPv6 and UDP returning path swap swaps the value of some fields in
the header. However, all the directions have the same fields
(e.g., source and destination address fields).
The
[RFC8724] defines the use of a direction indicator (DI) DI in the Field Descriptor, which
allows a single Rule to process a message header differently differently,
depending on the direction.
o
* Even when a field is "symmetric" (i.e., found in both directions),
the values carried in each direction are different. The
compression may use a "match-mapping" MO to limit the range of
expected values in a particular direction and reduce the
Compression Residue's size. Through the direction indicator (DI), DI, a field description Field Descriptor in
the Rules splits the possible field value into two parts, one for
each direction. For instance, if a client sends only CON requests, Confirmable
(CON) requests [RFC7252], the Type can be elided by compression,
and the answer may use one single bit to carry either the ACK or
RST
Reset (RST) type. The field Code has the same behavior, behavior: the 0.0X
code format value in the request, request and the Y.ZZ code format in the
response.
o
* In SCHC, the Rule defines the different header fields' length, so
SCHC does not need to send it. In IPv6 and UDP headers, the
fields have a fixed size, known by definition. On the other hand,
some CoAP header fields have variable lengths, and the Rule
description specifies it. For example, in a URI-path Uri-Path or URI-
query, Uri-
Query, the Token size may vary from 0 to 8 bytes, and the CoAP
options use the Type-Length-Value encoding format.
When doing SCHC compression of a variable-length field,
Section 7.5.2 from 7.4.2 of [RFC8724] offers the possibility to define option of defining a
function for the Field length Length in the Field Description Descriptor to know the
length before compression. If the field length Field Length is unknown, the
Rule will set it as a variable, and SCHC will send the compressed
field's length in the Compression Residue.
o
* A field can appear several times in the CoAP headers. It is found
typically for elements of a URI (path or queries). The SCHC
specification [RFC8724] allows a Field ID FID to appear several times in
the Rule and uses the Field Position (FP) to identify the correct
instance, thereby removing the matching operation's MO's ambiguity.
o
* Field lengths Lengths defined in the CoAP protocol can be too large regarding when it comes to
LPWAN traffic constraints. For instance, this is particularly
true for the Message-ID Message ID field and the Token field. SCHC uses
different Matching operators (MO) MOs to perform the compression. See section Section 7.4 of
[RFC8724]. In this case, SCHC can apply the Most Significant Bits (MSB)
(MSBs) MO to reduce the information carried on LPWANs.
4. Compression of CoAP header fields Header Fields
This section discusses the compression of the different CoAP header
fields. The CoAP compression with SCHC follows the information provided
in Section 7.1 of [RFC8724].
4.1. CoAP version field Version Field
The CoAP version is bidirectional and MUST be elided during the SCHC
compression
compression, since it always contains the same value. In the future,
or if a new version of CoAP is defined, new Rules will be needed to
avoid ambiguities between versions.
4.2. CoAP type field
The Type Field
CoAP protocol [RFC7252] has four types of messages: two requests (CON, NON),
one response (ACK), and one empty message (RST).
The SCHC compression scheme SHOULD elide this field if, for instance,
a client is sending only NON Non-confirmable (NON) messages or only CON
messages. For the RST message, SCHC may use a dedicated Rule. For
other usages, SCHC can use a "match-mapping" MO.
4.3. CoAP code field Code Field
The code field is Code field, defined in an IANA registry [RFC7252], and it indicates the
Request Method used in CoAP. The compression of the CoAP code Code field
follows the same principle as that of the CoAP type Type field. If the
Device plays a specific role, SCHC may split the code values into two
fields description,
Field Descriptors: (1) the request codes with the 0 class and (2) the
response values. SCHC will use the direction indicator DI to identify the correct value
in the packet.
If the Device only implements a CoAP client, SCHC compression may
reduce the request code to the set of requests the client can
process.
For known values, SCHC can use a "match-mapping" MO. If SCHC cannot
compress the code Code field, it will send the values in the Compression
Residue.
4.4. CoAP Message ID field Field
SCHC can compress the Message ID field with the "MSB" MO and the
"LSB" CDA. See section Section 7.4 of [RFC8724].
4.5. CoAP Token fields Fields
CoAP defines the Token using two CoAP fields, fields: Token Length in the
mandatory header and Token Value directly following the mandatory
CoAP header.
SCHC processes the Token length Length as it would any header field. If the
value does not change, the size can be stored in the TV and elided
during the transmission. Otherwise, SCHC will send the token length Token Length
in the Compression Residue.
For the Token Value, SCHC MUST NOT send it as a variable-length data in
the Compression Residue Residue, to avoid ambiguity with the Token Length.
Therefore, SCHC MUST use the Token length Length value to define the size of
the Compression Residue. SCHC designates a specific function "tkl" function, "tkl",
that the Rule MUST use to complete the field description. Field Descriptor. During the
decompression, this function returns the value contained in the Token
Length field.
5. CoAP options Options
CoAP defines options placed after the basic header in Option Numbers
order; header, ordered by option
number; see [RFC7252]. Each Option instance in a message uses the
format Delta-Type (D-T), Length (L), Value (V). The SCHC Rule builds
the description of the option by using the following:
* in the Field ID FID: the Option
Number option number built from the D-T;
* in TV, the Option Value; TV: the option value; and
* for the Option Length
uses section 7.4 Length: the information provided in Sections 7.4.1
and 7.4.2 of [RFC8724].
When the Option Length has a well-
known well-known size, the Rule may keep the
length value. Therefore, SCHC compression does not send it.
Otherwise, SCHC Compression compression carries the length of the Compression
Residue, in addition to the Compression Residue value.
CoAP requests and responses do not include the same options. So
Compression So,
compression Rules may reflect this asymmetry by tagging the direction
indicator. DI.
Note that length coding differs between CoAP options and SCHC
variable size Compression Residue.
The following sections present how SCHC compresses some specific CoAP
options.
If CoAP introduces a new option, the SCHC Rules MAY be updated, and
the new Field ID FID description MUST be assigned to allow its compression.
Otherwise, if no Rule describes this new option, the SCHC compression is
not achieved, and SCHC sends the CoAP header without compression.
5.1. CoAP Content and Accept options. Options
If the client expects a single value, it can be stored in the TV and
elided during the transmission. Otherwise, if the client expects
several possible values, a "match-mapping" MO SHOULD be used to limit
the Compression Residue's size. If not, SCHC has to send the option
value in the Compression Residue (fixed or variable length).
5.2. CoAP option Option Max-Age, Uri-Host, and Uri-Port fields Fields
SCHC compresses these three fields in the same way. When the value values
of these options is are known, SCHC can elide these fields. If the
option uses well-known values, SCHC can use a "match-mapping" MO.
Otherwise, SCHC will use the "value-sent" MO, and the Compression
Residue will send these options' values.
5.3. CoAP option Option Uri-Path and Uri-Query fields Fields
The Uri-Path and Uri-Query fields are repeatable options; this means
that in the CoAP header, they may appear several times with different
values. The SCHC Rule description uses the Field Position (FP) FP to distinguish the
different instances in the path.
To compress repeatable field values, SCHC may use a "match-mapping"
MO to reduce the size of variable Paths paths or Queries. queries. In these cases,
to optimize the compression, several elements can be regrouped into a
single entry. The Numbering numbering of elements does not change, and the
first matching element sets the MO comparison.
In Figure 4, Table 1, SCHC can use a single bit in the Compression Residue to
code one of the two paths. If regrouping were not allowed, 2 bits in
the Compression Residue would be needed. SCHC sends the third path
element as a variable size in the Compression Residue.
+--------+---+--+--+--------+-------------+------------+
+==========+=====+====+====+==========+=========+==============+
| Field |FL |FP|DI| Target | Matching FL | FP | DI | TV | MO | CDA |
+==========+=====+====+====+==========+=========+==============+
| Uri-Path | | 1 | Up | Value ["/a/b", | Operator match- | mapping-sent |
+--------+---+--+--+--------+-------------+------------+
|Uri-Path|
| 1|up|["/a/b",|match-mapping|mapping-sent| | | | | |"/c/d"] "/c/d"] | mapping | |
|Uri-Path|var| 3|up| |ignore |value-sent
+----------+-----+----+----+----------+---------+--------------+
|
+--------+---+--+--+--------+-------------+------------+
Figure 4: complex path example Uri-Path | var | 3 | Up | | ignore | value-sent |
+----------+-----+----+----+----------+---------+--------------+
Table 1: Complex Path Example
The length of URI-Path Uri-Path and URI-Query Uri-Query may be known when the rule Rule is
defined. In any case, SCHC MUST set the field length Field Length to variable. a variable
value. The unit to indicate the Compression Residue size is expressed in Byte. bytes.
SCHC compression can use the MSB MO to a Uri-Path or Uri-Query
element. However, attention to the length is important because the
MSB value is in bits, and the size MUST always be a multiple of 8
bits.
The length sent at the beginning of a variable-length Compression
Residue indicates the LSB's size in bytes.
For instance, for a CORECONF path /c/X6?k="eth0" /c/X6?k=eth0, the Rule description
can be:
+-------------+---+--+--+--------+---------+-------------+ be as follows (Table 2):
+===========+=====+====+====+======+=========+============+
| Field |FL |FP|DI| Target | Match FL | FP | DI | TV | MO | CDA |
+===========+=====+====+====+======+=========+============+
| Uri-Path | | 1 | Up | "c" | equal | not-sent |
+-----------+-----+----+----+------+---------+------------+
| Uri-Path | var | 2 | Up | | ignore | value-sent |
+-----------+-----+----+----+------+---------+------------+
| Value Uri-Query | Opera. var | 1 |
+-------------+---+--+--+--------+---------+-------------+
|Uri-Path Up | "k=" | 1|up|"c" |equal |not-sent MSB(16) |
|Uri-Path |var| 2|up| |ignore |value-sent |
|Uri-Query |var| 1|up|"k=\"" |MSB(24) |LSB LSB |
+-------------+---+--+--+--------+---------+-------------+
Figure 5:
+-----------+-----+----+----+------+---------+------------+
Table 2: CORECONF URI compression
Figure 5 Compression
Table 2 shows the Rule description for a URI-Path Uri-Path and a URI-Query. Uri-Query.
SCHC compresses the first part of the URI-Path Uri-Path with a "not-sent" CDA.
SCHC will send the second element of the URI-Path Uri-Path with the length
(i.e., 0x2 X 6) "X6") followed by the query option (i.e., 0x05 eth0"). 0x4 "eth0").
5.3.1. Variable number Number of Path or Query elements Elements
SCHC fixed the number of Uri-Path or Uri-Query elements in a Rule at
the Rule creation time. If the number varies, SCHC SHOULD either
* create several Rules to cover all the possibilities. Another one is to
define the length of possibilities or
* create a Rule that defines several entries for Uri-Path to variable cover
the longest path and sends send a Compression Residue with a length of 0
to indicate that this a Uri-Path entry is empty.
However, this adds 4 bits to the variable Compression Residue size.
See section 7.5.2 Section 7.4.2 of [RFC8724].
5.4. CoAP option Option Size1, Size2, Proxy-URI Proxy-URI, and Proxy-Scheme fields Fields
The SCHC Rule description MAY define sending some field values by
setting the TV to "not-sent," "not-sent", the MO to "ignore," "ignore", and the CDA to "value-
sent."
"value-sent". A Rule MAY also use a "match-mapping" MO when there
are different options for the same FID. Otherwise, the Rule sets the
TV to the value, the MO to "equal," "equal", and the CDA to "not-sent." "not-sent".
5.5. CoAP option Option ETag, If-Match, If-None-Match, Location-Path, and
Location-Query fields Fields
A Rule entry cannot store these fields' values. The Rule description
MUST always send these values in the Compression Residue.
6. SCHC compression Compression of CoAP extension RFCs Extensions
6.1. Block
When a packet uses a Block [RFC7959] option, option [RFC7959], SCHC compression MUST
send its content in the Compression Residue. The SCHC Rule describes
an empty TV with a the MO set to "ignore" and a the CDA set to "value-sent." "value-
sent". The Block option allows fragmentation at the CoAP level that
is compatible with SCHC fragmentation. Both fragmentation mechanisms
are complementary, and the node may use them for the same packet as
needed.
6.2. Observe
The
[RFC7641] defines the Observe option. Option. The SCHC Rule description will
not define the TV, TV but will set the MO to "ignore," "ignore" and the CDA to "value-
sent."
"value-sent". SCHC does not limit the maximum size for this option
(3 bytes). To reduce the transmission size, either the Device
implementation MAY limit the delta between two consecutive values, values or
a proxy can modify the increment.
Since the Observe option Option MAY use an a RST message to inform a server
that the client does not require the Observe response, a specific
SCHC Rule SHOULD exist to allow the message's compression with the
RST type.
6.3. No-Response
The
[RFC7967] defines a No-Response option limiting the responses made by
a server to a request. Different behaviors exist while using this
option to limit the responses made by a server to a request. If both
ends know the value, then the SCHC Rule will describe a TV to this
value, with a the MO set to "equal" and the CDA set to "not-sent." "not-sent".
Otherwise, if the value is changing over time, the SCHC Rule will set
the MO to "ignore" and the CDA to "value-sent." "value-sent". The Rule may also
use a "match-mapping" MO to compress this option.
6.4. OSCORE
OSCORE [RFC8613] defines end-to-end protection for CoAP messages.
This section describes how SCHC Rules can be applied to compress
OSCORE-protected messages.
The
Figure 6 4 shows the OSCORE Option Value option value encoding defined in
Section 6.1 of [RFC8613], where the first byte specifies the Content content
of the OSCORE options using flags. The three most significant bits
of this byte are reserved and always set to 0. Bit h, when set,
indicates the presence of the kid context field in the option. Bit
k, when set, indicates the presence of a kid field. The three least
significant bits n bits, n, indicate the length of the piv (Partial
Initialization Vector) field in bytes. When n = 0, no piv is
present.
0 1 2 3 4 5 6 7 <--------- n bytes ------------->
+-+-+-+-+-+-+-+-+---------------------------------
|0 0 0|h|k| n | Partial IV (if any) ...
+-+-+-+-+-+-+-+-+---------------------------------
| | |
|<-- CoAP -->|<------ CoAP OSCORE_piv ------> |
OSCORE_flags
<- 1 byte -> <------ s bytes ----->
+------------+----------------------+-----------------------+
| s (if any) | kid context (if any) | kid (if any) ... |
+------------+----------------------+-----------------------+
| | |
| <------ CoAP OSCORE_kidctx ------>|<-- CoAP OSCORE_kid -->|
Figure 6: 4: OSCORE Option
The flag byte is followed by the piv field, the kid context field,
and the kid field field, in this that order, and and, if present, the kid context
field's length (in bytes) is encoded in the first byte denoting byte, denoted by 's' the length of the
kid context in bytes.
"s".
To better perform OSCORE SCHC compression, the Rule description needs
to identify the OSCORE Option option and the fields it contains.
Conceptually, it discerns up to 4 four distinct pieces of information
within the OSCORE option: the flag bits, the piv, the kid context,
and the kid. The SCHC Rule splits into four field descriptions the OSCORE option into four Field
Descriptors in order to compress them:
o
* CoAP OSCORE_flags,
o OSCORE_flags
* CoAP OSCORE_piv,
o OSCORE_piv
* CoAP OSCORE_kidctx,
o OSCORE_kidctx
* CoAP OSCORE_kid. OSCORE_kid
Figure 6 4 shows the OSCORE Option option format with those four fields
superimposed on it. Note that the CoAP OSCORE_kidctx field directly
includes the size octet octet, s.
7. Examples of CoAP header compression Header Compression
7.1. Mandatory header Header with CON message Message
In this first scenario, the SCHC Compressor at compressor on the Network Gateway NGW side receives
a POST message from an Internet client, which is immediately
acknowledged by the Device. Figure 7 Table 3 describes the SCHC Rule
descriptions for this scenario.
RuleID
+===================================================================+
|RuleID 1
+-------------+--+--+--+------+---------+-------------++------------+ |
+==========+===+==+==+======+===============+===============+=======+
| Field |FL|FP|DI|Target| Match | FL|FP|DI| TV | MO | CDA || | Sent |
| | | | |Value | Opera. | || [bits] |
+-------------+--+--+--+------+---------+-------------++------------+ | [bits]|
+==========+===+==+==+======+===============+===============+=======+
|CoAP version |2 |1 |Bi|01 | equal | not-sent | |
|version | | | | | 2| 1|bi| 01 |equal |not-sent || | | |
+----------+---+--+--+------+---------------+---------------+=======+
|CoAP Type |2 |1 |Dw|CON | equal | not-sent | 2| 1|dw| CON |equal |not-sent || |
+----------+---+--+--+------+---------------+---------------+=======+
|CoAP Type |2 |1 |Up|[ACK, | 2| 1|up|[ACK, |match- |matching- || match-mapping | matching-sent |T |
| | | | RST] |mapping |sent || T |RST] | | | |
+----------+---+--+--+------+---------------+---------------+=======+
|CoAP TKL |4 |1 |Bi|0 | 4| 1|bi| 0 |equal |not-sent || equal | not-sent | |
+----------+---+--+--+------+---------------+---------------+=======+
|CoAP Code |8 |1 |Bi|[0.00,| match-mapping | 8| 1|bi|[0.00,| matching-sent |CC CCC | ||
| | | | |... | | ... |match- |matching- || | |
| | | | 5.05]|mapping |sent || CC CCC |5.05] | | | |
+----------+---+--+--+------+---------------+---------------+=======+
|CoAP MID |16| 1|bi| 0000 |MSB(7 ) |LSB || M-ID| |16 |1 |Bi|0000 | MSB(7) | LSB |MID |
+----------+---+--+--+------+---------------+---------------+=======+
|CoAP Uri-Path|var 1|dw| path |equal Uri- |var|1 |Dw|path | equal 1 |not-sent || |
+-------------+--+--+--+------+---------+-------------++------------+
Figure 7: not-sent | |
|Path | | | | | | | |
+----------+---+--+--+------+---------------+---------------+=======+
Table 3: CoAP Context to compress header Compress Header without Token
In this example, SCHC compression elides the version and the Token Length
fields. The 26 method 25 Method and response codes Response Codes defined in [RFC7252]
has have
been shrunk to 5 bits using a "match-mapping" MO. The Uri-Path
contains a single element indicated in the TV and elided with the CDA
"not-sent."
"not-sent".
SCHC Compression compression reduces the header header, sending only the Type, a mapped
code, and the least significant bits of the Message ID (9 bits in the
example above).
Note that a client located in an Application Server sending a request
to a server located in the Device may not be compressed through this
Rule
Rule, since the MID might not start with 7 bits equal to 0. A CoAP
proxy placed before the SCHC C/D can rewrite the message Message ID to fit the
value and match the Rule.
7.2. OSCORE Compression
OSCORE aims to solve the problem of end-to-end encryption for CoAP
messages. Therefore, the goal is to hide the message as much as
possible the
message while still enabling proxy operation.
Conceptually
Conceptually, this is achieved by splitting the CoAP message into an
Inner Plaintext and Outer OSCORE Message. message. The Inner Plaintext
contains sensitive information that is not necessary for proxy
operation. However, it is part of the message that can be encrypted
until it reaches its end destination. The Outer Message acts as a
shell matching the regular CoAP message format and includes all
Options
options and information needed for proxy operation and caching.
Figure 8 5 below illustrates this analysis.
The
CoAP protocol arranges the options into one of 3 classes; three classes, each granted a
specific type of protection by the protocol:
o
Class E: Encrypted options moved to the Inner Plaintext,
o Plaintext.
Class I: Integrity-protected options included in the AAD Additional
Authenticated Data (AAD) for the encryption of the Plaintext but
otherwise left untouched in the Outer Message,
o Message.
Class U: Unprotected options left untouched in the Outer Message.
These classes point out that the Outer option contains the OSCORE
Option
option and that the message is OSCORE protected; this option carries
the information necessary to retrieve the Security Context. The end-
point
endpoint will use this Security Context to decrypt the message
correctly.
Original CoAP Packet
+-+-+---+-------+---------------+
|v|t|TKL| code | Msg Id. Message ID |
+-+-+---+-------+---------------+....+
| Token |
+-------------------------------.....+
| Options (IEU) |
. .
. .
+------+-------------------+
| 0xFF |
+------+------------------------+
| |
| Payload |
| |
+-------------------------------+
/ \
/ \
/ \
/ \
Outer Header v v Plaintext
+-+-+---+--------+---------------+ +-------+
|v|t|TKL|new code| Msg Id. Message ID | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF 0xFF |
. . +-------+-----------+
. OSCORE Option . | |
+------+-------------------+ | Payload |
| 0xFF | | |
+------+ +-------------------+
Figure 8: A 5: CoAP packet is split Packet Split into an OSCORE outer Outer Header and plaintext Plaintext
Figure 8 5 shows the packet format for the OSCORE Outer header and
Plaintext.
In the Outer Header, header, the original header code is hidden and replaced
by a default dummy value. As seen in Sections 4.1.3.5 and 4.2 of
[RFC8613], the message code is replaced by POST for requests and
Changed for responses when CoAP is not using the Observe option. Option. If
CoAP uses Observe, the OSCORE message code is replaced by FETCH for
requests and Content for responses.
The first byte of the Plaintext contains the original packet code,
followed by the message code, the class E options, and, if present,
the original message Payload payload preceded by its payload marker.
An AEAD Authenticated Encryption with Associated Data (AEAD) algorithm now
encrypts the Plaintext. This integrity
protects integrity-protects the Security Context
parameters and, eventually, any class I options from the Outer Header.
header. The resulting Ciphertext ciphertext becomes the new payload of the
OSCORE message, as illustrated in Figure 9. 6.
As defined in [RFC5116], this Ciphertext ciphertext is the encrypted Plaintext's
concatenation of the authentication tag. Authentication Tag. Note that Inner Compression
only affects the Plaintext before encryption. Thus only the first
variable-length of the Ciphertext can be reduced. The authentication
tag is Authentication
Tag, fixed in length and uncompressed, is considered part of the cost
of protection.
Outer Header
+-+-+---+--------+---------------+
|v|t|TKL|new code| Msg Id. Message ID |
+-+-+---+--------+---------------+....+
| Token |
+--------------------------------.....+
| Options (IU) |
. .
. OSCORE Option .
+------+-------------------+
| 0xFF |
+------+---------------------------+
| |
| Ciphertext: Encrypted Inner |
| Header and Payload |
| + Authentication Tag |
| |
+----------------------------------+
Figure 9: 6: OSCORE message Message
The SCHC Compression compression scheme consists of compressing both the
Plaintext before encryption and the resulting OSCORE message after
encryption,
encryption; see Figure 10. 7.
The OSCORE message translates into a segmented process where SCHC
compression is applied independently in 2 two stages, each with its
corresponding set of Rules, with the Inner SCHC Rules and the Outer
SCHC Rules. This way, compression is applied to all fields of the
original CoAP message.
Outer Message OSCORE Plaintext
+-+-+---+--------+---------------+ +-------+
|v|t|TKL|new code| Msg Id. Message ID | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF 0xFF |
. . +-------+-----------+
. OSCORE Option . | |
+------+-------------------+ | Payload |
| 0xFF | | |
+------+------------+ +-------------------+
| Ciphertext |<---------\ |
| | | v
+-------------------+ | +-----------------+
| | | Inner SCHC |
v | | Compression |
+-----------------+ | +-----------------+
| Outer SCHC | | |
| Compression | | v
+-----------------+ | +-------+
| | |RuleID |
v | +-------+-----------+
+--------+ +------------+ |Compression Residue|
|RuleID' | | Encryption | <-- +----------+--------+
+--------+-----------+ +------------+ | |
|Compression Residue'| | Payload |
+-----------+--------+ | |
| Ciphertext | +-------------------+
| |
+--------------------+
Figure 10: 7: OSCORE Compression Diagram
Note that since the corresponding end-point endpoint can only decrypt the Inner
part of the message, this end-point endpoint will also have to implement Inner
SCHC Compression/Decompression.
7.3. Example OSCORE Compression
This section gives an example with a GET Request request and its consequent
Content Response response from a Device-based CoAP client to a cloud-based
CoAP server. The example also describes a possible set of Rules for
the
Inner SCHC Compression and Outer SCHC Compression. A dump of the
results and a contrast between SCHC + OSCORE performance with SCHC + COAP
CoAP performance is are also listed. This example gives an
approximation of the cost of security with SCHC-OSCORE.
Our first CoAP message is the GET request in Figure 11. 8.
Original message:
=================
0x4101000182bb74656d7065726174757265
Header:
0x4101
01 Ver
00 CON
0001 TKL
00000001 Request Code 1 "GET"
0x0001 = mid
0x82 = token
Options:
0xbb74656d7065726174757265
Option 11: URI_PATH
Value = temperature
Original msg message length: 17 bytes. bytes
Figure 11: 8: CoAP GET Request
Its corresponding response is the CONTENT Response Content response in Figure 12. 9.
Original message:
=================
0x6145000182ff32332043
Header:
0x6145
01 Ver
10 ACK
0001 TKL
01000101 Successful Response Code 69 "2.05 Content"
0x0001 = mid
0x82 = token
0xFF Payload marker
Payload:
0x32332043
Original msg message length: 10 bytes
Figure 12: 9: CoAP CONTENT Content Response
The SCHC Rules for the Inner Compression include all fields already
present in a regular CoAP message. The methods described in
Section 4 apply to these fields. As Table 4 provides an example, see Figure 13.
RuleID example.
+===================================================================+
|RuleID 0
+--------------+--+--+--+-----------+---------+---------++------+ |
+========+==+==+==+===========+===============+==============+======+
| Field |FL|FP|DI| Target TV | MO | CDA || | Sent |
| | | | | Value | | ||[bits]|
+--------------+--+--+--+-----------+---------+---------++------+ |[bits]|
+========+==+==+==+===========+===============+==============+======+
|CoAP Code | 8| 1|up| 1 |8 |1 |Up|1 | equal |not-sent || |
|CoAP Code not-sent | 8| 1|dw|[69, |
|Code | || | | | | | |132] |match- |mapping- || | |
+--------+--+--+--+-----------+---------------+--------------+======+
|CoAP |8 |1 |Dw|[69,132] | match-mapping | mapping-sent |c |
|Code | | | | |mapping |sent || c |
|CoAP Uri-Path | | 1|up|temperature| |
+--------+--+--+--+-----------+---------------+--------------+======+
|CoAP | |1 |Up|temperature| equal |not-sent || |
+--------------+--+--+--+-----------+---------+---------++------+
Figure 13: not-sent | |
|Uri-Path| | | | | | | |
+--------+--+--+--+-----------+---------------+--------------+======+
Table 4: Inner SCHC Rules Rule
Figure 14 10 shows the Plaintext obtained for the example GET request.
The packet follows the process of Inner Compression and Encryption encryption
until the payload. The outer Outer OSCORE Message message adds the result of the
Inner process.
________________________________________________________
| |
| OSCORE Plaintext |
| |
| 0x01bb74656d7065726174757265 (13 bytes) |
| |
| 0x01 Request Code GET |
| |
| bb74656d7065726174757265 Option 11: URI_PATH |
| Value = temperature |
|________________________________________________________|
|
|
| Inner SCHC Compression
|
v
_________________________________
| |
| Compressed Plaintext |
| |
| 0x00 |
| |
| RuleID = 0x00 (1 byte) |
| (No Compression Residue) |
|_________________________________|
|
| AEAD Encryption
| (piv = 0x04)
v
_________________________________________________
| |
| encrypted_plaintext = 0xa2 (1 byte) |
| tag = 0xc54fe1b434297b62 (8 bytes) |
| |
| ciphertext = 0xa2c54fe1b434297b62 (9 bytes) |
|_________________________________________________|
Figure 14: 10: Plaintext compression Compression and encryption Encryption for GET Request
In this case, the original message has no payload, and its resulting
Plaintext is compressed up to only 1 byte (size (the size of the RuleID).
The AEAD algorithm preserves this length in its first output and
yields a fixed-size tag. SCHC cannot compress the tag, and the
OSCORE message must include it without compression. The use of
integrity protection translates into an overhead in total message
length, limiting the amount of compression that can be achieved and plays
playing into the cost of adding security to the exchange.
Figure 15 11 shows the process for the example CONTENT Response. Content response. The
Compression Residue is 1 bit long. Note that since SCHC adds padding
after the payload, this misalignment causes the hexadecimal code from
the payload to differ from the original, even if SCHC cannot compress
the tag. The overhead for the tag bytes limits the SCHC's performance
but brings security to the transmission.
________________________________________________________
| |
| OSCORE Plaintext |
| |
| 0x45ff32332043 (6 bytes) |
| |
| 0x45 Successful Response Code 69 "2.05 Content" |
| |
| ff Payload marker |
| |
| 32332043 Payload |
|________________________________________________________|
|
|
| Inner SCHC Compression
|
v
_____________________________________________
_________________________________________________
| |
| Compressed Plaintext |
| |
| 0x001919902180 (6 bytes) |
| |
| 00 RuleID |
| |
| 0b0 (1 bit match-map match-mapping Compression Residue) |
| 0x32332043 >> 1 (shifted payload) |
| 0b0000000 Padding |
|_____________________________________________|
|_________________________________________________|
|
| AEAD Encryption
| (piv = 0x04)
v
_________________________________________________________
| |
| encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) |
| tag = 0xe9aef3f2461e0c29 (8 bytes) |
| |
| ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) |
|_________________________________________________________|
Figure 15: 11: Plaintext compression Compression and encryption Encryption for CONTENT Content Response
The Outer SCHC Rules (Figure 18) Rule (Table 5) must process the OSCORE Options options fields. Figure 16
Figures 12 and Figure 17 shows 13 show a dump of the OSCORE Messages messages generated from
the example messages. They include the Inner Compressed Ciphertext ciphertext
in the payload. These are the messages that have to be compressed by
via the Outer SCHC Compression.
Figure 18 Compression scheme.
Table 5 shows a possible set of Outer Rules Rule items to compress the
Outer
Header.
RuleID header.
+===================================================================+
|RuleID 0
+------------------+--+--+--+--------------+-------+--------++------+ |
+===============+===+==+==+================+=======+=========+======+
| Field |FL|FP|DI| Target | FL|FP|DI| TV | MO | CDA || | Sent |
| | | | | Value | | ||[bits]|
+------------------+--+--+--+--------------+-------+--------++------+ |[bits]|
+===============+===+==+==+================+=======+=========+======+
|CoAP version | 2| 1|bi| |2 |1 |Bi| 01 |equal |not-sent|| |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Type | 2| 1|up| |2 |1 |Up| 0 |equal |not-sent|| |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Type | 2| 1|dw| |2 |1 |Dw| 2 |equal |not-sent|| |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP TKL | 4| 1|bi| |4 |1 |Bi| 1 |equal |not-sent|| |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Code | 8| 1|up| |8 |1 |Up| 2 |equal |not-sent|| |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Code | 8| 1|dw| |8 |1 |Dw| 68 |equal |not-sent|| |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP MID |16| 1|bi| |16 |1 |Bi| 0000 |MSB(12)|LSB ||MMMM |MMMM |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Token |tkl 1|bi| |tkl|1 |Bi| 0x80 |MSB(5) |LSB ||TTT |TTT |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_flags | 8| 1|up| |8 |1 |Up| 0x09 |equal |not-sent|| |not-sent | |
|OSCORE_flags | | | | | | | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_piv |var 1|up| OSCORE_piv|var|1 |Up| 0x00 |MSB(4) |LSB ||PPPP |PPPP |
|COAP OSCORE_kid |var 1|up|0x636c69656e70|MSB(52)|LSB ||KKKK |
|COAP OSCORE_kidctx|var 1|bi|
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_kid|var|1 |Up| 0x636c69656e70 |MSB(52)|LSB |KKKK |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP |var|1 |Bi| b'' |equal |not-sent|| |not-sent | |
|OSCORE_kidctx | | | | | |
|CoAP OSCORE_flags | 8| 1|dw| |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP |8 |1 |Dw| b'' |equal |not-sent|| |not-sent | |
|OSCORE_flags | | | | | | | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_piv |var 1|dw| OSCORE_piv|var|1 |Dw| b'' |equal |not-sent|| |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_kid |var 1|dw| OSCORE_kid|var|1 |Dw| b'' |equal |not-sent|| |not-sent |
+------------------+--+--+--+--------------+-------+--------++------+
Figure 18: |
+---------------+---+--+--+----------------+-------+---------+======+
Table 5: Outer SCHC Rules Rule
Protected message:
==================
0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62
(25 bytes)
Header:
0x4102
01 Ver
00 CON
0001 TKL
00000010 Request Code 2 "POST"
0x0001 = mid
0x82 = token
Options:
0xd8080904636c69656e74 (10 bytes)
Option 21: OBJECT_SECURITY
Value = 0x0904636c69656e74
09 = 000 0 1 001 Flag flag byte
h k n
04 piv
636c69656e74 kid
0xFF Payload marker
Payload:
0xa2c54fe1b434297b62 (9 bytes)
Figure 16: 12: Protected and Inner SCHC Compressed GET Request
Protected message:
==================
0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29
(22 bytes)
Header:
0x6144
01 Ver
10 ACK
0001 TKL
01000100 Successful Response Code 68 "2.04 Changed"
0x0001 = mid
0x82 = token
Options:
0xd008 (2 bytes)
Option 21: OBJECT_SECURITY
Value = b''
0xFF Payload marker
Payload:
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Figure 17: 13: Protected and Inner SCHC Compressed CONTENT Content Response
For the flag bits, some SCHC compression methods are useful,
depending on the Application. application. The most straightforward alternative
is to provide a fixed value for the flags, combining a MO of "equal"
and a CDA "not-sent." of "not-sent". This SCHC definition saves most bits but
could prevent flexibility. Otherwise, SCHC could use a "match-mapping" "match-
mapping" MO to choose from several configurations for the exchange.
If not, the SCHC description may use an "MSB" MO to mask off the
three hard-coded most significant bits.
Note that fixing a flag bit will limit the choices of CoAP Options choice options
that can be used in the exchange exchange, since their the values of these choices
are dependent on specific options.
The piv field lends itself to having some bits masked off with an
"MSB" MO and an "LSB" CDA. This SCHC description could be useful in
applications where the message frequency is low low, such as LPWAN
technologies. Note that compressing the sequence numbers may reduce
the maximum number of sequence numbers that can be used in an
exchange. Once the sequence number exceeds the maximum value, the
OSCORE keys need to be re-established.
The size s size, s, that is included in the kid context field MAY be masked
off with an "LSB" CDA. The rest of the field could have additional
bits masked off or have the whole field fixed with a MO of "equal"
and a CDA "not-sent." of "not-sent". The same holds for the kid field.
The Outer Rule of Figure 18 Table 5 is applied to the example GET Request request and
CONTENT Response. Figure 19
Content response. Figures 14 and Figure 20 15 show the resulting messages.
Compressed message:
==================
0x001489458a9fc3686852f6c4 (12 bytes)
0x00 RuleID
1489 Compression Residue
458a9fc3686852f6c4 Padded payload
Compression Residue:
0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding)
mid tkn piv kid
Payload
0xa2c54fe1b434297b62 (9 bytes)
Compressed message length: 12 bytes
Figure 19: 14: SCHC-OSCORE Compressed GET Request
Compressed message:
==================
0x0014218daf84d983d35de7e48c3c1852 (16 bytes)
0x00 RuleID
14 Compression Residue
218daf84d983d35de7e48c3c1852 Padded payload
Compression Residue:
0b0001 010 (7 bits -> 1 byte with padding)
mid tkn
Payload
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Compressed msg message length: 16 bytes
Figure 20: 15: SCHC-OSCORE Compressed CONTENT Content Response
In contrast, comparing these results with what would be obtained by
SCHC compressing the original CoAP messages without protecting them
with OSCORE is done by compressing the CoAP messages according to the
SCHC Rules Rule in Figure 21.
RuleID Table 6.
+===================================================================+
|RuleID 1
+---------------+--+--+--+-----------+---------+-----------++-------+ |
+========+===+==+==+===========+===============+=============+======+
| Field |FL|FP|DI| Target | FL|FP|DI| TV | MO | CDA || | Sent |
| | | | | Value | | || [bits]|
+---------------+--+--+--+-----------+---------+-----------++-------+ |[bits]|
+========+===+==+==+===========+===============+=============+======+
|CoAP version |2 |1 |Bi|01 | 2| 1|bi| 01 |equal equal |not-sent || | |
|version | | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP Type |2 |1 |Up|0 | 2| 1|up| 0 |equal equal |not-sent || | |
|Type | | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP Type |2 |1 |Dw|2 | 2| 1|dw| 2 |equal equal |not-sent || | |
|Type | | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP TKL TKL|4 |1 |Bi|1 | 4| 1|bi| 1 |equal equal |not-sent || | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP Code |8 |1 |Up|2 | 8| 1|up| 2 |equal equal |not-sent || | |
|Code | | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP Code |8 |1 |Dw|[69,132] | match-mapping |mapping-sent |C |
|Code | 8| 1|dw| [69,132] |match- |mapping- || | | | | | | |mapping |sent ||C |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP MID |16| 1|bi| 0000 |MSB(12) MID|16 |1 |Bi|0000 | MSB(12) |LSB ||MMMM |MMMM |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP Token |tkl 1|bi| 0x80 |MSB(5) |tkl|1 |Bi|0x80 | MSB(5) |LSB ||TTT |TTT |
|Token | | | | | | |
|CoAP Uri-Path |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP | 1|up|temperature|equal |1 |Up|temperature| equal |not-sent || |
+---------------+--+--+--+-----------+---------+-----------++-------+
Figure 21: |
|Uri-Path| | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
Table 6: SCHC-CoAP Rules Rule (No OSCORE)
Figure 21
The Rule in Table 6 yields the SCHC compression results as shown in
Figure 22 16 for
request, the request and Figure 23 17 for the response.
Compressed message:
==================
0x0114
0x01 = RuleID
Compression Residue:
0b00010100 (1 byte)
Compressed msg message length: 2 bytes
Figure 22: 16: CoAP GET Compressed without OSCORE
Compressed message:
==================
0x010a32332043
0x01 = RuleID
Compression Residue:
0b00001010 (1 byte)
Payload
0x32332043
Compressed msg message length: 6 bytes
Figure 23: 17: CoAP CONTENT Content Compressed without OSCORE
As can be seen, the difference between applying SCHC + OSCORE as
compared to regular SCHC + COAP CoAP is about 10 bytes.
8. IANA Considerations
This document has no request to IANA. IANA actions.
9. Security considerations Considerations
The use of SCHC header compression for CoAP header fields only
affects the representation of the header information. SCHC header
compression itself does not increase or decrease the overall level of
security of the communication. When the connection does not use a
security protocol (such as OSCORE, (OSCORE, DTLS, etc.), it is necessary to use a layer-two
Layer 2 security mechanism to protect the SCHC messages.
If an LPWAN is the layer-two technology, Layer 2 technology being used, the SCHC security
considerations of discussed in [RFC8724] continue to apply. When using
another
layer-two Layer 2 protocol, the use of a cryptographic integrity-protection
mechanisms integrity-
protection mechanism to protect the SCHC headers is REQUIRED. Such
cryptographic integrity protection is necessary in order to continue
to provide the properties that [RFC8724] relies upon.
When SCHC is used with OSCORE, the security considerations of discussed
in [RFC8613] continue to apply.
When SCHC is used with the OSCORE outer Outer headers, the Initialization
Vector (IV) size in the Compression Residue must be carefully
selected. There is a tradeoff trade-off between compression efficiency (with
a longer "MSB" MO prefix) and the frequency at which the Device must
renew its key material (in order to prevent the IV from expanding to
an uncompressable uncompressible value). The key renewal key-renewal operation itself requires
several message exchanges and requires energy-intensive computation,
but the optimal tradeoff trade-off will depend on the specifics of the device Device
and expected usage patterns.
If an attacker can introduce a corrupted SCHC-compressed packet onto
a link, DoS attacks are possible can be mounted by causing excessive resource
consumption at the decompressor. However, an attacker able to inject
packets at the link layer is also capable of other, potentially more
damaging, attacks.
SCHC compression emits variable-length Compression Residues for some
CoAP fields. In the representation of the compressed header representation, header, the
length field that is sent is not the length of the original header
field but rather the length of the Compression Residue that is being
transmitted. If a corrupted packet arrives at the decompressor with
a longer or shorter length than the original compressed
representation possessed, the SCHC decompression procedures will
detect an error and drop the packet.
SCHC header compression rules Rules MUST remain tightly coupled between the
compressor and the decompressor. If the compression rules Rules get out of
sync, a Compression Residue might be decompressed differently at the
receiver than the initial message submitted to compression
procedures. Accordingly, any time the context Rules are updated on
an OSCORE endpoint, that endpoint MUST trigger OSCORE key re-
establishment. Similar procedures may be appropriate to signal Rule
udpates
updates when other message-protection mechanisms are in use.
11.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga,
Zúñiga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
10.
Acknowledgements
The authors would like to thank (in alphabetic order): Christian
Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
Fossati, Klaus Hartke, Benjamin Kaduk, Francesca Palombini, Alexander
Pelov, Goran Selander Göran Selander, and Eric Éric Vyncke.
Authors' Addresses
Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Laurent Toutain
Institut MINES TELECOM; IMT Atlantique
CS 17607
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
Ricardo Andreasen
Universidad de Buenos Aires
Av. Paseo Colon 850
C1063ACV Ciudad Autonoma de Buenos Aires
Argentina
Email: randreasen@fi.uba.ar