LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-16
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (lpwan WG) | |
|---|---|---|---|
| Authors | Ana Minaburo , Laurent Toutain , Ricardo Andreasen | ||
| Last updated | 2020-10-20 (Latest revision 2020-07-03) | ||
| Replaces | draft-toutain-lpwan-coap-static-context-hc | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
| Reviews |
SECDIR Telechat review
(of
-15)
Has Nits
SECDIR Last Call review
(of
-13)
Has Nits
TSVART Last Call review
(of
-12)
Almost Ready
GENART Last Call review
(of
-12)
Almost Ready
IOTDIR Last Call review
(of
-11)
Almost Ready
|
||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Pascal Thubert | ||
| Shepherd write-up | Show Last changed 2019-10-09 | ||
| IESG | IESG state | IESG Evaluation::AD Followup | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs 2 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Éric Vyncke | ||
| Send notices to | Pascal Thubert <pthubert@cisco.com> | ||
| IANA | IANA review state | Version Changed - Review Needed |
draft-ietf-lpwan-coap-static-context-hc-16
lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Standards Track L. Toutain
Expires: April 23, 2021 Institut MINES TELECOM; IMT Atlantique
R. Andreasen
Universidad de Buenos Aires
October 20, 2020
LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-16
Abstract
This draft defines how Static Context Header Compression (SCHC) can
be applied to the Constrained Application Protocol (CoAP). SCHC is a
header compression mechanism adapted for constrained devices. SCHC
uses a static description of the header to reduce the redundancy and
size of the header's information. While RFC 8724 describes the SCHC
compression and fragmentation framework, and its application for
IPv6/UDP headers, this document applies SCHC for CoAP headers. The
CoAP header structure differs from IPv6 and UDP since CoAP uses a
flexible header with a variable number of options, themselves of
variable length. The CoAP protocol messages format is asymmetric:
the request messages have a header format different from the one 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 23, 2021.
Minaburo, et al. Expires April 23, 2021 [Page 1]
Internet-Draft LPWAN CoAP compression October 2020
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. SCHC Applicability to CoAP . . . . . . . . . . . . . . . . . 4
3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 7
3.1. Differences between CoAP and UDP/IP Compression . . . . . 8
4. Compression of CoAP header fields . . . . . . . . . . . . . . 9
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 9
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 9
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 9
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 10
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 10
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 11
5.2. CoAP option Max-Age, Uri-Host, and Uri-Port fields . . . 11
5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 11
5.3.1. Variable-length Uri-Path and Uri-Query . . . . . . . 12
5.3.2. Variable number of Path or Query elements . . . . . . 12
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
fields . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path,
and Location-Query fields . . . . . . . . . . . . . . . . 13
6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 13
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 13
6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Examples of CoAP header compression . . . . . . . . . . . . . 15
7.1. Mandatory header with CON message . . . . . . . . . . . . 15
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 16
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
Minaburo, et al. Expires April 23, 2021 [Page 2]
Internet-Draft LPWAN CoAP compression October 2020
9. Security considerations . . . . . . . . . . . . . . . . . . . 29
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
11. Normative References . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
CoAP [rfc7252] is a command/response protocol designed for micro-
controllers with a small amount of RAM and ROM and is optimized for
REST-based (Representational state transfer) services. Although CoAP
was designed for Low-Power Wireless Personal Area Networks (6LoWPAN),
a CoAP header's size is still too large for LPWAN (Low Power Wide
Area Networks) and some compression of the CoAP header is required
either to increase performances or allow CoAP other some LPWAN
technologies.
The [rfc8724] defines SCHC, a header compression mechanism for the
LPWAN network based on a static context. Section 5 of the [rfc8724]
explains the architecture where compression and decompression are
done. The SCHC compression scheme assumes as a prerequisite that the
static context is known to both endpoints 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: one to compress IP and
UDP in the LPWAN network and another at the application level for
CoAP. These two compressions may be independent. Both follow the
same principle described in RFC8724. SCHC rules driving the
compression/decompression are different and may be managed by
different entities. The [rfc8724] describes how the IP and UDP
headers may be compressed. This document specifies how the SCHC
compression rules can be applied to CoAP traffic.
SCHC compresses and decompresses headers based on shared contexts
between devices.
Each context consists of multiple Rules. Each Rule can match header
fields and specific values or ranges of values.
If a Rule matches, the matched header fields are replaced by the
RuleID and some residual bits. Thus, different Rules may correspond
to divers protocols packets that a device expects to send or receive.
A Rule describes the packets's entire header with an ordered list of
fields descriptions; see section 7 of [rfc8724]. Thereby
each description contains the field ID (FID), its length (FL), and
its position (FP), a direction indicator (DI) (upstream, downstream,
and bidirectional), and some associated Target Values (TV). The
Minaburo, et al. Expires April 23, 2021 [Page 3]
Internet-Draft LPWAN CoAP compression October 2020
direction indicator is used for compression to give the best TV to
the FID when these values differ in the transmission direction. So a
field may be described several times depending on the asymmetry of
its possible TVs.
A Matching Operator (MO) is associated with each header field
description.
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 a
field unknown to the SCHC compressor.
In that case, a Compression/Decompression Action (CDA) associated
with each field give the method to compress and decompress each
field. Compression mainly results in one of 4 actions:
o send the field value,
o send nothing,
o send some least significant bits of the field or
o send an index.
After applying the compression, there may be some bits to be sent.
These values are called Compression Residues.
SCHC is a general mechanism applied to different protocols, the exact
Rules to be used depending on the protocol and the 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] when, and only when, they appear in all
capitals, as shown here.
2. SCHC Applicability to CoAP
The SCHC Compression Rules can be applied to CoAP headers. SCHC
Compression of the CoAP header 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 1,Figure 2 and Figure 3
Minaburo, et al. Expires April 23, 2021 [Page 4]
Internet-Draft LPWAN CoAP compression October 2020
In the first example, Figure 1, a Rule compresses the complete header
stack from IPv6 to CoAP. In this case, SCHC C/D (Static Context
Header Compression Compressor/Decompressor) is performed at the
device and the application. The host 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
The SCHC can be viewed as a layer above layer 2. This layer received
non-encrypted packets and can apply compression rule to all the
headers. On the other end, the NGW receives the SCHC packet and
reconstructs the headers from the rule, identified by its ID and the
header residues. The result is a regular IPv6 packet that can be
forwarded toward the destination. The same process applies in the
other direction. A not encrypted packet arrived at the NGW, thanks
to 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] document (represented
in dotted lines).
This use case needs an end-to-end context initialization between the
device and the application and is out-of-scope of this document.
Minaburo, et al. Expires April 23, 2021 [Page 5]
Internet-Draft LPWAN CoAP compression October 2020
(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 compression/decompression
In the third example, Figure 3, the Object Security for Constrained
RESTful Environments (OSCORE) [rfc8613] is used. In this case, two
rulesets are used to compress the CoAP message. A first ruleset
focused on the inner header compresses it. The result is encrypted
using the OSCORE mechanism. A second ruleset compresses the outer
header, including the OSCORE Options.
Minaburo, et al. Expires April 23, 2021 [Page 6]
Internet-Draft LPWAN CoAP compression October 2020
(device) (NGW) (App)
+--------+ +--------+
| CoAP | | CoAP |
| inner | | inner |
+--------+ +--------+
| SCHC | | SCHC |
| inner | | inner |
+--------+ +--------+
| CoAP | | CoAP |
| outer | | outer |
+--------+ +--------+
| SCHC | | SCHC |
| outer | | outer |
+--------+ +--------+
. udp . . udp .
.......... .................. ..........
. ipv6 . . ipv6 . . ipv6 .
.......... .................. ..........
. schc . . schc . . . .
.......... .......... . . .
. lpwan . . lpwan . . . .
.......... .................. ..........
((((LPWAN)))) ------ Internet ------
Figure 3: OSCORE compression/decompression.
In the case of several SCHC instances, as shown in Figure 3 and
Figure 3, the rulesets may come from different provisioning domains.
This document focuses on CoAP compression represented in the dashed
boxes in the previous figures.
3. CoAP Headers compressed with SCHC
The use of SCHC over the CoAP header uses the same description and
compression/decompression techniques like the one for IP and UDP
explained in the [rfc8724]. For CoAP, SCHC Rules description uses
the direction information to optimize the compression by reducing the
number of Rules needed to compress headers. The field description
MAY define both request/response headers and target values in the
same Rule, using the DI (direction indicator) to make the difference.
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
Minaburo, et al. Expires April 23, 2021 [Page 7]
Internet-Draft LPWAN CoAP compression October 2020
the Compression Residue is the complete header of the packet. See
section 6 of [rfc8724].
3.1. Differences between CoAP and UDP/IP Compression
CoAP compression differs from IPv6 and UDP compression on the
following aspects:
o The CoAP protocol is asymmetric; the headers are different for a
request or a response. For example, the URI-Path option is
mandatory in the request, and it may not be present in the
response. A request may contain an Accept option, and the
response may include a Content-Format option. In comparison, IPv6
and UDP returning path swap the value of some fields in the
header. But all the directions have the same fields (e.g., source
and destination address fields).
The [rfc8724] defines the use of a Direction Indicator (DI) in the
Field Descriptor, which allows a single Rule to process a message
headers 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 matching list in the TV to limit the
range of expected values in a particular direction and therefore
reduce the Compression Residue's size. Through the Direction
Indicator (DI), a field description in the Rules splits the
possible field value into two parts, one for each direction. For
instance, if a client sends only CON requests, the type can be
elided by compression, and the answer may use one single bit to
carry either the ACK or RST type. The field Code has the same
behavior, the 0.0X code format value in the request, and Y.ZZ code
format in the response.
o Headers in IPv6 and UDP have a fixed size. The size is not sent
as part of the Compression Residue but is defined in the Rule.
Some CoAP header fields have variable lengths, so the length is
also specified in the Field Description. For example, the Token
size may vary from 0 to 8 bytes. And the CoAP options have a
variable length since they use the Type-Length-Value encoding
format, as URI-path or URI-query.
Section 7.5.2 from [rfc8724] offers the possibility to define a
function for the Field length in the Field Description to know the
length before compression. When doing SCHC compression of a
variable-length field,
if the field size is unknown, the Field Length in the Rule is set
as variable, and the size is sent with the Compression Residue.
Minaburo, et al. Expires April 23, 2021 [Page 8]
Internet-Draft LPWAN CoAP compression October 2020
o A field can appear several times in the CoAP headers. This is
typical for elements of a URI (path or queries). The SCHC
specification [rfc8724] allows a Field ID to appear several times
in the Rule and uses the Field Position (FP) to identify the
correct instance, and thereby removing the ambiguity of the
matching operation.
o Field sizes defined in the CoAP protocol can be too
large regarding LPWAN traffic constraints. This is particularly
true for the Message-ID field and the Token field. SCHC uses
different Matching operators (MO) to perform the compression. See
section 7.4 of [rfc8724]. In this case, the Most Significant Bits
(MSB) MO can be applied to reduce the information carried on
LPWANs.
4. Compression of CoAP header fields
This section discusses the compression of the different CoAP header
fields. The CoAP compression with SCHC follows the Section 7.1 of
[rfc8724].
4.1. CoAP version field
CoAP version is bidirectional and MUST be elided during the SCHC
compression since it always contains the same value. In the future,
if new versions of CoAP are defined, new Rules will be needed to
avoid ambiguities between versions.
4.2. CoAP type field
The CoAP Protocol [rfc7252] has four types of messages: two requests
(CON, NON), one response (ACK), and one empty message (RST).
The field SHOULD be elided if, for instance, a client is sending only
NON or only CON messages. For the RST message, a dedicated Rule may
be needed. For other usages, a mapping list can be used.
4.3. CoAP code field
The code field indicates the Request Method used in CoAP, an IANA
registry [rfc7252]. The compression of the CoAP code field follows
the same principle as that of the CoAP type field. If the device
plays a specific role, the set of code values can be split into two
parts, the request codes with the 0 class and the response values.
If the device only implements a CoAP client, the request code can be
reduced to the set of requests the client can to process.
Minaburo, et al. Expires April 23, 2021 [Page 9]
Internet-Draft LPWAN CoAP compression October 2020
A mapping list can be used for known values. The field cannot be
compressed for other values, and the value needs to be sent in the
Compression Residue.
4.4. CoAP Message ID field
The Message ID field can be compressed with the MSB(x) MO and the
Least Significant Bits (LSB) CDA. See section 7.4 of [rfc8724].
4.5. CoAP Token fields
A Token is defined through two CoAP fields, Token Length in the
mandatory header and Token Value directly following the mandatory
CoAP header.
Token Length is processed as any protocol field. If the value does
not change, the size can be stored in the TV and elided during the
transmission. Otherwise, it will have to be sent in the Compression
Residue.
Token Value MUST NOT be sent as a variable-length residue to avoid
ambiguity with Token Length. Therefore, the Token Length value MUST
be used to define the size of the Compression Residue. A specific
function designated as "TKL" MUST be used in the Rule. During the
decompression, this function returns the value contained in the Token
Length field.
5. CoAP options
CoAP defines options that are placed after the based header in Option
Numbers order, see [rfc7252]. Each Option instance in a message uses
the format Delta-Type (D-T), Length (L), Value (V). When applying
SCHC compression to the Option, the D-T, L, and V format serve to
make the Rule description of the Option. The SCHC compression builds
the description of the Option by using in the Field ID the Option
Number built from D-T; in TV, the Option Value; and the Option Length
uses section 7.4 of [rfc8724]. When the Option Length has a
wellknown size, it can be stored in the Rule. Therefore, SCHC
compression does not send it. Otherwise, SCHC 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 Rules may reflect this asymmetry by tagging the direction
indicator.
Note that length coding differs between CoAP options and SCHC
variable size Compression Residue.
Minaburo, et al. Expires April 23, 2021 [Page 10]
Internet-Draft LPWAN CoAP compression October 2020
The following sections present how SCHC compresses some specific CoAP
options.
If a new option is introduced in CoAP, a new Field ID has to be
assigned in the Rules to allow its compression. Otherwise, if no
Rule describes this Option, the SCHC compression is not possible, and
the CoAP header is sent without compression.
5.1. CoAP Content and Accept 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 matching-list SHOULD be used to limit the
Compression Residue's size. Otherwise, the value has to be sent as a
Compression Residue (fixed or variable length).
5.2. CoAP option Max-Age, Uri-Host, and Uri-Port fields
If both ends know the value, the value can be elided.
A matching list can be used if some well-known values are defined.
Otherwise, these options can be sent as a Compression Residue.
5.3. CoAP option Uri-Path and Uri-Query fields
Uri-Path and Uri-Query elements are repeatable options. The Field
Position (FP) gives the position in the path.
A Mapping list can be used to reduce the size of variable Paths or
Queries. In that case, to optimize the compression, several elements
can be regrouped into a single entry. The Numbering of elements do
not change; MO comparison is set with the first element of the
matching.
+-------------+---+--+--+--------+---------+-------------+
| Field |FL |FP|DI| Target | Match | CDA |
| | | | | Value | Opera. | |
+-------------+---+--+--+--------+---------+-------------+
|Uri-Path | | 1|up|["/a/b",|equal |not-sent |
| | | | |"/c/d"] | | |
|Uri-Path |var| 3|up| |ignore |value-sent |
+-------------+---+--+--+--------+---------+-------------+
Figure 4: complex path example
Minaburo, et al. Expires April 23, 2021 [Page 11]
Internet-Draft LPWAN CoAP compression October 2020
In Figure 4, a single bit residue can be used to code one of the 2
paths. If regrouping were not allowed, a 2 bits residue would be
needed. The third path element is sent as a variable size residue.
5.3.1. Variable-length Uri-Path and Uri-Query
When the length is not known at the Rule creation, the Field Length
MUST be set to variable, and the unit is set to bytes.
The MSB MO can be applied to a Uri-Path or Uri-Query element. Since
MSB value is given in bit, the size MUST always be a multiple of 8
bits.
The length sent at the beginning of a variable-length residue
indicates the size of the LSB in bytes.
For instance, for a CORECONF path /c/X6?k="eth0" the Rule can be set
to:
+-------------+---+--+--+--------+---------+-------------+
| Field |FL |FP|DI| Target | Match | CDA |
| | | | | Value | Opera. | |
+-------------+---+--+--+--------+---------+-------------+
|Uri-Path | 8| 1|up|"c" |equal |not-sent |
|Uri-Path |var| 2|up| |ignore |value-sent |
|Uri-Query |var| 1|up|"k=" |MSB(16) |LSB |
+-------------+---+--+--+--------+---------+-------------+
Figure 5: CORECONF URI compression
Figure 5 shows the parsing and the compression of the URI, where c is
not sent. The second element is sent with the length (i.e., 0x2 X 6)
followed by the query option (i.e. 0x05 "eth0").
5.3.2. Variable number of Path or Query elements
The number of Uri-Path or Uri-Query elements in a Rule is fixed at
the Rule creation time. If the number varies, several Rules SHOULD
be created to cover all the possibilities. Another possibility is to
define the length of Uri-Path to variable and send a Compression
Residue with a length of 0 to indicate that this Uri-Path is empty.
This adds 4 bits to the variable Residue size. See section 7.5.2
[rfc8724]
Minaburo, et al. Expires April 23, 2021 [Page 12]
Internet-Draft LPWAN CoAP compression October 2020
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields
If the field value has to be sent, TV is not set, MO is set to
"ignore", and CDA is set to "value-sent." A mapping MAY also be
used.
Otherwise, the TV is set to the value, MO is set to "equal", and CDA
is set to "not-sent".
5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path, and
Location-Query fields
These fields' values cannot be stored in a Rule entry. They MUST
always be sent with the Compression Residues.
6. SCHC compression of CoAP extension RFCs
6.1. Block
Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also
includes a fragmentation protocol. They can be both used. If a
block option is used, its content MUST be sent as a Compression
Residue.
6.2. Observe
The [rfc7641] defines the Observe option. The TV is not set, MO is
set to "ignore", and the CDA is set to "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, or a proxy can modify the
increment.
Since an RST message may be sent to inform a server that the client
does not require Observe response; a 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. If both ends know the value, then TV
is set to this value, MO is set to "equal", and CDA is set to "not-
sent".
Otherwise, if the value is changing over time, TV is not set, MO is
set to "ignore", and CDA to "value-sent". A matching list can also
be used to reduce the size.
Minaburo, et al. Expires April 23, 2021 [Page 13]
Internet-Draft LPWAN CoAP compression October 2020
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.
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: OSCORE Option
The encoding of the OSCORE Option Value defined in Section 6.1 of
[rfc8613] is repeated in Figure 6.
The first byte specifies the 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 indicate the
length of the piv (Partial Initialization Vector) field in bytes.
When n = 0, no piv is present.
The flag byte is followed by the piv field, kid context field, and
kid field in this order, and if present, the length of the kid
context field is encoded in the first byte denoting by s the length
of the kid context in bytes.
This specification recommends identifying the OSCORE Option and the
fields it contains. Conceptually, it discerns up to 4 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 to compress them:
o CoAP OSCORE_flags,
Minaburo, et al. Expires April 23, 2021 [Page 14]
Internet-Draft LPWAN CoAP compression October 2020
o CoAP OSCORE_piv,
o CoAP OSCORE_kidctx,
o CoAP OSCORE_kid.
Figure 6 shows the OSCORE Option format with those four fields
superimposed on it. Note that the CoAP OSCORE_kidctx field includes
directly the size octet s.
7. Examples of CoAP header compression
7.1. Mandatory header with CON message
In this first scenario, the LPWAN Compressor at the Network Gateway
side receives from an Internet client a POST message, which is
immediately acknowledged by the Device. For this simple scenario,
the Rules are described in Figure 7.
RuleID 1
+-------------+--+--+--+------+---------+-------------++------------+
| Field |FL|FP|DI|Target| Match | CDA || Sent |
| | | | |Value | Opera. | || [bits] |
+-------------+--+--+--+------+---------+-------------++------------+
|CoAP version | 2| 1|bi| 01 |equal |not-sent || |
|CoAP Type | 2| 1|dw| CON |equal |not-sent || |
|CoAP Type | 2| 1|up|[ACK, | | || |
| | | | | RST] |match-map|matching-sent|| T |
|CoAP TKL | 4| 1|bi| 0 |equal |not-sent || |
|CoAP Code | 8| 1|bi|[0.00,| | || |
| | | | | ... | | || |
| | | | | 5.05]|match-map|matching-sent|| CC CCC |
|CoAP MID |16| 1|bi| 0000 |MSB(7 ) |LSB || M-ID|
|CoAP Uri-Path|var 1|dw| path |equal 1 |not-sent || |
+-------------+--+--+--+------+---------+-------------++------------+
Figure 7: CoAP Context to compress header without token
The version and Token Length fields are elided. The 26 method and
response codes defined in [rfc7252] has been shrunk to 5 bits using a
matching list. Uri-Path contains a single element indicated in the
matching operator.
Minaburo, et al. Expires April 23, 2021 [Page 15]
Internet-Draft LPWAN CoAP compression October 2020
SCHC Compression reduces the header sending only the Type, a mapped
code and the least significant bits of Message ID (9 bits in the
example above).
Note that a request sent by a client located in an Application Server
to a server located in the device, may not be compressed through this
Rule since the MID will not start with 7 bits equal to 0. A CoAP
proxy, before the core SCHC C/D can rewrite the message ID to a value
matched by the Rule.
7.2. OSCORE Compression
OSCORE aims to solve the problem of end-to-end encryption for CoAP
messages. The goal, therefore, is to hide as much of the message as
possible while still enabling proxy operation.
Conceptually this is achieved by splitting the CoAP message into an
Inner Plaintext and Outer OSCORE Message. The Inner Plaintext
contains sensitive information that is not necessary for proxy
operation. This, in turn, is the part of the message which 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 and information needed for proxy operation and caching.
This decomposition is illustrated in Figure 8.
CoAP options are sorted into one of 3 classes, each granted a
specific type of protection by the protocol:
o Class E: Encrypted options moved to the Inner Plaintext,
o Class I: Integrity-protected options included in the AAD for the
encryption of the Plaintext but otherwise left untouched in the
Outer Message,
o Class U: Unprotected options left untouched in the Outer Message.
Additionally, the OSCORE Option is added as an Outer option,
signaling that the message is OSCORE protected. This option carries
the information necessary to retrieve the Security Context with which
the message was encrypted to be correctly decrypted at the other end-
point.
Minaburo, et al. Expires April 23, 2021 [Page 16]
Internet-Draft LPWAN CoAP compression October 2020
Original CoAP Message
+-+-+---+-------+---------------+
|v|t|tkl| code | Msg Id. |
+-+-+---+-------+---------------+....+
| Token |
+-------------------------------.....+
| Options (IEU) |
. .
. .
+------+-------------------+
| 0xFF |
+------+------------------------+
| |
| Payload |
| |
+-------------------------------+
/ \
/ \
/ \
/ \
Outer Header v v Plaintext
+-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF |
. . +-------+-----------+
. OSCORE Option . | |
+------+-------------------+ | Payload |
| 0xFF | | |
+------+ +-------------------+
Figure 8: A CoAP message is split into an OSCORE outer and plaintext
Figure 8 shows the message format for the OSCORE Message and
Plaintext.
In the Outer Header, the original message 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 Observe is not used. If Observe is used,
the message code is replaced by FETCH for requests and Content for
responses.
The original message code is put into the first byte of the
Plaintext. Following the message code, the class E options come,
Minaburo, et al. Expires April 23, 2021 [Page 17]
Internet-Draft LPWAN CoAP compression October 2020
and, if present, the original message Payload is preceded by its
payload marker.
The Plaintext is now encrypted by an AEAD algorithm which integrity
protects Security Context parameters and, eventually, any class I
options from the Outer Header. Currently, no CoAP options are marked
class I. The resulting Ciphertext becomes the new Payload of the
OSCORE message, as illustrated in Figure 9.
As defined in [rfc5116], this Ciphertext is the concatenation of the
encrypted Plaintext and its 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 fixed in length and is considered part of the
cost of protection.
Outer Header
+-+-+---+--------+---------------+
|v|t|tkl|new code| Msg Id. |
+-+-+---+--------+---------------+....+
| Token |
+--------------------------------.....+
| Options (IU) |
. .
. OSCORE Option .
+------+-------------------+
| 0xFF |
+------+---------------------------+
| |
| Ciphertext: Encrypted Inner |
| Header and Payload |
| + Authentication Tag |
| |
+----------------------------------+
Figure 9: OSCORE message
The SCHC Compression scheme consists of compressing both the
Plaintext before encryption and the resulting OSCORE message after
encryption, see Figure 10.
This translates into a segmented process where SCHC compression is
applied independently in 2 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.
Minaburo, et al. Expires April 23, 2021 [Page 18]
Internet-Draft LPWAN CoAP compression October 2020
Note that since the corresponding end-point can only decrypt the
Inner part of the message, this end-point will also have to implement
Inner SCHC Compression/Decompression.
Outer Message OSCORE Plaintext
+-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF |
. . +-------+-----------+
. OSCORE Option . | |
+------+-------------------+ | Payload |
| 0xFF | | |
+------+------------+ +-------------------+
| Ciphertext |<---------\ |
| | | v
+-------------------+ | +-----------------+
| | | Inner SCHC |
v | | Compression |
+-----------------+ | +-----------------+
| Outer SCHC | | |
| Compression | | v
+-----------------+ | +-------+
| | |RuleID |
v | +-------+--+
+--------+ +------------+ | Residue |
|RuleID' | | Encryption | <--- +----------+--------+
+--------+--+ +------------+ | |
| Residue' | | Payload |
+-----------+-------+ | |
| Ciphertext | +-------------------+
| |
+-------------------+
Figure 10: OSCORE Compression Diagram
7.3. Example OSCORE Compression
An example is given with a GET Request and its consequent Content
Response from a device-based CoAP client to a cloud-based CoAP
server. A possible set of Rules for the Inner and Outer SCHC
Compression is shown. A dump of the results and a contrast between
SCHC + OSCORE performance with SCHC + COAP performance is also
listed. This gives an approximation to the cost of security with
SCHC-OSCORE.
Minaburo, et al. Expires April 23, 2021 [Page 19]
Internet-Draft LPWAN CoAP compression October 2020
Our first example CoAP message is the GET Request in Figure 11
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 length: 17 bytes.
Figure 11: CoAP GET Request
Its corresponding response is the CONTENT Response in Figure 12.
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 length: 10
Figure 12: CoAP CONTENT Response
Minaburo, et al. Expires April 23, 2021 [Page 20]
Internet-Draft LPWAN CoAP compression October 2020
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 an example, see Figure 13.
RuleID 0
+--------------+--+--+--+-----------+----------+----------++------+
| Field |FL|FP|DI| Target | MO | CDA || Sent |
| | | | | Value | | ||[bits]|
+--------------+--+--+--+-----------+----------+----------++------+
|CoAP Code | 8| 1|up| 1 | equal |not-sent || |
|CoAP Code | 8| 1|dw|[69,132] | match-map|match-sent|| c |
|CoAP Uri-Path |88| 1|up|temperature| equal |not-sent || |
+--------------+--+--+--+-----------+----------+----------++------+
Figure 13: Inner SCHC Rules
Figure 14 shows the Plaintext obtained for the example GET Request
and follows the process of Inner Compression and Encryption until the
end up with the Payload to be added in the outer OSCORE Message.
In this case, the original message has no payload, and its resulting
Plaintext can be compressed up to only 1 byte (size of the RuleID).
The AEAD algorithm preserves this length in its first output and
yields a fixed-size tag that cannot be compressed and has to be
included in the OSCORE message. This translates into an overhead in
total message length, limiting the amount of compression that can be
achieved and plays into the cost of adding security to the exchange.
Minaburo, et al. Expires April 23, 2021 [Page 21]
Internet-Draft LPWAN CoAP compression October 2020
________________________________________________________
| |
| 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 residue) |
|_________________________________|
|
| AEAD Encryption
| (piv = 0x04)
v
_________________________________________________
| |
| encrypted_plaintext = 0xa2 (1 byte) |
| tag = 0xc54fe1b434297b62 (8 bytes) |
| |
| ciphertext = 0xa2c54fe1b434297b62 (9 bytes) |
|_________________________________________________|
Figure 14: Plaintext compression and encryption for GET Request
In Figure 15, the process is repeated for the example CONTENT
Response. The 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 though it has
not been compressed.
Minaburo, et al. Expires April 23, 2021 [Page 22]
Internet-Draft LPWAN CoAP compression October 2020
On top of this, the overhead from the tag bytes is incurred as
before.
________________________________________________________
| |
| 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 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: Plaintext compression and encryption for CONTENT Response
Minaburo, et al. Expires April 23, 2021 [Page 23]
Internet-Draft LPWAN CoAP compression October 2020
The Outer SCHC Rules (Figure 18) must process the OSCORE Options
fields. The Figure 16 and Figure 17 show a dump of the OSCORE
Messages generated from the example messages once they have been
provided with the Inner Compressed Ciphertext in the payload. These
are the messages that have to be compressed by the Outer SCHC
Compression.
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 byte
h k n
04 piv
636c69656e74 kid
0xFF Payload marker
Payload:
0xa2c54fe1b434297b62 (9 bytes)
Figure 16: Protected and Inner SCHC Compressed GET Request
Minaburo, et al. Expires April 23, 2021 [Page 24]
Internet-Draft LPWAN CoAP compression October 2020
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: Protected and Inner SCHC Compressed CONTENT Response
For the flag bits, some SCHC compression methods are useful,
depending on the application. The simplest alternative is to provide
a fixed value for the flags, combining MO equal and CDA not- sent.
This saves most bits but could prevent flexibility. Otherwise,
match-mapping could be used to choose from an interesting number of
configurations for the exchange.
Otherwise, MSB could be used to mask off the 3 hard-coded most
significant bits.
Note that fixing a flag bit will limit CoAP Options choice that can
be used in the exchange since their values are dependent on certain
options.
The piv field lends itself to having some bits masked off with MO MSB
and CDA LSB. This could be useful in applications where the message
frequency is low such as LPWAN technologies. Note that compressing
the sequence numbers effectively reduces the maximum number of
sequence numbers used in an exchange. Once this amount is exceeded,
the OSCORE keys need to be re-established.
The size s included in the kid context field MAY be masked off with
CDA MSB. The rest of the field could have additional bits masked off
Minaburo, et al. Expires April 23, 2021 [Page 25]
Internet-Draft LPWAN CoAP compression October 2020
or have the whole field be fixed with MO equal and CDA not-sent. The
same holds for the kid field.
Figure 18 shows a possible set of Outer Rules to compress the Outer
Header.
RuleID 0
+------------------+--+--+--+--------------+-------+--------++------+
| Field |FL|FP|DI| Target | MO | CDA || Sent |
| | | | | Value | | ||[bits]|
+------------------+--+--+--+--------------+-------+--------++------+
|CoAP version | 2| 1|bi| 01 |equal |not-sent|| |
|CoAP Type | 2| 1|up| 0 |equal |not-sent|| |
|CoAP Type | 2| 1|dw| 2 |equal |not-sent|| |
|CoAP TKL | 4| 1|bi| 1 |equal |not-sent|| |
|CoAP Code | 8| 1|up| 2 |equal |not-sent|| |
|CoAP Code | 8| 1|dw| 68 |equal |not-sent|| |
|CoAP MID |16| 1|bi| 0000 |MSB(12)|LSB ||MMMM |
|CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT |
|CoAP OSCORE_flags | 8| 1|up| 0x09 |equal |not-sent|| |
|CoAP OSCORE_piv |var 1|up| 0x00 |MSB(4) |LSB ||PPPP |
|COAP OSCORE_kid |var 1|up|0x636c69656e70|MSB(52)|LSB ||KKKK |
|COAP OSCORE_kidctx|var 1|bi| b'' |equal |not-sent|| |
|CoAP OSCORE_flags | 8| 1|dw| b'' |equal |not-sent|| |
|CoAP OSCORE_piv |var 1|dw| b'' |equal |not-sent|| |
|CoAP OSCORE_kid |var 1|dw| b'' |equal |not-sent|| |
+------------------+--+--+--+--------------+-------+--------++------+
Figure 18: Outer SCHC Rules
These Outer Rules are applied to the example GET Request and CONTENT
Response. The resulting messages are shown in Figure 19 and
Figure 20.
Minaburo, et al. Expires April 23, 2021 [Page 26]
Internet-Draft LPWAN CoAP compression October 2020
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: 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 length: 16 bytes
Figure 20: SCHC-OSCORE Compressed 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 in Figure 21.
Minaburo, et al. Expires April 23, 2021 [Page 27]
Internet-Draft LPWAN CoAP compression October 2020
RuleID 1
+---------------+--+--+--+-----------+---------+-----------++-------+
| Field |FL|FP|DI| Target | MO | CDA || Sent |
| | | | | Value | | || [bits]|
+---------------+--+--+--+-----------+---------+-----------++-------+
|CoAP version | 2| 1|bi| 01 |equal |not-sent || |
|CoAP Type | 2| 1|up| 0 |equal |not-sent || |
|CoAP Type | 2| 1|dw| 2 |equal |not-sent || |
|CoAP TKL | 4| 1|bi| 1 |equal |not-sent || |
|CoAP Code | 8| 1|up| 2 |equal |not-sent || |
|CoAP Code | 8| 1|dw| [69,132] |match-map|map-sent ||C |
|CoAP MID |16| 1|bi| 0000 |MSB(12) |LSB ||MMMM |
|CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT |
|CoAP Uri-Path |88| 1|up|temperature|equal |not-sent || |
+---------------+--+--+--+-----------+---------+-----------++-------+
Figure 21: SCHC-CoAP Rules (No OSCORE)
This yields the results in Figure 22 for the Request, and Figure 23
for the Response.
Compressed message:
==================
0x0114
0x01 = RuleID
Compression Residue:
0b00010100 (1 byte)
Compressed msg length: 2
Figure 22: CoAP GET Compressed without OSCORE
Minaburo, et al. Expires April 23, 2021 [Page 28]
Internet-Draft LPWAN CoAP compression October 2020
Compressed message:
==================
0x010a32332043
0x01 = RuleID
Compression Residue:
0b00001010 (1 byte)
Payload
0x32332043
Compressed msg length: 6
Figure 23: CoAP CONTENT Compressed without OSCORE
As can be seen, the difference between applying SCHC + OSCORE as
compared to regular SCHC + COAP is about 10 bytes.
8. IANA Considerations
This document has no request to IANA.
9. Security considerations
When applied to LPWAN, the Security Considerations of SCHC header
compression [rfc8724] are valid for SCHC CoAP header compression.
When CoAP uses OSCORE, the security considerations defined in
[rfc8613] does not change when SCHC header compression is applied.
The definition of SCHC over CoAP header fields permits the
compression of header information only. The SCHC header compression
itself does not increase or reduce the level of security in the
communication. When the connection does not use any security
protocol as OSCORE, DTLS, or other, it is highly necessary to use a
layer two security.
DoS attacks are possible if an intruder can introduce a compressed
SCHC corrupted packet onto the link and cause a compression
efficiency reduction. However, an intruder having the ability to add
corrupted packets at the link layer raises additional security issues
than those related to the use of header compression.
SCHC compression returns variable-length Residues for some CoAP
fields. In the compressed header, the length sent is not the
original header field length but the length of the Residue. So if a
corrupted packet comes to the decompressor with a longer or shorter
Minaburo, et al. Expires April 23, 2021 [Page 29]
Internet-Draft LPWAN CoAP compression October 2020
length than the one in the original header, SCHC decompression will
detect an error and drops the packet.
OSCORE compression is also based on the same compression method
described in [rfc8724]. The size of the Initialisation Vector (IV)
residue must be considered carefully. A residue size obtained with
LSB CDA over the IV impacts on the compression efficiency and the
frequency the device will renew its key. This operation requires
several exchanges and is energy-consuming.
SCHC header and compression Rules MUST remain tightly coupled.
Otherwise, an encrypted residue may be decompressed differently by
the receiver. To avoid this situation, if the Rule is modified in
one location, the OSCORE keys MUST be re-established.
10. Acknowledgements
The authors would like to thank (in alphabetic order): Christian
Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran
Selander.
11. 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>.
Minaburo, et al. Expires April 23, 2021 [Page 30]
Internet-Draft LPWAN CoAP compression October 2020
[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, "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>.
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
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
Minaburo, et al. Expires April 23, 2021 [Page 31]