< draft-ietf-lpwan-coap-static-context-hc-16.txt   draft-ietf-lpwan-coap-static-context-hc-17.txt >
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Standards Track L. Toutain Intended status: Standards Track L. Toutain
Expires: April 23, 2021 Institut MINES TELECOM; IMT Atlantique Expires: July 25, 2021 Institut MINES TELECOM; IMT Atlantique
R. Andreasen R. Andreasen
Universidad de Buenos Aires Universidad de Buenos Aires
October 20, 2020 January 21, 2021
LPWAN Static Context Header Compression (SCHC) for CoAP LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-16 draft-ietf-lpwan-coap-static-context-hc-17
Abstract Abstract
This draft defines how Static Context Header Compression (SCHC) can This draft defines how to compress the Constrained Application
be applied to the Constrained Application Protocol (CoAP). SCHC is a Protocol (CoAP) using the Static Context Header Compression (SCHC).
header compression mechanism adapted for constrained devices. SCHC SCHC is a header compression mechanism adapted for Constrained
uses a static description of the header to reduce the redundancy and Devices. SCHC uses a static description of the header to reduce the
size of the header's information. While RFC 8724 describes the SCHC header's redundancy and size. While RFC 8724 describes the SCHC
compression and fragmentation framework, and its application for compression and fragmentation framework, and its application for
IPv6/UDP headers, this document applies SCHC for CoAP headers. The IPv6/UDP headers, this document applies SCHC for CoAP headers. The
CoAP header structure differs from IPv6 and UDP since CoAP uses a CoAP header structure differs from IPv6 and UDP since CoAP uses a
flexible header with a variable number of options, themselves of flexible header with a variable number of options, themselves of
variable length. The CoAP protocol messages format is asymmetric: variable length. The CoAP protocol messages format is asymmetric:
the request messages have a header format different from the one in the request messages have a header format different from the one in
the response messages. This specification gives guidance on applying the response messages. This specification gives guidance on applying
SCHC to flexible headers and how to leverage the asymmetry for more SCHC to flexible headers and how to leverage the asymmetry for more
efficient compression Rules. efficient compression Rules.
skipping to change at page 1, line 46 skipping to change at page 1, line 46
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 23, 2021. This Internet-Draft will expire on July 25, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
skipping to change at page 2, line 38 skipping to change at page 2, line 38
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 9 4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 9
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 9 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 9
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 9 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 9
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 10 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 10
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 10 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 10
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 10 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 11 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 11
5.2. CoAP option Max-Age, Uri-Host, and Uri-Port fields . . . 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. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 11
5.3.1. Variable-length Uri-Path and Uri-Query . . . . . . . 12 5.3.1. Variable-length Uri-Path and Uri-Query . . . . . . . 12
5.3.2. Variable number of Path or Query elements . . . . . . 12 5.3.2. Variable number of Path or Query elements . . . . . . 13
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
fields . . . . . . . . . . . . . . . . . . . . . . . . . 13 fields . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path, 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path,
and Location-Query fields . . . . . . . . . . . . . . . . 13 and Location-Query fields . . . . . . . . . . . . . . . . 13
6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 13 6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 13
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 13 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 14
6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Examples of CoAP header compression . . . . . . . . . . . . . 15 7. Examples of CoAP header compression . . . . . . . . . . . . . 15
7.1. Mandatory header with CON message . . . . . . . . . . . . 15 7.1. Mandatory header with CON message . . . . . . . . . . . . 15
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 16 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 16
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 19 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
9. Security considerations . . . . . . . . . . . . . . . . . . . 29 9. Security considerations . . . . . . . . . . . . . . . . . . . 31
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
11. Normative References . . . . . . . . . . . . . . . . . . . . 30 11. Normative References . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction 1. Introduction
CoAP [rfc7252] is a command/response protocol designed for micro- CoAP [RFC7252] is a command/response protocol designed for micro-
controllers with a small amount of RAM and ROM and is optimized for controllers with a small RAM and ROM and optimized for REST-based
REST-based (Representational state transfer) services. Although CoAP (Representative state transfer) services. Although the Constrained
was designed for Low-Power Wireless Personal Area Networks (6LoWPAN), Devices leads the CoAP design, a CoAP header's size is still too
a CoAP header's size is still too large for LPWAN (Low Power Wide large for LPWAN (Low Power Wide Area Networks). SCHC header
Area Networks) and some compression of the CoAP header is required compression over CoAP header is required to increase performance or
either to increase performances or allow CoAP other some LPWAN use CoAP over LPWAN technologies.
technologies.
The [rfc8724] defines SCHC, a header compression mechanism for the The [RFC8724] defines SCHC, a header compression mechanism for the
LPWAN network based on a static context. Section 5 of the [rfc8724] LPWAN network based on a static context. Section 5 of the [RFC8724]
explains the architecture where compression and decompression are explains where compression and decompression occur in the
done. The SCHC compression scheme assumes as a prerequisite that the architecture. The SCHC compression scheme assumes as a prerequisite
static context is known to both endpoints before transmission. The that both end-points know the static context before transmission.
way the context is configured, provisioned or exchanged is out of The way the context is configured, provisioned, or exchanged is out
this document's scope. of this document's scope.
CoAP is an application protocol, so CoAP compression requires CoAP is an application protocol, so CoAP compression requires
installing common rules between the two SCHC instances. SCHC installing common Rules between the two SCHC instances. SCHC
compression may apply at two different levels: one to compress IP and compression may apply at two different levels: at IP and UDP in the
UDP in the LPWAN network and another at the application level for LPWAN network and another at the application level for CoAP. These
CoAP. These two compressions may be independent. Both follow the two compressions may be independent. Both follow the same principle
same principle described in RFC8724. SCHC rules driving the described in [RFC8724]. As different entities manage the CoAP
compression/decompression are different and may be managed by compression at different levels, the SCHC Rules driving the
different entities. The [rfc8724] describes how the IP and UDP compression/decompression are also different. The [RFC8724]
headers may be compressed. This document specifies how the SCHC describes how to use SCHC for IP and UDP headers. This document
compression rules can be applied to CoAP traffic. specifies how to apply SCHC compression to CoAP headers.
SCHC compresses and decompresses headers based on shared contexts SCHC compresses and decompresses headers based on common contexts
between devices. between Devices. SCHC context includes multiple Rules. Each Rule
Each context consists of multiple Rules. Each Rule can match header can match the header fields to specific values or ranges of values.
fields and specific values or ranges of values.
If a Rule matches, the matched header fields are replaced by the If a Rule matches, the matched header fields are replaced by the
RuleID and some residual bits. Thus, different Rules may correspond RuleID and the Compression Residue that contains the residual bits of
to divers protocols packets that a device expects to send or receive. 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's entire header with an ordered list of A Rule describes the packets' entire header with an ordered list of
fields descriptions; see section 7 of [rfc8724]. Thereby fields descriptions; see section 7 of [RFC8724]. Thereby
each description contains the field ID (FID), its length (FL), and each description contains the field ID (FID), its length (FL), and
its position (FP), a direction indicator (DI) (upstream, downstream, its position (FP), a direction indicator (DI) (upstream, downstream,
and bidirectional), and some associated Target Values (TV). The and bidirectional), and some associated Target Values (TV). The
direction indicator is used for compression to give the best TV to direction indicator is used for compression to give the best TV to
the FID when these values differ in the transmission direction. So a the FID when these values differ in the transmission direction. So a
field may be described several times depending on the asymmetry of field may be described several times.
its possible TVs.
A Matching Operator (MO) is associated with each header field A Matching Operator (MO) is associated with each header field
description. description. The Rule is selected if all the MOs fit the TVs for all
The Rule is selected if all the MOs fit the TVs for all fields of the fields of the incoming header. A Rule cannot be selected if the
incoming header. A rule cannot be selected if the message contains a message contains an unknown field to the SCHC compressor.
field unknown to the SCHC compressor.
In that case, a Compression/Decompression Action (CDA) associated In that case, a Compression/Decompression Action (CDA) associated
with each field give the method to compress and decompress each with each field gives the method to compress and decompress each
field. Compression mainly results in one of 4 actions: field. Compression mainly results in one of 4 actions:
o send the field value, o send the field value (value-sent),
o send nothing, o send nothing (not-sent),
o send some least significant bits of the field or o send some least significant bits of the field (LSB) or,
o send an index. o send an index (mapping-sent).
After applying the compression, there may be some bits to be sent. After applying the compression, there may be some bits to be sent.
These values are called Compression Residues. These values are called Compression Residue.
SCHC is a general mechanism applied to different protocols, the exact SCHC is a general mechanism applied to different protocols, the exact
Rules to be used depending on the protocol and the application. Rules to be used depending on the protocol and the Application.
Section 10 of the [rfc8724] describes the compression scheme for IPv6 Section 10 of the [RFC8724] describes the compression scheme for IPv6
and UDP headers. and UDP headers. This document targets the CoAP header compression
This document targets the CoAP header compression using SCHC. using SCHC.
1.1. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][rfc8174] when, and only when, they appear in all 14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. SCHC Applicability to CoAP 2. SCHC Applicability to CoAP
The SCHC Compression Rules can be applied to CoAP headers. SCHC SCHC Compression for CoAP header MAY be done in conjunction with the
Compression of the CoAP header MAY be done in conjunction with the
lower layers (IPv6/UDP) or independently. The SCHC adaptation lower layers (IPv6/UDP) or independently. The SCHC adaptation
layers, described in Section 5 of [rfc8724], may be used, as shown in layers, described in Section 5 of [RFC8724], may be used as shown in
Figure 1,Figure 2 and Figure 3 Figure 1, Figure 2, and Figure 3.
In the first example, Figure 1, a Rule compresses the complete header 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 stack from IPv6 to CoAP. In this case, the Device and the NGW
Header Compression Compressor/Decompressor) is performed at the perform SCHC C/D (Static Context Header Compression Compressor/
device and the application. The host communicating with the device Decompressor). The host communicating with the Device does not
does not implement SCHC C/D. implement SCHC C/D.
(device) (NGW) (App) (Device) (NGW) (App)
+--------+ +--------+ +--------+ +--------+
| CoAP | | CoAP | | CoAP | | CoAP |
+--------+ +--------+ +--------+ +--------+
| UDP | | UDP | | UDP | | UDP |
+--------+ +----------------+ +--------+ +--------+ +----------------+ +--------+
| IPv6 | | IPv6 | | IPv6 | | IPv6 | | IPv6 | | IPv6 |
+--------+ +--------+-------+ +--------+ +--------+ +--------+-------+ +--------+
| SCHC | | SCHC | | | | | SCHC | | SCHC | | | |
+--------+ +--------+ + + + +--------+ +--------+ + + +
| LPWAN | | LPWAN | | | | | LPWAN | | LPWAN | | | |
+--------+ +--------+-------+ +--------+ +--------+ +--------+-------+ +--------+
((((LPWAN)))) ------ Internet ------ ((((LPWAN)))) ------ Internet ------
Figure 1: Compression/decompression at the LPWAN boundary Figure 1: Compression/Decompression at the LPWAN boundary.
The SCHC can be viewed as a layer above layer 2. This layer received Figure 1 shows the use of SCHC header compression above layer 2 in
non-encrypted packets and can apply compression rule to all the the Device and the NGW. The SCHC layer receives non-encrypted
headers. On the other end, the NGW receives the SCHC packet and packets and can apply compression Rules to all the headers in the
reconstructs the headers from the rule, identified by its ID and the stack. On the other end, the NGW receives the SCHC packet and
header residues. The result is a regular IPv6 packet that can be reconstructs the headers using the Rule and the Compression Residue.
forwarded toward the destination. The same process applies in the After the decompression, the NGW forwards the IPv6 packet toward the
other direction. A not encrypted packet arrived at the NGW, thanks destination. The same process applies in the other direction when a
to IP forwarding based on the IPv6 prefix. The NGW identifies the non-encrypted packet arrives at the NGW. Thanks to the IP forwarding
device and compresses headers using the device's rules. 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 In the second example, Figure 2, the SCHC compression is applied in
the CoAP layer, compressing the CoAP header independently of the the CoAP layer, compressing the CoAP header independently of the
other layers. The RuleID, the Compression Residue, and CoAP payload other layers. The RuleID, the Compression Residue, and CoAP payload
are encrypted using a mechanism such as DTLS. Only the other end are encrypted using a mechanism such as DTLS. Only the other end
(App) can decipher the information. If needed, layers below use SCHC (App) can decipher the information. If needed, layers below use SCHC
to compress the header as defined in [rfc8724] document (represented to compress the header as defined in [RFC8724] (represented in dotted
in dotted lines). lines).
This use case needs an end-to-end context initialization between the This use case needs an end-to-end context initialization between the
device and the application and is out-of-scope of this document. Device and the Application. The context initialization is out of the
scope of this document.
(device) (NGW) (App) (Device) (NGW) (App)
+--------+ +--------+ +--------+ +--------+
| CoAP | | CoAP | | CoAP | | CoAP |
+--------+ +--------+ +--------+ +--------+
| SCHC | | SCHC | | SCHC | | SCHC |
+--------+ +--------+ +--------+ +--------+
| DTLS | | DTLS | | DTLS | | DTLS |
+--------+ +--------+ +--------+ +--------+
. udp . . udp . . udp . . udp .
.......... .................. .......... .......... .................. ..........
. ipv6 . . ipv6 . . ipv6 . . ipv6 . . ipv6 . . ipv6 .
.......... .................. .......... .......... .................. ..........
. schc . . schc . . . . . schc . . schc . . . .
.......... .......... . . . .......... .......... . . .
. lpwan . . lpwan . . . . . lpwan . . lpwan . . . .
.......... .................. .......... .......... .................. ..........
((((LPWAN)))) ------ Internet ------ ((((LPWAN)))) ------ Internet ------
Figure 2: Standalone CoAP end-to-end compression/decompression Figure 2: Standalone CoAP end-to-end Compression/Decompression
In the third example, Figure 3, the Object Security for Constrained The third example, Figure 3, shows the use of Object Security for
RESTful Environments (OSCORE) [rfc8613] is used. In this case, two Constrained RESTful Environments (OSCORE) [RFC8613]. In this case,
rulesets are used to compress the CoAP message. A first ruleset SCHC needs two Rules to compress the CoAP header. A first Rule
focused on the inner header compresses it. The result is encrypted focused on the inner header. The result of this first compression is
using the OSCORE mechanism. A second ruleset compresses the outer encrypted using the OSCORE mechanism. Then a second Rule compresses
header, including the OSCORE Options. the outer header, including the OSCORE Options.
(device) (NGW) (App) (Device) (NGW) (App)
+--------+ +--------+ +--------+ +--------+
| CoAP | | CoAP | | CoAP | | CoAP |
| inner | | inner | | inner | | inner |
+--------+ +--------+ +--------+ +--------+
| SCHC | | SCHC | | SCHC | | SCHC |
| inner | | inner | | inner | | inner |
+--------+ +--------+ +--------+ +--------+
| CoAP | | CoAP | | CoAP | | CoAP |
| outer | | outer | | outer | | outer |
skipping to change at page 7, line 32 skipping to change at page 7, line 32
. ipv6 . . ipv6 . . ipv6 . . ipv6 . . ipv6 . . ipv6 .
.......... .................. .......... .......... .................. ..........
. schc . . schc . . . . . schc . . schc . . . .
.......... .......... . . . .......... .......... . . .
. lpwan . . lpwan . . . . . lpwan . . lpwan . . . .
.......... .................. .......... .......... .................. ..........
((((LPWAN)))) ------ Internet ------ ((((LPWAN)))) ------ Internet ------
Figure 3: OSCORE compression/decompression. Figure 3: OSCORE compression/decompression.
In the case of several SCHC instances, as shown in Figure 3 and In the case of several SCHC instances, as shown in Figure 2 and
Figure 3, the rulesets may come from different provisioning domains. Figure 3, the Rules may come from different provisioning domains.
This document focuses on CoAP compression represented in the dashed This document focuses on CoAP compression represented in the dashed
boxes in the previous figures. boxes in the previous figures.
3. CoAP Headers compressed with SCHC 3. CoAP Headers compressed with SCHC
The use of SCHC over the CoAP header uses the same description and The use of SCHC over the CoAP header uses the same description, and
compression/decompression techniques like the one for IP and UDP compression/decompression techniques like the one for IP and UDP
explained in the [rfc8724]. For CoAP, SCHC Rules description uses explained in the [RFC8724]. For CoAP, the SCHC Rules description
the direction information to optimize the compression by reducing the uses the direction information to optimize the compression by
number of Rules needed to compress headers. The field description reducing the number of Rules needed to compress headers. The field
MAY define both request/response headers and target values in the description MAY define both request/response headers and target
same Rule, using the DI (direction indicator) to make the difference. values in the same Rule, using the DI (direction indicator) to make
the difference.
As for other header compression protocols, when the compressor does As for other header compression protocols, when the compressor does
not find a correct Rule to compress the header, the packet MUST be not find a correct Rule to compress the header, the packet MUST be
sent uncompressed using the RuleID dedicated to this purpose. Where sent uncompressed using the RuleID dedicated to this purpose. Where
the Compression Residue is the complete header of the packet. See the Compression Residue is the complete header of the packet. See
section 6 of [rfc8724]. section 6 of [RFC8724].
3.1. Differences between CoAP and UDP/IP Compression 3.1. Differences between CoAP and UDP/IP Compression
CoAP compression differs from IPv6 and UDP compression on the CoAP compression differs from IPv6 and UDP compression in the
following aspects: following aspects:
o The CoAP protocol is asymmetric; the headers are different for a o The CoAP protocol is asymmetric; the headers are different for a
request or a response. For example, the URI-Path option is request or a response. For example, the URI-Path option is
mandatory in the request, and it may not be present in the mandatory in the request, and it might not be present in the
response. A request may contain an Accept option, and the response. A request might contain an Accept option, and the
response may include a Content-Format option. In comparison, IPv6 response might include a Content-Format option. In comparison,
and UDP returning path swap the value of some fields in the 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 header. However, all the directions have the same fields (e.g.,
and destination address fields). source and destination address fields).
The [rfc8724] defines the use of a Direction Indicator (DI) in the The [RFC8724] defines the use of a direction indicator (DI) in the
Field Descriptor, which allows a single Rule to process a message Field Descriptor, which allows a single Rule to process a message
headers differently depending on the direction. header differently depending on the direction.
o Even when a field is "symmetric" (i.e., found in both directions), o Even when a field is "symmetric" (i.e., found in both directions),
the values carried in each direction are different. the values carried in each direction are different. The
The compression may use a matching list in the TV to limit the compression may use a "match-mapping" MO to limit the range of
range of expected values in a particular direction and therefore expected values in a particular direction and reduce the
reduce the Compression Residue's size. Through the Direction Compression Residue's size. Through the direction indicator (DI),
Indicator (DI), a field description in the Rules splits the a field description in the Rules splits the possible field value
possible field value into two parts, one for each direction. For into two parts, one for each direction. For instance, if a client
instance, if a client sends only CON requests, the type can be sends only CON requests, the Type can be elided by compression,
elided by compression, and the answer may use one single bit to and the answer may use one single bit to carry either the ACK or
carry either the ACK or RST type. The field Code has the same RST type. The field Code has the same behavior, the 0.0X code
behavior, the 0.0X code format value in the request, and Y.ZZ code format value in the request, and the Y.ZZ code format in the
format in the response. response.
o Headers in IPv6 and UDP have a fixed size. The size is not sent o In SCHC, the Rule defines the different header fields' length, so
as part of the Compression Residue but is defined in the Rule. SCHC does not need to send it. In IPv6 and UDP headers, the
Some CoAP header fields have variable lengths, so the length is fields have a fixed size, known by definition. On the other hand,
also specified in the Field Description. For example, the Token some CoAP header fields have variable lengths, and the Rule
size may vary from 0 to 8 bytes. And the CoAP options have a description specifies it. For example, in a URI-path or URI-
variable length since they use the Type-Length-Value encoding query, the Token size may vary from 0 to 8 bytes, and the CoAP
format, as URI-path or URI-query. options use the Type-Length-Value encoding format.
Section 7.5.2 from [rfc8724] offers the possibility to define a When doing SCHC compression of a variable-length field,
Section 7.5.2 from [RFC8724] offers the possibility to define a
function for the Field length in the Field Description to know the function for the Field length in the Field Description to know the
length before compression. When doing SCHC compression of a length before compression. If the field length is unknown, the
variable-length field, Rule will set it as a variable, and SCHC will send the compressed
if the field size is unknown, the Field Length in the Rule is set field's length in the Compression Residue.
as variable, and the size is sent with the Compression Residue.
o A field can appear several times in the CoAP headers. This is o A field can appear several times in the CoAP headers. It is found
typical for elements of a URI (path or queries). The SCHC typically for elements of a URI (path or queries). The SCHC
specification [rfc8724] allows a Field ID to appear several times specification [RFC8724] allows a Field ID to appear several times
in the Rule and uses the Field Position (FP) to identify the in the Rule and uses the Field Position (FP) to identify the
correct instance, and thereby removing the ambiguity of the correct instance, thereby removing the matching operation's
matching operation. ambiguity.
o Field sizes defined in the CoAP protocol can be too o Field lengths defined in the CoAP protocol can be too
large regarding LPWAN traffic constraints. This is particularly large regarding LPWAN traffic constraints. For instance, this is
true for the Message-ID field and the Token field. SCHC uses particularly true for the Message-ID field and the Token field.
different Matching operators (MO) to perform the compression. See SCHC uses different Matching operators (MO) to perform the
section 7.4 of [rfc8724]. In this case, the Most Significant Bits compression. See section 7.4 of [RFC8724]. In this case, SCHC
(MSB) MO can be applied to reduce the information carried on can apply the Most Significant Bits (MSB) MO to reduce the
LPWANs. information carried on LPWANs.
4. Compression of CoAP header fields 4. Compression of CoAP header fields
This section discusses the compression of the different CoAP header This section discusses the compression of the different CoAP header
fields. The CoAP compression with SCHC follows the Section 7.1 of fields. The CoAP compression with SCHC follows Section 7.1 of
[rfc8724]. [RFC8724].
4.1. CoAP version field 4.1. CoAP version field
CoAP version is bidirectional and MUST be elided during the SCHC CoAP version is bidirectional and MUST be elided during the SCHC
compression since it always contains the same value. In the future, compression since it always contains the same value. In the future,
if new versions of CoAP are defined, new Rules will be needed to or if a new version of CoAP is defined, new Rules will be needed to
avoid ambiguities between versions. avoid ambiguities between versions.
4.2. CoAP type field 4.2. CoAP type field
The CoAP Protocol [rfc7252] has four types of messages: two requests The CoAP protocol [RFC7252] has four types of messages: two requests
(CON, NON), one response (ACK), and one empty message (RST). (CON, NON), one response (ACK), and one empty message (RST).
The field SHOULD be elided if, for instance, a client is sending only The SCHC compression SHOULD elide this field if, for instance, a
NON or only CON messages. For the RST message, a dedicated Rule may client is sending only NON or only CON messages. For the RST
be needed. For other usages, a mapping list can be used. message, SCHC may use a dedicated Rule. For other usages, SCHC can
use a "match-mapping" MO.
4.3. CoAP code field 4.3. CoAP code field
The code field indicates the Request Method used in CoAP, an IANA The code field is an IANA registry [RFC7252], and it indicates the
registry [rfc7252]. The compression of the CoAP code field follows Request Method used in CoAP. The compression of the CoAP code field
the same principle as that of the CoAP type field. If the device follows the same principle as that of the CoAP type field. If the
plays a specific role, the set of code values can be split into two Device plays a specific role, SCHC may split the code values into two
parts, the request codes with the 0 class and the response values. fields description, the request codes with the 0 class and the
response values. SCHC will use the direction indicator to identify
the correct value in the packet.
If the device only implements a CoAP client, the request code can be If the Device only implements a CoAP client, SCHC compression may
reduced to the set of requests the client can to process. reduce the request code to the set of requests the client can
process.
A mapping list can be used for known values. The field cannot be For known values, SCHC can use a "match-mapping" MO. If SCHC cannot
compressed for other values, and the value needs to be sent in the compress the code field, it will send the values in the Compression
Compression Residue. Residue.
4.4. CoAP Message ID field 4.4. CoAP Message ID field
The Message ID field can be compressed with the MSB(x) MO and the SCHC can compress the Message ID field with the "MSB" MO and the
Least Significant Bits (LSB) CDA. See section 7.4 of [rfc8724]. "LSB" CDA. See section 7.4 of [RFC8724].
4.5. CoAP Token fields 4.5. CoAP Token fields
A Token is defined through two CoAP fields, Token Length in the CoAP defines the Token using two CoAP fields, Token Length in the
mandatory header and Token Value directly following the mandatory mandatory header and Token Value directly following the mandatory
CoAP header. CoAP header.
Token Length is processed as any protocol field. If the value does SCHC processes the Token length as any header field. If the value
not change, the size can be stored in the TV and elided during the does not change, the size can be stored in the TV and elided during
transmission. Otherwise, it will have to be sent in the Compression the transmission. Otherwise, SCHC will send the token length in the
Residue. Compression Residue.
Token Value MUST NOT be sent as a variable-length residue to avoid For the Token Value, SCHC MUST NOT send it as a variable-length in
ambiguity with Token Length. Therefore, the Token Length value MUST the Compression Residue to avoid ambiguity with Token Length.
be used to define the size of the Compression Residue. A specific Therefore, SCHC MUST use the Token length value to define the size of
function designated as "TKL" MUST be used in the Rule. During the the Compression Residue. SCHC designates a specific function "tkl"
that the Rule MUST use to complete the field description. During the
decompression, this function returns the value contained in the Token decompression, this function returns the value contained in the Token
Length field. Length field.
5. CoAP options 5. CoAP options
CoAP defines options that are placed after the based header in Option CoAP defines options placed after the based header in Option Numbers
Numbers order, see [rfc7252]. Each Option instance in a message uses order; see [RFC7252]. Each Option instance in a message uses the
the format Delta-Type (D-T), Length (L), Value (V). When applying format Delta-Type (D-T), Length (L), Value (V). The SCHC Rule builds
SCHC compression to the Option, the D-T, L, and V format serve to the description of the option by using in the Field ID the Option
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 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 uses section 7.4 of [RFC8724]. When the Option Length has a well-
wellknown size, it can be stored in the Rule. Therefore, SCHC known size, the Rule may stock the length value. Therefore, SCHC
compression does not send it. Otherwise, SCHC Compression carries compression does not send it. Otherwise, SCHC Compression carries
the length of the Compression Residue, in addition to the Compression the length of the Compression Residue, in addition to the Compression
Residue value. Residue value.
CoAP requests and responses do not include the same options. So CoAP requests and responses do not include the same options. So
Compression Rules may reflect this asymmetry by tagging the direction Compression Rules may reflect this asymmetry by tagging the direction
indicator. indicator.
Note that length coding differs between CoAP options and SCHC Note that length coding differs between CoAP options and SCHC
variable size Compression Residue. variable size Compression Residue.
The following sections present how SCHC compresses some specific CoAP The following sections present how SCHC compresses some specific CoAP
options. options.
If a new option is introduced in CoAP, a new Field ID has to be If CoAP introduces a new option, the SCHC Rules MAY be updated, and
assigned in the Rules to allow its compression. Otherwise, if no the new Field ID description MUST be assigned to allow its
Rule describes this Option, the SCHC compression is not possible, and compression. Otherwise, if no Rule describes this new option, the
the CoAP header is sent without compression. SCHC compression is not achieved, and SCHC sends the CoAP header
without compression.
5.1. CoAP Content and Accept options. 5.1. CoAP Content and Accept options.
If the client expects a single value, it can be stored in the TV and If the client expects a single value, it can be stored in the TV and
elided during the transmission. Otherwise, if the client expects elided during the transmission. Otherwise, if the client expects
several possible values, a matching-list SHOULD be used to limit the several possible values, a "match-mapping" SHOULD be used to limit
Compression Residue's size. Otherwise, the value has to be sent as a the Compression Residue's size. If not, SCHC has to send the option
Compression Residue (fixed or variable length). value in the Compression Residue (fixed or variable length).
5.2. CoAP option Max-Age, Uri-Host, and Uri-Port fields 5.2. CoAP option Max-Age, Uri-Host, and Uri-Port fields
If both ends know the value, the value can be elided. SCHC compresses these three fields in the same way. When the value
of these options is known, SCHC can elide these fields. If the
A matching list can be used if some well-known values are defined. option uses well-known values, SCHC can use a "match-mapping" MO.
Otherwise, SCHC will use "value-sent" MO, and the Compression Residue
Otherwise, these options can be sent as a Compression Residue. will send these options' values.
5.3. CoAP option Uri-Path and Uri-Query fields 5.3. CoAP option Uri-Path and Uri-Query fields
Uri-Path and Uri-Query elements are repeatable options. The Field The Uri-Path and Uri-Query fields are repeatable options; this means
Position (FP) gives the position in the path. that in the CoAP header, they may appear several times with different
values. SCHC Rule description uses the Field Position (FP) to
distinguish the different instances in the path.
A Mapping list can be used to reduce the size of variable Paths or To compress repeatable field values, SCHC may use a "match-mapping"
Queries. In that case, to optimize the compression, several elements MO to reduce the size of variable Paths or Queries. In these cases,
can be regrouped into a single entry. The Numbering of elements do to optimize the compression, several elements can be regrouped into a
not change; MO comparison is set with the first element of the single entry. The Numbering of elements does not change, and the
matching. first matching element sets the MO comparison.
+-------------+---+--+--+--------+---------+-------------+ +--------+---+--+--+--------+-------------+------------+
| Field |FL |FP|DI| Target | Match | CDA | | Field |FL |FP|DI| Target | Matching | CDA |
| | | | | Value | Opera. | | | | | | | Value | Operator | |
+-------------+---+--+--+--------+---------+-------------+ +--------+---+--+--+--------+-------------+------------+
|Uri-Path | | 1|up|["/a/b",|equal |not-sent | |Uri-Path| | 1|up|["/a/b",|match-mapping|mapping-sent|
| | | | |"/c/d"] | | | | | | | |"/c/d"] | | |
|Uri-Path |var| 3|up| |ignore |value-sent | |Uri-Path|var| 3|up| |ignore |value-sent |
+-------------+---+--+--+--------+---------+-------------+ +--------+---+--+--+--------+-------------+------------+
Figure 4: complex path example Figure 4: complex path example
In Figure 4, a single bit residue can be used to code one of the 2 In Figure 4, SCHC can use a single bit in the Compression Residue to
paths. If regrouping were not allowed, a 2 bits residue would be code one of the two paths. If regrouping were not allowed, 2 bits in
needed. The third path element is sent as a variable size residue. the Compression Residue would be needed. SCHC sends the third path
element as a variable size in the Compression Residue.
5.3.1. Variable-length Uri-Path and Uri-Query 5.3.1. Variable-length Uri-Path and Uri-Query
When the length is not known at the Rule creation, the Field Length When SCHC creates the Rule, the length of URI-Path and URI-Query may
MUST be set to variable, and the unit is set to bytes. be known. Nevertheless, SCHC MUST set the field length to variable,
and the unit to bytes.
The MSB MO can be applied to a Uri-Path or Uri-Query element. Since SCHC compression can use the MSB MO to a Uri-Path or Uri-Query
MSB value is given in bit, the size MUST always be a multiple of 8 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. bits.
The length sent at the beginning of a variable-length residue The length sent at the beginning of a variable-length Compression
indicates the size of the LSB in bytes. Residue indicates the LSB's size in bytes.
For instance, for a CORECONF path /c/X6?k="eth0" the Rule can be set For instance, for a CORECONF path /c/X6?k="eth0" the Rule description
to: can be:
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
| Field |FL |FP|DI| Target | Match | CDA | | Field |FL |FP|DI| Target | Match | CDA |
| | | | | Value | Opera. | | | | | | | Value | Opera. | |
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
|Uri-Path | 8| 1|up|"c" |equal |not-sent | |Uri-Path | 8| 1|up|"c" |equal |not-sent |
|Uri-Path |var| 2|up| |ignore |value-sent | |Uri-Path |var| 2|up| |ignore |value-sent |
|Uri-Query |var| 1|up|"k=" |MSB(16) |LSB | |Uri-Query |var| 1|up|"k=" |MSB(16) |LSB |
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
Figure 5: CORECONF URI compression Figure 5: CORECONF URI compression
Figure 5 shows the parsing and the compression of the URI, where c is Figure 5 shows the Rule description for a URI-Path and a URI-Query.
not sent. The second element is sent with the length (i.e., 0x2 X 6) SCHC compresses the first part of the URI-Path with a "not-sent" CDA.
followed by the query option (i.e. 0x05 "eth0").
SCHC will send the second element of the URI-Path 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 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 SCHC fixed the number of Uri-Path or Uri-Query elements in a Rule at
the Rule creation time. If the number varies, several Rules SHOULD the Rule creation time. If the number varies, SCHC SHOULD create
be created to cover all the possibilities. Another possibility is to several Rules to cover all the possibilities. Another one is to
define the length of Uri-Path to variable and send a Compression define the length of Uri-Path to variable and sends a Compression
Residue with a length of 0 to indicate that this Uri-Path is empty. 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 However, this adds 4 bits to the variable Compression Residue size.
[rfc8724] See section 7.5.2 [RFC8724].
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields 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 The SCHC Rule description MAY define sending some field values by
"ignore", and CDA is set to "value-sent." A mapping MAY also be setting the TV to "not-sent," MO to "ignore," and CDA to "value-
used. sent." A Rule MAY also use a "match-mapping" when there are
different options for the same FID. Otherwise, the Rule sets the TV
Otherwise, the TV is set to the value, MO is set to "equal", and CDA to the value, MO to "equal," and CDA to "not-sent."
is set to "not-sent".
5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path, and 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path, and
Location-Query fields Location-Query fields
These fields' values cannot be stored in a Rule entry. They MUST A Rule entry cannot store these fields' values. The Rule description
always be sent with the Compression Residues. MUST always send these values in the Compression Residue.
6. SCHC compression of CoAP extension RFCs 6. SCHC compression of CoAP extension RFCs
6.1. Block 6.1. Block
Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also When a packet uses a Block [RFC7959] option, SCHC compression MUST
includes a fragmentation protocol. They can be both used. If a send its content in the Compression Residue. The SCHC Rule describes
block option is used, its content MUST be sent as a Compression an empty TV with a MO set to "ignore" and a CDA to "value-sent."
Residue. 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 6.2. Observe
The [rfc7641] defines the Observe option. The TV is not set, MO is The [RFC7641] defines the Observe option. The SCHC Rule description
set to "ignore", and the CDA is set to "value-sent". SCHC does not will not define the TV, but MO to "ignore," and the CDA to "value-
limit the maximum size for this option (3 bytes). To reduce the sent." SCHC does not limit the maximum size for this option (3
transmission size, either the device implementation MAY limit the bytes). To reduce the transmission size, either the Device
delta between two consecutive values, or a proxy can modify the implementation MAY limit the delta between two consecutive values, or
increment. a proxy can modify the increment.
Since an RST message may be sent to inform a server that the client Since the Observe option MAY use an RST message to inform a server
does not require Observe response; a Rule SHOULD exist to allow the that the client does not require the Observe response, a specific
message's compression with the RST type. SCHC Rule SHOULD exist to allow the message's compression with the
RST type.
6.3. No-Response 6.3. No-Response
The [rfc7967] defines a No-Response option limiting the responses The [RFC7967] defines a No-Response. Different behaviors exist while
made by a server to a request. If both ends know the value, then TV using this option to limit the responses made by a server to a
is set to this value, MO is set to "equal", and CDA is set to "not- request. If both ends know the value, then the SCHC Rule will
sent". describe a TV to this value, with a MO set to "equal" and CDA set to
"not-sent."
Otherwise, if the value is changing over time, TV is not set, MO is Otherwise, if the value is changing over time, the SCHC Rule will set
set to "ignore", and CDA to "value-sent". A matching list can also the MO to "ignore" and CDA to "value-sent." The Rule may also use a
be used to reduce the size. "match-mapping" to compress this option.
6.4. OSCORE 6.4. OSCORE
OSCORE [rfc8613] defines end-to-end protection for CoAP messages. OSCORE [RFC8613] defines end-to-end protection for CoAP messages.
This section describes how SCHC Rules can be applied to compress This section describes how SCHC Rules can be applied to compress
OSCORE-protected messages. OSCORE-protected messages.
0 1 2 3 4 5 6 7 <--------- n bytes -------------> 0 1 2 3 4 5 6 7 <--------- n bytes ------------->
+-+-+-+-+-+-+-+-+--------------------------------- +-+-+-+-+-+-+-+-+---------------------------------
|0 0 0|h|k| n | Partial IV (if any) ... |0 0 0|h|k| n | Partial IV (if any) ...
+-+-+-+-+-+-+-+-+--------------------------------- +-+-+-+-+-+-+-+-+---------------------------------
| | | | | |
|<-- CoAP -->|<------ CoAP OSCORE_piv ------> | |<-- CoAP -->|<------ CoAP OSCORE_piv ------> |
OSCORE_flags OSCORE_flags
<- 1 byte -> <------ s bytes -----> <- 1 byte -> <------ s bytes ----->
+------------+----------------------+-----------------------+ +------------+----------------------+-----------------------+
| s (if any) | kid context (if any) | kid (if any) ... | | s (if any) | kid context (if any) | kid (if any) ... |
+------------+----------------------+-----------------------+ +------------+----------------------+-----------------------+
| | | | | |
| <------ CoAP OSCORE_kidctx ------>|<-- CoAP OSCORE_kid -->| | <------ CoAP OSCORE_kidctx ------>|<-- CoAP OSCORE_kid -->|
Figure 6: OSCORE Option Figure 6: OSCORE Option
The encoding of the OSCORE Option Value defined in Section 6.1 of The Figure 6 shows the OSCORE Option Value encoding defined in
[rfc8613] is repeated in Figure 6. Section 6.1 of [RFC8613], where the first byte specifies the Content
of the OSCORE options using flags. The three most significant bits
The first byte specifies the content of the OSCORE options using of this byte are reserved and always set to 0. Bit h, when set,
flags. The three most significant bits of this byte are reserved and indicates the presence of the kid context field in the option. Bit
always set to 0. Bit h, when set, indicates the presence of the kid k, when set, indicates the presence of a kid field. The three least
context field in the option. Bit k, when set, indicates the presence significant bits n indicate the length of the piv (Partial
of a kid field. The three least significant bits n indicate the Initialization Vector) field in bytes. When n = 0, no piv is
length of the piv (Partial Initialization Vector) field in bytes. present.
When n = 0, no piv is present.
The flag byte is followed by the piv field, kid context field, and 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 kid field in this order, and if present, the kid context field's
context field is encoded in the first byte denoting by s the length length is encoded in the first byte denoting by 's' the length of the
of the kid context in bytes. kid context in bytes.
This specification recommends identifying the OSCORE Option and the To better perform OSCORE SCHC compression, the Rule description needs
fields it contains. Conceptually, it discerns up to 4 distinct to identify the OSCORE Option and the fields it contains.
pieces of information within the OSCORE option: the flag bits, the Conceptually, it discerns up to 4 distinct pieces of information
piv, the kid context, and the kid. The SCHC Rule splits into four within the OSCORE option: the flag bits, the piv, the kid context,
field descriptions the OSCORE option to compress them: and the kid. The SCHC Rule splits into four field descriptions the
OSCORE option to compress them:
o CoAP OSCORE_flags, o CoAP OSCORE_flags,
o CoAP OSCORE_piv, o CoAP OSCORE_piv,
o CoAP OSCORE_kidctx, o CoAP OSCORE_kidctx,
o CoAP OSCORE_kid. o CoAP OSCORE_kid.
Figure 6 shows the OSCORE Option format with those four fields Figure 6 shows the OSCORE Option format with those four fields
superimposed on it. Note that the CoAP OSCORE_kidctx field includes superimposed on it. Note that the CoAP OSCORE_kidctx field directly
directly the size octet s. includes the size octet s.
7. Examples of CoAP header compression 7. Examples of CoAP header compression
7.1. Mandatory header with CON message 7.1. Mandatory header with CON message
In this first scenario, the LPWAN Compressor at the Network Gateway In this first scenario, the SCHC Compressor at the Network Gateway
side receives from an Internet client a POST message, which is side receives a POST message from an Internet client, which is
immediately acknowledged by the Device. For this simple scenario, immediately acknowledged by the Device. Figure 7 describes the SCHC
the Rules are described in Figure 7. Rule descriptions for this scenario.
RuleID 1 RuleID 1
+-------------+--+--+--+------+---------+-------------++------------+ +-------------+--+--+--+------+---------+-------------++------------+
| Field |FL|FP|DI|Target| Match | CDA || Sent | | Field |FL|FP|DI|Target| Match | CDA || Sent |
| | | | |Value | Opera. | || [bits] | | | | | |Value | Opera. | || [bits] |
+-------------+--+--+--+------+---------+-------------++------------+ +-------------+--+--+--+------+---------+-------------++------------+
|CoAP version | 2| 1|bi| 01 |equal |not-sent || | |CoAP version | 2| 1|bi| 01 |equal |not-sent || |
|CoAP Type | 2| 1|dw| CON |equal |not-sent || | |CoAP Type | 2| 1|dw| CON |equal |not-sent || |
|CoAP Type | 2| 1|up|[ACK, | | || | |CoAP Type | 2| 1|up|[ACK, | | || |
| | | | | RST] |match-map|matching-sent|| T | | | | | | RST] |match-map|matching-sent|| T |
|CoAP TKL | 4| 1|bi| 0 |equal |not-sent || | |CoAP TKL | 4| 1|bi| 0 |equal |not-sent || |
|CoAP Code | 8| 1|bi|[0.00,| | || | |CoAP Code | 8| 1|bi|[0.00,| | || |
| | | | | ... | | || | | | | | | ... | | || |
| | | | | 5.05]|match-map|matching-sent|| CC CCC | | | | | | 5.05]|match-map|matching-sent|| CC CCC |
|CoAP MID |16| 1|bi| 0000 |MSB(7 ) |LSB || M-ID| |CoAP MID |16| 1|bi| 0000 |MSB(7 ) |LSB || M-ID|
|CoAP Uri-Path|var 1|dw| path |equal 1 |not-sent || | |CoAP Uri-Path|var 1|dw| path |equal 1 |not-sent || |
+-------------+--+--+--+------+---------+-------------++------------+ +-------------+--+--+--+------+---------+-------------++------------+
Figure 7: CoAP Context to compress header without token Figure 7: CoAP Context to compress header without Token
The version and Token Length fields are elided. The 26 method and In this example, SCHC compression elides the version and the Token
response codes defined in [rfc7252] has been shrunk to 5 bits using a Length fields. The 26 method and response codes defined in [RFC7252]
matching list. Uri-Path contains a single element indicated in the has been shrunk to 5 bits using a "match-mapping" MO. The Uri-Path
matching operator. contains a single element indicated in the TV and elided with the CDA
"not-sent."
SCHC Compression reduces the header sending only the Type, a mapped SCHC Compression reduces the header sending only the Type, a mapped
code and the least significant bits of Message ID (9 bits in the code, and the least significant bits of Message ID (9 bits in the
example above). example above).
Note that a request sent by a client located in an Application Server 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 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 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 proxy placed before the SCHC C/D can rewrite the message ID to fit
matched by the Rule. the value and match the Rule.
7.2. OSCORE Compression 7.2. OSCORE Compression
OSCORE aims to solve the problem of end-to-end encryption for CoAP 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 messages. Therefore, the goal is to hide as much as possible the
possible while still enabling proxy operation. message while still enabling proxy operation.
Conceptually this is achieved by splitting the CoAP message into an Conceptually this is achieved by splitting the CoAP message into an
Inner Plaintext and Outer OSCORE Message. The Inner Plaintext Inner Plaintext and Outer OSCORE Message. The Inner Plaintext
contains sensitive information that is not necessary for proxy contains sensitive information that is not necessary for proxy
operation. This, in turn, is the part of the message which can be operation. However, it is part of the message that can be encrypted
encrypted until it reaches its end destination. The Outer Message until it reaches its end destination. The Outer Message acts as a
acts as a shell matching the regular CoAP message format and includes shell matching the regular CoAP message format and includes all
all Options and information needed for proxy operation and caching. Options and information needed for proxy operation and caching.
This decomposition is illustrated in Figure 8. Figure 8 illustrates this analysis.
CoAP options are sorted into one of 3 classes, each granted a The CoAP protocol arranges the options into one of 3 classes; each
specific type of protection by the protocol: granted a specific type of protection by the protocol:
o Class E: Encrypted options moved to the Inner Plaintext, o Class E: Encrypted options moved to the Inner Plaintext,
o Class I: Integrity-protected options included in the AAD for the o Class I: Integrity-protected options included in the AAD for the
encryption of the Plaintext but otherwise left untouched in the encryption of the Plaintext but otherwise left untouched in the
Outer Message, Outer Message,
o Class U: Unprotected options 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, These classes point out that the Outer option contains the OSCORE
signaling that the message is OSCORE protected. This option carries Option and that the message is OSCORE protected; this option carries
the information necessary to retrieve the Security Context with which the information necessary to retrieve the Security Context. The end-
the message was encrypted to be correctly decrypted at the other end- point will use this Security Context to decrypt the message
point. correctly.
Original CoAP Message Original CoAP Packet
+-+-+---+-------+---------------+ +-+-+---+-------+---------------+
|v|t|tkl| code | Msg Id. | |v|t|TKL| code | Msg Id. |
+-+-+---+-------+---------------+....+ +-+-+---+-------+---------------+....+
| Token | | Token |
+-------------------------------.....+ +-------------------------------.....+
| Options (IEU) | | Options (IEU) |
. . . .
. . . .
+------+-------------------+ +------+-------------------+
| 0xFF | | 0xFF |
+------+------------------------+ +------+------------------------+
| | | |
| Payload | | Payload |
| | | |
+-------------------------------+ +-------------------------------+
/ \ / \
/ \ / \
/ \ / \
/ \ / \
Outer Header v v Plaintext Outer Header v v Plaintext
+-+-+---+--------+---------------+ +-------+ +-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code | |v|t|TKL|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+ +-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) | | Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+ +--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF | | Options (IU) | | OxFF |
. . +-------+-----------+ . . +-------+-----------+
. OSCORE Option . | | . OSCORE Option . | |
+------+-------------------+ | Payload | +------+-------------------+ | Payload |
| 0xFF | | | | 0xFF | | |
+------+ +-------------------+ +------+ +-------------------+
Figure 8: A CoAP message is split into an OSCORE outer and plaintext Figure 8: A CoAP packet is split into an OSCORE outer and plaintext
Figure 8 shows the message format for the OSCORE Message and Figure 8 shows the packet format for the OSCORE Outer header and
Plaintext. Plaintext.
In the Outer Header, the original message code is hidden and replaced In the Outer 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 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 [RFC8613], the message code is replaced by POST for requests and
Changed for responses when Observe is not used. If Observe is used, Changed for responses when CoAP is not using the Observe option. If
the message code is replaced by FETCH for requests and Content for CoAP uses Observe, the OSCORE message code is replaced by FETCH for
responses. requests and Content for responses.
The original message code is put into the first byte of the The first byte of the Plaintext contains the original packet code,
Plaintext. Following the message code, the class E options come, followed by the message code, the class E options, and, if present,
and, if present, the original message Payload is preceded by its the original message Payload preceded by its payload marker.
payload marker.
The Plaintext is now encrypted by an AEAD algorithm which integrity An AEAD algorithm now encrypts the Plaintext. This integrity
protects Security Context parameters and, eventually, any class I protects the Security Context parameters and, eventually, any class I
options from the Outer Header. Currently, no CoAP options are marked options from the Outer Header. The resulting Ciphertext becomes the
class I. The resulting Ciphertext becomes the new Payload of the new payload of the OSCORE message, as illustrated in Figure 9.
OSCORE message, as illustrated in Figure 9.
As defined in [rfc5116], this Ciphertext is the concatenation of the As defined in [RFC5116], this Ciphertext is the encrypted Plaintext's
encrypted Plaintext and its authentication tag. Note that Inner concatenation of the authentication tag. Note that Inner Compression
Compression only affects the Plaintext before encryption. Thus only only affects the Plaintext before encryption. Thus only the first
the first variable-length of the Ciphertext can be reduced. The variable-length of the Ciphertext can be reduced. The authentication
authentication tag is fixed in length and is considered part of the tag is fixed in length and is considered part of the cost of
cost of protection. protection.
Outer Header Outer Header
+-+-+---+--------+---------------+ +-+-+---+--------+---------------+
|v|t|tkl|new code| Msg Id. | |v|t|TKL|new code| Msg Id. |
+-+-+---+--------+---------------+....+ +-+-+---+--------+---------------+....+
| Token | | Token |
+--------------------------------.....+ +--------------------------------.....+
| Options (IU) | | Options (IU) |
. . . .
. OSCORE Option . . OSCORE Option .
+------+-------------------+ +------+-------------------+
| 0xFF | | 0xFF |
+------+---------------------------+ +------+---------------------------+
| | | |
skipping to change at page 18, line 45 skipping to change at page 19, line 46
| + Authentication Tag | | + Authentication Tag |
| | | |
+----------------------------------+ +----------------------------------+
Figure 9: OSCORE message Figure 9: OSCORE message
The SCHC Compression scheme consists of compressing both the The SCHC Compression scheme consists of compressing both the
Plaintext before encryption and the resulting OSCORE message after Plaintext before encryption and the resulting OSCORE message after
encryption, see Figure 10. encryption, see Figure 10.
This translates into a segmented process where SCHC compression is The OSCORE message translates into a segmented process where SCHC
applied independently in 2 stages, each with its corresponding set of compression is applied independently in 2 stages, each with its
Rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way, corresponding set of Rules, with the Inner SCHC Rules and the Outer
compression is applied to all fields of the original CoAP message. SCHC Rules. This way, compression is applied to all fields of the
original CoAP message.
Note that since the corresponding end-point can only decrypt the 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 part of the message, this end-point will also have to implement
Inner SCHC Compression/Decompression. Inner SCHC Compression/Decompression.
Outer Message OSCORE Plaintext Outer Message OSCORE Plaintext
+-+-+---+--------+---------------+ +-------+ +-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code | |v|t|TKL|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+ +-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) | | Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+ +--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF | | Options (IU) | | OxFF |
. . +-------+-----------+ . . +-------+-----------+
. OSCORE Option . | | . OSCORE Option . | |
+------+-------------------+ | Payload | +------+-------------------+ | Payload |
| 0xFF | | | | 0xFF | | |
+------+------------+ +-------------------+ +------+------------+ +-------------------+
| Ciphertext |<---------\ | | Ciphertext |<---------\ |
| | | v | | | v
+-------------------+ | +-----------------+ +-------------------+ | +-----------------+
| | | Inner SCHC | | | | Inner SCHC |
v | | Compression | v | | Compression |
+-----------------+ | +-----------------+ +-----------------+ | +-----------------+
| Outer SCHC | | | | Outer SCHC | | |
| Compression | | v | Compression | | v
+-----------------+ | +-------+ +-----------------+ | +-------+
| | |RuleID | | | |RuleID |
v | +-------+--+ v | +-------+-----------+
+--------+ +------------+ | Residue | +--------+ +------------+ |Compression Residue|
|RuleID' | | Encryption | <--- +----------+--------+ |RuleID' | | Encryption | <-- +----------+--------+
+--------+--+ +------------+ | | +--------+-----------+ +------------+ | |
| Residue' | | Payload | |Compression Residue'| | Payload |
+-----------+-------+ | | +-----------+--------+ | |
| Ciphertext | +-------------------+ | Ciphertext | +-------------------+
| | | |
+-------------------+ +--------------------+
Figure 10: OSCORE Compression Diagram Figure 10: OSCORE Compression Diagram
7.3. Example OSCORE Compression 7.3. Example OSCORE Compression
An example is given with a GET Request and its consequent Content This section gives an example with a GET Request and its consequent
Response from a device-based CoAP client to a cloud-based CoAP Content Response from a Device-based CoAP client to a cloud-based
server. A possible set of Rules for the Inner and Outer SCHC CoAP server. The example also describes a possible set of Rules for
Compression is shown. A dump of the results and a contrast between the Inner and Outer SCHC Compression. A dump of the results and a
SCHC + OSCORE performance with SCHC + COAP performance is also contrast between SCHC + OSCORE performance with SCHC + COAP
listed. This gives an approximation to the cost of security with performance is also listed. This example gives an approximation of
SCHC-OSCORE. the cost of security with SCHC-OSCORE.
Our first example CoAP message is the GET Request in Figure 11 Our first CoAP message is the GET request in Figure 11.
Original message: Original message:
================= =================
0x4101000182bb74656d7065726174757265 0x4101000182bb74656d7065726174757265
Header: Header:
0x4101 0x4101
01 Ver 01 Ver
00 CON 00 CON
0001 tkl 0001 TKL
00000001 Request Code 1 "GET" 00000001 Request Code 1 "GET"
0x0001 = mid 0x0001 = mid
0x82 = token 0x82 = token
Options: Options:
0xbb74656d7065726174757265 0xbb74656d7065726174757265
Option 11: URI_PATH Option 11: URI_PATH
Value = temperature Value = temperature
skipping to change at page 20, line 40 skipping to change at page 22, line 13
Its corresponding response is the CONTENT Response in Figure 12. Its corresponding response is the CONTENT Response in Figure 12.
Original message: Original message:
================= =================
0x6145000182ff32332043 0x6145000182ff32332043
Header: Header:
0x6145 0x6145
01 Ver 01 Ver
10 ACK 10 ACK
0001 tkl 0001 TKL
01000101 Successful Response Code 69 "2.05 Content" 01000101 Successful Response Code 69 "2.05 Content"
0x0001 = mid 0x0001 = mid
0x82 = token 0x82 = token
0xFF Payload marker 0xFF Payload marker
Payload: Payload:
0x32332043 0x32332043
Original msg length: 10 Original msg length: 10
Figure 12: CoAP CONTENT Response Figure 12: CoAP CONTENT Response
The SCHC Rules for the Inner Compression include all fields already The SCHC Rules for the Inner Compression include all fields already
present in a regular CoAP message. The methods described in present in a regular CoAP message. The methods described in
Section 4 apply to these fields. As an example, see Figure 13. Section 4 apply to these fields. As an example, see Figure 13.
RuleID 0 RuleID 0
+--------------+--+--+--+-----------+----------+----------++------+ +--------------+--+--+--+-----------+---------+---------++------+
| Field |FL|FP|DI| Target | MO | CDA || Sent | | Field |FL|FP|DI| Target | MO | CDA || Sent |
| | | | | Value | | ||[bits]| | | | | | Value | | ||[bits]|
+--------------+--+--+--+-----------+----------+----------++------+ +--------------+--+--+--+-----------+---------+---------++------+
|CoAP Code | 8| 1|up| 1 | equal |not-sent || | |CoAP Code | 8| 1|up| 1 | equal |not-sent || |
|CoAP Code | 8| 1|dw|[69,132] | match-map|match-sent|| c | |CoAP Code | 8| 1|dw|[69, | | || |
|CoAP Uri-Path |88| 1|up|temperature| equal |not-sent || | | | | | |132] |match-map|mapp-sent|| c |
+--------------+--+--+--+-----------+----------+----------++------+ |CoAP Uri-Path |88| 1|up|temperature| equal |not-sent || |
+--------------+--+--+--+-----------+---------+---------++------+
Figure 13: Inner SCHC Rules Figure 13: Inner SCHC Rules
Figure 14 shows the Plaintext obtained for the example GET Request Figure 14 shows the Plaintext obtained for the example GET request.
and follows the process of Inner Compression and Encryption until the The packet follows the process of Inner Compression and Encryption
end up with the Payload to be added in the outer OSCORE Message. until the payload. The outer OSCORE Message adds the result of the
Inner process.
In this case, the original message has no payload, and its resulting 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). Plaintext compressed up to only 1 byte (size of the RuleID). The
The AEAD algorithm preserves this length in its first output and AEAD algorithm preserves this length in its first output and yields a
yields a fixed-size tag that cannot be compressed and has to be fixed-size tag. SCHC cannot compress the tag, and the OSCORE message
included in the OSCORE message. This translates into an overhead in must include it without compression. The use of integrity translates
total message length, limiting the amount of compression that can be into an overhead in total message length, limiting the amount of
achieved and plays into the cost of adding security to the exchange. compression that can be achieved and plays into the cost of adding
security to the exchange.
________________________________________________________ ________________________________________________________
| | | |
| OSCORE Plaintext | | OSCORE Plaintext |
| | | |
| 0x01bb74656d7065726174757265 (13 bytes) | | 0x01bb74656d7065726174757265 (13 bytes) |
| | | |
| 0x01 Request Code GET | | 0x01 Request Code GET |
| | | |
| bb74656d7065726174757265 Option 11: URI_PATH | | bb74656d7065726174757265 Option 11: URI_PATH |
skipping to change at page 22, line 29 skipping to change at page 23, line 33
| Inner SCHC Compression | Inner SCHC Compression
| |
v v
_________________________________ _________________________________
| | | |
| Compressed Plaintext | | Compressed Plaintext |
| | | |
| 0x00 | | 0x00 |
| | | |
| RuleID = 0x00 (1 byte) | | RuleID = 0x00 (1 byte) |
| (No residue) | | (No Compression Residue) |
|_________________________________| |_________________________________|
| |
| AEAD Encryption | AEAD Encryption
| (piv = 0x04) | (piv = 0x04)
v v
_________________________________________________ _________________________________________________
| | | |
| encrypted_plaintext = 0xa2 (1 byte) | | encrypted_plaintext = 0xa2 (1 byte) |
| tag = 0xc54fe1b434297b62 (8 bytes) | | tag = 0xc54fe1b434297b62 (8 bytes) |
| | | |
| ciphertext = 0xa2c54fe1b434297b62 (9 bytes) | | ciphertext = 0xa2c54fe1b434297b62 (9 bytes) |
|_________________________________________________| |_________________________________________________|
Figure 14: Plaintext compression and encryption for GET Request Figure 14: Plaintext compression and encryption for GET Request
In Figure 15, the process is repeated for the example CONTENT Figure 15 shows the process for the example CONTENT Response. The
Response. The residue is 1 bit long. Note that since SCHC adds Compression Residue is 1 bit long. Note that since SCHC adds padding
padding after the payload, this misalignment causes the hexadecimal after the payload, this misalignment causes the hexadecimal code from
code from the payload to differ from the original, even though it has the payload to differ from the original, even if SCHC cannot compress
not been compressed. the tag. The overhead for the tag bytes limits the SCHC's
performance but brings security to the transmission.
On top of this, the overhead from the tag bytes is incurred as
before.
________________________________________________________ ________________________________________________________
| | | |
| OSCORE Plaintext | | OSCORE Plaintext |
| | | |
| 0x45ff32332043 (6 bytes) | | 0x45ff32332043 (6 bytes) |
| | | |
| 0x45 Successful Response Code 69 "2.05 Content" | | 0x45 Successful Response Code 69 "2.05 Content" |
| | | |
| ff Payload marker | | ff Payload marker |
| | | |
| 32332043 Payload | | 32332043 Payload |
|________________________________________________________| |________________________________________________________|
| |
| |
| Inner SCHC Compression | Inner SCHC Compression
| |
v v
__________________________________________ _____________________________________________
| | | |
| Compressed Plaintext | | Compressed Plaintext |
| | | |
| 0x001919902180 (6 bytes) | | 0x001919902180 (6 bytes) |
| | | |
| 00 RuleID | | 00 RuleID |
| | | |
| 0b0 (1 bit match-map residue) | | 0b0 (1 bit match-map Compression Residue) |
| 0x32332043 >> 1 (shifted payload) | | 0x32332043 >> 1 (shifted payload) |
| 0b0000000 Padding | | 0b0000000 Padding |
|__________________________________________| |_____________________________________________|
| |
| AEAD Encryption | AEAD Encryption
| (piv = 0x04) | (piv = 0x04)
v v
_________________________________________________________ _________________________________________________________
| | | |
| encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) | | encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) |
| tag = 0xe9aef3f2461e0c29 (8 bytes) | | tag = 0xe9aef3f2461e0c29 (8 bytes) |
| | | |
skipping to change at page 24, line 4 skipping to change at page 25, line 49
v v
_________________________________________________________ _________________________________________________________
| | | |
| encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) | | encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) |
| tag = 0xe9aef3f2461e0c29 (8 bytes) | | tag = 0xe9aef3f2461e0c29 (8 bytes) |
| | | |
| ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) | | ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) |
|_________________________________________________________| |_________________________________________________________|
Figure 15: Plaintext compression and encryption for CONTENT Response Figure 15: Plaintext compression and encryption for CONTENT Response
The Outer SCHC Rules (Figure 18) must process the OSCORE Options The Outer SCHC Rules (Figure 18) must process the OSCORE Options
fields. The Figure 16 and Figure 17 show a dump of the OSCORE fields. Figure 16 and Figure 17 shows a dump of the OSCORE Messages
Messages generated from the example messages once they have been generated from the example messages. They include the Inner
provided with the Inner Compressed Ciphertext in the payload. These Compressed Ciphertext in the payload. These are the messages that
are the messages that have to be compressed by the Outer SCHC have to be compressed by the Outer SCHC Compression.
Compression.
Protected message: Protected message:
================== ==================
0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62 0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62
(25 bytes) (25 bytes)
Header: Header:
0x4102 0x4102
01 Ver 01 Ver
00 CON 00 CON
0001 tkl 0001 TKL
00000010 Request Code 2 "POST" 00000010 Request Code 2 "POST"
0x0001 = mid 0x0001 = mid
0x82 = token 0x82 = token
Options: Options:
0xd8080904636c69656e74 (10 bytes) 0xd8080904636c69656e74 (10 bytes)
Option 21: OBJECT_SECURITY Option 21: OBJECT_SECURITY
Value = 0x0904636c69656e74 Value = 0x0904636c69656e74
09 = 000 0 1 001 Flag byte 09 = 000 0 1 001 Flag byte
skipping to change at page 25, line 14 skipping to change at page 27, line 14
Protected message: Protected message:
================== ==================
0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29 0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29
(22 bytes) (22 bytes)
Header: Header:
0x6144 0x6144
01 Ver 01 Ver
10 ACK 10 ACK
0001 tkl 0001 TKL
01000100 Successful Response Code 68 "2.04 Changed" 01000100 Successful Response Code 68 "2.04 Changed"
0x0001 = mid 0x0001 = mid
0x82 = token 0x82 = token
Options: Options:
0xd008 (2 bytes) 0xd008 (2 bytes)
Option 21: OBJECT_SECURITY Option 21: OBJECT_SECURITY
Value = b'' Value = b''
0xFF Payload marker 0xFF Payload marker
Payload: Payload:
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Figure 17: Protected and Inner SCHC Compressed CONTENT Response Figure 17: Protected and Inner SCHC Compressed CONTENT Response
For the flag bits, some SCHC compression methods are useful, For the flag bits, some SCHC compression methods are useful,
depending on the application. The simplest alternative is to provide depending on the Application. The most straightforward alternative
a fixed value for the flags, combining MO equal and CDA not- sent. is to provide a fixed value for the flags, combining MO "equal" and
This saves most bits but could prevent flexibility. Otherwise, CDA "not-sent." This SCHC definition saves most bits but could
match-mapping could be used to choose from an interesting number of prevent flexibility. Otherwise, SCHC could use a "match-mapping" MO
configurations for the exchange. to choose from several configurations for the exchange. If not, the
Otherwise, MSB could be used to mask off the 3 hard-coded most SCHC description may use an "MSB" MO to mask off the three hard-coded
significant bits. most significant bits.
Note that fixing a flag bit will limit CoAP Options choice that can 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 be used in the exchange since their values are dependent on specific
options. options.
The piv field lends itself to having some bits masked off with MO MSB The piv field lends itself to having some bits masked off with "MSB"
and CDA LSB. This could be useful in applications where the message MO and "LSB" CDA. This SCHC description could be useful in
frequency is low such as LPWAN technologies. Note that compressing applications where the message frequency is low such as LPWAN
the sequence numbers effectively reduces the maximum number of technologies. Note that compressing the sequence numbers may reduce
sequence numbers used in an exchange. Once this amount is exceeded, the maximum number of sequence numbers used in an exchange. Once the
the OSCORE keys need to be re-established. sequence number exceeds the maximum value, the OSCORE keys need to be
re-established.
The size s included in the kid context field MAY be masked off with 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 "LSB" CDA. The rest of the field could have additional bits masked
or have the whole field be fixed with MO equal and CDA not-sent. The off or have the whole field fixed with MO "equal" and CDA "not-sent."
same holds for the kid field. The same holds for the kid field.
Figure 18 shows a possible set of Outer Rules to compress the Outer Figure 18 shows a possible set of Outer Rules to compress the Outer
Header. Header.
RuleID 0 RuleID 0
+------------------+--+--+--+--------------+-------+--------++------+ +------------------+--+--+--+--------------+-------+--------++------+
| Field |FL|FP|DI| Target | MO | CDA || Sent | | Field |FL|FP|DI| Target | MO | CDA || Sent |
| | | | | Value | | ||[bits]| | | | | | Value | | ||[bits]|
+------------------+--+--+--+--------------+-------+--------++------+ +------------------+--+--+--+--------------+-------+--------++------+
|CoAP version | 2| 1|bi| 01 |equal |not-sent|| | |CoAP version | 2| 1|bi| 01 |equal |not-sent|| |
skipping to change at page 26, line 34 skipping to change at page 28, line 37
|CoAP OSCORE_piv |var 1|up| 0x00 |MSB(4) |LSB ||PPPP | |CoAP OSCORE_piv |var 1|up| 0x00 |MSB(4) |LSB ||PPPP |
|COAP OSCORE_kid |var 1|up|0x636c69656e70|MSB(52)|LSB ||KKKK | |COAP OSCORE_kid |var 1|up|0x636c69656e70|MSB(52)|LSB ||KKKK |
|COAP OSCORE_kidctx|var 1|bi| b'' |equal |not-sent|| | |COAP OSCORE_kidctx|var 1|bi| b'' |equal |not-sent|| |
|CoAP OSCORE_flags | 8| 1|dw| 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_piv |var 1|dw| b'' |equal |not-sent|| |
|CoAP OSCORE_kid |var 1|dw| b'' |equal |not-sent|| | |CoAP OSCORE_kid |var 1|dw| b'' |equal |not-sent|| |
+------------------+--+--+--+--------------+-------+--------++------+ +------------------+--+--+--+--------------+-------+--------++------+
Figure 18: Outer SCHC Rules Figure 18: Outer SCHC Rules
These Outer Rules are applied to the example GET Request and CONTENT The Outer Rule of Figure 18 is applied to the example GET Request and
Response. The resulting messages are shown in Figure 19 and CONTENT Response. Figure 19 and Figure 20 show the resulting
Figure 20. messages.
Compressed message: Compressed message:
================== ==================
0x001489458a9fc3686852f6c4 (12 bytes) 0x001489458a9fc3686852f6c4 (12 bytes)
0x00 RuleID 0x00 RuleID
1489 Compression Residue 1489 Compression Residue
458a9fc3686852f6c4 Padded payload 458a9fc3686852f6c4 Padded payload
Compression Residue: Compression Residue:
0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding) 0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding)
skipping to change at page 28, line 23 skipping to change at page 30, line 23
|CoAP TKL | 4| 1|bi| 1 |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|up| 2 |equal |not-sent || |
|CoAP Code | 8| 1|dw| [69,132] |match-map|map-sent ||C | |CoAP Code | 8| 1|dw| [69,132] |match-map|map-sent ||C |
|CoAP MID |16| 1|bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP MID |16| 1|bi| 0000 |MSB(12) |LSB ||MMMM |
|CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT |
|CoAP Uri-Path |88| 1|up|temperature|equal |not-sent || | |CoAP Uri-Path |88| 1|up|temperature|equal |not-sent || |
+---------------+--+--+--+-----------+---------+-----------++-------+ +---------------+--+--+--+-----------+---------+-----------++-------+
Figure 21: SCHC-CoAP Rules (No OSCORE) Figure 21: SCHC-CoAP Rules (No OSCORE)
This yields the results in Figure 22 for the Request, and Figure 23 Figure 21 Rule yields the SCHC compression results in Figure 22 for
for the Response. request, and Figure 23 for the response.
Compressed message: Compressed message:
================== ==================
0x0114 0x0114
0x01 = RuleID 0x01 = RuleID
Compression Residue: Compression Residue:
0b00010100 (1 byte) 0b00010100 (1 byte)
Compressed msg length: 2 Compressed msg length: 2
skipping to change at page 29, line 29 skipping to change at page 31, line 29
As can be seen, the difference between applying SCHC + OSCORE as As can be seen, the difference between applying SCHC + OSCORE as
compared to regular SCHC + COAP is about 10 bytes. compared to regular SCHC + COAP is about 10 bytes.
8. IANA Considerations 8. IANA Considerations
This document has no request to IANA. This document has no request to IANA.
9. Security considerations 9. Security considerations
When applied to LPWAN, the Security Considerations of SCHC header The use of SCHC header compression over CoAP header fields allow the
compression [rfc8724] are valid for SCHC CoAP header compression. compression of the header information only. The SCHC header
When CoAP uses OSCORE, the security considerations defined in compression itself does not increase or reduce the level of security
[rfc8613] does not change when SCHC header compression is applied. in the communication. When the connection does not use any security
protocol as OSCORE, DTLS, or other, it is necessary to use a layer
two security.
The definition of SCHC over CoAP header fields permits the If LPWAN is the layer two technology, the use of SCHC over the CoAP
compression of header information only. The SCHC header compression protocol keeps valid the Security Considerations of SCHC header
itself does not increase or reduce the level of security in the compression [RFC8724]. When using another layer two, integrity
communication. When the connection does not use any security protection is mandatory.
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 The use of SCHC when CoAP uses OSCORE keeps valid the security
SCHC corrupted packet onto the link and cause a compression considerations defined in [RFC8613].
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 DoS attacks are possible if an intruder can introduce a corrupted
fields. In the compressed header, the length sent is not the SCHC compressed packet onto the link and cause excessive resource
original header field length but the length of the Residue. So if a consumption at the decompressor. However, an intruder having the
corrupted packet comes to the decompressor with a longer or shorter ability to add corrupted packets at the link layer raises additional
length than the one in the original header, SCHC decompression will security issues than those related to header compression.
detect an error and drops the packet.
OSCORE compression is also based on the same compression method SCHC compression returns variable-length Compression Residues for
described in [rfc8724]. The size of the Initialisation Vector (IV) some CoAP fields. In the compressed header, the length sent is not
residue must be considered carefully. A residue size obtained with the original header field length but the Compression Residue's length
LSB CDA over the IV impacts on the compression efficiency and the that is transmitted. So If a corrupted packet comes to the
frequency the device will renew its key. This operation requires decompressor with a longer or shorter length than the original
several exchanges and is energy-consuming. header, SCHC decompression will detect an error and drop the packet.
Using SCHC over the OSCORE headers, OSCORE MUST consider the
Initialization Vector (IV) size carefully in the Compression Residue.
A Compression Residue size obtained with an "LSB" CDA over the IV
impacts the compression efficiency and the frequency that the Device
will renew its key. This operation requires several exchanges and is
energy-consuming.
SCHC header and compression Rules MUST remain tightly coupled. SCHC header and compression Rules MUST remain tightly coupled.
Otherwise, an encrypted residue may be decompressed differently by Otherwise, an encrypted Compression Residue may be decompressed
the receiver. To avoid this situation, if the Rule is modified in differently by the receiver. Any update in the context Rules on one
one location, the OSCORE keys MUST be re-established. side MUST trigger the OSCORE keys re-establishment.
10. Acknowledgements 10. Acknowledgements
The authors would like to thank (in alphabetic order): Christian The authors would like to thank (in alphabetic order): Christian
Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran Fossati, Klaus Hartke, Benjamin Kaduk, Francesca Palombini, Alexander
Selander. Pelov, Goran Selander and Eric Vyncke.
11. Normative References 11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[rfc5116] McGrew, D., "An Interface and Algorithms for Authenticated [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>. <https://www.rfc-editor.org/info/rfc5116>.
[rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014, DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>. <https://www.rfc-editor.org/info/rfc7252>.
[rfc7641] Hartke, K., "Observing Resources in the Constrained [RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641, Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015, DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>. <https://www.rfc-editor.org/info/rfc7641>.
[rfc7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959, the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016, DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>. <https://www.rfc-editor.org/info/rfc7959>.
[rfc7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T. [RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967, No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>. August 2016, <https://www.rfc-editor.org/info/rfc7967>.
[rfc8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[rfc8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments "Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>. <https://www.rfc-editor.org/info/rfc8613>.
[rfc8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. [RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "SCHC: Generic Framework for Static Context Header Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724, Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020, DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>. <https://www.rfc-editor.org/info/rfc8724>.
Authors' Addresses Authors' Addresses
Ana Minaburo Ana Minaburo
Acklio Acklio
1137A avenue des Champs Blancs 1137A avenue des Champs Blancs
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