rfc8824.original   rfc8824.txt 
lpwan Working Group A. Minaburo Internet Engineering Task Force (IETF) A. Minaburo
Internet-Draft Acklio Request for Comments: 8824 Acklio
Intended status: Standards Track L. Toutain Category: Standards Track L. Toutain
Expires: September 9, 2021 Institut MINES TELECOM; IMT Atlantique ISSN: 2070-1721 IMT Atlantique
R. Andreasen R. Andreasen
Universidad de Buenos Aires Universidad de Buenos Aires
March 08, 2021 June 2021
LPWAN Static Context Header Compression (SCHC) for CoAP Static Context Header Compression (SCHC) for the
draft-ietf-lpwan-coap-static-context-hc-19 Constrained Application Protocol (CoAP)
Abstract Abstract
This draft defines how to compress the Constrained Application This document defines how to compress Constrained Application
Protocol (CoAP) using the Static Context Header Compression (SCHC). Protocol (CoAP) headers using the Static Context Header Compression
SCHC is a header compression mechanism adapted for Constrained and fragmentation (SCHC) framework. SCHC defines a header
Devices. SCHC uses a static description of the header to reduce the compression mechanism adapted for Constrained Devices. SCHC uses a
header's redundancy and size. While RFC 8724 describes the SCHC static description of the header to reduce the header's redundancy
compression and fragmentation framework, and its application for and size. While RFC 8724 describes the SCHC compression and
IPv6/UDP headers, this document applies SCHC for CoAP headers. The fragmentation framework, and its application for IPv6/UDP headers,
CoAP header structure differs from IPv6 and UDP since CoAP uses a this document applies SCHC to CoAP headers. The CoAP header
flexible header with a variable number of options, themselves of structure differs from IPv6 and UDP, since CoAP uses a flexible
variable length. The CoAP protocol messages format is asymmetric: header with a variable number of options, themselves of variable
the request messages have a header format different from the one in length. The CoAP message format is asymmetric: the request messages
the response messages. This specification gives guidance on applying have a header format different from the format in the response
SCHC to flexible headers and how to leverage the asymmetry for more messages. This specification gives guidance on applying SCHC to
efficient compression Rules. flexible headers and how to leverage the asymmetry for more efficient
compression Rules.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on September 9, 2021. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8824.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terminology
2. SCHC Applicability to CoAP . . . . . . . . . . . . . . . . . 4 2. SCHC Applicability to CoAP
3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 7 3. CoAP Headers Compressed with SCHC
3.1. Differences between CoAP and UDP/IP Compression . . . . . 8 3.1. Differences between CoAP and UDP/IP Compression
4. Compression of CoAP header fields . . . . . . . . . . . . . . 9 4. Compression of CoAP Header Fields
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 9 4.1. CoAP Version Field
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 9 4.2. CoAP Type Field
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 9 4.3. CoAP Code Field
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 10 4.4. CoAP Message ID Field
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 10 4.5. CoAP Token Fields
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 10 5. CoAP Options
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 11 5.1. CoAP Content and Accept Options
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
5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 11 5.3. CoAP Option Uri-Path and Uri-Query Fields
5.3.1. Variable number of Path or Query elements . . . . . . 13 5.3.1. Variable Number of Path or Query Elements
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
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
6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 13 6. SCHC Compression of CoAP Extensions
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.1. Block
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.2. Observe
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 14 6.3. No-Response
6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.4. OSCORE
7. Examples of CoAP header compression . . . . . . . . . . . . . 15 7. Examples of CoAP Header Compression
7.1. Mandatory header with CON message . . . . . . . . . . . . 15 7.1. Mandatory Header with CON Message
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 16 7.2. OSCORE Compression
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 20 7.3. Example OSCORE Compression
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 8. IANA Considerations
9. Security considerations . . . . . . . . . . . . . . . . . . . 31 9. Security Considerations
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 10. Normative References
11. Normative References . . . . . . . . . . . . . . . . . . . . 32 Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 Authors' Addresses
1. Introduction 1. Introduction
CoAP [RFC7252] is a command/response protocol designed for micro- The Constrained Application Protocol (CoAP) [RFC7252] is a command/
controllers with a small RAM and ROM and optimized for REST-based response protocol designed for microcontrollers with small RAM and
(Representative state transfer) services. Although the Constrained ROM and optimized for services based on REST (Representational State
Devices leads the CoAP design, a CoAP header's size is still too Transfer). Although the Constrained Devices are a leading factor in
large for LPWAN (Low Power Wide Area Networks). SCHC header the design of CoAP, a CoAP header's size is still too large for
compression over CoAP header is required to increase performance or LPWANs (Low-Power Wide-Area Networks). Static Context Header
use CoAP over LPWAN technologies. Compression and fragmentation (SCHC) over CoAP headers is required to
increase performance or to use CoAP over LPWAN technologies.
The [RFC8724] defines SCHC, a header compression mechanism for the [RFC8724] defines the SCHC framework, which includes a header
LPWAN network based on a static context. Section 5 of the [RFC8724] compression mechanism for LPWANs that is based on a static context.
explains where compression and decompression occur in the Section 5 of [RFC8724] explains where compression and decompression
architecture. The SCHC compression scheme assumes as a prerequisite occur in the architecture. The SCHC compression scheme assumes as a
that both end-points know the static context before transmission. prerequisite that both endpoints know the static context before
The way the context is configured, provisioned, or exchanged is out transmission. The way the context is configured, provisioned, or
of this document's scope. exchanged is out 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: at IP and UDP in the compression may apply at two different levels: at IP and UDP in the
LPWAN network and another at the application level for CoAP. These LPWAN and another at the application level for CoAP. These two
two compressions may be independent. Both follow the same principle compression techniques may be independent. Both follow the same
described in [RFC8724]. As different entities manage the CoAP principle as that described in [RFC8724]. As different entities
compression at different levels, the SCHC Rules driving the manage the CoAP compression process at different levels, the SCHC
compression/decompression are also different. The [RFC8724] Rules driving the compression/decompression are also different.
describes how to use SCHC for IP and UDP headers. This document [RFC8724] describes how to use SCHC for IP and UDP headers. This
specifies how to apply SCHC compression to CoAP headers. document specifies how to apply SCHC compression to CoAP headers.
SCHC compresses and decompresses headers based on common contexts SCHC compresses and decompresses headers based on common contexts
between Devices. SCHC context includes multiple Rules. Each Rule between Devices. The SCHC context includes multiple Rules. Each
can match the header fields to specific values or ranges of values. Rule can match the header fields to specific values or ranges of
If a Rule matches, the matched header fields are replaced by the values. If a Rule matches, the matched header fields are replaced by
RuleID and the Compression Residue that contains the residual bits of the RuleID and the Compression Residue that contains the residual
the compression. Thus, different Rules may correspond to different bits of the compression. Thus, different Rules may correspond to
protocol headers in the packet that a Device expects to send or different protocol headers in the packet that a Device expects to
receive. send or receive.
A Rule describes the packets' 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 Field Descriptors; see Section 7 of [RFC8724]. Thereby, each
each description contains the field ID (FID), its length (FL), and description contains the Field ID (FID), Field Length (FL), and Field
its position (FP), a direction indicator (DI) (upstream, downstream, Position (FP), as well as a Direction Indicator (DI) (upstream,
and bidirectional), and some associated Target Values (TV). The downstream, and bidirectional) and some associated Target Values
direction indicator is used for compression to give the best TV to (TVs). The DI is used for compression to give the best TV to the FID
the FID when these values differ in the transmission direction. So a when these values differ in their transmission direction. So, a
field may be described several times. field may be described several times.
A Matching Operator (MO) is associated with each header field A Matching Operator (MO) is associated with each header Field
description. The Rule is selected if all the MOs fit the TVs for all Descriptor. The Rule is selected if all the MOs fit the TVs for all
fields of the incoming header. A Rule cannot be selected if the fields of the incoming header. A Rule cannot be selected if the
message contains an unknown field to the SCHC compressor. message contains a field that is 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 gives 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 four actions:
o send the field value (value-sent), * send the field value (value-sent),
o send nothing (not-sent), * send nothing (not-sent),
o send some least significant bits of the field (LSB) or, * send some Least Significant Bits (LSBs) of the field, or
o send an index (mapping-sent). * 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 Residue. 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, with the
Rules to be used depending on the protocol and the Application. exact 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 [RFC8724] describes the compression scheme for IPv6 and
and UDP headers. This document targets the CoAP header compression UDP headers. This document targets CoAP header compression using
using SCHC. 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
14 [RFC2119][RFC8174] when, and only when, they appear in all BCP 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
SCHC Compression for CoAP header MAY be done in conjunction with the SCHC compression for CoAP headers 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. Figures 1, 2, and 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, the Device and the NGW stack from IPv6 to CoAP. In this case, the Device and the Network
perform SCHC C/D (Static Context Header Compression Compressor/ Gateway (NGW) perform SCHC C/D (SCHC Compression/Decompression; see
Decompressor). The Application communicating with the Device does [RFC8724]). The application communicating with the Device does not
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
Figure 1 shows the use of SCHC header compression above layer 2 in Figure 1 shows the use of SCHC header compression above Layer 2 in
the Device and the NGW. The SCHC layer receives non-encrypted the Device and the NGW. The SCHC layer receives non-encrypted
packets and can apply compression Rules to all the headers in the packets and can apply compression Rules to all the headers in the
stack. On the other end, the NGW receives the SCHC packet and stack. On the other end, the NGW receives the SCHC packet and
reconstructs the headers using the Rule and the Compression Residue. reconstructs the headers using the Rule and the Compression Residue.
After the decompression, the NGW forwards the IPv6 packet toward the After the decompression, the NGW forwards the IPv6 packet toward the
destination. The same process applies in the other direction when a destination. The same process applies in the other direction when a
non-encrypted packet arrives at the NGW. Thanks to the IP forwarding non-encrypted packet arrives at the NGW. Thanks to the IP forwarding
based on the IPv6 prefix, the NGW identifies the Device and based on the IPv6 prefix, the NGW identifies the Device and
compresses headers using the Device's Rules. 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, SCHC compression is applied in the
the CoAP layer, compressing the CoAP header independently of the CoAP layer, compressing the CoAP header independently of the other
other layers. The RuleID, the Compression Residue, and CoAP payload layers. The RuleID, Compression Residue, and CoAP payload are
are encrypted using a mechanism such as DTLS. Only the other end encrypted using a mechanism such as DTLS. Only the other end (App)
(App) can decipher the information. If needed, layers below use SCHC can decipher the information. If needed, layers below use SCHC to
to compress the header as defined in [RFC8724] (represented in dotted compress the header as defined in [RFC8724] (represented by dotted
lines). lines in the figure).
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. The context initialization is out of the Device and the application. The context initialization is out of
scope of this document. scope for 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
The third example, Figure 3, shows the use of Object Security for The third example, Figure 3, shows the use of Object Security for
Constrained RESTful Environments (OSCORE) [RFC8613]. In this case, Constrained RESTful Environments (OSCORE) [RFC8613]. In this case,
SCHC needs two Rules to compress the CoAP header. A first Rule SCHC needs two Rules to compress the CoAP header. A first Rule
focused on the inner header. The result of this first compression is focuses on the Inner header. The result of this first compression is
encrypted using the OSCORE mechanism. Then a second Rule compresses encrypted using the OSCORE mechanism. Then, a second Rule compresses
the outer 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 |
+--------+ +--------+ +--------+ +--------+
| SCHC | | SCHC | | SCHC | | SCHC |
| outer | | outer | | Outer | | Outer |
+--------+ +--------+ +--------+ +--------+
. udp . . udp . . udp . . udp .
.......... .................. .......... .......... .................. ..........
. 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 2 and In the case of several SCHC instances, as shown in Figures 2 and 3,
Figure 3, the Rules may come from different provisioning domains. the Rules may come from different provisioning domains.
This document focuses on CoAP compression represented in the dashed This document focuses on CoAP compression, as represented by the
boxes in the previous figures. dashed 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 applies the same description and
compression/decompression techniques like the one for IP and UDP compression/decompression techniques as the technique used for IP and
explained in the [RFC8724]. For CoAP, the SCHC Rules description UDP, as explained in [RFC8724]. For CoAP, the SCHC Rules description
uses the direction information to optimize the compression by uses the direction information to optimize the compression by
reducing the number of Rules needed to compress headers. The field reducing the number of Rules needed to compress headers. The Field
description MAY define both request/response headers and target Descriptor MAY define both request/response headers and TVs in the
values in the same Rule, using the DI (direction indicator) to make same Rule, using the DI to indicate the header type.
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, and
the Compression Residue is the complete header of the packet. See where the Compression Residue is the complete header of the packet.
section 6 of [RFC8724]. See 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 in 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 * The CoAP message format is asymmetric; the headers are different
request or a response. For example, the URI-Path option is for a request or a response. For example, the Uri-Path option is
mandatory in the request, and it might not be present in the mandatory in the request, and it might not be present in the
response. A request might contain an Accept option, and the response. A request might contain an Accept option, and the
response might include a Content-Format option. In comparison, response might include a Content-Format option. In comparison,
IPv6 and UDP returning path swap the value of some fields in the the IPv6 and UDP returning path swaps the value of some fields in
header. However, all the directions have the same fields (e.g., the header. However, all the directions have the same fields
source and destination address fields). (e.g., source and destination address fields).
The [RFC8724] defines the use of a direction indicator (DI) in the [RFC8724] defines the use of a DI in the Field Descriptor, which
Field Descriptor, which allows a single Rule to process a message allows a single Rule to process a message header differently,
header differently depending on the direction. depending on the direction.
o Even when a field is "symmetric" (i.e., found in both directions), * Even when a field is "symmetric" (i.e., found in both directions),
the values carried in each direction are different. The the values carried in each direction are different. The
compression may use a "match-mapping" MO to limit the range of compression may use a "match-mapping" MO to limit the range of
expected values in a particular direction and reduce the expected values in a particular direction and reduce the
Compression Residue's size. Through the direction indicator (DI), Compression Residue's size. Through the DI, a Field Descriptor in
a field description in the Rules splits the possible field value the Rules splits the possible field value into two parts, one for
into two parts, one for each direction. For instance, if a client each direction. For instance, if a client sends only Confirmable
sends only CON requests, the Type can be elided by compression, (CON) requests [RFC7252], the Type can be elided by compression,
and the answer may use one single bit to carry either the ACK or and the answer may use one single bit to carry either the ACK or
RST type. The field Code has the same behavior, the 0.0X code Reset (RST) type. The field Code has the same behavior: the 0.0X
format value in the request, and the Y.ZZ code format in the code format value in the request and the Y.ZZ code format in the
response. response.
o In SCHC, the Rule defines the different header fields' length, so * In SCHC, the Rule defines the different header fields' length, so
SCHC does not need to send it. In IPv6 and UDP headers, the SCHC does not need to send it. In IPv6 and UDP headers, the
fields have a fixed size, known by definition. On the other hand, fields have a fixed size, known by definition. On the other hand,
some CoAP header fields have variable lengths, and the Rule some CoAP header fields have variable lengths, and the Rule
description specifies it. For example, in a URI-path or URI- description specifies it. For example, in a Uri-Path or Uri-
query, the Token size may vary from 0 to 8 bytes, and the CoAP Query, the Token size may vary from 0 to 8 bytes, and the CoAP
options use the Type-Length-Value encoding format. options use the Type-Length-Value encoding format.
When doing SCHC compression of a variable-length field, When doing SCHC compression of a variable-length field,
Section 7.5.2 from [RFC8724] offers the possibility to define a Section 7.4.2 of [RFC8724] offers the option of defining a
function for the Field length in the Field Description to know the function for the Field Length in the Field Descriptor to know the
length before compression. If the field length is unknown, the length before compression. If the Field Length is unknown, the
Rule will set it as a variable, and SCHC will send the compressed Rule will set it as a variable, and SCHC will send the compressed
field's length in the Compression Residue. field's length in the Compression Residue.
o A field can appear several times in the CoAP headers. It is found * A field can appear several times in the CoAP headers. It is found
typically 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 FID to appear several times in
in the Rule and uses the Field Position (FP) to identify the the Rule and uses the Field Position (FP) to identify the correct
correct instance, thereby removing the matching operation's instance, thereby removing the MO's ambiguity.
ambiguity.
o Field lengths defined in the CoAP protocol can be too * Field Lengths defined in CoAP can be too large when it comes to
large regarding LPWAN traffic constraints. For instance, this is LPWAN traffic constraints. For instance, this is particularly
particularly true for the Message-ID field and the Token field. true for the Message ID field and the Token field. SCHC uses
SCHC uses different Matching operators (MO) to perform the different MOs to perform the compression. See Section 7.4 of
compression. See section 7.4 of [RFC8724]. In this case, SCHC [RFC8724]. In this case, SCHC can apply the Most Significant Bits
can apply the Most Significant Bits (MSB) MO to reduce the (MSBs) MO to reduce the information carried on 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 Section 7.1 of fields. CoAP compression with SCHC follows the information provided
[RFC8724]. in Section 7.1 of [RFC8724].
4.1. CoAP version field 4.1. CoAP Version Field
CoAP version is bidirectional and MUST be elided during the SCHC The CoAP version is bidirectional and MUST be elided during SCHC
compression since it always contains the same value. In the future, compression, since it always contains the same value. In the future,
or if a new version of CoAP is defined, new Rules will be needed to 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 CoAP [RFC7252] has four types of messages: two requests (CON, NON),
(CON, NON), one response (ACK), and one empty message (RST). one response (ACK), and one empty message (RST).
The SCHC compression SHOULD elide this field if, for instance, a The SCHC compression scheme SHOULD elide this field if, for instance,
client is sending only NON or only CON messages. For the RST a client is sending only Non-confirmable (NON) messages or only CON
message, SCHC may use a dedicated Rule. For other usages, SCHC can messages. For the RST message, SCHC may use a dedicated Rule. For
use a "match-mapping" MO. other usages, SCHC can use a "match-mapping" MO.
4.3. CoAP code field 4.3. CoAP Code Field
The code field is an IANA registry [RFC7252], and it indicates the The Code field, defined in an IANA registry [RFC7252], indicates the
Request Method used in CoAP. The compression of the CoAP code field Request Method used in CoAP. The compression of the CoAP Code field
follows the same principle as that of the CoAP type field. If the follows the same principle as that of the CoAP Type field. If the
Device plays a specific role, SCHC may split the code values into two Device plays a specific role, SCHC may split the code values into two
fields description, the request codes with the 0 class and the Field Descriptors: (1) the request codes with the 0 class and (2) the
response values. SCHC will use the direction indicator to identify response values. SCHC will use the DI to identify the correct value
the correct value in the packet. in the packet.
If the Device only implements a CoAP client, SCHC compression may If the Device only implements a CoAP client, SCHC compression may
reduce the request code to the set of requests the client can reduce the request code to the set of requests the client can
process. process.
For known values, SCHC can use a "match-mapping" MO. If SCHC cannot For known values, SCHC can use a "match-mapping" MO. If SCHC cannot
compress the code field, it will send the values in the Compression compress the Code field, it will send the values in the Compression
Residue. Residue.
4.4. CoAP Message ID field 4.4. CoAP Message ID Field
SCHC can compress the Message ID field with the "MSB" MO and the SCHC can compress the Message ID field with the "MSB" MO and the
"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
CoAP defines the Token using 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.
SCHC processes the Token length as any header field. If the value SCHC processes the Token Length as it would any header field. If the
does not change, the size can be stored in the TV and elided during value does not change, the size can be stored in the TV and elided
the transmission. Otherwise, SCHC will send the token length in the during the transmission. Otherwise, SCHC will send the Token Length
Compression Residue. in the Compression Residue.
For the Token Value, SCHC MUST NOT send it as a variable-length in For the Token Value, SCHC MUST NOT send it as variable-length data in
the Compression Residue to avoid ambiguity with Token Length. the Compression Residue, to avoid ambiguity with the Token Length.
Therefore, SCHC MUST use the Token length value to define the size of Therefore, SCHC MUST use the Token Length value to define the size of
the Compression Residue. SCHC designates a specific function "tkl" the Compression Residue. SCHC designates a specific function, "tkl",
that the Rule MUST use to complete the field description. During the that the Rule MUST use to complete the Field Descriptor. 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 placed after the basic header in Option Numbers CoAP defines options placed after the basic header, ordered by option
order; see [RFC7252]. Each Option instance in a message uses the number; see [RFC7252]. Each Option instance in a message uses the
format Delta-Type (D-T), Length (L), Value (V). The SCHC Rule builds format Delta-Type (D-T), Length (L), Value (V). The SCHC Rule builds
the description of the option by using in the Field ID the Option the description of the option by using the following:
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 well-
known size, the Rule may keep the length value. Therefore, SCHC
compression does not send it. Otherwise, SCHC Compression carries
the length of the Compression Residue, in addition to the Compression
Residue value.
CoAP requests and responses do not include the same options. So * in the FID: the option number built from the D-T;
Compression Rules may reflect this asymmetry by tagging the direction
indicator. * in the TV: the option value; and
* for the Option Length: the information provided in Sections 7.4.1
and 7.4.2 of [RFC8724].
When the Option Length has a well-known size, the Rule may keep the
length value. Therefore, SCHC compression does not send it.
Otherwise, SCHC compression carries the length of the Compression
Residue, in addition to the Compression Residue value.
CoAP requests and responses do not include the same options. So,
compression Rules may reflect this asymmetry by tagging the DI.
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 CoAP introduces a new option, the SCHC Rules MAY be updated, and If CoAP introduces a new option, the SCHC Rules MAY be updated, and
the new Field ID description MUST be assigned to allow its the new FID description MUST be assigned to allow its compression.
compression. Otherwise, if no Rule describes this new option, the Otherwise, if no Rule describes this new option, SCHC compression is
SCHC compression is not achieved, and SCHC sends the CoAP header not achieved, and SCHC sends the CoAP header without compression.
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 "match-mapping" SHOULD be used to limit several possible values, a "match-mapping" MO SHOULD be used to limit
the Compression Residue's size. If not, SCHC has to send the option the Compression Residue's size. If not, SCHC has to send the option
value in the 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
SCHC compresses these three fields in the same way. When the value SCHC compresses these three fields in the same way. When the values
of these options is known, SCHC can elide these fields. If the of these options are known, SCHC can elide these fields. If the
option uses well-known values, SCHC can use a "match-mapping" MO. option uses well-known values, SCHC can use a "match-mapping" MO.
Otherwise, SCHC will use "value-sent" MO, and the Compression Residue Otherwise, SCHC will use the "value-sent" MO, and the Compression
will send these options' values. 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
The Uri-Path and Uri-Query fields are repeatable options; this means The Uri-Path and Uri-Query fields are repeatable options; this means
that in the CoAP header, they may appear several times with different that in the CoAP header, they may appear several times with different
values. SCHC Rule description uses the Field Position (FP) to values. The SCHC Rule description uses the FP to distinguish the
distinguish the different instances in the path. different instances in the path.
To compress repeatable field values, SCHC may use a "match-mapping" To compress repeatable field values, SCHC may use a "match-mapping"
MO to reduce the size of variable Paths or Queries. In these cases, MO to reduce the size of variable paths or queries. In these cases,
to optimize the compression, several elements can be regrouped into a to optimize the compression, several elements can be regrouped into a
single entry. The Numbering of elements does not change, and the single entry. The numbering of elements does not change, and the
first matching element sets the MO comparison. first matching element sets the MO comparison.
+--------+---+--+--+--------+-------------+------------+ In Table 1, SCHC can use a single bit in the Compression Residue to
| Field |FL |FP|DI| Target | Matching | CDA |
| | | | | Value | Operator | |
+--------+---+--+--+--------+-------------+------------+
|Uri-Path| | 1|up|["/a/b",|match-mapping|mapping-sent|
| | | | |"/c/d"] | | |
|Uri-Path|var| 3|up| |ignore |value-sent |
+--------+---+--+--+--------+-------------+------------+
Figure 4: complex path example
In Figure 4, SCHC can use a single bit in the Compression Residue to
code one of the two paths. If regrouping were not allowed, 2 bits in code one of the two paths. If regrouping were not allowed, 2 bits in
the Compression Residue would be needed. SCHC sends the third path the Compression Residue would be needed. SCHC sends the third path
element as a variable size in the Compression Residue. element as a variable size in the Compression Residue.
The length of URI-Path and URI-Query may be known when the rule is +==========+=====+====+====+==========+=========+==============+
defined. In any case, SCHC MUST set the field length to variable. | Field | FL | FP | DI | TV | MO | CDA |
The unit to indicate the Compression Residue size is in Byte. +==========+=====+====+====+==========+=========+==============+
| Uri-Path | | 1 | Up | ["/a/b", | match- | mapping-sent |
| | | | | "/c/d"] | mapping | |
+----------+-----+----+----+----------+---------+--------------+
| Uri-Path | var | 3 | Up | | ignore | value-sent |
+----------+-----+----+----+----------+---------+--------------+
Table 1: Complex Path Example
The length of Uri-Path and Uri-Query may be known when the Rule is
defined. In any case, SCHC MUST set the Field Length to a variable
value. The Compression Residue size is expressed in bytes.
SCHC compression can use the MSB MO to a Uri-Path or Uri-Query SCHC compression can use the MSB MO to a Uri-Path or Uri-Query
element. However, attention to the length is important because the 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 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 Compression The length sent at the beginning of a variable-length Compression
Residue indicates the LSB's size in bytes. Residue indicates the LSB's size in bytes.
For instance, for a CORECONF path /c/X6?k="eth0" the Rule description For instance, for a CORECONF path /c/X6?k=eth0, the Rule description
can be: can be as follows (Table 2):
+-------------+---+--+--+--------+---------+-------------+ +===========+=====+====+====+======+=========+============+
| Field |FL |FP|DI| Target | Match | CDA | | Field | FL | FP | DI | TV | MO | CDA |
| | | | | Value | Opera. | | +===========+=====+====+====+======+=========+============+
+-------------+---+--+--+--------+---------+-------------+ | Uri-Path | | 1 | Up | "c" | equal | not-sent |
|Uri-Path | | 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(24) |LSB | +-----------+-----+----+----+------+---------+------------+
+-------------+---+--+--+--------+---------+-------------+ | Uri-Query | var | 1 | Up | "k=" | MSB(16) | LSB |
+-----------+-----+----+----+------+---------+------------+
Figure 5: CORECONF URI compression Table 2: CORECONF URI Compression
Figure 5 shows the Rule description for a URI-Path and a URI-Query. Table 2 shows the Rule description for a Uri-Path and a Uri-Query.
SCHC compresses the first part of the URI-Path with a "not-sent" CDA. SCHC compresses the first part of the Uri-Path with a "not-sent" CDA.
SCHC will send the second element of the URI-Path with the length 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"). (i.e., 0x2 "X6") followed by the query option (i.e., 0x4 "eth0").
5.3.1. Variable number of Path or Query elements 5.3.1. Variable Number of Path or Query Elements
SCHC fixed the number of Uri-Path or Uri-Query elements in a Rule at SCHC fixed the number of Uri-Path or Uri-Query elements in a Rule at
the Rule creation time. If the number varies, SCHC SHOULD create the Rule creation time. If the number varies, SCHC SHOULD either
several Rules to cover all the possibilities. Another one is to
define the length of Uri-Path to variable and sends a Compression * create several Rules to cover all possibilities or
Residue with a length of 0 to indicate that this Uri-Path is empty.
* create a Rule that defines several entries for Uri-Path to cover
the longest path and send a Compression Residue with a length of 0
to indicate that a Uri-Path entry is empty.
However, this adds 4 bits to the variable Compression Residue size. However, this adds 4 bits to the variable Compression Residue size.
See section 7.5.2 [RFC8724]. See Section 7.4.2 of [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
The SCHC Rule description MAY define sending some field values by The SCHC Rule description MAY define sending some field values by
setting the TV to "not-sent," MO to "ignore," and CDA to "value- setting the TV to "not-sent", the MO to "ignore", and the CDA to
sent." A Rule MAY also use a "match-mapping" when there are "value-sent". A Rule MAY also use a "match-mapping" MO when there
different options for the same FID. Otherwise, the Rule sets the TV are different options for the same FID. Otherwise, the Rule sets the
to the value, MO to "equal," and CDA to "not-sent." TV to the value, the MO to "equal", and the CDA 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
A Rule entry cannot store these fields' values. The Rule description A Rule entry cannot store these fields' values. The Rule description
MUST always send these values in the Compression Residue. MUST always send these values in the Compression Residue.
6. SCHC compression of CoAP extension RFCs 6. SCHC Compression of CoAP Extensions
6.1. Block 6.1. Block
When a packet uses a Block [RFC7959] option, SCHC compression MUST When a packet uses a Block option [RFC7959], SCHC compression MUST
send its content in the Compression Residue. The SCHC Rule describes send its content in the Compression Residue. The SCHC Rule describes
an empty TV with a MO set to "ignore" and a CDA to "value-sent." an empty TV with the MO set to "ignore" and the CDA set to "value-
Block option allows fragmentation at the CoAP level that is sent". The Block option allows fragmentation at the CoAP level that
compatible with SCHC fragmentation. Both fragmentation mechanisms is compatible with SCHC fragmentation. Both fragmentation mechanisms
are complementary, and the node may use them for the same packet as are complementary, and the node may use them for the same packet as
needed. needed.
6.2. Observe 6.2. Observe
The [RFC7641] defines the Observe option. The SCHC Rule description [RFC7641] defines the Observe Option. The SCHC Rule description will
will not define the TV, but MO to "ignore," and the CDA to "value- not define the TV but will set the MO to "ignore" and the CDA to
sent." SCHC does not limit the maximum size for this option (3 "value-sent". SCHC does not limit the maximum size for this option
bytes). To reduce the transmission size, either the Device (3 bytes). To reduce the transmission size, either the Device
implementation MAY limit the delta between two consecutive values, or implementation MAY limit the delta between two consecutive values or
a proxy can modify the increment. a proxy can modify the increment.
Since the Observe option MAY use an RST message to inform a server Since the Observe Option MAY use a RST message to inform a server
that the client does not require the Observe response, a specific that the client does not require the Observe response, a specific
SCHC Rule SHOULD exist to allow the message's compression with the SCHC Rule SHOULD exist to allow the message's compression with the
RST type. RST type.
6.3. No-Response 6.3. No-Response
The [RFC7967] defines a No-Response option limiting the responses [RFC7967] defines a No-Response option limiting the responses made by
made by a server to a request. Different behaviors exist while using a server to a request. Different behaviors exist while using this
this option to limit the responses made by a server to a request. If option to limit the responses made by a server to a request. If both
both ends know the value, then the SCHC Rule will describe a TV to ends know the value, then the SCHC Rule will describe a TV to this
this value, with a MO set to "equal" and CDA set to "not-sent." value, with the MO set to "equal" and the CDA set to "not-sent".
Otherwise, if the value is changing over time, the SCHC Rule will set Otherwise, if the value is changing over time, the SCHC Rule will set
the MO to "ignore" and CDA to "value-sent." The Rule may also use a the MO to "ignore" and the CDA to "value-sent". The Rule may also
"match-mapping" to compress this option. use a "match-mapping" MO 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.
Figure 4 shows the OSCORE option value encoding defined in
Section 6.1 of [RFC8613], where the first byte specifies the content
of the OSCORE options using flags. The three most significant bits
of this byte are reserved and always set to 0. Bit h, when set,
indicates the presence of the kid context field in the option. Bit
k, when set, indicates the presence of a kid field. The three least
significant bits, n, indicate the length of the piv (Partial
Initialization Vector) field in bytes. When n = 0, no piv is
present.
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 4: OSCORE Option
The Figure 6 shows the OSCORE Option Value encoding defined in
Section 6.1 of [RFC8613], where the first byte specifies the Content
of the OSCORE options using flags. The three most significant bits
of this byte are reserved and always set to 0. Bit h, when set,
indicates the presence of the kid context field in the option. Bit
k, when set, indicates the presence of a kid field. The three least
significant bits n indicate the length of the piv (Partial
Initialization Vector) field in bytes. When n = 0, no piv is
present.
The flag byte is followed by the piv field, kid context field, and The flag byte is followed by the piv field, the kid context field,
kid field in this order, and if present, the kid context field's and the kid field, in that order, and, if present, the kid context
length is encoded in the first byte denoting by 's' the length of the field's length (in bytes) is encoded in the first byte, denoted by
kid context in bytes. "s".
To better perform OSCORE SCHC compression, the Rule description needs To better perform OSCORE SCHC compression, the Rule description needs
to identify the OSCORE Option and the fields it contains. to identify the OSCORE option and the fields it contains.
Conceptually, it discerns up to 4 distinct pieces of information Conceptually, it discerns up to four distinct pieces of information
within the OSCORE option: the flag bits, the piv, the kid context, within the OSCORE option: the flag bits, the piv, the kid context,
and the kid. The SCHC Rule splits into four field descriptions the and the kid. The SCHC Rule splits the OSCORE option into four Field
OSCORE option to compress them: Descriptors in order to compress them:
o CoAP OSCORE_flags, * CoAP OSCORE_flags
o CoAP OSCORE_piv, * CoAP OSCORE_piv
o CoAP OSCORE_kidctx, * CoAP OSCORE_kidctx
o CoAP OSCORE_kid. * CoAP OSCORE_kid
Figure 6 shows the OSCORE Option format with those four fields Figure 4 shows the OSCORE option format with those four fields
superimposed on it. Note that the CoAP OSCORE_kidctx field directly superimposed on it. Note that the CoAP OSCORE_kidctx field directly
includes 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 SCHC Compressor at the Network Gateway In this first scenario, the SCHC compressor on the NGW side receives
side receives a POST message from an Internet client, which is a POST message from an Internet client, which is immediately
immediately acknowledged by the Device. Figure 7 describes the SCHC acknowledged by the Device. Table 3 describes the SCHC Rule
Rule descriptions for this scenario. descriptions for this scenario.
RuleID 1 +===================================================================+
+-------------+--+--+--+------+---------+-------------++------------+ |RuleID 1 |
| Field |FL|FP|DI|Target| Match | CDA || Sent | +==========+===+==+==+======+===============+===============+=======+
| | | | |Value | Opera. | || [bits] | | Field | FL|FP|DI| TV | MO | CDA | Sent |
+-------------+--+--+--+------+---------+-------------++------------+ | | | | | | | | [bits]|
|CoAP version | 2| 1|bi| 01 |equal |not-sent || | +==========+===+==+==+======+===============+===============+=======+
|CoAP Type | 2| 1|dw| CON |equal |not-sent || | |CoAP |2 |1 |Bi|01 | equal | not-sent | |
|CoAP Type | 2| 1|up|[ACK, |match- |matching- || | |version | | | | | | | |
| | | | | RST] |mapping |sent || T | +----------+---+--+--+------+---------------+---------------+=======+
|CoAP TKL | 4| 1|bi| 0 |equal |not-sent || | |CoAP Type |2 |1 |Dw|CON | equal | not-sent | |
|CoAP Code | 8| 1|bi|[0.00,| | || | +----------+---+--+--+------+---------------+---------------+=======+
| | | | | ... |match- |matching- || | |CoAP Type |2 |1 |Up|[ACK, | match-mapping | matching-sent |T |
| | | | | 5.05]|mapping |sent || CC CCC | | | | | |RST] | | | |
|CoAP MID |16| 1|bi| 0000 |MSB(7 ) |LSB || M-ID| +----------+---+--+--+------+---------------+---------------+=======+
|CoAP Uri-Path|var 1|dw| path |equal 1 |not-sent || | |CoAP TKL |4 |1 |Bi|0 | equal | not-sent | |
+-------------+--+--+--+------+---------+-------------++------------+ +----------+---+--+--+------+---------------+---------------+=======+
|CoAP Code |8 |1 |Bi|[0.00,| match-mapping | matching-sent |CC CCC |
| | | | |... | | | |
| | | | |5.05] | | | |
+----------+---+--+--+------+---------------+---------------+=======+
|CoAP MID |16 |1 |Bi|0000 | MSB(7) | LSB |MID |
+----------+---+--+--+------+---------------+---------------+=======+
|CoAP Uri- |var|1 |Dw|path | equal 1 | not-sent | |
|Path | | | | | | | |
+----------+---+--+--+------+---------------+---------------+=======+
Figure 7: CoAP Context to compress header without Token Table 3: CoAP Context to Compress Header without Token
In this example, SCHC compression elides the version and the Token In this example, SCHC compression elides the version and Token Length
Length fields. The 26 method and response codes defined in [RFC7252] fields. The 25 Method and Response Codes defined in [RFC7252] have
has been shrunk to 5 bits using a "match-mapping" MO. The Uri-Path been shrunk to 5 bits using a "match-mapping" MO. The Uri-Path
contains a single element indicated in the TV and elided with the CDA contains a single element indicated in the TV and elided with the CDA
"not-sent." "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 the Message ID (9 bits in the
example above). example above).
Note that a client located in an Application Server sending a request 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 might not start with 7 bits equal to 0. A CoAP Rule, since the MID might not start with 7 bits equal to 0. A CoAP
proxy placed before the SCHC C/D can rewrite the message ID to fit proxy placed before SCHC C/D can rewrite the Message ID to fit the
the value and match the Rule. 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. Therefore, the goal is to hide as much as possible the messages. Therefore, the goal is to hide the message as much as
message while still enabling proxy operation. possible 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. However, it is part of the message that can be encrypted operation. However, it is part of the message that can be encrypted
until it reaches its end destination. The Outer Message acts as a until it reaches its end destination. The Outer Message acts as a
shell matching the regular CoAP message format and includes all shell matching the regular CoAP message format and includes all
Options and information needed for proxy operation and caching. options and information needed for proxy operation and caching.
Figure 8 illustrates this analysis. Figure 5 below illustrates this analysis.
The CoAP protocol arranges the options into one of 3 classes; each CoAP arranges the options into one of three classes, each granted a
granted a specific type of protection by the protocol: specific type of protection by the protocol:
o Class E: Encrypted options moved to the Inner Plaintext, Class E: Encrypted options moved to the Inner Plaintext.
o Class I: Integrity-protected options included in the AAD for the Class I: Integrity-protected options included in the Additional
encryption of the Plaintext but otherwise left untouched in the Authenticated Data (AAD) for the encryption of the Plaintext but
Outer Message, otherwise left untouched in the Outer Message.
o Class U: Unprotected options left untouched in the Outer Message. Class U: Unprotected options left untouched in the Outer Message.
These classes point out that the Outer option contains the OSCORE These classes point out that the Outer option contains the OSCORE
Option and 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. The end- the information necessary to retrieve the Security Context. The
point will use this Security Context to decrypt the message endpoint will use this Security Context to decrypt the message
correctly. correctly.
Original CoAP Packet Original CoAP Packet
+-+-+---+-------+---------------+ +-+-+---+-------+---------------+
|v|t|TKL| code | Msg Id. | |v|t|TKL| code | Message 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| Message ID | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+ +-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) | | Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+ +--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF | | Options (IU) | | 0xFF |
. . +-------+-----------+ . . +-------+-----------+
. OSCORE Option . | | . OSCORE Option . | |
+------+-------------------+ | Payload | +------+-------------------+ | Payload |
| 0xFF | | | | 0xFF | | |
+------+ +-------------------+ +------+ +-------------------+
Figure 8: A CoAP packet is split into an OSCORE outer and plaintext Figure 5: CoAP Packet Split into OSCORE Outer Header and Plaintext
Figure 8 shows the packet format for the OSCORE Outer header and Figure 5 shows the packet format for the OSCORE Outer header and
Plaintext. Plaintext.
In the Outer Header, the original header 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 CoAP is not using the Observe option. If Changed for responses when CoAP is not using the Observe Option. If
CoAP uses Observe, the OSCORE message code is replaced by FETCH for CoAP uses Observe, the OSCORE message code is replaced by FETCH for
requests and Content for responses. requests and Content for responses.
The first byte of the Plaintext contains the original packet code, The first byte of the Plaintext contains the original packet code,
followed by the message code, the class E options, and, if present, followed by the message code, the class E options, and, if present,
the original message Payload preceded by its payload marker. the original message payload preceded by its payload marker.
An AEAD algorithm now encrypts the Plaintext. This integrity An Authenticated Encryption with Associated Data (AEAD) algorithm now
protects the Security Context parameters and, eventually, any class I encrypts the Plaintext. This integrity-protects the Security Context
options from the Outer Header. The resulting Ciphertext becomes the parameters and, eventually, any class I options from the Outer
new payload of the OSCORE message, as illustrated in Figure 9. header. The resulting ciphertext becomes the new payload of the
OSCORE message, as illustrated in Figure 6.
As defined in [RFC5116], this Ciphertext is the encrypted Plaintext's As defined in [RFC5116], this ciphertext is the encrypted Plaintext's
concatenation of the authentication tag. Note that Inner Compression concatenation of the Authentication Tag. Note that Inner Compression
only affects the Plaintext before encryption. Thus only the first only affects the Plaintext before encryption. The Authentication
variable-length of the Ciphertext can be reduced. The authentication Tag, fixed in length and uncompressed, is considered part of the cost
tag is fixed in length and is considered part of the cost of of protection.
protection.
Outer Header Outer Header
+-+-+---+--------+---------------+ +-+-+---+--------+---------------+
|v|t|TKL|new code| Msg Id. | |v|t|TKL|new code| Message ID |
+-+-+---+--------+---------------+....+ +-+-+---+--------+---------------+....+
| Token | | Token |
+--------------------------------.....+ +--------------------------------.....+
| Options (IU) | | Options (IU) |
. . . .
. OSCORE Option . . OSCORE Option .
+------+-------------------+ +------+-------------------+
| 0xFF | | 0xFF |
+------+---------------------------+ +------+---------------------------+
| | | |
| Ciphertext: Encrypted Inner | | Ciphertext: Encrypted Inner |
| Header and Payload | | Header and Payload |
| + Authentication Tag | | + Authentication Tag |
| | | |
+----------------------------------+ +----------------------------------+
Figure 9: OSCORE message Figure 6: 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 7.
The OSCORE message translates into a segmented process where SCHC The OSCORE message translates into a segmented process where SCHC
compression is applied independently in 2 stages, each with its compression is applied independently in two stages, each with its
corresponding set of Rules, with the Inner SCHC Rules and the Outer corresponding set of Rules, with the Inner SCHC Rules and the Outer
SCHC Rules. This way, compression is applied to all fields of the SCHC Rules. This way, compression is applied to all fields of the
original CoAP message. original CoAP message.
Note that since the corresponding end-point can only decrypt the
Inner part of the message, this end-point will also have to implement
Inner SCHC Compression/Decompression.
Outer Message OSCORE Plaintext Outer Message OSCORE Plaintext
+-+-+---+--------+---------------+ +-------+ +-+-+---+--------+---------------+ +-------+
|v|t|TKL|new code| Msg Id. | | code | |v|t|TKL|new code| Message ID | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+ +-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) | | Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+ +--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF | | Options (IU) | | 0xFF |
. . +-------+-----------+ . . +-------+-----------+
. OSCORE Option . | | . OSCORE Option . | |
+------+-------------------+ | Payload | +------+-------------------+ | Payload |
| 0xFF | | | | 0xFF | | |
+------+------------+ +-------------------+ +------+------------+ +-------------------+
| Ciphertext |<---------\ | | Ciphertext |<---------\ |
| | | v | | | v
+-------------------+ | +-----------------+ +-------------------+ | +-----------------+
| | | Inner SCHC | | | | Inner SCHC |
v | | Compression | v | | Compression |
skipping to change at page 20, line 43 skipping to change at line 875
v | +-------+-----------+ v | +-------+-----------+
+--------+ +------------+ |Compression Residue| +--------+ +------------+ |Compression Residue|
|RuleID' | | Encryption | <-- +----------+--------+ |RuleID' | | Encryption | <-- +----------+--------+
+--------+-----------+ +------------+ | | +--------+-----------+ +------------+ | |
|Compression Residue'| | Payload | |Compression Residue'| | Payload |
+-----------+--------+ | | +-----------+--------+ | |
| Ciphertext | +-------------------+ | Ciphertext | +-------------------+
| | | |
+--------------------+ +--------------------+
Figure 10: OSCORE Compression Diagram Figure 7: OSCORE Compression Diagram
Note that since the corresponding endpoint can only decrypt the Inner
part of the message, this endpoint will also have to implement Inner
SCHC Compression/Decompression.
7.3. Example OSCORE Compression 7.3. Example OSCORE Compression
This section gives an example with a GET Request and its consequent This section gives an example with a GET request and its consequent
Content Response from a Device-based CoAP client to a cloud-based Content response from a Device-based CoAP client to a cloud-based
CoAP server. The example also describes a possible set of Rules for CoAP server. The example also describes a possible set of Rules for
the Inner and Outer SCHC Compression. A dump of the results and a Inner SCHC Compression and Outer SCHC Compression. A dump of the
contrast between SCHC + OSCORE performance with SCHC + COAP results and a contrast between SCHC + OSCORE performance with SCHC +
performance is also listed. This example gives an approximation of CoAP performance are also listed. This example gives an
the cost of security with SCHC-OSCORE. approximation of the cost of security with SCHC-OSCORE.
Our first CoAP message is the GET request in Figure 11. Our first CoAP message is the GET request in Figure 8.
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
Original msg length: 17 bytes. Original message length: 17 bytes
Figure 11: CoAP GET Request Figure 8: CoAP GET Request
Its corresponding response is the CONTENT Response in Figure 12. Its corresponding response is the Content response in Figure 9.
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 message length: 10 bytes
Figure 12: CoAP CONTENT Response Figure 9: 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. Table 4 provides an example.
RuleID 0 +===================================================================+
+--------------+--+--+--+-----------+---------+---------++------+ |RuleID 0 |
| Field |FL|FP|DI| Target | MO | CDA || Sent | +========+==+==+==+===========+===============+==============+======+
| | | | | Value | | ||[bits]| | Field |FL|FP|DI| TV | MO | CDA | Sent |
+--------------+--+--+--+-----------+---------+---------++------+ | | | | | | | |[bits]|
|CoAP Code | 8| 1|up| 1 | equal |not-sent || | +========+==+==+==+===========+===============+==============+======+
|CoAP Code | 8| 1|dw|[69, | | || | |CoAP |8 |1 |Up|1 | equal | not-sent | |
| | | | |132] |match- |mapping- || | |Code | | | | | | | |
| | | | | |mapping |sent || c | +--------+--+--+--+-----------+---------------+--------------+======+
|CoAP Uri-Path | | 1|up|temperature| equal |not-sent || | |CoAP |8 |1 |Dw|[69,132] | match-mapping | mapping-sent |c |
+--------------+--+--+--+-----------+---------+---------++------+ |Code | | | | | | | |
+--------+--+--+--+-----------+---------------+--------------+======+
|CoAP | |1 |Up|temperature| equal | not-sent | |
|Uri-Path| | | | | | | |
+--------+--+--+--+-----------+---------------+--------------+======+
Figure 13: Inner SCHC Rules Table 4: Inner SCHC Rule
Figure 14 shows the Plaintext obtained for the example GET request. Figure 10 shows the Plaintext obtained for the example GET request.
The packet follows the process of Inner Compression and Encryption The packet follows the process of Inner Compression and encryption
until the payload. The outer OSCORE Message adds the result of the until the payload. The Outer OSCORE message adds the result of the
Inner process. Inner process.
In this case, the original message has no payload, and its resulting
Plaintext compressed up to only 1 byte (size of the RuleID). The
AEAD algorithm preserves this length in its first output and yields a
fixed-size tag. SCHC cannot compress the tag, and the OSCORE message
must include it without compression. The use of integrity protection
translates into an overhead in total message length, limiting the
amount of compression that can be achieved and plays 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 |
| Value = temperature | | Value = temperature |
skipping to change at page 23, line 49 skipping to change at line 1006
| (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 10: Plaintext Compression and Encryption for GET Request
Figure 15 shows the process for the example CONTENT Response. The In this case, the original message has no payload, and its resulting
Plaintext is compressed up to only 1 byte (the size of the RuleID).
The AEAD algorithm preserves this length in its first output and
yields a fixed-size tag. SCHC cannot compress the tag, and the
OSCORE message must include it without compression. The use of
integrity protection translates into an overhead in total message
length, limiting the amount of compression that can be achieved and
playing into the cost of adding security to the exchange.
Figure 11 shows the process for the example Content response. The
Compression Residue is 1 bit long. Note that since SCHC adds padding Compression Residue is 1 bit long. Note that since SCHC adds padding
after the payload, this misalignment causes the hexadecimal code from after the payload, this misalignment causes the hexadecimal code from
the payload to differ from the original, even if SCHC cannot compress the payload to differ from the original, even if SCHC cannot compress
the tag. The overhead for the tag bytes limits the SCHC's the tag. The overhead for the tag bytes limits SCHC's performance
performance but brings security to the transmission. but brings security to the transmission.
________________________________________________________ ________________________________________________________
| | | |
| 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 Compression Residue) | | 0b0 (1 bit match-mapping 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) |
| | | |
| ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) | | ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) |
|_________________________________________________________| |_________________________________________________________|
Figure 15: Plaintext compression and encryption for CONTENT Response Figure 11: Plaintext Compression and Encryption for Content Response
The Outer SCHC Rules (Figure 18) must process the OSCORE Options The Outer SCHC Rule (Table 5) must process the OSCORE options fields.
fields. Figure 16 and Figure 17 shows a dump of the OSCORE Messages Figures 12 and 13 show a dump of the OSCORE messages generated from
generated from the example messages. They include the Inner the example messages. They include the Inner Compressed ciphertext
Compressed Ciphertext in the payload. These are the messages that in the payload. These are the messages that have to be compressed
have to be compressed by the Outer SCHC Compression. via the Outer SCHC Compression scheme.
Table 5 shows a possible set of Outer Rule items to compress the
Outer header.
+===================================================================+
|RuleID 0 |
+===============+===+==+==+================+=======+=========+======+
| Field | FL|FP|DI| TV | MO | CDA | Sent |
| | | | | | | |[bits]|
+===============+===+==+==+================+=======+=========+======+
|CoAP version |2 |1 |Bi| 01 |equal |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Type |2 |1 |Up| 0 |equal |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Type |2 |1 |Dw| 2 |equal |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP TKL |4 |1 |Bi| 1 |equal |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Code |8 |1 |Up| 2 |equal |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Code |8 |1 |Dw| 68 |equal |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP MID |16 |1 |Bi| 0000 |MSB(12)|LSB |MMMM |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP Token |tkl|1 |Bi| 0x80 |MSB(5) |LSB |TTT |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP |8 |1 |Up| 0x09 |equal |not-sent | |
|OSCORE_flags | | | | | | | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_piv|var|1 |Up| 0x00 |MSB(4) |LSB |PPPP |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_kid|var|1 |Up| 0x636c69656e70 |MSB(52)|LSB |KKKK |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP |var|1 |Bi| b'' |equal |not-sent | |
|OSCORE_kidctx | | | | | | | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP |8 |1 |Dw| b'' |equal |not-sent | |
|OSCORE_flags | | | | | | | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_piv|var|1 |Dw| b'' |equal |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
|CoAP OSCORE_kid|var|1 |Dw| b'' |equal |not-sent | |
+---------------+---+--+--+----------------+-------+---------+======+
Table 5: Outer SCHC Rule
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
h k n h k n
04 piv 04 piv
636c69656e74 kid 636c69656e74 kid
0xFF Payload marker 0xFF Payload marker
Payload: Payload:
0xa2c54fe1b434297b62 (9 bytes) 0xa2c54fe1b434297b62 (9 bytes)
Figure 16: Protected and Inner SCHC Compressed GET Request Figure 12: Protected and Inner SCHC Compressed GET Request
Protected message: Protected message:
================== ==================
0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29 0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29
(22 bytes) (22 bytes)
Header: Header:
0x6144 0x6144
01 Ver 01 Ver
10 ACK 10 ACK
skipping to change at page 27, line 29 skipping to change at line 1174
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 13: 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 most straightforward alternative depending on the application. The most straightforward alternative
is to provide a fixed value for the flags, combining MO "equal" and is to provide a fixed value for the flags, combining a MO of "equal"
CDA "not-sent." This SCHC definition saves most bits but could and a CDA of "not-sent". This SCHC definition saves most bits but
prevent flexibility. Otherwise, SCHC could use a "match-mapping" MO could prevent flexibility. Otherwise, SCHC could use a "match-
to choose from several configurations for the exchange. If not, the mapping" MO to choose from several configurations for the exchange.
SCHC description may use an "MSB" MO to mask off the three hard-coded If not, the SCHC description may use an "MSB" MO to mask off the
most significant bits. three hard-coded most significant bits.
Note that fixing a flag bit will limit CoAP Options choice that can Note that fixing a flag bit will limit the choices of CoAP options
be used in the exchange since their values are dependent on specific that can be used in the exchange, since the values of these choices
options. are dependent on specific options.
The piv field lends itself to having some bits masked off with "MSB" The piv field lends itself to having some bits masked off with an
MO and "LSB" CDA. This SCHC description could be useful in "MSB" MO and an "LSB" CDA. This SCHC description could be useful in
applications where the message frequency is low such as LPWAN applications where the message frequency is low, such as LPWAN
technologies. Note that compressing the sequence numbers may reduce technologies. Note that compressing the sequence numbers may reduce
the maximum number of sequence numbers that can be used in an the maximum number of sequence numbers that can be used in an
exchange. Once the sequence number exceeds the maximum value, the exchange. Once the sequence number exceeds the maximum value, the
OSCORE keys need to be re-established. OSCORE keys need to be re-established.
The size s included in the kid context field MAY be masked off with The size, s, that is included in the kid context field MAY be masked
"LSB" CDA. The rest of the field could have additional bits masked off with an "LSB" CDA. The rest of the field could have additional
off or have the whole field fixed with MO "equal" and CDA "not-sent." bits masked off or have the whole field fixed with a MO of "equal"
The same holds for the kid field. and a CDA of "not-sent". The same holds for the kid field.
Figure 18 shows a possible set of Outer Rules to compress the Outer
Header.
RuleID 0
+------------------+--+--+--+--------------+-------+--------++------+
| Field |FL|FP|DI| Target | MO | CDA || Sent |
| | | | | Value | | ||[bits]|
+------------------+--+--+--+--------------+-------+--------++------+
|CoAP version | 2| 1|bi| 01 |equal |not-sent|| |
|CoAP Type | 2| 1|up| 0 |equal |not-sent|| |
|CoAP Type | 2| 1|dw| 2 |equal |not-sent|| |
|CoAP TKL | 4| 1|bi| 1 |equal |not-sent|| |
|CoAP Code | 8| 1|up| 2 |equal |not-sent|| |
|CoAP Code | 8| 1|dw| 68 |equal |not-sent|| |
|CoAP MID |16| 1|bi| 0000 |MSB(12)|LSB ||MMMM |
|CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT |
|CoAP OSCORE_flags | 8| 1|up| 0x09 |equal |not-sent|| |
|CoAP OSCORE_piv |var 1|up| 0x00 |MSB(4) |LSB ||PPPP |
|COAP OSCORE_kid |var 1|up|0x636c69656e70|MSB(52)|LSB ||KKKK |
|COAP OSCORE_kidctx|var 1|bi| b'' |equal |not-sent|| |
|CoAP OSCORE_flags | 8| 1|dw| b'' |equal |not-sent|| |
|CoAP OSCORE_piv |var 1|dw| b'' |equal |not-sent|| |
|CoAP OSCORE_kid |var 1|dw| b'' |equal |not-sent|| |
+------------------+--+--+--+--------------+-------+--------++------+
Figure 18: Outer SCHC Rules
The Outer Rule of Figure 18 is applied to the example GET Request and The Outer Rule of Table 5 is applied to the example GET request and
CONTENT Response. Figure 19 and Figure 20 show the resulting Content response. Figures 14 and 15 show the resulting messages.
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)
mid tkn piv kid mid tkn piv kid
Payload Payload
0xa2c54fe1b434297b62 (9 bytes) 0xa2c54fe1b434297b62 (9 bytes)
Compressed message length: 12 bytes Compressed message length: 12 bytes
Figure 19: SCHC-OSCORE Compressed GET Request Figure 14: SCHC-OSCORE Compressed GET Request
Compressed message: Compressed message:
================== ==================
0x0014218daf84d983d35de7e48c3c1852 (16 bytes) 0x0014218daf84d983d35de7e48c3c1852 (16 bytes)
0x00 RuleID 0x00 RuleID
14 Compression Residue 14 Compression Residue
218daf84d983d35de7e48c3c1852 Padded payload 218daf84d983d35de7e48c3c1852 Padded payload
Compression Residue: Compression Residue:
0b0001 010 (7 bits -> 1 byte with padding) 0b0001 010 (7 bits -> 1 byte with padding)
mid tkn mid tkn
Payload Payload
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Compressed msg length: 16 bytes Compressed message length: 16 bytes
Figure 20: SCHC-OSCORE Compressed CONTENT Response Figure 15: SCHC-OSCORE Compressed Content Response
In contrast, comparing these results with what would be obtained by In contrast, comparing these results with what would be obtained by
SCHC compressing the original CoAP messages without protecting them SCHC compressing the original CoAP messages without protecting them
with OSCORE is done by compressing the CoAP messages according to the with OSCORE is done by compressing the CoAP messages according to the
SCHC Rules in Figure 21. SCHC Rule in Table 6.
RuleID 1 +===================================================================+
+---------------+--+--+--+-----------+---------+-----------++-------+ |RuleID 1 |
| Field |FL|FP|DI| Target | MO | CDA || Sent | +========+===+==+==+===========+===============+=============+======+
| | | | | Value | | || [bits]| | Field | FL|FP|DI| TV | MO | CDA | Sent |
+---------------+--+--+--+-----------+---------+-----------++-------+ | | | | | | | |[bits]|
|CoAP version | 2| 1|bi| 01 |equal |not-sent || | +========+===+==+==+===========+===============+=============+======+
|CoAP Type | 2| 1|up| 0 |equal |not-sent || | |CoAP |2 |1 |Bi|01 | equal |not-sent | |
|CoAP Type | 2| 1|dw| 2 |equal |not-sent || | |version | | | | | | | |
|CoAP TKL | 4| 1|bi| 1 |equal |not-sent || | +--------+---+--+--+-----------+---------------+-------------+======+
|CoAP Code | 8| 1|up| 2 |equal |not-sent || | |CoAP |2 |1 |Up|0 | equal |not-sent | |
|CoAP Code | 8| 1|dw| [69,132] |match- |mapping- || | |Type | | | | | | | |
| | | | | |mapping |sent ||C | +--------+---+--+--+-----------+---------------+-------------+======+
|CoAP MID |16| 1|bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP |2 |1 |Dw|2 | equal |not-sent | |
|CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT | |Type | | | | | | | |
|CoAP Uri-Path | | 1|up|temperature|equal |not-sent || | +--------+---+--+--+-----------+---------------+-------------+======+
+---------------+--+--+--+-----------+---------+-----------++-------+ |CoAP TKL|4 |1 |Bi|1 | equal |not-sent | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP |8 |1 |Up|2 | equal |not-sent | |
|Code | | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP |8 |1 |Dw|[69,132] | match-mapping |mapping-sent |C |
|Code | | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP MID|16 |1 |Bi|0000 | MSB(12) |LSB |MMMM |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP |tkl|1 |Bi|0x80 | MSB(5) |LSB |TTT |
|Token | | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
|CoAP | |1 |Up|temperature| equal |not-sent | |
|Uri-Path| | | | | | | |
+--------+---+--+--+-----------+---------------+-------------+======+
Figure 21: SCHC-CoAP Rules (No OSCORE) Table 6: SCHC-CoAP Rule (No OSCORE)
Figure 21 Rule yields the SCHC compression results in Figure 22 for The Rule in Table 6 yields the SCHC compression results as shown in
request, and Figure 23 for the response. Figure 16 for the request and Figure 17 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 message length: 2 bytes
Figure 22: CoAP GET Compressed without OSCORE Figure 16: CoAP GET Compressed without OSCORE
Compressed message: Compressed message:
================== ==================
0x010a32332043 0x010a32332043
0x01 = RuleID 0x01 = RuleID
Compression Residue: Compression Residue:
0b00001010 (1 byte) 0b00001010 (1 byte)
Payload Payload
0x32332043 0x32332043
Compressed msg length: 6 Compressed message length: 6 bytes
Figure 23: CoAP CONTENT Compressed without OSCORE Figure 17: CoAP Content Compressed without OSCORE
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 IANA actions.
9. Security considerations 9. Security Considerations
The use of SCHC header compression for CoAP header fields only The use of SCHC header compression for CoAP header fields only
affects the representation of the header information. SCHC header affects the representation of the header information. SCHC header
compression itself does not increase or decrease the overall level of compression itself does not increase or decrease the overall level of
security of the communication. When the connection does not use a security of the communication. When the connection does not use a
security protocol (such as OSCORE, DTLS, etc.), it is necessary to security protocol (OSCORE, DTLS, etc.), it is necessary to use a
use a layer-two security mechanism to protect the SCHC messages. Layer 2 security mechanism to protect the SCHC messages.
If LPWAN is the layer-two technology, the SCHC security If an LPWAN is the Layer 2 technology being used, the SCHC security
considerations of [RFC8724] continue to apply. When using another considerations discussed in [RFC8724] continue to apply. When using
layer-two protocol, use of a cryptographic integrity-protection another Layer 2 protocol, the use of a cryptographic integrity-
mechanisms to protect the SCHC headers is REQUIRED. Such protection mechanism to protect the SCHC headers is REQUIRED. Such
cryptographic integrity protection is necessary in order to continue cryptographic integrity protection is necessary in order to continue
to provide the properties that [RFC8724] relies upon. to provide the properties that [RFC8724] relies upon.
When SCHC is used with OSCORE, the security considerations of When SCHC is used with OSCORE, the security considerations discussed
[RFC8613] continue to apply. in [RFC8613] continue to apply.
When SCHC is used with the OSCORE outer headers, the Initialization When SCHC is used with the OSCORE Outer headers, the Initialization
Vector (IV) size in the Compression Residue must be carefully Vector (IV) size in the Compression Residue must be carefully
selected. There is a tradeoff between compression efficiency (with a selected. There is a trade-off between compression efficiency (with
longer "MSB" MO prefix) and the frequency at which the Device must a longer "MSB" MO prefix) and the frequency at which the Device must
renew its key material (in order to prevent the IV from expanding to renew its key material (in order to prevent the IV from expanding to
an uncompressable value). The key renewal operation itself requires an uncompressible value). The key-renewal operation itself requires
several message exchanges and requires energy-intensive computation, several message exchanges and requires energy-intensive computation,
but the optimal tradeoff will depend on the specifics of the device but the optimal trade-off will depend on the specifics of the Device
and expected usage patterns. and expected usage patterns.
If an attacker can introduce a corrupted SCHC-compressed packet onto If an attacker can introduce a corrupted SCHC-compressed packet onto
a link, DoS attacks are possible by causing excessive resource a link, DoS attacks can be mounted by causing excessive resource
consumption at the decompressor. However, an attacker able to inject consumption at the decompressor. However, an attacker able to inject
packets at the link layer is also capable of other, potentially more packets at the link layer is also capable of other, potentially more
damaging, attacks. damaging, attacks.
SCHC compression emits variable-length Compression Residues for some SCHC compression emits variable-length Compression Residues for some
CoAP fields. In the compressed header representation, the length CoAP fields. In the representation of the compressed header, the
field that is sent is not the length of the original header field but length field that is sent is not the length of the original header
rather the length of the Compression Residue that is being field but rather the length of the Compression Residue that is being
transmitted. If a corrupted packet arrives at the decompressor with transmitted. If a corrupted packet arrives at the decompressor with
a longer or shorter length than the original compressed a longer or shorter length than the original compressed
representation possessed, the SCHC decompression procedures will representation possessed, the SCHC decompression procedures will
detect an error and drop the packet. detect an error and drop the packet.
SCHC header compression rules MUST remain tightly coupled between SCHC header compression Rules MUST remain tightly coupled between the
compressor and decompressor. If the compression rules get out of compressor and the decompressor. If the compression Rules get out of
sync, a Compression Residue might be decompressed differently at the sync, a Compression Residue might be decompressed differently at the
receiver than the initial message submitted to compression receiver than the initial message submitted to compression
procedures. Accordingly, any time the context Rules are updated on procedures. Accordingly, any time the context Rules are updated on
an OSCORE endpoint, that endpoint MUST trigger OSCORE key re- an OSCORE endpoint, that endpoint MUST trigger OSCORE key re-
establishment. Similar procedures may be appropriate to signal Rule establishment. Similar procedures may be appropriate to signal Rule
udpates when other message-protection mechanisms are in use. updates when other message-protection mechanisms are in use.
10. Acknowledgements
The authors would like to thank (in alphabetic order): Christian
Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
Fossati, Klaus Hartke, Benjamin Kaduk, Francesca Palombini, Alexander
Pelov, Goran Selander and Eric Vyncke.
11. Normative References 10. 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>.
skipping to change at page 33, line 35 skipping to change at line 1410
[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 Zúñiga, "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>.
Acknowledgements
The authors would like to thank (in alphabetic order): Christian
Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
Fossati, Klaus Hartke, Benjamin Kaduk, Francesca Palombini, Alexander
Pelov, Göran Selander, and Éric Vyncke.
Authors' Addresses Authors' Addresses
Ana Minaburo Ana Minaburo
Acklio Acklio
1137A avenue des Champs Blancs 1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex 35510 Cesson-Sevigne Cedex
France France
Email: ana@ackl.io Email: ana@ackl.io
Laurent Toutain Laurent Toutain
Institut MINES TELECOM; IMT Atlantique Institut MINES TELECOM; IMT Atlantique
2 rue de la Chataigneraie
CS 17607 CS 17607
2 rue de la Chataigneraie
35576 Cesson-Sevigne Cedex 35576 Cesson-Sevigne Cedex
France France
Email: Laurent.Toutain@imt-atlantique.fr Email: Laurent.Toutain@imt-atlantique.fr
Ricardo Andreasen Ricardo Andreasen
Universidad de Buenos Aires Universidad de Buenos Aires
Av. Paseo Colon 850 Av. Paseo Colon 850
C1063ACV Ciudad Autonoma de Buenos Aires C1063ACV Ciudad Autonoma de Buenos Aires
Argentina Argentina
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