draft-ietf-lpwan-coap-static-context-hc-14.txt   draft-ietf-lpwan-coap-static-context-hc-15.txt 
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Standards Track L. Toutain Intended status: Standards Track L. Toutain
Expires: November 27, 2020 Institut MINES TELECOM; IMT Atlantique Expires: January 4, 2021 Institut MINES TELECOM; IMT Atlantique
R. Andreasen R. Andreasen
Universidad de Buenos Aires Universidad de Buenos Aires
May 26, 2020 July 03, 2020
LPWAN Static Context Header Compression (SCHC) for CoAP LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-14 draft-ietf-lpwan-coap-static-context-hc-15
Abstract Abstract
This draft defines the way Static Context Header Compression (SCHC) This draft defines the way Static Context Header Compression (SCHC)
header compression can be applied to the Constrained Application header compression can be applied to the Constrained Application
Protocol (CoAP). SCHC is a header compression mechanism adapted for Protocol (CoAP). SCHC is a header compression mechanism adapted for
constrained devices. SCHC uses a static description of the header to constrained devices. SCHC uses a static description of the header to
reduce the redundancy and the size of the information in the header. reduce the redundancy and the size of the information in the header.
While [rfc8724] describes the SCHC compression and fragmentation While [rfc8724] describes the SCHC compression and fragmentation
framework, and its application for IPv6/UDP headers, this document framework, and its application for IPv6/UDP headers, this document
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 27, 2020. This Internet-Draft will expire on January 4, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Applying SCHC to CoAP headers . . . . . . . . . . . . . . . . 4 2. Applying SCHC to CoAP headers . . . . . . . . . . . . . . . . 4
3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 5 3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 6
3.1. Differences between CoAP and UDP/IP Compression . . . . . 5 3.1. Differences between CoAP and UDP/IP Compression . . . . . 7
4. Compression of CoAP header fields . . . . . . . . . . . . . . 6 4. Compression of CoAP header fields . . . . . . . . . . . . . . 8
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 7 4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 8
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 7 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 8
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 7 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 8
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 7 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 9
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 9
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 8 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 8 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 10
5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 8 5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 10
5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 9 5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 10
5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 9 5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 11
5.3.2. Variable number of path or query elements . . . . . . 10 5.3.2. Variable number of path or query elements . . . . . . 11
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
fields . . . . . . . . . . . . . . . . . . . . . . . . . 10 fields . . . . . . . . . . . . . . . . . . . . . . . . . 11
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 . . . . . . . . . . . . . . . . 10 and Location-Query fields . . . . . . . . . . . . . . . . 12
6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 11 6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 12
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 11 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 12
6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Examples of CoAP header compression . . . . . . . . . . . . . 13 7. Examples of CoAP header compression . . . . . . . . . . . . . 14
7.1. Mandatory header with CON message . . . . . . . . . . . . 13 7.1. Mandatory header with CON message . . . . . . . . . . . . 14
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 14 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 15
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
9. Security considerations . . . . . . . . . . . . . . . . . . . 27 9. Security considerations . . . . . . . . . . . . . . . . . . . 28
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
11. Normative References . . . . . . . . . . . . . . . . . . . . 28 11. Normative References . . . . . . . . . . . . . . . . . . . . 29
Appendix A. Extension to the RFC8724 Annex D. . . . . . . . . . 29 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 1. Introduction
CoAP [rfc7252] is designed to easily interop with HTTP (Hypertext CoAP [rfc7252] is designed to easily interop with HTTP (Hypertext
Transfer Protocol) and is optimized for REST-based (Representational Transfer Protocol) and is optimized for REST-based (Representational
state transfer) services. Although CoAP was designed for constrained state transfer) services. Although CoAP was designed for constrained
devices, the size of a CoAP header still is too large for the devices, the size of a CoAP header still is too large for the
constraints of LPWAN (Low Power Wide Area Networks) and some constraints of LPWAN (Low Power Wide Area Networks) and some
compression is needed to reduce the header size. compression is needed to reduce the header size.
The [rfc8724] defines SCHC, a header compression mechanism for LPWAN The [rfc8724] defines SCHC, a header compression mechanism for LPWAN
network based on a static context. The section 5 of the [rfc8724] network based on a static context. Section 5 of the [rfc8724]
explains the architecture where compression and decompression are explains the architecture where compression and decompression are
done. The context is known by both ends before transmission. The done. The context is known by both ends before transmission. The
way the context is configured, provisioned or exchanged is out of the way the context is configured, provisioned or exchanged is out of the
scope of this document. scope of this document.
CoAP is an End-to-End protocol at the application level, so CoAP
compression requires to install common Rules between two hosts and IP
Routing may be needed to allow End-to-End communication. Therefore,
SCHC compression may apply at two different levels, one to compress
IP and UDP as described in [rfc8724] in the LPWAN network and another
at the application level. These two compressions may be independent.
The Compression Rules can be set up by two independent entities and
are out of the scope of this document. In both cases, SCHC mechanism
remains the same.
SCHC compresses and decompresses headers based on shared contexts SCHC compresses and decompresses headers based on shared contexts
between devices. Each context consists of multiple Rules. Each Rule between devices. Each context consists of multiple Rules. Each Rule
can match header fields and specific values or ranges of values. If can match header fields and specific values or ranges of values. If
a Rule matches, the matched header fields are substituted by the a Rule matches, the matched header fields are substituted by the
RuleID and optionally some residual bits. Thus, different Rules may RuleID and optionally some residual bits. Thus, different Rules may
correspond to different types of packets that a device expects to correspond to different types of packets that a device expects to
send or receive. send or receive.
A Rule describes the complete header of the packet with an ordered A Rule describes the complete header of the packet with an ordered
list of fields descriptions, see section 7 of [rfc8724], thereby each list of fields descriptions, see section 7 of [rfc8724], thereby each
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"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][rfc8174] when, and only when, they appear in all 14 [RFC2119][rfc8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Applying SCHC to CoAP headers 2. Applying SCHC to CoAP headers
The SCHC Compression Rules can be applied to CoAP headers. SCHC The SCHC Compression Rules can be applied to CoAP headers. SCHC
Compression of the CoAP header MAY be done in conjunction with the Compression of the CoAP header MAY be done in conjunction with the
lower layers (IPv6/UDP) or independently. The SCHC adaptation layers lower layers (IPv6/UDP) or independently. The SCHC adaptation layers
as described in Section 5 of [rfc8724] and may be used as shown in as described in Section 5 of [rfc8724] and may be used as shown in
Figure 1. Figure 1, Figure 2 and Figure 3.
^ +------------+ ^ +------------+ ^ +------------+ In the first example Figure 1, a Rule compresses the complete header
| | CoAP | | | CoAP | inner | | CoAP | stack from IPv6 to CoAP. In this case, SCHC C/D (Static Context
| +------------+ v +------------+ x | OSCORE | Header Compression Compressor/Decompressor) is performed at the
| | UDP | | DTLS | outer | +------------+ Sender and at the Receiver. The host communicating with the device
| +------------+ +------------+ | | UDP | do not implement SCHC C/D.
| | IPv6 | | UDP | | +------------+
v +------------+ +------------+ | | IPv6 |
| IPv6 | v +------------+
+------------+
Figure 1: Rule scope for CoAP (device) (NGW) (internet) (App)
Figure 1 shows some examples for CoAP protocol stacks and the SCHC +--------+ +--------+
Rule's scope. | CoAP | | CoAP |
+--------+ +--------+
| UDP | | UDP |
+--------+ +----------------+ +--------+
| IPv6 | | IPv6 | | IPv6 |
+--------+ +--------+-------+ +--------+
| SCHC | | SCHC | | | |
+--------+ +--------+ + + +
| LPWAN | | LPWAN | | | |
+--------+ +--------+-------+ +--------+
((((((())))))) ----- ------ ------ -----
In the first example, a Rule compresses the complete header stack Figure 1: Compression/decompression at the LPWAN bondary
from IPv6 to CoAP. In this case, SCHC C/D (Static Context Header
Compression Compressor/Decompressor) is performed at the Sender and
at the Receiver.
In the second example, the SCHC compression is applied in the CoAP In the second example, Figure 2, the SCHC compression is applied in
layer, compressing the CoAP header independently of the other layers. the CoAP layer, compressing the CoAP header independently of the
The RuleID and the Compression Residue are encrypted using a other layers. The RuleID and the Compression Residue are encrypted
mechanism such as DTLS. Only the other end can decipher the using a mechanism such as DTLS. Only the other end can decipher the
information. If needed, layers below use SCHC to compress the header information. If needed, layers below use SCHC to compress the header
as defined in [rfc8724] document. This use case realizes an End-to- as defined in [rfc8724] document. This use case realizes an End-to-
End context initialization between the sender and the receiver, see End context initialization between the sender and the receiver and is
Appendix A. out-of-scope of this document.
In the third example, the Object Security for Constrained RESTful (device) (NGW) (internet) (App)
Environments (OSCORE) [rfc8613] is used. In this case, two rulesets
are used to compress the CoAP message. A first ruleset focused on +--------+ +--------+
the inner header and is applied end to end by both ends. A second | CoAP | | CoAP |
ruleset compresses the outer header and the layers below and is done +--------+ +--------+
between the Sender and the Receiver. | SCHC | | SCHC |
+--------+ +--------+
| DTLS | | DTLS |
+--------+ +--------+
. udp . . udp .
.......... .................. ..........
. ipv6 . . ipv6 . . ipv6 .
.......... .................. ..........
. schc . . schc . . . .
.......... .......... . . .
. lpwan . . lpwan . . . .
.......... .................. ..........
((((((())))))) ----- ------ ------ -----
Figure 2: Standalone CoAP ene-to-end compression/decompression
In the third example, Figure 3 the Object Security for Constrained
RESTful Environments (OSCORE) [rfc8613] is used. In this case, two
rulesets are used to compress the CoAP message. A first ruleset
focused on the inner header and is applied end to end by both ends.
A second ruleset compresses the outer header and the layers below and
is done between the Sender and the Receiver.
(device) (NGW) (internet) (App)
+--------+ +--------+
| CoAP | | CoAP |
| inner | | inner |
+--------+ +--------+
| SCHC | | SCHC |
+--------+ +--------+
| CoAP | | CoAP |
| outer | | outer |
+--------+ +--------+
. udp . . udp .
.......... .................. ..........
. ipv6 . . ipv6 . . ipv6 .
.......... .................. ..........
. schc . . schc . . . .
.......... .......... . . .
. lpwan . . lpwan . . . .
.......... .................. ..........
((((((())))))) ----- ------ ------ -----
Figure 3: OSCORE compression/decompression.
In case of 2 rule-sets, as shown in Figure 2 and Figure 3, they may
come from different provisioning domains, and that they do not
include the cryptography part that is done in between the two SCHC
activities. This document focuses on CoAP compression represented in
the 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 uses the same description and
compression/decompression techniques as the one for IP and UDP compression/decompression techniques as the one for IP and UDP
explained in the [rfc8724]. For CoAP, SCHC Rules description uses explained in the [rfc8724]. For CoAP, SCHC Rules description uses
the direction information to optimize the compression by reducing the the direction information to optimize the compression by reducing the
number of Rules needed to compress headers. The field description number of Rules needed to compress headers. The field description
MAY define both request/response headers and target values in the MAY define both request/response headers and target values in the
same Rule, using the DI (direction indicator) to make the difference. same Rule, using the DI (direction indicator) to make the difference.
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the format Delta-Type (D-T), Length (L), Value (V). When applying the format Delta-Type (D-T), Length (L), Value (V). When applying
SCHC compression to the Option, the D-T, L, and V format serves to SCHC compression to the Option, the D-T, L, and V format serves to
make the Rule description of the Option. The SCHC compression builds make the Rule description of the Option. The SCHC compression builds
the description of the Option by using in the Field ID the Option the description of the Option by using in the Field ID the Option
Number built from D-T; in TV, the Option Value; and the Option Length Number built from D-T; in TV, the Option Value; and the Option Length
uses section 7.4 of RFC8724. When the Option Length has a wellknown uses section 7.4 of RFC8724. When the Option Length has a wellknown
size it can be stored in the Rule. Therefore, SCHC compression does size it can be stored in the Rule. Therefore, SCHC compression does
not send it. Otherwise, SCHC Compression carries the length of the not send it. Otherwise, SCHC Compression carries the length of the
Compression Residue in addition to the Compression Residue value. Compression Residue in addition to the Compression Residue value.
CoAP request and response do not include the same options. So
Compression Rules may reflect these assymetry by tagging the
direction indicator.
Note that length coding differs between CoAP options and SCHC Note that length coding differs between CoAP options and SCHC
variable size Compression Residue. variable size Compression Residue.
The following sections present how SCHC compresses some specific CoAP The following sections present how SCHC compresses some specific CoAP
Options. Options.
5.1. CoAP Content and Accept options. 5.1. CoAP Content and Accept options.
These fields are both unidirectional and MUST NOT be set to
bidirectional in a Rule entry.
If a single value is expected by the client, it can be stored in the If a single value is expected by the client, it can be stored in the
TV and elided during the transmission. Otherwise, if several TV and elided during the transmission. Otherwise, if several
possible values are expected by the client, a matching-list SHOULD be possible values are expected by the client, a matching-list SHOULD be
used to limit the size of the Compression Residue. Otherwise, the used to limit the size of the Compression Residue. Otherwise, the
value has to be sent as a Compression Residue (fixed or variable value has to be sent as a Compression Residue (fixed or variable
length). 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
These fields are unidirectional and MUST NOT be set to bidirectional
in a Rule DI entry, see section 7.1 of [rfc8724]. They are used only
by the server to inform of the caching duration and is never found in
client requests.
If the duration is known by both ends, the value can be elided. If the duration is known by both ends, the value can be elided.
A matching list can be used if some well-known values are defined. A matching list can be used if some well-known values are defined.
Otherwise these options can be sent as a Compression Residue (fixed Otherwise these options can be sent as a Compression Residue (fixed
or variable length). or variable length).
5.3. CoAP option Uri-Path and Uri-Query fields 5.3. CoAP option Uri-Path and Uri-Query fields
These fields are unidirectional and MUST NOT be set to bidirectional
in a Rule entry. They are used only by the client to access a
specific resource and are never found in server responses.
Uri-Path and Uri-Query elements are a repeatable options, the Field Uri-Path and Uri-Query elements are a repeatable options, the Field
Position (FP) gives the position in the path. Position (FP) gives the position in the path.
A Mapping list can be used to reduce the size of variable Paths or A Mapping list can be used to reduce the size of variable Paths or
Queries. In that case, to optimize the compression, several elements Queries. In that case, to optimize the compression, several elements
can be regrouped into a single entry. Numbering of elements do not can be regrouped into a single entry. Numbering of elements do not
change, MO comparison is set with the first element of the matching. change, MO comparison is set with the first element of the matching.
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
| Field |FL |FP|DI| Target | Match | CDA | | Field |FL |FP|DI| Target | Match | CDA |
| | | | | Value | Opera. | | | | | | | Value | Opera. | |
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
|URI-Path | | 1|up|["/a/b",|equal |not-sent | |URI-Path | | 1|up|["/a/b",|equal |not-sent |
| | | | |"/c/d"] | | | | | | | |"/c/d"] | | |
|URI-Path |var| 3|up| |ignore |value-sent | |URI-Path |var| 3|up| |ignore |value-sent |
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
Figure 2: complex path example Figure 4: complex path example
In Figure 2 a single bit residue can be used to code one of the 2 In Figure 4 a single bit residue can be used to code one of the 2
paths. If regrouping were not allowed, a 2 bits residue would be paths. If regrouping were not allowed, a 2 bits residue would be
needed. The third path element is sent as a variable size residue. needed. The third path element is sent as a variable size residue.
5.3.1. Variable length Uri-Path and Uri-Query 5.3.1. Variable length Uri-Path and Uri-Query
When the length is not known at the Rule creation, the Field Length When the length is not known at the Rule creation, the Field Length
MUST be set to variable, and the unit is set to bytes. MUST be set to variable, and the unit is set to bytes.
The MSB MO can be applied to a Uri-Path or Uri-Query element. Since The MSB MO can be applied to a Uri-Path or Uri-Query element. Since
MSB value is given in bit, the size MUST always be a multiple of 8 MSB value is given in bit, the size MUST always be a multiple of 8
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+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
| Field |FL |FP|DI| Target | Match | CDA | | Field |FL |FP|DI| Target | Match | CDA |
| | | | | Value | Opera. | | | | | | | Value | Opera. | |
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
|URI-Path | 8| 1|up|"c" |equal |not-sent | |URI-Path | 8| 1|up|"c" |equal |not-sent |
|URI-Path |var| 2|up| |ignore |value-sent | |URI-Path |var| 2|up| |ignore |value-sent |
|URI-Query |var| 1|up|"k=" |MSB(16) |LSB | |URI-Query |var| 1|up|"k=" |MSB(16) |LSB |
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
Figure 3: CORECONF URI compression Figure 5: CORECONF URI compression
Figure 3 shows the parsing and the compression of the URI, where c is Figure 5 shows the parsing and the compression of the URI, where c is
not sent. The second element is sent with the length (i.e. 0x2 X 6) not sent. The second element is sent with the length (i.e. 0x2 X 6)
followed by the query option (i.e. 0x05 "eth0"). followed by the query option (i.e. 0x05 "eth0").
5.3.2. Variable number of path or query elements 5.3.2. Variable number of path or query elements
The number of Uri-path or Uri-Query elements in a Rule is fixed at The number of Uri-path or Uri-Query elements in a Rule is fixed at
the Rule creation time. If the number varies, several Rules SHOULD the Rule creation time. If the number varies, several Rules SHOULD
be created to cover all the possibilities. Another possibility is to be created to cover all the possibilities. Another possibility is to
define the length of Uri-Path to variable and send a Compression define the length of Uri-Path to variable and send a Compression
Residue with a length of 0 to indicate that this Uri-Path is empty. Residue with a length of 0 to indicate that this Uri-Path is empty.
This adds 4 bits to the variable Residue size. See section 7.5.2 This adds 4 bits to the variable Residue size. See section 7.5.2
[rfc8724] [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
These fields are unidirectional and MUST NOT be set to bidirectional
in a Rule DI entry, see section 7.1 of the [rfc8724]. They are used
only by the client to access a specific resource and are never found
in server response.
If the field value has to be sent, TV is not set, MO is set to If the field value has to be sent, TV is not set, MO is set to
"ignore" and CDA is set to "value-sent". A mapping MAY also be used. "ignore" and CDA is set to "value-sent". A mapping MAY also be used.
Otherwise, the TV is set to the value, MO is set to "equal" and CDA Otherwise, the TV is set to the value, MO is set to "equal" and CDA
is set to "not-sent". is set to "not-sent".
5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and
Location-Query fields Location-Query fields
These fields are unidirectional.
These fields values cannot be stored in a Rule entry. They MUST These fields values cannot be stored in a Rule entry. They MUST
always be sent with the Compression Residues. always be sent with the Compression Residues.
6. SCHC compression of CoAP extension RFCs 6. SCHC compression of CoAP extension RFCs
6.1. Block 6.1. Block
Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also
includes a fragmentation protocol. They can be both used. If a includes a fragmentation protocol. They can be both used. If a
block option is used, its content MUST be sent as a Compression block option is used, its content MUST be sent as a Compression
skipping to change at page 12, line 20 skipping to change at page 13, line 20
|<-- 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_kidctxt ----->|<-- CoAP OSCORE_kid -->| | <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->|
Figure 4: OSCORE Option Figure 6: OSCORE Option
The encoding of the OSCORE Option Value defined in Section 6.1 of The encoding of the OSCORE Option Value defined in Section 6.1 of
[rfc8613] is repeated in Figure 4. [rfc8613] is repeated in Figure 6.
The first byte specifies the content of the OSCORE options using The first byte specifies the content of the OSCORE options using
flags. The three most significant bits of this byte are reserved and 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 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 context field in the option. Bit k, when set, indicates the presence
of a kid field. The three least significant bits n indicate the of a kid field. The three least significant bits n indicate the
length of the piv (Partial Initialization Vector) field in bytes. length of the piv (Partial Initialization Vector) field in bytes.
When n = 0, no piv is present. When n = 0, no piv is present.
The flag byte is followed by the piv field, kid context field, and The flag byte is followed by the piv field, kid context field, and
skipping to change at page 13, line 6 skipping to change at page 14, line 6
o CoAP OSCORE_flags, o CoAP OSCORE_flags,
o CoAP OSCORE_piv, o CoAP OSCORE_piv,
o CoAP OSCORE_kidctxt, o CoAP OSCORE_kidctxt,
o CoAP OSCORE_kid. o CoAP OSCORE_kid.
The OSCORE Option shows superimposed these four fields using the The OSCORE Option shows superimposed these four fields using the
format Figure 4, the CoAP OSCORE_kidctxt field includes the size bits format Figure 6, the CoAP OSCORE_kidctxt field includes the size bits
s. s.
7. Examples of CoAP header compression 7. Examples of CoAP header compression
7.1. Mandatory header with CON message 7.1. Mandatory header with CON message
In this first scenario, the LPWAN Compressor at the Network Gateway In this first scenario, the LPWAN Compressor at the Network Gateway
side receives from an Internet client a POST message, which is side receives from an Internet client a POST message, which is
immediately acknowledged by the Device. For this simple scenario, immediately acknowledged by the Device. For this simple scenario,
the Rules are described Figure 5. the Rules are described Figure 7.
RuleID 1 RuleID 1
+-------------+--+--+--+------+---------+-------------++------------+ +-------------+--+--+--+------+---------+-------------++------------+
| Field |FL|FP|DI|Target| Match | CDA || Sent | | Field |FL|FP|DI|Target| Match | CDA || Sent |
| | | | |Value | Opera. | || [bits] | | | | | |Value | Opera. | || [bits] |
+-------------+--+--+--+------+---------+-------------++------------+ +-------------+--+--+--+------+---------+-------------++------------+
|CoAP version | | |bi| 01 |equal |not-sent || | |CoAP version | | |bi| 01 |equal |not-sent || |
|CoAP Type | | |dw| CON |equal |not-sent || | |CoAP Type | | |dw| CON |equal |not-sent || |
|CoAP Type | | |up|[ACK, | | || | |CoAP Type | | |up|[ACK, | | || |
| | | | | RST] |match-map|matching-sent|| T | | | | | | RST] |match-map|matching-sent|| T |
|CoAP TKL | | |bi| 0 |equal |not-sent || | |CoAP TKL | | |bi| 0 |equal |not-sent || |
|CoAP Code | | |bi|[0.00,| | || | |CoAP Code | | |bi|[0.00,| | || |
| | | | | ... | | || | | | | | | ... | | || |
| | | | | 5.05]|match-map|matching-sent|| CC CCC | | | | | | 5.05]|match-map|matching-sent|| CC CCC |
|CoAP MID | | |bi| 0000 |MSB(7 ) |LSB || M-ID| |CoAP MID | | |bi| 0000 |MSB(7 ) |LSB || M-ID|
|CoAP Uri-Path| | |dw| path |equal 1 |not-sent || | |CoAP Uri-Path| | |dw| path |equal 1 |not-sent || |
+-------------+--+--+--+------+---------+-------------++------------+ +-------------+--+--+--+------+---------+-------------++------------+
Figure 5: CoAP Context to compress header without token Figure 7: CoAP Context to compress header without token
The version and Token Length fields are elided. The 26 method and The version and Token Length fields are elided. The 26 method and
response codes defined in [rfc7252] has been shrunk to 5 bits using a response codes defined in [rfc7252] has been shrunk to 5 bits using a
matching list. Uri-Path contains a single element indicated in the matching list. Uri-Path contains a single element indicated in the
matching operator. matching operator.
SCHC Compression reduces the header sending only the Type, a mapped SCHC Compression reduces the header sending only the Type, a mapped
code and the least significant bits of Message ID (9 bits in the code and the least significant bits of Message ID (9 bits in the
example above). example above).
skipping to change at page 14, line 18 skipping to change at page 15, line 18
messages. The goal, therefore, is to hide as much of the message as messages. The goal, therefore, is to hide as much of the message as
possible 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 which is not necessary for proxy contains sensitive information which is not necessary for proxy
operation. This, in turn, is the part of the message which can be operation. This, in turn, is the part of the message which can be
encrypted until it reaches its end destination. The Outer Message encrypted until it reaches its end destination. The Outer Message
acts as a shell matching the format of a regular CoAP message, and acts as a shell matching the format of a regular CoAP message, and
includes all Options and information needed for proxy operation and includes all Options and information needed for proxy operation and
caching. This decomposition is illustrated in Figure 6. caching. This decomposition is illustrated in Figure 8.
CoAP options are sorted into one of 3 classes, each granted a CoAP options are sorted into one of 3 classes, each 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, o Class E: Encrypted options moved to the Inner Plaintext,
o Class I: Integrity-protected options included in the AAD for the o Class I: Integrity-protected options included in the AAD for the
encryption of the Plaintext but otherwise left untouched in the encryption of the Plaintext but otherwise left untouched in the
Outer Message, Outer Message,
skipping to change at page 15, line 38 skipping to change at page 16, line 38
+-+-+---+--------+---------------+....+ +-------+-----......+ +-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) | | Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+ +--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF | | Options (IU) | | OxFF |
. . +-------+-----------+ . . +-------+-----------+
. OSCORE Option . | | . OSCORE Option . | |
+------+-------------------+ | Payload | +------+-------------------+ | Payload |
| 0xFF | | | | 0xFF | | |
+------+ +-------------------+ +------+ +-------------------+
Figure 6: A CoAP message is split into an OSCORE outer and plaintext Figure 8: A CoAP message is split into an OSCORE outer and plaintext
Figure 6 shows the message format for the OSCORE Message and Figure 8 shows the message format for the OSCORE Message and
Plaintext. Plaintext.
In the Outer Header, the original message code is hidden and replaced In the Outer Header, the original message code is hidden and replaced
by a default dummy value. As seen in Sections 4.1.3.5 and 4.2 of the by a default dummy value. As seen in Sections 4.1.3.5 and 4.2 of the
[rfc8613], the message code is replaced by POST for requests and [rfc8613], the message code is replaced by POST for requests and
Changed for responses when Observe is not used. If Observe is used, Changed for responses when Observe is not used. If Observe is used,
the message code is replaced by FETCH for requests and Content for the message code is replaced by FETCH for requests and Content for
responses. responses.
The original message code is put into the first byte of the The original message code is put into the first byte of the
Plaintext. Following the message code, the class E options comes and Plaintext. Following the message code, the class E options comes and
if present the original message Payload is preceded by its payload if present the original message Payload is preceded by its payload
marker. marker.
The Plaintext is now encrypted by an AEAD algorithm which integrity The Plaintext is now encrypted by an AEAD algorithm which integrity
protects Security Context parameters and eventually any class I protects Security Context parameters and eventually any class I
options from the Outer Header. Currently no CoAP options are marked options from the Outer Header. Currently no CoAP options are marked
class I. The resulting Ciphertext becomes the new Payload of the class I. The resulting Ciphertext becomes the new Payload of the
OSCORE message, as illustrated in Figure 7. OSCORE message, as illustrated in Figure 9.
This Ciphertext is, as defined in RFC 5116, the concatenation of the This Ciphertext is, as defined in RFC 5116, the concatenation of the
encrypted Plaintext and its authentication tag. Note that Inner encrypted Plaintext and its authentication tag. Note that Inner
Compression only affects the Plaintext before encryption, thus we can Compression only affects the Plaintext before encryption, thus we can
only aim to reduce this first, variable length component of the only aim to reduce this first, variable length component of the
Ciphertext. The authentication tag is fixed in length and considered Ciphertext. The authentication tag is fixed in length and considered
part of the cost of protection. part of the cost of protection.
Outer Header Outer Header
+-+-+---+--------+---------------+ +-+-+---+--------+---------------+
skipping to change at page 16, line 39 skipping to change at page 17, line 39
+------+-------------------+ +------+-------------------+
| 0xFF | | 0xFF |
+------+---------------------------+ +------+---------------------------+
| | | |
| Ciphertext: Encrypted Inner | | Ciphertext: Encrypted Inner |
| Header and Payload | | Header and Payload |
| + Authentication Tag | | + Authentication Tag |
| | | |
+----------------------------------+ +----------------------------------+
Figure 7: OSCORE message Figure 9: OSCORE message
The SCHC Compression scheme consists of compressing both the The SCHC Compression scheme consists of compressing both the
Plaintext before encryption and the resulting OSCORE message after Plaintext before encryption and the resulting OSCORE message after
encryption, see Figure 8. encryption, see Figure 10.
This translates into a segmented process where SCHC compression is This translates into a segmented process where SCHC compression is
applied independently in 2 stages, each with its corresponding set of applied independently in 2 stages, each with its corresponding set of
Rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way Rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way
compression is applied to all fields of the original CoAP message. compression is applied to all fields of the original CoAP message.
Note that since the Inner part of the message can only be decrypted Note that since the Inner part of the message can only be decrypted
by the corresponding end-point, this end-point will also have to by the corresponding end-point, this end-point will also have to
implement Inner SCHC Compression/Decompression. implement Inner SCHC Compression/Decompression.
skipping to change at page 17, line 41 skipping to change at page 18, line 41
v | +-------+--+ v | +-------+--+
+--------+ +------------+ | Residue | +--------+ +------------+ | Residue |
|RuleID' | | Encryption | <--- +----------+--------+ |RuleID' | | Encryption | <--- +----------+--------+
+--------+--+ +------------+ | | +--------+--+ +------------+ | |
| Residue' | | Payload | | Residue' | | Payload |
+-----------+-------+ | | +-----------+-------+ | |
| Ciphertext | +-------------------+ | Ciphertext | +-------------------+
| | | |
+-------------------+ +-------------------+
Figure 8: OSCORE Compression Diagram Figure 10: OSCORE Compression Diagram
7.3. Example OSCORE Compression 7.3. Example OSCORE Compression
An example is given with a GET Request and its consequent Content An example is given with a GET Request and its consequent Content
Response from a device-based CoAP client to a cloud-based CoAP Response from a device-based CoAP client to a cloud-based CoAP
server. A possible set of Rules for the Inner and Outer SCHC server. A possible set of Rules for the Inner and Outer SCHC
Compression is shown. A dump of the results and a contrast between Compression is shown. A dump of the results and a contrast between
SCHC + OSCORE performance with SCHC + COAP performance is also SCHC + OSCORE performance with SCHC + COAP performance is also
listed. This gives an approximation to the cost of security with listed. This gives an approximation to the cost of security with
SCHC-OSCORE. SCHC-OSCORE.
Our first example CoAP message is the GET Request in Figure 9 Our first example CoAP message is the GET Request in Figure 11
Original message: Original message:
================= =================
0x4101000182bb74656d7065726174757265 0x4101000182bb74656d7065726174757265
Header: Header:
0x4101 0x4101
01 Ver 01 Ver
00 CON 00 CON
0001 tkl 0001 tkl
skipping to change at page 18, line 28 skipping to change at page 19, line 28
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 msg length: 17 bytes.
Figure 9: CoAP GET Request Figure 11: CoAP GET Request
Its corresponding response is the CONTENT Response in Figure 10. Its corresponding response is the CONTENT Response in Figure 12.
Original message: Original message:
================= =================
0x6145000182ff32332043 0x6145000182ff32332043
Header: Header:
0x6145 0x6145
01 Ver 01 Ver
10 ACK 10 ACK
0001 tkl 0001 tkl
skipping to change at page 18, line 52 skipping to change at page 19, line 52
0x0001 = mid 0x0001 = mid
0x82 = token 0x82 = token
0xFF Payload marker 0xFF Payload marker
Payload: Payload:
0x32332043 0x32332043
Original msg length: 10 Original msg length: 10
Figure 10: CoAP CONTENT Response Figure 12: CoAP CONTENT Response
The SCHC Rules for the Inner Compression include all fields that are The SCHC Rules for the Inner Compression include all fields that are
already present in a regular CoAP message. The methods described in already present in a regular CoAP message. The methods described in
Section 4 applies to these fields. As an example, see Figure 11. Section 4 applies to these fields. As an example, see Figure 13.
RuleID 0 RuleID 0
+---------------+--+--+-----------+-----------+-----------++------+ +---------------+--+--+-----------+-----------+-----------++------+
| Field |FP|DI| Target | MO | CDA || Sent | | Field |FP|DI| Target | MO | CDA || Sent |
| | | | Value | | ||[bits]| | | | | Value | | ||[bits]|
+---------------+--+--+-----------+-----------+-----------++------+ +---------------+--+--+-----------+-----------+-----------++------+
|CoAP Code | |up| 1 | equal |not-sent || | |CoAP Code | |up| 1 | equal |not-sent || |
|CoAP Code | |dw|[69,132] | match-map |match-sent || c | |CoAP Code | |dw|[69,132] | match-map |match-sent || c |
|CoAP Uri-Path | |up|temperature| equal |not-sent || | |CoAP Uri-Path | |up|temperature| equal |not-sent || |
|COAP Option-End| |dw| 0xFF | equal |not-sent || | |COAP Option-End| |dw| 0xFF | equal |not-sent || |
+---------------+--+--+-----------+-----------+-----------++------+ +---------------+--+--+-----------+-----------+-----------++------+
Figure 11: Inner SCHC Rules Figure 13: Inner SCHC Rules
Figure 12 shows the Plaintext obtained for our example GET Request Figure 14 shows the Plaintext obtained for our example GET Request
and follows the process of Inner Compression and Encryption until we and follows the process of Inner Compression and Encryption until we
end up with the Payload to be added in the outer OSCORE Message. end up with the Payload to be added in the outer OSCORE Message.
In this case the original message has no payload and its resulting In this case the original message has no payload and its resulting
Plaintext can be compressed up to only 1 byte (size of the RuleID). Plaintext can be compressed up to only 1 byte (size of the RuleID).
The AEAD algorithm preserves this length in its first output, but The AEAD algorithm preserves this length in its first output, but
also yields a fixed-size tag which cannot be compressed and has to be also yields a fixed-size tag which cannot be compressed and has to be
included in the OSCORE message. This translates into an overhead in included in the OSCORE message. This translates into an overhead in
total message length, which limits the amount of compression that can total message length, which limits the amount of compression that can
be achieved and plays into the cost of adding security to the be achieved and plays into the cost of adding security to the
skipping to change at page 20, line 44 skipping to change at page 21, line 44
| (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 12: Plaintext compression and encryption for GET Request Figure 14: Plaintext compression and encryption for GET Request
In Figure 13 the process is repeated for the example CONTENT In Figure 15 the process is repeated for the example CONTENT
Response. The residue is 1 bit long. Note that since SCHC adds Response. The residue is 1 bit long. Note that since SCHC adds
padding after the payload, this misalignment causes the hexadecimal padding after the payload, this misalignment causes the hexadecimal
code from the payload to differ from the original, even though it has code from the payload to differ from the original, even though it has
not been compressed. not been compressed.
On top of this, the overhead from the tag bytes is incurred as On top of this, the overhead from the tag bytes is incurred as
before. before.
________________________________________________________ ________________________________________________________
| | | |
skipping to change at page 21, line 51 skipping to change at page 22, line 51
| (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 13: Plaintext compression and encryption for CONTENT Response Figure 15: Plaintext compression and encryption for CONTENT Response
The Outer SCHC Rules (Figure 16) must process the OSCORE Options The Outer SCHC Rules (Figure 18) must process the OSCORE Options
fields. In Figure 14 and Figure 15 we show a dump of the OSCORE fields. In Figure 16 and Figure 17 we show a dump of the OSCORE
Messages generated from our example messages once they have been Messages generated from our example messages once they have been
provided with the Inner Compressed Ciphertext in the payload. These provided with the Inner Compressed Ciphertext in the payload. These
are the messages that have to be compressed by the Outer SCHC are the messages that have to be compressed by the Outer SCHC
Compression. Compression.
Protected message: Protected message:
================== ==================
0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62 0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62
(25 bytes) (25 bytes)
skipping to change at page 22, line 39 skipping to change at page 23, line 39
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 14: Protected and Inner SCHC Compressed GET Request Figure 16: 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 23, line 29 skipping to change at page 24, line 29
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 15: Protected and Inner SCHC Compressed CONTENT Response Figure 17: Protected and Inner SCHC Compressed CONTENT Response
For the flag bits, a number of compression methods has been shown to For the flag bits, a number of compression methods has been shown to
be useful depending on the application. The simplest alternative is be useful depending on the application. The simplest alternative is
to provide a fixed value for the flags, combining MO equal and CDA to provide a fixed value for the flags, combining MO equal and CDA
not- sent. This saves most bits but could prevent flexibility. not- sent. This saves most bits but could prevent flexibility.
Otherwise, match-mapping could be used to choose from an interested Otherwise, match-mapping could be used to choose from an interested
number of configurations to the exchange. Otherwise, MSB could be number of configurations to the exchange. Otherwise, MSB could be
used to mask off the 3 hard-coded most significant bits. used to mask off the 3 hard-coded most significant bits.
Note that fixing a flag bit will limit the choice of CoAP Options Note that fixing a flag bit will limit the choice of CoAP Options
skipping to change at page 24, line 7 skipping to change at page 25, line 7
Note that compressing the sequence numbers effectively reduces the Note that compressing the sequence numbers effectively reduces the
maximum amount of sequence numbers that can be used in an exchange. maximum amount of sequence numbers that can be used in an exchange.
Once this amount is exceeded, the OSCORE keys need to be re- Once this amount is exceeded, the OSCORE keys need to be re-
established. established.
The size s included in the kid context field MAY be masked off with The size s included in the kid context field MAY be masked off with
CDA MSB. The rest of the field could have additional bits masked CDA MSB. The rest of the field could have additional bits masked
off, or have the whole field be fixed with MO equal and CDA not-sent. off, or have the whole field be fixed with MO equal and CDA not-sent.
The same holds for the kid field. The same holds for the kid field.
Figure 16 shows a possible set of Outer Rules to compress the Outer Figure 18 shows a possible set of Outer Rules to compress the Outer
Header. Header.
RuleID 0 RuleID 0
+-------------------+--+--+--------------+--------+---------++------+ +-------------------+--+--+--------------+--------+---------++------+
| Field |FP|DI| Target | MO | CDA || Sent | | Field |FP|DI| Target | MO | CDA || Sent |
| | | | Value | | ||[bits]| | | | | Value | | ||[bits]|
+-------------------+--+--+--------------+--------+---------++------+ +-------------------+--+--+--------------+--------+---------++------+
|CoAP version | |bi| 01 |equal |not-sent || | |CoAP version | |bi| 01 |equal |not-sent || |
|CoAP Type | |up| 0 |equal |not-sent || | |CoAP Type | |up| 0 |equal |not-sent || |
|CoAP Type | |dw| 2 |equal |not-sent || | |CoAP Type | |dw| 2 |equal |not-sent || |
skipping to change at page 24, line 33 skipping to change at page 25, line 33
|CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || | |CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || |
|CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP | |CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP |
|COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK | |COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK |
|COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || | |COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || |
|CoAP OSCORE_flags | |dw| b'' |equal |not-sent || | |CoAP OSCORE_flags | |dw| b'' |equal |not-sent || |
|CoAP OSCORE_piv | |dw| b'' |equal |not-sent || | |CoAP OSCORE_piv | |dw| b'' |equal |not-sent || |
|CoAP OSCORE_kid | |dw| b'' |equal |not-sent || | |CoAP OSCORE_kid | |dw| b'' |equal |not-sent || |
|COAP Option-End | |dw| 0xFF |equal |not-sent || | |COAP Option-End | |dw| 0xFF |equal |not-sent || |
+-------------------+--+--+--------------+--------+---------++------+ +-------------------+--+--+--------------+--------+---------++------+
Figure 16: Outer SCHC Rules Figure 18: Outer SCHC Rules
These Outer Rules are applied to the example GET Request and CONTENT These Outer Rules are applied to the example GET Request and CONTENT
Response. The resulting messages are shown in Figure 17 and Response. The resulting messages are shown in Figure 19 and
Figure 18. Figure 20.
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 17: SCHC-OSCORE Compressed GET Request Figure 19: 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 msg length: 16 bytes
Figure 18: SCHC-OSCORE Compressed CONTENT Response Figure 20: SCHC-OSCORE Compressed CONTENT Response
For contrast, we compare these results with what would be obtained by For contrast, we compare 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. To do this, we compress the CoAP messages according to with OSCORE. To do this, we compress the CoAP messages according to
the SCHC Rules in Figure 19. the SCHC Rules in Figure 21.
RuleID 1 RuleID 1
+---------------+--+--+-----------+---------+-----------++--------+ +---------------+--+--+-----------+---------+-----------++--------+
| Field |FP|DI| Target | MO | CDA || Sent | | Field |FP|DI| Target | MO | CDA || Sent |
| | | | Value | | || [bits] | | | | | Value | | || [bits] |
+---------------+--+--+-----------+---------+-----------++--------+ +---------------+--+--+-----------+---------+-----------++--------+
|CoAP version | |bi| 01 |equal |not-sent || | |CoAP version | |bi| 01 |equal |not-sent || |
|CoAP Type | |up| 0 |equal |not-sent || | |CoAP Type | |up| 0 |equal |not-sent || |
|CoAP Type | |dw| 2 |equal |not-sent || | |CoAP Type | |dw| 2 |equal |not-sent || |
|CoAP TKL | |bi| 1 |equal |not-sent || | |CoAP TKL | |bi| 1 |equal |not-sent || |
|CoAP Code | |up| 2 |equal |not-sent || | |CoAP Code | |up| 2 |equal |not-sent || |
|CoAP Code | |dw| [69,132] |match-map|map-sent ||C | |CoAP Code | |dw| [69,132] |match-map|map-sent ||C |
|CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM |
|CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT |
|CoAP Uri-Path | |up|temperature|equal |not-sent || | |CoAP Uri-Path | |up|temperature|equal |not-sent || |
|COAP Option-End| |dw| 0xFF |equal |not-sent || | |COAP Option-End| |dw| 0xFF |equal |not-sent || |
+---------------+--+--+-----------+---------+-----------++--------+ +---------------+--+--+-----------+---------+-----------++--------+
Figure 19: SCHC-CoAP Rules (No OSCORE) Figure 21: SCHC-CoAP Rules (No OSCORE)
This yields the results in Figure 20 for the Request, and Figure 21 This yields the results in Figure 22 for the Request, and Figure 23
for the Response. for the Response.
Compressed message: Compressed message:
================== ==================
0x0114 0x0114
0x01 = RuleID 0x01 = RuleID
Compression Residue: Compression Residue:
0b00010100 (1 byte) 0b00010100 (1 byte)
Compressed msg length: 2 Compressed msg length: 2
Figure 20: CoAP GET Compressed without OSCORE Figure 22: 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 msg length: 6
Figure 21: CoAP CONTENT Compressed without OSCORE Figure 23: 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 of cost. compared to regular SCHC + COAP is about 10 bytes of cost.
8. IANA Considerations 8. IANA Considerations
This document has no request to IANA. This document has no request to IANA.
9. Security considerations 9. Security considerations
The Security Considerations of SCHC header compression RFC8724 are When applied to LPWAN, the Security Considerations of SCHC header
valid for SCHC CoAP header compression. When CoAP uses OSCORE, the compression [rfc8724] are valid for SCHC CoAP header compression.
security considerations defined in RFC8613 does not change when SCHC When CoAP uses OSCORE, the security considerations defined in
header compression is applied. [rfc8613] does not change when SCHC header compression is applied.
The definition of SCHC over CoAP header fields permits the The definition of SCHC over CoAP header fields permits the
compression of header information only. The SCHC header compression compression of header information only. The SCHC header compression
itself does not increase or reduce the level of security in the itself does not increase or reduce the level of security in the
communication. When the communication does not use any security communication. When the connection does not use any security
protocol as OSCORE, DTLS, or other. It is highly necessary to use a protocol as OSCORE, DTLS, or other, it is highly necessary to use a
layer two security. layer two security.
DoS attacks are possible if an intruder can introduce a compressed DoS attacks are possible if an intruder can introduce a compressed
SCHC corrupted packet onto the link and cause a compression SCHC corrupted packet onto the link and cause a compression
efficiency reduction. However, an intruder having the ability to add efficiency reduction. However, an intruder having the ability to add
corrupted packets at the link layer raises additional security issues corrupted packets at the link layer raises additional security issues
than those related to the use of header compression. than those related to the use of header compression.
SCHC compression returns variable-length Residues for some CoAP SCHC compression returns variable-length Residues for some CoAP
fields. In the compressed header, the length sent is not the fields. In the compressed header, the length sent is not the
original header field length but the length of the Residue. So if a original header field length but the length of the Residue. So if a
corrupted packet comes to the decompressor with a longer or shorter corrupted packet comes to the decompressor with a longer or shorter
length than the one in the original header, SCHC decompression will length than the one in the original header, SCHC decompression will
detect an error and drops the packet. detect an error and drops the packet.
OSCORE compression is also based on the same compression method OSCORE compression is also based on the same compression method
described in [rfc8724]. The size of the Initialisation Vector (IV) described in [rfc8724]. The size of the Initialisation Vector (IV)
residue size must be considered carefully. A too large value has an residue must be considered carefully. A residue size obtained with
impact on the compression efficiency and a too small value will force LSB CDA over the IV has an impact on the compression efficiency and
the device to renew its key more often. This operation may be long the frequency the device will renew its key. This operation requires
and energy consuming. The size of the compressed IV MUST be choosen several exchanges and is energy-consuming.
regarding the highest expected traffic from the device.
SCHC header and compression Rules MUST remain tightly coupled. SCHC header and compression Rules MUST remain tightly coupled.
Otherwise, an encrypted residue may be decompressed in a different Otherwise, an encrypted residue may be decompressed differently by
way by the receiver. To avoid this situation, if the Rule is the receiver. To avoid this situation, if the Rule is modified in
modified in one location, the OSCORE keys MUST be re-established. one location, the OSCORE keys MUST be re-established.
10. Acknowledgements 10. Acknowledgements
The authors would like to thank (in alphabetic order): Christian The authors would like to thank (in alphabetic order): Christian
Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran
Selander. Selander.
11. Normative References 11. Normative References
skipping to change at page 29, line 25 skipping to change at page 30, line 20
"Object Security for Constrained RESTful Environments "Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>. <https://www.rfc-editor.org/info/rfc8613>.
[rfc8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. [rfc8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "SCHC: Generic Framework for Static Context Header Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724, Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020, DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>. <https://www.rfc-editor.org/info/rfc8724>.
Appendix A. Extension to the RFC8724 Annex D.
This section extends the RFC8724 Annex D list.
o How to establish the End-to-End context initialization using SCHC
for CoAP header only.
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
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