draft-ietf-lpwan-coap-static-context-hc-13.txt   draft-ietf-lpwan-coap-static-context-hc-14.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: September 6, 2020 Institut MINES TELECOM; IMT Atlantique Expires: November 27, 2020 Institut MINES TELECOM; IMT Atlantique
R. Andreasen R. Andreasen
Universidad de Buenos Aires Universidad de Buenos Aires
March 05, 2020 May 26, 2020
LPWAN Static Context Header Compression (SCHC) for CoAP LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-13 draft-ietf-lpwan-coap-static-context-hc-14
Abstract Abstract
This draft defines the way SCHC (Static Context Header Compression) This draft defines the way Static Context Header Compression (SCHC)
header compression can be applied to the CoAP protocol. SCHC is a header compression can be applied to the Constrained Application
header compression mechanism adapted for constrained devices. SCHC Protocol (CoAP). SCHC is a header compression mechanism adapted for
uses a static description of the header to reduce the redundancy and constrained devices. SCHC uses a static description of the header to
the size of the information in the header. While reduce the redundancy and the size of the information in the header.
[I-D.ietf-lpwan-ipv6-static-context-hc] describes the SCHC While [rfc8724] describes the SCHC compression and fragmentation
compression and fragmentation framework, and its application for framework, and its application for IPv6/UDP headers, this document
IPv6/UDP headers, this document applies the use of SCHC for CoAP applies the use of SCHC for CoAP headers. The CoAP header structure
headers. The CoAP header structure differs from IPv6 and UDP since differs from IPv6 and UDP since CoAP uses a flexible header with a
CoAP uses a flexible header with a variable number of options, variable number of options, themselves of variable length. The CoAP
themselves of variable length. The CoAP protocol messages format is protocol messages format is asymmetric: the request messages have a
asymmetric: the request messages have a header format different from header format different from the one in the response messages. This
the one in the response messages. This specification gives guidance specification gives guidance on how to apply SCHC to flexible headers
on how to apply SCHC to flexible headers and how to leverage the and how to leverage the asymmetry for more efficient compression
asymmetry for more efficient compression Rules. Rules.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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 September 6, 2020. This Internet-Draft will expire on November 27, 2020.
Copyright Notice Copyright Notice
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document authors. All rights reserved. document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . 4 2. Applying SCHC to CoAP headers . . . . . . . . . . . . . . . . 4
3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 5 3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 5
3.1. Differences between CoAP and UDP/IP . . . . . . . . . . . 5 3.1. Differences between CoAP and UDP/IP Compression . . . . . 5
4. Compression of CoAP header fields . . . . . . . . . . . . . . 6 4. Compression of CoAP header fields . . . . . . . . . . . . . . 6
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 7 4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 7
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 7 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 7
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 7 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 7
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 7 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 7
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 8 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 8 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 8
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 . . . . 8
5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8 5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 9
5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 9 5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 9
5.3.2. Variable number of path or query elements . . . . . . 10 5.3.2. Variable number of path or query elements . . . . . . 10
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 . . . . . . . . . . . . . . . . . . . . . . . . . 10
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 . . . . . . . . . . . . . . . . 10
6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 10 6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 11
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 11 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 11
6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Examples of CoAP header compression . . . . . . . . . . . . . 12 7. Examples of CoAP header compression . . . . . . . . . . . . . 13
7.1. Mandatory header with CON message . . . . . . . . . . . . 12 7.1. Mandatory header with CON message . . . . . . . . . . . . 13
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 13 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 14
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
9. Security considerations . . . . . . . . . . . . . . . . . . . 27 9. Security considerations . . . . . . . . . . . . . . . . . . . 27
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
11. Normative References . . . . . . . . . . . . . . . . . . . . 28 11. Normative References . . . . . . . . . . . . . . . . . . . . 28
Appendix A. Extension to the RFC8724 Annex D. . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 1. Introduction
CoAP [rfc7252] is a transfer protocol that implements a subset of CoAP [rfc7252] is designed to easily interop with HTTP (Hypertext
HTTP (Hypertext Transfer Protocol) and is optimized for REST-based Transfer Protocol) and is optimized for REST-based (Representational
(Representational state transfer) services. Although CoAP was state transfer) services. Although CoAP was designed for constrained
designed for constrained devices, the size of a CoAP header still is devices, the size of a CoAP header still is too large for the
too large for the constraints of LPWAN (Low Power Wide Area Networks) constraints of LPWAN (Low Power Wide Area Networks) and some
and some compression is needed to reduce the header size. compression is needed to reduce the header size.
The [I-D.ietf-lpwan-ipv6-static-context-hc] defines SCHC, a header The [rfc8724] defines SCHC, a header compression mechanism for LPWAN
compression mechanism for LPWAN network based on a static context. network based on a static context. The section 5 of the [rfc8724]
The section 5 of the [I-D.ietf-lpwan-ipv6-static-context-hc] explains explains the architecture where compression and decompression are
the architecture where compression and decompression are done. The done. The context is known by both ends before transmission. The
context is known by both ends before transmission. The way the way the context is configured, provisioned or exchanged is out of the
context is configured or exchanged is out of the scope for this scope of this document.
document.
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 rule a Rule matches, the matched header fields are substituted by the
ID 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 the list of fields descriptions, see section 7 of [rfc8724], thereby each
[I-D.ietf-lpwan-ipv6-static-context-hc], thereby each description description contains the field ID (FID), its length (FL) and its
contains the field ID (FID), its length (FL) and its position (FP), a position (FP), a direction indicator (DI) (upstream, downstream and
direction indicator (DI) (upstream, downstream and bidirectional) and bidirectional) and some associated Target Values (TV).
some associated Target Values (TV).
A Matching Operator (MO) is associated to each header field A Matching Operator (MO) is associated to each header field
description. The rule is selected if all the MOs fit the TVs for all description. The Rule is selected if all the MOs fit the TVs for all
fields of the incoming packet. fields of the incoming header.
In that case, a Compression/Decompression Action (CDA) associated to In that case, a Compression/Decompression Action (CDA) associated to
each field defines how the compressed and the decompressed values are each field defines how the compressed and the decompressed values are
computed out of each other, for each of the header fields. computed. Compression mainly results in one of 4 actions:
Compression mainly results in one of 4 actions: * send the field
value, * send nothing, * send some least significant bits of the
field or * send an index. After applying the compression there may
be some bits to be sent, these values are called Compression
Residues.
SCHC is a general concept mechanism that can be applied to different o send the field value,
o send nothing,
o send some least significant bits of the field or
o send an index.
After applying the compression there may be some bits to be sent,
these values are called Compression Residues.
SCHC is a general mechanism that can be applied to different
protocols, the exact Rules to be used depend on the protocol and the protocols, the exact Rules to be used depend on the protocol and the
application, and CoAP differs from UDP and IPv6, see Section 3. application. The section 10 of the [rfc8724] describes the
compression scheme for IPv6 and UDP headers. This document targets
the CoAP header compression using SCHC.
1.1. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [rfc2119][rfc8174] when, and only when, they appear in all 14 [RFC2119][rfc8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Applying SCHC to CoAP 2. Applying SCHC to CoAP headers
The SCHC Compression rules can be applied to CoAP flows. 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 [I-D.ietf-lpwan-ipv6-static-context-hc] as described in Section 5 of [rfc8724] and may be used as shown in
may be used as shown in Figure 1. Figure 1.
^ +------------+ ^ +------------+ ^ +------------+ ^ +------------+ ^ +------------+ ^ +------------+
| | CoAP | | | CoAP | inner | | CoAP | | | CoAP | | | CoAP | inner | | CoAP |
| +------------+ v +------------+ x | OSCORE | | +------------+ v +------------+ x | OSCORE |
| | UDP | | DTLS | outer | +------------+ | | UDP | | DTLS | outer | +------------+
| +------------+ +------------+ | | UDP | | +------------+ +------------+ | | UDP |
| | IPv6 | | UDP | | +------------+ | | IPv6 | | UDP | | +------------+
v +------------+ +------------+ | | IPv6 | v +------------+ +------------+ | | IPv6 |
| IPv6 | v +------------+ | IPv6 | v +------------+
+------------+ +------------+
Figure 1: rule scope for CoAP Figure 1: Rule scope for CoAP
Figure 1 shows some examples for CoAP architecture and the SCHC Figure 1 shows some examples for CoAP protocol stacks and the SCHC
rule's scope. Rule's scope.
In the first example, a rule compresses the complete header stack In the first example, a Rule compresses the complete header stack
from IPv6 to CoAP. In this case, SCHC C/D (Static Context Header from IPv6 to CoAP. In this case, SCHC C/D (Static Context Header
Compression Compressor/Decompressor) is performed at the Sender and Compression Compressor/Decompressor) is performed at the Sender and
at the Receiver. at the Receiver.
In the second example, an end-to-end encryption mechanisms is used In the second example, the SCHC compression is applied in the CoAP
between the Sender and the Receiver. The SCHC compression is applied layer, compressing the CoAP header independently of the other layers.
in the CoAP layer compressing the CoAP header independently of the The RuleID and the Compression Residue are encrypted using a
other layers. The rule ID 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. Layers below may also be compressed using other SCHC as defined in [rfc8724] document. This use case realizes an End-to-
rules (this is out of the scope of this document) as defined in the End context initialization between the sender and the receiver, see
SCHC [I-D.ietf-lpwan-ipv6-static-context-hc] document. Appendix A.
In the third example, OSCORE [rfc8613] is used. In this case, two In the third example, the Object Security for Constrained RESTful
rulesets are used to compress the CoAP message. A first ruleset Environments (OSCORE) [rfc8613] is used. In this case, two rulesets
focused on the inner header and is applied end to end by both ends. are used to compress the CoAP message. A first ruleset focused on
A second ruleset compresses the outer header and the layers below and the inner header and is applied end to end by both ends. A second
is done between the Sender and the Receiver. ruleset compresses the outer header and the layers below and is done
between the Sender and the Receiver.
3. CoAP Compression with SCHC 3. CoAP Headers compressed with SCHC
SCHC with CoAP will be used exactly the same way as it is applied in The use of SCHC over the CoAP header uses the same description and
any protocol as IP or UDP with the difference that the fields compression/decompression techniques as the one for IP and UDP
description needs to be defined based on both headers and target explained in the [rfc8724]. For CoAP, SCHC Rules description uses
values of the request and the responses. SCHC Rules description use the direction information to optimize the compression by reducing the
the direction information to optmize the compression by reducing the number of Rules needed to compress headers. The field description
number of Rules needed to compress traffic. CoAP compression follows MAY define both request/response headers and target values in the
the [I-D.ietf-lpwan-ipv6-static-context-hc] scheme and as for other same Rule, using the DI (direction indicator) to make the difference.
protocols, if no valid Rule was found, then the packet MUST be sent As for other protocols, when the compressor does not find a correct
uncompressed using the RuleID dedicated to this purpose and the Rule to compress the header, the packet MUST be sent uncompressed
Compression Residue is the complete header of the packet. See using the RuleID dedicated to this purpose, and the Compression
section 6 of [I-D.ietf-lpwan-ipv6-static-context-hc]. Residue is the complete header of the packet. See section 6 of
[rfc8724].
3.1. Differences between CoAP and UDP/IP 3.1. Differences between CoAP and UDP/IP Compression
CoAP differs from IPv6 and UDP protocols on the following aspects: CoAP compression differs from IPv6 and UDP compression on the
following aspects:
o IPv6 and UDP are not request and response protocols as CoAP, and o The CoAP protocol is asymmetric, the headers are different for a
so the same header fields are used in all packets for all request or a response. For example, the URI-path option is
directions, with the value of some fields being swapped on the mandatory in the request, and it is not present in the response, a
return path (e.g. source and destination addresses fields). The request may contain an Accept option, and the response may include
CoAP headers instead are asymmetric, the headers are different for a Content option. In comparison, IPv6 and UDP returning path swap
a request or a response. For example, the URI-path option is the value of some fields in the header.
mandatory in the request and is not found in the response, a But all the directions have the same fields (e.g., source and
request may contain an Accept option and the response may contain destination addresses fields).
a Content option.
The [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a The [rfc8724] defines the use of a Direction Indicator (DI) in the
Direction Indicator (DI) in the Field Description, which allows a Field Description, which allows a single Rule to process message
single Rule to process message headers differently depending on headers differently depending on the direction.
the direction.
o Even when a field is "symmetric" (i.e. found in both directions) o Even when a field is "symmetric" (i.e., found in both directions),
the values carried in each direction are different. To performs the values carried in each direction are different.
the compression a matching list in the TV might be use because The compression may use a matching list in the TV to limit the
this allows reducing the range of expected values in a particular range of expected values in a particular direction and therefore
direction and therefore reduces the size of the reduces the size of the Compression Residue. Through the
compression residue. For instance, if a client sends only CON Direction Indicator (DI), a field description in the Rules splits
requests, the type can be elided by compression and the answer may the possible field value into two parts, one for each direction.
use one single bit to carry either the ACK or RST type. In CoAP For instance, if a client sends only CON requests, the type can be
some fields have the same behavior, for example the field Code can elided by compression, and the answer may use one single bit to
have 0.0X code format value in the request and Y.ZZ code format in carry either the ACK or RST type. The field Code have as well the
the response. Through the direction indicator, a field same behavior, the 0.0X code format value in the request and Y.ZZ
description in the Rules splits the possible field value in two code format in the response.
parts. Resulting in a smaller compression residue.
o In IPv6 and UDP, header fields have a fixed size, defined in the o Headers in IPv6 and UDP have a fixed size. The size is not sent
Rule, which is not sent. In CoAP, some fields in the header have as part of the Compression Residue, but is defined in the Rule.
a variable length, for example the Token size may vary from 0 to 8 Some CoAP header fields have variable lengths, so the length is
bytes, the length is given by a field in the header. The CoAP also specified in the Field Description. For example, the Token
options are described using the Type-Length-Value encoding format. size may vary from 0 to 8 bytes. And the CoAP options have a
variable length since they use the Type-Length-Value encoding
format, as URI-path or URI-query.
Section 7.5.2 from [I-D.ietf-lpwan-ipv6-static-context-hc] offers Section 7.5.2 from [rfc8724] offers the possibility to define a
the possibility to define a function for the Field Length in the function for the Field length in the Field Description to have
Field Description to have knwoledge of the length before knowledge of the length before compression. When doing SCHC
compression. When doing SCHC compression of a variable length compression of a variable-length field,
field two cases may be raised after applying the CDA: * The result if the field size is not known, the Field Length in the Rule is
of the compression is of fixed length and the compressed value is set as variable and the size is sent with the Compression Residue.
sent in the residue. * Or the result of the compression is of
variable-length and in this case, the size is sent with the
compressed value in the residue.
o In CoAP headers, a field can appear several times. This is o A field can appear several time in the CoAP headers. This is
typical for elements of a URI (path or queries). The SCHC typical for elements of a URI (path or queries). The SCHC
specification [I-D.ietf-lpwan-ipv6-static-context-hc] allows a specification [rfc8724] allows a Field ID to appear several times
Field ID to appears several times in the rule, and uses the Field in the Rule, and uses the Field Position (FP) to identify the
Position (FP) to identify the correct instance, and thereby correct instance, and thereby removing the ambiguity of the
removing the ambiguity of the matching operation. matching operation.
o Field sizes defined in the CoAP protocol can be too large o Field sizes defined in the CoAP protocol can be too large
regarding LPWAN traffic constraints. This is particularly true regarding LPWAN traffic constraints. This is particularly true
for the Message ID field and the Token field. SCHC uses different for the Message ID field and the Token field. SCHC uses different
Matching operators (MO) to performs the compression, see section Matching operators (MO) to perform the compression, see section
7.4 of [I-D.ietf-lpwan-ipv6-static-context-hc]. In this case the 7.4 of [rfc8724]. In this case the Most Significant Bits (MSB) MO
Most Significant Bits (MSB) MO can be applied to reduce the can be applied to reduce the information carried on LPWANs.
information carried on LP
4. Compression of CoAP header fields 4. Compression of CoAP header fields
This section discusses the compression of the different CoAP header This section discusses the compression of the different CoAP header
fields. The CoAP compression with SCHC follows the Section 7.1 of fields. The CoAP compression with SCHC follows the Section 7.1 of
[I-D.ietf-lpwan-ipv6-static-context-hc]. [rfc8724].
4.1. CoAP version field 4.1. CoAP version field
CoAP version is bidirectional and MUST be elided during the SCHC CoAP version is bidirectional and MUST be elided during the SCHC
compression, since it always contains the same value. In the future, compression, since it always contains the same value. In the future,
if new versions of CoAP are defined, new rules will be needed to if new versions of CoAP are 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 type of messages: two request The CoAP Protocol [rfc7252] has four type of messages: two request
(CON, NON); one response (ACK) and one empty message (RST). (CON, NON); one response (ACK) and one empty message (RST).
The field SHOULD be elided if for instance a client is sending only The field SHOULD be elided if for instance a client is sending only
NON or only CON messages. For the RST message a dedicated Rule may NON or only CON messages. For the RST message a dedicated Rule may
be needed. For other usages a mapping list can be used. be needed. For other usages a mapping list can be used.
4.3. CoAP code field 4.3. CoAP code field
The code field indicates the Request Method used in CoAP, a registry The code field indicates the Request Method used in CoAP, a IANA
is given in section 12.1 of [rfc7252]. The compression of the CoAP registry [rfc7252]. The compression of the CoAP code field follows
code field follows the same principle as that of the CoAP type field. the same principle as that of the CoAP type field. If the device
If the device plays a specific role, the set of code values can be plays a specific role, the set of code values can be split in two
split in two parts, the request codes with the 0 class and the parts, the request codes with the 0 class and the response values.
response values.
If the device only implements a CoAP client, the request code can be If the device only implements a CoAP client, the request code can be
reduced to the set of requests the client is able to process. reduced to the set of requests the client is able to process.
A mapping list can be used for known values, for other values the A mapping list can be used for known values. For other values the
field cannot be compressed an the value needs to be sent in the field cannot be compressed an the value needs to be sent in the
residue. Compression Residue.
4.4. CoAP Message ID field 4.4. CoAP Message ID field
The Message ID field can be compressed with the MSB(x) MO and the The Message ID field can be compressed with the MSB(x) MO and the
Least Significant Bits (LSB) CDA, see section 7.4 of Least Significant Bits (LSB) CDA, see section 7.4 of [rfc8724].
[I-D.ietf-lpwan-ipv6-static-context-hc].
4.5. CoAP Token fields 4.5. CoAP Token fields
Token is defined through two CoAP fields, Token Length in the Token is defined through two CoAP fields, Token Length in the
mandatory header and Token Value directly following the mandatory mandatory header and Token Value directly following the mandatory
CoAP header. CoAP header.
Token Length is processed as any protocol field. If the value does Token Length is processed as any protocol field. If the value does
not change, the size can be stored in the TV and elided during the not change, the size can be stored in the TV and elided during the
transmission. Otherwise, it will have to be sent in the compression transmission. Otherwise, it will have to be sent in the Compression
residue. Residue.
Token Value MUST not be sent as a variable length residue to avoid Token Value MUST NOT be sent as a variable length residue to avoid
ambiguity with Token Length. Therefore, Token Length value MUST be ambiguity with Token Length. Therefore, Token Length value MUST be
used to define the size of the residue. A specific function used to define the size of the Compression Residue. A specific
designated as "TKL" MUST be used in the Rule. During the function designated as "TKL" MUST be used in the Rule. During the
decompression, this function returns the value contained in the Token decompression, this function returns the value contained in the Token
Length field. Length field.
5. CoAP options 5. CoAP options
CoAP defines options that are placed after the based header in Option
Numbers order, see [rfc7252]. Each Option instance in a message uses
the format Delta-Type (D-T), Length (L), Value (V). When applying
SCHC compression to the Option, the D-T, L, and V format serves to
make the Rule description of the Option. The SCHC compression builds
the description of the Option by using in the Field ID the Option
Number built from D-T; in TV, the Option Value; and the Option Length
uses section 7.4 of RFC8724. When the Option Length has a wellknown
size it can be stored in the Rule. Therefore, SCHC compression does
not send it. Otherwise, SCHC Compression carries the length of the
Compression Residue in addition to the Compression Residue value.
Note that length coding differs between CoAP options and SCHC
variable size Compression Residue.
The following sections present how SCHC compresses some specific CoAP
Options.
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 These fields are both unidirectional and MUST NOT be set to
bidirectional in a rule entry. 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 residue. Otherwise, the value has to used to limit the size of the Compression Residue. Otherwise, the
be sent as a residue (fixed or variable length). value has to be sent as a Compression Residue (fixed or variable
length).
5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields 5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields
These fields are unidirectional and MUST NOT be set to bidirectional These fields are unidirectional and MUST NOT be set to bidirectional
in a rule DI entry. see section 7.1 of in a Rule DI entry, see section 7.1 of [rfc8724]. They are used only
[I-D.ietf-lpwan-ipv6-static-context-hc]. They are used only by the by the server to inform of the caching duration and is never found in
server to inform of the caching duration and is never found in client client requests.
requests.
If the duration is known by both ends, the value can be elided on the If the duration is known by both ends, the value can be elided.
LPWAN.
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 residue (fixed or variable Otherwise these options can be sent as a Compression Residue (fixed
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 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 in a Rule entry. They are used only by the client to access a
specific resource and are never found in server responses. 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 | | 3|up| |ignore |value-sent | |URI-Path |var| 3|up| |ignore |value-sent |
+-------------+--+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
Figure 2: complex path example Figure 2: complex path example
In Figure 2 a single bit residue can be used to code one of the 2 In Figure 2 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. 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
bits. bits.
The length sent at the beginning of a variable length residue The length sent at the beginning of a variable length residue
indicates the size of the LSB in bytes. indicates the size of the LSB in bytes.
For instance for a CORECONF path /c/X6?k="eth0" the rule can be set For instance for a CORECONF path /c/X6?k="eth0" the Rule can be set
to: to:
+-------------+---+--+--+--------+---------+-------------+ +-------------+---+--+--+--------+---------+-------------+
| 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 3: CORECONF URI compression
Figure 3 shows the parsing and the compression of the URI, where c is Figure 3 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 the 4 bits of the variable residue size. See section 7.5.2 This adds 4 bits to the variable Residue size. See section 7.5.2
[I-D.ietf-lpwan-ipv6-static-context-hc] [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 These fields are unidirectional and MUST NOT be set to bidirectional
in a rule DI entry, see section 7.1 of the in a Rule DI entry, see section 7.1 of the [rfc8724]. They are used
[I-D.ietf-lpwan-ipv6-static-context-hc]. They are used only by the only by the client to access a specific resource and are never found
client to access a specific resource and are never found in server in server response.
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 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 are compatible. If a block includes a fragmentation protocol. They can be both used. If a
option is used, its content MUST be sent as a compression residue. block option is used, its content MUST be sent as a Compression
Residue.
6.2. Observe 6.2. Observe
The [rfc7641] defines the Observe option. The TV is not set, MO is The [rfc7641] defines the Observe option. The TV is not set, MO is
set to "ignore" and the CDA is set to "value-sent". SCHC does not set to "ignore" and the CDA is set to "value-sent". SCHC does not
limit the maximum size for this option (3 bytes). To reduce the limit the maximum size for this option (3 bytes). To reduce the
transmission size, either the device implementation MAY limit the transmission size, either the device implementation MAY limit the
delta between two consecutive values, or a proxy can modify the delta between two consecutive values, or a proxy can modify the
increment. increment.
Since an RST message may be sent to inform a server that the client Since an RST message may be sent to inform a server that the client
does not require Observe response, a rule MUST allow the transmission does not require Observe response, a Rule MUST allow the transmission
of this message. of this message.
6.3. No-Response 6.3. No-Response
The [rfc7967] defines a No-Response option limiting the responses The [rfc7967] defines a No-Response option limiting the responses
made by a server to a request. If the value is known by both ends, made by a server to a request. If the value is known by both ends,
then TV is set to this value, MO is set to "equal" and CDA is set to then TV is set to this value, MO is set to "equal" and CDA is set to
"not-sent". "not-sent".
Otherwise, if the value is changing over time, TV is not set, MO is Otherwise, if the value is changing over time, TV is not set, MO is
set to "ignore" and CDA to "value-sent". A matching list can also be set to "ignore" and CDA to "value-sent". A matching list can also be
used to reduce the size. used to reduce the size.
6.4. OSCORE 6.4. OSCORE
OSCORE [rfc8613] defines end-to-end protection for CoAP messages. OSCORE [rfc8613] defines end-to-end protection for CoAP messages.
This section describes how SCHC rules can be applied to compress This section describes how SCHC Rules can be applied to compress
OSCORE-protected messages. OSCORE-protected messages.
0 1 2 3 4 5 6 7 <--------- n bytes -------------> 0 1 2 3 4 5 6 7 <--------- n bytes ------------->
+-+-+-+-+-+-+-+-+--------------------------------- +-+-+-+-+-+-+-+-+---------------------------------
|0 0 0|h|k| n | Partial IV (if any) ... |0 0 0|h|k| n | Partial IV (if any) ...
+-+-+-+-+-+-+-+-+--------------------------------- +-+-+-+-+-+-+-+-+---------------------------------
| | | | | |
|<-- CoAP -->|<------ CoAP OSCORE_piv ------> | |<-- CoAP -->|<------ CoAP OSCORE_piv ------> |
OSCORE_flags OSCORE_flags
skipping to change at page 12, line 8 skipping to change at page 12, line 25
| 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 4: 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 4.
The first byte is used for flags that specify the contents of the The first byte specifies the content of the OSCORE options using
OSCORE option. The 3 most significant bits of this byte are reserved flags. The three most significant bits of this byte are reserved and
and always set to 0. Bit h, when set, indicates the presence of the always set to 0. Bit h, when set, indicates the presence of the kid
kid context field in the option. Bit k, when set, indicates the context field in the option. Bit k, when set, indicates the presence
presence of a kid field. The 3 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 kid The flag byte is followed by the piv field, kid context field, and
field in this order and if present; the length of the kid context kid field in this order, and if present, the length of the kid
field is encoded in the first byte denoting by s the length of the context field is encoded in the first byte denoting by s the length
kid context in bytes. of the kid context in bytes.
This specification recommends to identify the OSCORE Option and the
fields it contains.
Conceptually, it discerns up to 4 distinct pieces of information This specification recommends identifying the OSCORE Option and the
within the OSCORE option: the flag bits, the piv, the kid context, fields it contains Conceptually, it discerns up to 4 distinct pieces
and the kid. It is thus recommended that the parser split the OSCORE of information within the OSCORE option: the flag bits, the piv, the
option into the 4 subsequent fields: kid context, and the kid. The SCHC Rule splits into four field
descriptions the OSCORE option to compress them:
o CoAP OSCORE_flags, o CoAP OSCORE_flags,
o CoAP OSCORE_piv, o CoAP OSCORE_piv,
o CoAP OSCORE_kidctxt, o CoAP OSCORE_kidctxt,
o CoAP OSCORE_kid. o CoAP OSCORE_kid.
These fields are shown superimposed on the OSCORE Option format in The OSCORE Option shows superimposed these four fields using the
Figure 4, the CoAP OSCORE_kidctxt field including the size bits s. format Figure 4, the CoAP OSCORE_kidctxt field includes the size bits
Their size SHOULD be reduced using SCHC compression. 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 5.
Rule ID 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,| | || |
skipping to change at page 13, line 35 skipping to change at page 13, line 48
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).
Note that a request sent by a client located in an Application Server Note that a request sent by a client located in an Application Server
to a server located in the device, may not be compressed through this to a server located in the device, may not be compressed through this
rule since the MID will not start with 7 bits equal to 0. A CoAP Rule since the MID will not start with 7 bits equal to 0. A CoAP
proxy, before the core SCHC C/D can rewrite the message ID to a value proxy, before the core SCHC C/D can rewrite the message ID to a value
matched by the rule. matched by the Rule.
7.2. OSCORE Compression 7.2. OSCORE Compression
OSCORE aims to solve the problem of end-to-end encryption for CoAP OSCORE aims to solve the problem of end-to-end encryption for CoAP
messages. The goal, therefore, is to hide as much of the message as messages. 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
skipping to change at page 15, line 44 skipping to change at page 15, line 44
+------+-------------------+ | Payload | +------+-------------------+ | Payload |
| 0xFF | | | | 0xFF | | |
+------+ +-------------------+ +------+ +-------------------+
Figure 6: A CoAP message is split into an OSCORE outer and plaintext Figure 6: A CoAP message is split into an OSCORE outer and plaintext
Figure 6 shows the message format for the OSCORE Message and Figure 6 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.
skipping to change at page 16, line 47 skipping to change at page 16, line 47
+----------------------------------+ +----------------------------------+
Figure 7: OSCORE message Figure 7: 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 8.
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.
Outer Message OSCORE Plaintext Outer Message OSCORE Plaintext
+-+-+---+--------+---------------+ +-------+ +-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code | |v|t|tkl|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+ +-+-+---+--------+---------------+....+ +-------+-----......+
skipping to change at page 17, line 30 skipping to change at page 17, line 30
+------+------------+ +-------------------+ +------+------------+ +-------------------+
| Ciphertext |<---------\ | | Ciphertext |<---------\ |
| | | v | | | v
+-------------------+ | +-----------------+ +-------------------+ | +-----------------+
| | | Inner SCHC | | | | Inner SCHC |
v | | Compression | v | | Compression |
+-----------------+ | +-----------------+ +-----------------+ | +-----------------+
| Outer SCHC | | | | Outer SCHC | | |
| Compression | | v | Compression | | v
+-----------------+ | +-------+ +-----------------+ | +-------+
| | |Rule ID| | | |RuleID |
v | +-------+--+ v | +-------+--+
+--------+ +------------+ | Residue | +--------+ +------------+ | Residue |
|Rule ID'| | Encryption | <--- +----------+--------+ |RuleID' | | Encryption | <--- +----------+--------+
+--------+--+ +------------+ | | +--------+--+ +------------+ | |
| Residue' | | Payload | | Residue' | | Payload |
+-----------+-------+ | | +-----------+-------+ | |
| Ciphertext | +-------------------+ | Ciphertext | +-------------------+
| | | |
+-------------------+ +-------------------+
Figure 8: OSCORE Compression Diagram Figure 8: 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 9
Original message: Original message:
================= =================
0x4101000182bb74656d7065726174757265 0x4101000182bb74656d7065726174757265
skipping to change at page 19, line 5 skipping to change at page 19, line 5
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 10: CoAP CONTENT Response
TheSCHC Rules for the Inner Compression include all fields that are The SCHC Rules for the Inner Compression include all fields that are
alreadypresent in a regular CoAP message. The methods described in already present in a regular CoAP message. The methods described in
section Section 4 applies to these fields. As an example, see Section 4 applies to these fields. As an example, see Figure 11.
Figure 11.
Rule ID 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 11: Inner SCHC Rules
Figure 12 shows the Plaintext obtained for our example GET Request Figure 12 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 Rule ID). 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
exchange. exchange.
________________________________________________________ ________________________________________________________
| | | |
| OSCORE Plaintext | | OSCORE Plaintext |
skipping to change at page 20, line 28 skipping to change at page 20, line 28
| |
| Inner SCHC Compression | Inner SCHC Compression
| |
v v
_________________________________ _________________________________
| | | |
| Compressed Plaintext | | Compressed Plaintext |
| | | |
| 0x00 | | 0x00 |
| | | |
| Rule ID = 0x00 (1 byte) | | RuleID = 0x00 (1 byte) |
| (No residue) | | (No residue) |
|_________________________________| |_________________________________|
| |
| AEAD Encryption | AEAD Encryption
| (piv = 0x04) | (piv = 0x04)
v v
_________________________________________________ _________________________________________________
| | | |
| encrypted_plaintext = 0xa2 (1 byte) | | encrypted_plaintext = 0xa2 (1 byte) |
skipping to change at page 21, line 32 skipping to change at page 21, line 32
| |
| Inner SCHC Compression | Inner SCHC Compression
| |
v v
__________________________________________ __________________________________________
| | | |
| Compressed Plaintext | | Compressed Plaintext |
| | | |
| 0x001919902180 (6 bytes) | | 0x001919902180 (6 bytes) |
| | | |
| 00 Rule ID | | 00 RuleID |
| | | |
| 0b0 (1 bit match-map residue) | | 0b0 (1 bit match-map 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 13: Plaintext compression and encryption for CONTENT Response Figure 13: Plaintext compression and encryption for CONTENT Response
The Outer SCHC Rules (Figure 16) MUST process the OSCORE Options The Outer SCHC Rules (Figure 16) must process the OSCORE Options
fields. In Figure 14 and Figure 15 we show a dump of the OSCORE fields. In Figure 14 and Figure 15 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 24, line 10 skipping to change at page 24, line 10
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 16 shows a possible set of Outer Rules to compress the Outer
Header. Header.
Rule ID 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 || |
|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| 68 |equal |not-sent || | |CoAP Code | |dw| 68 |equal |not-sent || |
skipping to change at page 25, line 8 skipping to change at page 25, line 8
Figure 16: Outer SCHC Rules Figure 16: 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 17 and
Figure 18. Figure 18.
Compressed message: Compressed message:
================== ==================
0x001489458a9fc3686852f6c4 (12 bytes) 0x001489458a9fc3686852f6c4 (12 bytes)
0x00 Rule ID 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 17: SCHC-OSCORE Compressed GET Request
Compressed message: Compressed message:
================== ==================
0x0014218daf84d983d35de7e48c3c1852 (16 bytes) 0x0014218daf84d983d35de7e48c3c1852 (16 bytes)
0x00 Rule ID 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 18: 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 19.
Rule ID 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 |
skipping to change at page 26, line 30 skipping to change at page 26, line 30
+---------------+--+--+-----------+---------+-----------++--------+ +---------------+--+--+-----------+---------+-----------++--------+
Figure 19: SCHC-CoAP Rules (No OSCORE) Figure 19: SCHC-CoAP Rules (No OSCORE)
This yields the results in Figure 20 for the Request, and Figure 21 This yields the results in Figure 20 for the Request, and Figure 21
for the Response. for the Response.
Compressed message: Compressed message:
================== ==================
0x0114 0x0114
0x01 = Rule ID 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 20: CoAP GET Compressed without OSCORE
Compressed message: Compressed message:
================== ==================
0x010a32332043 0x010a32332043
0x01 = Rule ID 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 21: 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
This document does not have any more Security consideration than the The Security Considerations of SCHC header compression RFC8724 are
ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc]. valid for SCHC CoAP header compression. When CoAP uses OSCORE, the
Variable length residues may be used to compress URI elements. They security considerations defined in RFC8613 does not change when SCHC
cannot produce a packet expansion either on the LPWAN network or in header compression is applied.
the Internet network after decompression. The length send is not
used to indicate the information that should be reconstructed at the The definition of SCHC over CoAP header fields permits the
other end, but on the contrary the information sent as a Residue. compression of header information only. The SCHC header compression
Therefore, if a length is set to a high value, but the number of bits itself does not increase or reduce the level of security in the
on the SCHC packet is smaller, the packet must be dropped by the communication. When the communication does not use any security
decompressor. protocol as OSCORE, DTLS, or other. It is highly necessary to use a
layer two security.
DoS attacks are possible if an intruder can introduce a compressed
SCHC corrupted packet onto the link and cause a compression
efficiency reduction. However, an intruder having the ability to add
corrupted packets at the link layer raises additional security issues
than those related to the use of header compression.
SCHC compression returns variable-length Residues for some CoAP
fields. In the compressed header, the length sent is not the
original header field length but the length of the Residue. So if a
corrupted packet comes to the decompressor with a longer or shorter
length than the one in the original header, SCHC decompression will
detect an error and drops the packet.
OSCORE compression is also based on the same compression method OSCORE compression is also based on the same compression method
described in [I-D.ietf-lpwan-ipv6-static-context-hc]. The size of described in [rfc8724]. The size of the Initialisation Vector (IV)
the Initialisation Vector residue size must be considered carefully. residue size must be considered carefully. A too large value has an
A too large value has a impact on the compression efficiency and a impact on the compression efficiency and a too small value will force
too small value will force the device to renew its key more often. the device to renew its key more often. This operation may be long
This operation may be long and energy consuming. and energy consuming. The size of the compressed IV MUST be choosen
regarding the highest expected traffic from the device.
SCHC header and compression Rules MUST remain tightly coupled.
Otherwise, an encrypted residue may be decompressed in a different
way by the receiver. To avoid this situation, if the Rule is
modified in one location, the OSCORE keys MUST be re-established.
10. Acknowledgements 10. Acknowledgements
The authors would like to thank Dominique Barthel, Carsten Bormann, The authors would like to thank (in alphabetic order): Christian
Thomas Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov, Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
Goran Selander. Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran
Selander.
11. Normative References 11. Normative References
[I-D.ietf-lpwan-ipv6-static-context-hc] [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
Zuniga, "Static Context Header Compression (SCHC) and
fragmentation for LPWAN, application to UDP/IPv6", draft-
ietf-lpwan-ipv6-static-context-hc-24 (work in progress),
December 2019.
[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>.
[rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained [rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014, DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>. <https://www.rfc-editor.org/info/rfc7252>.
[rfc7641] Hartke, K., "Observing Resources in the Constrained [rfc7641] Hartke, K., "Observing Resources in the Constrained
skipping to change at page 29, line 5 skipping to change at page 29, line 19
[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.
Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
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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