< draft-ietf-lpwan-coap-static-context-hc-08.txt   draft-ietf-lpwan-coap-static-context-hc-09.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 30, 2019 Institut MINES TELECOM; IMT Atlantique Expires: January 7, 2020 Institut MINES TELECOM; IMT Atlantique
R. Andreasen R. Andreasen
Universidad de Buenos Aires Universidad de Buenos Aires
May 29, 2019 July 06, 2019
LPWAN Static Context Header Compression (SCHC) for CoAP LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-08 draft-ietf-lpwan-coap-static-context-hc-09
Abstract Abstract
This draft defines the way SCHC header compression can be applied to This draft defines the way SCHC header compression can be applied to
CoAP headers. The CoAP header structure differs from IPv6 and UDP CoAP headers. The CoAP header structure differs from IPv6 and UDP
protocols since CoAP protocols since CoAP uses a flexible header with a variable number of
uses a flexible header with a variable number of options themselves options, themselves of variable length. The CoAP protocol is
of variable length. The CoAP protocol is asymmetric in its message asymmetric in its message format: the format of the packet header in
format, the format of the header packet in the request messages is the request messages is different from that in the response messages.
different from that in the response messages. Most of the Most of the compression mechanisms have been introduced in
compression mechanisms have been introduced in
[I-D.ietf-lpwan-ipv6-static-context-hc], this document explains how [I-D.ietf-lpwan-ipv6-static-context-hc], this document explains how
to use the SCHC compression for CoAP. to use the SCHC compression for CoAP.
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 November 30, 2019. This Internet-Draft will expire on January 7, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 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
skipping to change at page 2, line 28 skipping to change at page 2, line 25
2. SCHC Compression Process . . . . . . . . . . . . . . . . . . 3 2. SCHC Compression Process . . . . . . . . . . . . . . . . . . 3
3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 4 3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 4
4. Compression of CoAP header fields . . . . . . . . . . . . . . 6 4. Compression of CoAP header fields . . . . . . . . . . . . . . 6
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 6 4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 6
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 6 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 6
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 6 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 6
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 6 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 6
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 7 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 7 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 7
5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri- 5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 7
Port fields . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8 5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8
5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 8 5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 8
5.3.2. Variable number of path or query elements . . . . . . 9 5.3.2. Variable number of path or query elements . . . . . . 9
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
fields . . . . . . . . . . . . . . . . . . . . . . . . . 9 fields . . . . . . . . . . . . . . . . . . . . . . . . . 9
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 . . . . . . . . . . . . . . . . 9 and Location-Query fields . . . . . . . . . . . . . . . . 9
6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 10 6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 10 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 10
6.4. Time Scale . . . . . . . . . . . . . . . . . . . . . . . 10 6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.5. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11 7. Examples of CoAP header compression . . . . . . . . . . . . . 11
7. Examples of CoAP header compression . . . . . . . . . . . . . 12 7.1. Mandatory header with CON message . . . . . . . . . . . . 11
7.1. Mandatory header with CON message . . . . . . . . . . . . 12 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 12
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 13 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 16
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 9. Security considerations . . . . . . . . . . . . . . . . . . . 26
9. Security considerations . . . . . . . . . . . . . . . . . . . 27 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27 11. Normative References . . . . . . . . . . . . . . . . . . . . 26
11. Normative References . . . . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction 1. Introduction
CoAP [rfc7252] is an implementation of the REST architecture for CoAP [rfc7252] is an implementation of the REST architecture for
constrained devices. Although CoAP was designed for constrained constrained devices. Although CoAP was designed for constrained
devices, the size of a CoAP header may still be too large for LPWAN devices, the size of a CoAP header may still be too large for the
constraints and some compression may be needed to reduce the header constraints of Low Power Wide Area Networks (LPWAN) and some
size. compression may be needed to reduce the header size.
[I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression [I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
mechanism for LPWAN network based on a static context. The context mechanism for LPWAN network based on a static context. The context
is said static since the field description composing the Rules are is said static since the field description composing the Rules are
not learned during the packet exchanges but are previously defined. not learned during the packet exchanges but are previously defined.
The context(s) is(are) known by both ends before transmission. The context(s) is(are) known by both ends before transmission.
A context is composed of a set of rules that are referenced by Rule A context is composed of a set of rules that are referenced by Rule
IDs (identifiers). A rule contains an ordered list of the fields IDs (identifiers). A rule contains an ordered list of the fields
descriptions containing a field ID (FID), its length (FL) and its descriptions containing a field ID (FID), its length (FL) and its
position (FP), a direction indicator (DI) (upstream, downstream and position (FP), a direction indicator (DI) (upstream, downstream and
bidirectional) and some associated Target Values (TV). Target Value bidirectional) and some associated Target Values (TV). Target Value
indicates the value that can be expected. TV can also be a list of indicates the value that can be expected. TV can also be a list of
values. A Matching Operator (MO) is associated to each header field values. 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. In that case, a Compression/Decompression Action (CDA) fields of the incoming packet. In that case, a Compression/
associated to each field defines the link between the compressed and Decompression Action (CDA) associated to each field defines how the
decompressed value for each of the header fields. Compression compressed and the decompressed values are computed out of each
results mainly in 4 actions: send the field value, send nothing, send other, for each of the header fields. Compression mainly results in
less significant bits of a field, send an index. Values sent are one of 4 actions: send the field value, send nothing, send some least
called Compression Residues and follows the rule ID. significant bits of the field or send an index. Values sent are
called Compression Residues and follow the rule ID in the transmitted
message.
The compression rules define a generic way to compress and decompress
the fields. If the device is modified, for example, to introduce new
functionalities or new CoAP options, the rules must be updated to
reflect the evolution. There is no risk to lock a device in a
particular version of CoAP.
2. SCHC Compression Process 2. SCHC Compression Process
The SCHC Compression rules can be applied to CoAP flows. SCHC The SCHC Compression rules can be applied to CoAP flows. 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
above layers (IPv6/UDP) or independently. The SCHC adaptation layers lower layers (IPv6/UDP) or independently. The SCHC adaptation layers
as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used
as shown in Figure 1. as shown in 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 architecture and the SCHC
rule's scope. A rule can cover all headers from IPv6 to CoAP, in rule's scope.
which case SCHC C/D is performed at the device and at the LPWAN
boundary. If an end-to-end encryption mechanisms is used between the In the first example, a rule compresses all headers from IPv6 to
device and the application, CoAP MAY be compressed independently of CoAP. In this case, SCHC C/D is performed at the device and at the
the other layers. The rule ID and the compression residue are LPWAN boundary.
encrypted using a mechanism such as DTLS. Only the other end can
decipher the information. In the second example, an end-to-end encryption mechanisms is used
between the device and the application. CoAP is compressed
independently of the other layers. The rule ID and the compression
residue are encrypted using a mechanism such as DTLS. Only the other
end can decipher the information.
Layers below may also be compressed using other SCHC rules (this is Layers below may also be compressed using other SCHC rules (this is
out of the scope of this document). OSCORE out of the scope of this document).
[I-D.ietf-core-object-security] can also define 2 rules to compress
the CoAP message. A first rule focuses on the inner header and is In the third example, OSCORE [I-D.ietf-core-object-security] is used.
end to end, a second rule may compress the outer header and the 2 rulesets are used to compress the CoAP message. A first ruleset
layers below. SCHC C/D for inner header is done by both ends, SCHC focuses on the inner header and is end to end, a second ruleset
C/D for outer header and other headers is done between the device and compresses the outer header and the layers below. SCHC C/D for inner
the LPWAN boundary. header is done by both ends, and SCHC C/D for outer header and other
headers is done between the device and the LPWAN boundary.
3. CoAP Compression with SCHC 3. CoAP Compression with SCHC
CoAP differs from IPv6 and UDP protocols on the following aspects: CoAP differs from IPv6 and UDP protocols on the following aspects:
o IPv6 and UDP are symmetrical protocols. The same fields are found o IPv6 and UDP are symmetrical protocols. The same fields are found
in the request and in the response, only the location in the in the request and in the response, with the value of some fields
header may vary (e.g. source and destination fields). A CoAP being swapped on the return path (e.g. source and destination
request is different from a response. For example, the URI-path fields). A CoAP request is intrinsically different from a
option is mandatory in the request and is not found in the response. For example, the URI-path option is mandatory in the
response, a request may contain an Accept option and the response request and is not found in the response, a request may contain an
a Content option. Accept option and the response a Content option.
[I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a
message direction (DI) in the Field Description, which allows a message direction (DI) in the Field Description, which allows a
single Rule to process message headers differently in both single Rule to process message headers differently depending of
directions. 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. Combined with the values carried in each direction are different. Combined with
a matching list in the TV, this allows reducing the range of a matching list in the TV, this allows reducing the range of
expected values in a particular direction and therefore reduce the expected values in a particular direction and therefore reduce the
size of the compression residue. For instance, if a client sends size of the compression residue. For instance, if a client sends
only CON request, the type can be elided by compression and the only CON request, the type can be elided by compression and the
answer may use one single bit to carry either the ACK or RST type. answer may use one single bit to carry either the ACK or RST type.
The same behavior can be applied to the CoAP Code field (0.0X code The same behavior can be applied to the CoAP Code field (0.0X code
are present in the request and Y.ZZ in the answer). The direction are present in the request and Y.ZZ in the answer). The direction
skipping to change at page 5, line 25 skipping to change at page 5, line 30
direction. direction.
o In IPv6 and UDP, header fields have a fixed size. In CoAP, Token o In IPv6 and UDP, header fields have a fixed size. In CoAP, Token
size may vary from 0 to 8 bytes, the length being given by a field size may vary from 0 to 8 bytes, the length being given by a field
in the header. More systematically, the CoAP options are in the header. More systematically, the CoAP options are
described using the Type-Length-Value. described using the Type-Length-Value.
[I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to [I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to
define a function for the Field Length in the Field Description. define a function for the Field Length in the Field Description.
o In CoAP headers, a field can be present several times. This is o In CoAP headers, a field can appear several times. This is
typical for elements of an URI (path or queries). The position typical for elements of a URI (path or queries).
defined in a rule, associated to a Field ID, can be used to
identify the proper instance.
[I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field ID to [I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field ID to
appears several times in the rule, the Field Position (FP) removes appears several times in the rule, the Field Position (FP)
ambiguities for the matching operation. identifies the proper instance, thereby removing the ambiguity of
the 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 or Token field. The MSB MO can be used for the message ID field or Token field. The MSB MO can be used
to reduce the information carried on LPWANs. to reduce the information carried on LPWANs.
o CoAP also obeys the client/server paradigm and the compression o CoAP also obeys the client/server paradigm and the compression
ratio can be different if the request is issued from an LPWAN ratio can be different if the request is issued from an LPWAN
device or from an non LPWAN device. For instance a Device (Dev) device or from a non LPWAN device. For instance a Device (Dev)
aware of LPWAN constraints can generate a 1 byte token, but a aware of LPWAN constraints can generate a 1 byte token, but a
regular CoAP client will certainly send a larger token to the Dev. regular CoAP client will certainly send a larger token to the Dev.
SCHC compression will not modify the values to offer a better The SCHC compression-decompression process does not modify the
compression rate. Nevertheless, a proxy placed before the values. Nevertheless, a proxy placed before the compressor may
compressor may change some field values to offer a better change some field values to allow SCHC achieving a better
compression ratio and maintain the necessary context for compression ratio, while maintaining the necessary context for
interoperability with existing CoAP implementations. interoperability with existing CoAP implementations.
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. fields.
4.1. CoAP version field 4.1. CoAP version field
This field is bidirectional and MUST be elided during the SCHC This field 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 defined to if new versions of CoAP are defined, new rules will be defined to
avoid ambiguities between versions. avoid ambiguities between versions.
4.2. CoAP type field 4.2. CoAP type field
[rfc7252] defines 4 types of messages: CON, NON, ACK and RST. The [rfc7252] defines 4 types of messages: CON, NON, ACK and RST. The
last two are a response to the first two. If the device plays a last two are a response to the first two. If the device plays a
specific role, a rule can exploit these properties with the mapping specific client or server role, a rule can exploit these properties
list: [CON, NON] for one direction and [ACK, RST] for the other with the mapping list: [CON, NON] for one direction and [ACK, RST]
direction. Compression residue is reduced to 1 bit. for the other direction. The compression residue is reduced to 1
bit.
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 CON messages. NON or only CON messages.
In any case, a rule MUST be defined to carry RST to a client. In any case, a rule MUST be defined to carry RST to a client.
4.3. CoAP code field 4.3. CoAP code field
The compression of the CoAP code field follows the same principle as The compression of the CoAP code field follows the same principle as
for the CoAP type field. If the device plays a specific role, the that of the CoAP type field. If the device plays a specific role,
set of code values can be split in two parts, the request codes with the set of code values can be split in two parts, the request codes
the 0 class and the response values. with the 0 class and the 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.
All the response codes MUST be compressed with a SCHC rule. All the response codes MUST be compressed with a SCHC rule.
4.4. CoAP Message ID field 4.4. CoAP Message ID field
This field is bidirectional and is used to manage acknowledgments. This field is bidirectional and is used to manage acknowledgments.
The server memorizes the value for a EXCHANGE_LIFETIME period (by The server memorizes the value for a EXCHANGE_LIFETIME period (by
skipping to change at page 8, line 5 skipping to change at page 7, line 48
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. If is not possible, the value used to limit the size of the residue. If is not possible, the value
has to be sent as a residue (fixed or variable length). has to be sent as a residue (fixed or variable length).
5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri-Port 5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields
fields
These fields is unidirectional and MUST NOT be set to bidirectional These fields are unidirectional and MUST NOT be set to bidirectional
in a rule entry. It is used only by the server to inform of the in a rule entry. They are used only by the server to inform of the
caching duration and is never found in client requests. caching duration and is never found in client 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 on the
LPWAN. 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 SHOULD be sent as a residue (fixed or Otherwise these options SHOULD be sent as a residue (fixed or
variable length). variable length).
skipping to change at page 10, line 37 skipping to change at page 10, line 29
[rfc7967] defines a No-Response option limiting the responses made by [rfc7967] defines a No-Response option limiting the responses made by
a server to a request. If the value is known by both ends, then TV 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 "not- is set to this value, MO is set to "equal" and CDA is set to "not-
sent". 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. Time Scale 6.4. OSCORE
The time scale [I-D.toutain-core-time-scale] option allows a client
to inform the server that it is in a constrained network and that
message ID MUST be kept for a duration given by the option.
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 "not-sent".
Otherwise, if the value is changing over time, TV is not set, MO is
set to "ignore" and CDA to "value-sent". A matching list can also be
used to reduce the size.
6.5. OSCORE
OSCORE [I-D.ietf-core-object-security] defines end-to-end protection OSCORE [I-D.ietf-core-object-security] defines end-to-end protection
for CoAP messages. This section describes how SCHC rules can be for CoAP messages. This section describes how SCHC rules can be
applied to compress OSCORE-protected messages. applied to compress 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) ...
+-+-+-+-+-+-+-+-+--------------------------------- +-+-+-+-+-+-+-+-+---------------------------------
| | | | | |
skipping to change at page 12, line 4 skipping to change at page 11, line 29
This draft recommends to implement a parser that is able to identify This draft recommends to implement a parser that is able to identify
the OSCORE Option and the fields it contains. the OSCORE Option and the fields it contains.
Conceptually, it discerns up to 4 distinct pieces of information Conceptually, it discerns up to 4 distinct pieces of information
within the OSCORE option: the flag bits, the piv, the kid context, within the OSCORE option: the flag bits, the piv, the kid context,
and the kid. It is thus recommended that the parser split the OSCORE and the kid. It is thus recommended that the parser split the OSCORE
option into the 4 subsequent fields: option into the 4 subsequent fields:
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 These fields are shown superimposed on the OSCORE Option format in
Figure 4, the CoAP OSCORE_kidctxt field including the size bits s. Figure 4, the CoAP OSCORE_kidctxt field including the size bits s.
Their size SHOULD be reduced using the MSB matching operator. Their size SHOULD be reduced using SCHC compression.
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 a client on the Internet a POST message, which is side receives from a client on the Internet 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 Rule ID 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 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| ML1 |match-map|matching-sent|| CC CCC | |CoAP Code | | |bi|[0.00,| | || |
|CoAP MID | | |bi| 0000 |MSB(7 ) |LSB(9) || M-ID| | | | | | ... | | || |
| | | | | 5.05]|match-map|matching-sent|| CC CCC |
|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 5: CoAP Context to compress header without token
The version and Token Length fields are elided. Code has shrunk to 5 The version and Token Length fields are elided. The 26 method and
bits using a matching list. Uri-Path contains a single element response codes defined in [rfc7252] has been shrunk to 5 bits using a
indicated in the matching operator. matching list. Uri-Path contains a single element indicated in the
matching operator.
Figure 6 shows the time diagram of the exchange. A client in the
Application Server sends a CON request. It can go through a proxy
which reduces the message ID to a smallest value, with at least the 9
most significant bits equal to 0. SCHC Compression reduces the
header sending only the Type, a mapped code and the least 9
significant bits of Message ID.
Device LPWAN SCHC C/D SCHC Compression reduces the header sending only the Type, a mapped
| | code and the least significant bits of Message ID (9 bits in the
| rule id=1 |<-------------------- example above).
|<-------------------| +-+-+--+----+------+
<------------------- | CCCCCMMMMMMMMM | |1|0| 4|0.01|0x0034|
+-+-+--+----+-------+ | 00001000110100 | | 0xb4 p a t|
|1|0| 1|0.01|0x0034 | | | | h |
| 0xb4 p a t | | | +------+
| h | | |
+------+ | |
| |
| |
---------------------->| rule id=1 |
+-+-+--+----+--------+ |------------------->|
|1|2| 0|2.05| 0x0034 | | TCCCCCMMMMMMMMM |--------------------->
+-+-+--+----+--------+ | 001100000110100 | +-+-+--+----+------+
| | |1|2| 0|2.05|0x0034|
v v +-+-+--+----+------+
Figure 6: Compression with global addresses Note that a request sent by a client located an Application Server to
a server in the device, may not be compressed through this rule since
the MID will not start with 7 bits equal to 0. A CoAP proxy, before
the core SCHC C/D can rewrite the message ID to a value matched by
the rule.
7.2. OSCORE Compression 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 sensible information which is not necessary for proxy contains sensible 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 7. caching. This decomposition is illustrated in Figure 6.
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 14, line 44 skipping to change at page 14, line 38
+-+-+---+--------+---------------+....+ +-------+-----......+ +-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) | | Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+ +--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF | | Options (IU) | | OxFF |
. . +-------+-----------+ . . +-------+-----------+
. OSCORE Option . | | . OSCORE Option . | |
+------+-------------------+ | Payload | +------+-------------------+ | Payload |
| 0xFF | | | | 0xFF | | |
+------+ +-------------------+ +------+ +-------------------+
Figure 7: OSCORE inner and outer header form a CoAP message Figure 6: A CoAP message is split into an OSCORE outer and plaintext
Figure 7 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 by a default dummy value. As seen in sections 4.1.3.5 and 4.2 of
[I-D.ietf-core-object-security], the message code is replaced by POST [I-D.ietf-core-object-security], the message code is replaced by POST
for requests and Changed for responses when Observe is not used. If for requests and Changed for responses when Observe is not used. If
Observe is used, the message code is replaced by FETCH for requests Observe is used, the message code is replaced by FETCH for requests
and Content for responses. and Content for 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 15, line 19 skipping to change at page 15, line 11
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 8. OSCORE message, as illustrated in Figure 7.
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
+-+-+---+--------+---------------+ +-+-+---+--------+---------------+
|v|t|tkl|new code| Msg Id. | |v|t|tkl|new code| Msg Id. |
+-+-+---+--------+---------------+....+ +-+-+---+--------+---------------+....+
| Token | | Token |
+--------------------------------.....+ +--------------------------------.....+
| Options (IU) | | Options (IU) |
. . . .
. OSCORE Option . . OSCORE Option .
+------+-------------------+ +------+-------------------+
| 0xFF | | 0xFF |
+------+-------------------------+ +------+---------------------------+
| | | |
| Encrypted Inner Header and | | Ciphertext: Encrypted Inner |
| Payload | | Header and Payload |
| | | + Authentication Tag |
+--------------------------------+ | |
+----------------------------------+
Figure 8: 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 9. 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.
skipping to change at page 16, line 50 skipping to change at page 16, line 41
v | +-------+--+ v | +-------+--+
+--------+ +------------+ | Residue | +--------+ +------------+ | Residue |
|Rule ID'| | Encryption | <--- +----------+--------+ |Rule ID'| | Encryption | <--- +----------+--------+
+--------+--+ +------------+ | | +--------+--+ +------------+ | |
| Residue' | | Payload | | Residue' | | Payload |
+-----------+-------+ | | +-----------+-------+ | |
| Ciphertext | +-------------------+ | Ciphertext | +-------------------+
| | | |
+-------------------+ +-------------------+
Figure 9: 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. A possible set of rules for the Inner and Outer SCHC Response from a device-based CoAP client to a cloud-based CoAP
server. A possible set of rules for the Inner and Outer SCHC
Compression is shown. A dump of the results and a contrast between 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 10 Our first example CoAP message is the GET Request in Figure 9
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 17, line 37 skipping to change at page 17, 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 10: CoAP GET Request Figure 9: CoAP GET Request
Its corresponding response is the CONTENT Response in Figure 11. Its corresponding response is the CONTENT Response in Figure 10.
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 25 skipping to change at page 17, 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 11: CoAP CONTENT Response Figure 10: 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, what is important is the already present in a regular CoAP message, what is important is the
order of appearance and inclusion of only those CoAP fields that go order of appearance and inclusion of only those CoAP fields that go
into the Plaintext, Figure 12. into the Plaintext, Figure 11.
Rule ID 0 Rule ID 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 12: Inner SCHC Rules Figure 11: Inner SCHC Rules
Figure 13 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 Rule ID).
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 19, line 48 skipping to change at page 19, 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 13: Plaintext compression and encryption for GET Request Figure 12: Plaintext compression and encryption for GET Request
In Figure 14 we repeat the process for the example CONTENT Response. In Figure 13 we repeat the process for the example CONTENT Response.
In this case the misalignment produced by the compression residue (1 In this case the misalignment produced by the compression residue (1
bit) makes it so that 7 bits of padding have to be applied after the bit) makes it so that 7 bits of padding have to be applied after the
payload, resulting in a compressed Plaintext that is the same size as payload, resulting in a compressed Plaintext that is the same size as
before compression. This misalignment also causes the hexcode from before compression. This misalignment also causes the hexcode from
the payload to differ from the original, even though it has not been the payload to differ from the original, even though it has not been
compressed. On top of this, the overhead from the tag bytes is compressed. On top of this, the overhead from the tag bytes is
incurred as before. incurred as before.
________________________________________________________ ________________________________________________________
| | | |
skipping to change at page 21, line 48 skipping to change at page 20, line 50
| (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 14: 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 17) MUST process the OSCORE Options fields. In Figure 14 and Figure 15 we show a dump of the OSCORE
fields. In Figure 15 and Figure 16 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 are to go through Outer SCHC Compression. are the messages that are to go through Outer SCHC Compression.
Protected message: Protected message:
================== ==================
0x4102000182d7080904636c69656e74ffa2c54fe1b434297b62 0x4102000182d7080904636c69656e74ffa2c54fe1b434297b62
(25 bytes) (25 bytes)
Header: Header:
0x4102 0x4102
01 Ver 01 Ver
00 CON 00 CON
0001 tkl 0001 tkl
00000010 Request Code 2 "POST" 00000010 Request Code 2 "POST"
0x0001 = mid 0x0001 = mid
0x82 = token 0x82 = token
Options: Options:
0xd7080904636c69656e74 (10 bytes) 0xd8080904636c69656e74 (10 bytes)
Option 21: OBJECT_SECURITY Option 21: OBJECT_SECURITY
Value = 0x0904636c69656e74 Value = 0x0904636c69656e74
09 = 000 0 1 001 Flag byte 09 = 000 0 1 001 Flag byte
h k n h k n
04 piv 04 piv
636c69656e74 kid 636c69656e74 kid
0xFF Payload marker 0xFF Payload marker
Payload: Payload:
0xa2c54fe1b434297b62 (9 bytes) 0xa2c54fe1b434297b62 (9 bytes)
Figure 15: Protected and Inner SCHC Compressed GET Request Figure 14: 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 22, 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 16: Protected and Inner SCHC Compressed CONTENT Response Figure 15: Protected and Inner SCHC Compressed CONTENT Response
For the flag bits, a number of compression methods could prove to be For the flag bits, a number of compression methods could prove to be
useful depending on the application. The simplest alternative is to useful depending on the application. The simplest alternative is to
provide a fixed value for the flags, combining MO equal and CDA not- provide a fixed value for the flags, combining MO equal and CDA not-
sent. This saves most bits but could hinder flexibility. Otherwise, sent. This saves most bits but could hinder flexibility. Otherwise,
match-mapping could allow to choose from a number of configurations match-mapping could allow to choose from a number of configurations
of interest to the exchange. If neither of these alternatives is of interest to the exchange. If neither of these alternatives is
desirable, MSB could be used to mask off the 3 hard-coded most desirable, MSB could be used to mask off the 3 hard-coded most
significant bits. significant bits.
skipping to change at page 24, line 10 skipping to change at page 23, line 10
technologies. Note that compressing the sequence numbers effectively technologies. Note that compressing the sequence numbers effectively
reduces the maximum amount of sequence numbers that can be used in an reduces the maximum amount of sequence numbers that can be used in an
exchange. Once this amount is exceeded, the SCHC Context would need exchange. Once this amount is exceeded, the SCHC Context would need
to be re-established. to be re-established.
The size s included in the kid context field MAY be masked off with The size s included in the kid context field MAY be masked off with
CDA MSB. The rest of the field could have additional bits masked 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 17 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 Rule ID 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 36 skipping to change at page 23, line 36
|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 17: 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 18 and Response. The resulting messages are shown in Figure 17 and
Figure 19. Figure 18.
Compressed message: Compressed message:
================== ==================
0x001489458a9fc3686852f6c4 (12 bytes) 0x001489458a9fc3686852f6c4 (12 bytes)
0x00 Rule ID 0x00 Rule ID
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 18: 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 Rule ID
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 19: 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 20. the SCHC rules in Figure 19.
Rule ID 1 Rule ID 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] |equal |not-sent || | |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 20: SCHC-CoAP Rules (No OSCORE) Figure 19: SCHC-CoAP Rules (No OSCORE)
This yields the results in Figure 21 for the Request, and Figure 22 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 = Rule ID
Compression residue: Compression residue:
0b00010100 (1 byte) 0b00010100 (1 byte)
Compressed msg length: 2 Compressed msg length: 2
Figure 21: CoAP GET Compressed without OSCORE Figure 20: CoAP GET Compressed without OSCORE
Compressed message: Compressed message:
================== ==================
0x010a32332043 0x010a32332043
0x01 = Rule ID 0x01 = Rule ID
Compression residue: Compression residue:
0b00001010 (1 byte) 0b00001010 (1 byte)
Payload Payload
0x32332043 0x32332043
Compressed msg length: 6 Compressed msg length: 6
Figure 22: 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 This document does not have any more Security consideration than the
ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc] ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc]
10. Acknowledgements 10. Acknowledgements
Thanks to all the persons that have give us feedback The authors would like to thank Dominique Barthel, Carsten Bormann,
Thomas Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov,
Goran Selander.
11. Normative References 11. Normative References
[I-D.ietf-core-object-security] [I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz, Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments "Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-16 (work in (OSCORE)", draft-ietf-core-object-security-16 (work in
progress), March 2019. progress), March 2019.
[I-D.ietf-lpwan-ipv6-static-context-hc] [I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J. Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
Zuniga, "LPWAN Static Context Header Compression (SCHC) Zuniga, "Static Context Header Compression (SCHC) and
and fragmentation for IPv6 and UDP", draft-ietf-lpwan- fragmentation for LPWAN, application to UDP/IPv6", draft-
ipv6-static-context-hc-18 (work in progress), December ietf-lpwan-ipv6-static-context-hc-19 (work in progress),
2018. July 2019.
[I-D.toutain-core-time-scale] [I-D.toutain-core-time-scale]
Minaburo, A. and L. Toutain, "CoAP Time Scale Option", Minaburo, A. and L. Toutain, "CoAP Time Scale Option",
draft-toutain-core-time-scale-00 (work in progress), draft-toutain-core-time-scale-00 (work in progress),
October 2017. October 2017.
[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>.
 End of changes. 68 change blocks. 
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