lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Standards Track L. Toutain
Expires: November 27, 2020 Institut MINES TELECOM; IMT Atlantique
R. Andreasen
Universidad de Buenos Aires
May 26, 2020
LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-14
Abstract
This draft defines the way Static Context Header Compression (SCHC)
header compression can be applied to the Constrained Application
Protocol (CoAP). SCHC is a header compression mechanism adapted for
constrained devices. SCHC uses a static description of the header to
reduce the redundancy and the size of the information in the header.
While [rfc8724] describes the SCHC compression and fragmentation
framework, and its application for IPv6/UDP headers, this document
applies the use of SCHC for CoAP headers. The CoAP header structure
differs from IPv6 and UDP since CoAP uses a flexible header with a
variable number of options, themselves of variable length. The CoAP
protocol messages format is asymmetric: the request messages have a
header format different from the one in the response messages. This
specification gives guidance on how to apply SCHC to flexible headers
and how to leverage the asymmetry for more efficient compression
Rules.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 27, 2020.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Applying SCHC to CoAP headers . . . . . . . . . . . . . . . . 4
3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 5
3.1. Differences between CoAP and UDP/IP Compression . . . . . 5
4. Compression of CoAP header fields . . . . . . . . . . . . . . 6
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 7
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 7
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 7
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 7
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 8
5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 8
5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 9
5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 9
5.3.2. Variable number of path or query elements . . . . . . 10
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
fields . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path
and Location-Query fields . . . . . . . . . . . . . . . . 10
6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 11
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 11
6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Examples of CoAP header compression . . . . . . . . . . . . . 13
7.1. Mandatory header with CON message . . . . . . . . . . . . 13
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 14
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
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9. Security considerations . . . . . . . . . . . . . . . . . . . 27
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
11. Normative References . . . . . . . . . . . . . . . . . . . . 28
Appendix A. Extension to the RFC8724 Annex D. . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
CoAP [rfc7252] is designed to easily interop with HTTP (Hypertext
Transfer Protocol) and is optimized for REST-based (Representational
state transfer) services. Although CoAP was designed for constrained
devices, the size of a CoAP header still is too large for the
constraints of LPWAN (Low Power Wide Area Networks) and some
compression is needed to reduce the header size.
The [rfc8724] defines SCHC, a header compression mechanism for LPWAN
network based on a static context. The section 5 of the [rfc8724]
explains the architecture where compression and decompression are
done. The context is known by both ends before transmission. The
way the context is configured, provisioned or exchanged is out of the
scope of this document.
SCHC compresses and decompresses headers based on shared contexts
between devices. Each context consists of multiple Rules. Each Rule
can match header fields and specific values or ranges of values. If
a Rule matches, the matched header fields are substituted by the
RuleID and optionally some residual bits. Thus, different Rules may
correspond to different types of packets that a device expects to
send or receive.
A Rule describes the complete header of the packet with an ordered
list of fields descriptions, see section 7 of [rfc8724], thereby each
description contains the field ID (FID), its length (FL) and its
position (FP), a direction indicator (DI) (upstream, downstream and
bidirectional) and some associated Target Values (TV).
A Matching Operator (MO) is associated to each header field
description. The Rule is selected if all the MOs fit the TVs for all
fields of the incoming header.
In that case, a Compression/Decompression Action (CDA) associated to
each field defines how the compressed and the decompressed values are
computed. Compression mainly results in one of 4 actions:
o send the field value,
o send nothing,
o send some least significant bits of the field or
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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
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
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][rfc8174] when, and only when, they appear in all
capitals, as shown here.
2. Applying SCHC to CoAP headers
The SCHC Compression Rules can be applied to CoAP headers. SCHC
Compression of the CoAP header MAY be done in conjunction with the
lower layers (IPv6/UDP) or independently. The SCHC adaptation layers
as described in Section 5 of [rfc8724] and may be used as shown in
Figure 1.
^ +------------+ ^ +------------+ ^ +------------+
| | CoAP | | | CoAP | inner | | CoAP |
| +------------+ v +------------+ x | OSCORE |
| | UDP | | DTLS | outer | +------------+
| +------------+ +------------+ | | UDP |
| | IPv6 | | UDP | | +------------+
v +------------+ +------------+ | | IPv6 |
| IPv6 | v +------------+
+------------+
Figure 1: Rule scope for CoAP
Figure 1 shows some examples for CoAP protocol stacks and the SCHC
Rule's scope.
In the first example, a Rule compresses the complete header stack
from IPv6 to CoAP. In this case, SCHC C/D (Static Context Header
Compression Compressor/Decompressor) is performed at the Sender and
at the Receiver.
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In the second example, the SCHC compression is applied in the CoAP
layer, compressing the CoAP header independently of the other layers.
The RuleID and the Compression Residue are encrypted using a
mechanism such as DTLS. Only the other end can decipher the
information. If needed, layers below use SCHC to compress the header
as defined in [rfc8724] document. This use case realizes an End-to-
End context initialization between the sender and the receiver, see
Appendix A.
In the third example, the Object Security for Constrained RESTful
Environments (OSCORE) [rfc8613] is used. In this case, two rulesets
are used to compress the CoAP message. A first ruleset focused on
the inner header and is applied end to end by both ends. A second
ruleset compresses the outer header and the layers below and is done
between the Sender and the Receiver.
3. CoAP Headers compressed with SCHC
The use of SCHC over the CoAP header uses the same description and
compression/decompression techniques as the one for IP and UDP
explained in the [rfc8724]. For CoAP, SCHC Rules description uses
the direction information to optimize the compression by reducing the
number of Rules needed to compress headers. The field description
MAY define both request/response headers and target values in the
same Rule, using the DI (direction indicator) to make the difference.
As for other protocols, when the compressor does not find a correct
Rule to compress the header, the packet MUST be sent uncompressed
using the RuleID dedicated to this purpose, and the Compression
Residue is the complete header of the packet. See section 6 of
[rfc8724].
3.1. Differences between CoAP and UDP/IP Compression
CoAP compression differs from IPv6 and UDP compression on the
following aspects:
o The CoAP protocol is asymmetric, the headers are different for a
request or a response. For example, the URI-path option is
mandatory in the request, and it is not present in the response, a
request may contain an Accept option, and the response may include
a Content option. In comparison, IPv6 and UDP returning path swap
the value of some fields in the header.
But all the directions have the same fields (e.g., source and
destination addresses fields).
The [rfc8724] defines the use of a Direction Indicator (DI) in the
Field Description, which allows a single Rule to process message
headers differently depending on the direction.
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o Even when a field is "symmetric" (i.e., found in both directions),
the values carried in each direction are different.
The compression may use a matching list in the TV to limit the
range of expected values in a particular direction and therefore
reduces the size of the Compression Residue. Through the
Direction Indicator (DI), a field description in the Rules splits
the possible field value into two parts, one for each direction.
For instance, if a client sends only CON requests, the type can be
elided by compression, and the answer may use one single bit to
carry either the ACK or RST type. The field Code have as well the
same behavior, the 0.0X code format value in the request and Y.ZZ
code format in the response.
o Headers in IPv6 and UDP have a fixed size. The size is not sent
as part of the Compression Residue, but is defined in the Rule.
Some CoAP header fields have variable lengths, so the length is
also specified in the Field Description. For example, the Token
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 [rfc8724] offers the possibility to define a
function for the Field length in the Field Description to have
knowledge of the length before compression. When doing SCHC
compression of a variable-length field,
if the field size is not known, the Field Length in the Rule is
set as variable and the size is sent with the Compression Residue.
o A field can appear several time in the CoAP headers. This is
typical for elements of a URI (path or queries). The SCHC
specification [rfc8724] allows a Field ID to appear several times
in the Rule, and uses the Field Position (FP) to identify the
correct instance, and thereby removing the ambiguity of the
matching operation.
o Field sizes defined in the CoAP protocol can be too large
regarding LPWAN traffic constraints. This is particularly true
for the Message ID field and the Token field. SCHC uses different
Matching operators (MO) to perform the compression, see section
7.4 of [rfc8724]. In this case the Most Significant Bits (MSB) MO
can be applied to reduce the information carried on LPWANs.
4. Compression of CoAP header fields
This section discusses the compression of the different CoAP header
fields. The CoAP compression with SCHC follows the Section 7.1 of
[rfc8724].
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4.1. CoAP version field
CoAP version is bidirectional and MUST be elided during the SCHC
compression, since it always contains the same value. In the future,
if new versions of CoAP are defined, new Rules will be needed to
avoid ambiguities between versions.
4.2. CoAP type field
The CoAP Protocol [rfc7252] has four type of messages: two request
(CON, NON); one response (ACK) and one empty message (RST).
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
be needed. For other usages a mapping list can be used.
4.3. CoAP code field
The code field indicates the Request Method used in CoAP, a IANA
registry [rfc7252]. The compression of the CoAP code field follows
the same principle as that of the CoAP type field. If the device
plays a specific role, the set of code values can be split in two
parts, the request codes with the 0 class and the response values.
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.
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
Compression Residue.
4.4. CoAP Message ID field
The Message ID field can be compressed with the MSB(x) MO and the
Least Significant Bits (LSB) CDA, see section 7.4 of [rfc8724].
4.5. CoAP Token fields
Token is defined through two CoAP fields, Token Length in the
mandatory header and Token Value directly following the mandatory
CoAP header.
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
transmission. Otherwise, it will have to be sent in the Compression
Residue.
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Token Value MUST NOT be sent as a variable length residue to avoid
ambiguity with Token Length. Therefore, Token Length value MUST be
used to define the size of the Compression Residue. A specific
function designated as "TKL" MUST be used in the Rule. During the
decompression, this function returns the value contained in the Token
Length field.
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.
These fields are both unidirectional and MUST NOT be set to
bidirectional in a Rule entry.
If a single value is expected by the client, it can be stored in the
TV and elided during the transmission. Otherwise, if several
possible values are expected by the client, a matching-list SHOULD be
used to limit the size of the Compression Residue. Otherwise, the
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
These fields are unidirectional and MUST NOT be set to bidirectional
in a Rule DI entry, see section 7.1 of [rfc8724]. They are used only
by the server to inform of the caching duration and is never found in
client requests.
If the duration is known by both ends, the value can be elided.
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A matching list can be used if some well-known values are defined.
Otherwise these options can be sent as a Compression Residue (fixed
or variable length).
5.3. CoAP option Uri-Path and Uri-Query fields
These fields are unidirectional and MUST NOT be set to bidirectional
in a Rule entry. They are used only by the client to access a
specific resource and are never found in server responses.
Uri-Path and Uri-Query elements are a repeatable options, the Field
Position (FP) gives the position in the path.
A Mapping list can be used to reduce the size of variable Paths or
Queries. In that case, to optimize the compression, several elements
can be regrouped into a single entry. Numbering of elements do not
change, MO comparison is set with the first element of the matching.
+-------------+---+--+--+--------+---------+-------------+
| Field |FL |FP|DI| Target | Match | CDA |
| | | | | Value | Opera. | |
+-------------+---+--+--+--------+---------+-------------+
|URI-Path | | 1|up|["/a/b",|equal |not-sent |
| | | | |"/c/d"] | | |
|URI-Path |var| 3|up| |ignore |value-sent |
+-------------+---+--+--+--------+---------+-------------+
Figure 2: complex path example
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
needed. The third path element is sent as a variable size residue.
5.3.1. Variable length Uri-Path and Uri-Query
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.
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
bits.
The length sent at the beginning of a variable length residue
indicates the size of the LSB in bytes.
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For instance for a CORECONF path /c/X6?k="eth0" the Rule can be set
to:
+-------------+---+--+--+--------+---------+-------------+
| Field |FL |FP|DI| Target | Match | CDA |
| | | | | Value | Opera. | |
+-------------+---+--+--+--------+---------+-------------+
|URI-Path | 8| 1|up|"c" |equal |not-sent |
|URI-Path |var| 2|up| |ignore |value-sent |
|URI-Query |var| 1|up|"k=" |MSB(16) |LSB |
+-------------+---+--+--+--------+---------+-------------+
Figure 3: CORECONF URI compression
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)
followed by the query option (i.e. 0x05 "eth0").
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 Rule creation time. If the number varies, several Rules SHOULD
be created to cover all the possibilities. Another possibility is to
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.
This adds 4 bits to the variable Residue size. See section 7.5.2
[rfc8724]
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields
These fields are unidirectional and MUST NOT be set to bidirectional
in a Rule DI entry, see section 7.1 of the [rfc8724]. They are used
only by the client to access a specific resource and are never found
in server response.
If the field value has to be sent, TV is not set, MO is set to
"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
is set to "not-sent".
5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and
Location-Query fields
These fields are unidirectional.
These fields values cannot be stored in a Rule entry. They MUST
always be sent with the Compression Residues.
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6. SCHC compression of CoAP extension RFCs
6.1. Block
Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also
includes a fragmentation protocol. They can be both used. If a
block option is used, its content MUST be sent as a Compression
Residue.
6.2. Observe
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
limit the maximum size for this option (3 bytes). To reduce the
transmission size, either the device implementation MAY limit the
delta between two consecutive values, or a proxy can modify the
increment.
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
of this message.
6.3. No-Response
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,
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.
6.4. OSCORE
OSCORE [rfc8613] defines end-to-end protection for CoAP messages.
This section describes how SCHC Rules can be applied to compress
OSCORE-protected messages.
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0 1 2 3 4 5 6 7 <--------- n bytes ------------->
+-+-+-+-+-+-+-+-+---------------------------------
|0 0 0|h|k| n | Partial IV (if any) ...
+-+-+-+-+-+-+-+-+---------------------------------
| | |
|<-- CoAP -->|<------ CoAP OSCORE_piv ------> |
OSCORE_flags
<- 1 byte -> <------ s bytes ----->
+------------+----------------------+-----------------------+
| s (if any) | kid context (if any) | kid (if any) ... |
+------------+----------------------+-----------------------+
| | |
| <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->|
Figure 4: OSCORE Option
The encoding of the OSCORE Option Value defined in Section 6.1 of
[rfc8613] is repeated in Figure 4.
The first byte specifies the content of the OSCORE options using
flags. The three most significant bits of this byte are reserved and
always set to 0. Bit h, when set, indicates the presence of the kid
context field in the option. Bit k, when set, indicates the presence
of a kid field. The three least significant bits n indicate the
length of the piv (Partial Initialization Vector) field in bytes.
When n = 0, no piv is present.
The flag byte is followed by the piv field, kid context field, and
kid field in this order, and if present, the length of the kid
context field is encoded in the first byte denoting by s the length
of the kid context in bytes.
This specification recommends identifying the OSCORE Option and the
fields it contains Conceptually, it discerns up to 4 distinct pieces
of information within the OSCORE option: the flag bits, the piv, the
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_piv,
o CoAP OSCORE_kidctxt,
o CoAP OSCORE_kid.
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The OSCORE Option shows superimposed these four fields using the
format Figure 4, the CoAP OSCORE_kidctxt field includes the size bits
s.
7. Examples of CoAP header compression
7.1. Mandatory header with CON message
In this first scenario, the LPWAN Compressor at the Network Gateway
side receives from an Internet client a POST message, which is
immediately acknowledged by the Device. For this simple scenario,
the Rules are described Figure 5.
RuleID 1
+-------------+--+--+--+------+---------+-------------++------------+
| Field |FL|FP|DI|Target| Match | CDA || Sent |
| | | | |Value | Opera. | || [bits] |
+-------------+--+--+--+------+---------+-------------++------------+
|CoAP version | | |bi| 01 |equal |not-sent || |
|CoAP Type | | |dw| CON |equal |not-sent || |
|CoAP Type | | |up|[ACK, | | || |
| | | | | RST] |match-map|matching-sent|| T |
|CoAP TKL | | |bi| 0 |equal |not-sent || |
|CoAP Code | | |bi|[0.00,| | || |
| | | | | ... | | || |
| | | | | 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 || |
+-------------+--+--+--+------+---------+-------------++------------+
Figure 5: CoAP Context to compress header without token
The version and Token Length fields are elided. The 26 method and
response codes defined in [rfc7252] has been shrunk to 5 bits using a
matching list. Uri-Path contains a single element indicated in the
matching operator.
SCHC Compression reduces the header sending only the Type, a mapped
code and the least significant bits of Message ID (9 bits in the
example above).
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
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.
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7.2. OSCORE Compression
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
possible while still enabling proxy operation.
Conceptually this is achieved by splitting the CoAP message into an
Inner Plaintext and Outer OSCORE Message. The Inner Plaintext
contains sensitive information which is not necessary for proxy
operation. This, in turn, is the part of the message which can be
encrypted until it reaches its end destination. The Outer Message
acts as a shell matching the format of a regular CoAP message, and
includes all Options and information needed for proxy operation and
caching. This decomposition is illustrated in Figure 6.
CoAP options are sorted into one of 3 classes, each granted a
specific type of protection by the protocol:
o Class E: Encrypted options moved to the Inner Plaintext,
o Class I: Integrity-protected options included in the AAD for the
encryption of the Plaintext but otherwise left untouched in the
Outer Message,
o Class U: Unprotected options left untouched in the Outer Message.
Additionally, the OSCORE Option is added as an Outer option,
signalling that the message is OSCORE protected. This option carries
the information necessary to retrieve the Security Context with which
the message was encrypted so that it may be correctly decrypted at
the other end-point.
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Original CoAP Message
+-+-+---+-------+---------------+
|v|t|tkl| code | Msg Id. |
+-+-+---+-------+---------------+....+
| Token |
+-------------------------------.....+
| Options (IEU) |
. .
. .
+------+-------------------+
| 0xFF |
+------+------------------------+
| |
| Payload |
| |
+-------------------------------+
/ \
/ \
/ \
/ \
Outer Header v v Plaintext
+-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF |
. . +-------+-----------+
. OSCORE Option . | |
+------+-------------------+ | Payload |
| 0xFF | | |
+------+ +-------------------+
Figure 6: A CoAP message is split into an OSCORE outer and plaintext
Figure 6 shows the message format for the OSCORE Message and
Plaintext.
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
[rfc8613], the message code is replaced by POST for requests and
Changed for responses when Observe is not used. If Observe is used,
the message code is replaced by FETCH for requests and Content for
responses.
The original message code is put into the first byte of the
Plaintext. Following the message code, the class E options comes and
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if present the original message Payload is preceded by its payload
marker.
The Plaintext is now encrypted by an AEAD algorithm which integrity
protects Security Context parameters and eventually any class I
options from the Outer Header. Currently no CoAP options are marked
class I. The resulting Ciphertext becomes the new Payload of the
OSCORE message, as illustrated in Figure 7.
This Ciphertext is, as defined in RFC 5116, the concatenation of the
encrypted Plaintext and its authentication tag. Note that Inner
Compression only affects the Plaintext before encryption, thus we can
only aim to reduce this first, variable length component of the
Ciphertext. The authentication tag is fixed in length and considered
part of the cost of protection.
Outer Header
+-+-+---+--------+---------------+
|v|t|tkl|new code| Msg Id. |
+-+-+---+--------+---------------+....+
| Token |
+--------------------------------.....+
| Options (IU) |
. .
. OSCORE Option .
+------+-------------------+
| 0xFF |
+------+---------------------------+
| |
| Ciphertext: Encrypted Inner |
| Header and Payload |
| + Authentication Tag |
| |
+----------------------------------+
Figure 7: OSCORE message
The SCHC Compression scheme consists of compressing both the
Plaintext before encryption and the resulting OSCORE message after
encryption, see Figure 8.
This translates into a segmented process where SCHC compression is
applied independently in 2 stages, each with its corresponding set of
Rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way
compression is applied to all fields of the original CoAP message.
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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
implement Inner SCHC Compression/Decompression.
Outer Message OSCORE Plaintext
+-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF |
. . +-------+-----------+
. OSCORE Option . | |
+------+-------------------+ | Payload |
| 0xFF | | |
+------+------------+ +-------------------+
| Ciphertext |<---------\ |
| | | v
+-------------------+ | +-----------------+
| | | Inner SCHC |
v | | Compression |
+-----------------+ | +-----------------+
| Outer SCHC | | |
| Compression | | v
+-----------------+ | +-------+
| | |RuleID |
v | +-------+--+
+--------+ +------------+ | Residue |
|RuleID' | | Encryption | <--- +----------+--------+
+--------+--+ +------------+ | |
| Residue' | | Payload |
+-----------+-------+ | |
| Ciphertext | +-------------------+
| |
+-------------------+
Figure 8: OSCORE Compression Diagram
7.3. Example OSCORE Compression
An example is given with a GET Request and its consequent Content
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
SCHC + OSCORE performance with SCHC + COAP performance is also
listed. This gives an approximation to the cost of security with
SCHC-OSCORE.
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Our first example CoAP message is the GET Request in Figure 9
Original message:
=================
0x4101000182bb74656d7065726174757265
Header:
0x4101
01 Ver
00 CON
0001 tkl
00000001 Request Code 1 "GET"
0x0001 = mid
0x82 = token
Options:
0xbb74656d7065726174757265
Option 11: URI_PATH
Value = temperature
Original msg length: 17 bytes.
Figure 9: CoAP GET Request
Its corresponding response is the CONTENT Response in Figure 10.
Original message:
=================
0x6145000182ff32332043
Header:
0x6145
01 Ver
10 ACK
0001 tkl
01000101 Successful Response Code 69 "2.05 Content"
0x0001 = mid
0x82 = token
0xFF Payload marker
Payload:
0x32332043
Original msg length: 10
Figure 10: CoAP CONTENT Response
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The SCHC Rules for the Inner Compression include all fields that are
already present in a regular CoAP message. The methods described in
Section 4 applies to these fields. As an example, see Figure 11.
RuleID 0
+---------------+--+--+-----------+-----------+-----------++------+
| Field |FP|DI| Target | MO | CDA || Sent |
| | | | Value | | ||[bits]|
+---------------+--+--+-----------+-----------+-----------++------+
|CoAP Code | |up| 1 | equal |not-sent || |
|CoAP Code | |dw|[69,132] | match-map |match-sent || c |
|CoAP Uri-Path | |up|temperature| equal |not-sent || |
|COAP Option-End| |dw| 0xFF | equal |not-sent || |
+---------------+--+--+-----------+-----------+-----------++------+
Figure 11: Inner SCHC Rules
Figure 12 shows the Plaintext obtained for our example GET Request
and follows the process of Inner Compression and Encryption until we
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
Plaintext can be compressed up to only 1 byte (size of the RuleID).
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
included in the OSCORE message. This translates into an overhead in
total message length, which limits the amount of compression that can
be achieved and plays into the cost of adding security to the
exchange.
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________________________________________________________
| |
| OSCORE Plaintext |
| |
| 0x01bb74656d7065726174757265 (13 bytes) |
| |
| 0x01 Request Code GET |
| |
| bb74656d7065726174757265 Option 11: URI_PATH |
| Value = temperature |
|________________________________________________________|
|
|
| Inner SCHC Compression
|
v
_________________________________
| |
| Compressed Plaintext |
| |
| 0x00 |
| |
| RuleID = 0x00 (1 byte) |
| (No residue) |
|_________________________________|
|
| AEAD Encryption
| (piv = 0x04)
v
_________________________________________________
| |
| encrypted_plaintext = 0xa2 (1 byte) |
| tag = 0xc54fe1b434297b62 (8 bytes) |
| |
| ciphertext = 0xa2c54fe1b434297b62 (9 bytes) |
|_________________________________________________|
Figure 12: Plaintext compression and encryption for GET Request
In Figure 13 the process is repeated for the example CONTENT
Response. The residue is 1 bit long. Note that since SCHC adds
padding after the payload, this misalignment causes the hexadecimal
code from the payload to differ from the original, even though it has
not been compressed.
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On top of this, the overhead from the tag bytes is incurred as
before.
________________________________________________________
| |
| OSCORE Plaintext |
| |
| 0x45ff32332043 (6 bytes) |
| |
| 0x45 Successful Response Code 69 "2.05 Content" |
| |
| ff Payload marker |
| |
| 32332043 Payload |
|________________________________________________________|
|
|
| Inner SCHC Compression
|
v
__________________________________________
| |
| Compressed Plaintext |
| |
| 0x001919902180 (6 bytes) |
| |
| 00 RuleID |
| |
| 0b0 (1 bit match-map residue) |
| 0x32332043 >> 1 (shifted payload) |
| 0b0000000 Padding |
|__________________________________________|
|
| AEAD Encryption
| (piv = 0x04)
v
_________________________________________________________
| |
| encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) |
| tag = 0xe9aef3f2461e0c29 (8 bytes) |
| |
| ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) |
|_________________________________________________________|
Figure 13: Plaintext compression and encryption for CONTENT Response
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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
Messages generated from our example messages once they have been
provided with the Inner Compressed Ciphertext in the payload. These
are the messages that have to be compressed by the Outer SCHC
Compression.
Protected message:
==================
0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62
(25 bytes)
Header:
0x4102
01 Ver
00 CON
0001 tkl
00000010 Request Code 2 "POST"
0x0001 = mid
0x82 = token
Options:
0xd8080904636c69656e74 (10 bytes)
Option 21: OBJECT_SECURITY
Value = 0x0904636c69656e74
09 = 000 0 1 001 Flag byte
h k n
04 piv
636c69656e74 kid
0xFF Payload marker
Payload:
0xa2c54fe1b434297b62 (9 bytes)
Figure 14: Protected and Inner SCHC Compressed GET Request
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Protected message:
==================
0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29
(22 bytes)
Header:
0x6144
01 Ver
10 ACK
0001 tkl
01000100 Successful Response Code 68 "2.04 Changed"
0x0001 = mid
0x82 = token
Options:
0xd008 (2 bytes)
Option 21: OBJECT_SECURITY
Value = b''
0xFF Payload marker
Payload:
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Figure 15: Protected and Inner SCHC Compressed CONTENT Response
For the flag bits, a number of compression methods has been shown to
be useful depending on the application. The simplest alternative is
to provide a fixed value for the flags, combining MO equal and CDA
not- sent. This saves most bits but could prevent flexibility.
Otherwise, match-mapping could be used to choose from an interested
number of configurations to the exchange. Otherwise, MSB could be
used to mask off the 3 hard-coded most significant bits.
Note that fixing a flag bit will limit the choice of CoAP Options
that can be used in the exchange, since their values are dependent on
certain options.
The piv field lends itself to having a number of bits masked off with
MO MSB and CDA LSB. This could be useful in applications where the
message frequency is low such as that found in LPWAN technologies.
Note that compressing the sequence numbers effectively reduces the
maximum amount of sequence numbers that can be used in an exchange.
Once this amount is exceeded, the OSCORE keys need to be re-
established.
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
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off, or have the whole field be fixed with MO equal and CDA not-sent.
The same holds for the kid field.
Figure 16 shows a possible set of Outer Rules to compress the Outer
Header.
RuleID 0
+-------------------+--+--+--------------+--------+---------++------+
| Field |FP|DI| Target | MO | CDA || Sent |
| | | | Value | | ||[bits]|
+-------------------+--+--+--------------+--------+---------++------+
|CoAP version | |bi| 01 |equal |not-sent || |
|CoAP Type | |up| 0 |equal |not-sent || |
|CoAP Type | |dw| 2 |equal |not-sent || |
|CoAP TKL | |bi| 1 |equal |not-sent || |
|CoAP Code | |up| 2 |equal |not-sent || |
|CoAP Code | |dw| 68 |equal |not-sent || |
|CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM |
|CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT |
|CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || |
|CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP |
|COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK |
|COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || |
|CoAP OSCORE_flags | |dw| b'' |equal |not-sent || |
|CoAP OSCORE_piv | |dw| b'' |equal |not-sent || |
|CoAP OSCORE_kid | |dw| b'' |equal |not-sent || |
|COAP Option-End | |dw| 0xFF |equal |not-sent || |
+-------------------+--+--+--------------+--------+---------++------+
Figure 16: Outer SCHC Rules
These Outer Rules are applied to the example GET Request and CONTENT
Response. The resulting messages are shown in Figure 17 and
Figure 18.
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Compressed message:
==================
0x001489458a9fc3686852f6c4 (12 bytes)
0x00 RuleID
1489 Compression Residue
458a9fc3686852f6c4 Padded payload
Compression Residue:
0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding)
mid tkn piv kid
Payload
0xa2c54fe1b434297b62 (9 bytes)
Compressed message length: 12 bytes
Figure 17: SCHC-OSCORE Compressed GET Request
Compressed message:
==================
0x0014218daf84d983d35de7e48c3c1852 (16 bytes)
0x00 RuleID
14 Compression Residue
218daf84d983d35de7e48c3c1852 Padded payload
Compression Residue:
0b0001 010 (7 bits -> 1 byte with padding)
mid tkn
Payload
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Compressed msg length: 16 bytes
Figure 18: SCHC-OSCORE Compressed CONTENT Response
For contrast, we compare these results with what would be obtained by
SCHC compressing the original CoAP messages without protecting them
with OSCORE. To do this, we compress the CoAP messages according to
the SCHC Rules in Figure 19.
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RuleID 1
+---------------+--+--+-----------+---------+-----------++--------+
| Field |FP|DI| Target | MO | CDA || Sent |
| | | | Value | | || [bits] |
+---------------+--+--+-----------+---------+-----------++--------+
|CoAP version | |bi| 01 |equal |not-sent || |
|CoAP Type | |up| 0 |equal |not-sent || |
|CoAP Type | |dw| 2 |equal |not-sent || |
|CoAP TKL | |bi| 1 |equal |not-sent || |
|CoAP Code | |up| 2 |equal |not-sent || |
|CoAP Code | |dw| [69,132] |match-map|map-sent ||C |
|CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM |
|CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT |
|CoAP Uri-Path | |up|temperature|equal |not-sent || |
|COAP Option-End| |dw| 0xFF |equal |not-sent || |
+---------------+--+--+-----------+---------+-----------++--------+
Figure 19: SCHC-CoAP Rules (No OSCORE)
This yields the results in Figure 20 for the Request, and Figure 21
for the Response.
Compressed message:
==================
0x0114
0x01 = RuleID
Compression Residue:
0b00010100 (1 byte)
Compressed msg length: 2
Figure 20: CoAP GET Compressed without OSCORE
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Compressed message:
==================
0x010a32332043
0x01 = RuleID
Compression Residue:
0b00001010 (1 byte)
Payload
0x32332043
Compressed msg length: 6
Figure 21: CoAP CONTENT Compressed without OSCORE
As can be seen, the difference between applying SCHC + OSCORE as
compared to regular SCHC + COAP is about 10 bytes of cost.
8. IANA Considerations
This document has no request to IANA.
9. Security considerations
The Security Considerations of SCHC header compression RFC8724 are
valid for SCHC CoAP header compression. When CoAP uses OSCORE, the
security considerations defined in RFC8613 does not change when SCHC
header compression is applied.
The definition of SCHC over CoAP header fields permits the
compression of header information only. The SCHC header compression
itself does not increase or reduce the level of security in the
communication. When the communication does not use any security
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
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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
described in [rfc8724]. The size of the Initialisation Vector (IV)
residue size must be considered carefully. A too large value has an
impact on the compression efficiency and a too small value will force
the device to renew its key more often. This operation may be long
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
The authors would like to thank (in alphabetic order): Christian
Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran
Selander.
11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[rfc7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[rfc7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
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[rfc7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>.
[rfc8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[rfc8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<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
Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Laurent Toutain
Institut MINES TELECOM; IMT Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
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Ricardo Andreasen
Universidad de Buenos Aires
Av. Paseo Colon 850
C1063ACV Ciudad Autonoma de Buenos Aires
Argentina
Email: randreasen@fi.uba.ar
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