lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Standards Track L. Toutain
Expires: January 7, 2020 Institut MINES TELECOM; IMT Atlantique
R. Andreasen
Universidad de Buenos Aires
July 06, 2019
LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-09
Abstract
This draft defines the way SCHC header compression can be applied to
CoAP headers. The CoAP header structure differs from IPv6 and UDP
protocols since CoAP uses a flexible header with a variable number of
options, themselves of variable length. The CoAP protocol is
asymmetric in its message format: the format of the packet header in
the request messages is different from that in the response messages.
Most of the compression mechanisms have been introduced in
[I-D.ietf-lpwan-ipv6-static-context-hc], this document explains how
to use the SCHC compression for CoAP.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 7, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. SCHC Compression Process . . . . . . . . . . . . . . . . . . 3
3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 4
4. Compression of CoAP header fields . . . . . . . . . . . . . . 6
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 6
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 6
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 6
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 6
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 7
5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 7
5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8
5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 8
5.3.2. Variable number of path or query elements . . . . . . 9
5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
fields . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path
and Location-Query fields . . . . . . . . . . . . . . . . 9
6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 10
6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Examples of CoAP header compression . . . . . . . . . . . . . 11
7.1. Mandatory header with CON message . . . . . . . . . . . . 11
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 12
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
9. Security considerations . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
11. Normative References . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
CoAP [rfc7252] is an implementation of the REST architecture for
constrained devices. Although CoAP was designed for constrained
devices, the size of a CoAP header may still be too large for the
constraints of Low Power Wide Area Networks (LPWAN) and some
compression may be needed to reduce the header size.
[I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
mechanism for LPWAN network based on a static context. The context
is said static since the field description composing the Rules are
not learned during the packet exchanges but are previously defined.
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
IDs (identifiers). A rule contains an ordered list of the fields
descriptions containing a field ID (FID), its length (FL) and its
position (FP), a direction indicator (DI) (upstream, downstream and
bidirectional) and some associated Target Values (TV). Target Value
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
description. The rule is selected if all the MOs fit the TVs for all
fields of the incoming packet. In that case, a Compression/
Decompression Action (CDA) associated to each field defines how the
compressed and the decompressed values are computed out of each
other, for each of the header fields. Compression mainly results in
one of 4 actions: send the field value, send nothing, send some least
significant bits of the field or send an index. 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
The SCHC Compression rules can be applied to CoAP flows. 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 [I-D.ietf-lpwan-ipv6-static-context-hc] may be used
as shown in Figure 1.
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^ +------------+ ^ +------------+ ^ +------------+
| | 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 architecture and the SCHC
rule's scope.
In the first example, a rule compresses all headers from IPv6 to
CoAP. In this case, SCHC C/D is performed at the device and at the
LPWAN boundary.
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
out of the scope of this document).
In the third example, OSCORE [I-D.ietf-core-object-security] is used.
2 rulesets are used to compress the CoAP message. A first ruleset
focuses on the inner header and is end to end, a second ruleset
compresses the outer header and the layers below. SCHC C/D for inner
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
CoAP differs from IPv6 and UDP protocols on the following aspects:
o IPv6 and UDP are symmetrical protocols. The same fields are found
in the request and in the response, with the value of some fields
being swapped on the return path (e.g. source and destination
fields). A CoAP request is intrinsically different from a
response. For example, the URI-path option is mandatory in the
request and is not found in the response, a request may contain an
Accept option and the response a Content option.
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[I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a
message direction (DI) in the Field Description, which allows a
single Rule to process message headers differently depending of
the direction.
o Even when a field is "symmetric" (i.e. found in both directions)
the values carried in each direction are different. Combined with
a matching list in the TV, this allows reducing the range of
expected values in a particular direction and therefore reduce the
size of the compression residue. For instance, if a client sends
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.
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
allows splitting in two parts the possible values for each
direction.
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
in the header. More systematically, the CoAP options are
described using the Type-Length-Value.
[I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to
define a function for the Field Length in the Field Description.
o In CoAP headers, a field can appear several times. This is
typical for elements of a URI (path or queries).
[I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field ID to
appears several times in the rule, the Field Position (FP)
identifies the proper instance, 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 or Token field. The MSB MO can be used
to reduce the information carried on LPWANs.
o CoAP also obeys the client/server paradigm and the compression
ratio can be different if the request is issued from an LPWAN
device or from a non LPWAN device. For instance a Device (Dev)
aware of LPWAN constraints can generate a 1 byte token, but a
regular CoAP client will certainly send a larger token to the Dev.
The SCHC compression-decompression process does not modify the
values. Nevertheless, a proxy placed before the compressor may
change some field values to allow SCHC achieving a better
compression ratio, while maintaining the necessary context for
interoperability with existing CoAP implementations.
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4. Compression of CoAP header fields
This section discusses the compression of the different CoAP header
fields.
4.1. CoAP version field
This field 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 defined to
avoid ambiguities between versions.
4.2. CoAP type field
[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
specific client or server role, a rule can exploit these properties
with the mapping list: [CON, NON] for one direction and [ACK, RST]
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
NON or only CON messages.
In any case, a rule MUST be defined to carry RST to a client.
4.3. CoAP code field
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.
All the response codes MUST be compressed with a SCHC rule.
4.4. CoAP Message ID field
This field is bidirectional and is used to manage acknowledgments.
The server memorizes the value for a EXCHANGE_LIFETIME period (by
default 247 seconds) for CON messages and a NON_LIFETIME period (by
default 145 seconds) for NON messages. During that period, a server
receiving the same Message ID value will process the message as a
retransmission. After this period, it will be processed as a new
message.
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In case the Device is a client, the size of the message ID field may
be too large regarding the number of messages sent. The client
SHOULD use only small message ID values, for instance 4 bit long.
Therefore, a MSB can be used to limit the size of the compression
residue.
In case the Device is a server, the client may be located outside of
the LPWAN area and view the Device as a regular device connected to
the internet. The client will generate Message ID using the 16 bits
space offered by this field. A CoAP proxy can be set before the SCHC
C/D to reduce the value of the Message ID, to allow its compression
with the MSB matching operator and LSB CDA.
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
remains the same during all the transaction, the size can be stored
in the context and elided during the transmission. Otherwise, it
will have to the sent as a compression residue.
Token Value size cannot be defined directly in the rule in the Field
Length (FL). Instead, a specific function designated as "TKL" MUST
be used and length does not have to the sent with the residue.
During the decompression, this function returns the value contained
in the Token Length field.
5. 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 residue. If is not possible, the value
has to be sent as a 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 entry. They are used only by the server to inform of the
caching duration and is never found in client requests.
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If the duration is known by both ends, the value can be elided on the
LPWAN.
A matching list can be used if some well-known values are defined.
Otherwise these options SHOULD be sent as a 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.
FID FL FP DI TV MO CDA
URI-Path 1 up ["/a/b", equal not-sent
"/c/d"]
URI-Path 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.
5.3.1. Variable length Uri-Path and Uri-Query
When the length is not known at the rule creation, the Field Length
SHOULD 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.
For instance for a CORECONF path /c/X6?k="eth0" the rule can be set
to:
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FID FL FP DI TV MO CDA
URI-Path 1 up "c" equal not-sent
URI-Path 2 up ignore value-sent
URI-Query 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 compression residue.
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 entry. 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.
6. Other RFCs
6.1. Block
Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also
includes a fragmentation protocol. They are compatible. If a block
option is used, its content MUST be sent as a compression residue.
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6.2. Observe
[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
[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 [I-D.ietf-core-object-security] defines end-to-end protection
for CoAP messages. This section describes how SCHC rules can be
applied to compress OSCORE-protected messages.
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
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The encoding of the OSCORE Option Value defined in Section 6.1 of
[I-D.ietf-core-object-security] is repeated in Figure 4.
The first byte is used for flags that specify the contents of the
OSCORE option. The 3 most significant bits 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 3 least significant bits n indicate the length of the
piv field in bytes. When n = 0, no piv is present.
After the flag byte follow the piv field, kid context field and kid
field in 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 draft recommends to implement a parser that is able to identify
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. It is thus recommended that the parser split the OSCORE
option into the 4 subsequent fields:
o CoAP OSCORE_flags,
o CoAP OSCORE_piv,
o CoAP OSCORE_kidctxt,
o CoAP OSCORE_kid.
These fields are shown superimposed on the OSCORE Option format in
Figure 4, the CoAP OSCORE_kidctxt field including the size bits s.
Their size SHOULD be reduced using SCHC compression.
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 a client on the Internet a POST message, which is
immediately acknowledged by the Device. For this simple scenario,
the rules are described Figure 5.
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Rule ID 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 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
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 sensible 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
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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,
signaling 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
[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
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
+-----------------+ | +-------+
| | |Rule ID|
v | +-------+--+
+--------+ +------------+ | Residue |
|Rule ID'| | 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, what is important is the
order of appearance and inclusion of only those CoAP fields that go
into the Plaintext, Figure 11.
Rule ID 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 Rule ID).
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 |
| |
| Rule ID = 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 we repeat the process for the example CONTENT Response.
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
payload, resulting in a compressed Plaintext that is the same size as
before compression. This misalignment also causes the hexcode from
the payload to differ from the original, even though it has not been
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compressed. 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 Rule ID |
| |
| 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 are to go through Outer SCHC Compression.
Protected message:
==================
0x4102000182d7080904636c69656e74ffa2c54fe1b434297b62
(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 could prove 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 hinder flexibility. Otherwise,
match-mapping could allow to choose from a number of configurations
of interest to the exchange. If neither of these alternatives is
desirable, 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 prove 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 SCHC Context would need
to be re-established.
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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
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.
Rule ID 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 Rule ID
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 Rule ID
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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Rule ID 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 = Rule ID
Compression residue:
0b00010100 (1 byte)
Compressed msg length: 2
Figure 20: CoAP GET Compressed without OSCORE
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Compressed message:
==================
0x010a32332043
0x01 = Rule ID
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
This document does not have any more Security consideration than the
ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc]
10. Acknowledgements
The authors would like to thank Dominique Barthel, Carsten Bormann,
Thomas Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov,
Goran Selander.
11. Normative References
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-16 (work in
progress), March 2019.
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[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
Zuniga, "Static Context Header Compression (SCHC) and
fragmentation for LPWAN, application to UDP/IPv6", draft-
ietf-lpwan-ipv6-static-context-hc-19 (work in progress),
July 2019.
[I-D.toutain-core-time-scale]
Minaburo, A. and L. Toutain, "CoAP Time Scale Option",
draft-toutain-core-time-scale-00 (work in progress),
October 2017.
[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>.
[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>.
Authors' Addresses
Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
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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
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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