lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational L. Toutain
Expires: April 25, 2019 Institut MINES TELECOM; IMT Atlantique
R. Andreasen
Universidad de Buenos Aires
October 22, 2018
LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-05
Abstract
This draft defines the way SCHC header compression can be applied to
CoAP headers. CoAP header structure differs from IPv6 and UDP
protocols since the CoAP
use a flexible header with a variable number of options themselves of
a variable length. Another important difference is the asymmetry in
the header format used in request and 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
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This Internet-Draft will expire on April 25, 2019.
Copyright Notice
Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SCHC Compression Process . . . . . . . . . . . . . . . . . . 3
3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 4
4. Compression of CoAP header fields . . . . . . . . . . . . . . 5
4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 5
4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 5
4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 6
4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 6
4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 6
5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. CoAP Content and Accept options. . . . . . . . . . . . . 7
5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri-
Port fields . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 7
5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 8
5.3.2. Variable number of path or query elements . . . . . . 8
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 . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 9
6.4. Time Scale . . . . . . . . . . . . . . . . . . . . . . . 10
6.5. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Examples of CoAP header compression . . . . . . . . . . . . . 11
7.1. Mandatory header with CON message . . . . . . . . . . . . 11
7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 13
7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17
8. Normative References . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
CoAP [rfc7252] is an implementation of the REST architecture for
constrained devices. Nevertheless, if limited, the size of a CoAP
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header may be too large for LPWAN constraints 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 and
the context 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. In that case, a Compression/Decompression Action (CDA)
associated to each field defines the link between the compressed and
decompressed value for each of the header fields. Compression
results mainly in 4 actions: send the field value, send nothing, send
less significant bits of a field, send an index. Values sent are
called Compression Residues and follows the rule ID.
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
above layers (IPv6/UDP) or independently. The SCHC adaptation layers
as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used
as as shown in the 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
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Figure 1 shows some examples for CoAP architecture and the SCHC
rule's scope. A rule can covers all headers from IPv6 to CoAP, SCHC
C/D is done in the device and at the LPWAN boundary. If an end-to-
end encryption mechanisms is used between the device and the
application. CoAP must be 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). OSCORE
[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
end to end, a second rule may compress the outer header and the layer
above. SCHC C/D for inner header is done by both ends, 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, only the location in the
header may vary (e.g. source and destination fields). A CoAP
request is 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.
[I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a
message direction (DI) when processing the rule which allows the
description of message header format 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
a matching list in the TV, this will allow to reduce the range of
expected values in a particular direction and therefore reduce the
size of a compression residue. For instance, if a client sends
only CON request, the type can be elided by compression and the
answer may use one bit to carry either the ACK or RST type. 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 to split 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, length is given by a field in the
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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 be duplicated several times, for
instances, elements of an URI (path or queries). The position
defined in a rule, associated to a Field ID, can be used to
identify the proper element.
[I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field id to
appears several times in the rule, the Field Position (FP) removes
ambiguities for the matching operation.
o Field size 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 use of MSB MO can be used to
reduce the information carried on LPWANs.
o CoAP also obeys to the client/server paradigm and the compression
rate can be different if the request is issued from an LPWAN node
or from an 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 Thing. SCHC
compression will not modify the values to offer a better
compression rate. Nevertheless a proxy placed before the
compressor may change some field values to offer a better
compression rate and maintain the necessary context for
interoperability with existing CoAP implementations.
4. Compression of CoAP header fields
This section discusses of 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 version of CoAP are defined, new rules ID will be defined
avoiding ambiguities between versions.
4.2. CoAP type field
[rfc7252] defines 4 types of messages: CON, NON, ACK and RST. The
latter two ones are a response of the two first ones. If the device
plays a specific role, a rule can exploit these property with the
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mapping list: [CON, NON] for one direction and [ACK, RST] for the
other direction. Compression residue is reduced to 1 bit.
The field must be elided if for instance a client is sending only NON
or 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
for 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 implement only a CoAP client, the request code can be
reduced to the set of request the client is able to process.
All the response codes should be compressed with a SCHC rule.
4.4. CoAP Message ID field
This field is bidirectional and is used to manage acknowledgments.
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
messages.
In case the Device is a client, the size of the message ID field may
the too large regarding the number of messages sent. Client may 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, 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.
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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 send as a compression residue.
Token Value size should not be defined directly in the rule in the
Field Length (FL). Instead a specific function designed as "TKL"
must be used and length do 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 field are both unidirectional and must not be set to
bidirectional in a rule entry.
If 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 field, CoAP option Uri-Host and Uri-Port
fields
This field is unidirectional and must not be set to bidirectional in
a rule entry. It is 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, 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
This fields are unidirectional and must not be set to bidirectional
in a rule entry. They are used only by the client to access to 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.
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A Mapping list can be used to reduce 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 was not allowed, a 2 bits residue is needed.
5.3.1. Variable length Uri-Path and Uri-Query
When the length is 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 apply 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 and the LSB CDA must not carry any value.
The length sent at the beginning of a variable length residue
indicates the size of the LSB in bytes.
For instance for a CoMi path /c/X6?k="eth0" the rule can be set to:
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: CoMi 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 element 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 possibilities 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 add 4 bits to the compression residue.
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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 to a
specific resource and are never found in server response.
If the field value must be sent, TV is not set, MO is set to "ignore"
and CDA is set to "value-sent. A mapping can 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
includes also a fragmentation protocol. They are compatible. If a
block option is used, its content must be sent as a compression
residue.
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 should limit the
delta between two consecutive value or a proxy can modify the
incrementation.
Since 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 an No-Response option limiting the responses made
by a server to a request. If the value is not known by both ends,
then TV is set to this value, MO is set to "equal" and CDA is set to
"not-sent".
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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. Time Scale
Time scale [I-D.toutain-core-time-scale] option allows a client to
inform the server that it is in a slow network and that message ID
should be kept for a duration given by the option.
If the value is not 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
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
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
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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 superposed on the OSCORE Option format in Figure 4,
the CoAP OSCORE_kidctxt field including the size bits s. Their size
may be reduced using the MSB matching operator.
7. Examples of CoAP header compression
7.1. Mandatory header with CON message
In this first scenario, the LPWAN compressor receives from outside
client 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 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| ML1 |match-map|matching-sent|| CC CCC |
|CoAP MID | | |bi| 0000 |MSB(7 ) |LSB(9) || 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. Code has shrunk to 5
bits using a 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.
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Device LPWAN SCHC C/D
| |
| rule id=1 |<--------------------
|<-------------------| +-+-+--+----+------+
<------------------- | 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
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
includes all Options and information needed for proxy operation and
caching. This decomposition is illustrated in Figure 7.
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.
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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.
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 7: OSCORE inner and outer header form a CoAP message
Figure 7 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
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[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
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 8.
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 |
+------+-------------------------+
| |
| Encrypted Inner Header and |
| Payload |
| |
+--------------------------------+
Figure 8: OSCORE message
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The SCHC Compression scheme consists of compressing both the
Plaintext before encryption and the resulting OSCORE message after
encryption, see Figure 9.
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.
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 9: OSCORE Compression Diagram
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7.3. Example OSCORE Compression
An example is given with a GET Request and its consequent CONTENT
Response. 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.
Our first example CoAP message is the GET Request in Figure 10
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 10: CoAP GET Request
Its corresponding response is the CONTENT Response in Figure 11.
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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 11: CoAP CONTENT Response
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 12.
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 12: Inner SCHC Rules
Figure 13 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 must be
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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.
________________________________________________________
| |
| 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 13: Plaintext compression and encryption for GET Request
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In Figure 14 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 must 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
compressed. On top of this, the overhead from the tag bytes is
incurred as before.
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________________________________________________________
| |
| 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 14: Plaintext compression and encryption for CONTENT Response
The Outer SCHC Rules (Figure 17) must process the OSCORE Options
fields. In Figure 15 and Figure 16 we show a dump of the OSCORE
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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:
0xd7080904636c69656e74 (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 15: 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 16: 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 17 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 17: Outer SCHC Rules
These Outer Rules are applied to the example GET Request and CONTENT
Response. The resulting messages are shown in Figure 18 and
Figure 19.
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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 18: 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 19: 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 20.
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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] |equal |not-sent || |
|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 20: SCHC-CoAP Rules (No OSCORE)
This yields the results in Figure 21 for the Request, and Figure 22
for the Response.
Compressed message:
==================
0x0114
0x01 = Rule ID
Compression residue:
0b00010100 (1 byte)
Compressed msg length: 2
Figure 21: CoAP GET Compressed without OSCORE
Compressed message:
==================
0x010a32332043
0x01 = Rule ID
Compression residue:
0b00001010 (1 byte)
Payload
0x32332043
Compressed msg length: 6
Figure 22: CoAP CONTENT Compressed without OSCORE
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As can be seen, the difference between applying SCHC + OSCORE as
compared to regular SCHC + COAP is about 10 bytes of cost.
8. 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-15 (work in
progress), August 2018.
[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., and D. Barthel,
"LPWAN Static Context Header Compression (SCHC) and
fragmentation for IPv6 and UDP", draft-ietf-lpwan-ipv6-
static-context-hc-16 (work in progress), June 2018.
[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
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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
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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