draft-ietf-lpwan-coap-static-context-hc-00.txt   draft-ietf-lpwan-coap-static-context-hc-01.txt 
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Informational L. Toutain Intended status: Informational L. Toutain
Expires: June 8, 2017 Institut MINES TELECOM ; TELECOM Bretagne Expires: September 11, 2017 Institut MINES TELECOM ; IMT Atlantique
December 5, 2016 March 10, 2017
6LPWA Static Context Header Compression (SCHC) for CoAP LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-00 draft-ietf-lpwan-coap-static-context-hc-01
Abstract Abstract
This draft discusses the way SCHC can be applied to CoAP headers and This draft discusses the way SCHC header compression can be applied
extend the number of functions (CDF) to optimize compression. to CoAP headers in an LPWAN flow regarding the generated traffic.
CoAP protocol differs from IPv6 and UDP protocols because the CoAP
Header has a flexible header due to variable options. Another
important difference is the asymmetric format in the header
information used in the request and the response packets. This draft
shows that the Client and the Server do not uses the same fields and
how the SCHC header compression can be used.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 8, 2017. This Internet-Draft will expire on September 11, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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1. Introduction 1. Introduction
[I-D.toutain-lpwan-ipv6-static-context-hc] defines a compression [I-D.toutain-lpwan-ipv6-static-context-hc] defines a header
technique for LPWA network based on static context. This context is compression mechanism for LPWAN network based on a static context.
said static since the element values composing the context are not Where the context is said static since the element values composing
learned during packet exchanges but previously installed. The the context are not learned during the packet exchanges but are
context is known by both ends. A context is composed of a set of previously defined. The context(s) is(are) known by both ends before
rules (referenced by rule ids). A rule describes the header fields transmission.
with some associated Target Values (TV). A Matching Operator (MO) is
associated to each field. The rule is selected if all the MO matches
. A Compression Decompression Function is associated to each field to
define the link between the compressed and decompressed value for a
specific field.
This draft discusses the way SCHC can be applied to CoAP headers and A context is composed of a set of rules (contexts) that are
extend the number of functions (CDF) to optimize compression. referenced by Rule IDs (identifiers). A rule describes the header
fields with 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. In that case, a Compression
Decompression Function (CDF) associated to each field defines the
link between the compressed and decompressed value for each of the
header fields.
2. Compressing CoAP This draft discusses the way SCHC can be applied to CoAP headers, how
to extend MOs to match a specific element when several fields of the
same type are presented in the header. It also introduces the notion
of bidirectional or unidirectional (upstream and downstream) fields.
CoAP [RFC7252] is an implementation of a the REST architecture for 2. CoAP Compressing
contrained devices. Gateway between CoAP and HTTP can be easily
build since both protocol uses the same address space (URL), caching CoAP [RFC7252] is an implementation of the REST architecture for
constrained devices. Gateway between CoAP and HTTP can be easily
built since both protocols uses the same address space (URL), caching
mechanisms and methods. mechanisms and methods.
Nevertheless, if limited, the size of a CoAP header may be Nevertheless, if limited, the size of a CoAP header may be too large
incompatible with LPWAN constraints and some compression may be for LPWAN constraints and some compression may be needed to reduce
needed to reduce the header size. CoAP compression is not the header size. CoAP compression is not straightforward. Some
straightforward. Some differences between IPv6/UDP and CoAP can be differences between IPv6/UDP and CoAP can be highlighted. CoAP
enlighten. CoAP differs from IPv6 and UDP protocols: differs from IPv6 and UDP protocols in the following
aspects:
o IPv6 and UDP are symmetrical protocols. The same fields are found o IPv6 and UDP are symmetrical protocols. The same fields are found
in the request and in the answer, only location in the header may in the request and in the response, only position in the header
change (e.g. source and destination fields). A CoAP request is may change (e.g. source and destination fields). A CoAP request
different from an answer. For instance, the URI-path option is is different from an response. For example, the URI-path option
mandatory in the request and may not be found in the response. is mandatory in the request and is not found in the response.
o CoAP also obeys to the client/server paradigm and the compression o CoAP also obeys to the client/server paradigm and the compression
rate can be different if the request is issued from a LPWAN node rate can be different if the request is issued from a LPWAN node
or from an external device. For instance in the former case the or from an non LPWAN device. For instance a Thing (ES) aware of
token size may be set to one byte. In the latter case, the token LPWAN constraints can generate a 1 byte token, but a regular CoAP
size cannot be constraint and be up to 15 byte long. cleint will certainly send a larger token to the Thing.
o In IPv6, main header and UDP fields have a fixed size. In CoAP, o In IPv6 and UDP header fields have a fixed size. In CoAP, Token
Token size may vary from 0 to 15 bytes, length is given by a field size may vary from 0 to 8 bytes, length is given by a field in the
in the header. More systematically, the options are described header. More systematically, the CoAP options are described using
using the Type-Length-Value principle. Evenmore regarding the the Type-Length-Value. When applying SCHC header compression, the
option size value, the coding will be different. token size is not known at the rule creation, the sender and the
receiver must agree on its compressed size.
o options type in CoAP are not defined with the same value. The o The options type in CoAP is not defined with the same value. The
Delta TLV coding makes that the type is not independant of Delta TLV coding makes that the type is not independent of
previous option and may vary regarding the options contained in previous option and may vary regarding the options contained in
the header. the header.
2.1. CoAP usages 2.1. CoAP behavior
A LPWAN node can either be a client or a server and sometimes both. A LPWAN node can either be a client or a server and sometimes both.
In the client mode, the LPWAN node sends request to a server and In the client mode, the LPWAN node sends request to a server and
expected answer or acknowledgements. Acknowledgements can be at 2 expects an answer or acknowledgements. Acknowledgements can be at 2
different levels: different levels:
o transport level, a CON message is acknowledged by an ACK message. o In the transport level, a CON message is acknowledged by an ACK
NON confirmable messages are not acknowledged. message. A NON confirmable message is not acknowledged at all.
o REST level, a REST request is acknowledged by an "error" code. o In REST level, a REST request is acknowledged by an "error" code.
[RFC7967] defines an option to limit the number of The [RFC7967] defines an option to limit the number of
acknowledgements. acknowledgements.
Note that acknowledgement can be optimized and a REST level Note that acknowledgement can be optimized and a REST level
acknowledgement can be used as a transport level acknowledgement. acknowledgement can be used as a transport level acknowledgement.
2.2. CoAP protocol analysis 2.2. CoAP protocol analysis
CoAP defines the following fields: CoAP header format defines the following fields:
o version (2 bits): this field can be elided during a compresssion o version (2 bits): this field can be elided during the SCHC
compresssion
o type (2 bits): defines the type of the transport messages, 4 o type (2 bits). It defines the type of the transport messages, 4
values are defined. Regarding the type of exchange, if only NON values are defined, regarding the type of exchange. If only NON
messages are sent or CON/ACK messages, this field can be reduced messages are sent or CON/ACK messages, this field can be reduced
to 0 or 1 bit. to 0 or 1 bit.
o token length (4 bytes). The standard allows up to 15 bytes for a o token length (4 bits). The standard allows up to 8 bytes for a
token length. If a fix token size is chosen, then this field can token. If a fixed token size is chosen, then this field can be
be elided. If some variation in length are needed then 1 or 2 elided. If some variation in length are needed then 1 or 2 bits
bits could be enough for most of LPWAN applications. could be enough for most of LPWAN applications.
o code (8 bits). This field codes the request and the response o code (8 bits). This field codes the request and the response
values. CoAP represents in a more compact way, coding used in values. In CoAP these values are represented in a more compact
HTTP, but the coding is not optimal. way then the coding used in HTTP, but the coding is not optimal.
o message id (16 bits). This value is used to acknowledge CON o message id (16 bits). This value of this header field is used to
frames. The size of this field is computed to allow the acknowledge CON frames. The size of this field is computed to
anticipation (how many frames can be sent without allow the anticipation (how many frames can be sent without
acknowledgement). When a value is used, [RFC7252] defines the acknowledgement). When a value is used, the [RFC7252] defines the
time before it can be reused without ambiguities. This size may time before it can be reused without ambiguities. This size
be too large for a LPWAN node sending or receiving few messages a defined may be too large for a LPWAN node sending or receiving few
day. messages a day.
o Token (0 to 15 bytes). Token identifies active flows. Regarding o Token (0 to 8 bytes). Token header field is used to identify
the usage (stability of in time and limited number), a short token active flows. Regarding the usage for LPWAN (stability in time
(1 Byte) can be enough. and limited number), a short token (1 Byte or less) can be enough.
o options are coded through delta-TLV. The delta-T depends of o options are coded using delta-TLV. The delta-T depends on
previous values, length is encoded inside the option. [RFC7252] previous values, length is encoded inside the option. The
distinguishes repeatable options that can appear several time in [RFC7252] distinguishes repeatable options that can appear several
the header. Among them we can distinguish: times in the header. Among them we can distinguish:
* list options which appear several time in the header but are * list options which appear several time in the header but are
exclusive such as the Accept option. exclusive such as the Accept option.
* cumulative options which appear several time in the header but * cumulative options which appear several times in the header but
are part of a more generic value such as Uri-Path and Uri- are part of a more generic value such as Uri-Path and Uri-
Query. Query. In that case, some elements may not change during the
Thing lifetime and other may change at each request. For
instance CoMi [I-D.ietf-core-comi] defines the following path
/c/X6?k="eth0", where the first path element "c" does not
change, the second element can vary over time with a different
length (it represents the base64 enconding of a SID) and the
query string can also vary over time.
For a given flow some value options are stable through time. For a given flow some value options are stable through time.
Observe, ETag, If-Match, If-None-Match and Size varies in each Observe, ETag, If-Match, If-None-Match and Size varies in each
message. Options can be stored in a SCHC context and cumulative message.
options can be stored globally.
The CoAP protocol must not be altered by the compression/ The CoAP protocol must not be altered by the compression/
decompression phase, but if no semantic is attributed to a value, it decompression phase, but if no semantic is attributed to a value, it
may be changed during this phase. For instance the compression phase may be changed during this phase. For instance, the compression
may reduce the size of a token but must maintain its unicity. The phase may reduce the size of a token but must maintain its unicity.
decompressor will not be able to restore the original value but The decompressor will not be able to restore the original value but
behavior will remain the same. If no special semantic is assigned to the behavior will remain the same. If no special semantic is
the token, this will be transparent. If a special semantic is assigned to the token, this will be transparent. If a special
assigned to the token, its compression may not be possible. semantic is assigned to the token, its compression may not be
possible.
This implies that the compressor/decompressor must be aware of the 3. SCHC rules for CoAP header compression
protocol state machine and do not processes request and response the
same way.
A conservative compression leaves the field value unchanged. Non This draft refines the rules definition by adding the direction of
conservative compression can be used when a CoAP implementation has the message, from the Thing point of view (uplink, downlink or
not been defined to work specifically with LPWAN network and uses bidirectional). It does not introduce new Machting Operator or new
large values for fields. Compression Decompression Function, but add some possibility to check
one particular element when several of them are present at the same
time.
2.2.1. CoAP Compression Decompression Function A rule can contain CoAP and IPv6/UDP entries. In that case, IPv6/UDP
entries are tagged bidirectional.
To compress more efficiently CoAP message, several Compression/ 3.1. Directional Rules
Decompression Function (CDF) must be defined.
2.2.1.1. Static-mapping By default, an entry in a rule is bidirectional which means that it
can be applied either on the uplink or downlink headers. By
specifying the direction, the LC will take into account the specific
field only if the direction match.
The goal of static-mapping is to reduce the size of a field by If the Thing is a client, the URI-Path option is only present on
allocating shorter value. The mapping is known by both ends and request and not on the response. Therefore, the exact matching
stored in a table in both end context. The Static-mapping is principle to select a rule cannot apply.
conservative.
Static-mapping may be applied to several fields. For instance the Some options are marked unidirectional, the value (uplink or
type field may be reduced from 2 bits to 1 bit if only CON/ACK type downlink) depends of the scenario. A Uri-Path option will be marked
is used, but the main benefit is compressing the code field. uplink if the Thing acts as a client and downlink if the Thing acts
as a server. If the Thing acts both as client and server, two
different rules will be defined.
3.2. Matching Operator
The Matching Operator behavior has not changed, but the value must
take a position value, if the entry is repeated :
FID TV MO CDF
URI-Path foo equal 1 not-sent
URI-Path bar equal 2 not-sent
Figure 1: Position entry.
For instance, the rule Figure 1 matches with /foo/bar, but not /bar/
foo.
The position is added after the natural argument of the MO, for
example MSB (4,3) indicates a most significant bit matching of 4 bits
in a field located in position 3.
3.3. Compressed field length
When the length is not clearly indicated in the rule, the value
length must be sent with the field data, which means for CoAP to send
directly the CoAP option where the delta-T is set to 0.
For the CoMi path /c/X6?k="eth0" the rule can be set to:
FID TV MO CDF
URI-Path c equal 1 not-sent
URI-Path ignore 2 value-sent
URI-Query k= MSB (16, 1) value-sent
Figure 2: CoMi URI compression
Figure 2 shows the parsing and the compression of the URI. where c is
not sent. The second element is sent with the length (i.e. 0x02 X 6)
followed by the query option (i.e. 0x08 k="eth0").
[[NOTE we don't process URI with a multiple number of path element
??]].
4. Application to CoAP header fields
This section lists the different CoAP header fields and how they can
be compressed.
4.1. CoAP version field
This field is bidirectional.
This field contains always the same value, therefore the TV may be 1,
the MO is set to "equal" and the CDF is set to "not-sent"
4.2. CoAP type field
This field is bidirectional or undirectional.
Several strategies can be applied to this field regarding the values
used:
o if only one type is sent, for example NON message, its
transmission can be avoided. TV is set to the value, MO is set to
"equal" and CDF is set to "not-sent".
o if two values are sent, for example CON and ACK and RST is not
used, this field can be reduced to one bit. TV is set to a
matching value {CON: 0, ACK: 1}, MO is set to match-mapping and
CDF is set to mapping-sent.
o It is also possible avoid transmission of this field by marking it
unidirectional. In one direction, the TV is set to CON, MO is set
to "equal" and CDF is set to "not-sent". On the other direction,
the TV is set to ACK, the MO is set to "equal" and the CDF is set
to "not-sent".
o Otherwise TV is not set, MO is set to "ignore" and CDF is set to
"value-sent".
4.3. CoAP token length field
This field is bi-directional.
Several strategies can be applied to this field regarding the values:
o no token or a wellknown length, the transmission can be avoided.
TV is set to the length, the MO is set to "equal" and CDF is set
to "not-sent"
o The length is variable from one message to another. TV is not
set, MO is set to "ignore" and CDF is set to "value-sent". The
size of the sent value must be known by ends. The size may be 4
bits. The receiver must take into account this value to retrieve
the token. A CoAP proxy may be used before the compression to
reduce the field size.
4.4. CoAP code field
This field is unidirectional. The client and the server do not use
the same values.
The CoAP code field defines a tricky way to ensure compatibility with The CoAP code field defines a tricky way to ensure compatibility with
HTTP values. Nevertheless only 21 values are defined by [RFC7252] HTTP values. Nevertheless only 21 values are defined by [RFC7252]
compared to the 255 possible values. So it could efficiently be compared to the 255 possible values. So, it could efficiently be
coded on 5 bits. To allow flexibility and evolution if new codes are coded on 5 bits. The number of code may vary over time, some new
introduced, a static mapping table is associated to each instance of codes may be introduced or some applications use a limited number of
this function. values.
Figure 1 gives a possible mapping, it can be changed to add new codes +------+------------------------------+-----------+
| Code | Description | Mapping |
+------+------------------------------+-----------+
| 0.00 | | 0x00 |
| 0.01 | GET | 0x01 |
| 0.02 | POST | 0x02 |
| 0.03 | PUT | 0x03 |
| 0.04 | DELETE | 0x04 |
| 0.05 | FETCH | 0x05 |
| 0.06 | PATCH | 0x06 |
| 0.07 | iPATCH | 0x07 |
| 2.01 | Created | 0x08 |
| 2.02 | Deleted | 0x09 |
| 2.03 | Valid | 0x0A |
| 2.04 | Changed | 0x0B |
| 2.05 | Content | 0x0C |
| 4.00 | Bad Request | 0x0D |
| 4.01 | Unauthorized | 0x0E |
| 4.02 | Bad Option | 0x0F |
| 4.03 | Forbidden | 0x10 |
| 4.04 | Not Found | 0x11 |
| 4.05 | Method Not Allowed | 0x12 |
| 4.06 | Not Acceptable | 0x13 |
| 4.12 | Precondition Failed | 0x14 |
| 4.13 | Request Entity Too Large | 0x15 |
| 4.15 | Unsupported Content-Format | 0x16 |
| 5.00 | Internal Server Error | 0x17 |
| 5.01 | Not Implemented | 0x18 |
| 5.02 | Bad Gateway | 0x19 |
| 5.03 | Service Unavailable | 0x1A |
| 5.04 | Gateway Timeout | 0x1B |
| 5.05 | Proxying Not Supported | 0x1C |
+------+------------------------------+-----------+
Figure 3: Example of CoAP code mapping
Figure 3 gives a possible mapping, it can be changed to add new codes
or reduced if some values are never used by both ends. or reduced if some values are never used by both ends.
+------+------------------------------+-----------+ The field can be treated differently in upstream than in downstream.
| Code | Description | Mapping | If the Thing is a client an entry can be set on the uplink message
+------+------------------------------+-----------+ with a code matching for 0.0X values and another for downlink values
| 0.00 | | 0x00 | for Y.ZZ codes. It is the opposite if the thing is a server.
| 0.01 | GET | 0x01 |
| 0.02 | POST | 0x02 |
| 0.03 | PUT | 0x03 |
| 0.04 | DELETE | 0x04 |
| 0.05 | FETCH | 0x05 |
| 0.06 | PATCH | 0x06 |
| 0.07 | iPATCH | 0x07 |
| 2.01 | Created | 0x08 |
| 2.02 | Deleted | 0x09 |
| 2.03 | Valid | 0x0A |
| 2.04 | Changed | 0x0B |
| 2.05 | Content | 0x0C |
| 4.00 | Bad Request | 0x0D |
| 4.01 | Unauthorized | 0x0E |
| 4.02 | Bad Option | 0x0F |
| 4.03 | Forbidden | 0x10 |
| 4.04 | Not Found | 0x11 |
| 4.05 | Method Not Allowed | 0x12 |
| 4.06 | Not Acceptable | 0x13 |
| 4.12 | Precondition Failed | 0x14 |
| 4.13 | Request Entity Too Large | 0x15 |
| 4.15 | Unsupported Content-Format | 0x16 |
| 5.00 | Internal Server Error | 0x17 |
| 5.01 | Not Implemented | 0x18 |
| 5.02 | Bad Gateway | 0x19 |
| 5.03 | Service Unavailable | 0x1A |
| 5.04 | Gateway Timeout | 0x1B |
| 5.05 | Proxying Not Supported | 0x1C |
+------+------------------------------+-----------+
Figure 1: CoAP code mapping 4.5. CoAP Message ID field
This CDF can also be applied to path to send a reference instead of This field is bidirectional.
the path value.
2.2.1.2. Remapping Message ID is used for two purposes:
With dynamic mapping, the mapping is done dynamically, which means o To acknowledge a CON message with an ACK.
that the other end has no way to the learn the original value. This
function is not conservative. The mapping context must be stored in
a reliable way on the compressor, if lost the session with LPWAN node
will be lost, which can generate a traffic increase on the LPWA
network.
This function converts a large number to a smaller one and maintain o To avoid duplicate messages.
bi-directional mapping. If the field has no semantic, such as a CoAP
token or a message ID, this will reduce the size of the information
sent on the link. This mapping only applies for request compression,
answers must keep the value original value.
For instance a compression receives a CoAP request with a large In LPWAN, since a message can be received by several radio gateway,
token. The compressor reduces the token size by allocating a unused some LPWAN technologies include a sequence number in L2 to avoid
value in a smaller space. When the response come back, the duplicate frames. Therefore if the message does not need to be
compressor exchange the smallest token with the original one. acknowledged (NON or RST message), the Message ID field can be
avoided. In that case TV is not set, MO is set to ignore and CDF is
set to "not-sent". The decompressor can generate a number.
This mean that the compressor must be aware of the CoAP state [[Note; check id this field is not used by OSCOAP .]]
machine, to identify a request and its associated response, but also
determine when a token value can be reused.
2.2.1.3. Reduce-entropy To optimize information sent on the LPWAN, shorter values may be used
during the exchange, but Message ID values generated a common CoAP
implementation will not take into account this limitation. Before
the compression, a proxy may be needed to reduce the size. In that
case, the TV is set to 0x0000, MO is set to "MSB(l)" and CDF is set
to "LSB(16-l)", where "l" is the size of the compressed header.
Reduce-entropy is a non-conservative function. the goal is to Otherwise if no compression is needed the TV is not set, MO is set to
minimize the increase in a field value. It may be used for the ignore and CDF is set to "not-sent".
observe option, all increase in the original sequence number will
lead to an increase of 1 in the compressed value.
For instance a LPWAN node is a CoAP server and receives Observe 4.6. CoAP Token field
responses coming from an outside client. The client uses a clock to
generate Observe sequence number. If that value has non particular
meaning for the CoAP server, increase of 1 will not change the
protocol behavior. Reordering works the same way as for original
Observe.
2.2.2. CoAP mandatory header This field is bi-directional.
Figure 2 proposes some function assignments to the CoAP header Token is used to identify transactions and varies from one
fields. transaction to another. Therefore, it is usually necessary to send
the value of the token field on the LPWAN network. The optimization
will occur by using small values.
/--------------------+---------------------+----------------------------------------\ Common CoAP implementations may generate large tokens, even if
| Field |Function | Behavior | shorter tokens could be used regarding the LPWAN characteristics. A
+--------------------+---------------------+----------------------------------------+ proxy may be needed to reduce the size of the token before
|version |not-sent |version is always the same | compression.
+--------------------+---------------------+----------------------------------------+
|type |value-sent |if all the types are used |
| |static-mapping |to reduce to one bit if 2 type are used |
| |not-sent |if only one type is used (e.g. NON) |
+--------------------+---------------------+----------------------------------------+
|token length |not-sent |no tokens or fixed size |
| |compute-token-length |if token size is reduced |
| |value-sent |token is sent integrally |
+--------------------+---------------------+----------------------------------------+
|code |value-sent |no modification |
| |static-mapping |code size reduction |
+--------------------+---------------------+----------------------------------------+
|message id |value-sent |no modification |
|token |remapping |reduces message id size |
+====================+=====================+========================================+
|Content-Format |value-sent |no modification |
|Accept |not-sent |defined in the rule |
|Max-Age |static-mapping |map the possible value |
+--------------------+---------------------+----------------------------------------+
|Path: |value-sent |no modification |
|Uri-Host+Uri-Port+ |not-sent |defined in the rule |
|Uri-Path*+Uri-Query*|static-mapping |a value to define a path |
| | | |
|Proxy-Uri | |Note: only the full path is stored in |
|Proxy-Scheme | |context |
+--------------------+---------------------+----------------------------------------+
|ETag |value-sent |Always sent |
|Location-Path | | |
|Location-Query | | |
|If-Match | | |
|If-None-Match | | |
|Size1 | | |
+--------------------+---------------------+----------------------------------------+
Figure 2: SCHC functions' example assignment for CoAP Otherwise the TV is not set, the MO is set to ignore and CDF is set
to "value-sent".
2.2.3. Examples of CoAP header compression The decompression may know the length of the token field from the
token length field.
2.2.3.1. Mandatory header with CON message 4.7. CoAP option Content-format field.
This field is unidirectional and must not be set to bidirectional in
a rule entry. It is used only by the server to inform the client
about of the payload type and is never found in client requests.
If the value is known by both sides, the TV contains that value and
MO is set to "equal" and the CDF is set to "not-sent".
Otherwise the TV is not set, MO is set to "ignore" and CDF is set to
"value-sent"
A mapping list can also be used to reduce the size.
4.8. CoAP option Accept field
This field is unidirectional and must not be set to bidirectional in
a rule entry. It is used only by the client to inform of the
possible payload type and is never found in server response.
The number of accept options is not limited and can vary regarding
the usage. To be selected a rule must contain the exact number about
accept options with their positions.
if the accept value must be sent, the TV contains that value, MO is
set to "ignore x" where "x" is the accept option's position and CDF
is set to value-sent. Since the value length is not known, it must
be sent as a CoAP TLV with delta-T set to 0.
Otherwise the TV is not set, MO is set to "equal x" where x is the
accept option's position and CDF is set to "not-sent"
[[note: it could be more liberal and do not provide the same order
after decompression]]
4.9. CoAP option Max-Age field
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, the TV is set with this
duration, the MO is set to "equal" and the CDF is set to "not-sent".
Otherwise the TV is not set, the MO is set to "ignore" and the CDF is
set to "value-sent". Since the value length is not known, it must be
sent as a CoAP TLV with delta-T set to 0.
[[note: we can reduce (or create a new option) the unit to minute,
second is small for LPWAN ]]
4.10. CoAP option Uri-Host and Uri-Port 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 server and are never found in server response.
For each option, if the value is known by both ends, the TV is set
with this value, the MO is set to "equal" and the CDF is set to "not-
sent".
Otherwise the TV is not set, the MO is set to "ignore" and the CDF is
set to "value-sent". Since the value length is not known, it must be
sent as a CoAP TLV with delta-T set to 0.
4.11. 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 response.
Path and Query option may have different formats. Section 3.2 gives
some examples.
If the URI path as well as the query is composed of 2 or more
elements, then the position must be set in the MO parameters.
If a Path or Query element is stable over the time, then TV is set
with its value, MO is set to "equal x" where x is the position in the
Path or the Query and CDF is set to "not-sent".
Otherwise if the value varies over time, TV is not set, MO is set to
"ignore x" where x is the position in the Path or in the Query and
CDF is set to "value-sent". Since the value length is not known, it
must be sent as a CoAP TLV with deltaT set to 0.
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.
For instance, the following Path /foo/bar/variable/stable can leads
to the rule defined Figure 4.
FID TV MO CDF
URI-Path {"/foo/bar":1, match-mapping 1 mapping-sent
"/bar/foo":2}
URI-Path ignore 3 value-sent
URI-Path stable equal 4 not-sent
Figure 4: complex path example
4.12. CoAP option 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 CDF 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 CDF is
set to "not-sent"
4.13. 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 request.
[[Can include OSCOAP Object security in that category ]]
5. Other RFCs
5.1. Block
Block option should be avoided in LPWAN. The minimum size of 16
bytes can be incompatible with some LPWAN technologies.
[[Note: do we recommand LPWAN fragmentation since the smallest value
of 16 is too big?]]
5.2. Observe
[RFC7641] defines the Observe option. The TV is not set, MO is set
to "ignore" and the CDF is set to "value-sent". SCHC does not limit
the maximum size for this option (3 bytes). To reduce the
transmission size either the Thing implementation should limit the
value increase or a proxy can be used limit the increase.
Since RST message may be sent to inform a server that the client do
not require Observe response, a rule must allow the transmission of
this message.
5.3. No-Response
[RFC7967] defines an No-Response option limiting the responses made
by a server to a request. If the value is not by both ends, then TV
is set to this value, MO is set to "equal" and CDF is set to "not-
sent".
Otherwise, if the value is changing over time, TV is not set, MO is
set to "ignore" and CDF to "value-sent". A matching list can also be
used to reduce the size.
6. Examples of CoAP header compression
6.1. Mandatory header with CON message
In this first scenario, the LPWAN compressor receives from outside In this first scenario, the LPWAN compressor receives from outside
client a POST message, which is immediately acknowledged by the ES. client a POST message, which is immediately acknowledged by the
For this simple scenario, the rules are described Figure 3 Thing. For this simple scenario, the rules are described Figure 5.
rule id 1
+-------------+-------+-----+---------------+----------------+
| Field |TV |MO |CDF | Sent |
+=============+=======+=====+===============+================+
|CoAP version | 01 |= |not-sent | |
|CoAP Type | | |value-sent |TT |
|CoAP TKL | 0000 |= |not-sent | |
|CoAP Code | | |static-map | CC CCC |
|CoAP MID | | |dynamic-map | M-ID |
|CoAP Path |/path | |not-sent | |
+-------------+-------+-----+---------------+----------------+
Figure 3: CoAP Context to compress header without token rule id 1
+-------------+------+---------+-------------+-----+----------------+
| Field |TV |MO |CDF |dir | Sent |
+=============+======+=========+=============+=====+================+
|CoAP version | 01 |equal |not-sent |bi | |
|CoAP Type | |ignore |value-sent |bi |TT |
|CoAP TKL | 0 |equal |not-sent |bi | |
|CoAP Code | ML1 |match-map|matching-sent|bi | CC CCC |
|CoAP MID | 0000 |MSB(7 ) |LSB(9) |bi | M-ID |
|CoAP Uri-Path| path |equal 1 |not-sent |down | |
+-------------+------+---------+-------------+-----+----------------+
Figure 3 gives a simple compression rule for CoAP headers without Figure 5: CoAP Context to compress header without token
tokens.
The version fields and Token Length are elided. Code has shrunk to 5 The version and Token Length fields are elided. Code has shrunk to 5
bits using the static-mapping function. Message-ID has shrunk to 9 bits using the matching list (as the one given Figure 3: 0.01 is
bits to preserve alignment on byte boundary. value 0x01 and 2.05 is value 0x0c) Message-ID has shrunk to 9 bits to
preserve alignment on byte boundary. The most significant bit must
be set to 0 through a CoAP proxy. Uri-Path contains a single element
indicated in the matching operator.
Figure 4 shows the time diagram of the exchange. A LPWAN Application Figure 6 shows the time diagram of the exchange. A LPWAN Application
Server sends a CON message. Compression reduces the header sending Server sends a CON message. Compression reduces the header sending
only the Type, a mapped code and the Message ID is change to a value only the Type, a mapped code and the least 9 significant bits of
on 9 bits. The receiver decompress the header. The message ID value Message ID. The receiver decompresses the header. .
is changed.
The CON message is a request, therefore the LC process to a dynamic The CON message is a request, therefore the LC process to a dynamic
mapping. When the ES receives the ACK message, this will not mapping. When the ES receives the ACK message, this will not
initiate locally a the message ID mapping since it is a response. initiate locally a message ID mapping since it is a response. The LC
The LC receives the ACK and uncompress it to restore the original receives the ACK and uncompressed it to restore the original value.
value. Dynamic Mapping context lifetime follows the same rules as Dynamic Mapping context lifetime follows the same rules as message ID
message ID duration. duration.
End System LPWA LC End System LPWA LC
| | | |
| rule id=1 |<---------------------- | rule id=1 |<----------------------
|<---------------------------| +-+-+--+----+--------+ |<--------------------| +-+-+--+----+--------+
<-------------------- | TTCC CCCM MMMM MMMM | |1|0| 4|0.01| 0x1234 | <-------------------- | TTCC CCCM MMMM MMMM| |1|0| 4|0.01| 0x0034 |
+-+-+--+----+--------+ | 0000 0010 0000 0001 | | 0xb4 p a t | +-+-+--+----+--------+ | 0000 0010 0011 0100| | 0xb4 p a t |
|1|0| 1|0.01| 0x0001 | | | | h | |1|0| 1|0.01| 0x0034 | | | | h |
| 0xb4 p a t | | | +------+ | 0xb4 p a t | | | +------+
| h | | | dynamic mapping | h | | |
+------+ | | +--------+--------+ +------+ | |
| | |0x1234 | 0x01 | | |
| | +--------+--------+ | |
+-+-+--+----+--------+ |--------------------------->| ----------------------->| rule id=1 |
|1|2| 0|2.05| 0x0001 | | TTCC CCCM MMMM MMMM |------------------------> +-+-+--+----+--------+ |-------------------->|
+-+-+--+----+--------+ | 1000 0000 0000 0001 | +-+-+--+----+--------+ |1|2| 0|2.05| 0x0034 | | TTCC CCCM MMMM MMMM|------------------------>
| | |1|2| 0|2.05| 0x1234 | +-+-+--+----+--------+ | 1001 1000 0011 0100| +-+-+--+----+--------+
v v +-+-+--+----+--------+ | | |1|2| 0|2.05| 0x0034 |
v v +-+-+--+----+--------+
Figure 4: Compression with global addresses Figure 6: Compression with global addresses
Note that the compressor and decompressor must understand the CoAP The message can be further optimized by setting some fields
protocol: unidirectional, as described in Figure 7. Note that Type is no more
sent in the compressed format, Compressed Code size in not changed in
that example (8 values are needed to code all the requests and 21 to
code all the responses in the matching list Figure 3)
rule id 1
+-------------+------+---------+-------------+---+----------------+
| Field |TV |MO |CDF |dir| Sent |
+=============+======+=========+=============+===+================+
|CoAP version | 01 |equal |not-sent |bi | |
|CoAP Type | CON |equal |not-sent |dw | |
|CoAP Type | ACK |equal |not-sent |up | |
|CoAP TKL | 0 |equal |not-sent |bi | |
|CoAP Code | ML2 |match-map|matching-sent|dw |CCCC C |
|CoAP Code | ML3 |match-map|matching-sent|up |CCCC C |
|CoAP MID | 0000 |MSB(5) |LSB(11) |bi | M-ID |
|CoAP Uri-Path| path |equal 1 |not-sent |dw | |
+-------------+------+---------+-------------+---+----------------+
o The LC compressor detects a new transport request and allocate a ML1 = {CON : 0, ACK:1} ML2 = {POST:0, 2.04:1, 0.00:3}
new dynamic mapping value.
o When receiving a response the ES compressor ES detects that this Figure 7: CoAP Context to compress header without token
is a response (type=2) therefore the message ID value in
unchanged.
o The upstream compressor detects that is an REST answer (code 2.05) 6.2. Complete exchange
therefore the path option is not inserted in the uncompress header
2.2.3.2. Exchange with token In that example, the Thing is using CoMi and sends queries for 2 SID.
The following scenario introduces tokens. The LC manages two CON
remapping contexts. One for Message ID and the other for token. ES MID=0x0012 | |
manages one context for Message ID. Mapping is trigged by the POST | |
reception of CON messages to compress or CoAP requests to compress. Accept X | |
Note that the compressed message ID size has been reduced to 7 bits, /c/k=AS |------------------------>|
compared to the previous example, to maintain byte boundary | |
alignment. | |
|<------------------------| ACK MID=0x0012
| | 0.00
| |
| |
|<------------------------| CON
| | MID=0X0034
| | Content-Format X
ACK MID=0x0034 |------------------------>|
0.00
+----------------+------------------------+----------------+-----------------+ rule id 3
| Field | Function | Ctxt Value | Sent compressed | +-------------+------+---------+-------------+---+----------------+
+----------------+------------------------+----------------+-----------------+ | Field |TV |MO |CDF |dir| Sent |
|CoAP version | not-sent | | | +=============+======+=========+=============+===+================+
|CoAP Type | value-sent | |TT | |CoAP version | 01 |equal |not-sent |bi | |
|CoAP TKL | compute-token-length | | LL | |CoAP Type | CON |equal |not-sent |up | |
|CoAP Code | map-code | mapping table | CCCC C | |CoAP Type | ACK |equal |not-sent |dw | |
|CoAP MID | remapping | 7 bits | M-ID | |CoAP TKL | 1 |equal |not-sent |bi | |
|CoAP Token | remapping | 8 bits | token| |CoAP Code | POST |equal |not-sent |up | |
|CoAP Path | not-sent |/data/humidity | |CoAP Code | 0.00 |equal |not-sent |dw | |
+----------------+------------------------+----------------+-----------------+ |CoAP MID | 0000 |MSB(8) |LSB(8) |bi |MMMMMMMM |
|CoAP Token | |ignore |send-value |up |TTTTTTTT |
|CoAP Uri-Path| /c |equal 1 |not-sent |dw | |
|CoAP Uri-query ML4 |equal 1 |not-sent |dw |P |
|CoAP Content | X |equal |not-sent |up | |
+-------------+------+---------+-------------+---+----------------+
Figure 5: CoAP Context to compress header with token rule id 4
End System LPWA LC +-------------+------+---------+-------------+---+----------------+
| | | Field |TV |MO |CDF |dir| Sent |
| SHIM=1 |<---------------------- +=============+======+=========+=============+===+================+
|<---------------------------| +-+-+--+----+--------+ |CoAP version | 01 |equal |not-sent |bi | |
<-------------------- | TT LL CCCC C MMMMMMM | |1|0| 4|0.01| 0x1234 | |CoAP Type | CON |equal |not-sent |dw | |
+-+-+--+----+--------+ | 00 01 0000 1 0000001 | | DEADBEEF | |CoAP Type | ACK |equal |not-sent |up | |
|1|0| 1|0.01| 0x0001 | | 0000 0001 | | 0xb4 d a t | |CoAP TKL | 1 |equal |not-sent |bi | |
| 01 0xb4 d a | | Token | | a 0x08 h u | |CoAP Code | 2.05 |equal |not-sent |dw | |
| t a 0x08 h | | | | m i d i | |CoAP Code | 0.00 |equal |not-sent |up | |
| u m i d | | | | t y | |CoAP MID | 0000 |MSB(8) |LSB(8) |bi |MMMMMMMM |
| i t y | | | +------------+ |CoAP Token | |ignore |send-value |dw |TTTTTTTT |
+-----------------+ | | Mid mapping: 1234 -> 1 |COAP Accept | X |equal |not-sent |dw | |
| | Tk mapping: DEADBEEF -> 1 +-------------+------+---------+-------------+---+----------------+
+-+-+--+----+--------+ |--------------------------->|
|1|2| 0|0.00| 0x0001 | | TT LL CCCC C MMMMMMMM |------------------------>
+-+-+--+----+--------+ | 10 01 0000 0 00000001 | +-+-+--+----+--------+
| | |1|2| 0|0.00| 0x1234 |
| | +-+-+--+----+--------+
+-+-+--+----+--------+ |--------------------------->|
|1|0| 0|2.05| 0xCAFE | | TT LL CCCC C MMMMMMMM |------------------------>
| 0x01 2 5 | | 00 01 1100 0 00000002 | +-+-+--+----+--------+
+--------------------+ | 0000 0001 | |1|0| 4|2.05| 0x0001 |
| 2 5 | | DEADBEEF |
| | | 2 5 |
Mid mapping: CAFE -> 1 | | +-----------+
| |
| |<------------------------
|<---------------------------| +-+-+--+----+--------+
<-----------------------| TT LL CCCC C MMMMMMMM | |1|2| 0|0.00|0x0001 |
+-+-+--+----+--------+ | 10 00 0000 0 00000002 | +-+-+--+----+--------+
|1|2| 0|0.00| 0xCAFE | | |
+-+-+--+----+--------+ | |
v v
Figure 6: Compression with token alternative rule:
3. Normative References rule id 4
+-------------+------+---------+-------------+---+----------------+
| Field |TV |MO |CDF |dir| Sent |
+=============+======+=========+=============+===+================+
|CoAP version | 01 |equal |not-sent |bi | |
|CoAP Type | ML1 |equal |match-sent(1)|bi |t |
|CoAP TKL | 1 |equal |not-sent |bi | |
|CoAP Code | ML2 |equal |match-sent(1)|up | cc |
|CoAP Code | ML3 |equal |match-sent(2)|dw | cc |
|CoAP MID | 0000 |MSB(8) |LSB(8) |bi |MMMMMMMM |
|CoAP Token | |ignore |send-value |dw |TTTTTTTT |
|CoAP Uri-Path| /c |equal 1 |not-sent |dw | |
|CoAP Uri-query ML4 |equal 1 |not-sent |dw |P |
|CoAP Content | X |equal |not-sent |up | |
|COAP Accept | x |equal |not-sent |dw | |
+-------------+------+---------+-------------+---+----------------+
ML1 {CON:0, ACK:1} ML2 {POST:0, 0.00: 1} ML3 {2.05:0, 0.00:1}
ML4 {NULL:0, k=AS:1, K=AZE:2}
7. Normative References
[I-D.ietf-core-comi]
Stok, P., Bierman, A., Veillette, M., and A. Pelov, "CoAP
Management Interface", draft-ietf-core-comi-00 (work in
progress), January 2017.
[I-D.toutain-lpwan-ipv6-static-context-hc] [I-D.toutain-lpwan-ipv6-static-context-hc]
Minaburo, A. and L. Toutain, "LPWAN Static Context Header Minaburo, A. and L. Toutain, "LPWAN Static Context Header
Compression (SCHC) for IPv6 and UDP", draft-toutain-lpwan- Compression (SCHC) for IPv6 and UDP", draft-toutain-lpwan-
ipv6-static-context-hc-00 (work in progress), September ipv6-static-context-hc-00 (work in progress), September
2016. 2016.
[RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol [RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC 1332, DOI 10.17487/RFC1332, May 1992, (IPCP)", RFC 1332, DOI 10.17487/RFC1332, May 1992,
<http://www.rfc-editor.org/info/rfc1332>. <http://www.rfc-editor.org/info/rfc1332>.
skipping to change at page 13, line 42 skipping to change at page 18, line 10
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011, DOI 10.17487/RFC6282, September 2011,
<http://www.rfc-editor.org/info/rfc6282>. <http://www.rfc-editor.org/info/rfc6282>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014, DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>. <http://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,
<http://www.rfc-editor.org/info/rfc7641>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T. [RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967, No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <http://www.rfc-editor.org/info/rfc7967>. August 2016, <http://www.rfc-editor.org/info/rfc7967>.
Authors' Addresses Authors' Addresses
Ana Minaburo Ana Minaburo
Acklio Acklio
2bis rue de la Chataigneraie 2bis rue de la Chataigneraie
35510 Cesson-Sevigne Cedex 35510 Cesson-Sevigne Cedex
France France
Email: ana@ackl.io Email: ana@ackl.io
Laurent Toutain Laurent Toutain
Institut MINES TELECOM ; TELECOM Bretagne Institut MINES TELECOM ; IMT Atlantique
2 rue de la Chataigneraie 2 rue de la Chataigneraie
CS 17607 CS 17607
35576 Cesson-Sevigne Cedex 35576 Cesson-Sevigne Cedex
France France
Email: Laurent.Toutain@telecom-bretagne.eu Email: Laurent.Toutain@imt-atlantique.fr
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