LPWAN Static Context Header Compression (SCHC) for CoAPAcklio2bis rue de la Chataigneraie35510 Cesson-Sevigne CedexFranceana@ackl.ioInstitut MINES TELECOM ; IMT Atlantique2 rue de la ChataigneraieCS 1760735576 Cesson-Sevigne CedexFranceLaurent.Toutain@imt-atlantique.frlpwan Working GroupThis 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 Header
is flexible header with a variable number of options themself of a variable length.
Another important difference is
the asymmetry in the header information used for request and
response messages. This draft takes into account the fact that a thing can play the
role of a CoAP client, a CoAP client or both roles.CoAP 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.Nevertheless, if limited, the size of a CoAP header may be
too large for LPWAN constraints and some compression may be
needed to reduce the header size. defines a header compression
mechanism for LPWAN network based on a static context. The context is
said static since the element values composing 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 (contexts) that are referenced by Rule IDs
(identifiers). A rule contains an ordered list of the header fields containing à field ID (FID)
and its position when repeated, a direction indicator (DI) (upstream, downstream and bidirectional)
and some associated Target Values (TV) which are expected in the message header. 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.CoAP differs from IPv6 and UDP protocols on the following
aspects:IPv6 and UDP are symmetrical protocols. The same fields are found in the
request and in
the response, only location in the header may vary (e.g. source and destination
fields). A CoAP request is different from an response. For example, the URI-path
option is mandatory in the request and is not found in the response, request may contain
an Accept option and the response a Content-format option.
Even when a field is “symmetric” (i.e. found in both directions) the values carried are
different. For instance the Type field will contain a CON value in the request and a
ACK or RST value in the response. Exploiting the asymmetry in compression will allow to
send no bit in the compressed request and a single bit in the answer. Same behavior can be
applied to the CoAP Code field (O.OX code are present in the request and Y.ZZ in the answer).CoAP also obeys to the client/server paradigm and the compression rate can
be different if the request is issued from a LPWAN node or from an non LPWAN
device. For instance a Thing (ES) 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.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 header. More
systematically, the CoAP options are described using the Type-Length-Value.
When applying SCHC header compression.
By sending compressed field information following the rule order, SCHC offers a
serialization/deserialization mechanism. Since a field exists to indicate the token
length there is no ambiguity. For options, the rule indicates also the expected options
found the int CoAP header. Therefore only the length is needed to recognise an option.
The length will be send using the same CoAP encoding (size less than 12 are directly sent,
higher values uses the escape mechanisms defined by ). Delta Type is omitted,
the value will
be recovered by the decompressor. This reduce the option length of 4, 12 or 20 bits regarding
the orignial size of the delta type encoding in the option.In CoAP headers a field can be duplicated several times, for instances, elements
of an URI (path or queries) or accepted
formats. The position
defined in a rule, associated to a Field ID, can be used to identify the proper element.This section discusses of the compression of the different CoAP header fields. These are just
examples. The compression should take into account the nature of the traffic and not
only the field values. Next chapter will define some compression rules for some common
exchanges.This field is bidirectional and can be elided during the SCHC compression, since it always
contains the same value. It appears only in first position.This field can be managed bidirectionally or unidirectionally.Several strategies can be
applied to this field regarding the values used:If the ES is a client or a Server and non confirmable message are used, the transmission
of the Type field can be avoided: Pos is always 1,DI can either be “uplink” if the ES is a CoAP client or “downlink” if the ES
is a CoAP server, or “bidirectional”TV is set to the value,MO is set to “equal”CDF is set to “not-sent”.If the ES is either a client or a Server and confirmable message are used, the
DI can be used to elide the type on the request and compress it to 1 bit on the response.
The example above shows the rule for a ES acting as a client, directions need to be
reversed for a ES acting as a server.Otherwise if the ES is acting simultaneously as a client and a server and the rule handle
these two traffics, Type field must be sent uncompressed.This field is bi-directional.Several strategies can be applied to this field regarding the values:no token or a wellknown length, the transmission can be avoided. A special care must be
taken, if CON messages are acknowledged with an empty ACK message. In that case the token
is not always present.If the length is changing from one message to an other, the Token Length field must be
sent. If the Token length can be limited, then only the least significant bits have
to be sent. The example below allows values between 0 and 3.otherwise the field value has to be sent.This field is bidirectional, but compression can be enhanced using DI.The CoAP Code field defines a tricky way to
ensure compatibility with
HTTP values. Nevertheless only 21 values are defined by
compared to the 255 possible values. gives a possible mapping, it can be changed
to add new codes or reduced if some values are never used by both ends.
It could efficiently be coded on 5 bits.Even if the number of code can be increase
with other RFC, implementations may use a limited number of values, which can
help to reduce the number of bits sent on the LPWAN.The number of code may vary over time, some new codes
may be introduced or some applications use a limited number of values.The client and the server do not use the
same values. This asymmetry can be exploited to reduce the size sent on
the LPWAN.The field can be treated differently in upstream than in downstream.
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 for Y.ZZ
codes. It is the opposite if the thing is a server.If the ES always sends or receives requests with the same method, the Code
field can be elided. The entry below shows a rule for a client sending only
GET request.If the client may send different methods, a matching-list can be applied. For
table , 3 bits are necessary, but it could be less
if fewer methods are used. Example below gives an example where the ES is a server
and receives only GET and POST requests.The same approach can be applied to responses.This field is bidirectional.Message ID is used for two purposes:To acknowledge a CON message with an ACK.To avoid duplicate messages.In LPWAN, since a message can be received by several radio gateway, some
LPWAN technologies include a sequence number in L2 to avoid duplicate frames. Therefore
if the message does not need to be acknowledged (NON or RST message), the Message
ID field can be avoided.The decompressor must generate a value.[[Note; check id this field is not used by OSCOAP .]]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.Otherwise if no compression is possible, the field has to be sentThis field is bi-directional.Token is used to identify transactions and varies from one
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 shorter tokens could
be used regarding the LPWAN characteristics. A proxy may be needed to reduce
the size of the token before compression.The size of the compress token sent is known by a combination of the Token Length field
and the rule entry. For instance, with the entry below:The uncompressed token is 2 bytes long, but the compressed size will be 4 bits.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 single value is expected by the client, the TV contains that value and MO is set to
“equal” and the CDF is set to “not-sent”. The examples below describe the rules for an
ES acting as a server.If several possible value are expected by the client, a matching-list can be used.Otherwise the value can be sent.The value-sent CDF in the compressor do not send the
option type and the decompressor reconstruct it regarding the position in the rule.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. Since the order in which the Accept value are sent, the position order
can be modified. The rule belowwill be selected only if two accept options are in the CoAP header if this order.The rule below:will accept a-only CoAP messages with 2 accept options, but the order will not influence
the rule selection. The decompression will reconstruct the header regarding the rule order.Otherwise a matching-list can be applied to the different values, in that case the order
is important to recover the appropriate value and the position must be clearly indicate.Finally, the option can be explicitly sent.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 wellknown values are defined.Otherwise the option length and value can be sent on the LPWAN.[[note: we can reduce (or create a new option) the unit to minute,
second is small for LPWAN ]]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.The Matching Operator behavior has not changed, but the value must take a position value,
if the entry is repeated :For instance, the rule matches with /foo/bar, but not /bar/foo.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 with length and value.For instance for a CoMi path /c/X6?k=”eth0” the rule can be set to: 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”).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
.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”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 ]]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?]] 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. 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.In this first scenario, the LPWAN compressor receives from outside client
a POST message, which is immediately acknowledged by the Thing. For this simple
scenario, the rules are described .The version and Token Length fields are elided. Code has shrunk to 5 bits
using the matching list (as the one given : 0.01
is 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. shows the time diagram of the exchange. A LPWAN Application Server sends
a CON message. Compression reduces the header sending only the Type, a mapped
code and the least 9 significant bits of Message ID. The receiver decompresses
the header. .The CON message is a request, therefore the LC process to a dynamic
mapping. When the ES receives the ACK message, this will not
initiate locally a message ID mapping since it is a response.
The LC receives the ACK and uncompressed it to restore the original
value. Dynamic Mapping context lifetime follows the same rules as
message ID duration.The message can be further optimized by setting some fields unidirectional, as
described in . 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
)In that example, the Thing is using CoMi and sends queries for 2 SID.The Constrained Application Protocol (CoAP)The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation.CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments.Constrained Application Protocol (CoAP) Option for No Server ResponseThere can be machine-to-machine (M2M) scenarios where server responses to client requests are redundant. This kind of open-loop exchange (with no response path from the server to the client) may be desired to minimize resource consumption in constrained systems while updating many resources simultaneously or performing high-frequency updates. CoAP already provides Non-confirmable (NON) messages that are not acknowledged by the recipient. However, the request/response semantics still require the server to respond with a status code indicating "the result of the attempt to understand and satisfy the request", per RFC 7252.This specification introduces a CoAP option called 'No-Response'. Using this option, the client can explicitly express to the server its disinterest in all responses against the particular request. This option also provides granular control to enable expression of disinterest to a particular response class or a combination of response classes. The server MAY decide to suppress the response by not transmitting it back to the client according to the value of the No-Response option in the request. This option may be effective for both unicast and multicast requests. This document also discusses a few examples of applications that benefit from this option.Observing Resources in the Constrained Application Protocol (CoAP)The Constrained Application Protocol (CoAP) is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients and provides eventual consistency between the state observed by each client and the actual resource state at the server.LPWAN Static Context Header Compression (SCHC) for IPv6 and UDPThis document describes a header compression scheme for IPv6, IPv6/ UDP based on static contexts. This technique is especially tailored for LPWA networks and could be extended to other protocol stacks. During the IETF history several compression mechanisms have been proposed. First mechanisms, such as RoHC, are using a context to store header field values and send smaller incremental differences on the link. Values in the context evolve dynamically with information contained in the compressed header. The challenge is to maintain sender's and receiver's contexts synchronized even with packet losses. Based on the fact that IPv6 contains only static fields, 6LoWPAN developed an efficient context-free compression mechanisms, allowing better flexibility and performance. The Static Context Header Compression (SCHC) combines the advantages of RoHC context which offers a great level of flexibility in the processing of fields, and 6LoWPAN behavior to elide fields that are known from the other side. Static context means that values in the context field do not change during the transmission, avoiding complex resynchronization mechanisms, incompatible with LPWA characteristics. In most of the cases, IPv6/UDP headers are reduced to a small identifier. This document focuses on IPv6/UDP headers compression, but the mechanism can be applied to other protocols such as CoAP. It will be described in a separate document.