LPWAN Static Context Header Compression (SCHC) for CoAPAcklio1137A avenue des Champs Blancs35510 Cesson-Sevigne CedexFranceana@ackl.ioInstitut MINES TELECOM; IMT Atlantique2 rue de la ChataigneraieCS 1760735576 Cesson-Sevigne CedexFranceLaurent.Toutain@imt-atlantique.frUniversidad de Buenos AiresAv. Paseo Colon 850C1063ACV Ciudad Autonoma de Buenos AiresArgentinarandreasen@fi.uba.arlpwan Working GroupThis draft defines the way SCHC header compression can be applied to CoAP
headers. The CoAP header structure differs from IPv6 and UDP protocols since CoAP
uses a flexible header with a variable number of options themselves of variable length.
The CoAP protocol is asymmetric in its message format, the format of the header packet in the request messages
is different from that in the response messages.
Most of the compression mechanisms have been introduced in
, this document explains how to use the SCHC compression
for CoAP.CoAP is an implementation of the REST architecture for constrained
devices. Although CoAP was designed for constrained devices, the size of a CoAP header may still be
too large for 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 field description composing the Rules are not
learned during the packet exchanges but are previously defined. The
context(s) is(are) known by both ends before transmission.A context is composed of a set of rules that are referenced by Rule IDs
(identifiers). A rule contains an ordered list of the fields descriptions containing a
field ID (FID), its length (FL)
and its position (FP), a direction indicator (DI) (upstream, downstream and bidirectional)
and some associated Target Values (TV). Target Value indicates the value that can be expected.
TV can also be a list of values. A Matching Operator (MO) is
associated to each header field description. The rule is selected if all the MOs fit
the TVs for all fields. 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.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
may be used as shown in . shows some examples for CoAP architecture and the SCHC rule’s scope. A rule can cover all headers from
IPv6 to CoAP, in which case SCHC C/D is performed at the device and at the LPWAN boundary. If an end-to-end
encryption mechanisms is used between the device and the application,
CoAP MAY 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 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 layers below. 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.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 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. defines the use of a message direction (DI) in the Field Description,
which allows a single Rule to process message headers differently in both directions.Even when a field is “symmetric” (i.e. found in both directions) the values carried in each direction are
different. Combined with a matching list in the TV, this allows reducing the range of
expected values in a particular direction and therefore reduce the size of the compression residue.
For instance,
if a client sends only CON request, the type can be elided by compression and the answer
may use one single bit to carry either the ACK or RST type. The same behavior can be
applied to the CoAP Code field (0.0X code are present in the request and Y.ZZ in the answer).
The direction allows splitting in two parts the possible values for each direction.In IPv6 and UDP, header fields have a fixed size. In CoAP, Token size
may vary from 0 to 8 bytes, the length being given by a field in the header. More
systematically, the CoAP options are described using the Type-Length-Value.
offers the possibility to define a function for the Field Length in the Field Description.In CoAP headers, a field can be present several times. This is typical for 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 instance. allows a Field ID to appears several times in the
rule, the Field Position (FP) removes ambiguities for the matching operation.Field sizes defined in the CoAP protocol can be too large regarding LPWAN traffic constraints.
This is particularly true for the message ID field or Token field. The MSB MO can be
used to reduce the information carried on LPWANs.CoAP also obeys the client/server paradigm and the compression ratio can
be different if the request is issued from an LPWAN device or from 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 Dev. 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 ratio and
maintain the necessary context for interoperability with existing CoAP implementations.This section discusses the compression of the different CoAP header fields.This field is bidirectional and MUST be elided during the SCHC compression, since it always
contains the same value. In the future, if new versions of CoAP are defined, new rules will
be defined to avoid ambiguities between versions. defines 4 types of messages: CON, NON, ACK and RST. The last two are a response to the first two. If the device plays a specific role, a rule can exploit these properties with the mapping list: [CON, NON] for one direction and [ACK, RST] for the other direction. Compression residue is reduced to 1 bit.The field SHOULD 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.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 only implements a CoAP client, the request code can be reduced to the set of requests the client is able to process.All the response codes MUST be compressed with a SCHC rule.This field is bidirectional and is used to manage acknowledgments. The server memorizes the value for a EXCHANGE_LIFETIME period (by default 247 seconds) for CON messages and a NON_LIFETIME period (by default 145 seconds) for NON messages. During that period, a server receiving the same Message ID value will process the message as a retransmission. After this period, it will be processed as a new message.In case the Device is a client, the size of the message ID field may be too large regarding the number of messages sent. The client SHOULD use only small message ID values, for instance 4 bit long. Therefore, a MSB can be used to limit the size of the compression residue.In case the Device is a server, the client may be located outside of the LPWAN area and view the Device as a regular device connected to the internet. The client will generate Message ID using the 16 bits space offered by this field. A CoAP proxy can be set before the SCHC C/D to reduce the value of the Message ID, to allow its compression with the MSB matching operator and LSB CDA.Token is defined through two CoAP fields, Token Length in the mandatory header and Token Value directly following the mandatory CoAP header.Token Length is processed as any protocol field. If the value remains the same during all the transaction, the size can be stored in the context and elided during the transmission. Otherwise, it will have to the sent as a compression residue.Token Value size cannot be defined directly in the rule in the Field Length (FL). Instead, a specific function designated as “TKL” MUST be used and length does not have to the sent with the residue. During the decompression, this function returns the value contained in the Token Length field.These fields are both unidirectional and MUST NOT be set to bidirectional in a rule entry.If a single value is expected by the client, it can be stored in the TV and elided during the transmission. Otherwise, if several possible values are expected by the client, a matching-list SHOULD be used to limit the size of the residue. If is not possible, the value has to be sent as a residue (fixed or variable length).These fields 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 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).These fields are unidirectional and MUST NOT be set to bidirectional in a rule entry.
They are used only by the client to access a specific resource and are never found
in server responses.Uri-Path and Uri-Query elements are a repeatable options, the Field Position (FP) gives the
position in the path.A Mapping list can be used to reduce the size of variable Paths or Queries. In that case, to
optimize the compression, several elements can be regrouped into a single entry.
Numbering of elements do not change, MO comparison is set with the first element
of the matching.In a single bit residue can be used to code one of the 2 paths. If regrouping were not allowed, a 2 bits residue would be needed.When the length is not known at the rule creation, the Field Length SHOULD be set to variable,
and the unit is set to bytes.The MSB MO can be applied to a Uri-Path or Uri-Query element. Since MSB value is given in bit,
the size MUST always be a multiple of 8 bits.The length sent at the beginning of a variable length residue indicates the size of the LSB in bytes.For instance for a CORECONF path /c/X6?k=”eth0” the rule can be set to: 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”).The number of Uri-path or Uri-Query elements in a rule is fixed at the rule creation time. If the number
varies, several rules SHOULD be created to cover all the possibilities. Another possibility is
to define the length of Uri-Path to variable and send a compression residue with a length of 0 to
indicate that this Uri-Path is empty. This adds 4 bits to the compression residue.These fields are unidirectional and MUST NOT be set to bidirectional in a rule entry.
They are used only by the client to access a specific resource and are never found
in server response.If the field value has to be sent, TV is not set, MO is set to “ignore” and CDA is set
to “value-sent”. A mapping MAY also be used.Otherwise, the TV is set to the value, MO is set to “equal” and CDA is set to “not-sent”.These fields are unidirectional.These fields values cannot be stored in a rule entry. They MUST always be sent with the
compression residues.Block allows a fragmentation at the CoAP level. SCHC also includes a fragmentation protocol.
They are compatible. If a block option is used, its content MUST be sent as a compression residue. defines the Observe option. The TV is not set, MO is set to “ignore” and the
CDA is set to “value-sent”. SCHC does not limit the maximum size for this option (3 bytes).
To reduce the transmission size, either the device implementation MAY limit the delta between two consecutive values,
or a proxy can modify the increment.Since an RST message may be sent to inform a server that the client does not require Observe
response, a rule MUST allow the transmission of this message. defines a No-Response option limiting the responses made by a server to
a request. If the value is known by both ends, then TV is set to this value, MO is
set to “equal” and CDA is set to “not-sent”.Otherwise, if the value is changing over time, TV is not set, MO is set to “ignore” and
CDA to “value-sent”. A matching list can also be used to reduce the size.The time scale option allows a client to inform the server that
it is in a constrained network and that message ID MUST be kept for a duration given by the option.If the value is known by both ends, then TV is set to this value, MO is
set to “equal” and CDA is set to “not-sent”.Otherwise, if the value is changing over time, TV is not set, MO is set to “ignore” and
CDA to “value-sent”. A matching list can also be used to reduce the size.OSCORE defines end-to-end protection for CoAP messages.
This section describes how SCHC rules can be applied to compress OSCORE-protected messages.The encoding of the OSCORE Option Value defined in Section 6.1 of
is repeated in .The first byte is used for flags that specify the contents of the OSCORE
option. The 3 most significant bits are reserved and always set to 0. Bit h,
when set, indicates the presence of the kid context field in the option. Bit k,
when set, indicates the presence of a kid field. The 3 least significant bits
n indicate the length of the piv field in bytes. When n = 0, no piv
is present.After the flag byte follow the piv field, kid context field and kid field in
order and if present; the length of the kid context field is encoded in the
first byte denoting by s the length of the kid context in bytes.This draft recommends to implement a parser that is able to identify the OSCORE
Option and the fields it contains.Conceptually, it discerns up to 4 distinct pieces of information within the OSCORE option: the flag bits, the piv, the kid context, and the kid. It is thus recommended that the parser split the OSCORE option into the 4 subsequent fields:CoAP OSCORE_flags,CoAP OSCORE_piv,CoAP OSCORE_kidctxt,CoAP OSCORE_kid.These fields are shown superimposed on the OSCORE Option format in , the CoAP OSCORE_kidctxt field including the size bits s. Their size SHOULD be reduced using the MSB matching operator.In this first scenario, the LPWAN compressor at the Network Gateway side receives from a client on the Internet
a POST message, which is immediately acknowledged by the Device. For this simple
scenario, the rules are described .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. 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.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
.CoAP options are sorted into one of 3 classes, each granted a specific
type of protection by the protocol:Class E: Encrypted options moved to the Inner Plaintext,Class I: Integrity-protected options included in the AAD for the encryption
of the Plaintext but otherwise left untouched in the Outer Message,Class U: Unprotected options left untouched in the Outer Message.Additionally, the OSCORE Option is added as an Outer option, signaling that the
message is OSCORE protected. This option carries the information necessary to
retrieve the Security Context with which the message was encrypted so that it
may be correctly decrypted at the other end-point. shows the message format for the OSCORE Message and
Plaintext.In the Outer Header, the original message code is hidden and replaced by a default
dummy value. As seen in sections 4.1.3.5 and 4.2 of ,
the 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
.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.The SCHC Compression scheme consists of compressing both the Plaintext before
encryption and the resulting OSCORE message after encryption, see .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.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 Its corresponding response is the CONTENT Response in .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, . shows the Plaintext obtained for our example GET Request and follows the process of Inner Compression and Encryption until we end up with the Payload to be added in the outer OSCORE Message.In this case the original message has no payload and its resulting Plaintext can be compressed up to only 1 byte (size of the Rule ID). The AEAD algorithm preserves this length in its first output, but also yields a fixed-size tag which cannot be compressed and has to be included in the OSCORE message. This translates into an overhead in total message length, which limits the amount of compression that can be achieved and plays into the cost of adding security to the exchange.In we repeat the process for the example CONTENT Response. In this case the misalignment produced by the compression residue (1 bit) makes it so that 7 bits of padding have to be applied after the payload, resulting in a compressed Plaintext that is the same size as before compression. This misalignment also causes the hexcode from the payload to differ from the original, even though it has not been compressed. On top of this, the overhead from the tag bytes is incurred as before.The Outer SCHC Rules () MUST process the OSCORE Options
fields. In and we show a dump of the OSCORE Messages generated from our example messages once they have been provided with the Inner Compressed Ciphertext in the payload. These are the messages that are to go through Outer SCHC Compression.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.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. shows a possible set of Outer Rules to compress the Outer Header.These Outer Rules are applied to the example GET Request and CONTENT Response. The resulting messages are shown in and .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 .This yields the results in for the Request, and
for the Response.As can be seen, the difference between applying SCHC + OSCORE as compared to
regular SCHC + COAP is about 10 bytes of cost.This document has no request to IANA.This document does not have any more Security consideration than the ones already raised on Thanks to all the persons that have give us feedbackThe 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.Block-Wise Transfers in the Constrained Application Protocol (CoAP)The Constrained Application Protocol (CoAP) is a RESTful transfer protocol for constrained nodes and networks. Basic CoAP messages work well for small payloads from sensors and actuators; however, applications will need to transfer larger payloads occasionally -- for instance, for firmware updates. In contrast to HTTP, where TCP does the grunt work of segmenting and resequencing, CoAP is based on datagram transports such as UDP or Datagram Transport Layer Security (DTLS). These transports only offer fragmentation, which is even more problematic in constrained nodes and networks, limiting the maximum size of resource representations that can practically be transferred.Instead of relying on IP fragmentation, this specification extends basic CoAP with a pair of "Block" options for transferring multiple blocks of information from a resource representation in multiple request-response pairs. In many important cases, the Block options enable a server to be truly stateless: the server can handle each block transfer separately, with no need for a connection setup or other server-side memory of previous block transfers. Essentially, the Block options provide a minimal way to transfer larger representations in a block-wise fashion.A CoAP implementation that does not support these options generally is limited in the size of the representations that can be exchanged, so there is an expectation that the Block options will be widely used in CoAP implementations. Therefore, this specification updates RFC 7252.Object Security for Constrained RESTful Environments (OSCORE)This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although being an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates [RFC7252].LPWAN Static Context Header Compression (SCHC) and fragmentation for IPv6 and UDPThis document defines the Static Context Header Compression (SCHC) framework, which provides both header compression and fragmentation functionalities. SCHC has been designed for Low Power Wide Area Networks (LPWAN). SCHC compression is based on a common static context stored in both the LPWAN device and the network side. This document defines a header compression mechanism and its application to compress IPv6/UDP headers. This document also specifies a fragmentation and reassembly mechanism that is used to support the IPv6 MTU requirement over the LPWAN technologies. Fragmentation is needed for IPv6 datagrams that, after SCHC compression or when such compression was not possible, still exceed the layer-2 maximum payload size. The SCHC header compression and fragmentation mechanisms are independent of the specific LPWAN technology over which they are used. This document defines generic functionalities and offers flexibility with regard to parameter settings and mechanism choices. This document standardizes the exchange over the LPWAN between two SCHC entities. Settings and choices specific to a technology or a product are expected to be grouped into profiles, which are specified in other documents. Data models for the context and profiles are out of scope.CoAP Time Scale OptionSCHC compression mechanism for LPWAN network enables IPv6 on devices connected to a constrained network (LPWAN). They can communicate with a CoAP server located anywhere in the Internet. LPWAN network characteristics limits the number of exchanges and may impose a long RTT. The CoAP server must be aware of these properties to manage correctly requests. The Time Scale option allows a device to inform a CoAP server of the duration the message ID value should be kept in memory to manage correctly message duplication.