| Internet-Draft | Updates on using SCHC for CoAP | July 2023 |
| Tiloca, et al. | Expires 11 January 2024 | [Page] |
This document clarifies, updates and extends the method specified in RFC 8824 for compressing Constrained Application Protocol (CoAP) headers using the Static Context Header Compression and fragmentation (SCHC) framework. In particular, it considers recently defined CoAP options and specifies how CoAP headers are compressed in the presence of intermediaries. Therefore, this document updates RFC 8824.¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the Static Context Header Compression Working Group mailing list ([email protected]), which is archived at https://mailarchive.ietf.org/arch/browse/schc/.¶
Source for this draft and an issue tracker can be found at https://gitlab.com/crimson84/draft-tiloca-schc-8824-update.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 11 January 2024.¶
Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
The Constrained Application Protocol (CoAP) [RFC7252] is a web-transfer protocol intended for applications based on the REST (Representational State Transfer) paradigm, and designed to be affordable also for resource-constrained devices.¶
In order to enable the use of CoAP in LPWANs (Low-Power Wide-Area Networks) as well as to improve performance, [RFC8824] defines how to use the Static Context Header Compression and fragmentation (SCHC) framework [RFC8724] for compressing CoAP headers.¶
This document clarifies, updates and extends the SCHC compression of CoAP headers defined in [RFC8824] at the application level, by: providing specific clarifications; updating specific details of the compression processing, based on recent developments related to the security protocol OSCORE [RFC8613] for end-to-end protection of CoAP messages; and extending the compression processing to take into account additional CoAP options and the presence of CoAP proxies.¶
In particular, this document updates [RFC8824] as follows.¶
This document does not alter the core approach, design choices and features of the SCHC compression applied to CoAP headers.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Readers are expected to be familiar with the terms and concepts related to the SCHC framework [RFC8724], the web-transfer protocol CoAP [RFC7252], the security protocol OSCORE [RFC8613] and the use of SCHC for CoAP [RFC8824].¶
This section updates and extends Section 5 of [RFC8824], as to how SCHC compresses some specific CoAP options. In particular, Section 2.1 updates Section 5.4 of [RFC8824].¶
The SCHC Rule description MAY define sending some field values by not setting the TV, while setting the MO to "ignore" and the CDA to "value-sent". A Rule MAY also use a "match-mapping" MO when there are different options for the same FID. Otherwise, the Rule sets the TV to the value, the MO to "equal", and the CDA to "not-sent".¶
The Hop-Limit field is an option defined in [RFC8768] that can be used to detect forwarding loops through a chain of CoAP proxies. The first proxy in the chain that understands the option includes it in a received request with a proper value set, before forwarding the request. Any following proxy that understands the option decrements the option value and forwards the request if the new value is different than zero, or returns a 5.08 (Hop Limit Reached) error response otherwise.¶
When a packet uses the Hop-Limit option, SCHC compression SHOULD send its content in the Compression Residue. That is, in the SCHC Rule, the TV is not set, while the MO is set to "ignore", and the CDA is set to "value-sent". As an exception, and consistently with the default value 16 defined for the Hop-Limit option in Section 3 of [RFC8768], a Rule MAY describe a TV with value 16, with the MO set to "equal" and the CDA set to "not-sent".¶
The Echo field is an option defined in [RFC9175] that a server can include in a response as a challenge to the client, and that the client echoes back to the server in one or more requests. This enables the server to verify the freshness of a request and to cryptographically verify the aliveness of the client. Also, it forces the client to demonstrate reachability at its claimed network address.¶
When a packet uses the Echo option, SCHC compression SHOULD send its content in the Compression Residue. That is, in the SCHC Rule, the TV is not set, while the MO is set to "ignore", and the CDA is set to "value-sent". An exception applies in case the server generates the values to use for the Echo option by means of a persistent counter (see Appendix A of [RFC9175]). In such a case, a Rule MAY use the "MSB" MO and the "LSB" CDA. This would be effectively applicable until the persistent counter at the server becomes greater than the maximum threshold value that produces an MSB-matching.¶
The Request-Tag field is an option defined in [RFC9175] that the client can set in request messages of block-wise operations, with value an ephemeral short-lived identifier of the specific block-wise operation in question. This allows the server to match message fragments belonging to the same request operation and, if the server supports it, to reliably process simultaneous block-wise request operations on a single resource. If requests are integrity protected, this also protects against interchange of fragments between different block-wise request operations.¶
When a packet uses the Request-Tag option, SCHC compression MAY send its content in the Compression Residue. That is, in the SCHC Rule, the TV is not set, while the MO is set to "ignore", and the CDA is set to "value-sent". Alternatively, if a pre-defined set of Request-Tag values used by the client is known, a Rule MAY use a "match-mapping" MO when there are different options for the same FID.¶
The EDHOC field is an option defined in [I-D.ietf-core-oscore-edhoc] that a client can include in a request, in order to perform an optimized, shortened execution of the authenticated key establishment protocol EDHOC [I-D.ietf-lake-edhoc]. Such a request conveys both the final EDHOC message and actual application data, where the latter is protected with OSCORE [RFC8613] using a Security Context derived from the result of the current EDHOC execution.¶
The option occurs at most once and is always empty. The SCHC Rule MUST describe an empty TV, with the MO set to "equal" and the CDA set to "not-sent".¶
This section updates and extends Section 6 of [RFC8824], as to how SCHC compresses some specific CoAP options providing protocol extensions. In particular, Section 3.1 updates Section 6.1 of [RFC8824], while Section 3.2 updates Section 6.4 of [RFC8824].¶
When a packet uses a Block1 or Block2 option [RFC7959] or a Q-Block1 or Q-Block2 option [RFC9177], SCHC compression MUST send its content in the Compression Residue. In the SCHC Rule, the TV is not set, while the MO is set to "ignore" and the CDA is set to "value-sent". The Block1, Block2, Q-Block1 and Q-Block2 options allow fragmentation at the CoAP level that is compatible with SCHC fragmentation. Both fragmentation mechanisms are complementary, and the node may use them for the same packet as needed.¶
The security protocol OSCORE [RFC8613] provides end-to-end protection for CoAP messages. Group OSCORE [I-D.ietf-core-oscore-groupcomm] builds on OSCORE and defines end-to-end protection of CoAP messages in group communication [I-D.ietf-core-groupcomm-bis]. This section describes how SCHC Rules can be applied to compress messages protected with OSCORE or Group OSCORE.¶
Figure 1 shows the OSCORE option value encoding, which was originally defined in Section 6.1 of [RFC8613] and has been extended in [I-D.ietf-core-oscore-key-update][I-D.ietf-core-oscore-groupcomm]. The first byte of the OSCORE option value specifies the content of the OSCORE option using flags, as follows.¶
Assuming the presence of a single flag byte, this is followed by the piv field, the kid context field, and the kid field, in that order. Also, if present, the kid context field's length (in bytes) is encoded in the first byte, denoted by "s".¶
0 1 2 3 4 5 6 7 <--------- n bytes -------------> +-+-+-+-+-+-+-+-+---------------------------------+ |0 0 0|h|k| n | Partial IV (if any) | +-+-+-+-+-+-+-+-+---------------------------------+ | | | |<-- CoAP -->|<------- CoAP OSCORE_piv ------> | OSCORE_flags <-- 1 byte --> <------ s bytes -----> +--------------+----------------------+-----------------------+ | s (if any) | kid context (if any) | kid (if any) ... | +--------------+----------------------+-----------------------+ | | | |<-------- CoAP OSCORE_kidctx ------->|<-- CoAP OSCORE_kid -->|
Figure 2 shows the OSCORE option value encoding, with the second byte of flags also present. As defined in Section 4.1 of [I-D.ietf-core-oscore-key-update], the least significant bit d of this byte, when set, indicates that two additional fields are included in the option, following the kid context field (if any).¶
These two fields, namely x and nonce, are used when running the key update protocol KUDOS defined in [I-D.ietf-core-oscore-key-update], with x specifying the length of the nonce field in bytes as well as the specific behavior to adopt during the KUDOS execution. In particular, the figure provides the breakdown of the x field, where its three least significant bits form the sub-field m, which specifies the size of nonce in bytes, minus 1.¶
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 <----- n bytes ----->
+-+-+-+-+-+-+-+-+---+---+---+---+---+---+---+---+---------------------+
|1|0|0|h|k| n | 0 | 0 | 0 | 0 | 0 | 0 | 0 | d | Partial IV (if any) |
+-+-+-+-+-+-+-+-+---+---+---+---+---+---+---+---+---------------------+
| | |
|<------------------- CoAP -------------------->|<- CoAP OSCORE_piv ->|
OSCORE_flags
<- 1 byte -> <----------- s bytes ------------> <------ 1 byte ----->
+------------+----------------------------------+---------------------+
| s (if any) | kid context (if any) | x (if any) |
+------------+----------------------------------+---------------------+
| | |
|<------------- CoAP OSCORE_kidctx ------------>|<-- CoAP OSCORE_x -->|
| |
| 0 1 2 3 4 5 6 7 |
| +-+-+-+-+-+-+-+-+ |
| |0|0|b|p| m | |
| +-+-+-+-+-+-+-+-+ |
<----- m + 1 bytes ----->
+-------------------------+-----------------------+
| nonce (if any) | kid (if any) ... |
+-------------------------+-----------------------+
| | |
|<-- CoAP OSCORE_nonce -->|<-- CoAP OSCORE_kid -->|
To better perform OSCORE SCHC compression, the Rule description needs to identify the OSCORE option and the fields it contains. Conceptually, it discerns up to six distinct pieces of information within the OSCORE option: the flag bits, the piv, the kid context, the x byte, the nonce, and the kid. The SCHC Rule splits the OSCORE option into six Field Descriptors in order to compress them:¶
Figure 1 shows the OSCORE option format with the four fields OSCORE_flags, OSCORE_piv, OSCORE_kidctx and OSCORE_kid superimposed on it. Also, Figure 2 shows the OSCORE option format with all the six fields superimposed on it, with reference to a message exchanged during an execution of the KUDOS key update protocol.¶
In both cases, the CoAP OSCORE_kidctx field directly includes the size octet, s. In the latter case, the following applies.¶
For the x field, if both endpoints know the value, then the SCHC Rule will describe a TV to this value, with the MO set to "equal" and the CDA set to "not-sent". This models the case where the two endpoints run KUDOS with a pre-agreed size of the nonce field, as well as with a pre-agreed combination of its modes of operations, as per the bits b and p of the m sub-field.¶
Otherwise, if the value is changing over time, the SCHC Rule will set the MO to "ignore" and the CDA to "value-sent". The Rule may also use a "match-mapping" MO to compress this field, in case the two endpoints pre-agree on a set of alternative ways to run KUDOS, with respect to the size of the nonce field and the combination of the KUDOS modes of operation to use.¶
For the nonce field, the SCHC Rule has the TV not set, while the MO is set to "ignore" and the CDA is set to "value-sent".¶
In addition, for the value of the nonce field, SCHC MUST NOT send it as variable-length data in the Compression Residue, to avoid ambiguity with the length of the nonce field encoded in the x field. Therefore, SCHC MUST use the m sub-field of the x field to define the size of the Compression Residue. SCHC designates a specific function, "osc.x.m", that the Rule MUST use to complete the Field Descriptor. During the decompression, this function returns the length of the nonce field in bytes, as the value of the three least significant bits of the m sub-field of the x field, plus 1.¶
As originally intended in [RFC8824], the following applies with respect to the 0xFF payload marker. A SCHC compression Rule for CoAP includes all the expected CoAP options, therefore the payload marker does not have to be specified.¶
If the CoAP message to compress with SCHC is not going to be protected with OSCORE and includes a payload, then the 0xFF payload marker MUST NOT be included in the compressed message, which is composed of the Compression RuleID, the Compression Residue (if any), and the CoAP payload.¶
After having decompressed an incoming message, the recipient endpoint MUST prepend the 0xFF payload marker to the CoAP payload, if any was present after the consumed Compression Residue.¶
If the CoAP message has to be protected with OSCORE, the same rationale described in Section 4.1 applies to both the Inner SCHC Compression and the Outer SCHC Compression defined in Section 7.2 of [RFC8824]. That is:¶
After having completed the Outer SCHC Decompression of an incoming message, the recipient endpoint MUST prepend the 0xFF payload marker to the OSCORE Ciphertext.¶
After having completed the Inner SCHC Decompression of an incoming message, the recipient endpoint MUST prepend the 0xFF payload marker to the CoAP payload, if any was present after the consumed Compression Residue.¶
Building on [RFC8824], this section clarifies how SCHC Compression/Decompression is performed when CoAP proxies are deployed. The following refers to the origin client and origin server as application endpoints.¶
Note that SCHC Compression/Decompression of CoAP headers is not necessarily used between each pair of hops in the communication chain. For example, if a proxy is deployed between an origin client and an origin server, SCHC might be used on the communication leg between the origin client and the proxy, but not on the communication leg between the proxy and the origin server.¶
In case OSCORE is not used end-to-end between client and server, the SCHC processing occurs hop-by-hop, by relying on SCHC Rules that are consistently shared between two adjacent hops.¶
In particular, SCHC is used as defined below.¶
Each proxy decompresses the incoming compressed message, by using the SCHC Rules that it shares with the (previous hop towards the) sender application endpoint.¶
Then, the proxy compresses the CoAP message to be forwarded, by using the SCHC Rules that it shares with the (next hop towards the) recipient application endpoint.¶
The resulting, compressed message is sent to the (next hop towards the) recipient application endpoint.¶
In case OSCORE is used end-to-end between client and server (see Section 7.2 of [RFC8824]), the following applies.¶
The SCHC processing occurs end-to-end as to the Inner SCHC Compression/Decompression, by relying on Inner SCHC Rules that are consistently shared between the two application endpoints acting as OSCORE endpoints and sharing the used OSCORE Security Context.¶
Instead, the SCHC processing occurs hop-by-hop as to the Outer SCHC Compression/Decompression, by relying on Outer SCHC Rules that are consistently shared between two adjacent hops.¶
In particular, SCHC is used as defined below.¶
The sender application endpoint performs the Inner SCHC Compression on the original CoAP message, by using the Inner SCHC Rules that it shares with the recipient application endpoint.¶
Following the AEAD Encryption of the compressed input obtained from the previous step, the sender application endpoint performs the Outer SCHC Compression on the resulting OSCORE-protected message, by using the Outer SCHC Rules that it shares with the next hop towards the recipient application endpoint.¶
The resulting, compressed message is sent to the next hop towards the recipient application endpoint.¶
Each proxy performs the Outer SCHC Decompression on the incoming compressed message, by using the SCHC Rules that it shares with the (previous hop towards the) sender application endpoint.¶
Then, the proxy performs the Outer SCHC Compression of the OSCORE-protected message to be forwarded, by using the SCHC Rules that it shares with the (next hop towards the) recipient application endpoint.¶
The resulting, compressed message is sent to the (next hop towards the) recipient application endpoint.¶
The recipient application endpoint performs the Outer SCHC Decompression on the incoming compressed message, by using the Outer SCHC Rules that it shares with the previous hop towards the sender application endpoint.¶
Then, the recipient application endpoint performs the AEAD Decryption of the OSCORE-protected message obtained from the previous step.¶
Finally, the recipient application endpoint performs the Inner SCHC Decompression on the compressed input obtained from the previous step, by using the Inner SCHC Rules that it shares with the sender application endpoint. The result is the original CoAP message produced by the sender application endpoint.¶
This section provides examples of SCHC Compression/Decompression in the presence of a CoAP proxy.¶
The presented examples refer to the same deployment considered in Section 2 of [RFC8824], including a Device communicating over LPWAN with a Network Gateway (NGW), which in turn communicates with an Application Server over the Internet. The Application Server and the Device exchange CoAP messages through the NGW.¶
In addition, the following also applies in the presented examples.¶
Like [RFC8824], the presented examples focus on SCHC Compression/Decompression of CoAP headers, i.e., irrespective of possible SCHC Compression/Decompression applied to further protocol headers.¶
The example in Section 6.1 considers an exchange of two unprotected messages, while the example in Section 6.2 considers an exchange of two messages protected end-to-end with OSCORE. In the examples, the character | denotes bit concatenation.¶
Figure 3 and Figure 4 show the two CoAP messages exchanged between the Device and the Application Server, via the proxy. The figures show the two messages as originally generated by the application at the two origin endpoints, i.e., before they are possibly protected end-to-end with OSCORE as considered by the example in Section 6.2.¶
In particular, note that:¶
Original message:
=================
0x41010001823b6578616d706c652e636f6d
8b74656d7065726174757265d40f636f6170
Header:
0x4101
01 Ver
00 CON
0001 TKL
00000001 Request Code 1 "GET"
0x0001 = mid
0x82 = token
Options:
0x3b6578616d706c652e636f6d
Option 3: Uri-Host
Value = example.com
0x8b74656d7065726174757265
Option 11: Uri-Path
Value = temperature
0xd40f636f6170
Option 39: Proxy-Scheme
Value = coap
Original message length: 35 bytes
Original message:
=================
0x6145000475ff32332043
Header:
0x6145
01 Ver
10 ACK
0001 TKL
01000101 Successful Response Code 69 "2.05 Content"
0x0004 = mid
0x75 = token
0xFF Payload marker
Payload:
0x32332043
Original message length: 10 bytes
In case OSCORE is not used end-to-end between the Device and the Application Server, the following SCHC Rules are shared between the different entities. Based on those Rules, the SCHC Compression/Decompression is performed as per Section 5.1.¶
The Device and the proxy share the SCHC Rule shown in Figure 5, with RuleID 0.¶
+=====================================================================+ | RuleID 0 | +==========+=====+===+===+=============+=========+===========+========+ | Field | FL | FP| DI| TV | MO | CDA | Sent | | | | | | | | | [bits] | +==========+=====+===+===+=============+=========+===========+========+ | CoAP | 2 | 1 | Bi| 01 | equal | not-sent | | | version | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 2 | 1 | Up| 0 | equal | not-sent | | | Type | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 2 | 1 | Dw| [0, 2] | match | matching- | T | | Type | | | | | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 4 | 1 | Bi| 1 | equal | not-sent | | | TKL | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Up| [1, 2, | match- | matching- | CC | | Code | | | | 3, 4] | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Dw| [65, 68, | match- | matching- | CC | | Code | | | | 69, 132] | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 16 | 1 | Bi| 0x00 | MSB(12) | LSB | MMMM | | MID | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | tkl | 1 | Bi| 0x80 | MSB(5) | LSB | TTT | | Token | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| | ignore | value- | | | Uri-Host | (B) | | | | | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| temperature | equal | not-sent | | | Uri-Path | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| coap | equal | not-sent | | | Proxy- | | | | | | | | | Scheme | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+
Instead, the proxy and the Application Server share the SCHC Rule shown in Figure 6, with RuleID 1.¶
+=====================================================================+ | RuleID 1 | +==========+=====+===+===+=============+=========+===========+========+ | Field | FL | FP| DI| TV | MO | CDA | Sent | | | | | | | | | [bits] | +==========+=====+===+===+=============+=========+===========+========+ | CoAP | 2 | 1 | Bi| 01 | equal | not-sent | | | version | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 2 | 1 | Up| 0 | equal | not-sent | | | Type | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 2 | 1 | Dw| [0, 2] | match | matching- | T | | Type | | | | | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 4 | 1 | Bi| 1 | equal | not-sent | | | TKL | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Up| [1, 2, | match- | matching- | CC | | Code | | | | 3, 4] | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Dw| [65, 68, | match- | matching- | CC | | Code | | | | 69, 132] | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 16 | 1 | Bi| 0x00 | MSB(12) | LSB | MMMM | | MID | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | tkl | 1 | Bi| 0x70 | MSB(5) | LSB | TTT | | Token | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| | ignore | value- | | | Uri-Host | (B) | | | | | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| temperature | equal | not-sent | | | Uri-Path | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+
First, the Device applies the Rule in Figure 5 shared with the proxy to the CoAP request in Figure 3. The result is the compressed CoAP request in Figure 7, which the Device sends to the proxy.¶
Compressed message:
=================
0x00055b2bc30b6b836329731b7b68 (14 bytes)
0x00 RuleID
055b2bc30b6b836329731b7b68 compression residue
and padded payload
Compression Residue (101 bits -> 13 bytes with padding)
0b 00 0001 010 1011 | 0x6578616d706c652e636f6d
code mid tkn Uri-Host (residue size and residue)
Compressed message length: 14 bytes
Upon receiving the message in Figure 7, the proxy decompresses it with the Rule in Figure 5 shared with the Device, and obtains the same CoAP request in Figure 3.¶
After that, the proxy removes the Proxy-Scheme Option from the decompressed CoAP request. Also, the proxy replaces the values of the CoAP Message ID and of the CoAP Token to 0x0004 and 0x75, respectively. The result is the CoAP request shown in Figure 8.¶
Message to forward:
=================
0x41010004753b6578616d706c652e636f6d
8b74656d7065726174757265
Header:
0x4101
01 Ver
00 CON
0001 TKL
00000001 Request Code 1 "GET"
0x0004 = mid
0x75 = token
Options:
0x3b6578616d706c652e636f6d
Option 3: Uri-Host
Value = example.com
0x8b74656d7065726174757265
Option 11: Uri-Path
Value = temperature
Original message length: 29 bytes
Then, the proxy applies the Rule in Figure 6 shared with the Application Server to the CoAP request in Figure 8.¶
The result is the compressed CoAP request in Figure 9, which the proxy forwards to the Application Server.¶
Compressed message to forward:
=================
0x0112db2bc30b6b836329731b7b68 (14 bytes)
0x01 RuleID
12db2bc30b6b836329731b7b68 compression residue
and padded payload
Compression Residue (101 bits -> 13 bytes with padding)
0b 00 0100 101 1011 | 0x6578616d706c652e636f6d
code mid tkn Uri-Host (residue size and residue)
Compressed message length: 14 bytes
Upon receiving the message in Figure 9, the Application Server decompresses it using the Rule in Figure 6 shared with the proxy. The result is the same CoAP request in Figure 8, which the Application Server delivers to the application.¶
After that, the Application Server produces the CoAP response in Figure 4, and compresses it using the Rule in Figure 6 shared with the proxy. The result is the compressed CoAP response shown in Figure 10, which the Application Server sends to the proxy.¶
Compressed message:
=================
0x01c94c8cc810c0 (7 bytes)
0x01 RuleID
c94c8cc810c0 compression residue
and padded payload
Compression Residue (10 bits -> 2 bytes with padding)
0b 1 10 0100 101
type code mid tkn
Payload
0x32332043 (4 bytes)
Compressed message length: 7 bytes
Upon receiving the message in Figure 10, the proxy decompresses it using the Rule in Figure 6 shared with the Application Server. The result is the same CoAP response in Figure 4.¶
Then, the proxy replaces the values of the CoAP Message ID and of the CoAP Token to 0x0001 and 0x82, respectively. The result is the CoAP response shown in Figure 11.¶
Message to forward:
=================
0x6145000182ff32332043
Header:
0x6145
01 Ver
10 ACK
0001 TKL
01000101 Successful Response Code 69 "2.05 Content"
0x0001 = mid
0x82 = token
0xFF Payload marker
Payload:
0x32332043
Original message length: 10 bytes
Then, the proxy compresses the CoAP response in Figure 11 with the Rule in Figure 5 shared with the Device. The result is the compressed CoAP response shown in Figure 12, which the proxy forwards to the Device.¶
Compressed message:
=================
0x00c28c8cc810c0 (7 bytes)
0x00 RuleID
c28c8cc810c0 compression residue
and padded payload
Compression Residue (10 bits -> 2 bytes with padding)
0b 1 10 0001 010
type code mid tkn
Payload
0x32332043 (4 bytes)
Compressed message length: 7 bytes
Upon receiving the message in Figure 12, the Device decompresses it using the Rule in Figure 5 shared with the proxy. The result is the same CoAP request in Figure 11, which the Device delivers to the application.¶
In case OSCORE is used end-to-end between the Device and the Application Server, the following SCHC Rules are shared between the different entities. Based on those Rules, the SCHC Compression/Decompression is performed as per Section 5.2.¶
The Device and the Application Server share the SCHC Rule shown in Figure 13, with RuleID 2. The Device and the Application Server use this Rule to perform the Inner SCHC Compression/Decompression end-to-end.¶
+=====================================================================+ | RuleID 2 | +==========+=====+===+===+=============+=========+===========+========+ | Field | FL | FP| DI| TV | MO | CDA | Sent | | | | | | | | | [bits] | +==========+=====+===+===+=============+=========+===========+========+ | CoAP | 8 | 1 | Up| [1, 2, | match- | matching- | CC | | Code | | | | 3, 4] | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Dw| [65, 68, | match- | matching- | CC | | Code | | | | 69, 132] | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| temperature | equal | not-sent | | | Uri-Path | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+
The Device and the proxy share the SCHC Rule shown in Figure 14, with RuleID 3. The Device and the proxy use this Rule to perform the Outer SCHC Compression/Decompression hop-by-hop on their communication leg.¶
+=====================================================================+ | RuleID 3 | +==========+=====+===+===+=============+=========+===========+========+ | Field | FL | FP| DI| TV | MO | CDA | Sent | | | | | | | | | [bits] | +==========+=====+===+===+=============+=========+===========+========+ | CoAP | 2 | 1 | Bi| 01 | equal | not-sent | | | version | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 2 | 1 | Up| 0 | equal | not-sent | | | Type | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 2 | 1 | Dw| [0, 2] | match | matching- | T | | Type | | | | | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 4 | 1 | Bi| 1 | equal | not-sent | | | TKL | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Up| 2 | equal | not-sent | | | Code | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Dw| 68 | equal | not-sent | | | Code | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 16 | 1 | Bi| 0x00 | MSB(12) | LSB | MMMM | | MID | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | tkl | 1 | Bi| 0x80 | MSB(5) | LSB | TTT | | Token | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| | ignore | value- | | | Uri-Host | (B) | | | | | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Up| 0x09 | equal | not-sent | | | OSCORE_ | | | | | | | | | flags | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| 0x00 | MSB(4) | LSB | PPPP | | OSCORE | | | | | | | | | piv | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| 0x0000 | MSB(12) | LSB | KKKK | | OSCORE_ | | | | | | | | | kid | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Bi| b'' | equal | not-sent | | | OSCORE_ | | | | | | | | | kidctx | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Dw| b'' | equal | not-sent | | | OSCORE_ | | | | | | | | | flags | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Dw| b'' | equal | not-sent | | | OSCORE_ | | | | | | | | | piv | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Dw| b'' | equal | not-sent | | | OSCORE_ | | | | | | | | | kid | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| coap | equal | not-sent | | | Proxy- | | | | | | | | | Scheme | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+
The proxy and the Application Server share the SCHC Rule shown in Figure 15, with RuleID 4. The proxy and the Application Server use this Rule to perform the Outer SCHC Compression/Decompression hop-by-hop on their communication leg.¶
+=====================================================================+ | RuleID 4 | +==========+=====+===+===+=============+=========+===========+========+ | Field | FL | FP| DI| TV | MO | CDA | Sent | | | | | | | | | [bits] | +==========+=====+===+===+=============+=========+===========+========+ | CoAP | 2 | 1 | Bi| 01 | equal | not-sent | | | version | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 2 | 1 | Up| 0 | equal | not-sent | | | Type | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 2 | 1 | Dw| [0, 2] | match | matching- | T | | Type | | | | | mapping | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 4 | 1 | Bi| 1 | equal | not-sent | | | TKL | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Up| 2 | equal | not-sent | | | Code | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Dw| 68 | equal | not-sent | | | Code | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 16 | 1 | Bi| 0x00 | MSB(12) | LSB | MMMM | | MID | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | tkl | 1 | Bi| 0x70 | MSB(5) | LSB | TTT | | Token | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| | ignore | value- | | | Uri-Host | (B) | | | | | sent | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Up| 0x09 | equal | not-sent | | | OSCORE_ | | | | | | | | | flags | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| 0x00 | MSB(4) | LSB | PPPP | | OSCORE | | | | | | | | | piv | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Up| 0x0000 | MSB(12) | LSB | KKKK | | OSCORE_ | | | | | | | | | kid | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Bi| b'' | equal | not-sent | | | OSCORE_ | | | | | | | | | kidctx | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | 8 | 1 | Dw| b'' | equal | not-sent | | | OSCORE_ | | | | | | | | | flags | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Dw| b'' | equal | not-sent | | | OSCORE_ | | | | | | | | | piv | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+ | CoAP | var | 1 | Dw| b'' | equal | not-sent | | | OSCORE_ | | | | | | | | | kid | | | | | | | | +----------+-----+---+---+-------------+---------+-----------+--------+
When the Device applies the Rule in Figure 13 shared with the Application Server to the CoAP request in Figure 3, this results in the Compressed Plaintext shown in Figure 16.¶
As per Section 7.2 of [RFC8824], the message follows the process of SCHC Inner Compression and encryption until the payload (if any). The ciphertext resulting from the overall Inner process is used as payload of the Outer OSCORE message.¶
_____________________________________________________
| |
| OSCORE Plaintext |
| |
| 0x01bb74656d7065726174757265 (13 bytes) |
| |
| 0x01 Request Code GET |
| |
| 0xbb74656d7065726174757265 Option 11: URI_PATH |
| Value = temperature |
|_____________________________________________________|
|
| Inner SCHC Compression
|
v
_________________________________________________
| |
| Compressed Plaintext |
| |
| 0x0200 (2 bytes) |
| |
| |
| RuleID = 0x02 (1 byte) |
| |
| |
| Compression residue |
| and padded payload = 0x00 (1 byte) |
| |
| 0b00 (2 bits match-mapping Compression Residue) |
| 0b000000 (6 bit padding) |
|_________________________________________________|
|
| AEAD Encryption
| (piv = 0x04)
|
v
________________________________________________
| |
| encrypted_plaintext = 0xa2cf (2 bytes) |
| tag = 0xc54fe1b434297b62 (8 bytes) |
| |
| ciphertext = 0xa2cfc54fe1b434297b62 (10 bytes) |
|________________________________________________|
When the Application Server applies the Rule in Figure 13 shared with the Device to the CoAP response in Figure 4, this results in the Compressed Plaintext shown in Figure 17.¶
As per Section 7.2 of [RFC8824], the message follows the process of SCHC Inner Compression and encryption until the payload (if any). The ciphertext resulting from the overall Inner process is used as payload of the Outer OSCORE message.¶
_________________________________________________
| |
| OSCORE Plaintext |
| |
| 0x45ff32332043 (6 bytes) |
| |
| 0x45 Successful Response Code 69 "2.05 Content" |
| |
| 0xff Payload marker |
| |
| 0x32332043 Payload |
|_________________________________________________|
|
| Inner SCHC Compression
|
v
_________________________________________________
| |
| Compressed Plaintext |
| |
| 0x028c8cc810c0 (6 bytes) |
| |
| |
| RuleID = 0x02 |
| |
| |
| Compression residue |
| and padded payload = 0x8c8cc810c0 (5 bytes) |
| |
| 0b10 (2 bits match-mapping Compression Residue) |
| 0x32332043 >> 2 (shifted payload) |
| 0b000000 Padding |
|_________________________________________________|
|
| AEAD Encryption
| (piv = 0x04)
|
v
_________________________________________________________
| |
| encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) |
| tag = 0xe9aef3f2461e0c29 (8 bytes) |
| |
| ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) |
|_________________________________________________________|
After having performed the SCHC Inner Compression of the CoAP request in Figure 3, the Device protects it with OSCORE by considering the Compressed Plaintext in Figure 16. The result is the OSCORE-protected CoAP request shown in Figure 18.¶
Protected message:
==================
0x41020001823b6578616d706c652e636f6d
6409040005d411636f6170ffa2cfc54fe1b434297b62
(39 bytes)
Header:
0x4102
01 Ver
00 CON
0001 TKL
00000010 Request Code 2 "POST"
0x0001 = mid
0x82 = token
Options:
0x3b6578616d706c652e636f6d
Option 3: Uri-Host
Value = example.com
0x6409040005
Option 9: OSCORE
Value = 0x09040005
09 = 000 0 1 001 flag byte
h k n
04 piv
0005 kid
0xd411636f6170
Option 39: Proxy-Scheme
Value = coap
0xFF Payload marker
Payload:
0xa2cfc54fe1b434297b62 (10 bytes)
Then, the Device applies the Rule in Figure 14 shared with the proxy to the OSCORE-protected CoAP request in Figure 18, thus performing the SCHC Outer Compression of such request. The result is the OSCORE-protected and Outer Compressed CoAP request shown in Figure 19, which the Device sends to the proxy.¶
Compressed message:
=================
0x03156caf0c2dae0d8ca5cc6deda8b459f8a9fc3686852f6c40 (25 bytes)
0x03 RuleID
156caf0c2dae0d8ca5cc6deda8b459f8a9fc3686852f6c40 compression
residue and
padded payload
Compression Residue (107 bits -> 14 bytes with padding)
0b 0001 010 1011 | 0x6578616d706c652e636f6d | 0b 0100 0101
mid tkn Uri-Host (residue size and residue) piv kid
Payload
0xa2cfc54fe1b434297b62 (10 bytes)
Compressed message length: 25 bytes
Upon receiving the message in Figure 19, the proxy decompresses it with the Rule in Figure 14 shared with the Device, thus performing the SCHC Outer Decompression. The result is the same OSCORE-protected CoAP request in Figure 18.¶
After that, the proxy removes the Proxy-Scheme Option from the decompressed OSCORE-protected CoAP request. Also, the proxy replaces the values of the CoAP Message ID and of the CoAP Token to 0x0004 and 0x75, respectively. The result is the OSCORE-protected CoAP request shown in Figure 20.¶
Protected message:
==================
0x41020004753b6578616d706c652e636f6d
6409040005ffa2cfc54fe1b434297b62
(33 bytes)
Header:
0x4102
01 Ver
00 CON
0001 TKL
00000010 Request Code 2 "POST"
0x0004 = mid
0x75 = token
Options:
0x3b6578616d706c652e636f6d
Option 3: Uri-Host
Value = example.com
0x6409040005
Option 9: OSCORE
Value = 0x09040005
09 = 000 0 1 001 flag byte
h k n
04 piv
0005 kid
0xFF Payload marker
Payload:
0xa2cfc54fe1b434297b62 (10 bytes)
Then, the proxy applies the Rule in Figure 15 shared with the Application Server to the OSCORE-protected CoAP request in Figure 20, thus performing the SCHC Outer Compression of such request. The result is the OSCORE-protected and Outer Compressed CoAP request shown in Figure 21, which the proxy forwards to the Application Server.¶
Compressed message:
=================
0x044b6caf0c2dae0d8ca5cc6deda8b459f8a9fc3686852f6c40 (25 bytes)
0x04 RuleID
4b6caf0c2dae0d8ca5cc6deda8b459f8a9fc3686852f6c40 compression
residue and
padded payload
Compression Residue (107 bits -> 14 bytes with padding)
0b 0100 101 1011 | 0x6578616d706c652e636f6d | 0b 0100 0101
mid tkn Uri-Host (residue size and residue) piv kid
Payload
0xa2cfc54fe1b434297b62 (10 bytes)
Compressed message length: 25 bytes
Upon receiving the message in Figure 21, the Application Server decompresses it using the Rule in Figure 15 shared with the proxy, thus performing the SCHC Outer Decompression. The result is the same OSCORE-protected CoAP request in Figure 20.¶
The Application Server decrypts and verifies such a request, which results in the same Compressed Plaintext in Figure 16. Then, the Application Server applies the Rule in Figure 13 shared with the Device to such a Compressed Plaintext, thus performing the SCHC Inner Decompression. The result is used to rebuild the same CoAP request in Figure 3, which the Application Server delivers to the application.¶
After having performed the SCHC Inner Compression of the CoAP response in Figure 4, the Application Server protects it with OSCORE by considering the Compressed Plaintext in Figure 17. The result is the OSCORE-protected CoAP response shown in Figure 22.¶
Protected message:
==================
0x614400047590ff10c6d7c26cc1e9aef3f2461e0c29
(21 bytes)
Header:
0x6144
01 Ver
10 ACK
0001 TKL
01000100 Successful Response Code 68 "2.04 Changed"
0x0004 = mid
0x75 = token
Options:
0x90
Option 9: OSCORE
Value = b''
0xFF Payload marker
Payload:
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Then, the Application Server applies the Rule in Figure 15 shared with the proxy to the OSCORE-protected CoAP response in Figure 22, thus performing the SCHC Outer Compression of such response. The result is the OSCORE-protected and Outer Compressed CoAP response shown in Figure 23, which the Application Server sends to the proxy.¶
Compressed message:
=================
0x04a510c6d7c26cc1e9aef3f2461e0c29 (16 bytes)
0x04 RuleID
a510c6d7c26cc1e9aef3f2461e0c29 compression residue
and padded payload
Compression Residue (8 bits -> 1 byte with padding)
0b 1 0100 101
type mid tkn
Payload
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Compressed message length: 16 bytes
Upon receiving the message in Figure 23, the proxy decompresses it with the Rule in Figure 15 shared with the Application Server, thus performing the SCHC Outer Decompression. The result is the same OSCORE-protected CoAP response in Figure 22.¶
After that, the proxy replaces the values of the CoAP Message ID and of the CoAP Token to 0x0001 and 0x82, respectively. The result is the OSCORE-protected CoAP response shown in Figure 24.¶
Protected message:
==================
0x614400018290ff10c6d7c26cc1e9aef3f2461e0c29
(21 bytes)
Header:
0x6144
01 Ver
10 ACK
0001 TKL
01000100 Successful Response Code 68 "2.04 Changed"
0x0001 = mid
0x82 = token
Options:
0x90
Option 9: OSCORE
Value = b''
0xFF Payload marker
Payload:
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Then, the proxy applies the Rule in Figure 14 shared with the Device to the OSCORE-protected CoAP response in Figure 24, thus performing the SCHC Outer Compression of such response. The result is the OSCORE-protected and Outer Compressed CoAP response shown in Figure 25, which the proxy forwards to the Device.¶
Compressed message:
=================
0x038a10c6d7c26cc1e9aef3f2461e0c29 (16 bytes)
0x03 RuleID
8a10c6d7c26cc1e9aef3f2461e0c29 compression residue
and padded payload
Compression Residue (8 bits -> 1 byte with padding)
0b 1 0001 010
type mid tkn
Payload
0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
Compressed message length: 15 bytes
Upon receiving the message in Figure 25, the Device decompresses it using the Rule in Figure 14 shared with the proxy, thus performing the SCHC Outer Decompression. The result is the same OSCORE-protected CoAP response in Figure 24.¶
The Device decrypts and verifies such a response, which results in the same Compressed Plaintext in Figure 17. Then, the Device applies the Rule in Figure 13 shared with the Application Server to such a Compressed Plaintext, thus performing the SCHC Inner Decompression. The result is used to rebuild the same CoAP response in Figure 4, which the Device delivers to the application.¶
The security considerations discussed in [RFC8724] and [RFC8824] continue to apply. When SCHC is used in the presence of CoAP proxies, the security considerations discussed in Section 11.2 of [RFC7252] continue to apply. When SCHC is used with OSCORE, the security considerations discussed in [RFC8613] continue to apply.¶
The security considerations in [RFC8824] specifically discuss how the use of SCHC for CoAP when OSCORE is also used may result in (more frequently) triggering key-renewal operations for the two endpoints. This can be due to an earlier exhaustion of the OSCORE Sender Sequence Number space, or to the installation of new compression Rules on one of the endpoints.¶
In either case, the two endpoints can run the key update protocol KUDOS defined in [I-D.ietf-core-oscore-key-update], as the recommended method to update their shared OSCORE Security Context.¶
This document has no actions for IANA.¶
The authors sincerely thank Christian Amsüss, Quentin Lampin, John Preuß Mattsson, Carles Gomez Montenegro, Göran Selander, Pascal Thubert, and Éric Vyncke for their comments and feedback.¶
The work on this document has been partly supported by the H2020 projects SIFIS-Home (Grant agreement 952652) and ARCADIAN-IoT (Grant agreement 101020259).¶