Static Context Header Compression (SCHC) for the Constrained Application Protocol (CoAP)
draft-ietf-lpwan-coap-static-context-hc-19
The information below is for an old version of the document that is already published as an RFC.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8824.
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Authors | Ana Minaburo , Laurent Toutain , Ricardo Andreasen | ||
Last updated | 2023-08-04 (Latest revision 2021-03-08) | ||
Replaces | draft-toutain-lpwan-coap-static-context-hc | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Intended RFC status | Proposed Standard | ||
Formats | |||
Reviews |
TSVART Last Call review
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by Joseph Touch
Almost ready
GENART Last Call review
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IOTDIR Last Call review
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Additional resources | Mailing list discussion | ||
Stream | WG state | Submitted to IESG for Publication | |
Document shepherd | Pascal Thubert | ||
Shepherd write-up | Show Last changed 2019-10-09 | ||
IESG | IESG state | Became RFC 8824 (Proposed Standard) | |
Action Holders |
(None)
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Consensus boilerplate | Yes | ||
Telechat date | (None) | ||
Responsible AD | Éric Vyncke | ||
Send notices to | Pascal Thubert <pthubert@cisco.com> | ||
IANA | IANA review state | Version Changed - Review Needed | |
IANA action state | No IANA Actions |
draft-ietf-lpwan-coap-static-context-hc-19
lpwan Working Group A. Minaburo Internet-Draft Acklio Intended status: Standards Track L. Toutain Expires: September 9, 2021 Institut MINES TELECOM; IMT Atlantique R. Andreasen Universidad de Buenos Aires March 08, 2021 LPWAN Static Context Header Compression (SCHC) for CoAP draft-ietf-lpwan-coap-static-context-hc-19 Abstract This draft defines how to compress the Constrained Application Protocol (CoAP) using the Static Context Header Compression (SCHC). SCHC is a header compression mechanism adapted for Constrained Devices. SCHC uses a static description of the header to reduce the header's redundancy and size. While RFC 8724 describes the SCHC compression and fragmentation framework, and its application for IPv6/UDP headers, this document applies SCHC for CoAP headers. The CoAP header structure differs from IPv6 and UDP since CoAP uses a flexible header with a variable number of options, themselves of variable length. The CoAP protocol messages format is asymmetric: the request messages have a header format different from the one in the response messages. This specification gives guidance on applying SCHC to flexible headers and how to leverage the asymmetry for more efficient compression Rules. Status of This Memo 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 September 9, 2021. Minaburo, et al. Expires September 9, 2021 [Page 1] Internet-Draft LPWAN CoAP compression March 2021 Copyright Notice Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. SCHC Applicability to CoAP . . . . . . . . . . . . . . . . . 4 3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 7 3.1. Differences between CoAP and UDP/IP Compression . . . . . 8 4. Compression of CoAP header fields . . . . . . . . . . . . . . 9 4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 9 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 9 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 9 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 10 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 10 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 10 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 11 5.2. CoAP option Max-Age, Uri-Host, and Uri-Port fields . . . 11 5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 11 5.3.1. Variable number of Path or Query elements . . . . . . 13 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path, and Location-Query fields . . . . . . . . . . . . . . . . 13 6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 13 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 14 6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 14 7. Examples of CoAP header compression . . . . . . . . . . . . . 15 7.1. Mandatory header with CON message . . . . . . . . . . . . 15 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 16 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 20 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 9. Security considerations . . . . . . . . . . . . . . . . . . . 31 Minaburo, et al. Expires September 9, 2021 [Page 2] Internet-Draft LPWAN CoAP compression March 2021 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 11. Normative References . . . . . . . . . . . . . . . . . . . . 32 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 1. Introduction CoAP [RFC7252] is a command/response protocol designed for micro- controllers with a small RAM and ROM and optimized for REST-based (Representative state transfer) services. Although the Constrained Devices leads the CoAP design, a CoAP header's size is still too large for LPWAN (Low Power Wide Area Networks). SCHC header compression over CoAP header is required to increase performance or use CoAP over LPWAN technologies. The [RFC8724] defines SCHC, a header compression mechanism for the LPWAN network based on a static context. Section 5 of the [RFC8724] explains where compression and decompression occur in the architecture. The SCHC compression scheme assumes as a prerequisite that both end-points know the static context before transmission. The way the context is configured, provisioned, or exchanged is out of this document's scope. CoAP is an application protocol, so CoAP compression requires installing common Rules between the two SCHC instances. SCHC compression may apply at two different levels: at IP and UDP in the LPWAN network and another at the application level for CoAP. These two compressions may be independent. Both follow the same principle described in [RFC8724]. As different entities manage the CoAP compression at different levels, the SCHC Rules driving the compression/decompression are also different. The [RFC8724] describes how to use SCHC for IP and UDP headers. This document specifies how to apply SCHC compression to CoAP headers. SCHC compresses and decompresses headers based on common contexts between Devices. SCHC context includes multiple Rules. Each Rule can match the header fields to specific values or ranges of values. If a Rule matches, the matched header fields are replaced by the RuleID and the Compression Residue that contains the residual bits of the compression. Thus, different Rules may correspond to different protocol headers in the packet that a Device expects to send or receive. A Rule describes the packets' entire header with an ordered list of fields descriptions; see section 7 of [RFC8724]. Thereby each description contains the field ID (FID), its length (FL), and its position (FP), a direction indicator (DI) (upstream, downstream, and bidirectional), and some associated Target Values (TV). The direction indicator is used for compression to give the best TV to Minaburo, et al. Expires September 9, 2021 [Page 3] Internet-Draft LPWAN CoAP compression March 2021 the FID when these values differ in the transmission direction. So a field may be described several times. A Matching Operator (MO) is associated with each header field description. The Rule is selected if all the MOs fit the TVs for all fields of the incoming header. A Rule cannot be selected if the message contains an unknown field to the SCHC compressor. In that case, a Compression/Decompression Action (CDA) associated with each field gives the method to compress and decompress each field. Compression mainly results in one of 4 actions: o send the field value (value-sent), o send nothing (not-sent), o send some least significant bits of the field (LSB) or, o send an index (mapping-sent). After applying the compression, there may be some bits to be sent. These values are called Compression Residue. SCHC is a general mechanism applied to different protocols, the exact Rules to be used depending on the protocol and the Application. Section 10 of the [RFC8724] describes the compression scheme for IPv6 and UDP headers. This document targets the CoAP header compression using SCHC. 1.1. Terminology 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. 2. SCHC Applicability to CoAP SCHC Compression for CoAP header MAY be done in conjunction with the lower layers (IPv6/UDP) or independently. The SCHC adaptation layers, described in Section 5 of [RFC8724], may be used as shown in Figure 1, Figure 2, and Figure 3. In the first example, Figure 1, a Rule compresses the complete header stack from IPv6 to CoAP. In this case, the Device and the NGW perform SCHC C/D (Static Context Header Compression Compressor/ Minaburo, et al. Expires September 9, 2021 [Page 4] Internet-Draft LPWAN CoAP compression March 2021 Decompressor). The Application communicating with the Device does not implement SCHC C/D. (Device) (NGW) (App) +--------+ +--------+ | CoAP | | CoAP | +--------+ +--------+ | UDP | | UDP | +--------+ +----------------+ +--------+ | IPv6 | | IPv6 | | IPv6 | +--------+ +--------+-------+ +--------+ | SCHC | | SCHC | | | | +--------+ +--------+ + + + | LPWAN | | LPWAN | | | | +--------+ +--------+-------+ +--------+ ((((LPWAN)))) ------ Internet ------ Figure 1: Compression/Decompression at the LPWAN boundary. Figure 1 shows the use of SCHC header compression above layer 2 in the Device and the NGW. The SCHC layer receives non-encrypted packets and can apply compression Rules to all the headers in the stack. On the other end, the NGW receives the SCHC packet and reconstructs the headers using the Rule and the Compression Residue. After the decompression, the NGW forwards the IPv6 packet toward the destination. The same process applies in the other direction when a non-encrypted packet arrives at the NGW. Thanks to the IP forwarding based on the IPv6 prefix, the NGW identifies the Device and compresses headers using the Device's Rules. In the second example, Figure 2, the SCHC compression is applied in the CoAP layer, compressing the CoAP header independently of the other layers. The RuleID, the Compression Residue, and CoAP payload are encrypted using a mechanism such as DTLS. Only the other end (App) can decipher the information. If needed, layers below use SCHC to compress the header as defined in [RFC8724] (represented in dotted lines). This use case needs an end-to-end context initialization between the Device and the Application. The context initialization is out of the scope of this document. Minaburo, et al. Expires September 9, 2021 [Page 5] Internet-Draft LPWAN CoAP compression March 2021 (Device) (NGW) (App) +--------+ +--------+ | CoAP | | CoAP | +--------+ +--------+ | SCHC | | SCHC | +--------+ +--------+ | DTLS | | DTLS | +--------+ +--------+ . udp . . udp . .......... .................. .......... . ipv6 . . ipv6 . . ipv6 . .......... .................. .......... . schc . . schc . . . . .......... .......... . . . . lpwan . . lpwan . . . . .......... .................. .......... ((((LPWAN)))) ------ Internet ------ Figure 2: Standalone CoAP end-to-end Compression/Decompression The third example, Figure 3, shows the use of Object Security for Constrained RESTful Environments (OSCORE) [RFC8613]. In this case, SCHC needs two Rules to compress the CoAP header. A first Rule focused on the inner header. The result of this first compression is encrypted using the OSCORE mechanism. Then a second Rule compresses the outer header, including the OSCORE Options. Minaburo, et al. Expires September 9, 2021 [Page 6] Internet-Draft LPWAN CoAP compression March 2021 (Device) (NGW) (App) +--------+ +--------+ | CoAP | | CoAP | | inner | | inner | +--------+ +--------+ | SCHC | | SCHC | | inner | | inner | +--------+ +--------+ | CoAP | | CoAP | | outer | | outer | +--------+ +--------+ | SCHC | | SCHC | | outer | | outer | +--------+ +--------+ . udp . . udp . .......... .................. .......... . ipv6 . . ipv6 . . ipv6 . .......... .................. .......... . schc . . schc . . . . .......... .......... . . . . lpwan . . lpwan . . . . .......... .................. .......... ((((LPWAN)))) ------ Internet ------ Figure 3: OSCORE compression/decompression. In the case of several SCHC instances, as shown in Figure 2 and Figure 3, the Rules may come from different provisioning domains. This document focuses on CoAP compression represented in the dashed boxes in the previous figures. 3. CoAP Headers compressed with SCHC The use of SCHC over the CoAP header uses the same description, and compression/decompression techniques like the one for IP and UDP explained in the [RFC8724]. For CoAP, the SCHC Rules description uses the direction information to optimize the compression by reducing the number of Rules needed to compress headers. The field description MAY define both request/response headers and target values in the same Rule, using the DI (direction indicator) to make the difference. As for other header compression protocols, when the compressor does not find a correct Rule to compress the header, the packet MUST be Minaburo, et al. Expires September 9, 2021 [Page 7] Internet-Draft LPWAN CoAP compression March 2021 sent uncompressed using the RuleID dedicated to this purpose. Where the Compression Residue is the complete header of the packet. See section 6 of [RFC8724]. 3.1. Differences between CoAP and UDP/IP Compression CoAP compression differs from IPv6 and UDP compression in the following aspects: o The CoAP protocol is asymmetric; the headers are different for a request or a response. For example, the URI-Path option is mandatory in the request, and it might not be present in the response. A request might contain an Accept option, and the response might include a Content-Format option. In comparison, IPv6 and UDP returning path swap the value of some fields in the header. However, all the directions have the same fields (e.g., source and destination address fields). The [RFC8724] defines the use of a direction indicator (DI) in the Field Descriptor, which allows a single Rule to process a message header differently depending on the direction. o Even when a field is "symmetric" (i.e., found in both directions), the values carried in each direction are different. The compression may use a "match-mapping" MO to limit the range of expected values in a particular direction and reduce the Compression Residue's size. Through the direction indicator (DI), a field description in the Rules splits the possible field value into two parts, one for each direction. For instance, if a client sends only CON requests, the Type can be elided by compression, and the answer may use one single bit to carry either the ACK or RST type. The field Code has the same behavior, the 0.0X code format value in the request, and the Y.ZZ code format in the response. o In SCHC, the Rule defines the different header fields' length, so SCHC does not need to send it. In IPv6 and UDP headers, the fields have a fixed size, known by definition. On the other hand, some CoAP header fields have variable lengths, and the Rule description specifies it. For example, in a URI-path or URI- query, the Token size may vary from 0 to 8 bytes, and the CoAP options use the Type-Length-Value encoding format. When doing SCHC compression of a variable-length field, Section 7.5.2 from [RFC8724] offers the possibility to define a function for the Field length in the Field Description to know the length before compression. If the field length is unknown, the Minaburo, et al. Expires September 9, 2021 [Page 8] Internet-Draft LPWAN CoAP compression March 2021 Rule will set it as a variable, and SCHC will send the compressed field's length in the Compression Residue. o A field can appear several times in the CoAP headers. It is found typically for elements of a URI (path or queries). The SCHC specification [RFC8724] allows a Field ID to appear several times in the Rule and uses the Field Position (FP) to identify the correct instance, thereby removing the matching operation's ambiguity. o Field lengths defined in the CoAP protocol can be too large regarding LPWAN traffic constraints. For instance, this is particularly true for the Message-ID field and the Token field. SCHC uses different Matching operators (MO) to perform the compression. See section 7.4 of [RFC8724]. In this case, SCHC can apply the Most Significant Bits (MSB) MO to reduce the information carried on LPWANs. 4. Compression of CoAP header fields This section discusses the compression of the different CoAP header fields. The CoAP compression with SCHC follows Section 7.1 of [RFC8724]. 4.1. CoAP version field CoAP version is bidirectional and MUST be elided during the SCHC compression since it always contains the same value. In the future, or if a new version of CoAP is defined, new Rules will be needed to avoid ambiguities between versions. 4.2. CoAP type field The CoAP protocol [RFC7252] has four types of messages: two requests (CON, NON), one response (ACK), and one empty message (RST). The SCHC compression SHOULD elide this field if, for instance, a client is sending only NON or only CON messages. For the RST message, SCHC may use a dedicated Rule. For other usages, SCHC can use a "match-mapping" MO. 4.3. CoAP code field The code field is an IANA registry [RFC7252], and it indicates the Request Method used in CoAP. The compression of the CoAP code field follows the same principle as that of the CoAP type field. If the Device plays a specific role, SCHC may split the code values into two fields description, the request codes with the 0 class and the Minaburo, et al. Expires September 9, 2021 [Page 9] Internet-Draft LPWAN CoAP compression March 2021 response values. SCHC will use the direction indicator to identify the correct value in the packet. If the Device only implements a CoAP client, SCHC compression may reduce the request code to the set of requests the client can process. For known values, SCHC can use a "match-mapping" MO. If SCHC cannot compress the code field, it will send the values in the Compression Residue. 4.4. CoAP Message ID field SCHC can compress the Message ID field with the "MSB" MO and the "LSB" CDA. See section 7.4 of [RFC8724]. 4.5. CoAP Token fields CoAP defines the Token using two CoAP fields, Token Length in the mandatory header and Token Value directly following the mandatory CoAP header. SCHC processes the Token length as any header field. If the value does not change, the size can be stored in the TV and elided during the transmission. Otherwise, SCHC will send the token length in the Compression Residue. For the Token Value, SCHC MUST NOT send it as a variable-length in the Compression Residue to avoid ambiguity with Token Length. Therefore, SCHC MUST use the Token length value to define the size of the Compression Residue. SCHC designates a specific function "tkl" that the Rule MUST use to complete the field description. During the decompression, this function returns the value contained in the Token Length field. 5. CoAP options CoAP defines options placed after the basic header in Option Numbers order; see [RFC7252]. Each Option instance in a message uses the format Delta-Type (D-T), Length (L), Value (V). The SCHC Rule builds the description of the option by using in the Field ID the Option Number built from D-T; in TV, the Option Value; and the Option Length uses section 7.4 of [RFC8724]. When the Option Length has a well- known size, the Rule may keep the length value. Therefore, SCHC compression does not send it. Otherwise, SCHC Compression carries the length of the Compression Residue, in addition to the Compression Residue value. Minaburo, et al. Expires September 9, 2021 [Page 10] Internet-Draft LPWAN CoAP compression March 2021 CoAP requests and responses do not include the same options. So Compression Rules may reflect this asymmetry by tagging the direction indicator. Note that length coding differs between CoAP options and SCHC variable size Compression Residue. The following sections present how SCHC compresses some specific CoAP options. If CoAP introduces a new option, the SCHC Rules MAY be updated, and the new Field ID description MUST be assigned to allow its compression. Otherwise, if no Rule describes this new option, the SCHC compression is not achieved, and SCHC sends the CoAP header without compression. 5.1. CoAP Content and Accept options. If the client expects a single value, it can be stored in the TV and elided during the transmission. Otherwise, if the client expects several possible values, a "match-mapping" SHOULD be used to limit the Compression Residue's size. If not, SCHC has to send the option value in the Compression Residue (fixed or variable length). 5.2. CoAP option Max-Age, Uri-Host, and Uri-Port fields SCHC compresses these three fields in the same way. When the value of these options is known, SCHC can elide these fields. If the option uses well-known values, SCHC can use a "match-mapping" MO. Otherwise, SCHC will use "value-sent" MO, and the Compression Residue will send these options' values. 5.3. CoAP option Uri-Path and Uri-Query fields The Uri-Path and Uri-Query fields are repeatable options; this means that in the CoAP header, they may appear several times with different values. SCHC Rule description uses the Field Position (FP) to distinguish the different instances in the path. To compress repeatable field values, SCHC may use a "match-mapping" MO to reduce the size of variable Paths or Queries. In these cases, to optimize the compression, several elements can be regrouped into a single entry. The Numbering of elements does not change, and the first matching element sets the MO comparison. Minaburo, et al. Expires September 9, 2021 [Page 11] Internet-Draft LPWAN CoAP compression March 2021 +--------+---+--+--+--------+-------------+------------+ | Field |FL |FP|DI| Target | Matching | CDA | | | | | | Value | Operator | | +--------+---+--+--+--------+-------------+------------+ |Uri-Path| | 1|up|["/a/b",|match-mapping|mapping-sent| | | | | |"/c/d"] | | | |Uri-Path|var| 3|up| |ignore |value-sent | +--------+---+--+--+--------+-------------+------------+ Figure 4: complex path example In Figure 4, SCHC can use a single bit in the Compression Residue to code one of the two paths. If regrouping were not allowed, 2 bits in the Compression Residue would be needed. SCHC sends the third path element as a variable size in the Compression Residue. The length of URI-Path and URI-Query may be known when the rule is defined. In any case, SCHC MUST set the field length to variable. The unit to indicate the Compression Residue size is in Byte. SCHC compression can use the MSB MO to a Uri-Path or Uri-Query element. However, attention to the length is important because the MSB value is in bits, and the size MUST always be a multiple of 8 bits. The length sent at the beginning of a variable-length Compression Residue indicates the LSB's size in bytes. For instance, for a CORECONF path /c/X6?k="eth0" the Rule description can be: +-------------+---+--+--+--------+---------+-------------+ | Field |FL |FP|DI| Target | Match | CDA | | | | | | Value | Opera. | | +-------------+---+--+--+--------+---------+-------------+ |Uri-Path | | 1|up|"c" |equal |not-sent | |Uri-Path |var| 2|up| |ignore |value-sent | |Uri-Query |var| 1|up|"k=\"" |MSB(24) |LSB | +-------------+---+--+--+--------+---------+-------------+ Figure 5: CORECONF URI compression Figure 5 shows the Rule description for a URI-Path and a URI-Query. SCHC compresses the first part of the URI-Path with a "not-sent" CDA. SCHC will send the second element of the URI-Path with the length (i.e., 0x2 X 6) followed by the query option (i.e., 0x05 eth0"). Minaburo, et al. Expires September 9, 2021 [Page 12] Internet-Draft LPWAN CoAP compression March 2021 5.3.1. Variable number of Path or Query elements SCHC fixed the number of Uri-Path or Uri-Query elements in a Rule at the Rule creation time. If the number varies, SCHC SHOULD create several Rules to cover all the possibilities. Another one is to define the length of Uri-Path to variable and sends a Compression Residue with a length of 0 to indicate that this Uri-Path is empty. However, this adds 4 bits to the variable Compression Residue size. See section 7.5.2 [RFC8724]. 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields The SCHC Rule description MAY define sending some field values by setting the TV to "not-sent," MO to "ignore," and CDA to "value- sent." A Rule MAY also use a "match-mapping" when there are different options for the same FID. Otherwise, the Rule sets the TV to the value, MO to "equal," and CDA to "not-sent." 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path, and Location-Query fields A Rule entry cannot store these fields' values. The Rule description MUST always send these values in the Compression Residue. 6. SCHC compression of CoAP extension RFCs 6.1. Block When a packet uses a Block [RFC7959] option, SCHC compression MUST send its content in the Compression Residue. The SCHC Rule describes an empty TV with a MO set to "ignore" and a CDA to "value-sent." Block option allows 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. 6.2. Observe The [RFC7641] defines the Observe option. The SCHC Rule description will not define the TV, but MO to "ignore," and the CDA 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 the Observe option MAY use an RST message to inform a server that the client does not require the Observe response, a specific Minaburo, et al. Expires September 9, 2021 [Page 13] Internet-Draft LPWAN CoAP compression March 2021 SCHC Rule SHOULD exist to allow the message's compression with the RST type. 6.3. No-Response The [RFC7967] defines a No-Response option limiting the responses made by a server to a request. Different behaviors exist while using this option to limit the responses made by a server to a request. If both ends know the value, then the SCHC Rule will describe a TV to this value, with a MO set to "equal" and CDA set to "not-sent." Otherwise, if the value is changing over time, the SCHC Rule will set the MO to "ignore" and CDA to "value-sent." The Rule may also use a "match-mapping" to compress this option. 6.4. OSCORE OSCORE [RFC8613] defines end-to-end protection for CoAP messages. This section describes how SCHC Rules can be applied to compress OSCORE-protected messages. 0 1 2 3 4 5 6 7 <--------- n bytes -------------> +-+-+-+-+-+-+-+-+--------------------------------- |0 0 0|h|k| n | Partial IV (if any) ... +-+-+-+-+-+-+-+-+--------------------------------- | | | |<-- CoAP -->|<------ CoAP OSCORE_piv ------> | OSCORE_flags <- 1 byte -> <------ s bytes -----> +------------+----------------------+-----------------------+ | s (if any) | kid context (if any) | kid (if any) ... | +------------+----------------------+-----------------------+ | | | | <------ CoAP OSCORE_kidctx ------>|<-- CoAP OSCORE_kid -->| Figure 6: OSCORE Option The Figure 6 shows the OSCORE Option Value encoding defined in Section 6.1 of [RFC8613], where the first byte specifies the Content of the OSCORE options using flags. The three most significant bits of this byte 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 three least significant bits n indicate the length of the piv (Partial Initialization Vector) field in bytes. When n = 0, no piv is present. Minaburo, et al. Expires September 9, 2021 [Page 14] Internet-Draft LPWAN CoAP compression March 2021 The flag byte is followed by the piv field, kid context field, and kid field in this order, and if present, the kid context field's length is encoded in the first byte denoting by 's' the length of the kid context in bytes. 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 4 distinct pieces of information within the OSCORE option: the flag bits, the piv, the kid context, and the kid. The SCHC Rule splits into four field descriptions the OSCORE option to compress them: o CoAP OSCORE_flags, o CoAP OSCORE_piv, o CoAP OSCORE_kidctx, o CoAP OSCORE_kid. Figure 6 shows the OSCORE Option format with those four fields superimposed on it. Note that the CoAP OSCORE_kidctx field directly includes the size octet s. 7. Examples of CoAP header compression 7.1. Mandatory header with CON message In this first scenario, the SCHC Compressor at the Network Gateway side receives a POST message from an Internet client, which is immediately acknowledged by the Device. Figure 7 describes the SCHC Rule descriptions for this scenario. Minaburo, et al. Expires September 9, 2021 [Page 15] Internet-Draft LPWAN CoAP compression March 2021 RuleID 1 +-------------+--+--+--+------+---------+-------------++------------+ | Field |FL|FP|DI|Target| Match | CDA || Sent | | | | | |Value | Opera. | || [bits] | +-------------+--+--+--+------+---------+-------------++------------+ |CoAP version | 2| 1|bi| 01 |equal |not-sent || | |CoAP Type | 2| 1|dw| CON |equal |not-sent || | |CoAP Type | 2| 1|up|[ACK, |match- |matching- || | | | | | | RST] |mapping |sent || T | |CoAP TKL | 4| 1|bi| 0 |equal |not-sent || | |CoAP Code | 8| 1|bi|[0.00,| | || | | | | | | ... |match- |matching- || | | | | | | 5.05]|mapping |sent || CC CCC | |CoAP MID |16| 1|bi| 0000 |MSB(7 ) |LSB || M-ID| |CoAP Uri-Path|var 1|dw| path |equal 1 |not-sent || | +-------------+--+--+--+------+---------+-------------++------------+ Figure 7: CoAP Context to compress header without Token In this example, SCHC compression elides the version and the Token Length fields. The 26 method and response codes defined in [RFC7252] has been shrunk to 5 bits using a "match-mapping" MO. The Uri-Path contains a single element indicated in the TV and elided with the CDA "not-sent." SCHC Compression reduces the header sending only the Type, a mapped code, and the least significant bits of Message ID (9 bits in the example above). Note that a client located in an Application Server sending a request to a server located in the Device may not be compressed through this Rule since the MID might not start with 7 bits equal to 0. A CoAP proxy placed before the SCHC C/D can rewrite the message ID to fit the value and match the Rule. 7.2. OSCORE Compression OSCORE aims to solve the problem of end-to-end encryption for CoAP messages. Therefore, the goal is to hide as much as possible the message 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 sensitive information that is not necessary for proxy operation. However, it is part of the message that can be encrypted Minaburo, et al. Expires September 9, 2021 [Page 16] Internet-Draft LPWAN CoAP compression March 2021 until it reaches its end destination. The Outer Message acts as a shell matching the regular CoAP message format and includes all Options and information needed for proxy operation and caching. Figure 8 illustrates this analysis. The CoAP protocol arranges the options into one of 3 classes; each granted a specific type of protection by the protocol: o Class E: Encrypted options moved to the Inner Plaintext, o Class I: Integrity-protected options included in the AAD for the encryption of the Plaintext but otherwise left untouched in the Outer Message, o Class U: Unprotected options left untouched in the Outer Message. These classes point out that the Outer option contains the OSCORE Option and that the message is OSCORE protected; this option carries the information necessary to retrieve the Security Context. The end- point will use this Security Context to decrypt the message correctly. Minaburo, et al. Expires September 9, 2021 [Page 17] Internet-Draft LPWAN CoAP compression March 2021 Original CoAP Packet +-+-+---+-------+---------------+ |v|t|TKL| code | Msg Id. | +-+-+---+-------+---------------+....+ | Token | +-------------------------------.....+ | Options (IEU) | . . . . +------+-------------------+ | 0xFF | +------+------------------------+ | | | Payload | | | +-------------------------------+ / \ / \ / \ / \ Outer Header v v Plaintext +-+-+---+--------+---------------+ +-------+ |v|t|TKL|new code| Msg Id. | | code | +-+-+---+--------+---------------+....+ +-------+-----......+ | Token | | Options (E) | +--------------------------------.....+ +-------+------.....+ | Options (IU) | | OxFF | . . +-------+-----------+ . OSCORE Option . | | +------+-------------------+ | Payload | | 0xFF | | | +------+ +-------------------+ Figure 8: A CoAP packet is split into an OSCORE outer and plaintext Figure 8 shows the packet format for the OSCORE Outer header and Plaintext. In the Outer Header, the original header code is hidden and replaced by a default dummy value. As seen in Sections 4.1.3.5 and 4.2 of [RFC8613], the message code is replaced by POST for requests and Changed for responses when CoAP is not using the Observe option. If CoAP uses Observe, the OSCORE message code is replaced by FETCH for requests and Content for responses. Minaburo, et al. Expires September 9, 2021 [Page 18] Internet-Draft LPWAN CoAP compression March 2021 The first byte of the Plaintext contains the original packet code, followed by the message code, the class E options, and, if present, the original message Payload preceded by its payload marker. An AEAD algorithm now encrypts the Plaintext. This integrity protects the Security Context parameters and, eventually, any class I options from the Outer Header. The resulting Ciphertext becomes the new payload of the OSCORE message, as illustrated in Figure 9. As defined in [RFC5116], this Ciphertext is the encrypted Plaintext's concatenation of the authentication tag. Note that Inner Compression only affects the Plaintext before encryption. Thus only the first variable-length of the Ciphertext can be reduced. The authentication tag is fixed in length and is considered part of the cost of protection. Outer Header +-+-+---+--------+---------------+ |v|t|TKL|new code| Msg Id. | +-+-+---+--------+---------------+....+ | Token | +--------------------------------.....+ | Options (IU) | . . . OSCORE Option . +------+-------------------+ | 0xFF | +------+---------------------------+ | | | Ciphertext: Encrypted Inner | | Header and Payload | | + Authentication Tag | | | +----------------------------------+ Figure 9: OSCORE message The SCHC Compression scheme consists of compressing both the Plaintext before encryption and the resulting OSCORE message after encryption, see Figure 10. The OSCORE message 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 Minaburo, et al. Expires September 9, 2021 [Page 19] Internet-Draft LPWAN CoAP compression March 2021 SCHC Rules. This way, compression is applied to all fields of the original CoAP message. Note that since the corresponding end-point can only decrypt the Inner part of the message, this end-point will also have to implement Inner SCHC Compression/Decompression. Outer Message OSCORE Plaintext +-+-+---+--------+---------------+ +-------+ |v|t|TKL|new code| Msg Id. | | code | +-+-+---+--------+---------------+....+ +-------+-----......+ | Token | | Options (E) | +--------------------------------.....+ +-------+------.....+ | Options (IU) | | OxFF | . . +-------+-----------+ . OSCORE Option . | | +------+-------------------+ | Payload | | 0xFF | | | +------+------------+ +-------------------+ | Ciphertext |<---------\ | | | | v +-------------------+ | +-----------------+ | | | Inner SCHC | v | | Compression | +-----------------+ | +-----------------+ | Outer SCHC | | | | Compression | | v +-----------------+ | +-------+ | | |RuleID | v | +-------+-----------+ +--------+ +------------+ |Compression Residue| |RuleID' | | Encryption | <-- +----------+--------+ +--------+-----------+ +------------+ | | |Compression Residue'| | Payload | +-----------+--------+ | | | Ciphertext | +-------------------+ | | +--------------------+ Figure 10: OSCORE Compression Diagram 7.3. Example OSCORE Compression This section gives an example with a GET Request and its consequent Content Response from a Device-based CoAP client to a cloud-based CoAP server. The example also describes a possible set of Rules for the Inner and Outer SCHC Compression. A dump of the results and a Minaburo, et al. Expires September 9, 2021 [Page 20] Internet-Draft LPWAN CoAP compression March 2021 contrast between SCHC + OSCORE performance with SCHC + COAP performance is also listed. This example gives an approximation of the cost of security with SCHC-OSCORE. Our first CoAP message is the GET request in Figure 11. Original message: ================= 0x4101000182bb74656d7065726174757265 Header: 0x4101 01 Ver 00 CON 0001 TKL 00000001 Request Code 1 "GET" 0x0001 = mid 0x82 = token Options: 0xbb74656d7065726174757265 Option 11: URI_PATH Value = temperature Original msg length: 17 bytes. Figure 11: CoAP GET Request Its corresponding response is the CONTENT Response in Figure 12. Minaburo, et al. Expires September 9, 2021 [Page 21] Internet-Draft LPWAN CoAP compression March 2021 Original message: ================= 0x6145000182ff32332043 Header: 0x6145 01 Ver 10 ACK 0001 TKL 01000101 Successful Response Code 69 "2.05 Content" 0x0001 = mid 0x82 = token 0xFF Payload marker Payload: 0x32332043 Original msg length: 10 Figure 12: CoAP CONTENT Response The SCHC Rules for the Inner Compression include all fields already present in a regular CoAP message. The methods described in Section 4 apply to these fields. As an example, see Figure 13. RuleID 0 +--------------+--+--+--+-----------+---------+---------++------+ | Field |FL|FP|DI| Target | MO | CDA || Sent | | | | | | Value | | ||[bits]| +--------------+--+--+--+-----------+---------+---------++------+ |CoAP Code | 8| 1|up| 1 | equal |not-sent || | |CoAP Code | 8| 1|dw|[69, | | || | | | | | |132] |match- |mapping- || | | | | | | |mapping |sent || c | |CoAP Uri-Path | | 1|up|temperature| equal |not-sent || | +--------------+--+--+--+-----------+---------+---------++------+ Figure 13: Inner SCHC Rules Figure 14 shows the Plaintext obtained for the example GET request. The packet follows the process of Inner Compression and Encryption until the payload. The outer OSCORE Message adds the result of the Inner process. In this case, the original message has no payload, and its resulting Plaintext compressed up to only 1 byte (size of the RuleID). The AEAD algorithm preserves this length in its first output and yields a Minaburo, et al. Expires September 9, 2021 [Page 22] Internet-Draft LPWAN CoAP compression March 2021 fixed-size tag. SCHC cannot compress the tag, and the OSCORE message must include it without compression. The use of integrity protection translates into an overhead in total message length, limiting the amount of compression that can be achieved and plays into the cost of adding security to the exchange. ________________________________________________________ | | | OSCORE Plaintext | | | | 0x01bb74656d7065726174757265 (13 bytes) | | | | 0x01 Request Code GET | | | | bb74656d7065726174757265 Option 11: URI_PATH | | Value = temperature | |________________________________________________________| | | | Inner SCHC Compression | v _________________________________ | | | Compressed Plaintext | | | | 0x00 | | | | RuleID = 0x00 (1 byte) | | (No Compression Residue) | |_________________________________| | | AEAD Encryption | (piv = 0x04) v _________________________________________________ | | | encrypted_plaintext = 0xa2 (1 byte) | | tag = 0xc54fe1b434297b62 (8 bytes) | | | | ciphertext = 0xa2c54fe1b434297b62 (9 bytes) | |_________________________________________________| Figure 14: Plaintext compression and encryption for GET Request Minaburo, et al. Expires September 9, 2021 [Page 23] Internet-Draft LPWAN CoAP compression March 2021 Figure 15 shows the process for the example CONTENT Response. The Compression Residue is 1 bit long. Note that since SCHC adds padding after the payload, this misalignment causes the hexadecimal code from the payload to differ from the original, even if SCHC cannot compress the tag. The overhead for the tag bytes limits the SCHC's performance but brings security to the transmission. Minaburo, et al. Expires September 9, 2021 [Page 24] Internet-Draft LPWAN CoAP compression March 2021 ________________________________________________________ | | | OSCORE Plaintext | | | | 0x45ff32332043 (6 bytes) | | | | 0x45 Successful Response Code 69 "2.05 Content" | | | | ff Payload marker | | | | 32332043 Payload | |________________________________________________________| | | | Inner SCHC Compression | v _____________________________________________ | | | Compressed Plaintext | | | | 0x001919902180 (6 bytes) | | | | 00 RuleID | | | | 0b0 (1 bit match-map Compression Residue) | | 0x32332043 >> 1 (shifted payload) | | 0b0000000 Padding | |_____________________________________________| | | AEAD Encryption | (piv = 0x04) v _________________________________________________________ | | | encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) | | tag = 0xe9aef3f2461e0c29 (8 bytes) | | | | ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) | |_________________________________________________________| Figure 15: Plaintext compression and encryption for CONTENT Response The Outer SCHC Rules (Figure 18) must process the OSCORE Options fields. Figure 16 and Figure 17 shows a dump of the OSCORE Messages Minaburo, et al. Expires September 9, 2021 [Page 25] Internet-Draft LPWAN CoAP compression March 2021 generated from the example messages. They include the Inner Compressed Ciphertext in the payload. These are the messages that have to be compressed by the Outer SCHC Compression. Protected message: ================== 0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62 (25 bytes) Header: 0x4102 01 Ver 00 CON 0001 TKL 00000010 Request Code 2 "POST" 0x0001 = mid 0x82 = token Options: 0xd8080904636c69656e74 (10 bytes) Option 21: OBJECT_SECURITY Value = 0x0904636c69656e74 09 = 000 0 1 001 Flag byte h k n 04 piv 636c69656e74 kid 0xFF Payload marker Payload: 0xa2c54fe1b434297b62 (9 bytes) Figure 16: Protected and Inner SCHC Compressed GET Request Minaburo, et al. Expires September 9, 2021 [Page 26] Internet-Draft LPWAN CoAP compression March 2021 Protected message: ================== 0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29 (22 bytes) Header: 0x6144 01 Ver 10 ACK 0001 TKL 01000100 Successful Response Code 68 "2.04 Changed" 0x0001 = mid 0x82 = token Options: 0xd008 (2 bytes) Option 21: OBJECT_SECURITY Value = b'' 0xFF Payload marker Payload: 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) Figure 17: Protected and Inner SCHC Compressed CONTENT Response For the flag bits, some SCHC compression methods are useful, depending on the Application. The most straightforward alternative is to provide a fixed value for the flags, combining MO "equal" and CDA "not-sent." This SCHC definition saves most bits but could prevent flexibility. Otherwise, SCHC could use a "match-mapping" MO to choose from several configurations for the exchange. If not, the SCHC description may use an "MSB" MO to mask off the three hard-coded most significant bits. Note that fixing a flag bit will limit CoAP Options choice that can be used in the exchange since their values are dependent on specific options. The piv field lends itself to having some bits masked off with "MSB" MO and "LSB" CDA. This SCHC description could be useful in applications where the message frequency is low such as LPWAN technologies. Note that compressing the sequence numbers may reduce the maximum number of sequence numbers that can be used in an exchange. Once the sequence number exceeds the maximum value, the OSCORE keys need to be re-established. Minaburo, et al. Expires September 9, 2021 [Page 27] Internet-Draft LPWAN CoAP compression March 2021 The size s included in the kid context field MAY be masked off with "LSB" CDA. The rest of the field could have additional bits masked off or have the whole field fixed with MO "equal" and CDA "not-sent." The same holds for the kid field. Figure 18 shows a possible set of Outer Rules to compress the Outer Header. RuleID 0 +------------------+--+--+--+--------------+-------+--------++------+ | Field |FL|FP|DI| Target | MO | CDA || Sent | | | | | | Value | | ||[bits]| +------------------+--+--+--+--------------+-------+--------++------+ |CoAP version | 2| 1|bi| 01 |equal |not-sent|| | |CoAP Type | 2| 1|up| 0 |equal |not-sent|| | |CoAP Type | 2| 1|dw| 2 |equal |not-sent|| | |CoAP TKL | 4| 1|bi| 1 |equal |not-sent|| | |CoAP Code | 8| 1|up| 2 |equal |not-sent|| | |CoAP Code | 8| 1|dw| 68 |equal |not-sent|| | |CoAP MID |16| 1|bi| 0000 |MSB(12)|LSB ||MMMM | |CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP OSCORE_flags | 8| 1|up| 0x09 |equal |not-sent|| | |CoAP OSCORE_piv |var 1|up| 0x00 |MSB(4) |LSB ||PPPP | |COAP OSCORE_kid |var 1|up|0x636c69656e70|MSB(52)|LSB ||KKKK | |COAP OSCORE_kidctx|var 1|bi| b'' |equal |not-sent|| | |CoAP OSCORE_flags | 8| 1|dw| b'' |equal |not-sent|| | |CoAP OSCORE_piv |var 1|dw| b'' |equal |not-sent|| | |CoAP OSCORE_kid |var 1|dw| b'' |equal |not-sent|| | +------------------+--+--+--+--------------+-------+--------++------+ Figure 18: Outer SCHC Rules The Outer Rule of Figure 18 is applied to the example GET Request and CONTENT Response. Figure 19 and Figure 20 show the resulting messages. Minaburo, et al. Expires September 9, 2021 [Page 28] Internet-Draft LPWAN CoAP compression March 2021 Compressed message: ================== 0x001489458a9fc3686852f6c4 (12 bytes) 0x00 RuleID 1489 Compression Residue 458a9fc3686852f6c4 Padded payload Compression Residue: 0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding) mid tkn piv kid Payload 0xa2c54fe1b434297b62 (9 bytes) Compressed message length: 12 bytes Figure 19: SCHC-OSCORE Compressed GET Request Compressed message: ================== 0x0014218daf84d983d35de7e48c3c1852 (16 bytes) 0x00 RuleID 14 Compression Residue 218daf84d983d35de7e48c3c1852 Padded payload Compression Residue: 0b0001 010 (7 bits -> 1 byte with padding) mid tkn Payload 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) Compressed msg length: 16 bytes Figure 20: SCHC-OSCORE Compressed CONTENT Response In contrast, comparing these results with what would be obtained by SCHC compressing the original CoAP messages without protecting them with OSCORE is done by compressing the CoAP messages according to the SCHC Rules in Figure 21. Minaburo, et al. Expires September 9, 2021 [Page 29] Internet-Draft LPWAN CoAP compression March 2021 RuleID 1 +---------------+--+--+--+-----------+---------+-----------++-------+ | Field |FL|FP|DI| Target | MO | CDA || Sent | | | | | | Value | | || [bits]| +---------------+--+--+--+-----------+---------+-----------++-------+ |CoAP version | 2| 1|bi| 01 |equal |not-sent || | |CoAP Type | 2| 1|up| 0 |equal |not-sent || | |CoAP Type | 2| 1|dw| 2 |equal |not-sent || | |CoAP TKL | 4| 1|bi| 1 |equal |not-sent || | |CoAP Code | 8| 1|up| 2 |equal |not-sent || | |CoAP Code | 8| 1|dw| [69,132] |match- |mapping- || | | | | | | |mapping |sent ||C | |CoAP MID |16| 1|bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP Uri-Path | | 1|up|temperature|equal |not-sent || | +---------------+--+--+--+-----------+---------+-----------++-------+ Figure 21: SCHC-CoAP Rules (No OSCORE) Figure 21 Rule yields the SCHC compression results in Figure 22 for request, and Figure 23 for the response. Compressed message: ================== 0x0114 0x01 = RuleID Compression Residue: 0b00010100 (1 byte) Compressed msg length: 2 Figure 22: CoAP GET Compressed without OSCORE Minaburo, et al. Expires September 9, 2021 [Page 30] Internet-Draft LPWAN CoAP compression March 2021 Compressed message: ================== 0x010a32332043 0x01 = RuleID Compression Residue: 0b00001010 (1 byte) Payload 0x32332043 Compressed msg length: 6 Figure 23: CoAP CONTENT Compressed without OSCORE As can be seen, the difference between applying SCHC + OSCORE as compared to regular SCHC + COAP is about 10 bytes. 8. IANA Considerations This document has no request to IANA. 9. Security considerations The use of SCHC header compression for CoAP header fields only affects the representation of the header information. SCHC header compression itself does not increase or decrease the overall level of security of the communication. When the connection does not use a security protocol (such as OSCORE, DTLS, etc.), it is necessary to use a layer-two security mechanism to protect the SCHC messages. If LPWAN is the layer-two technology, the SCHC security considerations of [RFC8724] continue to apply. When using another layer-two protocol, use of a cryptographic integrity-protection mechanisms to protect the SCHC headers is REQUIRED. Such cryptographic integrity protection is necessary in order to continue to provide the properties that [RFC8724] relies upon. When SCHC is used with OSCORE, the security considerations of [RFC8613] continue to apply. When SCHC is used with the OSCORE outer headers, the Initialization Vector (IV) size in the Compression Residue must be carefully selected. There is a tradeoff between compression efficiency (with a longer "MSB" MO prefix) and the frequency at which the Device must renew its key material (in order to prevent the IV from expanding to Minaburo, et al. Expires September 9, 2021 [Page 31] Internet-Draft LPWAN CoAP compression March 2021 an uncompressable value). The key renewal operation itself requires several message exchanges and requires energy-intensive computation, but the optimal tradeoff will depend on the specifics of the device and expected usage patterns. If an attacker can introduce a corrupted SCHC-compressed packet onto a link, DoS attacks are possible by causing excessive resource consumption at the decompressor. However, an attacker able to inject packets at the link layer is also capable of other, potentially more damaging, attacks. SCHC compression emits variable-length Compression Residues for some CoAP fields. In the compressed header representation, the length field that is sent is not the length of the original header field but rather the length of the Compression Residue that is being transmitted. If a corrupted packet arrives at the decompressor with a longer or shorter length than the original compressed representation possessed, the SCHC decompression procedures will detect an error and drop the packet. SCHC header compression rules MUST remain tightly coupled between compressor and decompressor. If the compression rules get out of sync, a Compression Residue might be decompressed differently at the receiver than the initial message submitted to compression procedures. Accordingly, any time the context Rules are updated on an OSCORE endpoint, that endpoint MUST trigger OSCORE key re- establishment. Similar procedures may be appropriate to signal Rule udpates when other message-protection mechanisms are in use. 10. Acknowledgements The authors would like to thank (in alphabetic order): Christian Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas Fossati, Klaus Hartke, Benjamin Kaduk, Francesca Palombini, Alexander Pelov, Goran Selander and Eric Vyncke. 11. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, <https://www.rfc-editor.org/info/rfc5116>. Minaburo, et al. Expires September 9, 2021 [Page 32] Internet-Draft LPWAN CoAP compression March 2021 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, <https://www.rfc-editor.org/info/rfc7252>. [RFC7641] Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015, <https://www.rfc-editor.org/info/rfc7641>. [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, August 2016, <https://www.rfc-editor.org/info/rfc7959>. [RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T. Bose, "Constrained Application Protocol (CoAP) Option for No Server Response", RFC 7967, DOI 10.17487/RFC7967, August 2016, <https://www.rfc-editor.org/info/rfc7967>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, <https://www.rfc-editor.org/info/rfc8613>. [RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. Zuniga, "SCHC: Generic Framework for Static Context Header Compression and Fragmentation", RFC 8724, DOI 10.17487/RFC8724, April 2020, <https://www.rfc-editor.org/info/rfc8724>. Authors' Addresses Ana Minaburo Acklio 1137A avenue des Champs Blancs 35510 Cesson-Sevigne Cedex France Email: ana@ackl.io Minaburo, et al. Expires September 9, 2021 [Page 33] Internet-Draft LPWAN CoAP compression March 2021 Laurent Toutain Institut MINES TELECOM; IMT Atlantique 2 rue de la Chataigneraie CS 17607 35576 Cesson-Sevigne Cedex France Email: Laurent.Toutain@imt-atlantique.fr Ricardo Andreasen Universidad de Buenos Aires Av. Paseo Colon 850 C1063ACV Ciudad Autonoma de Buenos Aires Argentina Email: randreasen@fi.uba.ar Minaburo, et al. Expires September 9, 2021 [Page 34]