lpwan Working Group A. Minaburo Internet-Draft Acklio Intended status: Standards Track L. Toutain Expires: September 6, 2020 Institut MINES TELECOM; IMT Atlantique R. Andreasen Universidad de Buenos Aires March 05, 2020 LPWAN Static Context Header Compression (SCHC) for CoAP draft-ietf-lpwan-coap-static-context-hc-13 Abstract This draft defines the way SCHC (Static Context Header Compression) header compression can be applied to the CoAP protocol. SCHC is a header compression mechanism adapted for constrained devices. SCHC uses a static description of the header to reduce the redundancy and the size of the information in the header. While [I-D.ietf-lpwan-ipv6-static-context-hc] describes the SCHC compression and fragmentation framework, and its application for IPv6/UDP headers, this document applies the use of 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 how to apply 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 6, 2020. Minaburo, et al. Expires September 6, 2020 [Page 1] Internet-Draft LPWAN CoAP compression March 2020 Copyright Notice Copyright (c) 2020 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. Applying SCHC to CoAP . . . . . . . . . . . . . . . . . . . . 4 3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 5 3.1. Differences between CoAP and UDP/IP . . . . . . . . . . . 5 4. Compression of CoAP header fields . . . . . . . . . . . . . . 6 4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 7 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 7 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 7 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 7 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 8 5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 8 5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8 5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 9 5.3.2. Variable number of path or query elements . . . . . . 10 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and Location-Query fields . . . . . . . . . . . . . . . . 10 6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 10 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 11 6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11 7. Examples of CoAP header compression . . . . . . . . . . . . . 12 7.1. Mandatory header with CON message . . . . . . . . . . . . 12 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 13 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 Minaburo, et al. Expires September 6, 2020 [Page 2] Internet-Draft LPWAN CoAP compression March 2020 9. Security considerations . . . . . . . . . . . . . . . . . . . 27 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 11. Normative References . . . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 1. Introduction CoAP [rfc7252] is a transfer protocol that implements a subset of HTTP (Hypertext Transfer Protocol) and is optimized for REST-based (Representational state transfer) services. Although CoAP was designed for constrained devices, the size of a CoAP header still is too large for the constraints of LPWAN (Low Power Wide Area Networks) and some compression is needed to reduce the header size. The [I-D.ietf-lpwan-ipv6-static-context-hc] defines SCHC, a header compression mechanism for LPWAN network based on a static context. The section 5 of the [I-D.ietf-lpwan-ipv6-static-context-hc] explains the architecture where compression and decompression are done. The context is known by both ends before transmission. The way the context is configured or exchanged is out of the scope for this document. SCHC compresses and decompresses headers based on shared contexts between devices. Each context consists of multiple Rules. Each rule can match header fields and specific values or ranges of values. If a rule matches, the matched header fields are substituted by the rule ID and optionally some residual bits. Thus, different Rules may correspond to different types of packets that a device expects to send or receive. A Rule describes the complete header of the packet with an ordered list of fields descriptions, see section 7 of the [I-D.ietf-lpwan-ipv6-static-context-hc], 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). A Matching Operator (MO) is associated to each header field description. The rule is selected if all the MOs fit the TVs for all fields of the incoming packet. In that case, a Compression/Decompression Action (CDA) associated to each field defines how the compressed and the decompressed values are computed out of each other, for each of the header fields. Compression mainly results in one of 4 actions: * send the field value, * send nothing, * send some least significant bits of the field or * send an index. After applying the compression there may be some bits to be sent, these values are called Compression Residues. Minaburo, et al. Expires September 6, 2020 [Page 3] Internet-Draft LPWAN CoAP compression March 2020 SCHC is a general concept mechanism that can be applied to different protocols, the exact Rules to be used depend on the protocol and the application, and CoAP differs from UDP and IPv6, see Section 3. 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. Applying SCHC to CoAP The SCHC Compression rules can be applied to CoAP flows. SCHC Compression of the CoAP header MAY be done in conjunction with the lower layers (IPv6/UDP) or independently. The SCHC adaptation layers as described in section 5 of [I-D.ietf-lpwan-ipv6-static-context-hc] may be used as shown in Figure 1. ^ +------------+ ^ +------------+ ^ +------------+ | | CoAP | | | CoAP | inner | | CoAP | | +------------+ v +------------+ x | OSCORE | | | UDP | | DTLS | outer | +------------+ | +------------+ +------------+ | | UDP | | | IPv6 | | UDP | | +------------+ v +------------+ +------------+ | | IPv6 | | IPv6 | v +------------+ +------------+ Figure 1: rule scope for CoAP Figure 1 shows some examples for CoAP architecture and the SCHC rule's scope. In the first example, a rule compresses the complete header stack from IPv6 to CoAP. In this case, SCHC C/D (Static Context Header Compression Compressor/Decompressor) is performed at the Sender and at the Receiver. In the second example, an end-to-end encryption mechanisms is used between the Sender and the Receiver. The SCHC compression is applied in the CoAP layer compressing the CoAP header independently of the other layers. The rule ID and the compression residue are encrypted using a mechanism such as DTLS. Only the other end can decipher the information. Layers below may also be compressed using other SCHC Minaburo, et al. Expires September 6, 2020 [Page 4] Internet-Draft LPWAN CoAP compression March 2020 rules (this is out of the scope of this document) as defined in the SCHC [I-D.ietf-lpwan-ipv6-static-context-hc] document. In the third example, OSCORE [rfc8613] is used. In this case, two rulesets are used to compress the CoAP message. A first ruleset focused on the inner header and is applied end to end by both ends. A second ruleset compresses the outer header and the layers below and is done between the Sender and the Receiver. 3. CoAP Compression with SCHC SCHC with CoAP will be used exactly the same way as it is applied in any protocol as IP or UDP with the difference that the fields description needs to be defined based on both headers and target values of the request and the responses. SCHC Rules description use the direction information to optmize the compression by reducing the number of Rules needed to compress traffic. CoAP compression follows the [I-D.ietf-lpwan-ipv6-static-context-hc] scheme and as for other protocols, if no valid Rule was found, then the packet MUST be sent uncompressed using the RuleID dedicated to this purpose and the Compression Residue is the complete header of the packet. See section 6 of [I-D.ietf-lpwan-ipv6-static-context-hc]. 3.1. Differences between CoAP and UDP/IP CoAP differs from IPv6 and UDP protocols on the following aspects: o IPv6 and UDP are not request and response protocols as CoAP, and so the same header fields are used in all packets for all directions, with the value of some fields being swapped on the return path (e.g. source and destination addresses fields). The CoAP headers instead are asymmetric, the headers are different for a request or a response. For example, the URI-path option is mandatory in the request and is not found in the response, a request may contain an Accept option and the response may contain a Content option. The [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a Direction Indicator (DI) in the Field Description, which allows a single Rule to process message headers 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. To performs the compression a matching list in the TV might be use because this allows reducing the range of expected values in a particular direction and therefore reduces the size of the Minaburo, et al. Expires September 6, 2020 [Page 5] Internet-Draft LPWAN CoAP compression March 2020 compression residue. 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. In CoAP some fields have the same behavior, for example the field Code can have 0.0X code format value in the request and Y.ZZ code format in the response. Through the direction indicator, a field description in the Rules splits the possible field value in two parts. Resulting in a smaller compression residue. o In IPv6 and UDP, header fields have a fixed size, defined in the Rule, which is not sent. In CoAP, some fields in the header have a variable length, for example the Token size may vary from 0 to 8 bytes, the length is given by a field in the header. The CoAP options are described using the Type-Length-Value encoding format. Section 7.5.2 from [I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to define a function for the Field Length in the Field Description to have knwoledge of the length before compression. When doing SCHC compression of a variable length field two cases may be raised after applying the CDA: * The result of the compression is of fixed length and the compressed value is sent in the residue. * Or the result of the compression is of variable-length and in this case, the size is sent with the compressed value in the residue. o In CoAP headers, a field can appear several times. This is typical for elements of a URI (path or queries). The SCHC specification [I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field ID to appears several times in the rule, and uses the Field Position (FP) to identify the correct instance, and thereby removing the ambiguity of the matching operation. o Field sizes defined in the CoAP protocol can be too large regarding LPWAN traffic constraints. This is particularly true for the Message ID field and the Token field. SCHC uses different Matching operators (MO) to performs the compression, see section 7.4 of [I-D.ietf-lpwan-ipv6-static-context-hc]. In this case the Most Significant Bits (MSB) MO can be applied to reduce the information carried on LP 4. Compression of CoAP header fields This section discusses the compression of the different CoAP header fields. The CoAP compression with SCHC follows the Section 7.1 of [I-D.ietf-lpwan-ipv6-static-context-hc]. Minaburo, et al. Expires September 6, 2020 [Page 6] Internet-Draft LPWAN CoAP compression March 2020 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, if new versions of CoAP are defined, new rules will be needed to avoid ambiguities between versions. 4.2. CoAP type field The CoAP Protocol [rfc7252] has four type of messages: two request (CON, NON); one response (ACK) and one empty message (RST). The field SHOULD be elided if for instance a client is sending only NON or only CON messages. For the RST message a dedicated Rule may be needed. For other usages a mapping list can be used. 4.3. CoAP code field The code field indicates the Request Method used in CoAP, a registry is given in section 12.1 of [rfc7252]. 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, the set of code values can be split in two parts, the request codes with the 0 class and the response values. If the device only implements a CoAP client, the request code can be reduced to the set of requests the client is able to process. A mapping list can be used for known values, for other values the field cannot be compressed an the value needs to be sent in the residue. 4.4. CoAP Message ID field The Message ID field can be compressed with the MSB(x) MO and the Least Significant Bits (LSB) CDA, see section 7.4 of [I-D.ietf-lpwan-ipv6-static-context-hc]. 4.5. CoAP Token fields Token is defined through two CoAP fields, Token Length in the mandatory header and Token Value directly following the mandatory CoAP header. Token Length is processed as any protocol field. If the value does not change, the size can be stored in the TV and elided during the transmission. Otherwise, it will have to be sent in the compression residue. Minaburo, et al. Expires September 6, 2020 [Page 7] Internet-Draft LPWAN CoAP compression March 2020 Token Value MUST not be sent as a variable length residue to avoid ambiguity with Token Length. Therefore, Token Length value MUST be used to define the size of the residue. A specific function designated as "TKL" MUST be used in the Rule. During the decompression, this function returns the value contained in the Token Length field. 5. CoAP options 5.1. CoAP Content and Accept options. These fields are both unidirectional and MUST NOT be set to bidirectional in a rule entry. If a single value is expected by the client, it can be stored in the TV and elided during the transmission. Otherwise, if several possible values are expected by the client, a matching-list SHOULD be used to limit the size of the residue. Otherwise, the value has to be sent as a residue (fixed or variable length). 5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields These fields are unidirectional and MUST NOT be set to bidirectional in a rule DI entry. see section 7.1 of [I-D.ietf-lpwan-ipv6-static-context-hc]. They are used only by the server to inform of the caching duration and is never found in client requests. If the duration is known by both ends, the value can be elided on the LPWAN. A matching list can be used if some well-known values are defined. Otherwise these options can be sent as a residue (fixed or variable length). 5.3. CoAP option Uri-Path and Uri-Query fields These fields are unidirectional and MUST NOT be set to bidirectional in a rule entry. They are used only by the client to access a specific resource and are never found in server responses. Uri-Path and Uri-Query elements are a repeatable options, the Field Position (FP) gives the position in the path. A Mapping list can be used to reduce the size of variable Paths or Queries. In that case, to optimize the compression, several elements Minaburo, et al. Expires September 6, 2020 [Page 8] Internet-Draft LPWAN CoAP compression March 2020 can be regrouped into a single entry. Numbering of elements do not change, MO comparison is set with the first element of the matching. +-------------+--+--+--+--------+---------+-------------+ | Field |FL|FP|DI| Target | Match | CDA | | | | | | Value | Opera. | | +-------------+--+--+--+--------+---------+-------------+ |URI-Path | | 1|up|["/a/b",|equal |not-sent | | | | | |"/c/d"] | | | |URI-Path | | 3|up| |ignore |value-sent | +-------------+--+--+--+--------+---------+-------------+ Figure 2: complex path example In Figure 2 a single bit residue can be used to code one of the 2 paths. If regrouping were not allowed, a 2 bits residue would be needed. 5.3.1. Variable length Uri-Path and Uri-Query When the length is not known at the rule creation, the Field Length MUST be set to variable, and the unit is set to bytes. The MSB MO can be applied to a Uri-Path or Uri-Query element. Since MSB value is given in bit, the size MUST always be a multiple of 8 bits. The length sent at the beginning of a variable length residue indicates the size of the LSB in bytes. For instance for a CORECONF path /c/X6?k="eth0" the rule can be set to: +-------------+---+--+--+--------+---------+-------------+ | Field |FL |FP|DI| Target | Match | CDA | | | | | | Value | Opera. | | +-------------+---+--+--+--------+---------+-------------+ |URI-Path | 8| 1|up|"c" |equal |not-sent | |URI-Path |var| 2|up| |ignore |value-sent | |URI-Query |var| 1|up|"k=" |MSB(16) |LSB | +-------------+---+--+--+--------+---------+-------------+ Figure 3: CORECONF URI compression Minaburo, et al. Expires September 6, 2020 [Page 9] Internet-Draft LPWAN CoAP compression March 2020 Figure 3 shows the parsing and the compression of the URI, where c is not sent. The second element is sent with the length (i.e. 0x2 X 6) followed by the query option (i.e. 0x05 "eth0"). 5.3.2. Variable number of path or query elements The number of Uri-path or Uri-Query elements in a rule is fixed at the rule creation time. If the number varies, several rules SHOULD be created to cover all the possibilities. Another possibility is to define the length of Uri-Path to variable and send a compression residue with a length of 0 to indicate that this Uri-Path is empty. This adds the 4 bits of the variable residue size. See section 7.5.2 [I-D.ietf-lpwan-ipv6-static-context-hc] 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields These fields are unidirectional and MUST NOT be set to bidirectional in a rule DI entry, see section 7.1 of the [I-D.ietf-lpwan-ipv6-static-context-hc]. They are used only by the client to access a specific resource and are never found in server response. If the field value has to be sent, TV is not set, MO is set to "ignore" and CDA is set to "value-sent". A mapping MAY also be used. Otherwise, the TV is set to the value, MO is set to "equal" and CDA is set to "not-sent". 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and Location-Query fields These fields are unidirectional. These fields values cannot be stored in a rule entry. They MUST always be sent with the compression residues. 6. SCHC compression of CoAP extension RFCs 6.1. Block Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also includes a fragmentation protocol. They are compatible. If a block option is used, its content MUST be sent as a compression residue. Minaburo, et al. Expires September 6, 2020 [Page 10] Internet-Draft LPWAN CoAP compression March 2020 6.2. Observe The [rfc7641] defines the Observe option. The TV is not set, MO is set to "ignore" and the CDA is set to "value-sent". SCHC does not limit the maximum size for this option (3 bytes). To reduce the transmission size, either the device implementation MAY limit the delta between two consecutive values, or a proxy can modify the increment. Since an RST message may be sent to inform a server that the client does not require Observe response, a rule MUST allow the transmission of this message. 6.3. No-Response The [rfc7967] defines a No-Response option limiting the responses made by a server to a request. If the value is known by both ends, then TV is set to this value, MO is set to "equal" and CDA is set to "not-sent". Otherwise, if the value is changing over time, TV is not set, MO is set to "ignore" and CDA to "value-sent". A matching list can also be used to reduce the size. 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_kidctxt ----->|<-- CoAP OSCORE_kid -->| Figure 4: OSCORE Option Minaburo, et al. Expires September 6, 2020 [Page 11] Internet-Draft LPWAN CoAP compression March 2020 The encoding of the OSCORE Option Value defined in Section 6.1 of [rfc8613] is repeated in Figure 4. The first byte is used for flags that specify the contents of the OSCORE option. The 3 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 3 least significant bits n indicate the length of the piv (Partial Initialization Vector) field in bytes. When n = 0, no piv is present. The flag byte is followed by the piv field, kid context field and kid field in this order and if present; the length of the kid context field is encoded in the first byte denoting by s the length of the kid context in bytes. This specification recommends to identify the OSCORE Option and the fields it contains. Conceptually, it discerns up to 4 distinct pieces of information within the OSCORE option: the flag bits, the piv, the kid context, and the kid. It is thus recommended that the parser split the OSCORE option into the 4 subsequent fields: o CoAP OSCORE_flags, o CoAP OSCORE_piv, o CoAP OSCORE_kidctxt, o CoAP OSCORE_kid. These fields are shown superimposed on the OSCORE Option format in Figure 4, the CoAP OSCORE_kidctxt field including the size bits s. Their size SHOULD be reduced using SCHC compression. 7. Examples of CoAP header compression 7.1. Mandatory header with CON message In this first scenario, the LPWAN compressor at the Network Gateway side receives from an Internet client a POST message, which is immediately acknowledged by the Device. For this simple scenario, the rules are described Figure 5. Minaburo, et al. Expires September 6, 2020 [Page 12] Internet-Draft LPWAN CoAP compression March 2020 Rule ID 1 +-------------+--+--+--+------+---------+-------------++------------+ | Field |FL|FP|DI|Target| Match | CDA || Sent | | | | | |Value | Opera. | || [bits] | +-------------+--+--+--+------+---------+-------------++------------+ |CoAP version | | |bi| 01 |equal |not-sent || | |CoAP Type | | |dw| CON |equal |not-sent || | |CoAP Type | | |up|[ACK, | | || | | | | | | RST] |match-map|matching-sent|| T | |CoAP TKL | | |bi| 0 |equal |not-sent || | |CoAP Code | | |bi|[0.00,| | || | | | | | | ... | | || | | | | | | 5.05]|match-map|matching-sent|| CC CCC | |CoAP MID | | |bi| 0000 |MSB(7 ) |LSB || M-ID| |CoAP Uri-Path| | |dw| path |equal 1 |not-sent || | +-------------+--+--+--+------+---------+-------------++------------+ Figure 5: CoAP Context to compress header without token The version and Token Length fields are elided. The 26 method and response codes defined in [rfc7252] has been shrunk to 5 bits using a matching list. Uri-Path contains a single element indicated in the matching operator. 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 request sent by a client located in an Application Server to a server located in the device, may not be compressed through this rule since the MID will not start with 7 bits equal to 0. A CoAP proxy, before the core SCHC C/D can rewrite the message ID to a value matched by the rule. 7.2. OSCORE Compression OSCORE aims to solve the problem of end-to-end encryption for CoAP messages. The goal, therefore, is to hide as much of the message as possible while still enabling proxy operation. Conceptually this is achieved by splitting the CoAP message into an Inner Plaintext and Outer OSCORE Message. The Inner Plaintext contains sensitive information which is not necessary for proxy operation. This, in turn, is the part of the message which can be encrypted until it reaches its end destination. The Outer Message acts as a shell matching the format of a regular CoAP message, and Minaburo, et al. Expires September 6, 2020 [Page 13] Internet-Draft LPWAN CoAP compression March 2020 includes all Options and information needed for proxy operation and caching. This decomposition is illustrated in Figure 6. CoAP options are sorted 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. Additionally, the OSCORE Option is added as an Outer option, signalling that the message is OSCORE protected. This option carries the information necessary to retrieve the Security Context with which the message was encrypted so that it may be correctly decrypted at the other end-point. Minaburo, et al. Expires September 6, 2020 [Page 14] Internet-Draft LPWAN CoAP compression March 2020 Original CoAP Message +-+-+---+-------+---------------+ |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 6: A CoAP message is split into an OSCORE outer and plaintext Figure 6 shows the message format for the OSCORE Message and Plaintext. In the Outer Header, the original message code is hidden and replaced by a default dummy value. As seen in sections 4.1.3.5 and 4.2 of the [rfc8613], the message code is replaced by POST for requests and Changed for responses when Observe is not used. If Observe is used, the message code is replaced by FETCH for requests and Content for responses. The original message code is put into the first byte of the Plaintext. Following the message code, the class E options comes and Minaburo, et al. Expires September 6, 2020 [Page 15] Internet-Draft LPWAN CoAP compression March 2020 if present the original message Payload is preceded by its payload marker. The Plaintext is now encrypted by an AEAD algorithm which integrity protects Security Context parameters and eventually any class I options from the Outer Header. Currently no CoAP options are marked class I. The resulting Ciphertext becomes the new Payload of the OSCORE message, as illustrated in Figure 7. This Ciphertext is, as defined in RFC 5116, the concatenation of the encrypted Plaintext and its authentication tag. Note that Inner Compression only affects the Plaintext before encryption, thus we can only aim to reduce this first, variable length component of the Ciphertext. The authentication tag is fixed in length and considered part of the cost of protection. Outer Header +-+-+---+--------+---------------+ |v|t|tkl|new code| Msg Id. | +-+-+---+--------+---------------+....+ | Token | +--------------------------------.....+ | Options (IU) | . . . OSCORE Option . +------+-------------------+ | 0xFF | +------+---------------------------+ | | | Ciphertext: Encrypted Inner | | Header and Payload | | + Authentication Tag | | | +----------------------------------+ Figure 7: OSCORE message The SCHC Compression scheme consists of compressing both the Plaintext before encryption and the resulting OSCORE message after encryption, see Figure 8. This translates into a segmented process where SCHC compression is applied independently in 2 stages, each with its corresponding set of rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way compression is applied to all fields of the original CoAP message. Minaburo, et al. Expires September 6, 2020 [Page 16] Internet-Draft LPWAN CoAP compression March 2020 Note that since the Inner part of the message can only be decrypted by the corresponding end-point, this end-point will also have to implement Inner SCHC Compression/Decompression. 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 +-----------------+ | +-------+ | | |Rule ID| v | +-------+--+ +--------+ +------------+ | Residue | |Rule ID'| | Encryption | <--- +----------+--------+ +--------+--+ +------------+ | | | Residue' | | Payload | +-----------+-------+ | | | Ciphertext | +-------------------+ | | +-------------------+ Figure 8: OSCORE Compression Diagram 7.3. Example OSCORE Compression An example is given with a GET Request and its consequent CONTENT Response from a device-based CoAP client to a cloud-based CoAP server. A possible set of rules for the Inner and Outer SCHC Compression is shown. A dump of the results and a contrast between SCHC + OSCORE performance with SCHC + COAP performance is also listed. This gives an approximation to the cost of security with SCHC-OSCORE. Minaburo, et al. Expires September 6, 2020 [Page 17] Internet-Draft LPWAN CoAP compression March 2020 Our first example CoAP message is the GET Request in Figure 9 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 9: CoAP GET Request Its corresponding response is the CONTENT Response in Figure 10. 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 10: CoAP CONTENT Response Minaburo, et al. Expires September 6, 2020 [Page 18] Internet-Draft LPWAN CoAP compression March 2020 TheSCHC Rules for the Inner Compression include all fields that are alreadypresent in a regular CoAP message. The methods described in section Section 4 applies to these fields. As an example, see Figure 11. Rule ID 0 +---------------+--+--+-----------+-----------+-----------++------+ | Field |FP|DI| Target | MO | CDA || Sent | | | | | Value | | ||[bits]| +---------------+--+--+-----------+-----------+-----------++------+ |CoAP Code | |up| 1 | equal |not-sent || | |CoAP Code | |dw|[69,132] | match-map |match-sent || c | |CoAP Uri-Path | |up|temperature| equal |not-sent || | |COAP Option-End| |dw| 0xFF | equal |not-sent || | +---------------+--+--+-----------+-----------+-----------++------+ Figure 11: Inner SCHC Rules Figure 12 shows the Plaintext obtained for our example GET Request and follows the process of Inner Compression and Encryption until we end up with the Payload to be added in the outer OSCORE Message. In this case the original message has no payload and its resulting Plaintext can be compressed up to only 1 byte (size of the Rule ID). The AEAD algorithm preserves this length in its first output, but also yields a fixed-size tag which cannot be compressed and has to be included in the OSCORE message. This translates into an overhead in total message length, which limits the amount of compression that can be achieved and plays into the cost of adding security to the exchange. Minaburo, et al. Expires September 6, 2020 [Page 19] Internet-Draft LPWAN CoAP compression March 2020 ________________________________________________________ | | | OSCORE Plaintext | | | | 0x01bb74656d7065726174757265 (13 bytes) | | | | 0x01 Request Code GET | | | | bb74656d7065726174757265 Option 11: URI_PATH | | Value = temperature | |________________________________________________________| | | | Inner SCHC Compression | v _________________________________ | | | Compressed Plaintext | | | | 0x00 | | | | Rule ID = 0x00 (1 byte) | | (No residue) | |_________________________________| | | AEAD Encryption | (piv = 0x04) v _________________________________________________ | | | encrypted_plaintext = 0xa2 (1 byte) | | tag = 0xc54fe1b434297b62 (8 bytes) | | | | ciphertext = 0xa2c54fe1b434297b62 (9 bytes) | |_________________________________________________| Figure 12: Plaintext compression and encryption for GET Request In Figure 13 the process is repeated for the example CONTENT Response. The 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 though it has not been compressed. Minaburo, et al. Expires September 6, 2020 [Page 20] Internet-Draft LPWAN CoAP compression March 2020 On top of this, the overhead from the tag bytes is incurred as before. ________________________________________________________ | | | 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 Rule ID | | | | 0b0 (1 bit match-map 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 13: Plaintext compression and encryption for CONTENT Response Minaburo, et al. Expires September 6, 2020 [Page 21] Internet-Draft LPWAN CoAP compression March 2020 The Outer SCHC Rules (Figure 16) MUST process the OSCORE Options fields. In Figure 14 and Figure 15 we show a dump of the OSCORE Messages generated from our example messages once they have been provided with the Inner Compressed Ciphertext in the payload. These are the messages that 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 14: Protected and Inner SCHC Compressed GET Request Minaburo, et al. Expires September 6, 2020 [Page 22] Internet-Draft LPWAN CoAP compression March 2020 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 15: Protected and Inner SCHC Compressed CONTENT Response For the flag bits, a number of compression methods has been shown to be useful depending on the application. The simplest alternative is to provide a fixed value for the flags, combining MO equal and CDA not- sent. This saves most bits but could prevent flexibility. Otherwise, match-mapping could be used to choose from an interested number of configurations to the exchange. Otherwise, MSB could be used to mask off the 3 hard-coded most significant bits. Note that fixing a flag bit will limit the choice of CoAP Options that can be used in the exchange, since their values are dependent on certain options. The piv field lends itself to having a number of bits masked off with MO MSB and CDA LSB. This could be useful in applications where the message frequency is low such as that found in LPWAN technologies. Note that compressing the sequence numbers effectively reduces the maximum amount of sequence numbers that can be used in an exchange. Once this amount is exceeded, the OSCORE keys need to be re- established. The size s included in the kid context field MAY be masked off with CDA MSB. The rest of the field could have additional bits masked Minaburo, et al. Expires September 6, 2020 [Page 23] Internet-Draft LPWAN CoAP compression March 2020 off, or have the whole field be fixed with MO equal and CDA not-sent. The same holds for the kid field. Figure 16 shows a possible set of Outer Rules to compress the Outer Header. Rule ID 0 +-------------------+--+--+--------------+--------+---------++------+ | Field |FP|DI| Target | MO | CDA || Sent | | | | | Value | | ||[bits]| +-------------------+--+--+--------------+--------+---------++------+ |CoAP version | |bi| 01 |equal |not-sent || | |CoAP Type | |up| 0 |equal |not-sent || | |CoAP Type | |dw| 2 |equal |not-sent || | |CoAP TKL | |bi| 1 |equal |not-sent || | |CoAP Code | |up| 2 |equal |not-sent || | |CoAP Code | |dw| 68 |equal |not-sent || | |CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || | |CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP | |COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK | |COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || | |CoAP OSCORE_flags | |dw| b'' |equal |not-sent || | |CoAP OSCORE_piv | |dw| b'' |equal |not-sent || | |CoAP OSCORE_kid | |dw| b'' |equal |not-sent || | |COAP Option-End | |dw| 0xFF |equal |not-sent || | +-------------------+--+--+--------------+--------+---------++------+ Figure 16: Outer SCHC Rules These Outer Rules are applied to the example GET Request and CONTENT Response. The resulting messages are shown in Figure 17 and Figure 18. Minaburo, et al. Expires September 6, 2020 [Page 24] Internet-Draft LPWAN CoAP compression March 2020 Compressed message: ================== 0x001489458a9fc3686852f6c4 (12 bytes) 0x00 Rule ID 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 17: SCHC-OSCORE Compressed GET Request Compressed message: ================== 0x0014218daf84d983d35de7e48c3c1852 (16 bytes) 0x00 Rule ID 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 18: SCHC-OSCORE Compressed CONTENT Response For contrast, we compare these results with what would be obtained by SCHC compressing the original CoAP messages without protecting them with OSCORE. To do this, we compress the CoAP messages according to the SCHC rules in Figure 19. Minaburo, et al. Expires September 6, 2020 [Page 25] Internet-Draft LPWAN CoAP compression March 2020 Rule ID 1 +---------------+--+--+-----------+---------+-----------++--------+ | Field |FP|DI| Target | MO | CDA || Sent | | | | | Value | | || [bits] | +---------------+--+--+-----------+---------+-----------++--------+ |CoAP version | |bi| 01 |equal |not-sent || | |CoAP Type | |up| 0 |equal |not-sent || | |CoAP Type | |dw| 2 |equal |not-sent || | |CoAP TKL | |bi| 1 |equal |not-sent || | |CoAP Code | |up| 2 |equal |not-sent || | |CoAP Code | |dw| [69,132] |match-map|map-sent ||C | |CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP Uri-Path | |up|temperature|equal |not-sent || | |COAP Option-End| |dw| 0xFF |equal |not-sent || | +---------------+--+--+-----------+---------+-----------++--------+ Figure 19: SCHC-CoAP Rules (No OSCORE) This yields the results in Figure 20 for the Request, and Figure 21 for the Response. Compressed message: ================== 0x0114 0x01 = Rule ID Compression residue: 0b00010100 (1 byte) Compressed msg length: 2 Figure 20: CoAP GET Compressed without OSCORE Minaburo, et al. Expires September 6, 2020 [Page 26] Internet-Draft LPWAN CoAP compression March 2020 Compressed message: ================== 0x010a32332043 0x01 = Rule ID Compression residue: 0b00001010 (1 byte) Payload 0x32332043 Compressed msg length: 6 Figure 21: 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 of cost. 8. IANA Considerations This document has no request to IANA. 9. Security considerations This document does not have any more Security consideration than the ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc]. Variable length residues may be used to compress URI elements. They cannot produce a packet expansion either on the LPWAN network or in the Internet network after decompression. The length send is not used to indicate the information that should be reconstructed at the other end, but on the contrary the information sent as a Residue. Therefore, if a length is set to a high value, but the number of bits on the SCHC packet is smaller, the packet must be dropped by the decompressor. OSCORE compression is also based on the same compression method described in [I-D.ietf-lpwan-ipv6-static-context-hc]. The size of the Initialisation Vector residue size must be considered carefully. A too large value has a impact on the compression efficiency and a too small value will force the device to renew its key more often. This operation may be long and energy consuming. Minaburo, et al. Expires September 6, 2020 [Page 27] Internet-Draft LPWAN CoAP compression March 2020 10. Acknowledgements The authors would like to thank Dominique Barthel, Carsten Bormann, Thomas Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov, Goran Selander. 11. Normative References [I-D.ietf-lpwan-ipv6-static-context-hc] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J. Zuniga, "Static Context Header Compression (SCHC) and fragmentation for LPWAN, application to UDP/IPv6", draft- ietf-lpwan-ipv6-static-context-hc-24 (work in progress), December 2019. [rfc2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, . [rfc7641] Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015, . [rfc7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, August 2016, . [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, . [rfc8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . Minaburo, et al. Expires September 6, 2020 [Page 28] Internet-Draft LPWAN CoAP compression March 2020 Authors' Addresses Ana Minaburo Acklio 1137A avenue des Champs Blancs 35510 Cesson-Sevigne Cedex France Email: ana@ackl.io 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 6, 2020 [Page 29]