--- 1/draft-ietf-lpwan-coap-static-context-hc-12.txt 2020-03-05 14:13:16.009440347 -0800 +++ 2/draft-ietf-lpwan-coap-static-context-hc-13.txt 2020-03-05 14:13:16.073441955 -0800 @@ -1,360 +1,377 @@ lpwan Working Group A. Minaburo Internet-Draft Acklio Intended status: Standards Track L. Toutain -Expires: June 12, 2020 Institut MINES TELECOM; IMT Atlantique +Expires: September 6, 2020 Institut MINES TELECOM; IMT Atlantique R. Andreasen Universidad de Buenos Aires - December 10, 2019 + March 05, 2020 LPWAN Static Context Header Compression (SCHC) for CoAP - draft-ietf-lpwan-coap-static-context-hc-12 + draft-ietf-lpwan-coap-static-context-hc-13 Abstract - This draft defines the way SCHC header compression can be applied to - CoAP headers. The CoAP header structure differs from IPv6 and UDP - protocols since CoAP uses a flexible header with a variable number of - options, themselves of variable length. The CoAP protocol messages - format is asymmetric: the request messages have a header format - different from the one in the response messages. This document - explains how to use the SCHC compression mechanism for CoAP. + 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 June 12, 2020. + This Internet-Draft will expire on September 6, 2020. Copyright Notice - Copyright (c) 2019 IETF Trust and the persons identified as the + 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 - 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 - 2. SCHC Compression Process . . . . . . . . . . . . . . . . . . 3 - 3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 4 + 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 . . . . . . . . . . . . . . . . . . . 6 - 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 6 - 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . 7 - 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 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 . . . . . . 9 + 5.3.2. Variable number of path or query elements . . . . . . 10 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme - fields . . . . . . . . . . . . . . . . . . . . . . . . . 9 + fields . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and Location-Query fields . . . . . . . . . . . . . . . . 10 - 6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 10 + 6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 10 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 10 - 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 10 - 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . 16 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 - 9. Security considerations . . . . . . . . . . . . . . . . . . . 26 - 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26 - 11. Normative References . . . . . . . . . . . . . . . . . . . . 26 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 + 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 + 9. Security considerations . . . . . . . . . . . . . . . . . . . 27 + 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 + 11. Normative References . . . . . . . . . . . . . . . . . . . . 28 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 1. Introduction - CoAP [rfc7252] is an implementation of the REST architecture for - constrained devices. Although CoAP was designed for constrained - devices, the size of a CoAP header still is too large for the - constraints of Low Power Wide Area Networks (LPWAN) and some - compression is needed to reduce the header size. + 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. - [I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression - mechanism for LPWAN network based on a static context. The context - is said static since the field description composing the Rules are - not learned during the packet exchanges but are previously defined. - The context(s) is(are) known by both ends before transmission. + 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. - A context is composed of a set of rules that are referenced by Rule - IDs (identifiers). A rule contains an ordered list of the fields - descriptions containing a field ID (FID), its length (FL) and its - position (FP), a direction indicator (DI) (upstream, downstream and - bidirectional) and some associated Target Values (TV). Target Value - indicates the value that can be expected. TV can also be a list of - values. A Matching Operator (MO) is associated to each header field + 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 and are transmitted after the Rule ID in - the compressed messages. + 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. - The compression rules define a generic way to compress and decompress - the fields. If the device is modified, for example, to introduce new - functionalities or new CoAP options, the rules must be updated to - reflect the evolution. + 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. SCHC Compression Process +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 [I-D.ietf-lpwan-ipv6-static-context-hc] may be used - as shown in Figure 1. + 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 device and - at the LPWAN boundary. + 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 device and the application. 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 rules (this is out of the scope of this document) as - defined in the SCHC [I-D.ietf-lpwan-ipv6-static-context-hc] document. + 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 + 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 device and the LPWAN boundary. + 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 symmetrical protocols. The same fields are found - in the request and in the response, with the value of some fields - being swapped on the return path (e.g. source and destination - fields). A CoAP request is intrinsically different from a - response. For example, the URI-path option is mandatory in the - request and is not found in the response, a request may contain an - Accept option and the response a Content option. + 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. - [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a - message direction (DI) in the Field Description, which allows a - single Rule to process message headers differently depending of + 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. Combined with - a matching list in the TV, this allows reducing the range of - expected values in a particular direction and therefore reduce the - size of the compression residue. For instance, if a client sends - only CON request, the type can be elided by compression and the - answer may use one single bit to carry either the ACK or RST type. - The same behavior can be applied to the CoAP Code field 0.0X code - Format is found in the request and Y.ZZ code format in the answer. - The direction allows splitting in two parts the possible values - for each direction in the same Rule. + 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 + 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 and it is not - sent. In CoAP, some fields in the header have a varying size, for - example the Token size may vary from 0 to 8 bytes, the length is - given by a field in the header. More systematically, the CoAP - options are described using the Type-Length-Value. + 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. - [I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to - define a function for the Field Length in the Field Description. + 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. The MSB MO can be - applied to reduce the information carried on LPWANs. - - o CoAP also obeys the client/server paradigm and the compression - ratio can be different if the request is issued from an LPWAN - device or from a non LPWAN device. For instance, a Device (Dev) - aware of LPWAN constraints can generate a 1-byte token, but a - regular CoAP client will certainly send a larger token to the Dev. - The SCHC compression-decompression process never modifies the - Values it only reduces their sizes. Nevertheless, a proxy placed - before the compressor may change some field values to allow SCHC - achieving a better compression ratio, while maintaining the - necessary context for interoperability with existing CoAP - implementations. - - o 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]. + 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. + fields. The CoAP compression with SCHC follows the Section 7.1 of + [I-D.ietf-lpwan-ipv6-static-context-hc]. 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 - CoAP Protocol [rfc7252] defines 4 types of messages: CON, NON, ACK - and RST. ACK and RST are a response to the CON and NON. If the - device plays a specific client or server role, a rule can take - advantage of these properties with the mapping list: [CON, NON] for - one direction and [ACK, RST] for the other direction and so, the - compression residue is reduced to 1 bit. + 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. - - In any case, a rule MUST be defined to carry RST to a client. + 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 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. + 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. - All the response codes MUST be compressed with a SCHC rule. + 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 is bidirectional and is used to manage - acknowledgments. The server memorizes the value for an - EXCHANGE_LIFETIME period (by default 247 seconds) for CON messages - and a NON_LIFETIME period (by default 145 seconds) for NON messages. - During that period, a server receiving the same Message ID value will - process the message as a retransmission. After this period, it will - be processed as a new message. - - In case where the Device is a client, the size of the Message ID - field may be too large regarding the number of messages sent. The - client SHOULD use only small Message ID values, for instance 4 bit - long. Therefore, an MSB can be used to limit the size of the - compression residue. - - In case where the Device is a server, the client may be located - outside of the LPWAN area and it views the Device as a regular device - connected to the Internet. The client will generate Message ID using - the 16 bits space offered by this field. A CoAP proxy can be set - before the SCHC C/D to reduce the value of the Message ID, to allow - its compression with the MSB matching operator and LSB CDA. + 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 - remains the same during all the transaction, the size can be stored - in the context and elided during the transmission. Otherwise, it - will have to be sent as a compression residue. + 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. - Token Value size cannot be defined directly in the rule in the Field - Length (FL). Instead, a specific function designated as "TKL" MUST - be used and length does not have to be sent with the residue. During - the decompression, this function returns the value contained in the - Token Length field. + 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 entry. They are used only by the server to inform of the - caching duration and is never found in client requests. + 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 SHOULD be sent as a residue (fixed or - variable length). + 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. @@ -374,21 +391,21 @@ 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 - SHOULD be set to variable, and the unit is set to bytes. + 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: @@ -408,43 +425,46 @@ 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 4 bits to the compression residue. + 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 entry. They are used only by the client to access a - specific resource and are never found in server response. + 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. Other RFCs +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. 6.2. Observe The [rfc7641] defines the Observe option. The TV is not set, MO is @@ -501,22 +521,22 @@ 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 draft recommends to implement a parser that is able to identify - the OSCORE Option and the fields it contains. + 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, @@ -787,23 +807,23 @@ 0xFF Payload marker Payload: 0x32332043 Original msg length: 10 Figure 10: CoAP CONTENT Response The SCHC Rules for the Inner Compression include all fields that are - already present in a regular CoAP message, what is important is their - order and the definition of only those CoAP fields are into - Plaintext, Figure 11. + 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 || | @@ -858,28 +878,28 @@ _________________________________________________ | | | 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 we repeat the process for the example CONTENT Response. - In this case the misalignment produced by the compression residue (1 - bit) makes it so that 7 bits of padding have to be applied after the - payload, resulting in a compressed Plaintext that is the same size as - before compression. This misalignment also causes the hexcode from - the payload to differ from the original, even though it has not been - compressed. On top of this, the overhead from the tag bytes is - incurred as before. + 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. + + 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 | @@ -920,21 +940,21 @@ Figure 13: Plaintext compression and encryption for CONTENT Response 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: ================== - 0x4102000182d7080904636c69656e74ffa2c54fe1b434297b62 + 0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62 (25 bytes) Header: 0x4102 01 Ver 00 CON 0001 tkl 00000010 Request Code 2 "POST" 0x0001 = mid @@ -991,21 +1011,21 @@ 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 SCHC Context would need to be re- + 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 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. @@ -1130,21 +1150,36 @@ 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] + 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. 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]