--- 1/draft-ietf-lpwan-coap-static-context-hc-13.txt 2020-05-26 10:13:04.671670148 -0700 +++ 2/draft-ietf-lpwan-coap-static-context-hc-14.txt 2020-05-26 10:13:04.735671771 -0700 @@ -1,505 +1,524 @@ lpwan Working Group A. Minaburo Internet-Draft Acklio Intended status: Standards Track L. Toutain -Expires: September 6, 2020 Institut MINES TELECOM; IMT Atlantique +Expires: November 27, 2020 Institut MINES TELECOM; IMT Atlantique R. Andreasen Universidad de Buenos Aires - March 05, 2020 + May 26, 2020 LPWAN Static Context Header Compression (SCHC) for CoAP - draft-ietf-lpwan-coap-static-context-hc-13 + draft-ietf-lpwan-coap-static-context-hc-14 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. + This draft defines the way Static Context Header Compression (SCHC) + header compression can be applied to the Constrained Application + Protocol (CoAP). 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 [rfc8724] 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. + This Internet-Draft will expire on November 27, 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 + 2. Applying SCHC to CoAP headers . . . . . . . . . . . . . . . . 4 + 3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 5 + 3.1. Differences between CoAP and UDP/IP Compression . . . . . 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. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 9 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. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 11 + 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 11 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. Examples of CoAP header compression . . . . . . . . . . . . . 13 + 7.1. Mandatory header with CON message . . . . . . . . . . . . 13 + 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 14 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 9. Security considerations . . . . . . . . . . . . . . . . . . . 27 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 11. Normative References . . . . . . . . . . . . . . . . . . . . 28 + Appendix A. Extension to the RFC8724 Annex D. . . . . . . . . . 29 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. + CoAP [rfc7252] is designed to easily interop with 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. + The [rfc8724] defines SCHC, a header compression mechanism for LPWAN + network based on a static context. The section 5 of the [rfc8724] + explains the architecture where compression and decompression are + done. The context is known by both ends before transmission. The + way the context is configured, provisioned or exchanged is out of the + scope of this document. SCHC compresses and decompresses headers based on shared contexts - between devices. Each context consists of multiple Rules. Each rule + 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 + a Rule matches, the matched header fields are substituted by the + RuleID 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). + 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). 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. + description. The Rule is selected if all the MOs fit the TVs for all + fields of the incoming header. 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. + computed. Compression mainly results in one of 4 actions: - SCHC is a general concept mechanism that can be applied to different + o send the field value, + + o send nothing, + + o send some least significant bits of the field or + o send an index. + + After applying the compression there may be some bits to be sent, + these values are called Compression Residues. + + SCHC is a general 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. + application. The 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 + 14 [RFC2119][rfc8174] when, and only when, they appear in all capitals, as shown here. -2. Applying SCHC to CoAP +2. Applying SCHC to CoAP headers - The SCHC Compression rules can be applied to CoAP flows. SCHC + The SCHC Compression Rules can be applied to CoAP headers. 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. + as described in Section 5 of [rfc8724] and 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: Rule scope for CoAP - Figure 1 shows some examples for CoAP architecture and the SCHC - rule's scope. + Figure 1 shows some examples for CoAP protocol stacks and the SCHC + Rule's scope. - In the first example, a rule compresses the complete header stack + 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 - 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 second example, the SCHC compression is applied in the CoAP + layer, compressing the CoAP header independently of the other layers. + The RuleID and the Compression Residue are encrypted using a + mechanism such as DTLS. Only the other end can decipher the + information. If needed, layers below use SCHC to compress the header + as defined in [rfc8724] document. This use case realizes an End-to- + End context initialization between the sender and the receiver, see + Appendix A. - 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. + In the third example, the Object Security for Constrained RESTful + Environments (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 +3. CoAP Headers compressed 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]. + The use of SCHC over the CoAP header uses the same description and + compression/decompression techniques as the one for IP and UDP + explained in the [rfc8724]. For CoAP, 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 protocols, when the compressor does not find a correct + Rule to compress the header, 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 + [rfc8724]. -3.1. Differences between CoAP and UDP/IP +3.1. Differences between CoAP and UDP/IP Compression - CoAP differs from IPv6 and UDP protocols on the following aspects: + CoAP compression differs from IPv6 and UDP compression 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. + 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 is not present in the response, a + request may contain an Accept option, and the response may include + a Content option. In comparison, IPv6 and UDP returning path swap + the value of some fields in the header. + But all the directions have the same fields (e.g., source and + destination addresses fields). - 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. + The [rfc8724] 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 - 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 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 matching list in the TV to limit the + range of expected values in a particular direction and therefore + reduces the size of the Compression Residue. 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 have as well the + same behavior, the 0.0X code format value in the request and Y.ZZ + code format in the response. - 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. + o Headers in IPv6 and UDP have a fixed size. The size is not sent + as part of the Compression Residue, but is defined in the Rule. + Some CoAP header fields have variable lengths, so the length is + also specified in the Field Description. For example, the Token + size may vary from 0 to 8 bytes. And the CoAP options have a + variable length since they use the Type-Length-Value encoding + format, as URI-path or URI-query. - 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. + Section 7.5.2 from [rfc8724] offers the possibility to define a + function for the Field length in the Field Description to have + knowledge of the length before compression. When doing SCHC + compression of a variable-length field, + if the field size is not known, the Field Length in the Rule is + set as variable and the size is sent with the Compression Residue. - o In CoAP headers, a field can appear several times. This is + o A field can appear several time in the CoAP headers. 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. + specification [rfc8724] allows a Field ID to appear 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 + Matching operators (MO) to perform the compression, see section + 7.4 of [rfc8724]. In this case the Most Significant Bits (MSB) MO + can be applied 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 the Section 7.1 of - [I-D.ietf-lpwan-ipv6-static-context-hc]. + [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, - if new versions of CoAP are defined, new rules will be needed to + 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. + The code field indicates the Request Method used in CoAP, a IANA + registry [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 + 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. + Compression 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]. + Least Significant Bits (LSB) CDA, see section 7.4 of [rfc8724]. 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. + transmission. Otherwise, it will have to be sent in the Compression + Residue. - Token Value MUST not be sent as a variable length residue to avoid + 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 + used to define the size of the Compression 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 + CoAP defines options that are placed after the based header in Option + Numbers order, see [rfc7252]. Each Option instance in a message uses + the format Delta-Type (D-T), Length (L), Value (V). When applying + SCHC compression to the Option, the D-T, L, and V format serves to + make the Rule description of the Option. The SCHC compression 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 wellknown + size it can be stored in the Rule. Therefore, SCHC compression does + not send it. Otherwise, SCHC Compression carries the length of the + Compression Residue in addition to the Compression Residue value. + + 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. + 5.1. CoAP Content and Accept options. These fields are both unidirectional and MUST NOT be set to - bidirectional in a rule entry. + 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). + used to limit the size of the Compression Residue. Otherwise, the + value has to be sent as a Compression 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. + in a Rule DI entry, see section 7.1 of [rfc8724]. 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. + If the duration is known by both ends, the value can be elided. 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). + Otherwise these options can be sent as a Compression 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 + in a Rule entry. They are used only by the client to access a specific resource and are never found in server responses. Uri-Path and Uri-Query elements are a repeatable options, the Field Position (FP) gives the position in the path. A Mapping list can be used to reduce the size of variable Paths or Queries. In that case, to optimize the compression, several elements can be regrouped into a single entry. Numbering of elements do not change, MO comparison is set with the first element of the matching. - +-------------+--+--+--+--------+---------+-------------+ + +-------------+---+--+--+--------+---------+-------------+ | 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 | - +-------------+--+--+--+--------+---------+-------------+ + |URI-Path |var| 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. + needed. The third path element is sent as a variable size residue. 5.3.1. Variable length Uri-Path and Uri-Query - When the length is not known at the rule creation, the Field Length + 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 + 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 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 + 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] + 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 variable Residue size. See section 7.5.2 + [rfc8724] 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. + in a Rule DI entry, see section 7.1 of the [rfc8724]. 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. + 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. + includes a fragmentation protocol. They can be both used. 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 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 + 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 + 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 @@ -508,61 +527,59 @@ | s (if any) | kid context (if any) | kid (if any) ... | +------------+----------------------+-----------------------+ | | | | <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->| Figure 4: OSCORE Option 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 + 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. - 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. + 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. - 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: + This specification recommends identifying 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_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. + The OSCORE Option shows superimposed these four fields using the + format Figure 4, the CoAP OSCORE_kidctxt field includes the size bits + s. 7. Examples of CoAP header compression 7.1. Mandatory header with CON message - In this first scenario, the LPWAN compressor at the Network Gateway + 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. + the Rules are described Figure 5. 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 | @@ -580,23 +597,23 @@ 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 + 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. + 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 @@ -655,21 +672,21 @@ +------+-------------------+ | 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 + 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 if present the original message Payload is preceded by its payload marker. @@ -706,21 +723,21 @@ +----------------------------------+ 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 + Rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way compression is applied to all fields of the original CoAP message. Note that since the Inner part of the message can only be decrypted by the corresponding end-point, this end-point will also have to implement Inner SCHC Compression/Decompression. Outer Message OSCORE Plaintext +-+-+---+--------+---------------+ +-------+ |v|t|tkl|new code| Msg Id. | | code | +-+-+---+--------+---------------+....+ +-------+-----......+ @@ -749,23 +766,23 @@ | 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 + 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 + 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. Our first example CoAP message is the GET Request in Figure 9 Original message: ================= 0x4101000182bb74656d7065726174757265 @@ -808,22 +825,21 @@ 0xFF Payload marker Payload: 0x32332043 Original msg length: 10 Figure 10: CoAP CONTENT Response 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. + 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 || | @@ -931,21 +947,21 @@ v _________________________________________________________ | | | encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) | | tag = 0xe9aef3f2461e0c29 (8 bytes) | | | | ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) | |_________________________________________________________| Figure 13: Plaintext compression and encryption for CONTENT Response - The Outer SCHC Rules (Figure 16) MUST process the OSCORE Options + 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) @@ -1058,52 +1074,52 @@ Response. The resulting messages are shown in Figure 17 and Figure 18. Compressed message: ================== 0x001489458a9fc3686852f6c4 (12 bytes) 0x00 Rule ID 1489 Compression Residue 458a9fc3686852f6c4 Padded payload - Compression residue: + 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 + 14 Compression Residue 218daf84d983d35de7e48c3c1852 Padded payload - Compression residue: + 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. + the SCHC Rules in Figure 19. 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 || | @@ -1118,85 +1134,99 @@ 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: + Compression Residue: 0b00010100 (1 byte) Compressed msg length: 2 Figure 20: CoAP GET Compressed without OSCORE Compressed message: ================== 0x010a32332043 0x01 = Rule ID - Compression residue: + 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. + The Security Considerations of SCHC header compression RFC8724 are + valid for SCHC CoAP header compression. When CoAP uses OSCORE, the + security considerations defined in RFC8613 does not change when SCHC + header compression is applied. + + The definition of SCHC over CoAP header fields permits the + compression of header information only. The SCHC header compression + itself does not increase or reduce the level of security in the + communication. When the communication does not use any security + protocol as OSCORE, DTLS, or other. It is highly necessary to use a + layer two security. + + DoS attacks are possible if an intruder can introduce a compressed + SCHC corrupted packet onto the link and cause a compression + efficiency reduction. However, an intruder having the ability to add + corrupted packets at the link layer raises additional security issues + than those related to the use of header compression. + + SCHC compression returns variable-length Residues for some CoAP + fields. In the compressed header, the length sent is not the + original header field length but the length of the Residue. So if a + corrupted packet comes to the decompressor with a longer or shorter + length than the one in the original header, SCHC decompression will + detect an error and drops the packet. 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. + described in [rfc8724]. The size of the Initialisation Vector (IV) + residue size must be considered carefully. A too large value has an + 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. The size of the compressed IV MUST be choosen + regarding the highest expected traffic from the device. + + SCHC header and compression Rules MUST remain tightly coupled. + Otherwise, an encrypted residue may be decompressed in a different + way by the receiver. To avoid this situation, if the Rule is + modified in one location, the OSCORE keys MUST be re-established. 10. Acknowledgements - The authors would like to thank Dominique Barthel, Carsten Bormann, - Thomas Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov, - Goran Selander. + The authors would like to thank (in alphabetic order): Christian + Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas + Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and 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 + [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 @@ -1216,20 +1246,33 @@ [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, . + [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, + . + +Appendix A. Extension to the RFC8724 Annex D. + + This section extends the RFC8724 Annex D list. + + o How to establish the End-to-End context initialization using SCHC + for CoAP header only. + Authors' Addresses Ana Minaburo Acklio 1137A avenue des Champs Blancs 35510 Cesson-Sevigne Cedex France Email: ana@ackl.io