--- 1/draft-ietf-lpwan-coap-static-context-hc-03.txt 2018-07-02 07:17:19.052012056 -0700 +++ 2/draft-ietf-lpwan-coap-static-context-hc-04.txt 2018-07-02 07:17:19.440021348 -0700 @@ -1,797 +1,984 @@ lpwan Working Group A. Minaburo Internet-Draft Acklio Intended status: Informational L. Toutain -Expires: September 5, 2018 Institut MINES TELECOM; IMT Atlantique - March 04, 2018 +Expires: January 3, 2019 Institut MINES TELECOM; IMT Atlantique + R. Andreasen + Universidad de Buenos Aires + July 02, 2018 LPWAN Static Context Header Compression (SCHC) for CoAP - draft-ietf-lpwan-coap-static-context-hc-03 + draft-ietf-lpwan-coap-static-context-hc-04 Abstract This draft defines the way SCHC header compression can be applied to CoAP headers. CoAP header structure differs from IPv6 and UDP - protocols since the CoAP Header is flexible header with a variable - number of options themself of a variable length. Another important - difference is the asymmetry in the header information used for - request and response messages. This draft takes into account the - fact that a thing can play the role of a CoAP client, a CoAP client - or both roles. + protocols since the CoAP + use a flexible header with a variable number of options themself of a + variable length. Another important difference is the asymmetry in + the header format used in request and response messages. Most of the + compression mechanisms have been introduced in + [I-D.ietf-lpwan-ipv6-static-context-hc], this document explains how + to use the SCHC compression for CoAP. 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 5, 2018. + This Internet-Draft will expire on January 3, 2019. Copyright Notice Copyright (c) 2018 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 - 2. CoAP Compressing . . . . . . . . . . . . . . . . . . . . . . 3 - 3. Compression of CoAP header fields . . . . . . . . . . . . . . 4 - 3.1. CoAP version field (2 bits) . . . . . . . . . . . . . . . 4 - 3.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 5 - 3.3. CoAP token length field . . . . . . . . . . . . . . . . . 5 - 3.4. CoAP code field . . . . . . . . . . . . . . . . . . . . . 6 - 3.5. CoAP Message ID field . . . . . . . . . . . . . . . . . . 8 - 3.6. CoAP Token field . . . . . . . . . . . . . . . . . . . . 9 - 4. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 9 - 4.1. CoAP option Content-format field. . . . . . . . . . . . . 9 - 4.2. CoAP option Accept field . . . . . . . . . . . . . . . . 10 - 4.3. CoAP option Max-Age field, CoAP option Uri-Host and Uri- - Port fields . . . . . . . . . . . . . . . . . . . . . . . 11 - 5. CoAP option Uri-Path and Uri-Query fields . . . . . . . . . . 11 - 5.1. CoAP option Proxy-URI and Proxy-Scheme fields . . . . . . 12 - 5.2. CoAP option ETag, If-Match, If-None-Match, Location-Path - and Location-Query fields . . . . . . . . . . . . . . . . 13 - 6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 13 - 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13 - 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13 - 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 13 - 7. Protocol analysis . . . . . . . . . . . . . . . . . . . . . . 13 - 8. Examples of CoAP header compression . . . . . . . . . . . . . 14 - 8.1. Mandatory header with CON message . . . . . . . . . . . . 14 - 8.2. Complete exchange . . . . . . . . . . . . . . . . . . . . 16 - 9. Normative References . . . . . . . . . . . . . . . . . . . . 17 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 + 2. SCHC Compression Process . . . . . . . . . . . . . . . . . . 3 + 3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 4 + 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 + 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 6 + 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7 + 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 7 + 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 7 + 5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri- + Port fields . . . . . . . . . . . . . . . . . . . . . . . 7 + 5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8 + 5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 8 + 5.3.2. Variable number of path or query elements . . . . . . 9 + 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme + fields . . . . . . . . . . . . . . . . . . . . . . . . . 9 + 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path + and Location-Query fields . . . . . . . . . . . . . . . . 9 + 6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 9 + 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 9 + 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 10 + 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 10 + 6.4. Time Scale . . . . . . . . . . . . . . . . . . . . . . . 10 + 6.5. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 10 + 7. Examples of CoAP header compression . . . . . . . . . . . . . 12 + 7.1. Mandatory header with CON message . . . . . . . . . . . . 12 + 7.2. Complete exchange . . . . . . . . . . . . . . . . . . . . 13 + 7.3. OSCORE Compression . . . . . . . . . . . . . . . . . . . 14 + 7.4. Example OSCORE Compression . . . . . . . . . . . . . . . 17 + 8. Normative References . . . . . . . . . . . . . . . . . . . . 22 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 1. Introduction CoAP [rfc7252] is an implementation of the REST architecture for - constrained devices. A Gateway between CoAP and HTTP can be easily - built since both protocols uses the same address space (URL), caching - mechanisms and methods. - - Nevertheless, if limited, the size of a CoAP header may be too large - for LPWAN constraints and some compression may be needed to reduce - the header size. + constrained devices. Nevertheless, if limited, the size of a CoAP + header may be too large for LPWAN constraints and some compression + may be needed to reduce the header size. - [I-D.toutain-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 and the context are not learned during the packet exchanges but - are previously defined. The context(s) is(are) known by both ends - before transmission. + [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 and + the context are not learned during the packet exchanges but are + previously defined. The context(s) is(are) known by both ends before + transmission. 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) and its position when - repeated, a direction indicator (DI) (upstream, downstream and - bidirectional) and some associated Target Values (TV) which are - expected in the message header. A Matching Operator (MO) is - associated to each header field description. The rule is selected if - all the MOs fit the TVs. In that case, a Compression/Decompression - Action (CDA) associated to each field defines the link between the - compressed and decompressed value for each of the header 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 + description. The rule is selected if all the MOs fit the TVs for all + fields. In that case, a Compression/Decompression Action (CDA) + associated to each field defines the link between the compressed and + decompressed value for each of the header fields. Compression + results mainly in 4 actions: send the field value, send nothing, send + less significant bits of a field, send an index. Values sent are + called Compression Residues and follows the rule ID. - This document describes how the rules can be applied to CoAP flows. +2. SCHC Compression Process + + The SCHC Compression rules can be applied to CoAP flows. SCHC Compression of the CoAP header may be done in conjunction with the - above layers or independantly. + above layers (IPv6/UDP) or independently. The SCHC adaptation layers + as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used + as as shown in the Figure 1. -2. CoAP Compressing + ^ +------------+ ^ +------------+ ^ +------------+ + | | 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. A rule can covers all headers from IPv6 to CoAP, SCHC + C/D is done in the device and at the LPWAN boundary. If an end-to- + end encryption mechanisms is used between the device and the + application. CoAP must be compressed 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). OSCORE + [I-D.ietf-core-object-security] can also define 2 rules to compress + the CoAP message. A first rule focuses on the inner header and is + end to end, a second rule may compress the outer header and the layer + above. SCHC C/D for inner header is done by both ends, SCHC C/D for + outer header and other headers is done between the device and the + LPWAN boundary. + +3. CoAP Compression with SCHC 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, only the location in the header may vary (e.g. source and destination fields). A CoAP request is 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-format option. + a Content option. - Even when a field is "symmetric" (i.e. found in both directions) - the values carried are different. For instance the Type field - will contain a CON value in the request and a ACK or RST value in - the response. Exploiting the asymmetry in compression will allow - to send no bit in the compressed request and a single bit in the - answer. Same behavior can be applied to the CoAP Code field (O.OX - code are present in the request and Y.ZZ in the answer). + [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a + message direction (DI) when processing the rule which allows the + description of message header format in both directions. + + 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 will allow to reduce the range of + expected values in a particular direction and therefore reduce the + size of a compression residue. For instance, if a client sends + only CON request, the type can be elided by compression and the + answer may use one bit to carry either the ACK or RST type. Same + behavior can be applied to the CoAP Code field (0.0X code are + present in the request and Y.ZZ in the answer). The direction + allows to split in two parts the possible values for each + direction. + + o In IPv6 and UDP header fields have a fixed size. In CoAP, Token + size may vary from 0 to 8 bytes, length is given by a field in the + header. More systematically, the CoAP options are described using + the Type-Length-Value. + + [I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to + define a function for the Field Length in the Field Description. + + o In CoAP headers, a field can be duplicated several times, for + instances, elements of an URI (path or queries). The position + defined in a rule, associated to a Field ID, can be used to + identify the proper element. + + [I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field id to + appears several times in the rule, the Field Position (FP) removes + ambiguities for the matching operation. + + o Field size defined in the CoAP protocol can be to large regarding + LPWAN traffic constraints. This is particularly true for the + message ID field or Token field. The use of MSB MO can be used to + reduce the information carried on LPWANs. o CoAP also obeys to the client/server paradigm and the compression rate can be different if the request is issued from an LPWAN node - or from an non LPWAN device. For instance a Thing (ES) aware of + or from an 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 Thing. SCHC compression will not modify the values to offer a better compression rate. Nevertheless a proxy placed before the compressor may change some field values to offer a better compression rate and maintain the necessary context for interoperability with existing CoAP implementations. - o In IPv6 and UDP header fields have a fixed size. In CoAP, Token - size may vary from 0 to 8 bytes, length is given by a field in the - header. More systematically, the CoAP options are described using - the Type-Length-Value. When applying SCHC header compression. - - By sending compressed field information following the rule order, - SCHC offers a serialization/deserialization mechanism. Since a - field exists to indicate the token length there is no ambiguity. - For options, the rule indicates also the expected options found - the int CoAP header. Therefore only the length is needed to - recognize an option. The length will be sent using the same CoAP - encoding (size less than 12 are directly sent, higher values use - the escape mechanisms defined by [rfc7252]). Delta Type is - omitted, the value will be recovered by the decompressor. This - reduces the option length of 4, 12 or 20 bits regarding the - original size of the delta type encoding in the option. - - o In CoAP headers a field can be duplicated several times, for - instances, elements of an URI (path or queries) or accepted - formats. The position defined in a rule, associated to a Field - ID, can be used to identify the proper element. - -3. Compression of CoAP header fields +4. Compression of CoAP header fields This section discusses of the compression of the different CoAP - header fields. These are just examples. The compression should take - into account the nature of the traffic and not only the field values. - Next chapter will define some compression rules for some common - exchanges. - -3.1. CoAP version field (2 bits) - - This field is bidirectional and can be elided during the SCHC - compression, since it always contains the same value. It appears - only in first position. - - FID FL FP DI Value MO CDA Sent - ver 2 1 bi 1 equal not-sent - -3.2. CoAP type field - - This field can be managed bidirectionally or unidirectionally.Several - strategies can be applied to this field regarding the values used: - - o If the ES is a client or a Server and non confirmable message are - used, the transmission of the Type field can be avoided: - - * Pos is always 1, - - * DI can either be "uplink" if the ES is a CoAP client or - "downlink" if the ES is a CoAP server, or "bidirectional" - - * TV is set to the value, - - * MO is set to "equal" - - * CDA is set to "not-sent". - - FID FL FP DI Target Value MO CDA Sent - type 2 1 bi NON equal not-sent - - o If the ES is either a client or a Server and confirmable message - are used, the DI can be used to elide the type on the request and - compress it to 1 bit on the response. The example above shows the - rule for a ES acting as a client, directions need to be reversed - for a ES acting as a server. - - FID FL FP DI TV MO CDA Sent - type 2 1 up CON equal not-sent - type 2 1 dw [ACK,RST] match-mapping mapping-sent [1] - - o Otherwise if the ES is acting simultaneously as a client and a - server and the rule handle these two traffics, Type field must be - sent uncompressed. - - FID FL FP DI TV MO CDA Sent - type 2 1 bi ignore send-value [2] - -3.3. CoAP token length field - - This field is bi-directional. - - Several strategies can be applied to this field regarding the values: - - o no token or a wellknown length, the transmission can be avoided. - A special care must be taken, if CON messages are acknowledged - with an empty ACK message. In that case the token is not always - present. - - FID FL FP DI TV MO CDA Sent - TKL 4 1 bi value ignore send-value [4] - - o If the length is changing from one message to an other, the Token - Length field must be sent. If the Token length can be limited, - then only the least significant bits have to be sent. The example - below allows values between 0 and 3. - - FID FL FP DI TV MO CDA Sent - TKL 4 1 bi 0x0 MSB(2) LSB(2) [2] - - o otherwise the field value has to be sent. - - FID FL FP DI TV MO CDA Sent - TKL 4 1 bi ignore value-sent [4] - -3.4. CoAP code field - - This field is bidirectional, but compression can be enhanced using - DI. - - The CoAP Code field defines a tricky way to ensure compatibility with - HTTP values. Nevertheless only 21 values are defined by [rfc7252] - compared to the 255 possible values. - - +------+------------------------------+-----------+ - | Code | Description | Mapping | - +------+------------------------------+-----------+ - | 0.00 | | 0x00 | - | 0.01 | GET | 0x01 | - | 0.02 | POST | 0x02 | - | 0.03 | PUT | 0x03 | - | 0.04 | DELETE | 0x04 | - | 0.05 | FETCH | 0x05 | - | 0.06 | PATCH | 0x06 | - | 0.07 | iPATCH | 0x07 | - | 2.01 | Created | 0x08 | - | 2.02 | Deleted | 0x09 | - | 2.03 | Valid | 0x0A | - | 2.04 | Changed | 0x0B | - | 2.05 | Content | 0x0C | - | 4.00 | Bad Request | 0x0D | - | 4.01 | Unauthorized | 0x0E | - | 4.02 | Bad Option | 0x0F | - | 4.03 | Forbidden | 0x10 | - | 4.04 | Not Found | 0x11 | - | 4.05 | Method Not Allowed | 0x12 | - | 4.06 | Not Acceptable | 0x13 | - | 4.12 | Precondition Failed | 0x14 | - | 4.13 | Request Entity Too Large | 0x15 | - | 4.15 | Unsupported Content-Format | 0x16 | - | 5.00 | Internal Server Error | 0x17 | - | 5.01 | Not Implemented | 0x18 | - | 5.02 | Bad Gateway | 0x19 | - | 5.03 | Service Unavailable | 0x1A | - | 5.04 | Gateway Timeout | 0x1B | - | 5.05 | Proxying Not Supported | 0x1C | - +------+------------------------------+-----------+ - - Figure 1: Example of CoAP code mapping - - Figure 1 gives a possible mapping, it can be changed to add new codes - or reduced if some values are never used by both ends. It could - efficiently be coded on 5 bits. - - Even if the number of code can be increase with other RFC, - implementations may use a limited number of values, which can help to - reduce the number of bits sent on the LPWAN. - - The number of code may vary over time, some new codes may be - introduced or some applications use a limited number of values. - - The client and the server do not use the same values. This asymmetry - can be exploited to reduce the size sent on the LPWAN. - - The field can be treated differently in upstream than in downstream. - If the Thing is a client an entry can be set on the uplink message - with a code matching for 0.0X values and another for downlink values - for Y.ZZ codes. It is the opposite if the thing is a server. - - If the ES always sends or receives requests with the same method, the - Code field can be elided. The entry below shows a rule for a client - sending only GET request. - - FID FL FP DI TV MO CDA Sent - code 8 1 up GET equal not-sent - - If the client may send different methods, a matching-list can be - applied. For table Figure 1, 3 bits are necessary, but it could be - less if fewer methods are used. Example below gives an example where - the ES is a server and receives only GET and POST requests. - - FID FL FP DI Target Value MO CDA Sent - code 8 1 dw [0.01, 0.02] match-mapping mapping-sent [1] - - The same approach can be applied to responses. - -3.5. CoAP Message ID field - - This field is bidirectional. - - Message ID is used for two purposes: - - o To acknowledge a CON message with an ACK. - - o To avoid duplicate messages. - - In LPWAN, since a message can be received by several radio gateway, - some LPWAN technologies include a sequence number in L2 to avoid - duplicate frames. Therefore if the message does not need to be - acknowledged (NON or RST message), the Message ID field can be - avoided. - - FID FL FP DI TV MO CDA Sent - Mid 8 1 bi ignore not-sent - - The decompressor must generate a value. - - [[Note; check id this field is not used by OSCOAP .]] - To optimize information sent on the LPWAN, shorter values may be used - during the exchange, but Message ID values generated a common CoAP - implementation will not take into account this limitation. Before - the compression, a proxy may be needed to reduce the size. - - FID FL FP DI TV MO CDA Sent - Mid 8 1 bi 0x0000 MSB(12) LSB(4) [4] - - Otherwise if no compression is possible, the field has to be sent - - FID FL FP DI TV MO CDA Sent - Mid 8 1 bi ignore value-sent [8] - -3.6. CoAP Token field - - This field is bi-directional. - - Token is used to identify transactions and varies from one - transaction to another. Therefore, it is usually necessary to send - the value of the token field on the LPWAN network. The optimization - will occur by using small values. - - Common CoAP implementations may generate large tokens, even if - shorter tokens could be used regarding the LPWAN characteristics. A - proxy may be needed to reduce the size of the token before - compression. - - The size of the compress token sent is known by a combination of the - Token Length field and the rule entry. For instance, with the entry - below: - - FID FL FP DI TV MO CDA Sent - tkl 4 1 bi 2 equal not-sent - token 8 1 bi 0x00 MSB(12) LSB(4) [4] + header fields. - The uncompressed token is 2 bytes long, but the compressed size will - be 4 bits. +4.1. CoAP version field -4. CoAP options + This field is bidirectional and must be elided during the SCHC + compression, since it always contains the same value. In the future, + if new version of CoAP are defined, new rules ID will be defined + avoiding ambiguities between versions. -4.1. CoAP option Content-format field. +4.2. CoAP type field - This field is unidirectional and must not be set to bidirectional in - a rule entry. It is used only by the server to inform the client - about of the payload type and is never found in client requests. + [rfc7252] defines 4 types of messages: CON, NON, ACK and RST. The + latter two ones are a response of the two first ones. If the device + plays a specific role, a rule can exploit these property with the + mapping list: [CON, NON] for one direction and [ACK, RST] for the + other direction. Compression residue is reduced to 1 bit. - If single value is expected by the client, the TV contains that value - and MO is set to "equal" and the CDF is set to "not-sent". The - examples below describe the rules for an ES acting as a server. + The field must be elided if for instance a client is sending only NON + or CON messages. - FID FL FP DI TV MO CDA Sent - content 16 1 up value equal not-sent + In any case, a rule must be defined to carry RST to a client. - If several possible value are expected by the client, a matching-list - can be used. +4.3. CoAP code field - FID FL FP DI TV MO CDA Sent - content 16 1 up [50, 41] match-mapping mapping-sent [1] + The compression of the CoAP code field follows the same principle as + for 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. - Otherwise the value can be sent.The value-sent CDF in the compressor - do not send the option type and the decompressor reconstruct it - regarding the position in the rule. + If the device implement only a CoAP client, the request code can be + reduced to the set of request the client is able to process. - FID FL FP DI TV MO CDA Sent - content 16 1 up ignore value-sent [0-16] + All the response codes should be compressed with a SCHC rule. -4.2. CoAP option Accept field +4.4. CoAP Message ID field - This field is unidirectional and must not be set to bidirectional in - a rule entry. It is used only by the client to inform of the - possible payload type and is never found in server response. + This field is bidirectional and is used to manage acknowledgments. + Server memorizes the value for a 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 has a + retransmission. After this period, it will be processed as a new + messages. - The number of accept options is not limited and can vary regarding - the usage. To be selected a rule must contain the exact number about - accept options with their positions. Since the order in which the - Accept value are sent, the position order can be modified. The rule - below + In case the Device is a client, the size of the message ID field may + the too large regarding the number of messages sent. Client may use + only small message ID values, for instance 4 bit long. Therefore a + MSB can be used to limit the size of the compression residue. - FID FL FP DI TV MO CDA Sent - accept 16 1 up 41 egal not-sent - accept 16 2 up 50 egal not-sent + In case the Device is a server, client may be located outside of the + LPWAN area and view 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. - will be selected only if two accept options are in the CoAP header if - this order. +4.5. CoAP Token fields - The rule below: + Token is defined through two CoAP fields, Token Length in the + mandatory header and Token Value directly following the mandatory + CoAP header. - FID FL FP DI TV MO CDA Sent - accept 16 0 up 41 egal not-sent - accept 16 0 up 50 egal not-sent + Token Length is processed as a tradition 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 the send as a compression residue. - will accept a-only CoAP messages with 2 accept options, but the order - will not influence the rule selection. The decompression will - reconstruct the header regarding the rule order. + Token Value size should not be defined directly in the rule in the + Field Length (FL). Instead a specific function designed as "TKL" + must be used. This function informs the SCHC C/D that the length of + this field has to be read from the Token Length field. - Otherwise a matching-list can be applied to the different values, in - that case the order is important to recover the appropriate value and - the position must be clearly indicate. +5. CoAP options - FID FL FP DI TV MO CDA Sent - accept 16 1 up [50,41] match-mapping mapping-sent [1] - accept 16 2 up [50,61] match-mapping mapping-sent [1] - accept 16 3 up [61,71] match-mapping mapping-sent [1] +5.1. CoAP Content and Accept options. - Finally, the option can be explicitly sent. + These field are both unidirectional and must not be set to + bidirectional in a rule entry. - FID FL FP DI TV MO CDA Sent - accept 1 up ignore value-sent + If 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. If not the possible, the value as to + be sent as a residue (fixed or variable length). -4.3. CoAP option Max-Age field, CoAP option Uri-Host and Uri-Port +5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri-Port fields This field is unidirectional and must not be set to bidirectional in a rule entry. It is 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, value can be elided on the LPWAN. - A matching list can be used if some wellknown values are defined. - - Otherwise the option length and value can be sent on the LPWAN. + A matching list can be used if some well-known values are defined. - [[note: we can reduce (or create a new option) the unit to minute, - second is small for LPWAN ]] + Otherwise these options should be sent as a residue (fixed or + variable length). -5. CoAP option Uri-Path and Uri-Query fields +5.3. CoAP option Uri-Path and Uri-Query fields This fields are unidirectional and must not be set to bidirectional in a rule entry. They are used only by the client to access to a - specific resource and are never found in server response. + specific resource and are never found in server responses. - The Matching Operator behavior has not changed, but the value must - take a position value, if the entry is repeated : + Uri-Path and Uri-Query elements are a repeatable options, the Field + Position (FP) gives the position in the path. - FID FL FP DI TV MO CDA Sent - URI-Path 1 up foo equal not-sent - URI-Path 2 up bar equal not-sent + A Mapping list can be used to reduce 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. - Figure 2: Position entry. + FID FL FP DI TV MO CDA + URI-Path 1 up ["/a/b", equal not-sent + "/c/d"] + URI-Path 3 up ignore value-sent - For instance, the rule Figure 2 matches with /foo/bar, but not /bar/ - foo. + Figure 2: complex path example - When the length is not clearly indicated in the rule, the value - length must be sent with the field data, which means for CoAP to send - directly the CoAP option with length and value. + In Figure 2 a single bit residue can be used to code one of the 2 + paths. If regrouping was not allowed, a 2 bits residue whould have + been needed. + +5.3.1. Variable length Uri-Path and Uri-Query + + When the length is 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 apply 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 and the LSB CDA must not carry any value. + + The length sent at the beginning of a variable length residue + indicates the size of the LSB in bytes. For instance for a CoMi path /c/X6?k="eth0" the rule can be set to: - FID FL FP DI TV MO CDA Sent - URI-Path 1 up c equal not-sent + FID FL FP DI TV MO CDA + URI-Path 1 up "c" equal not-sent URI-Path 2 up ignore value-sent - URI-Query 1 up k= MSB (16) LSB + URI-Query 1 up "k=" MSB (16) LSB Figure 3: CoMi 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"). - A Mapping list can be used to reduce 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. - - FID FL FP DI TV MO CDA Sent - URI-Path 1 up {0:"/c/c", equal not-sent - 1:"/c/d" - URI-Path 3 up ignore value-sent - URI-Query 1 up k= MSB (16) LSB - - Figure 4: complex path example +5.3.2. Variable number of path or query elements - For instance, the following Path /foo/bar/variable/stable can leads - to the rule defined Figure 4. + The number of Uri-path or Uri-Query element 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 possibilities 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 add 4 bits to the compression residue. -5.1. CoAP option Proxy-URI and Proxy-Scheme fields +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 to a specific resource and are never found in server response. If the field value must be sent, TV is not set, MO is set to "ignore" and CDF is set to "value-sent. A mapping can also be used. Otherwise the TV is set to the value, MO is set to "equal" and CDF is set to "not-sent" -5.2. CoAP option ETag, If-Match, If-None-Match, Location-Path and +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 request. - - [[Can include OSCOAP Object security in that category ]] + always be sent with the compression residues. 6. Other RFCs 6.1. Block - Block option should be avoided in LPWAN. The minimum size of 16 - bytes can be incompatible with some LPWAN technologies. - - [[Note: do we recommand LPWAN fragmentation since the smallest value - of 16 is too big?]] + Block [rfc7959] allows a fragmentation at the CoAP level. SCHC + includes also a fragmentation protocol. They are compatible. If a + block option is used, its content must be sent as a compression + residue. 6.2. Observe [rfc7641] defines the Observe option. The TV is not set, MO is set to "ignore" and the CDF is set to "value-sent". SCHC does not limit the maximum size for this option (3 bytes). To reduce the - transmission size either the Thing implementation should limit the - value increase or a proxy can be used limit the increase. + transmission size either the device implementation should limit the + value increase or a proxy can modify the incrementation. Since RST message may be sent to inform a server that the client do not require Observe response, a rule must allow the transmission of this message. 6.3. No-Response [rfc7967] defines an No-Response option limiting the responses made - by a server to a request. If the value is not by both ends, then TV - is set to this value, MO is set to "equal" and CDF is set to "not- - sent". + by a server to a request. If the value is not known by both ends, + then TV is set to this value, MO is set to "equal" and CDF is set to + "not-sent". Otherwise, if the value is changing over time, TV is not set, MO is - set to "ignore" and CDF to "value-sent". A matching list can also be + set to "ignore" and CDA to "value-sent". A matching list can also be used to reduce the size. -7. Protocol analysis -8. Examples of CoAP header compression +6.4. Time Scale -8.1. Mandatory header with CON message + Time scale [I-D.toutain-core-time-scale] option allows a client to + inform the server that it is in a slow network and that message ID + should be kept for a duration given by the option. + + If the value is not 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. + +6.5. OSCORE + + OSCORE [I-D.ietf-core-object-security] 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 OSCORE_piv ------------------> | + + <- 1 byte -> <------ s bytes -----> + +------------+----------------------+-----------------------+ + | 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 + [I-D.ietf-core-object-security] 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 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 to length of the + piv field in bytes, n = 0 taken to mean that no piv is present. + + After the flag byte follow the piv field, kid context field and kid + field in 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 makes it possible + to do a preliminary processing of the message in preparation for + regular SCHC compression. + + Conceptually, the OSCORE option can transmit up to 3 distinct pieces + of information at a time: the piv, the kid context, and the kid. + This draft proposes that the SCHC Parser split the contents of this + option into 3 SCHC fields: + + o CoAP OSCORE_piv, + + o CoAP OSCORE_ctxt, + + o CoAP OSCORE_kid. + + These fields are superposed on the OSCORE Option format in Figure 4, + and include the corresponding flag and size bits for each part of the + option. Both the flag and size bits can be omitted by use of the MSB + matching operator on each field. + +7. Examples of CoAP header compression + +7.1. Mandatory header with CON message In this first scenario, the LPWAN compressor receives from outside client a POST message, which is immediately acknowledged by the - Thing. For this simple scenario, the rules are described Figure 5. + Device. For this simple scenario, 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 version | | |bi| 01 |equal |not-sent || | - |CoAP Type | | |bi| |ignore |value-sent ||TT | + |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| ML1 |match-map|matching-sent|| CC CCC | |CoAP MID | | |bi| 0000 |MSB(7 ) |LSB(9) || 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. Code has shrunk to 5 - bits using the matching list (as the one given Figure 1: 0.01 is - value 0x01 and 2.05 is value 0x0c) Message-ID has shrunk to 9 bits to - preserve alignment on byte boundary. The most significant bit must - be set to 0 through a CoAP proxy. Uri-Path contains a single element + bits using a matching list. Uri-Path contains a single element indicated in the matching operator. - Figure 6 shows the time diagram of the exchange. A LPWAN Application - Server sends a CON message. Compression reduces the header sending - only the Type, a mapped code and the least 9 significant bits of - Message ID. The receiver decompresses the header. . - - The CON message is a request, therefore the LC process to a dynamic - mapping. When the ES receives the ACK message, this will not - initiate locally a message ID mapping since it is a response. The LC - receives the ACK and uncompressed it to restore the original value. - Dynamic Mapping context lifetime follows the same rules as message ID - duration. + Figure 6 shows the time diagram of the exchange. A client in the + Application Server sends a CON request. It can go through a proxy + which reduces the message ID to a smallest value, with at least the 9 + most significant bits equal to 0. SCHC Compression reduces the + header sending only the Type, a mapped code and the least 9 + significant bits of Message ID. - End System LPWA LC + Device LPWAN SCHC C/D | | | rule id=1 |<-------------------- |<-------------------| +-+-+--+----+------+ - <------------------- | TTCC CCCM MMMM MMMM| |1|0| 4|0.01|0x0034| - +-+-+--+----+-------+ | 0000 0010 0011 0100| | 0xb4 p a t| + <------------------- | CCCCCMMMMMMMMM | |1|0| 4|0.01|0x0034| + +-+-+--+----+-------+ | 00001000110100 | | 0xb4 p a t| |1|0| 1|0.01|0x0034 | | | | h | | 0xb4 p a t | | | +------+ | h | | | +------+ | | | | | | ---------------------->| rule id=1 | +-+-+--+----+--------+ |------------------->| - |1|2| 0|2.05| 0x0034 | | TTCC CCCM MMMM MMMM|---------------------> - +-+-+--+----+--------+ | 1001 1000 0011 0100| +-+-+--+----+------+ + |1|2| 0|2.05| 0x0034 | | TCCCCCMMMMMMMMM |---------------------> + +-+-+--+----+--------+ | 001100000110100 | +-+-+--+----+------+ | | |1|2| 0|2.05|0x0034| v v +-+-+--+----+------+ Figure 6: Compression with global addresses - The message can be further optimized by setting some fields - unidirectional, as described in Figure 7. Note that Type is no more - sent in the compressed format, Compressed Code size in not changed in - that example (8 values are needed to code all the requests and 21 to - code all the responses in the matching list Figure 1) - - Rule ID 2 - +-------------+--+--+--+------+---------+------------++------------+ - | Field |FL|FP|DI|Target| MO | CDA || Sent | - | | | | |Value | | || [bits] | - +-------------+--+--+--+------+---------+------------++------------+ - |CoAP version | | |bi|01 |equal |not-sent || | - |CoAP Type | | |dw|CON |equal |not-sent || | - |CoAP Type | | |up| ACK |equal |not-sent || | - |CoAP TKL | | |bi|0 |equal |not-sent || | - |CoAP Code | | |dw|ML2 |match-map|mapping-sent||CCCC C | - |CoAP Code | | |up|ML3 |match-map|mapping-sent||CCCC C | - |CoAP MID | | |bi|0000 |MSB(5) |LSB(11) || M-ID | - |CoAP Uri-Path| | |dw|path |equal 1 |not-sent || | - +-------------+--+--+--+------+---------+------------++------------+ - - ML1 = {CON : 0, ACK:1} ML2 = {POST:0, 2.04:1, 0.00:3} - - Figure 7: CoAP Context to compress header without token - -8.2. Complete exchange +7.2. Complete exchange In that example, the Thing is using CoMi and sends queries for 2 SID. CON MID=0x0012 | | POST | | Accept X | | /c/k=AS |------------------------>| | | | | |<------------------------| ACK MID=0x0012 | | 0.00 | | | | |<------------------------| CON | | MID=0X0034 | | Content-Format X ACK MID=0x0034 |------------------------>| 0.00 - Rule ID 3 - +--------------+--+--+--+------+--------+-----------++------------+ - | Field |FL|FP|DI|Target| MO | CDA || Sent | - | | | | |Value | | || [bits] | - +--------------+--+--+--+------+--------+-----------++------------+ - |CoAP version | | |bi| 01 |equal |not-sent || | - |CoAP Type | | |up| CON |equal |not-sent || | - |CoAP Type | | |dw| ACK |equal |not-sent || | - |CoAP TKL | | |bi| 1 |equal |not-sent || | - |CoAP Code | | |up| POST |equal |not-sent || | - |CoAP Code | | |dw| 0.00 |equal |not-sent || | - |CoAP MID | | |bi| 0000 |MSB(8) |LSB ||MMMMMMMM | - |CoAP Token | | |up| |ignore |send-value ||TTTTTTTT | - |CoAP Uri-Path | | |dw| /c |equal 1 |not-sent || | - |CoAP Uri-query| | |dw| ML4 |equal 1 |not-sent ||P | - |CoAP Content | | |up| X |equal |not-sent || | - +--------------+--+--+--+------+--------+-----------++------------+ +7.3. OSCORE Compression - Rule ID 4 - +--------------+--+--+--+------+--------+-----------++------------+ - | Field |FL|FP|DI|Target| MO | CDA || Sent | - | | | | |Value | | || [bits] | - +--------------+--+--+--+------+--------+-----------++------------+ - |CoAP version | | |bi| 01 |equal |not-sent || | - |CoAP Type | | |dw| CON |equal |not-sent || | - |CoAP Type | | |up| ACK |equal |not-sent || | - |CoAP TKL | | |bi| 1 |equal |not-sent || | - |CoAP Code | | |dw| 2.05 |equal |not-sent || | - |CoAP Code | | |up| 0.00 |equal |not-sent || | - |CoAP MID | | |bi| 0000 |MSB(8) |LSB ||MMMMMMMM | - |CoAP Token | | |dw| |ignore |send-value||TTTTTTTT | - |COAP Accept | | |dw| X |equal |not-sent || | - +--------------+--+--+--+------+---------+----------++------------+ + OSCORE aims to solve the problem of end-to-end encryption for CoAP + messages, which are otherwise required to terminate their TLS or DTLS + protection at the proxy, as discussed in Section 11.2 of [rfc7252]. + CoAP proxies are men-in-the-middle, but not all of the information + they have access to is necessary for their operation. The goal, + therefore, is to hide as much of the message as possible while still + enabling proxy operation. - alternative rule: + Conceptually this is achieved by splitting the CoAP message into an + Inner Plaintext and Outer OSCORE Message. The Inner Plaintext + contains sensible information which is not necessary for proxy + operation. This, in turn, is the part of the message which can be + encrypted and need not be decrypted until it reaches its end + destination. The Outer Message acts as a shell matching the format + of a regular CoAP message, and includes all Options and information + needed for proxy operation and caching. This decomposition is + illustrated in Figure 7. - Rule ID 4 - +--------------+--+--+--+------+---------+-----------++------------+ - | Field |FL|FP|DI|Target| MO | CDA || Sent | - | | | | |Value | | || [bits] | - +--------------+--+--+--+------+---------+-----------++------------+ - |CoAP version | | |bi| 01 |equal |not-sent || | - |CoAP Type | | |bi| ML1 |match-map|match-sent ||t | - |CoAP TKL | | |bi| 1 |equal |not-sent || | - |CoAP Code | | |up| ML2 |match-map|match-sent || cc | - |CoAP Code | | |dw| ML3 |match-map|match-sent || cc | - |CoAP MID | | |bi| 0000 |MSB(8) |LSB ||MMMMMMMM | - |CoAP Token | | |dw| |ignore |send-value ||TTTTTTTT | - |CoAP Uri-Path | | |dw| /c |equal 1 |not-sent || | - |CoAP Uri-query| | |dw| ML4 |equal 1 |not-sent ||P | - |CoAP Content | | |up| X |equal |not-sent || | - |COAP Accept | | |dw| x |equal |not-sent || | - +--------------+--+--+--+------+---------+-----------++------------+ + CoAP options are sorted into one of 3 classes, each granted a + specific type of protection by the protocol: - ML1 {CON:0, ACK:1} ML2 {POST:0, 0.00: 1} ML3 {2.05:0, 0.00:1} - ML4 {NULL:0, k=AS:1, K=AZE:2} + o Class E: Enrypted options moved to the Inner Plaintext, -9. Normative References + o Class I: Intergrity-protected options included in the AAD for the + encryption of the Plaintext but otherwise left untouched in the + Outer Message, - [I-D.toutain-lpwan-ipv6-static-context-hc] - Minaburo, A. and L. Toutain, "LPWAN Static Context Header - Compression (SCHC) for IPv6 and UDP", draft-toutain-lpwan- - ipv6-static-context-hc-00 (work in progress), September - 2016. + o Class U: Unprotected options left untouched in the Outer Message. + + Additionally, the OSCORE Option is added as an Outer option, + signaling 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. + + Orignal 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 7: OSCORE inner and outer header form a CoAP message + + Figure 7 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 value (POST or FETCH) depending on whether + the original message was a Request or a Response. The original + message code is put into the first byte of the Plaintext. Following + the message code come the class E options and if present the original + message Payload 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 8. + + Outer Header + +-+-+---+--------+---------------+ + |v|t|tkl|new code| Msg Id. | + +-+-+---+--------+---------------+....+ + | Token | + +--------------------------------.....+ + | Options (IU) | + . . + . OSCORE Option . + +------+-------------------+ + | 0xFF | + +------+-------------------------+ + | | + | Encrypted Inner Header and | + | Payload | + | | + +--------------------------------+ + + Figure 8: OSCORE message + + The SCHC Compression scheme consists of compressing both the + Plaintext before encryption and the resulting OSCORE message after + encryption, see Figure 9. This way compression reaches all fields of + the original CoAP message. + + 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 9: OSCORE Compression Diagram + +7.4. Example OSCORE Compression + + In what follows we present an example GET Request and consequent + CONTENT Response and show a possible set of rules for the Inner and + Outer SCHC Compression. We then show a dump of the results and + contrast SCHC + OSCORE performance with SCHC + COAP performance. + This gives an approximation to the cost of security with SCHC-OSCORE. + + Our first example CoAP message is the GET Request in Figure 10 + 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 10: CoAP GET Request + + Its corresponding response is the CONTENT Response in Figure 11. + + 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 11: CoAP CONTENT Response + + The SCHC Rules for the Inner Compression include all fields that are + already present in a regular CoAP message, what matters is the order + of appearance and inclusion of only those CoAP fields that go into + the Plaintext, Figure 12. + + 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 12: Inner SCHC Rules + + The Outer SCHC Rules (Figure 13) must process the OSCORE Options + fields. Here we mask off the repeated bits (most importantly the + flag and size bits) with the MSB Mathing Operator. + +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_piv| |up| 0x0900 |MSB(12) |LSB ||PPPP | +|COAP OSCORE_kid| |up|b'\x06client' |MSB(52) |LSB ||KKKK | +|CoAP OSCORE_piv| |dw| b'' |equal |not-sent || | +|COAP Option-End| |dw| 0xFF |equal |not-sent || | ++---------------+--+--+--------------+---------+-----------++------------+ + + Figure 13: Outer SCHC Rules + + Next we show a dump of the compressed message: + + Compressed message: + ================== + 0x00291287f0a5c4833760d170 + 0x00 = Rule ID + + piv = 0x04 + + Compression residue: + 0b0001 010 0100 0100 (15 bits -> 2 bytes with padding) + mid tkn piv kid + + Payload + 0xa1fc297120cdd8345c + + Compressed message length: 12 bytes + + Figure 14: SCHC-OSCORE Compressed GET Request + + Compressed message: + ================== + 0x0015f4de9cb814c96aed9b1d981a3a58 + 0x00 = Rule ID + + Compression residue: + 0b0001 010 (7 bits -> 1 byte with padding) + mid tkn + + Payload + 0xfa6f4e5c0a64b576cd8ecc0d1d2c + + Compressed msg length: 16 bytes + + Figure 15: 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 mesages according to + the SCHC rules in Figure 16. + + 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] |equal |not-sent || | + |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 16: SCHC-CoAP Rules (No OSCORE) + + This yields the results in Figure 17 for the Request, and Figure 18 + for the Response. + + Compressed message: + ================== + 0x0114 + 0x01 = Rule ID + + Compression residue: + 0b00010100 (1 byte) + + Compressed msg length: 2 + + Figure 17: CoAP GET Compressed without OSCORE + + Compressed message: + ================== + 0x010a32332043 + 0x01 = Rule ID + + Compression residue: + 0b00001010 (1 byte) + + Payload + 0x32332043 + + Compressed msg length: 6 + + Figure 18: 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. Normative References + + [I-D.ietf-core-object-security] + Selander, G., Mattsson, J., Palombini, F., and L. Seitz, + "Object Security for Constrained RESTful Environments + (OSCORE)", draft-ietf-core-object-security-13 (work in + progress), June 2018. + + [I-D.ietf-lpwan-ipv6-static-context-hc] + Minaburo, A., Toutain, L., Gomez, C., and D. Barthel, + "LPWAN Static Context Header Compression (SCHC) and + fragmentation for IPv6 and UDP", draft-ietf-lpwan-ipv6- + static-context-hc-16 (work in progress), June 2018. + + [I-D.toutain-core-time-scale] + Minaburo, A. and L. Toutain, "CoAP Time Scale Option", + draft-toutain-core-time-scale-00 (work in progress), + October 2017. [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, . Authors' Addresses - Ana Minaburo Acklio - 2bis rue de la Chataigneraie + 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