--- 1/draft-ietf-lpwan-coap-static-context-hc-08.txt 2019-07-06 03:13:11.235524734 -0700 +++ 2/draft-ietf-lpwan-coap-static-context-hc-09.txt 2019-07-06 03:13:11.291525971 -0700 @@ -1,51 +1,50 @@ lpwan Working Group A. Minaburo Internet-Draft Acklio Intended status: Standards Track L. Toutain -Expires: November 30, 2019 Institut MINES TELECOM; IMT Atlantique +Expires: January 7, 2020 Institut MINES TELECOM; IMT Atlantique R. Andreasen Universidad de Buenos Aires - May 29, 2019 + July 06, 2019 LPWAN Static Context Header Compression (SCHC) for CoAP - draft-ietf-lpwan-coap-static-context-hc-08 + draft-ietf-lpwan-coap-static-context-hc-09 Abstract This draft defines the way SCHC header compression can be applied to CoAP headers. The CoAP header structure differs from IPv6 and UDP - protocols since CoAP - uses a flexible header with a variable number of options themselves - of variable length. The CoAP protocol is asymmetric in its message - format, the format of the header packet in the request messages is - different from that in the response messages. Most of the - compression mechanisms have been introduced in + protocols since CoAP uses a flexible header with a variable number of + options, themselves of variable length. The CoAP protocol is + asymmetric in its message format: the format of the packet header in + the request messages is different from that in the 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 November 30, 2019. + This Internet-Draft will expire on January 7, 2020. Copyright Notice Copyright (c) 2019 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 @@ -61,127 +60,138 @@ 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 . . . . . . . . . . . . . . . . . . . . . . . 8 + 5.2. CoAP option Max-Age, 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 . . . . . . . . . . . . . . . . . . . . . . . . . 10 - 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 10 + 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 . . . . . . . . . . . . . . . . . . . . . . . . . 11 - 7. Examples of CoAP header compression . . . . . . . . . . . . . 12 - 7.1. Mandatory header with CON message . . . . . . . . . . . . 12 - 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 13 - 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 - 9. Security considerations . . . . . . . . . . . . . . . . . . . 27 - 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27 - 11. Normative References . . . . . . . . . . . . . . . . . . . . 27 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 + 6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 10 + 7. Examples of CoAP header compression . . . . . . . . . . . . . 11 + 7.1. Mandatory header with CON message . . . . . . . . . . . . 11 + 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 12 + 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 16 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 + 9. Security considerations . . . . . . . . . . . . . . . . . . . 26 + 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26 + 11. Normative References . . . . . . . . . . . . . . . . . . . . 26 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 1. Introduction CoAP [rfc7252] is an implementation of the REST architecture for constrained devices. Although CoAP was designed for constrained - devices, the size of a CoAP header may still be too large for LPWAN - constraints and some compression may be needed to reduce the header - size. + devices, the size of a CoAP header may still be too large for the + constraints of Low Power Wide Area Networks (LPWAN) and some + compression may be needed to reduce the header size. [I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression mechanism for LPWAN network based on a static context. The context is said static since the field description composing the Rules are not learned during the packet exchanges but are previously defined. The context(s) is(are) known by both ends before transmission. A context is composed of a set of rules that are referenced by Rule IDs (identifiers). A rule contains an ordered list of the fields descriptions containing a field ID (FID), its length (FL) and its position (FP), a direction indicator (DI) (upstream, downstream and bidirectional) and some associated Target Values (TV). Target Value indicates the value that can be expected. TV can also be a list of values. A Matching Operator (MO) is associated to each header field 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. + fields of the incoming packet. In that case, a Compression/ + Decompression Action (CDA) associated to each field defines how the + compressed and the decompressed values are computed out of each + other, for each of the header fields. Compression mainly results in + one of 4 actions: send the field value, send nothing, send some least + significant bits of the field or send an index. Values sent are + called Compression Residues and follow the rule ID in the transmitted + message. + + The compression rules define a generic way to compress and decompress + the fields. If the device is modified, for example, to introduce new + functionalities or new CoAP options, the rules must be updated to + reflect the evolution. There is no risk to lock a device in a + particular version of CoAP. 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 (IPv6/UDP) or independently. The SCHC adaptation layers + lower layers (IPv6/UDP) or independently. The SCHC adaptation layers as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used as shown in Figure 1. ^ +------------+ ^ +------------+ ^ +------------+ | | 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 cover all headers from IPv6 to CoAP, in - which case SCHC C/D is performed at the device and at the LPWAN - boundary. If an end-to-end encryption mechanisms is used between the - device and the application, CoAP MAY 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. + rule's scope. + + In the first example, a rule compresses all headers from IPv6 to + CoAP. In this case, SCHC C/D is performed at the device and at the + LPWAN boundary. + + In the second example, an end-to-end encryption mechanisms is used + between the device and the application. CoAP is 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 - layers below. 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. + out of the scope of this document). + + In the third example, OSCORE [I-D.ietf-core-object-security] is used. + 2 rulesets are used to compress the CoAP message. A first ruleset + focuses on the inner header and is end to end, a second ruleset + compresses the outer header and the layers below. SCHC C/D for inner + header is done by both ends, and 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 option. + in the request and in the response, with the value of some fields + being swapped on the return path (e.g. source and destination + fields). A CoAP request is intrinsically different from a + response. For example, the URI-path option is mandatory in the + request and is not found in the response, a request may contain an + Accept option and the response a Content option. [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a message direction (DI) in the Field Description, which allows a - single Rule to process message headers differently in both - directions. + single Rule to process message headers differently depending of + the direction. o Even when a field is "symmetric" (i.e. found in both directions) the values carried in each direction are different. Combined with a matching list in the TV, this allows reducing the range of expected values in a particular direction and therefore reduce the size of the compression residue. For instance, if a client sends only CON request, the type can be elided by compression and the answer may use one single bit to carry either the ACK or RST type. The same behavior can be applied to the CoAP Code field (0.0X code are present in the request and Y.ZZ in the answer). The direction @@ -189,76 +199,75 @@ direction. o In IPv6 and UDP, header fields have a fixed size. In CoAP, Token size may vary from 0 to 8 bytes, the length being 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 present several times. This is - typical for 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 instance. - + o In CoAP headers, a field can appear several times. This is + typical for elements of a URI (path or queries). [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. + appears several times in the rule, the Field Position (FP) + identifies the proper instance, 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 or Token field. The MSB MO can be used to reduce the information carried on LPWANs. o CoAP also obeys the client/server paradigm and the compression ratio can be different if the request is issued from an LPWAN - device or from an non LPWAN device. For instance a Device (Dev) + device or from a non LPWAN device. For instance a Device (Dev) aware of LPWAN constraints can generate a 1 byte token, but a regular CoAP client will certainly send a larger token to the Dev. - 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 ratio and maintain the necessary context for + The SCHC compression-decompression process does not modify the + values. Nevertheless, a proxy placed before the compressor may + change some field values to allow SCHC achieving a better + compression ratio, while maintaining the necessary context for interoperability with existing CoAP implementations. 4. Compression of CoAP header fields This section discusses the compression of the different CoAP header fields. 4.1. CoAP version field This field 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 defined to avoid ambiguities between versions. 4.2. CoAP type field [rfc7252] defines 4 types of messages: CON, NON, ACK and RST. The last two are a response to the first two. If the device plays a - specific role, a rule can exploit these properties with the mapping - list: [CON, NON] for one direction and [ACK, RST] for the other - direction. Compression residue is reduced to 1 bit. + specific client or server role, a rule can exploit these properties + with the mapping list: [CON, NON] for one direction and [ACK, RST] + for the other direction. The compression residue is reduced to 1 + bit. The field SHOULD be elided if for instance a client is sending only - NON or CON messages. + NON or only CON messages. In any case, a rule MUST be defined to carry RST to a client. 4.3. CoAP code field 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. + that of the CoAP type field. If the device plays a specific role, + the set of code values can be split in two parts, the request codes + with the 0 class and the response values. If the device only implements a CoAP client, the request code can be reduced to the set of requests the client is able to process. All the response codes MUST be compressed with a SCHC rule. 4.4. CoAP Message ID field This field is bidirectional and is used to manage acknowledgments. The server memorizes the value for a EXCHANGE_LIFETIME period (by @@ -304,25 +313,24 @@ These fields are both unidirectional and MUST NOT be set to bidirectional in a rule entry. If a single value is expected by the client, it can be stored in the TV and elided during the transmission. Otherwise, if several possible values are expected by the client, a matching-list SHOULD be used to limit the size of the residue. If is not possible, the value has to be sent as a residue (fixed or variable length). -5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri-Port - fields +5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields - These fields is unidirectional and MUST NOT be set to bidirectional - in a rule entry. It is used only by the server to inform of the + These fields are unidirectional and MUST NOT be set to bidirectional + in a rule entry. They are used only by the server to inform of the caching duration and is never found in client requests. If the duration is known by both ends, the value can be elided on the LPWAN. A matching list can be used if some well-known values are defined. Otherwise these options SHOULD be sent as a residue (fixed or variable length). @@ -431,34 +439,21 @@ [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. Time Scale - - The time scale [I-D.toutain-core-time-scale] option allows a client - to inform the server that it is in a constrained network and that - message ID MUST be kept for a duration given by the option. - - 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. - -6.5. OSCORE +6.4. 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) ... +-+-+-+-+-+-+-+-+--------------------------------- | | | @@ -491,103 +486,88 @@ This draft recommends to implement a parser that is able to identify the OSCORE Option and the fields it contains. Conceptually, it discerns up to 4 distinct pieces of information within the OSCORE option: the flag bits, the piv, the kid context, and the kid. It is thus recommended that the parser split the OSCORE option into the 4 subsequent fields: o CoAP OSCORE_flags, + o CoAP OSCORE_piv, o CoAP OSCORE_kidctxt, o CoAP OSCORE_kid. These fields are shown superimposed on the OSCORE Option format in Figure 4, the CoAP OSCORE_kidctxt field including the size bits s. - Their size SHOULD be reduced using the MSB matching operator. + Their size SHOULD be reduced using SCHC compression. 7. Examples of CoAP header compression 7.1. Mandatory header with CON message In this first scenario, the LPWAN compressor at the Network Gateway side receives from a client on the Internet a POST message, which is immediately acknowledged by the 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 | | |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 Code | | |bi|[0.00,| | || | + | | | | | ... | | || | + | | | | | 5.05]|match-map|matching-sent|| CC CCC | + |CoAP MID | | |bi| 0000 |MSB(7 ) |LSB || M-ID| |CoAP Uri-Path| | |dw| path |equal 1 |not-sent || | +-------------+--+--+--+------+---------+-------------++------------+ Figure 5: CoAP Context to compress header without token - The version and Token Length fields are elided. Code has shrunk to 5 - 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 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. + The version and Token Length fields are elided. The 26 method and + response codes defined in [rfc7252] has been shrunk to 5 bits using a + matching list. Uri-Path contains a single element indicated in the + matching operator. - Device LPWAN SCHC C/D - | | - | rule id=1 |<-------------------- - |<-------------------| +-+-+--+----+------+ - <------------------- | 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 | | TCCCCCMMMMMMMMM |---------------------> - +-+-+--+----+--------+ | 001100000110100 | +-+-+--+----+------+ - | | |1|2| 0|2.05|0x0034| - v v +-+-+--+----+------+ + 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). - Figure 6: Compression with global addresses + Note that a request sent by a client located an Application Server to + a server in the device, may not be compressed through this rule since + the MID will not start with 7 bits equal to 0. A CoAP proxy, before + the core SCHC C/D can rewrite the message ID to a value matched by + the rule. 7.2. OSCORE Compression OSCORE aims to solve the problem of end-to-end encryption for CoAP messages. The goal, therefore, is to hide as much of the message as possible while still enabling proxy operation. Conceptually this is achieved by splitting the CoAP message into an Inner Plaintext and Outer OSCORE Message. The Inner Plaintext contains sensible information which is not necessary for proxy operation. This, in turn, is the part of the message which can be encrypted until it reaches its end destination. The Outer Message acts as a shell matching the format of a regular CoAP message, and includes all Options and information needed for proxy operation and - caching. This decomposition is illustrated in Figure 7. + caching. This decomposition is illustrated in Figure 6. CoAP options are sorted into one of 3 classes, each granted a specific type of protection by the protocol: o Class E: Encrypted options moved to the Inner Plaintext, o Class I: Integrity-protected options included in the AAD for the encryption of the Plaintext but otherwise left untouched in the Outer Message, @@ -625,28 +605,27 @@ +-+-+---+--------+---------------+....+ +-------+-----......+ | Token | | Options (E) | +--------------------------------.....+ +-------+------.....+ | Options (IU) | | OxFF | . . +-------+-----------+ . OSCORE Option . | | +------+-------------------+ | Payload | | 0xFF | | | +------+ +-------------------+ - Figure 7: OSCORE inner and outer header form a CoAP message + Figure 6: A CoAP message is split into an OSCORE outer and plaintext - Figure 7 shows the message format for the OSCORE Message and + 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 - [I-D.ietf-core-object-security], 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. @@ -647,52 +626,53 @@ 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. 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. + OSCORE message, as illustrated in Figure 7. This Ciphertext is, as defined in RFC 5116, the concatenation of the encrypted Plaintext and its authentication tag. Note that Inner Compression only affects the Plaintext before encryption, thus we can only aim to reduce this first, variable length component of the Ciphertext. The authentication tag is fixed in length and considered part of the cost of protection. Outer Header +-+-+---+--------+---------------+ |v|t|tkl|new code| Msg Id. | +-+-+---+--------+---------------+....+ | Token | +--------------------------------.....+ | Options (IU) | . . . OSCORE Option . +------+-------------------+ | 0xFF | - +------+-------------------------+ + +------+---------------------------+ | | - | Encrypted Inner Header and | - | Payload | + | Ciphertext: Encrypted Inner | + | Header and Payload | + | + Authentication Tag | | | - +--------------------------------+ + +----------------------------------+ - Figure 8: OSCORE message + 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 9. + encryption, see Figure 8. This translates into a segmented process where SCHC compression is applied independently in 2 stages, each with its corresponding set of rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way compression is applied to all fields of the original CoAP message. 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. @@ -721,32 +701,33 @@ v | +-------+--+ +--------+ +------------+ | Residue | |Rule ID'| | Encryption | <--- +----------+--------+ +--------+--+ +------------+ | | | Residue' | | Payload | +-----------+-------+ | | | Ciphertext | +-------------------+ | | +-------------------+ - Figure 9: OSCORE Compression Diagram + Figure 8: OSCORE Compression Diagram 7.3. Example OSCORE Compression An example is given with a GET Request and its consequent CONTENT - Response. A possible set of rules for the Inner and Outer SCHC + Response from a device-based CoAP client to a cloud-based CoAP + server. A possible set of rules for the Inner and Outer SCHC Compression is shown. A dump of the results and a contrast between SCHC + OSCORE performance with SCHC + COAP performance is also listed. This gives an approximation to the cost of security with SCHC-OSCORE. - Our first example CoAP message is the GET Request in Figure 10 + Our first example CoAP message is the GET Request in Figure 9 Original message: ================= 0x4101000182bb74656d7065726174757265 Header: 0x4101 01 Ver 00 CON 0001 tkl @@ -755,23 +736,23 @@ 0x0001 = mid 0x82 = token Options: 0xbb74656d7065726174757265 Option 11: URI_PATH Value = temperature Original msg length: 17 bytes. - Figure 10: CoAP GET Request + Figure 9: CoAP GET Request - Its corresponding response is the CONTENT Response in Figure 11. + Its corresponding response is the CONTENT Response in Figure 10. Original message: ================= 0x6145000182ff32332043 Header: 0x6145 01 Ver 10 ACK 0001 tkl @@ -779,41 +760,41 @@ 0x0001 = mid 0x82 = token 0xFF Payload marker Payload: 0x32332043 Original msg length: 10 - Figure 11: CoAP CONTENT Response + Figure 10: CoAP CONTENT Response The SCHC Rules for the Inner Compression include all fields that are already present in a regular CoAP message, what is important is the order of appearance and inclusion of only those CoAP fields that go - into the Plaintext, Figure 12. + into the Plaintext, Figure 11. Rule ID 0 +---------------+--+--+-----------+-----------+-----------++------+ | Field |FP|DI| Target | MO | CDA || Sent | | | | | Value | | ||[bits]| +---------------+--+--+-----------+-----------+-----------++------+ |CoAP Code | |up| 1 | equal |not-sent || | |CoAP Code | |dw|[69,132] | match-map |match-sent || c | |CoAP Uri-Path | |up|temperature| equal |not-sent || | |COAP Option-End| |dw| 0xFF | equal |not-sent || | +---------------+--+--+-----------+-----------+-----------++------+ - Figure 12: Inner SCHC Rules + Figure 11: Inner SCHC Rules - Figure 13 shows the Plaintext obtained for our example GET Request + Figure 12 shows the Plaintext obtained for our example GET Request and follows the process of Inner Compression and Encryption until we end up with the Payload to be added in the outer OSCORE Message. In this case the original message has no payload and its resulting Plaintext can be compressed up to only 1 byte (size of the Rule ID). The AEAD algorithm preserves this length in its first output, but also yields a fixed-size tag which cannot be compressed and has to be included in the OSCORE message. This translates into an overhead in total message length, which limits the amount of compression that can be achieved and plays into the cost of adding security to the @@ -851,23 +832,23 @@ | (piv = 0x04) v _________________________________________________ | | | encrypted_plaintext = 0xa2 (1 byte) | | tag = 0xc54fe1b434297b62 (8 bytes) | | | | ciphertext = 0xa2c54fe1b434297b62 (9 bytes) | |_________________________________________________| - Figure 13: Plaintext compression and encryption for GET Request + Figure 12: Plaintext compression and encryption for GET Request - In Figure 14 we repeat the process for the example CONTENT Response. + In Figure 13 we repeat the process for the example CONTENT Response. In this case the misalignment produced by the compression residue (1 bit) makes it so that 7 bits of padding have to be applied after the payload, resulting in a compressed Plaintext that is the same size as before compression. This misalignment also causes the hexcode from the payload to differ from the original, even though it has not been compressed. On top of this, the overhead from the tag bytes is incurred as before. ________________________________________________________ | | @@ -905,57 +886,56 @@ | (piv = 0x04) v _________________________________________________________ | | | encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) | | tag = 0xe9aef3f2461e0c29 (8 bytes) | | | | ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) | |_________________________________________________________| - Figure 14: Plaintext compression and encryption for CONTENT Response - - The Outer SCHC Rules (Figure 17) MUST process the OSCORE Options - fields. In Figure 15 and Figure 16 we show a dump of the OSCORE + Figure 13: Plaintext compression and encryption for CONTENT Response + The Outer SCHC Rules (Figure 16) MUST process the OSCORE Options + fields. In Figure 14 and Figure 15 we show a dump of the OSCORE Messages generated from our example messages once they have been provided with the Inner Compressed Ciphertext in the payload. These are the messages that are to go through Outer SCHC Compression. Protected message: ================== 0x4102000182d7080904636c69656e74ffa2c54fe1b434297b62 (25 bytes) Header: 0x4102 01 Ver 00 CON 0001 tkl 00000010 Request Code 2 "POST" 0x0001 = mid 0x82 = token Options: - 0xd7080904636c69656e74 (10 bytes) + 0xd8080904636c69656e74 (10 bytes) Option 21: OBJECT_SECURITY Value = 0x0904636c69656e74 09 = 000 0 1 001 Flag byte h k n 04 piv 636c69656e74 kid 0xFF Payload marker Payload: 0xa2c54fe1b434297b62 (9 bytes) - Figure 15: Protected and Inner SCHC Compressed GET Request + Figure 14: Protected and Inner SCHC Compressed GET Request Protected message: ================== 0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29 (22 bytes) Header: 0x6144 01 Ver 10 ACK @@ -967,21 +947,21 @@ Options: 0xd008 (2 bytes) Option 21: OBJECT_SECURITY Value = b'' 0xFF Payload marker Payload: 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) - Figure 16: Protected and Inner SCHC Compressed CONTENT Response + Figure 15: Protected and Inner SCHC Compressed CONTENT Response For the flag bits, a number of compression methods could prove to be useful depending on the application. The simplest alternative is to provide a fixed value for the flags, combining MO equal and CDA not- sent. This saves most bits but could hinder flexibility. Otherwise, match-mapping could allow to choose from a number of configurations of interest to the exchange. If neither of these alternatives is desirable, MSB could be used to mask off the 3 hard-coded most significant bits. @@ -995,21 +975,21 @@ technologies. Note that compressing the sequence numbers effectively reduces the maximum amount of sequence numbers that can be used in an exchange. Once this amount is exceeded, the SCHC Context would need to be re-established. The size s included in the kid context field MAY be masked off with CDA MSB. The rest of the field could have additional bits masked off, or have the whole field be fixed with MO equal and CDA not-sent. The same holds for the kid field. - Figure 17 shows a possible set of Outer Rules to compress the Outer + Figure 16 shows a possible set of Outer Rules to compress the Outer Header. Rule ID 0 +-------------------+--+--+--------------+--------+---------++------+ | Field |FP|DI| Target | MO | CDA || Sent | | | | | Value | | ||[bits]| +-------------------+--+--+--------------+--------+---------++------+ |CoAP version | |bi| 01 |equal |not-sent || | |CoAP Type | |up| 0 |equal |not-sent || | |CoAP Type | |dw| 2 |equal |not-sent || | @@ -1021,145 +1001,147 @@ |CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || | |CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP | |COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK | |COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || | |CoAP OSCORE_flags | |dw| b'' |equal |not-sent || | |CoAP OSCORE_piv | |dw| b'' |equal |not-sent || | |CoAP OSCORE_kid | |dw| b'' |equal |not-sent || | |COAP Option-End | |dw| 0xFF |equal |not-sent || | +-------------------+--+--+--------------+--------+---------++------+ - Figure 17: Outer SCHC Rules + Figure 16: Outer SCHC Rules These Outer Rules are applied to the example GET Request and CONTENT - Response. The resulting messages are shown in Figure 18 and - Figure 19. + 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: 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 18: SCHC-OSCORE Compressed GET Request + Figure 17: SCHC-OSCORE Compressed GET Request Compressed message: ================== 0x0014218daf84d983d35de7e48c3c1852 (16 bytes) 0x00 Rule ID 14 Compression residue 218daf84d983d35de7e48c3c1852 Padded payload Compression residue: 0b0001 010 (7 bits -> 1 byte with padding) mid tkn Payload 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) Compressed msg length: 16 bytes - Figure 19: SCHC-OSCORE Compressed CONTENT Response + 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 20. + 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 || | |CoAP Code | |up| 2 |equal |not-sent || | - |CoAP Code | |dw| [69,132] |equal |not-sent || | + |CoAP Code | |dw| [69,132] |match-map|map-sent ||C | |CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP Uri-Path | |up|temperature|equal |not-sent || | |COAP Option-End| |dw| 0xFF |equal |not-sent || | +---------------+--+--+-----------+---------+-----------++--------+ - Figure 20: SCHC-CoAP Rules (No OSCORE) + Figure 19: SCHC-CoAP Rules (No OSCORE) - This yields the results in Figure 21 for the Request, and Figure 22 + This yields the results in Figure 20 for the Request, and Figure 21 for the Response. Compressed message: ================== 0x0114 0x01 = Rule ID Compression residue: 0b00010100 (1 byte) Compressed msg length: 2 - Figure 21: CoAP GET Compressed without OSCORE + Figure 20: CoAP GET Compressed without OSCORE Compressed message: ================== 0x010a32332043 0x01 = Rule ID Compression residue: 0b00001010 (1 byte) Payload 0x32332043 Compressed msg length: 6 - Figure 22: CoAP CONTENT Compressed without OSCORE + 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] 10. Acknowledgements - Thanks to all the persons that have give us feedback + The authors would like to thank Dominique Barthel, Carsten Bormann, + Thomas Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov, + Goran Selander. 11. Normative References [I-D.ietf-core-object-security] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", draft-ietf-core-object-security-16 (work in progress), March 2019. [I-D.ietf-lpwan-ipv6-static-context-hc] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J. - Zuniga, "LPWAN Static Context Header Compression (SCHC) - and fragmentation for IPv6 and UDP", draft-ietf-lpwan- - ipv6-static-context-hc-18 (work in progress), December - 2018. + Zuniga, "Static Context Header Compression (SCHC) and + fragmentation for LPWAN, application to UDP/IPv6", draft- + ietf-lpwan-ipv6-static-context-hc-19 (work in progress), + July 2019. [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, .