--- 1/draft-ietf-lpwan-coap-static-context-hc-06.txt 2019-05-24 03:13:09.202779854 -0700 +++ 2/draft-ietf-lpwan-coap-static-context-hc-07.txt 2019-05-24 03:13:09.262781374 -0700 @@ -1,51 +1,51 @@ lpwan Working Group A. Minaburo Internet-Draft Acklio -Intended status: Informational L. Toutain -Expires: August 9, 2019 Institut MINES TELECOM; IMT Atlantique +Intended status: Standards Track L. Toutain +Expires: November 25, 2019 Institut MINES TELECOM; IMT Atlantique R. Andreasen Universidad de Buenos Aires - February 05, 2019 + May 24, 2019 LPWAN Static Context Header Compression (SCHC) for CoAP - draft-ietf-lpwan-coap-static-context-hc-06 + draft-ietf-lpwan-coap-static-context-hc-07 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 - use a flexible header with a variable number of options themselves of - a variable length. The CoAP protocol is asymmetric in its format - messages, the format of the header packet in the request messages is - different than in the response messages. Most of the compression - mechanisms have been introduced in + 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 [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 August 9, 2019. + This Internet-Draft will expire on November 25, 2019. 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 @@ -62,394 +62,397 @@ 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 + Port fields . . . . . . . . . . . . . . . . . . . . . . . 8 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. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 10 + 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 10 6.4. Time Scale . . . . . . . . . . . . . . . . . . . . . . . 10 - 6.5. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 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 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 + 9. Security considerations . . . . . . . . . . . . . . . . . . . 28 + 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 + 11. Normative References . . . . . . . . . . . . . . . . . . . . 29 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 1. Introduction CoAP [rfc7252] is an implementation of the REST architecture for - 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. + 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. [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. + 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. 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 + Compression of the CoAP header MAY be done in conjunction with the 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. + 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 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. + 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. 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. + 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. 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. [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. + message direction (DI) in the Field Description, which allows a + single Rule to process message headers differently 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 + a matching list in the TV, this allows reducing the range of expected values in a particular direction and therefore reduce the - size of a compression residue. For instance, if a client sends + 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 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 + 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 + allows splitting 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. + 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 duplicated several times, for - instances, elements of an URI (path or queries). The position + 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 element. + identify the proper instance. - [I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field id to + [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 too 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 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 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 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 + 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) + 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 rate and maintain the necessary context for + compression ratio and maintain the necessary context for interoperability with existing CoAP implementations. 4. Compression of CoAP header fields - This section discusses of the compression of the different 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 + 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. + 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 - 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. + 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. - The field must be elided if for instance a client is sending only NON - or CON messages. + The field SHOULD be elided if for instance a client is sending only + NON or CON messages. - In any case, a rule must be defined to carry RST to a client. + 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. - 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. + 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 should be compressed with a SCHC rule. + 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. - 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 + The 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 as a retransmission. After this period, it will be processed as a new - messages. + message. 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. + be too large regarding the number of messages sent. The client + SHOULD 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. - 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 + In case the Device is a server, the 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. 4.5. CoAP Token fields Token is defined through two CoAP fields, Token Length in the mandatory header and Token Value directly following the mandatory CoAP header. Token Length is processed as any protocol field. If the value remains the same during all the transaction, the size can be stored - in the context and elided during the transmission. Otherwise it will - have to the send as a compression residue. + in the context and elided during the transmission. Otherwise, it + will have to the sent as a compression residue. - 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 and length do not have to the sent with the residue. + Token Value size cannot be defined directly in the rule in the Field + Length (FL). Instead, a specific function designated as "TKL" MUST + be used and length does not have to the sent with the residue. During the decompression, this function returns the value contained in the Token Length field. 5. CoAP options 5.1. CoAP Content and Accept options. - These field are both unidirectional and must not be set to + These fields are both unidirectional and MUST NOT be set to bidirectional in a rule entry. - 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 is not possible, the value has to - be sent as a residue (fixed or variable length). + 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 - 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. + 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 + caching duration and is never found in client requests. - If the duration is known by both ends, value can be elided on the + 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 + Otherwise these options SHOULD be sent as a residue (fixed or variable length). 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 + These fields are unidirectional and MUST NOT be set to bidirectional + in a rule entry. They are used only by the client to access a specific resource and are never found in server responses. Uri-Path and Uri-Query elements are a repeatable options, the Field Position (FP) gives the position in the path. - A Mapping list can be used to reduce size of variable Paths or + A Mapping list can be used to reduce the size of variable Paths or Queries. In that case, to optimize the compression, several elements can be regrouped into a single entry. Numbering of elements do not change, MO comparison is set with the first element of the matching. 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 Figure 2: complex path example 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 is needed. + paths. If regrouping were not allowed, a 2 bits residue would be + needed. 5.3.1. Variable length Uri-Path and Uri-Query - When the length is known at the rule creation, the Field Length must - be set to variable, and the unit is set to bytes. + When the length is not known at the rule creation, the Field Length + SHOULD be set to variable, and the unit is set to bytes. - 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 MSB MO can be applied to a Uri-Path or Uri-Query element. Since + MSB value is given in bit, the size MUST always be a multiple of 8 + bits. The length sent at the beginning of a variable length residue indicates the size of the LSB in bytes. - For instance for a CoMi path /c/X6?k="eth0" the rule can be set to: + For instance for a CORECONF path /c/X6?k="eth0" the rule can be set + to: 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 - Figure 3: CoMi URI compression + Figure 3: CORECONF URI compression - Figure 3 shows the parsing and the compression of the URI. where c is + Figure 3 shows the parsing and the compression of the URI, where c is not sent. The second element is sent with the length (i.e. 0x2 X 6) followed by the query option (i.e. 0x05 "eth0"). 5.3.2. Variable number of path or query elements - The number of Uri-path or Uri-Query 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 + The number of Uri-path or Uri-Query elements in a rule is fixed at + the rule creation time. If the number varies, several rules SHOULD + be created to cover all the possibilities. Another possibility is to define the length of Uri-Path to variable and send a compression residue with a length of 0 to indicate that this Uri-Path is empty. - This add 4 bits to the compression residue. + This adds 4 bits to the compression residue. 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 + These fields are unidirectional and MUST NOT be set to bidirectional + in a rule entry. They are used only by the client to access a specific resource and are never found in server response. - If the field value must be sent, TV is not set, MO is set to "ignore" - and CDA is set to "value-sent. A mapping can also be used. + If the field value has to be sent, TV is not set, MO is set to + "ignore" and CDA is set to "value-sent". A mapping MAY also be used. - Otherwise the TV is set to the value, MO is set to "equal" and CDA is - set to "not-sent" + Otherwise, the TV is set to the value, MO is set to "equal" and CDA + is set to "not-sent". 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and Location-Query fields These fields are unidirectional. - These fields values cannot be stored in a rule entry. They must + These fields values cannot be stored in a rule entry. They MUST always be sent with the compression residues. 6. Other RFCs 6.1. Block - 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. + Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also + includes a fragmentation protocol. They are compatible. If a block + option is used, its content MUST be sent as a compression residue. 6.2. Observe [rfc7641] defines the Observe option. The TV is not set, MO is set to "ignore" and the CDA is set to "value-sent". SCHC does not limit the maximum size for this option (3 bytes). To reduce the - transmission size either the device implementation should limit the - delta between two consecutive value or a proxy can modify the - incrementation. + transmission size, either the device implementation MAY limit the + delta between two consecutive values, or a proxy can modify the + increment. - Since RST message may be sent to inform a server that the client does - not require Observe response, a rule must allow the transmission of - this message. + Since an RST message may be sent to inform a server that the client + does not require Observe response, a rule MUST allow the transmission + of this message. 6.3. No-Response - [rfc7967] defines an No-Response option limiting the responses made - 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 CDA is set to - "not-sent". + [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 - 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. + 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 not known by both ends, then TV is set to this value, - MO is set to "equal" and CDA is set to "not-sent". + 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 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. @@ -488,42 +491,42 @@ 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 superposed on the OSCORE Option format in Figure 4, - the CoAP OSCORE_kidctxt field including the size bits s. Their size - may be reduced using the MSB matching operator. + 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. 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 - Device. For this simple scenario, the rules are described Figure 5. + 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 | + | 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| @@ -784,40 +787,52 @@ 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 is important is the order of appearance and inclusion of only those CoAP fields that go into the Plaintext, Figure 12. Rule ID 0 + <<<<<<< Updated upstream +---------------+--+--+-----------+-----------+-----------++------+ | 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 || | +---------------+--+--+-----------+-----------+-----------++------+ + ======= + +----------------+--+--+-----------+-----------+-----------++--------+ + | 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 || | + +----------------+--+--+-----------+-----------+-----------++--------+ + >>>>>>> Stashed changes Figure 12: Inner SCHC Rules Figure 13 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 must be + also yields a fixed-size tag which cannot be compressed and has to be included in the OSCORE message. This translates into an overhead in total message length, which limits the amount of compression that can be achieved and plays into the cost of adding security to the exchange. ________________________________________________________ | | | OSCORE Plaintext | | | | 0x01bb74656d7065726174757265 (13 bytes) | @@ -852,21 +867,21 @@ | encrypted_plaintext = 0xa2 (1 byte) | | tag = 0xc54fe1b434297b62 (8 bytes) | | | | ciphertext = 0xa2c54fe1b434297b62 (9 bytes) | |_________________________________________________| Figure 13: Plaintext compression and encryption for GET Request In Figure 14 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 must be applied after the + 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. ________________________________________________________ | | | OSCORE Plaintext | | | @@ -903,22 +918,21 @@ 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 + 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 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) @@ -987,29 +1001,30 @@ certain options. The piv field lends itself to having a number of bits masked off with MO MSB and CDA LSB. This could prove useful in applications where the message frequency is low such as that found in LPWAN technologies. Note that compressing the sequence numbers effectively reduces the maximum amount of sequence numbers that can be used in an exchange. Once this amount is exceeded, the SCHC Context would need to be re-established. - The size s included in the kid context field may be masked off with + 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 Header. Rule ID 0 +<<<<<<< Updated upstream +-------------------+--+--+--------------+--------+---------++------+ | 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 || | @@ -1017,20 +1032,43 @@ |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || | |CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP | |COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK | |COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || | |CoAP OSCORE_flags | |dw| b'' |equal |not-sent || | |CoAP OSCORE_piv | |dw| b'' |equal |not-sent || | |CoAP OSCORE_kid | |dw| b'' |equal |not-sent || | |COAP Option-End | |dw| 0xFF |equal |not-sent || | +-------------------+--+--+--------------+--------+---------++------+ +======= ++-------------------+--+--+--------------+---------+-----------++--------+ +| Field |FP|DI| Target | MO | CDA || Sent | +| | | | Value | | || [bits] | ++-------------------+--+--+--------------+---------+-----------++--------+ +|CoAP version | |bi| 01 |equal |not-sent || | +|CoAP Type | |up| 0 |equal |not-sent || | +|CoAP Type | |dw| 2 |equal |not-sent || | +|CoAP TKL | |bi| 1 |equal |not-sent || | +|CoAP Code | |up| 2 |equal |not-sent || | +|CoAP Code | |dw| 68 |equal |not-sent || | +|CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | +|CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | +|CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || | +|CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP | +|COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK | +|COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || | +|CoAP OSCORE_flags | |dw| b'' |equal |not-sent || | +|CoAP OSCORE_piv | |dw| b'' |equal |not-sent || | +|CoAP OSCORE_kid | |dw| b'' |equal |not-sent || | +|COAP Option-End | |dw| 0xFF |equal |not-sent || | ++-------------------+--+--+--------------+---------+-----------++--------+ +>>>>>>> Stashed changes Figure 17: 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. Compressed message: ================== 0x001489458a9fc3686852f6c4 (12 bytes) @@ -1065,35 +1103,53 @@ Compressed msg length: 16 bytes Figure 19: 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. Rule ID 1 + <<<<<<< Updated upstream +---------------+--+--+-----------+---------+-----------++--------+ | 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 || | +---------------+--+--+-----------+---------+-----------++--------+ + ======= + +---------------+--+--+-----------+---------+-----------++------------+ + | 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 || | + +---------------+--+--+-----------+---------+-----------++------------+ + >>>>>>> Stashed changes Figure 20: SCHC-CoAP Rules (No OSCORE) This yields the results in Figure 21 for the Request, and Figure 22 for the Response. Compressed message: ================== 0x0114 0x01 = Rule ID @@ -1134,22 +1190,22 @@ 10. Acknowledgements Thanks to all the persons that have give us feedback 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-15 (work in - progress), August 2018. + (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. [I-D.toutain-core-time-scale] Minaburo, A. and L. Toutain, "CoAP Time Scale Option",