--- 1/draft-ietf-lpwan-coap-static-context-hc-04.txt 2018-10-22 01:13:08.737326331 -0700 +++ 2/draft-ietf-lpwan-coap-static-context-hc-05.txt 2018-10-22 01:13:08.793327676 -0700 @@ -1,101 +1,100 @@ lpwan Working Group A. Minaburo Internet-Draft Acklio Intended status: Informational L. Toutain -Expires: January 3, 2019 Institut MINES TELECOM; IMT Atlantique +Expires: April 25, 2019 Institut MINES TELECOM; IMT Atlantique R. Andreasen Universidad de Buenos Aires - July 02, 2018 + October 22, 2018 LPWAN Static Context Header Compression (SCHC) for CoAP - draft-ietf-lpwan-coap-static-context-hc-04 + draft-ietf-lpwan-coap-static-context-hc-05 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 - use a flexible header with a variable number of options themself of a - variable length. Another important difference is the asymmetry in + use a flexible header with a variable number of options themselves 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 January 3, 2019. + This Internet-Draft will expire on April 25, 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 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. Compression of CoAP header fields . . . . . . . . . . . . . . 5 + 4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 5 + 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 5 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 6 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 6 - 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7 + 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 6 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. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 7 5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 8 - 5.3.2. Variable number of path or query elements . . . . . . 9 + 5.3.2. Variable number of path or query elements . . . . . . 8 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 + 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 9 + 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 9 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 + 7. Examples of CoAP header compression . . . . . . . . . . . . . 11 + 7.1. Mandatory header with CON message . . . . . . . . . . . . 11 + 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 13 + 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 + 8. Normative References . . . . . . . . . . . . . . . . . . . . 27 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 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. [I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression mechanism for LPWAN network based on a static context. The context @@ -194,21 +193,21 @@ 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 + 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 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 @@ -253,21 +252,21 @@ reduced to the set of request the client is able to process. All the response codes should 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 - receiving the same Message ID value will process the message has a + receiving the same Message ID value will process the message as a retransmission. After this period, it will be processed as a new messages. 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. 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 @@ -275,41 +274,42 @@ 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 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. + 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. 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. + must be used and length do 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 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 not the possible, the value as 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. If the duration is known by both ends, value can be elided on the @@ -335,22 +335,21 @@ 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 whould have - been needed. + paths. If regrouping was not allowed, a 2 bits residue is 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. @@ -379,23 +378,23 @@ residue with a length of 0 to indicate that this Uri-Path is empty. This add 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 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. + and CDA 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 + Otherwise the TV is set to the value, MO is set to "equal" and CDA is set to "not-sent" 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and Location-Query fields These fields are unidirectional. These fields values cannot be stored in a rule entry. They must always be sent with the compression residues. @@ -404,34 +403,35 @@ 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. 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 + 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 - value increase or a proxy can modify the incrementation. + delta between two consecutive value or a proxy can modify the + incrementation. - Since RST message may be sent to inform a server that the client do + 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. 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 CDF is set to + 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 @@ -447,67 +447,67 @@ 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 ------------------> | + | | | + |<-- CoAP -->|<------ CoAP OSCORE_piv ------> | + OSCORE_flags <- 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. + kid field. The 3 least significant bits n indicate the length of the + piv field in bytes. When n = 0, 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. + the OSCORE Option and the fields it contains. - 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: + 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_ctxt, + o CoAP OSCORE_kidctxt, 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. + the CoAP OSCORE_kidctxt field including the size bits s. Their size + may 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. Rule ID 1 @@ -553,79 +553,53 @@ | | ---------------------->| rule id=1 | +-+-+--+----+--------+ |------------------->| |1|2| 0|2.05| 0x0034 | | TCCCCCMMMMMMMMM |---------------------> +-+-+--+----+--------+ | 001100000110100 | +-+-+--+----+------+ | | |1|2| 0|2.05|0x0034| v v +-+-+--+----+------+ Figure 6: Compression with global addresses -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 - -7.3. OSCORE Compression +7.2. OSCORE Compression 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. + 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 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. + 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. CoAP options are sorted into one of 3 classes, each granted a specific type of protection by the protocol: - o Class E: Enrypted options moved to the Inner Plaintext, + o Class E: Encrypted options moved to the Inner Plaintext, - o Class I: Intergrity-protected options included in the AAD for the + o Class I: Integrity-protected options included in the AAD for the encryption of the Plaintext but otherwise left untouched in the Outer Message, 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 + Original CoAP Message +-+-+---+-------+---------------+ |v|t|tkl| code | Msg Id. | +-+-+---+-------+---------------+....+ | Token | +-------------------------------.....+ | Options (IEU) | . . . . +------+-------------------+ | 0xFF | @@ -647,33 +621,48 @@ | 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. + 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. 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. + 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 . +------+-------------------+ @@ -682,22 +671,30 @@ | | | 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. + encryption, see Figure 9. + + 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. Outer Message OSCORE Plaintext +-+-+---+--------+---------------+ +-------+ |v|t|tkl|new code| Msg Id. | | code | +-+-+---+--------+---------------+....+ +-------+-----......+ | Token | | Options (E) | +--------------------------------.....+ +-------+------.....+ | Options (IU) | | OxFF | . . +-------+-----------+ . OSCORE Option . | | @@ -719,29 +716,31 @@ |Rule ID'| | Encryption | <--- +----------+--------+ +--------+--+ +------------+ | | | Residue' | | Payload | +-----------+-------+ | | | Ciphertext | +-------------------+ | | +-------------------+ Figure 9: OSCORE Compression Diagram -7.4. Example OSCORE Compression +7.3. 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. + An example is given with a GET Request and its consequent CONTENT + Response. 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 + Original message: ================= 0x4101000182bb74656d7065726174757265 Header: 0x4101 01 Ver 00 CON 0001 tkl 00000001 Request Code 1 "GET" @@ -776,162 +775,364 @@ 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. + 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 +----------------+--+--+-----------+-----------+-----------++--------+ | 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. + 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 + 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) | + | | + | 0x01 Request Code GET | + | | + | bb74656d7065726174757265 Option 11: URI_PATH | + | Value = temperature | + |________________________________________________________| + + | + | + | Inner SCHC Compression + | + v + _________________________________ + | | + | Compressed Plaintext | + | | + | 0x00 | + | | + | Rule ID = 0x00 (1 byte) | + | (No residue) | + |_________________________________| + + | + | AEAD Encryption + | (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 + + 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 + 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 | + | | + | 0x45ff32332043 (6 bytes) | + | | + | 0x45 Successful Response Code 69 "2.05 Content" | + | | + | ff Payload marker | + | | + | 32332043 Payload | + |________________________________________________________| + + | + | + | Inner SCHC Compression + | + v + __________________________________________ + | | + | Compressed Plaintext | + | | + | 0x001919902180 (6 bytes) | + | | + | 00 Rule ID | + | | + | 0b0 (1 bit match-map residue) | + | 0x32332043 >> 1 (shifted payload) | + | 0b0000000 Padding | + |__________________________________________| + + | + | AEAD Encryption + | (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 + 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) + 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 + + Protected message: + ================== + 0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29 + (22 bytes) + + Header: + 0x6144 + 01 Ver + 10 ACK + 0001 tkl + 01000100 Successful Response Code 68 "2.04 Changed" + + 0x0001 = mid + 0x82 = token + + 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 + + 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. + + Note that fixing a flag bit will limit the choice of CoAP Options + that can be used in the exchange, since their values are dependent on + certain options. + + The piv field lends itself to having a number of bits masked off with + MO MSB and CDA LSB. This could 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 + 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 -+---------------+--+--+--------------+---------+-----------++------------+ ++-------------------+--+--+--------------+---------+-----------++--------+ | 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_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 13: Outer SCHC Rules + Figure 17: Outer SCHC Rules - Next we show a dump of the compressed message: + 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: ================== - 0x00291287f0a5c4833760d170 - 0x00 = Rule ID - - piv = 0x04 + 0x001489458a9fc3686852f6c4 (12 bytes) + 0x00 Rule ID + 1489 Compression Residue + 458a9fc3686852f6c4 Padded payload Compression residue: 0b0001 010 0100 0100 (15 bits -> 2 bytes with padding) mid tkn piv kid Payload - 0xa1fc297120cdd8345c + 0xa2c54fe1b434297b62 (9 bytes) Compressed message length: 12 bytes - Figure 14: SCHC-OSCORE Compressed GET Request + Figure 18: SCHC-OSCORE Compressed GET Request Compressed message: ================== - 0x0015f4de9cb814c96aed9b1d981a3a58 - 0x00 = Rule ID - + 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 - 0xfa6f4e5c0a64b576cd8ecc0d1d2c + 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) Compressed msg length: 16 bytes - Figure 15: SCHC-OSCORE Compressed CONTENT Response + 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 mesages according to - the SCHC rules in Figure 16. + with OSCORE. To do this, we compress the CoAP messages according to + the SCHC rules in Figure 20. 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) + Figure 20: SCHC-CoAP Rules (No OSCORE) - This yields the results in Figure 17 for the Request, and Figure 18 + This yields the results in Figure 21 for the Request, and Figure 22 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 + Figure 21: 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 + Figure 22: 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. + (OSCORE)", draft-ietf-core-object-security-15 (work in + progress), August 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),