| lpwan Working Group | A. Minaburo |
| Internet-Draft | Acklio |
| Intended status: Informational | L. Toutain |
| Expires: January 3, 2019 | Institut MINES TELECOM; IMT Atlantique |
| R. Andreasen | |
| Universidad de Buenos Aires | |
| July 02, 2018 |
LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-04
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 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.
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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 is said static since the field description composing the Rules and the context are not learned during the packet exchanges but are previously defined. The context(s) is(are) known by both ends before transmission.
A context is composed of a set of rules that are referenced by Rule IDs (identifiers). A rule contains an ordered list of the fields descriptions containing a field ID (FID), 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.
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 as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used as as shown in the 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.
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.
CoAP differs from IPv6 and UDP protocols on the following aspects:
This section discusses of the compression of the different CoAP header fields.
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.
[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.
The field must 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.
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.
All the response codes should be compressed with a SCHC rule.
This field is bidirectional and is used to manage acknowledgments. Server memorizes the value for a EXCHANGE_LIFETIME period (by default 247 seconds) for CON messages and a NON_LIFETIME period (by default 145 seconds) for NON messages. During that period, a server receiving the same Message ID value will process the message has a retransmission. After this period, it will be processed as a new messages.
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 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.
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 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.
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 be sent as a residue (fixed or variable length).
This field is unidirectional and must not be set to bidirectional in a rule entry. It is used only by the server to inform of the caching duration and is never found in client requests.
If the duration is known by both ends, value can be elided on the LPWAN.
A matching list can be used if some well-known values are defined.
Otherwise these options should be sent as a residue (fixed or variable length).
This fields are unidirectional and must not be set to bidirectional in a rule entry. They are used only by the client to access to a specific resource and are never found in server 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 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 whould have been needed.
When the length is known at the rule creation, the Field Length must be set to variable, and the unit is set to bytes.
The MSB MO can be apply to a Uri-Path or Uri-Query element. Since MSB value is given in bit, the size must always be a multiple of 8 bits and the LSB CDA must not carry any value.
The length sent at the beginning of a variable length residue indicates the size of the LSB in bytes.
For instance for a CoMi path /c/X6?k=”eth0” the rule can be set to:
FID FL FP DI TV MO CDA 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 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”).
The number of Uri-path or Uri-Query element in a rule is fixed at the rule creation time. If the number varies, several rules should be created to cover all the possibilities. Another possibilities is to define the length of Uri-Path to variable and send a compression residue with a length of 0 to indicate that this Uri-Path is empty. This add 4 bits to the compression residue.
These fields are unidirectional and must not be set to bidirectional in a rule entry. They are used only by the client to access to a specific resource and are never found in server response.
If the field value must be sent, TV is not set, MO is set to “ignore” and CDF is set to “value-sent. A mapping can also be used.
Otherwise the TV is set to the value, MO is set to “equal” and CDF is set to “not-sent”
These fields are unidirectional.
These fields values cannot be stored in a rule entry. They must always be sent with the compression residues.
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.
[rfc7641] defines the Observe option. The TV is not set, MO is set to “ignore” and the CDF is set to “value-sent”. SCHC does not limit the maximum size for this option (3 bytes). To reduce the transmission size either the device implementation should limit the value increase or a proxy can modify the incrementation.
Since RST message may be sent to inform a server that the client do not require Observe response, a rule must allow the transmission of this message.
[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 “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.
Time scale [I-D.toutain-core-time-scale] option allows a client to inform the server that it is in a slow network and that message ID should be kept for a duration given by the option.
If the value is not known by both ends, then TV is set to this value, MO is set to “equal” and CDA is set to “not-sent”.
Otherwise, if the value is changing over time, TV is not set, MO is set to “ignore” and CDA to “value-sent”. A matching list can also be used to reduce the size.
OSCORE [I-D.ietf-core-object-security] defines end-to-end protection for CoAP messages. This section describes how SCHC rules can be applied to compress OSCORE-protected messages.
0 1 2 3 4 5 6 7 <--------- n bytes ------------->
+-+-+-+-+-+-+-+-+---------------------------------
|0 0 0|h|k| n | Partial IV (if any) ...
+-+-+-+-+-+-+-+-+---------------------------------
| |
| <--------- CoAP OSCORE_piv ------------------> |
<- 1 byte -> <------ s bytes ----->
+------------+----------------------+-----------------------+
| s (if any) | kid context (if any) | kid (if any) ... |
+------------+----------------------+-----------------------+
| | |
| <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->|
Figure 4: OSCORE Option
The encoding of the OSCORE Option Value defined in Section 6.1 of [I-D.ietf-core-object-security] is repeated in Figure 4.
The first byte is used for flags that specify the contents of the OSCORE option. The 3 most significant bits are reserved and always set to 0. Bit h, when set, indicates the presence of the kid context field in the option. Bit k, when set, indicates the presence of a kid field. The 3 least significant bits n indicate to length of the piv field in bytes, n = 0 taken to mean that no piv is present.
After the flag byte follow the piv field, kid context field and kid field in order and if present; the length of the kid context field is encoded in the first byte denoting by s the length of the kid context in bytes.
This draft recommends to implement a parser that is able to identify the OSCORE Option and the fields it contains - this makes it possible to do a preliminary processing of the message in preparation for regular SCHC compression.
Conceptually, the OSCORE option can transmit up to 3 distinct pieces of information at a time: the piv, the kid context, and the kid. This draft proposes that the SCHC Parser split the contents of this option into 3 SCHC fields:
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.
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 +-------------+--+--+--+------+---------+-------------++------------+ | 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 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.
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 +-+-+--+----+------+
Figure 6: Compression with global addresses
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
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.
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.
CoAP options are sorted into one of 3 classes, each granted a specific type of protection by the protocol:
Additionally, the OSCORE Option is added as an Outer option, signaling that the message is OSCORE protected. This option carries the information necessary to retrieve the Security Context with which the message was encrypted so that it may be correctly decrypted at the other end-point.
Orignal CoAP Message
+-+-+---+-------+---------------+
|v|t|tkl| code | Msg Id. |
+-+-+---+-------+---------------+....+
| Token |
+-------------------------------.....+
| Options (IEU) |
. .
. .
+------+-------------------+
| 0xFF |
+------+------------------------+
| |
| Payload |
| |
+-------------------------------+
/ \
/ \
/ \
/ \
Outer Header v v Plaintext
+-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF |
. . +-------+-----------+
. OSCORE Option . | |
+------+-------------------+ | Payload |
| 0xFF | | |
+------+ +-------------------+
Figure 7: OSCORE inner and outer header form a CoAP message
Figure 7 shows the message format for the OSCORE Message and Plaintext. In the Outer Header, the original message code is hidden and replaced by a default value (POST or FETCH) depending on whether the original message was a Request or a Response. The original message code is put into the first byte of the Plaintext. Following the message code come the class E options and if present the original message Payload preceded by its payload marker.
The Plaintext is now encrypted by an AEAD algorithm which integrity protects Security Context parameters and eventually any class I options from the Outer Header. Currently no CoAP options are marked class I. The resulting Ciphertext becomes the new Payload of the OSCORE message, as illustrated in Figure 8.
Outer Header
+-+-+---+--------+---------------+
|v|t|tkl|new code| Msg Id. |
+-+-+---+--------+---------------+....+
| Token |
+--------------------------------.....+
| Options (IU) |
. .
. OSCORE Option .
+------+-------------------+
| 0xFF |
+------+-------------------------+
| |
| Encrypted Inner Header and |
| Payload |
| |
+--------------------------------+
Figure 8: OSCORE message
The SCHC Compression scheme consists of compressing both the Plaintext before encryption and the resulting OSCORE message after encryption, see Figure 9. This way compression reaches all fields of the original CoAP message.
Outer Message OSCORE Plaintext
+-+-+---+--------+---------------+ +-------+
|v|t|tkl|new code| Msg Id. | | code |
+-+-+---+--------+---------------+....+ +-------+-----......+
| Token | | Options (E) |
+--------------------------------.....+ +-------+------.....+
| Options (IU) | | OxFF |
. . +-------+-----------+
. OSCORE Option . | |
+------+-------------------+ | Payload |
| 0xFF | | |
+------+------------+ +-------------------+
| Ciphertext |<---------\ |
| | | v
+-------------------+ | +-----------------+
| | | Inner SCHC |
v | | Compression |
+-----------------+ | +-----------------+
| Outer SCHC | | |
| Compression | | v
+-----------------+ | +-------+
| | |Rule ID|
v | +-------+--+
+--------+ +------------+ | Residue |
|Rule ID'| | Encryption | <--- +----------+--------+
+--------+--+ +------------+ | |
| Residue' | | Payload |
+-----------+-------+ | |
| Ciphertext | +-------------------+
| |
+-------------------+
Figure 9: OSCORE Compression Diagram
In what follows we present an example GET Request and consequent CONTENT Response and show a possible set of rules for the Inner and Outer SCHC Compression. We then show a dump of the results and contrast SCHC + OSCORE performance with SCHC + COAP performance. This gives an approximation to the cost of security with SCHC-OSCORE.
Our first example CoAP message is the GET Request in Figure 10
Original message:
=================
0x4101000182bb74656d7065726174757265
Header:
0x4101
01 Ver
00 CON
0001 tkl
00000001 Request Code 1 "GET"
0x0001 = mid
0x82 = token
Options:
0xbb74656d7065726174757265
Option 11: URI_PATH
Value = temperature
Original msg length: 17 bytes.
Figure 10: CoAP GET Request
Its corresponding response is the CONTENT Response in Figure 11.
Original message:
=================
0x6145000182ff32332043
Header:
0x6145
01 Ver
10 ACK
0001 tkl
01000101 Successful Response Code 69 "2.05 Content"
0x0001 = mid
0x82 = token
0xFF Payload marker
Payload:
0x32332043
Original msg length: 10
Figure 11: CoAP CONTENT Response
The SCHC Rules for the Inner Compression include all fields that are already present in a regular CoAP message, what matters is the order of appearance and inclusion of only those CoAP fields that go into the Plaintext, Figure 12.
Rule ID 0 +----------------+--+--+-----------+-----------+-----------++--------+ | Field |FP|DI| Target | MO | CDA || Sent | | | | | Value | | || [bits] | +----------------+--+--+-----------+-----------+-----------++--------+ |CoAP Code | |up| 1 | equal |not-sent || | |CoAP Code | |dw|[69,132] | match-map |match-sent || c | |CoAP Uri-Path | |up|temperature| equal |not-sent || | |COAP Option-End | |dw| 0xFF | equal |not-sent || | +----------------+--+--+-----------+-----------+-----------++--------+
Figure 12: Inner SCHC Rules
The Outer SCHC Rules (Figure 13) must process the OSCORE Options fields. Here we mask off the repeated bits (most importantly the flag and size bits) with the MSB Mathing Operator.
Rule ID 0 +---------------+--+--+--------------+---------+-----------++------------+ | Field |FP|DI| Target | MO | CDA || Sent | | | | | Value | | || [bits] | +---------------+--+--+--------------+---------+-----------++------------+ |CoAP version | |bi| 01 |equal |not-sent || | |CoAP Type | |up| 0 |equal |not-sent || | |CoAP Type | |dw| 2 |equal |not-sent || | |CoAP TKL | |bi| 1 |equal |not-sent || | |CoAP Code | |up| 2 |equal |not-sent || | |CoAP Code | |dw| 68 |equal |not-sent || | |CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP OSCORE_piv| |up| 0x0900 |MSB(12) |LSB ||PPPP | |COAP OSCORE_kid| |up|b'\x06client' |MSB(52) |LSB ||KKKK | |CoAP OSCORE_piv| |dw| b'' |equal |not-sent || | |COAP Option-End| |dw| 0xFF |equal |not-sent || | +---------------+--+--+--------------+---------+-----------++------------+
Figure 13: Outer SCHC Rules
Next we show a dump of the compressed message:
Compressed message: ================== 0x00291287f0a5c4833760d170 0x00 = Rule ID piv = 0x04 Compression residue: 0b0001 010 0100 0100 (15 bits -> 2 bytes with padding) mid tkn piv kid Payload 0xa1fc297120cdd8345c Compressed message length: 12 bytes
Figure 14: SCHC-OSCORE Compressed GET Request
Compressed message: ================== 0x0015f4de9cb814c96aed9b1d981a3a58 0x00 = Rule ID Compression residue: 0b0001 010 (7 bits -> 1 byte with padding) mid tkn Payload 0xfa6f4e5c0a64b576cd8ecc0d1d2c Compressed msg length: 16 bytes
Figure 15: SCHC-OSCORE Compressed CONTENT Response
For contrast, we compare these results with what would be obtained by SCHC compressing the original CoAP messages without protecting them with OSCORE. To do this, we compress the CoAP mesages according to the SCHC rules in Figure 16.
Rule ID 1 +---------------+--+--+-----------+---------+-----------++------------+ | Field |FP|DI| Target | MO | CDA || Sent | | | | | Value | | || [bits] | +---------------+--+--+-----------+---------+-----------++------------+ |CoAP version | |bi| 01 |equal |not-sent || | |CoAP Type | |up| 0 |equal |not-sent || | |CoAP Type | |dw| 2 |equal |not-sent || | |CoAP TKL | |bi| 1 |equal |not-sent || | |CoAP Code | |up| 2 |equal |not-sent || | |CoAP Code | |dw| [69,132] |equal |not-sent || | |CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP Uri-Path | |up|temperature|equal |not-sent || | |COAP Option-End| |dw| 0xFF |equal |not-sent || | +---------------+--+--+-----------+---------+-----------++------------+
Figure 16: SCHC-CoAP Rules (No OSCORE)
This yields the results in Figure 17 for the Request, and Figure 18 for the Response.
Compressed message: ================== 0x0114 0x01 = Rule ID Compression residue: 0b00010100 (1 byte) Compressed msg length: 2
Figure 17: CoAP GET Compressed without OSCORE
Compressed message: ================== 0x010a32332043 0x01 = Rule ID Compression residue: 0b00001010 (1 byte) Payload 0x32332043 Compressed msg length: 6
Figure 18: CoAP CONTENT Compressed without OSCORE
As can be seen, the difference between applying SCHC + OSCORE as compared to regular SCHC + COAP is about 10 bytes of cost.