lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational L. Toutain
Expires: June 8, 2017 Institut MINES TELECOM ; TELECOM Bretagne
December 5, 2016
6LPWA Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-00
Abstract
This draft discusses the way SCHC can be applied to CoAP headers and
extend the number of functions (CDF) to optimize compression.
Status of This Memo
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1. Introduction
[I-D.toutain-lpwan-ipv6-static-context-hc] defines a compression
technique for LPWA network based on static context. This context is
said static since the element values composing the context are not
learned during packet exchanges but previously installed. The
context is known by both ends. A context is composed of a set of
rules (referenced by rule ids). A rule describes the header fields
with some associated Target Values (TV). A Matching Operator (MO) is
associated to each field. The rule is selected if all the MO matches
. A Compression Decompression Function is associated to each field to
define the link between the compressed and decompressed value for a
specific field.
This draft discusses the way SCHC can be applied to CoAP headers and
extend the number of functions (CDF) to optimize compression.
2. Compressing CoAP
CoAP [RFC7252] is an implementation of a the REST architecture for
contrained devices. Gateway between CoAP and HTTP can be easily
build since both protocol uses the same address space (URL), caching
mechanisms and methods.
Nevertheless, if limited, the size of a CoAP header may be
incompatible with LPWAN constraints and some compression may be
needed to reduce the header size. CoAP compression is not
straightforward. Some differences between IPv6/UDP and CoAP can be
enlighten. CoAP differs from IPv6 and UDP protocols:
o IPv6 and UDP are symmetrical protocols. The same fields are found
in the request and in the answer, only location in the header may
change (e.g. source and destination fields). A CoAP request is
different from an answer. For instance, the URI-path option is
mandatory in the request and may not be found in the response.
o CoAP also obeys to the client/server paradigm and the compression
rate can be different if the request is issued from a LPWAN node
or from an external device. For instance in the former case the
token size may be set to one byte. In the latter case, the token
size cannot be constraint and be up to 15 byte long.
o In IPv6, main header and UDP fields have a fixed size. In CoAP,
Token size may vary from 0 to 15 bytes, length is given by a field
in the header. More systematically, the options are described
using the Type-Length-Value principle. Evenmore regarding the
option size value, the coding will be different.
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o options type in CoAP are not defined with the same value. The
Delta TLV coding makes that the type is not independant of
previous option and may vary regarding the options contained in
the header.
2.1. CoAP usages
A LPWAN node can either be a client or a server and sometimes both.
In the client mode, the LPWAN node sends request to a server and
expected answer or acknowledgements. Acknowledgements can be at 2
different levels:
o transport level, a CON message is acknowledged by an ACK message.
NON confirmable messages are not acknowledged.
o REST level, a REST request is acknowledged by an "error" code.
[RFC7967] defines an option to limit the number of
acknowledgements.
Note that acknowledgement can be optimized and a REST level
acknowledgement can be used as a transport level acknowledgement.
2.2. CoAP protocol analysis
CoAP defines the following fields:
o version (2 bits): this field can be elided during a compresssion
o type (2 bits): defines the type of the transport messages, 4
values are defined. Regarding the type of exchange, if only NON
messages are sent or CON/ACK messages, this field can be reduced
to 0 or 1 bit.
o token length (4 bytes). The standard allows up to 15 bytes for a
token length. If a fix token size is chosen, then this field can
be elided. If some variation in length are needed then 1 or 2
bits could be enough for most of LPWAN applications.
o code (8 bits). This field codes the request and the response
values. CoAP represents in a more compact way, coding used in
HTTP, but the coding is not optimal.
o message id (16 bits). This value is used to acknowledge CON
frames. The size of this field is computed to allow the
anticipation (how many frames can be sent without
acknowledgement). When a value is used, [RFC7252] defines the
time before it can be reused without ambiguities. This size may
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be too large for a LPWAN node sending or receiving few messages a
day.
o Token (0 to 15 bytes). Token identifies active flows. Regarding
the usage (stability of in time and limited number), a short token
(1 Byte) can be enough.
o options are coded through delta-TLV. The delta-T depends of
previous values, length is encoded inside the option. [RFC7252]
distinguishes repeatable options that can appear several time in
the header. Among them we can distinguish:
* list options which appear several time in the header but are
exclusive such as the Accept option.
* cumulative options which appear several time in the header but
are part of a more generic value such as Uri-Path and Uri-
Query.
For a given flow some value options are stable through time.
Observe, ETag, If-Match, If-None-Match and Size varies in each
message. Options can be stored in a SCHC context and cumulative
options can be stored globally.
The CoAP protocol must not be altered by the compression/
decompression phase, but if no semantic is attributed to a value, it
may be changed during this phase. For instance the compression phase
may reduce the size of a token but must maintain its unicity. The
decompressor will not be able to restore the original value but
behavior will remain the same. If no special semantic is assigned to
the token, this will be transparent. If a special semantic is
assigned to the token, its compression may not be possible.
This implies that the compressor/decompressor must be aware of the
protocol state machine and do not processes request and response the
same way.
A conservative compression leaves the field value unchanged. Non
conservative compression can be used when a CoAP implementation has
not been defined to work specifically with LPWAN network and uses
large values for fields.
2.2.1. CoAP Compression Decompression Function
To compress more efficiently CoAP message, several Compression/
Decompression Function (CDF) must be defined.
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2.2.1.1. Static-mapping
The goal of static-mapping is to reduce the size of a field by
allocating shorter value. The mapping is known by both ends and
stored in a table in both end context. The Static-mapping is
conservative.
Static-mapping may be applied to several fields. For instance the
type field may be reduced from 2 bits to 1 bit if only CON/ACK type
is used, but the main benefit is compressing the code field.
The CoAP code field defines a tricky way to ensure compatibility with
HTTP values. Nevertheless only 21 values are defined by [RFC7252]
compared to the 255 possible values. So it could efficiently be
coded on 5 bits. To allow flexibility and evolution if new codes are
introduced, a static mapping table is associated to each instance of
this function.
Figure 1 gives a possible mapping, it can be changed to add new codes
or reduced if some values are never used by both ends.
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+------+------------------------------+-----------+
| Code | Description | Mapping |
+------+------------------------------+-----------+
| 0.00 | | 0x00 |
| 0.01 | GET | 0x01 |
| 0.02 | POST | 0x02 |
| 0.03 | PUT | 0x03 |
| 0.04 | DELETE | 0x04 |
| 0.05 | FETCH | 0x05 |
| 0.06 | PATCH | 0x06 |
| 0.07 | iPATCH | 0x07 |
| 2.01 | Created | 0x08 |
| 2.02 | Deleted | 0x09 |
| 2.03 | Valid | 0x0A |
| 2.04 | Changed | 0x0B |
| 2.05 | Content | 0x0C |
| 4.00 | Bad Request | 0x0D |
| 4.01 | Unauthorized | 0x0E |
| 4.02 | Bad Option | 0x0F |
| 4.03 | Forbidden | 0x10 |
| 4.04 | Not Found | 0x11 |
| 4.05 | Method Not Allowed | 0x12 |
| 4.06 | Not Acceptable | 0x13 |
| 4.12 | Precondition Failed | 0x14 |
| 4.13 | Request Entity Too Large | 0x15 |
| 4.15 | Unsupported Content-Format | 0x16 |
| 5.00 | Internal Server Error | 0x17 |
| 5.01 | Not Implemented | 0x18 |
| 5.02 | Bad Gateway | 0x19 |
| 5.03 | Service Unavailable | 0x1A |
| 5.04 | Gateway Timeout | 0x1B |
| 5.05 | Proxying Not Supported | 0x1C |
+------+------------------------------+-----------+
Figure 1: CoAP code mapping
This CDF can also be applied to path to send a reference instead of
the path value.
2.2.1.2. Remapping
With dynamic mapping, the mapping is done dynamically, which means
that the other end has no way to the learn the original value. This
function is not conservative. The mapping context must be stored in
a reliable way on the compressor, if lost the session with LPWAN node
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will be lost, which can generate a traffic increase on the LPWA
network.
This function converts a large number to a smaller one and maintain
bi-directional mapping. If the field has no semantic, such as a CoAP
token or a message ID, this will reduce the size of the information
sent on the link. This mapping only applies for request compression,
answers must keep the value original value.
For instance a compression receives a CoAP request with a large
token. The compressor reduces the token size by allocating a unused
value in a smaller space. When the response come back, the
compressor exchange the smallest token with the original one.
This mean that the compressor must be aware of the CoAP state
machine, to identify a request and its associated response, but also
determine when a token value can be reused.
2.2.1.3. Reduce-entropy
Reduce-entropy is a non-conservative function. the goal is to
minimize the increase in a field value. It may be used for the
observe option, all increase in the original sequence number will
lead to an increase of 1 in the compressed value.
For instance a LPWAN node is a CoAP server and receives Observe
responses coming from an outside client. The client uses a clock to
generate Observe sequence number. If that value has non particular
meaning for the CoAP server, increase of 1 will not change the
protocol behavior. Reordering works the same way as for original
Observe.
2.2.2. CoAP mandatory header
Figure 2 proposes some function assignments to the CoAP header
fields.
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/--------------------+---------------------+----------------------------------------\
| Field |Function | Behavior |
+--------------------+---------------------+----------------------------------------+
|version |not-sent |version is always the same |
+--------------------+---------------------+----------------------------------------+
|type |value-sent |if all the types are used |
| |static-mapping |to reduce to one bit if 2 type are used |
| |not-sent |if only one type is used (e.g. NON) |
+--------------------+---------------------+----------------------------------------+
|token length |not-sent |no tokens or fixed size |
| |compute-token-length |if token size is reduced |
| |value-sent |token is sent integrally |
+--------------------+---------------------+----------------------------------------+
|code |value-sent |no modification |
| |static-mapping |code size reduction |
+--------------------+---------------------+----------------------------------------+
|message id |value-sent |no modification |
|token |remapping |reduces message id size |
+====================+=====================+========================================+
|Content-Format |value-sent |no modification |
|Accept |not-sent |defined in the rule |
|Max-Age |static-mapping |map the possible value |
+--------------------+---------------------+----------------------------------------+
|Path: |value-sent |no modification |
|Uri-Host+Uri-Port+ |not-sent |defined in the rule |
|Uri-Path*+Uri-Query*|static-mapping |a value to define a path |
| | | |
|Proxy-Uri | |Note: only the full path is stored in |
|Proxy-Scheme | |context |
+--------------------+---------------------+----------------------------------------+
|ETag |value-sent |Always sent |
|Location-Path | | |
|Location-Query | | |
|If-Match | | |
|If-None-Match | | |
|Size1 | | |
+--------------------+---------------------+----------------------------------------+
Figure 2: SCHC functions' example assignment for CoAP
2.2.3. Examples of CoAP header compression
2.2.3.1. 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 ES.
For this simple scenario, the rules are described Figure 3
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rule id 1
+-------------+-------+-----+---------------+----------------+
| Field |TV |MO |CDF | Sent |
+=============+=======+=====+===============+================+
|CoAP version | 01 |= |not-sent | |
|CoAP Type | | |value-sent |TT |
|CoAP TKL | 0000 |= |not-sent | |
|CoAP Code | | |static-map | CC CCC |
|CoAP MID | | |dynamic-map | M-ID |
|CoAP Path |/path | |not-sent | |
+-------------+-------+-----+---------------+----------------+
Figure 3: CoAP Context to compress header without token
Figure 3 gives a simple compression rule for CoAP headers without
tokens.
The version fields and Token Length are elided. Code has shrunk to 5
bits using the static-mapping function. Message-ID has shrunk to 9
bits to preserve alignment on byte boundary.
Figure 4 shows the time diagram of the exchange. A LPWAN Application
Server sends a CON message. Compression reduces the header sending
only the Type, a mapped code and the Message ID is change to a value
on 9 bits. The receiver decompress the header. The message ID value
is changed.
The CON message is a request, therefore the LC process to a dynamic
mapping. When the ES receives the ACK message, this will not
initiate locally a the message ID mapping since it is a response.
The LC receives the ACK and uncompress it to restore the original
value. Dynamic Mapping context lifetime follows the same rules as
message ID duration.
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End System LPWA LC
| |
| rule id=1 |<----------------------
|<---------------------------| +-+-+--+----+--------+
<-------------------- | TTCC CCCM MMMM MMMM | |1|0| 4|0.01| 0x1234 |
+-+-+--+----+--------+ | 0000 0010 0000 0001 | | 0xb4 p a t |
|1|0| 1|0.01| 0x0001 | | | | h |
| 0xb4 p a t | | | +------+
| h | | | dynamic mapping
+------+ | | +--------+--------+
| | |0x1234 | 0x01 |
| | +--------+--------+
----------------------->| rule id=1 |
+-+-+--+----+--------+ |--------------------------->|
|1|2| 0|2.05| 0x0001 | | TTCC CCCM MMMM MMMM |------------------------>
+-+-+--+----+--------+ | 1000 0000 0000 0001 | +-+-+--+----+--------+
| | |1|2| 0|2.05| 0x1234 |
v v +-+-+--+----+--------+
Figure 4: Compression with global addresses
Note that the compressor and decompressor must understand the CoAP
protocol:
o The LC compressor detects a new transport request and allocate a
new dynamic mapping value.
o When receiving a response the ES compressor ES detects that this
is a response (type=2) therefore the message ID value in
unchanged.
o The upstream compressor detects that is an REST answer (code 2.05)
therefore the path option is not inserted in the uncompress header
2.2.3.2. Exchange with token
The following scenario introduces tokens. The LC manages two
remapping contexts. One for Message ID and the other for token. ES
manages one context for Message ID. Mapping is trigged by the
reception of CON messages to compress or CoAP requests to compress.
Note that the compressed message ID size has been reduced to 7 bits,
compared to the previous example, to maintain byte boundary
alignment.
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+----------------+------------------------+----------------+-----------------+
| Field | Function | Ctxt Value | Sent compressed |
+----------------+------------------------+----------------+-----------------+
|CoAP version | not-sent | | |
|CoAP Type | value-sent | |TT |
|CoAP TKL | compute-token-length | | LL |
|CoAP Code | map-code | mapping table | CCCC C |
|CoAP MID | remapping | 7 bits | M-ID |
|CoAP Token | remapping | 8 bits | token|
|CoAP Path | not-sent |/data/humidity |
+----------------+------------------------+----------------+-----------------+
Figure 5: CoAP Context to compress header with token
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End System LPWA LC
| |
| SHIM=1 |<----------------------
|<---------------------------| +-+-+--+----+--------+
<-------------------- | TT LL CCCC C MMMMMMM | |1|0| 4|0.01| 0x1234 |
+-+-+--+----+--------+ | 00 01 0000 1 0000001 | | DEADBEEF |
|1|0| 1|0.01| 0x0001 | | 0000 0001 | | 0xb4 d a t |
| 01 0xb4 d a | | Token | | a 0x08 h u |
| t a 0x08 h | | | | m i d i |
| u m i d | | | | t y |
| i t y | | | +------------+
+-----------------+ | | Mid mapping: 1234 -> 1
| | Tk mapping: DEADBEEF -> 1
----------------------->| SHIM=1 |
+-+-+--+----+--------+ |--------------------------->|
|1|2| 0|0.00| 0x0001 | | TT LL CCCC C MMMMMMMM |------------------------>
+-+-+--+----+--------+ | 10 01 0000 0 00000001 | +-+-+--+----+--------+
| | |1|2| 0|0.00| 0x1234 |
| | +-+-+--+----+--------+
----------------------->| |
+-+-+--+----+--------+ |--------------------------->|
|1|0| 0|2.05| 0xCAFE | | TT LL CCCC C MMMMMMMM |------------------------>
| 0x01 2 5 | | 00 01 1100 0 00000002 | +-+-+--+----+--------+
+--------------------+ | 0000 0001 | |1|0| 4|2.05| 0x0001 |
| 2 5 | | DEADBEEF |
| | | 2 5 |
Mid mapping: CAFE -> 1 | | +-----------+
| |
| |<------------------------
|<---------------------------| +-+-+--+----+--------+
<-----------------------| TT LL CCCC C MMMMMMMM | |1|2| 0|0.00|0x0001 |
+-+-+--+----+--------+ | 10 00 0000 0 00000002 | +-+-+--+----+--------+
|1|2| 0|0.00| 0xCAFE | | |
+-+-+--+----+--------+ | |
v v
Figure 6: Compression with token
3. Normative References
[I-D.toutain-lpwan-ipv6-static-context-hc]
Minaburo, A. and L. Toutain, "LPWAN Static Context Header
Compression (SCHC) for IPv6 and UDP", draft-toutain-lpwan-
ipv6-static-context-hc-00 (work in progress), September
2016.
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[RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC 1332, DOI 10.17487/RFC1332, May 1992,
<http://www.rfc-editor.org/info/rfc1332>.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, DOI 10.17487/RFC3095,
July 2001, <http://www.rfc-editor.org/info/rfc3095>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<http://www.rfc-editor.org/info/rfc4944>.
[RFC4997] Finking, R. and G. Pelletier, "Formal Notation for RObust
Header Compression (ROHC-FN)", RFC 4997,
DOI 10.17487/RFC4997, July 2007,
<http://www.rfc-editor.org/info/rfc4997>.
[RFC5225] Pelletier, G. and K. Sandlund, "RObust Header Compression
Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
UDP-Lite", RFC 5225, DOI 10.17487/RFC5225, April 2008,
<http://www.rfc-editor.org/info/rfc5225>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<http://www.rfc-editor.org/info/rfc6282>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <http://www.rfc-editor.org/info/rfc7967>.
Authors' Addresses
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Ana Minaburo
Acklio
2bis rue de la Chataigneraie
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Laurent Toutain
Institut MINES TELECOM ; TELECOM Bretagne
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@telecom-bretagne.eu
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