A Low-Power CoAP for Contiki
Matthias Kovatsch, Simon Duquennoy, and Adam Dunkels
kovatsch@inf.ethz.ch
Monday, 17 Oct 2011
simonduq@sics.se
adam@sics.se
Internet of Things Protocol Stack
Layer
Protocol
Application
CoAP / REST Engine (Erbium)
Transport
UDP
Network
IPv6 / RPL
Adaption
6LoWPAN
MAC
CSMA / link-layer bursts
Radio Duty Cycling
ContikiMAC
Physical
IEEE 802.15.4
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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Internet of Things Protocol Stack
Layer
Protocol
Application
CoAP / REST Engine (Erbium)
Transport
UDP
Network
IPv6 / RPL
Adaption
6LoWPAN
MAC
CSMA / link-layer bursts
Radio Duty Cycling
ContikiMAC
Physical
IEEE 802.15.4
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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ContikiMAC
Radio on
D Data frame
A ACK frame
Sender
D
Receiver
Channel check
Matthias Kovatsch – ETH Zürich
D
D
D A
D A
Transmission detected
A Low-Power CoAP for Contiki
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Link-layer Bursts
Radio on
D Data frame
A ACK frame
Sender
D
Receiver
Channel check
Matthias Kovatsch – ETH Zürich
D
D
D A D A D A D A
D A D A D A D A
Transmission detected
A Low-Power CoAP for Contiki
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Constrained Application Protocol (CoAP)
 RESTful Web services for networked embedded devices
 Idealized architectural style of the Web
 HTTP for the Internet of Things
coap://sky2/
coap://sky1/
coap://sky3/
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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«Erbium» CoAP for Contiki
 Implements draft-ietf-core-coap-07
 Available in the Contiki repository on SourceForge
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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«Erbium» CoAP
 Reliable UDP transport
 Observing resources
 Blockwise transfers
 Resource discovery
 Separate responses
 Blocking client requests
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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«Erbium» REST Engine
CoRE Link Format
title="Hello world";
rt="TemperatureC";
 Resource abstraction
RESOURCE(handle, METHODs, URI-Path, Web Linking info);
 Resource handler
PERIODIC_RESOURCE(handle, METHODs, URI-Path, info, period);
 Additional periodic handler
EVENT_RESOURCE(handle, METHODs, URI-Path, info);
 Additional event handler
Observable
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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«Erbium» Memory Footprint
ROM [kB]
RAM [kB]
CoAP REST Engine total
8.5
1.5
Measured stack usage
–
0.1
REST Engine
0.7
0
CoAP-07 base
4.5
0
CoAP-07 server
1.9
0.3
CoAP-07 transport
0.4
0.9
CoAP-07 observing
0.9
0.2
CoAP-07 separate
0.1
0
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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Experimental Set-up
4 hops
Energy data over USB
100 Requests each
3 hops
Static routing
Laptop with
border router
1 hop
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
2 hops
11
No duty cycling
ContikiMAC
Energy vs Latency for 64B Payload
1.0
400
8 Hz channel check rate
0.6% RDC
300
Latency [s]
Energy [mJ]
0.8
200
100
0.6
0.4
0.2
0
0
1
Matthias Kovatsch – ETH Zürich
2
3
4
Number of hops
1
A Low-Power CoAP for Contiki
2
3
4
Number of hops
12
4 hops
2 hops
1 hop
Transmitting Large Data
70
2
65
3
4
5
6
1 .8
60
Bursts avoid
steps in latency
1 .6
55
1 .4
50
1 .2
40
35
Steps become
less noticable
30
25
Latency [s]
Energy [mJ]
45
1 .0
0 .8
0 .6
20
15
0 .4
10
0 .2
5
0
0
128
256
384
512
0
Payload [Bytes]
Matthias Kovatsch – ETH Zürich
0
128
256
384
512
Payload [Bytes]
A Low-Power CoAP for Contiki
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Transmitting Large Data
Radio on
D Data frame
A ACK frame
Sender
D
Receiver
Channel check
Matthias Kovatsch – ETH Zürich
D
D
D A D A D A D A
D A D AD A D A
Transmission detected
A Low-Power CoAP for Contiki
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Separate Responses
Client
Server
Client
Server
Request
Request
ACK
Retry
Request
processing
Request
processing
Retry
…
Response
Benchmark
and switch
automatically
Response
ACK
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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Without
With
Separate Responses over 4 Hops
150
Savings marginal
Energy [mJ]
125
100
75
50
25
0
5
10
15
20
25
Server processing time [s]
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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Conclusion
 Internet of Things stack




Isolated layers with different goals work together
Layered architecture lowers complexity
Optimize for single fragments
No need to optimize payload size once fragmented
 CoAP
 Draft 07 implementation for Contiki
 Block size most important parameter for memory-constrained devices:
Exponential sizes too coarse-grained
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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Future Work
 Extend «Erbium» CoAP
 Blocking resource handler for UART
 Hierarchical resources / URI-Path handling
 Improve observer handling (only one buffer)
 Study RESTful approach
 Long-term deployment with smart appliances
 Separate application logic from device firmware
and combine with «Californium» App Server
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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THANK YOU
Questions?
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
19
Radio Duty Cycling
Current consumption (ca.) [mA]
25
20
15
10
5
0
CPU
(normal/LPM)
Matthias Kovatsch – ETH Zürich
Radio
(RX/TX)
Flash
(R/W/idle)
A Low-Power CoAP for Contiki
Sensors
(tmp./PIR/acc.)
LEDs
20
Constrained Application Protocol (CoAP)
 RESTful Web services for networked embedded devices
 Idealized architectural style of the Web
 Usually implemented with HTTP
 Central mechanisms





coap://sky2/
Reliable UDP transport («confirmable»)
Group communication (IP multicast)
Push notifications («observing»)
Resource discovery («CoAP link format»)
Larger data transport («blockwise transfers»)
coap://sky3/
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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Blockwise Transfers
Automatic or controlled by handler
RESOURCE(handle, METHODs, URI-Path, Web Linking info);
 Resource handler
handle_handler(void* request, void* response,
uint8_t *buffer, uint16_t preferred_size,
int32_t *offset);
Unaware: fill buffer up to
REST_MAX_CHUNK_SIZE
Chunk-aware: use offset and
fill buffer up to preferred_size
(update offset and set to -1 when finished)
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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Observing Resources
Post-handler registers observers automatically
PERIODIC_RESOURCE(handle, METHODs, URI-Path, info, period);
 Additional periodic handler
EVENT_RESOURCE(handle, METHODs, URI-Path, info);
 Additional event handler
int handle_event_handler(resource_t *r) {
...
REST.notify_subscribers(r->url, CON/NON, seqno, buf, len);
}
Every x-th is CON to resolve orphans
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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Erbium REST Engine
 Manipulate messages with REST API
 REST.set_header_etag(response, etag_buf, etag_len);
 REST.get_query_variable(request, name, value_buffer);
 Link implementation





coap-03
coap-06
coap-07
…
HTTP (not yet implemented for Erbium)
Matthias Kovatsch – ETH Zürich
A Low-Power CoAP for Contiki
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