Presentation at the IEEE Workshop on Internet of Things Technology and Architectures (IoTech 2011), Valencia, Spain. http://iotech-ws.com/

1. A Low-Power CoAP for Contiki
Matthias Kovatsch, Simon Duquennoy, and Adam Dunkels
kovatsch@inf.ethz.ch simonduq@sics.se adam@sics.se
Monday, 17 Oct 2011

2. 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 2

3. ContikiMAC Radio on
D Data frame
A ACK frame
Sender D D D D A
Receiver D A
Channel check Transmission detected
Matthias Kovatsch – ETH Zürich A Low-Power CoAP for Contiki 3

4. Link-layer Bursts Radio on
D Data frame
A ACK frame
Sender D D D D A D A D A D A
Receiver D A D A D A D A
Channel check Transmission detected
Matthias Kovatsch – ETH Zürich A Low-Power CoAP for Contiki 4

5. 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 5

6. «Erbium» CoAP for Contiki
 Implements draft-ietf-core-coap-07
 Available in the Contiki repository on SourceForge
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7. «Erbium» CoAP
 Reliable UDP transport
 Observing resources
 Blockwise transfers
 Resource discovery
 Separate responses
 Blocking client requests
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8. «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 8

9. «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 9

10. Experimental Set-up
4 hops
Energy data over USB
100 Requests each
3 hops
Laptop with Static routing
border router
1 hop 2 hops
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11. No duty cycling
ContikiMAC
Energy vs Latency for 64B Payload
8 Hz channel check rate
1.0
0.6% RDC
400
0.8
300
Energy [mJ]
Latency [s]
0.6
200
0.4
100
0.2
0
0
1 2 3 4 1 2 3 4
Number of hops Number of hops
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12. 4 hops
2 hops
Transmitting Large Data 1 hop
70
65
2 3 4 5 6 1.8
Bursts avoid
60
1.6 steps in latency
55
1.4
50
45 1.2
Energy [mJ]
Latency [s]
40
1.0
35
30 Steps become 0.8
25 less noticable
0.6
20
15 0.4
10
0.2
5
0 0
0 128 256 384 512 0 128 256 384 512
Payload [Bytes] Payload [Bytes]
Matthias Kovatsch – ETH Zürich A Low-Power CoAP for Contiki 12

13. Transmitting Large Data Radio on
D Data frame
Number of frames sent depends on
link quality, not number of fragments A ACK frame
Sender D D D D A D D D A
Receiver D A D A
Channel check Transmission detected
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14. Separate Responses
Client Server Client Server
Request Request
ACK
Retry
Request Request
processing processing
Retry
… Benchmark
and switch
Response automatically Response
ACK
Matthias Kovatsch – ETH Zürich A Low-Power CoAP for Contiki 14

15. Without
With
Separate Responses over 4 Hops
Savings marginal for CoAP’s default parameters
150
125
Energy [mJ]
100
75 Retransmissions after
2-3, 4-6, 8-12, and
50 16-24 seconds
25
0 5 10 15 20 25
Server processing time [s]
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16. 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 16

17. 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»-based App Server
https://github.com/mkovatsc/Californium
Matthias Kovatsch – ETH Zürich A Low-Power CoAP for Contiki 17

18. THANK YOU
Questions?
Matthias Kovatsch – ETH Zürich A Low-Power CoAP for Contiki 18

19. 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 Chunk-aware: use offset and
REST_MAX_CHUNK_SIZE fill buffer up to preferred_size
(update offset and set to -1 when finished)
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20. 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 20

21. «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)
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