A Low-Power CoAP for Contiki

Monday, 17 Oct 2011

A Low-Power CoAP for ContikiMatthias Kovatsch, Simon Duquennoy, and Adam [email protected] [email protected] [email protected]

mailto:[email protected]



A Low-Power CoAP for ContikiMatthias Kovatsch – ETH Zürich 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


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

/ link-layer bursts

A Low-Power CoAP for ContikiMatthias Kovatsch – ETH Zürich

ContikiMAC

4

A

D

A

D A

ACK frame

Data frame

Radio on

Sender

Receiver

Transmission detected

DD DD

Channel check


Link-layer Bursts

5

A

AD

Sender

Receiver


DD DD

Channel check

A

AD

D A

AD

D A

AD

D

D

A ACK frame

Data frame

Radio on


Constrained Application Protocol (CoAP)

RESTful Web services for networked embedded devices Idealized architectural style of the Web HTTP for the Internet of Things

6

coap://sky3/

coap://sky2/

coap://sky1/

GET /sensors/temperature


Implements draft-ietf-core-coap-07 Available in the Contiki repository on SourceForge

«Erbium» CoAP for Contiki

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«Erbium» CoAP

Reliable UDP transport

Observing resources

Blockwise transfers

Resource discovery

Separate responses

Blocking client requests

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/* Handle requests. */if (message->code >= COAP_GET && message->code <= COAP_DELETE) { /* Use transaction buffer for response to confirmable request. */ if ( (transaction = coap_new_transaction(message->tid, &UIP_IP_BUF->srcipaddr, UIP_UDP_BUF->srcport)) ) { static uint32_t block_num = 0; static uint16_t block_size = REST_MAX_CHUNK_SIZE; static uint32_t block_offset = 0; static int32_t new_offset = 0; if (message->type==COAP_TYPE_CON) { /* Reliable CON requests are answered with an ACK. */ coap_init_message(response, COAP_TYPE_ACK, CONTENT_2_05, message->tid); } else { /* Unreliable NON requests are answered with a NON as well. */ coap_init_message(response, COAP_TYPE_NON, CONTENT_2_05, coap_get_tid()); } /* resource handlers must take care of different handling (e.g., TOKEN_OPTION_REQUIRED_240) */ if (IS_OPTION(message, COAP_OPTION_TOKEN)) { coap_set_header_token(response, message->token, message->token_len); SET_OPTION(response, COAP_OPTION_TOKEN); } /* get offset for blockwise transfers */ if (coap_get_header_block2(message, &block_num, NULL, &block_size, &block_offset)) { block_size = MIN(block_size, REST_MAX_CHUNK_SIZE); new_offset = block_offset; } else { new_offset = 0; } /* Invoke resource handler. */ if (service_cbk) { /* Call REST framework and check if found and allowed. */ if (service_cbk(message, response, transaction->packet+COAP_MAX_HEADER_SIZE, block_size, &new_offset)) { /* Apply blockwise transfers. */ if ( IS_OPTION(message, COAP_OPTION_BLOCK2) ) { /* unchanged new_offset indicates that resource is unaware of blockwise transfer */ if (new_offset==block_offset) { if (block_offset >= response->payload_len) { response->code = BAD_OPTION_4_02; coap_set_payload(response, (uint8_t*)"Block out of scope", 18); } else { coap_set_header_block2(response, block_num, response->payload_len - block_offset > block_size, block_size); coap_set_payload(response, response->payload+block_offset, MIN(response->payload_len - block_offset, block_size)); } /* if (valid offset) */ } else { /* resource provides chunk-wise data */ coap_set_header_block2(response, block_num, new_offset!=-1 || response->payload_len > block_size, block_size); if (response->payload_len > block_size) coap_set_payload(response, response->payload, block_size); } /* if (resource aware of blockwise) */ } else if (new_offset!=0) { coap_set_header_block2(response, 0, new_offset!=-1, REST_MAX_CHUNK_SIZE); coap_set_payload(response, response->payload, MIN(response->payload_len, REST_MAX_CHUNK_SIZE)); } /* if (blockwise request) */ } } else { coap_error_code = INTERNAL_SERVER_ERROR_5_00; coap_error_message = "Service callback undefined"; } /* if (service callback) */ /* serialize Response. */ if ((transaction->packet_len = coap_serialize_message(response, transaction->packet))==0) { coap_error_code = PACKET_SERIALIZATION_ERROR; } } else { coap_error_code = MEMORY_ALLOC_ERR; coap_error_message = "Transaction buffer allocation failed"; } /* if (transaction buffer) */} else { /* Responses */ if (message->type==COAP_TYPE_ACK) { /* Handled by transaction layer */ } else if (message->type==COAP_TYPE_RST) { /* Cancel possible subscriptions. */ if (IS_OPTION(message, COAP_OPTION_TOKEN)) { coap_remove_observer_by_token(&UIP_IP_BUF->srcipaddr, UIP_UDP_BUF->srcport, message->token, message->token_len); }}


«Erbium» REST Engine

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

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Observable

CoRE Link Formattitle="Hello world"; rt="TemperatureC";


«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

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Experimental Set-up

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3 hops

2 hops1 hop

Energy data over USB

100 Requests each

Laptop withborder router

Static routing

4 hops


Energy vs Latency for 64B Payload

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Number of hopsNumber of hops

Ene

rgy

[mJ]

Late

ncy

[s]

No duty cyclingContikiMAC

1 2 3 4

0

100

200

300

400

1 2 3 4

0

0.2

0.4

0.6

0.8

1.08 Hz channel check rate

0.6% RDC


Transmitting Large Data

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0 128 256 384 5120

5

10

15

20

25

30

35

40

45

50

55

60

65

70

0 128 256 384 5120

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Payload [Bytes]Payload [Bytes]

Ene

rgy

[mJ]

Late

ncy

[s]

Bursts avoid steps in latency

2 3 4 5 6

Steps become less noticable

4 hops2 hops1 hop


Transmitting Large Data

14

A

AD

Sender

Receiver


DD DD

Channel check

AD

AD

AD A

AD

D

D

A ACK frame

Data frame

Radio on

AD


Separate Responses

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Client ClientServer Server

Requestprocessing

Request

Retry

Retry…

Response

Requestprocessing

Response

ACKRequest

ACK

Benchmarkand switch

automatically


Separate Responses over 4 Hops

16

WithoutWith

0 5 10 15 20 25

25

50

75

100

125

150

Ene

rgy

[mJ]

Server processing time [s]

Savings marginal


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

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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

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THANK YOU

Questions?


Radio Duty Cycling

20

CPU (normal/LPM)

Radio (RX/TX)

Flash (R/W/idle)

Sensors (tmp./

PIR/acc.)

LEDs0

5

10

15

20

25

Cur

rent

con

sum

ptio

n (c

a.) [

mA

]


Constrained Application Protocol (CoAP)

RESTful Web services for networked embedded devices Idealized architectural style of the Web Usually implemented with HTTP

Central mechanisms Reliable UDP transport («confirmable») Group communication (IP multicast) Push notifications («observing») Resource discovery («CoAP link format») Larger data transport («blockwise transfers»)

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coap://sky3/

coap://sky2/


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);

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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)


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);}

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Every x-th is CON to resolve orphans


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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A Low-Power CoAP for Contiki

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