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Protocols used in Constrained Environments
Vidhya Gholkar @vgholkar
CONSTRAINED ENVIRONMENT
http://tools.ietf.org/html/draft-ietf-lwig-terminology-06| Class | Rough translation |
| 0 | Can't run IP stack securely |
| 1 | Integrated Security but can't employ full stack using HTTP over TLS |
| 2 | Small but benefit from efficient protocols that free resources for application or reduce op costs. Arduino image  http://creativecommons.org/licenses/by-sa/3.0/legalcode |
Constrained Application Protocol
- Binary Protocol
- Request-response, RESTful
- does not sit on TCP
- Can proxy to and from HTTP
- NOT HTTP Compression
- draft-ietf-core-coap-18
- draft-ietf-core-observe-11
- RFC 6690
Features
- URI support
- Discovery
- Observation
- Security - DTLS
URI
coap+sms://+441234567/garden/peas/watercoap://building.uk:5633/~room/occ.xml
Request/Response
CoAP message
| Ver | Version |
| T | Transaction Type |
| OC | Option Count |
| Code | Request Method |
| Message ID | Identifier |
CoAP Methods
| Message | Code |
|---|---|
| GET | 0.01 |
| POST | 0.02 |
| PUT | 0.03 |
| DELETE | 0.04 |
Resource Discovery
- CoAP servers provide list of all known resources.
GET /.well-known/core
rt
ct
if
sz
GET /.well-known/core?rt=radiation
2.05 Content
</sensors/radiation>;rt="geiger";if="sensor"
Observe
draft-ietf-core-observe-11
Security
- Datagram TLS
- Records are independent so that if i'th record is lost i+1'th can still be decrypted.
- Handshake messages queued.
- Application is responsible for dealing with loss, reordering and so on.
Code to play with
- Cantcoap https://github.com/staropram/cantcoap
- C COAP Erbium git://github.com/contiki-os/contiki.git
- NodeJS https://github.com/morkai/h5.coap
- Firefox plugin (!) https://addons.mozilla.org/en-US/firefox/addon/copper-270430/
- libcoap http://sourceforge.net/projects/libcoap/
- Arduino implementation https:// github.com/ dgiannakop/ Arduino-CoAP.
Light Weight Device Management
© 2013 Open Mobile Alliance Ltd. All Rights Reserved.
technical.openmobilealliance.org/Technical/release_program/docs/LightweightM2M/V1_0-20131210-C/OMA-TS-LightweightM2M-V1_0-20131210-C.pdf
MQ Telemetry Transport
- Light weight, Binary protocol.
- Topic based Publish - Subscribe.
- For "Q" cf. AMQP, STOMP etc.
- Most recent message can be stored by broker and a Last Will can be set.
- TCP with concomitant TLS/SSL Security.
- 3 levels of Quality of Service (QoS).
- CONNECT, PUBLISH, SUBSCRIBE, UNSUBSCRIBE and DISCONNECT (others).
- MQTT-S addresses TCP constraint to be standardised.
- IBM driven, now at OASIS.
PubSub
Topics and Wild cards
Topic Publish:
/Lille/nord/BldA/hallway/temperature
/Lille/sud/BldB/meetingroom/pressure
Wildcard subscribe:
/Lille/+/+/temperature
/Lille/sud/# Header and Payload
Core MQTT messages
| Message | Code | Description |
|---|---|---|
| CONNECT | 1 | Client request to connect to Server |
| PUBLISH | 3 | Publish message |
| SUBSCRIBE | 8 | Client Subscribe request |
| UNSUBSCRIBE | 10 | Client Unsubscribe |
| DISCONNECT | 14 | Client is Disconnecting |
Quality of Service
for many constrained devices likely that only QoS 0 available.
Interesting Flags
| PUBLISH | Retain so broker remembers the last published value. |
| CONNECT | CleanSession - Forget sessions/subscriptions. |
Code to play with
- (C, JS, Python etc) http://git.eclipse.org/c/paho/
- Lots of client libraries including .NET, Erlang and Lua! Some are device specific e.g. Netduino. See mqtt.org
- Broker http://mosquitto.org/ http://mqtt.org/wiki/doku.php/public_brokers
Summary
- MQTT is binary PubSub based messaging protocol that requires TCP/IP.
- CoAP is a binary HTTP-like P2P protocol that can also emulate the PubSub pattern.
- Both can be used today.
- Beware of Apples to Oranges comparisons.
- Work with both protocols to see what is right for your application.