Application Layer Protocols for the IoT - Intranet DEIBhome.deib.polimi.it/cesana/teaching/IoT/2017/classes/7.Application... · Application Layer Protocols for the IoT 1. ... o Embedded

Politecnico di MilanoAdvanced Network Technologies Laboratory

Application Layer Protocols for the IoT

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Politecnico di MilanoAdvanced Network Technologies Laboratory

COnstrained ApplicationProtocol(COAP)

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o GOAL:toenableweb-basedservicesinconstrainedwirelessnetworksn 8bitmicro-controllersn limitedmemoryn low-powernetworks

o Problem:WEBsolutionarehardlyapplicable

o Solution:re-designweb-basedservicesforconstrainednetworks->COAP

Background

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o ResourcesintheWebare:n managedbyserversn identifiedbyURIsn accessedsynchronouslyby

clientsthroughrequest/responseparadigms

o Inaword,RepresentationalStateTransfer(REST)

HowDoestheWebWork?

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URL Resolution

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The CoAP Architecture

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o Embedded web transfer protocol (coap://)o Asynchronous transaction model

o UDP binding with reliability and multicast supporto GET, POST, PUT, DELETE methodso URI support

o 4 byte headero Subset of MIME types and HTTP response codeso Built-in discovery o Optional observation and block transfer

CoAP At a Glance

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o Transport: n (mainly) UDP binding

o Message Exchange between Endpointsn Messages with 4 bytes header (shared by request and

responses) containing a message ID (16 bits)

n Reliable exchange through Confirmable Messages which must be acknowledged (through ACK or Reset Messages). Simple Stop-and-Wait retransmission with exponential back-off.

n Unreliable exchange through Non-Confirmable Messagen Duplicate detection for both confirmable and non-confirmable

messages (through message ID)

COAPMessagingBasics

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COAPMessaging

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Message ID

o REST Request/Response piggybacked on CoAP Messages

o Method, Response Code and Options (URI, content-type etc.)

COAP Message Semantics

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COAPRequest/Response Examples

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Message ID

Token

COAP:SeparateResponse

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COAP:Non-confirmableRequest

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0 1 2 30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|Ver| T | TKL | Code | Message ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Token (if any, TKL bytes) ...+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Options (if any) ...+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|1 1 1 1 1 1 1 1| Payload (if any) ...+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Ver - Version (1)

T – Message Type (Confirmable, Non-Confirmable, Acknowledgement, Reset)

TKL- Token Length, if any, the number of Token bytes after this header

Code - Request Method (1-10) or Response Code (40-255)

Message ID – 16-bit identifier for matching responses

Token – Optional response matching token

Message Header (4 bytes)

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0 1 2 3 4 5 6 7+---------------+---------------+| | || Option Delta | Option Length | 1 byte| | |+---------------+---------------+\ \/ Option Delta / 0-2 bytes\ (extended) \+-------------------------------+\ \/ Option Length / 0-2 bytes\ (extended) \+-------------------------------+\ \/ /\ \/ Option Value / 0 or more bytes\ \/ /\ \+-------------------------------+

Option Delta - Difference between this option type and the previousLength - Length of the option value

Value - The value of Length bytes immediately follows Length

Option Format

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+-----+---+---+---+---+----------------+--------+--------+----------+| No. | C | U | N | R | Name | Format | Length | Default |+-----+---+---+---+---+----------------+--------+--------+----------+| 1 | x | | | x | If-Match | opaque | 0-8 | (none) || 3 | x | x | - | | Uri-Host | string | 1-255 | (see || | | | | | | | | below) || 4 | | | | x | ETag | opaque | 1-8 | (none) || 5 | x | | | | If-None-Match | empty | 0 | (none) || 7 | x | x | - | | Uri-Port | uint | 0-2 | (see || | | | | | | | | below) || 8 | | | | x | Location-Path | string | 0-255 | (none) || 11 | x | x | - | x | Uri-Path | string | 0-255 | (none) || 12 | | | | | Content-Format | uint | 0-2 | (none) || 14 | | x | - | | Max-Age | uint | 0-4 | 60 || 15 | x | x | - | x | Uri-Query | string | 0-255 | (none) || 16 | | | | | Accept | uint | 0-2 | (none) || 20 | | | | x | Location-Query | string | 0-255 | (none) || 35 | x | x | - | | Proxy-Uri | string | 1-1034 | (none) || 39 | x | x | - | | Proxy-Scheme | string | 1-255 | (none) |+-----+---+---+---+---+----------------+--------+--------+----------+

C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

Base Specification Options

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o StopandWaitapproacho Repeatarequestafteratime-outincase

ACK(orRST)isnotcomingback

Dealing with Packet Loss

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o Initialtime-outsetto:n Rand[ACK_TIMEOUT,ACK_TIMEOUT*

ACK_RANDOM_FACTOR]([2s,3s])o Whentime-outexpiresandthetransmissioncounteris

lessthanMAX_RETRANSMIT(4)n retransmitn Increasetransmissioncountern doublethetime-outvalue

o Theprocedureisrepeateduntiln AACKisreceivedn ARSTmessageisreceivedn thetransmissioncounterexceedsMAX_RETRANSMITn thetotalattemptdurationexceedsMAX_TRANSMIT_WAIT

(93s)

Back-OffDetails

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Proxyng and caching

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o PROBLEM:n RESTparadigmisoften“PULL”type,thatis,

dataisobtainedbyissuinganexplicitrequestn Information/datainWSNisoften

periodic/triggered(e.g.,getmeatemperaturesampleevery2secondsorgetmeawarningiftemperaturegoesbelow5°C)

o SOLUTION:useObservationonCOAPresources

COAPObservation

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Observation

21See draft-ietf-core-observe

o PROBLEM:avoidsegmentationinthelowerlayers(IPv6)

o SOLUTION:COAPBlockTransferModen bringsupfragmentationattheapplication

layer

COAPBlockTransfer

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o Block2 Optionaddedtomessagesn nr=incrementalblocknumberwithinoriginaldatan m=moreblocksflagn sz=blocksize

Block transfer

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o Resource Discoveryn GOAL: Discovering the links hosted by

CoAP (or HTTP) serversGET /.well-known/core?optional_query_string

n Returns a link-header style formato URL, relation, type, interface, content-type etc.

Discovery & Semantics

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</dev/bat>;obs;if="";rt="ipso:dev-bat";ct="0",</dev/mdl>;if="";rt="ipso:dev-mdl";ct="0",</dev/mfg>;if="";rt="ipso:dev-mfg";ct="0”,</pwr/0/rel>;obs;if="";rt="ipso:pwr-rel";ct="0",</pwr/0/w>;obs;if="";rt="ipso:pwr-w";ct="0",</sen/temp>;obs;if="";rt="ucum:Cel";ct="0"

CoRE Resource Discovery

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o Open source implementations:n Java CoAP Library Californium n C CoAP Library Erbiumn libCoAP C Library n jCoAP Java Libraryn OpenCoAP C Libraryn TinyOS and Contiki include CoAP support

o Firefox has a CoAP plugin called Coppero Wireshark has CoAP plugin

Getting Started with CoAP

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Politecnico di MilanoAdvanced Network Technologies Laboratory

TheMessageQueuingTelemetryTransport(MQTT)

Inventedin1999(IBMproprietarystandard)Releasedin2010OfficialOASISstandardsince2014CurrentversionMQTT3.1.1 http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html

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MQTTisaClientServerpublish/subscribemessaging transportprotocol.

o Morefeatures:n Simpletoimplement(especiallyatthesensor

side)n QoS Supportn Lightweightandbandwidthefficientn Dataagnosticn Sessionawareness

MQTTinshort

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o Publish/Subscribeparadigmn Clientsdon’tknoweachothern One-to-Manyparadigmn Everyclientpublishes&subscribesn PUSHinformationparadigmcomparedtoPULL’s

oneinCOAP

CommunicationPattern

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Publish: “30° C” Publish: “30° C”

Publish: “30° C”

Subscribe: “?°C”

Subscribe: “?°C”MQTT

Senosr nodeMobile device

Laptop

o Publish/Subscribeparadigmn Clientsdon’tknoweachothern One-to-Manyparadigmn Everyclientpublishes&subscribesn PUSHinformationparadigmcomparedtoPULL’s

oneinCOAP

MQTTComponents

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Publish: “30° C” Publish: “30° C”

Publish: “30° C”

Subscribe: “?°C”

Subscribe: “?°C”

Senosr nodeMQTT Client Mobile device

MQTT Client

LaptopMQTT Client

MQTTBroker

deib/antlab/room5/temperature

o Wildcardsallowedonlywhensubscribing

deib/antlab/ + /temperature

Plussigncanbeusedinmultiplelevels

MQTTTopics

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Topic level Topic separator

Single level Wildcard

deib/antlab/room5/ #

Multi-levelWildcards

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Multi-level Wildcard(can be used only at the end)

o EachMQTTclientopensoneconnectiontotheMQTTBroker

o Pushcapabilitieso Workseventhrough

firewalls

MQTTConnections

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MQTT

TCP

IP

o CONNECTmessagefields:n clientId “clientMatteo”n cleanSession truen username (opt) “matteo”n password (opt) “1234”n lastWillTopic (opt) “matteo/temp”n lastWillQoS (opt) 1n lastWillMessage (opt) “something wrong”n keepAlive 30

OpenConnections

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MQTTBroker

MQTTClient

CONNECT

CONNACK

o CONNACKmessagefields:n sessionPresent truen returnCode 0-4

0: everything ok1: unacceptable version2: id rejected3: server unavailable4: bad username and pwd5: unauthorized

OpenConnections

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MQTTBroker

MQTTClient

CONNECT

CONNACK

o PUBLISHmessagefieldsn packeId 2n topicName “matteo/temp”n QoS 1n retainFlag falsen Payload “temperature:30”n dupFlag false

Publishing

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MQTTBroker

MQTTClient

PUBLISH

MQTTClient

MQTTClient

MQTTClient

PUBLISH

o Bestefforttransfer(samereliabilityprovidedbytheunderlyingtrasportprotocol)

QoS 0:“atmostonce”

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MQTTBroker

MQTTClient

PUBLISH QoS=0

o TheMQTTclientstoresthemessageandkeepsretransmittingituntilitisacknowledgedbytheMQTTbroker(messagecanbereceivedmultipletimes)

o PUBACKmessagefields:n packetId 2

QoS 1:“atleastonce”

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MQTTBroker

MQTTClient

PUBLISH QoS=1

PUBACK

o PUBREC,PUBREL,PUBCOMPmessagefields:n PacketId 2

o PUBLISHreception@MQTTbroker:processthepacketaccordingly, sendPUBRECmessageback,storelocallypacketId toavoidduplicateprocessing

o PUBRECreception@MQTTclient:discardtheinitialpacketandsendPUBRELo PUBRELreception@MQTTclient:clearanycurrent stateandsendPUBCOMP

QoS 2:“exactlyonce”

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MQTTBroker

MQTTClient

PUBLISH QoS=1

PUBREC

PUBREL

PUBCOMP

o SUBSCRIBEmessagefields:n packetId 2n QoS1 0n Topic1 “matteo/temp/1”n QoS2 1n Topic2 “kitchen/temp/2”n …....... ........

Subscribing

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MQTTBroker

MQTTClient

SUBSCRIBE

SUBACK

o SUBACKmessage fields:n packetId 2n returnCode 1 2n returnCode 2 0

one returnCodefor each topic inthe subscription

one QoS/topic couplefor each subscription

o UNSUBSCRIBEmessagefields:n packetId 2n Topic1 “matteo/temp/1”n Topic2 “kitchen/temp/2”n …....... ........

Unsubscribing

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MQTTBroker

MQTTClient

UNSUBSCRIBE

UNSUBACK

o UNSUBACKmessage fields:n packetId 2

one returnCodefor each topic inthe subscription

o Indefaultoperationmodewhentheclientdisconnects,alltheclient-relatedstatusatthebrokerisflushed(listofsubscription,QoS pendingmessages,etc.)

o Inpersistentsessionsbothclientandbrokermaintainasession:n Broker:

o Existenceofasession,eveniftherearenosubscriptionso Allsubscriptionso AllmessagesinQoS 1or2flow,whicharenotconfirmedbythecliento AllnewQoS 1or2messages,whichtheclientmissedwhileitwasofflneo AllreceivedQoS 2messages,whicharenotyetconfirmedtotheclient

n Cliento AllmessagesinaQoS 1or2flow,whicharenotconfirmedbythebrokero AllreceivedQoS 2messages,whicharenotyetconfirmedtothebroker

o Thatmeanseveniftheclientisofflinealltheabovewillbestoredbythebrokerandareavailablerightaftertheclientreconnects.

PersistentSessions

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o Problem:publishingandsubscribingareasynchronousprocesses

o Aclientsubscribingtoatopicpatternmaynotgetanymessageonthattopicuntilsomeotherclientpublishesonit

o Retaind messagesarePUBLISHmessageswiththeretainedFlag settoone

o Thebrokerstoreslocallytheretainedmessageandsendittoanyotherclientwhichsubscribestoatopicpatternmatchingthatoftheretainedmessage

RetainedMessges

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o TheLastWillandTestament(LWT)notifiesotherclientsaboutanharddisconnec byaspecificclient

o Eachclientcanspecifyitslastwillmessagewhenconnectingtoabroker

o Thebrokerwillstorethemessageuntilitdetectsclientharddisconnection

o Thebrokersendsthemessagetoallsubscribedclientsonthespecifictopic

o ThestoredLWTmessagewillbediscardedifaclientdisconnectsgracefullybysendingaDISCONNECTmessage.

LastWillMessage

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o CONNECTmessagefields:n clientId “clientMatteo”n cleanSession truen username (opt) “matteo”n password (opt) “1234”n lastWillTopic (opt) “matteo/temp”n lastWillQoS (opt) 1n lastWillMessage (opt) “something wrong”n keepAlive 30

LWTSetUp

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MQTTBroker

MQTTClient

CONNECT

CONNACK

o AnI/Oerrorornetworkfailureisdetectedbytheserver.

o TheclientfailstocommunicatewithintheKeepAlivetime.

o TheclientclosesthenetworkconnectionwithoutsendingaDISCONNECTpacketfirst.

o Theserverclosesthenetworkconnectionbecauseofaprotocolerror.

LWTMessageissentwhen..

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o Itisresponsability oftheclienttokeeptheMQTTconnectionactive

o Uponexpirationofthekeepalive,ifnootherinteractionhashappenedwithbroker,theclient”pings”thebrokerwhich“pingsitback”

o PINGREQandPINGRESPmessageshavenullpayload

Keepalive

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MQTTBroker

MQTTClient

PINGREQ

PINGRESP

o Maindifferencesn Extendedarchitecture

withGatewaysandForwarders

n Newgatewaydiscoveryprocedures(andmessages)

n Somemessagesaremore“compressed”

n Extendedkeepaliveprocedurestosupportsleepingclients

MQTT-SN

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o Brokerslistn Mosquitton HiveMQ

o MQTTandNodeRed.Let’suseit

Gettingstarted

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