Politecnico di Milano Advanced Network Technologies Laboratory. Constrained Application Protocol (CoAP)

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1 Politecnico di Milano Advanced Network Technologies Laboratory Constrained Application Protocol (CoAP) 1

2 Background o GOAL: to enable web- based services in constrained wireless networks n 8 bit micro- controllers n limited memory n low- power networks o Problem: WEB solu@on are hardly applicable o Solu@on: re- design web- based services for constrained networks - > COAP 2

3 How Does the Web Work? o Resources in the Web are: n managed by servers n iden@fied by URIs n accessed synchronously by clients through request/ response paradigms o In a word, Representa@onal State Transfer (REST) 3

4 URL Resolution 4

5 Request/Response Transaction o Other common HTTP methods: PUT, POST, DELETE 5

6 The CoAP Architecture 6

7 CoAP Design Requirements See draft-shelby-core-coap-req 7

8 CoAP At a Glance o Embedded web transfer protocol (coap://) o Asynchronous transaction model o UDP binding with reliability and multicast support o GET, POST, PUT, DELETE methods o URI support o 4 byte header o Subset of MIME types and HTTP response codes o Built-in discovery o Optional observation and block transfer 8

9 COAP Messaging Basics o Transport: n (mainly) UDP binding o Message Exchange between Endpoints n n n n Messages with 4 bytes header (shared by request and responses) containing a message ID (16 bits) Reliable exchange through Confirmable Messages which must be acknowledged (through ACK or Reset Messages). Simple Stop-and-Wait retransmission with exponential backoff. Unreliable exchange through Non-Confirmable Message Duplicate detection for both confirmable and non-confirmable messages (through message ID) 9

10 COAP Messaging Message ID 10

11 COAP Message Semantics o REST Request/Response piggybacked on CoAP Messages o Method, Response Code and Options (URI, content-type etc.) 11

12 COAP Request/Response Examples Message ID Token 12

13 COAP: Separate Response 13

14 COAP: Non- confirmable Request 14

15 Message Header (4 bytes) Ver T TKL Code Message ID Token (if any, TKL bytes) Options (if any) 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 15

16 Option Format 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 previous Length - Length of the option value Value - The value of Length bytes immediately follows Length 16

17 Base Specification Options No. C U N R Name Format Length Default x x If-Match opaque 0-8 (none) 3 x x - Uri-Host string (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 (none) 11 x x - x Uri-Path string (none) 12 Content-Format uint 0-2 (none) 14 x - Max-Age uint x x - x Uri-Query string (none) 16 Accept uint 0-2 (none) 20 x Location-Query string (none) 35 x x - Proxy-Uri string (none) 39 x x - Proxy-Scheme string (none) C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable 17

18 Dealing with Packet Loss o Stop and Wait approach o Repeat a request aser out in case ACK (or RST) is not coming back 18

19 Back- Off Details o out set to: n Rand [ACK_TIMEOUT, ACK_TIMEOUT * ACK_RANDOM_FACTOR] ([2s, 3s]) o out expires and the transmission counter is less than MAX_RETRANSMIT (4) n retransmit n Increase transmission counter n double out value o The procedure is repeated un@l n A ACK is received n A RST message is received n the transmission counter exceeds MAX_RETRANSMIT n the total aaempt dura@on exceeds MAX_TRANSMIT_WAIT (93s) 19

20 Proxying and caching 20

21 COAP ObservaKon o PROBLEM: n REST paradigm is osen PULL type, that is, data is obtained by issuing an explicit request n Informa@on/data in WSN is osen periodic/ triggered (e.g., get me a temperature sample every 2 seconds or get me a warning if temperature goes below 5 C) o SOLUTION: use Observa@on on COAP resources 21

22 Observation See draft-ietf-core-observe 22

23 COAP Block Transfer o PROBLEM: avoid segmenta@on in the lower layers (IPv6) o SOLUTION: COAP Block Transfer Mode n brings up fragmenta@on at the applica@on layer 23

24 Block transfer o Block2 Op@on added to messages n nr=incremental block number within original data n m=more blocks flag n sz=block size 24

25 Discovery & Semantics o Resource Discovery n GOAL: Discovering the links hosted by CoAP (or HTTP) servers GET /.well-known/core?optional_query_string n Returns a link-header style format o URL, relation, type, interface, content-type etc. 25

26 CoRE Resource Discovery </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" 26

27 Getting Started with CoAP o o o Open source implementations: n Java CoAP Library Californium n C CoAP Library Erbium n n n n libcoap C Library jcoap Java Library OpenCoAP C Library TinyOS and Contiki include CoAP support Firefox has a CoAP plugin called Copper Wireshark has CoAP plugin 27

Politecnico di Milano Advanced Network Technologies Laboratory. Application Layer Protocols for the IoT

Politecnico di Milano Advanced Network Technologies Laboratory Application Layer Protocols for the IoT 1 Politecnico di Milano Advanced Network Technologies Laboratory COnstrained Application Protocol