Connectivity Framework 7 : Connectivity Standards
7.2 IIOT CONNECTIVITY TRANSPORT STANDARDS 7.2.1 TCP AND UDP OVER IP
In the context of IIoT , the network layer is the internet protocol ( IP ). The IP suite also provides the UDP 1 and the TCP 2 transport on top of the IP layer . These IP transports provide the foundation for the other connectivity transports and frameworks .
UDP , Universal Datagram Protocol is a connectionless transport ( see section 5.1.4 ) that provides best-effort delivery quality of service . A message is not resent if it is lost in the transmission . Messages may be received out-of-order . Messages are sent as quickly as possible , and so it is suitable for low latency real-time communications . A message shall be less than 64KB long . A connectivity transport or framework on top of UDP should therefore deal with fragmentation by caching and assembling portions of larger messages .
TCP , Transmission Control Protocol is a connection-oriented ( see section 5.1.4 ) transport that provides reliable and ordered delivery quality of service . A message is resent if it is lost in transmission . Messages are delivered in order . This can lead to head-of-line blocking — high priority , time-critical messages may be blocked behind low priority , non-critical messages . Retries hold up all messages in the channel . Thus , message latencies can vary greatly , leading to large jitter , especially when messages are lost in transmission . The connection sequence can be expensive in time and resources . There is no inherent limit on the message size .
The choice between UDP and TCP at the connectivity transport level has significant implications for the connectivity framework and its suitability ( see Figure 1-1 ). As shown in Figure 7-1 , some connectivity frameworks require TCP and inherit its characteristics ; some require UDP and inherit those characteristics ; and some can use either to support the varying application requirements .
7.2.2 CONSTRAINED APPLICATION PROTOCOL ( COAP )
Constrained Application Protocol ( CoAP ) is a connectivity transport standard inspired by HTTP , but designed to be more lightweight and efficient ( see Figure 7-1 ). It was established using the UDP transport . The IETF maintains the CoAP open standard specification . Since its definition alternative transports using TCP with TLS 3 , SMS , and Web Sockets have been developed .
CoAP is generally used in the operations domain ( see Figure 1-1 ). Like HTTP , a client sends a request to a server , specifying a data object , an operation , and a payload . The server replies with failure or success and a response payload . In addition , a client can also register to be notified of any changes in data object . Unlike HTTP , it is suitable for device-to-device queries . However , retries and reordering are implemented in the application stack . CoAP is designed to interoperate with HTTP and the RESTful web services through simple proxies .
1
See [ IETF-RFC768 ]
2
See [ IETF-RFC793 ]
3
See [ IETF-RFC4279 ] IIC : PUB : G5 : V1.0 : PB : 20170228 - 52 -