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E. Fraccaroli and D. Quaglia
the PDU header contains a special address identifying a group of interfaces. Each
receiver interested in this kind of communication should listen to PDUs reporting
this group address.
The Simplified ISO/OSI Model is shown in Fig. 3.6, and its layers are described
below.
3.2.2.1 Application Layer
It is responsible for defining how application data are exchanged between the end
nodes, the message types that implement the application logic (e.g., the format of the
message used to transfer a temperature), and the semantic of application data (e.g.,
a temperature value with respect to a humidity value). In the Simplified ISO/OSI
Model, this layer includes aspects that are split in different layers in the full ISO/OSI
Model, i.e., Presentation Layer devoted to data representation and encryption and
Session Layer devoted to session establishment, management, and recovery.
3.2.2.2 Transport Layer
It performs protocol multiplexing, i.e., addressing the different processes running
at the Application Layer. It is worth referring to well-known network architecture,
i.e., TCP/IP, to better understand the role of this layer. In the Transport Layer of the
TCP/IP architecture, there are two alternative protocols, i.e., Transmission Control
Protocol (TCP) and User Datagram Protocol (UDP). TCP is a connection-oriented
protocol; therefore, it provides a reliable and byte-oriented transport service. About
80% of the IP PDUs belonging to a TCP stream do not deliver actual data but are
devoted to connection management and acknowledgment. Furthermore, connection
establishment and retransmission of lost PDUs take time, and they are not suitable
for real-time communications. UDP just provides encapsulation of application
data into IP PDUs without any effective error control. It only addresses the basic
functionality of protocol multiplexing. In IoT applications, UDP is usually the best
choice [5] since:
• devices usually transmit small messages (e.g., a temperature sample) that do not
need to be fragmented and re-assembled as well as acknowledged;
• TCP connection overhead is not acceptable for low power wireless transmissions;
• devices may frequently go into sleep mode due to energy constraints; thus, it is
infeasible to maintain a long-lived TCP connection;
• some applications (e.g., device actuation) may have a low-latency requirement,
which may not tolerate the delay caused by TCP connection establishment.
However, TCP is still used in IoT applications when web services are exploited (see
Sect. 3.4.3) since they are conveyed by HTTP/HTTPS application protocols which
are delivered over TCP connections.
E. Fraccaroli and D. Quaglia
the PDU header contains a special address identifying a group of interfaces. Each
receiver interested in this kind of communication should listen to PDUs reporting
this group address.
The Simplified ISO/OSI Model is shown in Fig. 3.6, and its layers are described
below.
3.2.2.1 Application Layer
It is responsible for defining how application data are exchanged between the end
nodes, the message types that implement the application logic (e.g., the format of the
message used to transfer a temperature), and the semantic of application data (e.g.,
a temperature value with respect to a humidity value). In the Simplified ISO/OSI
Model, this layer includes aspects that are split in different layers in the full ISO/OSI
Model, i.e., Presentation Layer devoted to data representation and encryption and
Session Layer devoted to session establishment, management, and recovery.
3.2.2.2 Transport Layer
It performs protocol multiplexing, i.e., addressing the different processes running
at the Application Layer. It is worth referring to well-known network architecture,
i.e., TCP/IP, to better understand the role of this layer. In the Transport Layer of the
TCP/IP architecture, there are two alternative protocols, i.e., Transmission Control
Protocol (TCP) and User Datagram Protocol (UDP). TCP is a connection-oriented
protocol; therefore, it provides a reliable and byte-oriented transport service. About
80% of the IP PDUs belonging to a TCP stream do not deliver actual data but are
devoted to connection management and acknowledgment. Furthermore, connection
establishment and retransmission of lost PDUs take time, and they are not suitable
for real-time communications. UDP just provides encapsulation of application
data into IP PDUs without any effective error control. It only addresses the basic
functionality of protocol multiplexing. In IoT applications, UDP is usually the best
choice [5] since:
• devices usually transmit small messages (e.g., a temperature sample) that do not
need to be fragmented and re-assembled as well as acknowledged;
• TCP connection overhead is not acceptable for low power wireless transmissions;
• devices may frequently go into sleep mode due to energy constraints; thus, it is
infeasible to maintain a long-lived TCP connection;
• some applications (e.g., device actuation) may have a low-latency requirement,
which may not tolerate the delay caused by TCP connection establishment.
However, TCP is still used in IoT applications when web services are exploited (see
Sect. 3.4.3) since they are conveyed by HTTP/HTTPS application protocols which
are delivered over TCP connections.