Which OSI layer adds control information at the beginning of the payload and includes TCP segments and UDP datagrams?

Which OSI layer adds control information at the beginning of the payload and includes TCP segments and UDP datagrams?

• A. Transport layer
• B. Network layer
• C. Data Link layer
• D. Session layer

Answer: (A)

The transport layer is responsible for end-to-end delivery and adds control information to the data it carries. It prefixes the payload with a header that includes things like port numbers (to identify the communicating processes on each end), sequencing or order information, and flow/control mechanisms such as acknowledgments and windowing. This is what allows two hosts to establish a conversation and have data delivered reliably and in the correct order, or to send data in a lightweight, connectionless way, depending on which transport protocol is used. The specific data units at this layer are TCP segments when using the TCP protocol and UDP datagrams when using the UDP protocol. That naming—segment or datagram—reflects how the transport layer encapsulates application data with its own header and, in the TCP case, provides reliability and ordering, while UDP offers a simpler, faster, but less feature-rich option. In contrast, other layers handle different responsibilities: the network layer deals with addressing and routing (IP addresses and path selection), the data link layer handles physical addressing and frame delivery on a local link (MAC addresses and error detection on the link), and the session layer manages dialog control and session maintenance.

The transport layer is responsible for end-to-end delivery and adds control information to the data it carries. It prefixes the payload with a header that includes things like port numbers (to identify the communicating processes on each end), sequencing or order information, and flow/control mechanisms such as acknowledgments and windowing. This is what allows two hosts to establish a conversation and have data delivered reliably and in the correct order, or to send data in a lightweight, connectionless way, depending on which transport protocol is used.

The specific data units at this layer are TCP segments when using the TCP protocol and UDP datagrams when using the UDP protocol. That naming—segment or datagram—reflects how the transport layer encapsulates application data with its own header and, in the TCP case, provides reliability and ordering, while UDP offers a simpler, faster, but less feature-rich option.

In contrast, other layers handle different responsibilities: the network layer deals with addressing and routing (IP addresses and path selection), the data link layer handles physical addressing and frame delivery on a local link (MAC addresses and error detection on the link), and the session layer manages dialog control and session maintenance.