Which of the following statements about packets is true?

Packets are essential units of data transmitted across networks, and they consist of not just the user data, but also a header that contains control information. This header is crucial as it includes metadata necessary for routing and delivering the packet to its destination. The header typically contains source and destination addresses, packet sequencing information, and other control details that assist routers and switches in making decisions about how to forward the packet through the network.

The presence of this header information allows devices to understand not only where the packet came from and where it needs to go, but also how to handle it within the context of the greater data transmission process, such as checking for errors or reassembling packets that may have arrived out of order.

The other options incorrectly describe the nature and function of packets. For instance, packets do not solely consist of user data; they always include overhead in the form of headers. Additionally, packets are not restricted to wireless communications—they are used in both wired and wireless networks. Lastly, packets are crucial in routing protocols because they are the means by which routing information is communicated across the network. Hence, the correct character of packets is encapsulated in the statement about them including a header with control information.