Which statement describes a characteristic of the frame header fields of the data link layer?

• A. They all include the flow control and logical connection fields.
• B. Ethernet frame header fields contain Layer 3 source and destination addresses.
• C. They vary depending on protocols.
• D. They include information on user applications.

Answer: (C)

Frame header fields at the data link layer vary by protocol. Different link technologies define their own header formats, so the exact fields, their order, and what they carry are not universal. For example, Ethernet frames have destination and source MAC addresses and an EtherType field to indicate the next protocol, while other data link technologies (like 802.11, PPP/HDLC, or VLAN-tagged Ethernet) include different or additional fields. This variability is why a statement that the frame header fields change depending on the protocol is the best description. The other ideas aren’t universally true: flow control and logical connection fields aren’t standard parts of every data link frame header; Layer 3 addresses belong to the network layer, not the data link header; and information about user applications resides in the payload, not in the frame header.