Which statement best describes IP reliability?

• A. This protocol is not reliable and relies on higher level protocols to ensure reliability.
• B. IP guarantees in-order delivery.
• C. IP uses acknowledgments for every packet.
• D. IP provides built-in congestion control.

Answer: (B)

IP is a best-effort delivery service. It does not guarantee that a packet will arrive, nor that packets will arrive in the order they were sent, and duplicates can occur. The network can lose, reorder, or duplicate datagrams as they traverse routers and paths. Reliability and in-order delivery are achieved at higher layers, most notably with TCP, which uses acknowledgments, retransmissions, and sequence numbers to ensure data arrives correctly and in the right order. Routing and congestion control are not built into IP itself; those functions come from the transport layer (and, in some cases, network features that work with it). So saying that IP guarantees in-order delivery isn’t accurate—the true behavior is best-effort delivery, with reliability provided by upper-layer protocols.