Best effort means IP makes no guarantees about delivery — it will try its utmost to deliver each datagram, but if something goes wrong (congestion, no route, TTL expiry, corruption) the packet is silently dropped, with no notification to the sender and no retransmission by IP itself.

• IP provides: connectionless, unordered, unreliable delivery with no duplicate protection, no flow/congestion control and no error recovery — only header checksum for basic corruption detection.
• Any required reliability (retransmission, ordering, flow control) must be added by upper layers (TCP/UDP at the transport layer, or the application).
• This keeps the network core simple and scalable ("smart edges, dumb core"), which is the fundamental design philosophy of the Internet.