BGP provides reliable delivery via what TCP mechanism?

• A. Three-way handshake
• B. Retransmission timers
• C. Acks in the application layer
• D. Window scaling

Answer: (A)

BGP runs over TCP, so the reliable delivery of BGP messages depends on the TCP connection that carries them. The three-way handshake is the TCP mechanism that sets up this connection and ensures both sides are ready to exchange data in a reliable, ordered stream. Once the connection is established, TCP handles reliability through its core features—sequence numbers, acknowledgments, retransmission timers, and flow/congestion control—so BGP messages are delivered correctly even over imperfect networks.

Retransmission timers and window scaling are parts of TCP’s behavior once the connection exists, not the initial step that creates the reliable channel. Acknowledgments belong to TCP’s transport-layer mechanism rather than an application-layer ack. Therefore, the handshake is the critical step that enables the reliable channel that BGP then uses.