Upon the successful establishment of a TCP session between peers, what type of BGP message is exchanged?

Upon the successful establishment of a transport layer (TCP) connection, each BGP peer initiates the BGP session by sending an OPEN message. This message is the first BGP-specific communication exchanged between the peers. The OPEN message contains crucial parameters for the session, such as the BGP version, the local Autonomous System (AS) number, the Hold Time, and the BGP Identifier. The successful exchange and validation of these parameters in the OPEN messages allow the peers to transition to an established state, after which they can exchange routing information.

A. OpenSent: This is the name of a BGP state, not a message type. A router enters the OpenSent state after it has sent its OPEN message.
B. Keepalive: KEEPALIVE messages are exchanged periodically only after the BGP session has been fully established to ensure the peer is still reachable.
D. Notification: A NOTIFICATION message is sent to indicate a fatal error, which causes the BGP session to be terminated, not established.
E. Update: UPDATE messages are used to advertise, update, or withdraw routing information (NLRI) and are only sent after the BGP session is in the Established state.

1. IETF RFC 4271: A Border Gateway Protocol 4 (BGP-4)

Section 4.1, "Message Header Format": Lists the four BGP message types: OPEN, UPDATE, NOTIFICATION, and KEEPALIVE.

Section 8.2.2, "Finite State Machine": Describes the events for the BGP states. It explicitly states that upon a "TcpConnectionConfirmed" event, the local system "sends an OPEN message" and transitions to the OpenSent state. (DOI: https://doi.org/10.17487/RFC4271)

2. Nokia Border Gateway Protocol Fundamentals for Services Student Guide (Course Number: TTT30101AAAA)

Module 2: BGP Neighbor Relationships, Section: BGP Neighbor States: This section details the BGP finite state machine. It explains that after the TCP three-way handshake is complete (Connect state), the client sends a BGP OPEN message and moves to the OpenSent state. The server, upon receiving the TCP connection, also sends its OPEN message and moves to OpenSent.

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson.

Chapter 5.3, "The Border Gateway Protocol (BGP) in the Internet": This university-level textbook explains the BGP session establishment process, stating, "To initiate a BGP session with a peer, a router sends an OPEN message to the peer... The OPEN message allows a BGP router to identify itself." This confirms the OPEN message is the initial message for session establishment.