A participant has identified a conversation partner and is about to initiate a conversation.

How can two services sync up so they can start a conversation from the same point?

• Reliable protocols over unreliable channels often require sequence numbers to detect lost messages. Both parties need to start with the same sequence number so they can detect if the first message is already lost.
• Conversation partners may want to agree on parameters of the conversation, based on both parties' capabilities. For example, if both parties support compression or binary encoding, they can agree to switch to this more efficient message format.

A Three-Way Hand Shake has the requestor initiating a conversation, followed by an acknowledgment (Ack) by the conversation partner, to which the initiator replies with an acknowledgment (Ack-Ack).

The Three-Way Hand Shake conversation involves the following participants:

• The Initiator
• The conversation Partner

Example

https://tools.ietf.org/html/rfc761#page-29