UML Sequence Diagram A: SCTP FourWay Handshale

This UML sequence diagram has two lifelines called Client and Server. $($These should be "instances" not "threads".$)$ The sequence logic is as follows $($timewise in the order listed$)$:

The client sends an asynchronous message to the server that takes time with the label "INIT$($Ta$)".$

The server sends an synchronous message to the server that takes time with the label "INIT$-$ACK$($Tb$,$cookie$)".$

The client sends an asynchronous message to the server that takes time with the label "COOKIE$($Tb$,$cookie$)".$

The server sends an synchronous message to the server that takes time with the label "COOKIE$-$ACK$($Ta$)".$

This diagram therefore has two lifetimes with two arrows going from the client to the server and two from the server to the client.

ASIDE: SCTP is a network protocol that is similar to but different $($and more flexible$)$ than TCP$.$ TCP has a three$-$way handshake at the start of a connect and SCTP has a different four$-$way handshake. That said, this is not a computer networking course so I will not elaborate further except to say again many aspects of computer networking are nicely expressed as sequence diagrams. $$