SCTP is connection-oriented like TCP, so it also has association establishment and termination handshakes. However, SCTP's handshakes are different than TCP's, so we describe them here.
Four-Way Handshake
The following scenario, similar to TCP, occurs when an SCTP association is established:
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The server must be prepared to accept an incoming association. This preparation is normally done by calling socket, bind, and listen and is called a passive open.
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The client issues an active open by calling connect or by sending a message, which implicitly opens the association. This causes the client SCTP to send an INIT message (which stands for "initialization") to tell the server the client's list of IP addresses, initial sequence number, initiation tag to identify all packets in this association, number of outbound streams the client is requesting, and number of inbound streams the client can support.
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The server acknowledges the client's INIT message with an INIT-ACK message, which contains the server's list of IP addresses, initial sequence number, initiation tag, number of outbound streams the server is requesting, number of inbound streams the server can support, and a state cookie. The state cookie contains all of the state that the server needs to ensure that the association is valid, and is digitally signed to ensure its validity.
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The client echos the server's state cookie with a COOKIE-ECHO message. This message may also contain user data bundled within the same packet.
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The server acknowledges that the cookie was correct and that the association was established with a COOKIE-ACK message. This message may also contain user data bundled within the same packet.
The minimum number of packets required for this exchange is four; hence, this process is called SCTP's four-way handshake.
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