TCP/UDP Port Finder

TCP port 321 uses the Transmission Control Protocol. TCP is one of the main protocols in TCP/IP networks. TCP is a connection-oriented protocol, it requires handshaking to set up end-to-end communications. Only when a connection is set up user's data can be sent bi-directionally over the connection.
Attention! TCP guarantees delivery of data packets on port 321 in the same order in which they were sent. Guaranteed communication over TCP port 321 is the main difference between TCP and UDP. UDP port 321 would not have guaranteed communication as TCP.
UDP on port 321 provides an unreliable service and datagrams may arrive duplicated, out of order, or missing without notice. UDP on port 321 thinks that error checking and correction is not necessary or performed in the application, avoiding the overhead of such processing at the network interface level.
UDP (User Datagram Protocol) is a minimal message-oriented Transport Layer protocol (protocol is documented in IETF RFC 768).
Application examples that often use UDP: voice over IP (VoIP), streaming media and real-time multiplayer games. Many web applications use UDP, e.g. the Domain Name System (DNS), the Routing Information Protocol (RIP), the Dynamic Host Configuration Protocol (DHCP), the Simple Network Management Protocol (SNMP).
TCP vs UDP - TCP: reliable, ordered, heavyweight, streaming; UDP - unreliable, not ordered, lightweight, datagrams.