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Citadel Groupware
Port 504 is primarily used by Citadel groupware, a collaborative client-server platform offering email, calendar, contacts, bulletin boards, and more. This port facilitates dedicated Citadel protocol communications between specialized clients and the Citadel server, enabling multiservice access within the unified groupware environment. Both TCP and UDP are supported for flexible and reliable data transfer, underpinning internal messaging, directory access, and real-time collaboration functions..
Citadel is an open-source, multi-user collaboration server providing integrated functionalities such as email, calendars, address books, bulletin boards, instant messaging, and more. The system supports multiple access protocols (IMAP, POP3, SMTP, XMPP), but port 504 is specifically designated for its native Citadel protocol, mainly used by specialized Citadel clients.
Communications over port 504 facilitate seamless synchronization of groupware data, enabling clients to perform real-time updates, retrieve messages, manage calendars, or access conferencing features. The protocol is designed to efficiently multiplex different groupware services over a single dedicated channel, optimizing internal client-server interactions.
Both TCP and UDP are supported: TCP ensures reliable, connection-oriented data delivery for complex operations, while UDP can be used for lightweight or real-time status updates within the Citadel system. However, communications on this port are typically unencrypted by default, necessitating additional measures if confidentiality is required.