Like all remote display protocols, HDX relies on one of two Open Systems Interconnection stack Layer 4 protocols. HDX is based on the Transmission Control Protocol (TCP), but can also use the User Datagram Protocol (UDP).
TCP is the more reliable option of the two. It maintains the connection between the server and the endpoint until it receives confirmation of data delivery. If anything goes wrong, TCP resends data until successfully delivered. UDP does not wait for confirmation. It just sends the data. Because it skips the confirmation step, UDP is faster and particularly helpful for delivering media-heavy messages.
TCP also creates data packets and delivers them to the endpoints where they are reassembled in sequence. UDP does not take the time to sequence the data it sends.
HDX can use both UDP and TCP but is primarily based on TCP, whereas VMware's PC over IP protocol which is based on UDP.