SIP, which is the signaling path of a telephone call is usually handled via UDP (User Datagram Protocol). This protocol is similar to a "telegram" or normal "mail cover". Data is divided into packets that a theoretical maximum size of 1500 bytes may have. This size is limited, however, by additional information and further logical information to a usable size of 1300 bytes. Is now such a packet sent, the sender and the receiver is included in the package. However, there is no feedback to the sender if the package has really reached its recipient. This has also achieved the result that the UDP protocol is used, it needs to ensure and check that a package's recipient. This happens by means of the SIP protocol acknowledgment responses. An "INVITE" (call from A to B) follows in the case of a "TRYING", then a "PROGRESS", etc Should one fail to acknowledge the last packet is defined by timers at intervals until an acknowledgment is sent again. When a timeout expires, the session is terminated.
For SIP over TCP, the transport behaves fundamentally differently. Similar to a "registered letter". (Transmission Control Protocol), TCP is due to the concept in many ways "robust" as UDP. However, in contrast to UDP, but also some more traffic (overhead) of bandwidth for the same user data as UDP. However, this is negligible in the case of SIP far as possible. The advantages of TCP is the protocol architecture of the ansich. It contains built Mechannismen to recover lost packets independently re-send. Similarly, a TCP connection is set up in a dedicated form. This means before data is transmitted to the receiver already knows what the coming. There are more benefits of TCP are as compared to TCP for NAT (Network Address Translation) and correspondingly for firewalls easier to handle. What is now the "better" protocol does not really matter. It must meet the requirements for you as a customer. The dus.net GmbH offers you the opportunity to use both protocols for SIP. You make the settings for your particular device, the transport protocol to be used for SIP.
For audio data, so the language is generally used UDP. The reason is inter alia the few "overhead" of the large payload fraction and less time-consuming trials and eventual replacement issues of packets that are undesirable so.