 packet as missing if it is reported missing in four SACK chunks, thus triggering the fast retransmission algorithm. SCTP also retransmits a lost packet when it has not been acknowl- edged within a retransmission timeout period, and puts the SCTP association into slow start. In order not to generate too much SACK traffic, SCTP may apply the delayed acknowledgment algorithm specified in [33], which gener- ates a SACK after the second unacknowledged data chunk has been received. Generally, a SACK is bundled with data chunks in an SCTP packet returned to the other endpoint. If no SCTP packet is available for the SACK bundling within a certain time limit, a dedicated acknowledgment packet may be sent. The recommended delay limit is 200 ms but should not be more than 500 ms. In addition, the recommended retransmission timeout of SCTP is within the range of 1 second to 60 seconds. From the above descriptions of the retransmission mechanisms of SIP and SCTP, we can see that SCTP is more effective in loss recovery than SIP when SIP traffic is high, due to fast retransmissions. Thus, SIP over SCTP can yield a good performance. However, when SIP traffic is low, there are less opportunities for SACK bundling and SCTP has to wait between 200 ms and 500 ms to send a dedicated SACK. If the time taken for SCTP error recovery exceeds the SIP retransmission timeout, then redundant SIP retransmissions will be generated. In this case, SIP over UDP, which relies solely on SIP retransmissions to recover errors, can be more effective than SIP over SCTP. Although SCTP was originally designed to provide transport services for SS7 signalling messages over IP networks, it is useful for the transport of other signalling messages, such as SIP messages. However, the following problems exist in sending SIP messages over SCTP: (1) Transporting obsolete SIP mes- sages may waste network resources. Once a SIP message is included in a SCTP data packet, SCTP will keep resending the packet until it is acknowledged. Since SIP messages are only useful