 initial SIP retransmission timeout. PR-SCTP attempts to provide reliable delivery of the data packet within the lifetime using the same error recovery mechanism as in SCTP, and drops the packet if unsuccessful within the lifetime. At this time the SIP timeout period is also reached, and SIP takes over by retransmitting the SIP message dropped by the transport layer. As SIP increases its retransmission timeout to 2 X T1, accordingly, the lifetime of the new data packet in PR-SCTP carrying the retransmitted SIP message is also increased to 2 X T1. This process continues until the message has been sent successfully or the sixth retransmission of this SIP message has failed. If all the retransmissions have failed, an error message would be reported to the application layer to indicate transmission failure of the SIP message. PR-SCTP sets the lifetime of the data packets containing these messages to a constant when SIP messages that do not need to be transmitted are sent. These messages are then sent once and not retransmitted by PR-SCTP. In the proposed scheme, the application can require either an ordered or unordered transport service. The unordered service avoids the head-of-line (HOL) queue blocking problem for messages in the same stream. Unordered service is used to send SIP provisional messages, since the loss of these message should not block the delivery of other SIP messages. Other SIP messages can be sent either ordered or unordered. If multiple SIP sessions are mapped C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security into one SCTP stream, unordered service is recommended since it prevents the SIP messages of one session from blocking other SIP messages belonging to different SIP sessions. The schemes for mapping SIP sessions into SCTP streams are discussed in [34]. The lifetime of the PR-SCTP data packets containing the SIP message is set to the SIP retransmission timeout. This will result in the application and transport layers discarding the SIP messages at the sa