3. Creation of validation tables: each UA pairs each one of its transport addresses with the ones in the ALEX-it fields received by the other UA. Pairs are referred to as candidate channels and are the entries of the validation tables. Validation tables on both UAs contain the same data. 4. Channel validation: candidate channels are checked using STUN mes- sages to verify connectivity, beginning from the candidate channels for the SIP session. The successful channels with the highest priority, obtained by combining the priority of the component transport addresses, are referred to as optimal channels. The time required by the checks, especially when multiple retransmissions of STUN messages are needed to confirm failed checks (i.e., to make sure that the transport address cannot be reached rather than packets being dropped due to congestion), might not be negligible. In order to reduce the delay in beginning SIP C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security and media sessions, the validation phase has been split into two sub- phases: a. The fore validation sub-phase consists of a non-thorough proce- dure aiming at discovering in a short time, if possible, one optimal channel during which STUN messages are retransmitted quickly up to three times in case of missing answers. The fore validation sub-phase begins the probing procedure from the default chan- nels (i.e., channels made up of default transport addresses) and stops as soon as an optimal channel is discovered or when a timer fires. b. The background validation sub-phase follows, while concurrent SIP and media sessions start using the best channels successfully validated in the previous sub-phase, to probe again high-priority channels that may have not been successfully checked in the fore validation sub-phase and to probe the remaining channels. The validation procedure during the background validation sub-phase is less aggressive in order to limit network overhead due to connec- tivity checks, e.g., retransmission