Timer Value Section Meaning
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T1 500ms default Section 17.1.1.1 RTT Estimate
T2 4s Section 17.1.2.2 The maximum retransmit
interval for non-INVITE
requests and INVITE
responses
T4 5s Section 17.1.2.2 Maximum duration a
message will
remain in the network
Timer A initially T1 Section 17.1.1.2 INVITE request retransmit
interval, for UDP only
Timer B 64*T1 Section 17.1.1.2 INVITE transaction
timeout timer
Timer C > 3min Section 16.6 proxy INVITE transaction
bullet 11 timeout
Timer D > 32s for UDP Section 17.1.1.2 Wait time for response
0s for TCP/SCTP retransmits
Timer E initially T1 Section 17.1.2.2 non-INVITE request
retransmit interval,
UDP only
Timer F 64*T1 Section 17.1.2.2 non-INVITE transaction
timeout timer
Timer G initially T1 Section 17.2.1 INVITE response
retransmit interval
Timer H 64*T1 Section 17.2.1 Wait time for
ACK receipt
Timer I T4 for UDP Section 17.2.1 Wait time for
0s for TCP/SCTP ACK retransmits
Timer J 64*T1 for UDP Section 17.2.2 Wait time for
0s for TCP/SCTP non-INVITE request
retransmits
Timer K T4 for UDP Section 17.1.2.2 Wait time for
0s for TCP/SCTP response retransmits
Table 4: Summary of timers