Session Initiation Protocol (SIP) timer summary
Request for Comments (RFC) 3261, SIP: Session Initiation Protocol, specifies various timers that SIP uses.
Table 1 summarizes
for each SIP timer the default value, the section of RFC 3261 that
describes the timer, and the meaning of the timer.
| Timer | Default value | Section | Meaning |
|---|---|---|---|
| T1 | 500 ms | 17.1.1.1 | Round-trip time (RTT) estimate |
| T2 | 4 sec. | 17.1.2.2 | Maximum retransmission interval for non-INVITE requests and INVITE responses |
| T4 | 5 sec. | 17.1.2.2 | Maximum duration that a message can remain in the network |
| Timer A | initially T1 | 17.1.1.2 | INVITE request retransmission interval, for UDP only |
| Timer B | 64*T1 | 17.1.1.2 | INVITE transaction timeout timer |
| Timer D | > 32 sec. for UDP | 17.1.1.2 | Wait time for response retransmissions |
| 0 sec. for TCP and SCTP | |||
| Timer E | initially T1 | 17.1.2.2 | Non-INVITE request retransmission interval, UDP only |
| Timer F | 64*T1 | 17.1.2.2 | Non-INVITE transaction timeout timer |
| Timer G | initially T1 | 17.2.1 | INVITE response retransmission interval |
| Timer H | 64*T1 | 17.2.1 | Wait time for ACK receipt |
| Timer I | T4 for UDP | 17.2.1 | Wait time for ACK retransmissions |
| 0 sec. for TCP and SCTP | |||
| Timer J | 64*T1 for UDP | 17.2.2 | Wait time for retransmissions of non-INVITE requests |
| 0 sec. for TCP and SCTP | |||
| Timer K | T4 for UDP | 17.1.2.2 | Wait time for response retransmissions |
| 0 sec. for TCP and SCTP |