SIP Timers
Request for Comments (RFC) 3261, "SIP: Session Initiation Protocol," specifies various timers that SIP uses.
Summary of SIP timers summarizes for each SIP timer the default value, the section of RFC 3261 that describes the timer, and the meaning of the timer.
Table 1. Summary of SIP timers
| 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 |
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