SIP timer summary

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 C	> 3 min.	16.6 bullet 11	Proxy INVITE transaction timeout
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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