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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 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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