# SIP retransmission timers and transaction state Canonical: https://warmtransfer.net/knowledge/sip-retransmission-timers Last verified: 2026-09-15 3 constants underlie every SIP transaction timer: a round trip estimate defaulting to 500 milliseconds, a maximum retransmit interval of 4 seconds for non-INVITE requests and INVITE responses, and a maximum network lifetime of 5 seconds for a message[^5]. RFC 3261 names 8 lettered timers: A and E set a request retransmit interval starting at the round trip estimate, B and F set a transaction timeout at 64 times it, C sets a proxy timeout above 3 minutes, G sets a response retransmit interval, H sets the wait for an acknowledgement at 64 times the estimate, and D, I, J, and K set wait times for retransmissions to die out[^1]. ## Timer operations and intervals Timer C is the only transaction timer given as an absolute duration rather than a multiple of the round trip estimate, and it is stated as greater than 3 minutes[^3]. Timer G sets the interval at which a server retransmits an INVITE response starting at the round trip estimate and capped by the 4 second maximum[^4]. Timer H sets the wait for an acknowledgement at 64 times the round trip estimate, and its expiry is what ends a server transaction that never received one[^6]. ## Transport dependencies 4 timers are scoped to an unreliable transport and become 0 on a reliable one: A and E do not retransmit at all, and D, I, J, and K wait 0 seconds instead of their unreliable values[^7]. WarmTransfer's reading of the sources is that because all 4 retransmission timers are 0 on a reliable transport, a capture taken over TCP or TLS contains no request retransmission at all, so the absence of retransmissions in such a capture is a property of the transport and is not evidence about the network or the peer[^2]. ## See also See also [SIP transactions dialogs and call setup](https://warmtransfer.net/knowledge/sip-transactions-dialogs). ## Applicability Applies to: IETF Session Initiation Protocol and IETF SIP. Sources checked 2026-09-15. ## What remains uncertain Whether Webex and Unified Communications Manager run SIP over a reliable transport by default is not covered by the sources below. What round trip estimate real implementations use is not covered by the sources below. How a proxy chooses its own transaction timeout is not covered by the sources below. ## Sources [^1]: RFC 3261 names 8 lettered timers: A and E set a request retransmit interval starting at the round trip estimate B and F set a transaction timeout at 64 times it C sets a proxy timeout above 3 minutes G sets a response retransmit interval H sets the wait for an acknowledgement at 64 times the estimate and D I J and K set wait times for retransmissions to die out. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 appendix A, the timer table. Checked 2026-09-15. [^2]: Because all four retransmission timers are 0 on a reliable transport, a capture taken over TCP or TLS contains no request retransmission at all, so the absence of retransmissions in such a capture is a property of the transport and is not evidence about the network or the peer (inferred). Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 section 17 timer table, transport column. Checked 2026-09-15. [^3]: Timer C is the only transaction timer given as an absolute duration rather than a multiple of the round trip estimate and it is stated as greater than 3 minutes. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 sections 16.6 and appendix A. Checked 2026-09-15. [^4]: Timer G sets the interval at which a server retransmits an INVITE response starting at the round trip estimate and capped by the 4 second maximum. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 sections 17.2.1 and appendix A. Checked 2026-09-15. [^5]: Three constants underlie every SIP transaction timer: a round trip estimate defaulting to 500 milliseconds a maximum retransmit interval of 4 seconds for non-INVITE requests and INVITE responses and a maximum network lifetime of 5 seconds for a message. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 appendix A, the timer table. Checked 2026-09-15. [^6]: Timer H sets the wait for an acknowledgement at 64 times the round trip estimate and its expiry is what ends a server transaction that never received one. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 sections 17.2.1 and appendix A. Checked 2026-09-15. [^7]: Four timers are scoped to an unreliable transport and become 0 on a reliable one: A and E do not retransmit at all and D I J and K wait 0 seconds instead of their unreliable values. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), RFC 3261 appendix A, the timer table. Checked 2026-09-15.