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SIP retransmission timers and transaction state

Verified 2026-09-15 · 7 sources · tier 1

Also known as T1, Timer H.

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.

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.

See also

Part of

Related to

Referenced by

Sources

  1. 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.
    RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 appendix A, the timer table · Checked 2026-09-15
  2. 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
    RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 section 17 timer table, transport column · Checked 2026-09-15
  3. 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.
    RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 sections 16.6 and appendix A · Checked 2026-09-15
  4. 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.
    RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 sections 17.2.1 and appendix A · Checked 2026-09-15
  5. 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.
    RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 appendix A, the timer table · Checked 2026-09-15
  6. 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.
    RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 sections 17.2.1 and appendix A · Checked 2026-09-15
  7. 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.
    RFC 3261 — SIP: Session Initiation Protocol · RFC 3261 appendix A, the timer table · Checked 2026-09-15

Documents

tier 1 standards and regulators

RFC 3261 — SIP: Session Initiation Protocol

IETF / RFC Editor · 2002-06 · accessed 2026-09-04

Cite this page

APA

WarmTransfer. (2026, September 15). SIP retransmission timers and transaction state. WarmTransfer. https://warmtransfer.net/knowledge/sip-retransmission-timers

BibTeX

@misc{warmtransfer-sip-retransmission-timers,
  title  = {SIP retransmission timers and transaction state},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/sip-retransmission-timers},
  note   = {Verified 2026-09-15}
}