With RFC 3261 defaults over UDP, an unanswered INVITE is expected to be retransmitted at roughly 0.5, 1.5, 3.5, 7.5, 15.5 and 31.5 seconds after the first send (seven transmissions) before Timer B ends the transaction at 32 seconds; Cisco's default 'retry invite 6' with 'timers trying 500' produces the same count.
inferred · Checked 2026-10-02
- Vendor
- not restricted
- Product
- not restricted
- Subsystem
- SIP INVITE client transaction
- Deployment
- any
- Region
- not restricted
- Release range
- RFC 3261 defaults; implementations may differ
- Status
- inferred
- Checked
- 2026-10-02
Sources
- RFC 3261 — SIP: Session Initiation Protocol — IETF / RFC Editor · tier 1 standards and regulators · Section 17.1.1.2 (Timer A doubling, Timer B 64*T1), combined with Cisco 'retry invite' and 'timers trying' entries
Cite this note
APA
WarmTransfer. (2026, October 2). Troubleshooting SIP 408 Request Timeout and calls that get no response: source note sip-408-timeout-troubleshooting-inf-udp-invite-schedule. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-408-timeout-troubleshooting-inf-udp-invite-schedule
BibTeX
@misc{warmtransfer-claim-sip-408-timeout-troubleshooting-inf-udp-invite-schedule,
title = {Troubleshooting SIP 408 Request Timeout and calls that get no response: source note sip-408-timeout-troubleshooting-inf-udp-invite-schedule},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-408-timeout-troubleshooting-inf-udp-invite-schedule},
note = {Source note sip-408-timeout-troubleshooting-inf-udp-invite-schedule, checked 2026-10-02}
}