Under RFC 3263, if no NAPTR or SRV records are found and no transport is specified, a client should use UDP for a sip URI and TCP for a sips URI.
Checked 2026-10-02
If no SRV records are found, the client SHOULD use TCP for a SIPS URI, and UDP for a SIP URI.
- Vendor
- not restricted
- Product
- not restricted
- Subsystem
- SIP DNS server location
- Deployment
- any
- Region
- not restricted
- Release range
- RFC 3263, current as of 2026-10-02
- Checked
- 2026-10-02
Sources
- RFC 3263: Session Initiation Protocol (SIP): Locating SIP Servers — RFC Editor / IETF · tier 1 standards and regulators · Section 4.1 Selecting a Transport Protocol
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-rfc3263-no-srv-udp-default. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-408-timeout-troubleshooting-rfc3263-no-srv-udp-default
BibTeX
@misc{warmtransfer-claim-sip-408-timeout-troubleshooting-rfc3263-no-srv-udp-default,
title = {Troubleshooting SIP 408 Request Timeout and calls that get no response: source note sip-408-timeout-troubleshooting-rfc3263-no-srv-udp-default},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-408-timeout-troubleshooting-rfc3263-no-srv-udp-default},
note = {Source note sip-408-timeout-troubleshooting-rfc3263-no-srv-udp-default, checked 2026-10-02}
}