Without rport, a SIP server returns UDP responses to the request's source address but to the port in the top Via, which does not match the NAT binding the request created, so the response may never reach a client behind NAT.
Checked 2026-10-02
- Vendor
- not restricted
- Product
- not restricted
- Subsystem
- SIP NAT traversal
- Deployment
- any
- Region
- not restricted
- Release range
- RFC 3581, current as of 2026-10-02
- Checked
- 2026-10-02
Sources
- RFC 3581: An Extension to the Session Initiation Protocol (SIP) for Symmetric Response Routing — IETF / RFC Editor · tier 1 standards and regulators · Section 1 Introduction
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-rfc3581-via-port-breaks-nat. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-408-timeout-troubleshooting-rfc3581-via-port-breaks-nat
BibTeX
@misc{warmtransfer-claim-sip-408-timeout-troubleshooting-rfc3581-via-port-breaks-nat,
title = {Troubleshooting SIP 408 Request Timeout and calls that get no response: source note sip-408-timeout-troubleshooting-rfc3581-via-port-breaks-nat},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-408-timeout-troubleshooting-rfc3581-via-port-breaks-nat},
note = {Source note sip-408-timeout-troubleshooting-rfc3581-via-port-breaks-nat, checked 2026-10-02}
}