RFC 4459 says PMTUD signalling for tunnels is unreliable in IPv4 because firewalls and other boxes are often configured to drop all ICMP.
Checked 2026-10-02
- Vendor
- not restricted
- Product
- not restricted
- Subsystem
- Tunnel MTU
- Deployment
- on-premises, multi-tenant
- Region
- not restricted
- Release range
- Informational RFC as published
- Checked
- 2026-10-02
Sources
- RFC 4459: MTU and Fragmentation Issues with In-the-Network Tunneling — RFC Editor (IETF) · tier 1 standards and regulators · Section 3.2
Cite this note
APA
WarmTransfer. (2026, October 2). Troubleshooting failed calls caused by large SIP messages over UDP: source note sip-fragmentation-troubleshooting-rfc4459-icmp-drop-pmtud. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-fragmentation-troubleshooting-rfc4459-icmp-drop-pmtud
BibTeX
@misc{warmtransfer-claim-sip-fragmentation-troubleshooting-rfc4459-icmp-drop-pmtud,
title = {Troubleshooting failed calls caused by large SIP messages over UDP: source note sip-fragmentation-troubleshooting-rfc4459-icmp-drop-pmtud},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-fragmentation-troubleshooting-rfc4459-icmp-drop-pmtud},
note = {Source note sip-fragmentation-troubleshooting-rfc4459-icmp-drop-pmtud, checked 2026-10-02}
}