For UDP applications that cannot discover path MTU, RFC 8085 gives the effective send MTU as the smaller of 576 bytes and the first-hop MTU for IPv4, and 1280 bytes for IPv6.
Checked 2026-10-02
For IPv4, EMTU_S is the smaller of 576 bytes and the first-hop MTU.
- Vendor
- not restricted
- Product
- not restricted
- Subsystem
- UDP message sizing
- Deployment
- on-premises, multi-tenant
- Region
- not restricted
- Release range
- BCP 145 as published
- Checked
- 2026-10-02
Sources
- RFC 8085: UDP Usage Guidelines — RFC Editor (IETF) · tier 1 standards and regulators · Section 3.2 Message Size Guidelines
Cite this note
APA
WarmTransfer. (2026, October 2). Troubleshooting failed calls caused by large SIP messages over UDP: source note sip-fragmentation-troubleshooting-rfc8085-emtu-s-values. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-fragmentation-troubleshooting-rfc8085-emtu-s-values
BibTeX
@misc{warmtransfer-claim-sip-fragmentation-troubleshooting-rfc8085-emtu-s-values,
title = {Troubleshooting failed calls caused by large SIP messages over UDP: source note sip-fragmentation-troubleshooting-rfc8085-emtu-s-values},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-fragmentation-troubleshooting-rfc8085-emtu-s-values},
note = {Source note sip-fragmentation-troubleshooting-rfc8085-emtu-s-values, checked 2026-10-02}
}