RFC 3261 requires a SIP request to be sent over an RFC 2914 congestion-controlled transport such as TCP if it is within 200 bytes of the path MTU, or larger than 1300 bytes when the path MTU is unknown.
Checked 2026-10-02
If a request is within 200 bytes of the path MTU, or if it is larger than 1300 bytes and the path MTU is unknown, the request MUST be sent using an RFC 2914 congestion controlled transport protocol, such as TCP.
- Vendor
- not restricted
- Product
- not restricted
- Subsystem
- SIP transport selection
- Deployment
- on-premises, multi-tenant
- Region
- not restricted
- Release range
- RFC 3261 as published
- Checked
- 2026-10-02
Sources
- RFC 3261 — SIP: Session Initiation Protocol — IETF / RFC Editor · tier 1 standards and regulators · Section 18.1.1 Sending Requests, message-size paragraph
Cite this note
APA
WarmTransfer. (2026, October 2). Troubleshooting failed calls caused by large SIP messages over UDP: source note sip-fragmentation-troubleshooting-rfc3261-1300-byte-tcp-rule. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-fragmentation-troubleshooting-rfc3261-1300-byte-tcp-rule
BibTeX
@misc{warmtransfer-claim-sip-fragmentation-troubleshooting-rfc3261-1300-byte-tcp-rule,
title = {Troubleshooting failed calls caused by large SIP messages over UDP: source note sip-fragmentation-troubleshooting-rfc3261-1300-byte-tcp-rule},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-fragmentation-troubleshooting-rfc3261-1300-byte-tcp-rule},
note = {Source note sip-fragmentation-troubleshooting-rfc3261-1300-byte-tcp-rule, checked 2026-10-02}
}