RFC 3326: The Reason Header Field for the Session Initiation Protocol (SIP)

Publisher

IETF / RFC Editor

URL

https://www.rfc-editor.org/rfc/rfc3326.html

Published

2002-12-01

Updated

unknown

Accessed

2026-09-24

HTTP status

200

License

IETF Trust; BCP 78 and the Trust Legal Provisions; excerpts permitted with attribution under IETF Trust terms; short excerpts and locators only

Source notes citing this source

• The Reason header may appear in any request within a dialog, in any CANCEL request, and in responses whose status code explicitly allows it; RFC 3326 highlights its use in BYE and CANCEL. in context
• The SIP Reason header carries a protocol value of SIP or Q.850 with a numeric cause parameter and an optional text parameter, so a Q.850 cause can be conveyed in SIP signaling. in context
• Clearing causes cannot be compared across platforms without a mapping step, because Unified CM reports Q.850 plus Cisco-defined causes while Teams Direct Routing reports SIP final codes plus Microsoft subcodes; RFC 3326 lets SIP carry both but defines no one-to-one mapping. inferred in context
• The SIP Reason header field (RFC 3326) carries a cause for a request under a protocol token, where SIP carries a SIP status code and Q.850 carries an ITU-T Q.850 cause value in decimal. in context
• RFC 3326 allows the Reason header in any request within a dialog and in any CANCEL request, which lets a gateway pass the ISUP release cause onward in BYE or CANCEL. in context
• RFC 3326 defines the SIP Reason header with registered protocol values SIP and Q.850, where Q.850 carries an ITU-T Q.850 cause value in the cause parameter. in context

Cite this source record

APA

WarmTransfer. (2002, December 1). RFC 3326: The Reason Header Field for the Session Initiation Protocol (SIP). WarmTransfer. https://warmtransfer.net/knowledge/sources/rfc-3326-reason-header

BibTeX

@misc{warmtransfer-rfc-3326-reason-header,
  title  = {RFC 3326: The Reason Header Field for the Session Initiation Protocol (SIP)},
  author = {{WarmTransfer}},
  year   = {2002},
  url    = {https://warmtransfer.net/knowledge/sources/rfc-3326-reason-header},
  note   = {IETF / RFC Editor, accessed 2026-09-24}
}