I have reviewed this document as part of the security directorate's ongoing effort to review all IETF documents being processed by the IESG. These comments were written primarily for the benefit of the security area directors. Document editors and WG chairs should treat these comments just like any other last call comments. This document updates RFC 7315 by changing restrictions on where certain SIP private header extensions may be included, in order to address new 3GPP use cases. This document is Ready with nits. I know little about SIP or 3GPP. I do know security, though. After reading this document and also reading the Security Considerations section of RFC 7315, I believe that this document is OK from a security standpoint. Few new security issues are raised by this document and those that arise are properly documented in the Security Considerations section of this document. However, there are a few typos in the Security Considerations section. * The second sentence of the Security Considerations section ends with "the security considerations and assumptions (e.g. regarding only sending information to trusted entities) also to those messages." This clause is missing a verb. Maybe the word "apply" should appear before "to those messages". Also, greater clarity could be achieved by changing "the security considerations and assumptions" in that sentence fragment to "the security considerations and assumptions described in RFC 7315". * In the third sentence of the Security Considerations section, "disallow" should be "disallows" and "message" should be "messages". * In the fourth sentence of the Security Considerations section, "if a header field occur" should be "if a header field occurs". With these minor changes, I think the document will be ready to go from a security standpoint. Thanks, Steve