This document specifies the Content-ID header field for usage in the Session Initiation Protocol (SIP). This document also updates RFC 5621, which only allows a Content-ID URL to reference a body part that is part of a multipart message-body. This update enables a Content-ID URL to reference a complete message-body and metadata provided by some additional SIP header fields. This document updates RFC 5368 and RFC 6442 by clarifying their usage of the SIP Content-ID header field.