Why does SIP not have a Content-Transfer-Encoding header?
The Content-Transfer-Encoding header was primarily meant to allow message bodies to be transformed into formats that could be transferred on channels that were not 8 bit clean. HTTP, which makes use of many of the MIME headers, is 8 bit clean, and thus did not need Content-Transfer-Encoding. SIP followed suit, and so does not use it either. Content-Encoding is used for things like compression, which is different. (J. Rosenberg) See also RFC 2616 (HTTP/1.1), Section 19.4.5.
