SIP WG C. Jennings Internet-Draft Cisco Systems Expires: December 21, 2003 June 22, 2003 Recommendations for using MIME body parts in SIP draft-jennings-sip-mime-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 21, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document describes conventions for using MIME body parts in SIP messages. It uses a transport encoding of "binary" since SIP messages are always passed over an 8bit clean transport. 1. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [2]. This document adopts the terminology defined in RFC 2045 [1], particularly for the terms "transport encoding" and "binary". Jennings Expires December 21, 2003 [Page 1] Internet-Draft MIME in SIP June 2003 2. Introduction The Session Initiation Protocol (SIP) [3] protocol makes use of MIME [1] body parts. MIME provides several alternatives that were required given the characteristics of existing mail transport protocols and mail gateways that MIME operates through. SIP is always transported over an 8bit safe transport and thus does not need all the options available. This draft clarifies what should be used in the SIP context. 3. Discussion MIME offers several transport encoding options and any of them will work in SIP. However, having several options where one is needed does not contribute to interoperability. Binary encoding is faster to encode and decode, requires less code, and results in smaller messages than the other options. There has been a practice in the published SIP examples of using a base64 encoding due to the ease of displaying the example in publication. Some SIP implementers have taken this to mean that this is the preferred encoding and as a result only work with base64. Given the need to improve interoperability, it is reasonable to suggest that SIP implementations send one type of encoding. There are situations in which the body from a SIP message might be passed to another non SIP transport that might expose additional limitations. Currently the only example of this is the transfer of bodies from instant messaging messages to other instant messaging systems. Since other instant messaging protocols are also 8bit clean, gateways from SIP instant messaging [5] to these other protocols do not have this problem. Gateways to other protocols (for example SMTP [4]) need to modify the content of these messages anyway, regardless of the MIME encoding which is used on the original message. 4. Recommendations Devices MUST use a content transfer encoding of "binary" for MIME body parts in SIP messages they send. 5. Security Considerations This document limits options that exist in RFC 3261 so it does not introduce any additional security concerns beyond what is in RFC 3261. 6. Open Issues Jennings Expires December 21, 2003 [Page 2] Internet-Draft MIME in SIP June 2003 Should we bother to do this or just write it up as a bug on RFC 3261? Normative References [1] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. Informative References [4] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [5] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, December 2002. Author's Address Cullen Jennings Cisco Systems 170 West Tasman Drive MS: SJC-21/3 San Jose, CA 95134 USA Phone: +1 408 527-9132 EMail: fluffy@cisco.com Jennings Expires December 21, 2003 [Page 3] Internet-Draft MIME in SIP June 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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