SIP WG C. Jennings
Internet-Draft Cisco Systems
Expires: January 14, 2005 July 16, 2004
Recommendations for using MIME body parts in SIP
draft-jennings-sip-mime-02
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Abstract
This document describes conventions for using MIME body parts in SIP
messages. It recommends a transport encoding of "binary" since SIP
messages are always passed over an 8bit clean transport.
This work is being discussed on the sip@ietf.org mailing list.
1. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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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".
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 examples 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. There is no need to receive
messages that do not have an encoding of "binary". This will be
documented in an errata of RFC 3261.
The above recommendation was the consensus of the room at the
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previous meeting.
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. References
6.1 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.
6.2 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-21/2
San Jose, CA 95134
USA
Phone: +1 408 902-3341
EMail: fluffy@cisco.com
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