SIP Offer/Answer with Multipart MIME
draft-jennings-sipping-multipart-00

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Copyright Notice

Copyright (C) The Internet Society (2005). All Rights Reserved.

Abstract

This document addresses the issues around using multipart with the SIP offer/answer framework. It specifies a way to make an offer with a multipart/alternative MIME body and for the answer to indicate which of the parts was selected for the answer. This is needed for general backwards compatibility to allow migration in situations such as moving from SDP to SDPng or moving from non end-to-end encrypted SDP to encrypted SDP.

Table of Contents

1  Introduction and Overview
2  Conventions
3  Mechanisms
 3.1  Sending Offers
 3.2  Receiving Offers and Sending Answers
 3.3  Receiving Answers
4  Syntax
5  Multipart Outside of Offer/Answer
6  Example SIP SRTP Call
7  Security considerations
8  IANA
9  Acknowledgments
10  References
 10.1  Normative References
 10.2  Informational References
§  Author's Address
§  Intellectual Property and Copyright Statements

1 Introduction and Overview

SIP (RFC 3261 [5]) uses an offer/answer negotiation mechanism described in [1]. This system carries offers in formats such as SDP [6] and various signed and encrypted versions of SDP. However, the current offer/answer scheme does not allow a backwards compatibility mode in which a UA can make both old and new types of offers and allow the other UA to select the type of offer that it supports. This specification extends the SIP to allow for these backwards compatible offer/answer schemes.

The mechanism for doing this is based on multipart alternative MIME types[2]. The UA making the offer uses a multipart alternative and includes a Content-ID for each one of the parts. The UA receiving the offer selects one of the parts in the offer and sends an answer based on that part. When the UA sends the answer, it indicates the Content-ID of the selected offer part by including this in a new Content-Related-To header that is defined in this specification.

This approach can allow migration from SDP to SDPng[7]. It can also allow migration from offers that are not S/MIME protected (as described in RFC 3261) to ones that are, and allow SRTP [8] keying material to be passed in the S/MIME protected SDP using a mechanism such as SDES [8].

2 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 [4].

3 Mechanisms

3.1 Sending Offers

A UA that can support multiple types of offers SHOULD construct a multipart/alternative body with a body for each type it supports. Each body MUST include a unique Content-ID header.

If the UA receives an error response that indicates that multipart/alternative is not supported, it MAY retry the request using an offer that consists of only a SDP body.

3.2 Receiving Offers and Sending Answers

If a UA receives multiple SDP offers in an multipart/alternative body, it MUST interpret these as it would a normal multipart alternative, as defined in RFC 2046 [2], which means it picks the last alternative that it can support. Any S/MIME bodies that cannot be decrypted MUST be treated as unsupportable.

When the UA constructs the answer, it MUST include a MIME Content-Related-To header field as defined in Section 4 with the value set to the value of the header field for the Content-ID of the offer that was selected. The answer MAY also be a multipart/alternative.

3.3 Receiving Answers

When the UA receives an answer, it MUST look at the Content-Related-To header field value to find which answer has been used. If the answer is a multipart/alternative, the UA selects the preferred body in the same way it would for an offer. It then proceeds with normal offer/answer processing.

4 Syntax

This specification defines a new MIME header called "Content-Related-To". This updates RFC 2045 [3] with:

 rid := "Content-Related-To" ":" msg-id 

and adds "[rid CRLF]" to the Identity headers in RFC 2045.

The Content-Related-To header is used in answers and has a msg-id value that is the same as the Content-ID value of the offer to which this answer is related.

5 Multipart Outside of Offer/Answer

Applications such as presence and 911 location information result in information with significant privacy requirements being sent in SIP. Particular MIME types may define special meanings when both encrypted and unencrypted bodies are received, but, unless otherwise specified, the UA SHOULD use the encrypted version if it can decrypt it, and ignore the unencrypted version. There is no requirement that the two versions have the same information.

6 Example SIP SRTP Call

In this example, large parts of the message are omitted to highlight what is relevant to this specification. The lines in the example that are prefixed by $ represent encrypted blocks of data.

In this example, Alice calls Bob and offers both an RTP and an SRTP session. The SDP for the SRTP session contains the SRTP keying material, and the SDP is encrypted with S/MIME. It is assumed that Alice has Bob's public key.

Alice sends an INVITE to Bob that offers two alternative SDP bodies, one of which is encrypted and contains the SRTP keying information. The encrypted version is preferred so it comes second. Both parts contain Content-ID headers.

 INVITE sip:bob@biloxi.example.com SIP/2.0
 ...
 Content-Type: multipart/alternative;boundary=boundary

 --boundary
 Content-ID: 123
 Content-Type: application/sdp
 Content-Disposition: session

 < SDP offer for ordinary RTP only >
 --boundary
 Content-ID: 456
 Content-Type: application/pkcs7-mime
 Content-Disposition: session

 $ Content-Type: application/sdp
 $
 $ < encrypted SDP with key for SRTP >
 --boundary

Assuming that Bob's UA supports encryption and had Alice's public key, it would select the second alternative part as the offer and then construct an appropriate answer. The 200 includes the MIME Content-Related-To header that indicates which alternative MIME body was chosen.

 200 OK
 ...
 Content-ID: 789
 Content-Related-To: 456
 Content-Type: application/pkcs7-mime
 Content-Disposition: session

 $ Content-Type: application/sdp
 $
 $ < encrypted SDP with key for SRTP >
 

7 Security considerations

TODO

8 IANA

The MIME Content-Related-To header does not require any IANA actions.

9 Acknowledgments

Thanks for comments from Flemming Andreasen, Paul Kyzivat, Mark Baugher, and Dan Wing.

10  References

10.1  Normative References

10.2  Informational References

Author's Address

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Acknowledgement

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