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Olson 3 Internet-Draft Microsoft 4 Expires: August 28, 2003 February 27, 2003 6 Requirements for Content Indirection in Session Initiation Protocol 7 (SIP) Messages 8 draft-ietf-sipping-content-indirect-03 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as 18 Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at http:// 26 www.ietf.org/ietf/1id-abstracts.txt. 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 This Internet-Draft will expire on August 28, 2003. 33 Copyright Notice 35 Copyright (C) The Internet Society (2003). All Rights Reserved. 37 Abstract 39 This specification defines requirements for a mechanism to indirectly 40 specify the content of a SIP message for the purpose of transferring 41 the content via a non-SIP channel. 43 1. Terminology 45 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 46 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 47 document are to be interpreted as described in RFC 2119 [1]. 49 2. Introduction 51 The purpose of the Session Initiation Protocol [2] (SIP) is to 52 create, modify, or terminate sessions with one or more participants. 53 SIP messages, like HTTP, are sytnactically composed of a start line, 54 one or more headers, and an optional body. Unlike HTTP, SIP is not 55 designed as a general purpose transport of data. 57 There are numerous reasons why it might be desirable to indirectly 58 specify the content of a SIP message body. For bandwidth limited 59 applications such as cellular wireless, indirection provides a means 60 to annotate the (indirect) content with meta-data which may be used 61 by the recipient to determine whether or not to retrieve the content 62 over the resource limited link. 64 It is also possible that the content size to be transferred might 65 potentially overwhelm intermediate signaling proxies, thereby 66 unnecessarily increasing network latency. For time-sensitive SIP 67 applications, this may be unacceptable. Indirect content can remedy 68 this by moving the transfer of this content out of the SIP signaling 69 network and into a potentially separate data transfer channel. 71 There may also be scenarios where the session related data (body) 72 that needs to be conveyed does not directly reside on the endpoint or 73 User Agent. In such scenarios, it is desirable to have a mechanism 74 whereby the SIP message can contain an indirect reference to the 75 desired content. The receiving party would then use this indirect 76 reference to retrieve the content via a non-SIP transfer channel such 77 as HTTP, FTP, or LDAP. 79 The purpose of content indirection is purely to provide an 80 alternative transport mechanism for SIP MIME body parts. With the 81 exception of the transport mechanism, indirected body parts are 82 equivalent, and should have the same treatment, as in-line body 83 parts. 85 3. Example Use Cases 87 There are several example users of such a content indirection 88 mechanism. These are examples only and are not intended to limit the 89 scope or applicability of the mechanism. 91 3.1 Presence Notification 93 The information carried in a presence document could potentially 94 exceed the recommended size for a SIP (NOTIFY) request, particularly 95 if the document carries aggregated information from multiple 96 endpoints. In such a situation, it would be desirable to send the 97 NOTIFY request with an indirect pointer to the presence document 98 which could then be retrieved by, for example, HTTP. 100 Figure 1: Example information flow for presence notification 102 Watcher Presence Server 103 | | 104 | SUBSCRIBE | 105 |-------------------------->| 106 | 200 OK | 107 |<--------------------------| 108 | | 109 | NOTIFY | 110 |-------------------------->| 111 | 200 OK | 112 |<--------------------------| 113 | | 114 | NOTIFY (w/URI) | 115 |<--------------------------| 116 | 200 | 117 |-------------------------->| 118 | | 119 | HTTP GET | 120 |-------------------------->| 121 | | 122 | application/cpim-pidf+xml | 123 |<--------------------------| 124 | | 126 In this example, the presence server returns an HTTP URI pointing to 127 a presence document on the presence server which the watcher can then 128 fetch using an HTTP GET. 130 3.2 Document Sharing 132 During an instant messaging conversation, a useful service is 133 document sharing wherein one party sends an IM (MESSAGE request) with 134 an indirect pointer to a document which is meant to be rendered by 135 the remote party. Carrying such a document directly in the MESSAGE 136 request is not appropriate for most documents. Furthermore, the 137 document to be shared may reside on a completely independent server 138 from the originating party. 140 Figure 2: Example information flow for document sharing 142 UAC UAS Web Server 143 | | | 144 | MESSAGE w/URI | | 145 |------------------->| | 146 | 200 | | 147 |<-------------------| | 148 | | | 149 | | HTTP GET | 150 | |--------------->| 151 | | image/jpeg | 152 | |<---------------| 153 | | | 155 In this example, a user wishes to exchange a JPEG image that she has 156 stored on her web server with another user she has a IM conversation 157 with. The JPEG is intended to be rendered inline in the IM 158 conversation. The recepient of the MESSAGE request launches a HTTP 159 GET request to the web server to retrieve the JPEG image. 161 4. Requirements 163 It MUST be possible to specify the location of content via a URI 164 [3]. 166 It MUST be possible to specify the length of the indirect content. 168 It MUST be possible to specify the type of the indirect content. 170 It MUST be possible to specify the disposition of each URI 171 independently. 173 It MUST be possible to label each URI to identify if and when the 174 content referred to by that URI has changed. Applications of this 175 mechanism may send the same URI more than once. The intention of 176 this requirement is to allow the receiving party to determine if 177 the content referenced by the URI has changed without having to 178 actually retrieve that content. Example ways the URI could be 179 labelled include a sequence number, timestamp, version number, 180 etc. 182 It MUST be possible to specify the timespan for which a given URI 183 is valid. This may or may not be the same as the lifetime for the 184 content itself. 186 It MUST be possible for the UAC and the UAS to indicate support of 187 this content indirection mechanism. A fallback mechanism SHOULD 188 be specified in the event that one of the parties is unable to 189 support content indirection. 191 It MUST be possible for the UAC and UAS to negotiate the type of 192 the indirect content when using the content indirection mechanism. 194 It MUST be possible for the UAC and UAS to negotiate support for 195 URI scheme(s) to be used in the content indirection mechanism. 196 This is in addition to the ability to negotiate the content type. 198 It SHOULD be possible to ensure the integrity and privacy of the 199 URI when it is received by the remote party. 201 It MUST be possible to process the content indirection without 202 human intervention. 204 It MUST allow for indirect transference of content in any SIP 205 message which would otherwise carry that content as a body. 207 5. Security Considerations 209 Any content indirection mechanism introduces additional security 210 concerns. By its nature, content indirection requires an extra 211 processing step and information transfer. There are a number of 212 potential abuses of a content indirection mechanism: 214 Content indirection allows the initiator to choose an alternative 215 protocol with weaker security or known vulnerabilities for the 216 content transfer. For example, asking the recipient to issue an 217 HTTP request which results in a Basic authentication challenge. 219 Content indirection allows the initiator to ask the recipient to 220 consume additional resources in the information transfer and 221 content processing, potentially creating an avenue for denial of 222 service attacks. For example, an active FTP URL consuming 2 223 connections for every indirect content message. 225 Content indirection could be used as a form of port scanning 226 attack where the indirect content URL is actually a bogus URL 227 pointing to an internal resource of the recipient. The response 228 to the content indirection request could reveal information about 229 open (and vulnerable) ports on these internal resources. 231 A content indirection URL can disclose sensitive information about 232 the initiator such as an internal user name (as part of an HTTP 233 URL) or possibly geolocation information. 235 Fortunately, all of these potential threats can be mitigated through 236 careful screening of both the indirect content URIs that are received 237 as well as those that are sent. Integrity and privacy protection of 238 the indirect content URI can prevent additional attacks as well. 240 References 242 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 243 Levels", BCP 14, RFC 2119, March 1997. 245 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 246 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 247 Session Initiation Protocol", RFC 3261, June 2002. 249 [3] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource 250 Identifiers (URI): Generic Syntax", RFC 2396, August 1998. 252 Author's Address 254 Sean Olson 255 Microsoft 256 One Microsoft Way 257 Redmond, WA 98052 258 US 260 Phone: +1-425-707-2846 261 EMail: seanol@microsoft.com 262 URI: http://www.microsoft.com/rtc 264 Intellectual Property Statement 266 The IETF takes no position regarding the validity or scope of any 267 intellectual property or other rights that might be claimed to 268 pertain to the implementation or use of the technology described in 269 this document or the extent to which any license under such rights 270 might or might not be available; neither does it represent that it 271 has made any effort to identify any such rights. Information on the 272 IETF's procedures with respect to rights in standards-track and 273 standards-related documentation can be found in BCP-11. 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