Network Working Group
Request for Comments: 5365
Category: Standards Track

M. Garcia-Martin
G. Camarillo
Ericsson
October 2008

Multiple-Recipient MESSAGE Requests in

the Session Initiation Protocol (SIP)

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document specifies a mechanism that allows a SIP User Agent Client (UAC) to send a SIP MESSAGE request to a set of destinations, by using a SIP URI-list (Uniform Resource Identifier list) service. The UAC sends a SIP MESSAGE request that includes the payload along with the URI list to the MESSAGE URI-list service, which sends a MESSAGE request including the payload to each of the URIs included in the list.

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  URI-List Document  . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Option-Tag . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Procedures at the User Agent Client  . . . . . . . . . . . . .  6
   7.  Procedures at the MESSAGE URI-List Service . . . . . . . . . .  7
     7.1.  Determining the Intended Recipient . . . . . . . . . . . .  8
     7.2.  Creating an Outgoing MESSAGE Request . . . . . . . . . . .  8
     7.3.  Composing Bodies in the Outgoing MESSAGE Request . . . . . 10
   8.  Procedures at the UAS  . . . . . . . . . . . . . . . . . . . . 11
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     13.2. Informative References . . . . . . . . . . . . . . . . . . 17

1. Introduction

RFC 3261 (SIP) [RFC3261] is extended by RFC 3248 [RFC3428] to carry instant messages in MESSAGE requests. SIP-based messaging, as described in RFC 3428 [RFC3428], does not provide a mechanism to send the same request to multiple recipients or replying to all recipients of a SIP MESSAGE request. This memo addresses these functions.

A first requirement can be expressed as:

REQ-1: It must be possible for a user to send an instant message request to an ad hoc group, where the identities of the recipients are carried in the message itself.

One possibility to fulfill the above requirement is to establish a session of instant messages with an instant messaging conference server, and exchange the messages, for example, using MSRP (Message Session Relay Protocol) [RFC4975]. While this option seems to be reasonable in many cases, in other situations the sending user just wants to send a small pager-mode instant message to an ad hoc group without the burden of setting up a session. This document focuses on sending a pager-mode instant message to a number of intended recipients.

To meet the requirement with a pager-mode instant message, we allow SIP MESSAGE requests carry recipient-list bodies, i.e., URI lists in body parts whose Content-Disposition (RFC 2183) [RFC2183] is 'recipient-list', as specified in RFC 5363 [RFC5363]. A SIP MESSAGE URI-list service, which is a specialized application service, receives the request and sends a MESSAGE request including the received payload to each of the URIs in the list. Each of these MESSAGE requests contains a copy of the body included in the original MESSAGE request.

A second requirement addresses the "Reply-To-All" functionality:

REQ-2: It MUST be possible for the recipient of a group instant message to send a message to all other participants that received the same group instant message (i.e., Reply-To-All).

To meet this requirement, we provide a mechanism whereby the MESSAGE URI-list service also includes a URI list in body parts whose Content-Disposition (RFC 2183) [RFC2183] is 'recipient-list-history', as specified in RFC 5364 [RFC5364]. The 'recipient-list-history' body is sent along with the instant message payload in each of the instant messages sent to the recipients.

The User Agent Client (UAC) that sends a MESSAGE request to a MESSAGE URI-list service needs to be configured with the SIP URI of the service that provides the functionality. Discovering and provisioning of this URI to the UAC is outside the scope of this document.

2. Terminology

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 BCP 14, RFC 2119 [RFC2119] and indicate requirement levels for compliant implementations.

This document reuses the following terminology defined in RFC 3261 [RFC3261]:

Address-of-Record (AOR)

User Agent (UA)

User Agent Client (UAC)

User Agent Server (UAS)

This document defines the following new terms:

   MESSAGE URI-list service:  A specialized URI-list service that
   
      receives a MESSAGE request with a URI list and sends a similar
      MESSAGE request to each URI in the list.  In this context, similar
      indicates that some SIP header fields can change, but the MESSAGE
      URI-list service will not change the instant message payload.
      MESSAGE URI-list services behave effectively as specialized B2BUAs
      (Back-to-Back-User-Agents).  A server providing MESSAGE URI-list
      services can also offer URI-list services for other methods,
      although this functionality is outside the scope of this document.
      In this document, we only discuss MESSAGE URI-list services.
   
   Incoming MESSAGE request:  A SIP MESSAGE request that a UAC creates
      and addresses to a MESSAGE URI-list service.  Besides the regular
      instant message payload, an incoming MESSAGE request contains a
      URI list.
   
   Outgoing MESSAGE request:  A SIP MESSAGE request that a MESSAGE URI-
      list service creates and addresses to a UAS (User Agent Server).
      It contains the regular instant message payload.
   
   Intended recipient:  The intended final recipient of the request to
      be generated by MESSAGE URI-list service.
   
   Reply-To-All:  The ability of an intended recipient to receive a
      MESSAGE request that includes the payload and the list of
      recipients, and compose and send a MESSAGE request to the sender
      and the rest of the recipients.  The replying entity can use a
      MESSAGE URI-list service if one is at its disposal or can create a
      sequence of regular single-recipient MESSAGE requests to each SIP
      AOR.

3. Overview

A UAC creates a MESSAGE request that contains a multipart body including a list of URIs (intended recipients) and an instant message. The list of URIs is formatted according to the resource list document format specified in RFC 4826 [RFC4826] and extended with the attributes defined in RFC 5364 [RFC5364]. The UAC sends this MESSAGE request to the MESSAGE URI-list service. On reception of this incoming MESSAGE request, the MESSAGE URI-list service creates a MESSAGE request per intended recipient (listed in the URI list) and copies the instant message payload to each of those MESSAGES. The MESSAGE URI-list service also manipulates the XML resource list according to the procedures indicated in RFC 5364 [RFC5364], and attaches the result to each of the MESSAGE requests, along with the instant message payload. Then the MESSAGE URI-list service sends each of the created outgoing MESSAGE request to the respective receiver.

The MESSAGE URI-list mechanism allows a sender to specify multiple targets for a MESSAGE request by including an XML resource list document according to RFC 4826 [RFC4826] in the body of the MESSAGE request extended with the attributes defined in RFC 5364 [RFC5364]. This resource list, whose Content-Disposition (RFC 2183) [RFC2183] is 'recipient-list', as specified in RFC 5363 [RFC5363], includes the URIs of the targets. Each target URI may also be marked to indicate in what role the URI-list service will place the target (e.g., "to", "cc", or "bcc"), and whether the target URI is expected to be anonymized or not, according to the procedures described in RFC 5364 [RFC5364]. When the MESSAGE URI-list server expands the MESSAGE request to each recipient, it includes (along with the instant message payload) a new URI list (based on the received one), whose Content-Disposition (RFC 2183) [RFC2183] is 'recipient-list-history', as specified in RFC 5364 [RFC5364]. This new URI list includes the list of non-anonymous "to" and "cc" targets, allowing recipients both to get knowledge of other recipients and to reply to them.

4. URI-List Document

As described in RFC 5363 [RFC5363], specifications of individual URI- list services, like the MESSAGE URI-list service described here, need to specify a default format for 'recipient-list' bodies used within the particular service.

The default format for 'recipient-list' bodies for MESSAGE URI-list services is the resource list document specified in RFC 4826 [RFC4826] extended with the copy control attributes [RFC5364]. UACs and MESSAGE URI-list services handling 'recipient-list' bodies MUST support both of these formats and MAY support other formats.

As described in RFC 5364 [RFC5364], each URI can be tagged with a 'copyControl' attribute set to either "to", "cc", or "bcc", indicating the role in which the recipient will get the MESSAGE request. Additionally, URIs can be tagged with the 'anonymize' attribute to prevent that the MESSAGE URI-list server discloses the target URI in a URI list.

Additionally, RFC 5364 [RFC5364] defines a 'recipient-list-history' body that contains the list of intended recipients. The default format for 'recipient-list-history' bodies for MESSAGE URI-list services is also the resource list document specified in RFC 4826 [RFC4826] extended with the copy control attributes [RFC5364]. MESSAGE URI-list services MUST support both of these formats; UASs MAY support these formats. MESSAGE URI-list servers and UASs MAY support other formats.

The resource list document specified in RFC 4826 [RFC4826] provides a number of features that are not needed by the MESSAGE URI-list service defined in this document. The MESSAGE URI-list service needs to transfer a simple flat list of URIs between a UAC and the MESSAGE URI-list server and between the MESSAGE URI-list server and the UAS. The service does not need hierarchical lists or the ability to include entries by reference relative to the Extensible Configuration Access Protocol (XCAP) [RFC4825] root URI. Therefore, the MESSAGE URI-list service specified herein only uses flat resource lists documents that do not contain relative references.

5. Option-Tag

This document defines the 'recipient-list-message' option-tag for use in the Require and Supported SIP header fields.

This option-tag is used to ensure that a server can process the 'recipient-list' body used in a MESSAGE request. It also provides a mechanism to discover the capability of the server in responses to OPTIONS requests.

Section 6 provides normative procedures for the usage of this option tag.

6. Procedures at the User Agent Client

A UAC that wants to create a multiple-recipient MESSAGE request creates a MESSAGE request that MUST be formatted according to RFC 3428 [RFC3428] Section 4. The UAC populates the Request-URI with the SIP or SIPS URI of the MESSAGE URI-list service. In addition to the regular instant message body, the UAC adds a recipient-list body whose Content-Disposition type is 'recipient-list', specified in RFC 5363 [RFC5363]. This body contains a URI list with the recipients of the MESSAGE. Target URIs in this body MAY also be tagged with the 'copyControl' and 'anonymize' attributes specified in RFC 5364 [RFC5364]. The UAC MUST also include the 'recipient-list-message' option-tag, defined in Section 5, in a Require header field.

UACs generating MESSAGE requests that carry recipient-list bodies, as described in previous sections, MUST include this option-tag in a Require header field. UAs that are able to receive and process MESSAGEs with a recipient-list body, as described in previous sections, SHOULD include this option-tag in a Supported header field when responding to OPTIONS requests.

Multiple-recipient MESSAGE requests contain a multipart body that contains the body carrying the list and the actual instant message payload. In some cases, the MESSAGE request can contain bodies other than the text and the list bodies (e.g., when the request is protected with S/MIME as per RFC 3851 [RFC3851]).

Typically, the MESSAGE URI-list service will copy all the significant header fields in the outgoing MESSAGE request. However, there might be cases where the SIP UA wants the MESSAGE URI-list service to add a particular header field with a particular value, even if the header field wasn't present in the MESSAGE request sent by the UAC. In this case, the UAC MAY use the "?" mechanism described in Section 19.1.1 of RFC 3261 [RFC3261] to encode extra information in any URI in the list. However, the UAC MUST NOT use the special "body" hname (see Section 19.1.1 of RFC 3261 [RFC3261]) to encode a body, since the body is present in the MESSAGE request itself.

The following is an example of a URI that uses the "?" mechanism:

   sip:bob@example.com?Accept-Contact=*%3bmobility%3d%22mobile%22

The previous URI requests the MESSAGE URI-list service to add the following header field to a MESSAGE request to be sent to bob@example.com:

   Accept-Contact: *;mobility="mobile"

The resource list document format specified in RFC 4826 [RFC4826] provides features, such as hierarchical lists and the ability to include entries by reference relative to the XCAP root URI. However, these features are not needed by the multiple MESSAGE URI-list service defined in this document. Therefore, when using the default resource list document, UAs SHOULD use flat lists (i.e., no hierarchical lists) and SHOULD NOT use <entry-ref> elements.

7. Procedures at the MESSAGE URI-List Service

On reception of a MESSAGE request containing a URI list, the MESSAGE URI-list service answers to the UAC with a 202 (Accepted) response.

Note that the status code in the response to the MESSAGE does not provide any information about whether or not the MESSAGEs generated by the URI-list service were successfully delivered to the URIs in the list. That is, a 202 (Accepted) response means that the MESSAGE URI-list service has received the MESSAGE and that it will try to send a similar MESSAGE to the URIs in the list. Designing a mechanism to inform a client about the delivery status of an instant message is outside the scope of this document.

Since the MESSAGE URI-list service does not use hierarchical lists nor lists that include entries by reference to the XCAP root URI, a MESSAGE URI-list server receiving a URI list with more information than what has just been described MAY discard all the extra information.

If a MESSAGE request contains a Request-URI containing a URI that uses the "?" mechanism (see Section 19.1.1 of RFC 3261 [RFC3261]) and such URI contains the special "body" hname to include an additional body, the MESSAGE URI-list server MAY discard the contents of the "body" parameter.

7.1. Determining the Intended Recipient

On reception of a MESSAGE request containing a URI list, a MESSAGE URI-list service determines the list of intended recipients by inspecting the URI list contained in the body.

Section 4.1 of RFC 5363 [RFC5363] discusses cases when duplicated URIs are found in a URI list. In order to avoid duplicated requests, MESSAGE URI-list services MUST take those actions specified in RFC 5363 [RFC5363] into account to avoid sending duplicated requests to the same recipient.

7.2. Creating an Outgoing MESSAGE Request

Since the MESSAGE URI-list service behaves as a UAC for outgoing MESSAGE requests, for each of the intended recipients, the MESSAGE URI-list service creates a new MESSAGE request according to the procedures described in Section 4 of RFC 3428 [RFC3428]. Additionally, Section 5.3 of RFC 5363 [RFC5363] provides additional general guidance in creating outgoing requests. This document also specifies the following procedures:

A MESSAGE URI-list service MUST include a From header field whose value is the same as the From header field included in the incoming MESSAGE request, subject to the privacy requirements (see RFC 3323 [RFC3323] and RFC 3325 [RFC3325]) expressed in the incoming MESSAGE request.

Note that this does not apply to the "tag" parameter.

Failure to copy the From header field of the sender results in unacceptable security and privacy failures. Note also that this requirement does not intend to contradict requirements for additional services running on the same physical node. Specifically, a privacy service (see RFC 3323 [RFC3323]) can be co-located with the MESSAGE URI-list service, in which case, the privacy service has precedence over the MESSAGE URI-list service.

A MESSAGE URI-list service SHOULD generate a new To header field value set to the intended recipient's URI. According to the procedures of RFC 3261 [RFC3261] Section 8.1.1.1, this value is also expected to be equal to the Request-URI of the outgoing MESSAGE request.

The MESSAGE URI-list service behaves as a User Agent Client; thus, the To header field should be populated with the recipient's URI.

A MESSAGE URI-list service SHOULD create a new Call-ID header field value.

A Call-ID header field might contain addressing information that the sender wants to remain private. Since there is no need to keep the same Call-ID on both sides of the MESSAGE URI- list service, and since the MESSAGE URI-list service behaves as a User Agent Client, it is recommended to create a new Call-ID header field value according to the regular SIP procedures.

If a P-Asserted-Identity header field was present in the incoming MESSAGE request and the request was received from a trusted source, as specified in RFC 3325 [RFC3325], and the first hop of the outgoing MESSAGE request is also trusted, a MESSAGE URI-list service MUST include a P-Asserted-Identity header field in the outgoing MESSAGE request with the same received value. However, if the first hop of the outgoing MESSAGE request is not trusted and the incoming MESSAGE request included a Privacy header field with a value different than 'none', the MESSAGE URI-list service MUST NOT include a P-Asserted-Identity header field in the outgoing MESSAGE request.

If a MESSAGE URI-list service is able to assert the identity of a user (e.g., using HTTP Digest authentication scheme as per RFC 2617 [RFC2617], S/MIME as per RFC 3851 [RFC3851], etc.) and the service implements a mechanism where it can map that authentication scheme to a user's SIP or SIPS URI, and subject to the privacy requirements expressed in the incoming MESSAGE request (see RFC 3323 [RFC3323]), the MESSAGE URI-list service MAY insert a P-Asserted-Identity header with the value of the user's asserted URI.

If the incoming MESSAGE request contains an Authorization or Proxy-Authorization header field whose realm is set to the MESSAGE URI-list server's realm, then the MESSAGE URI-list service SHOULD NOT copy it to the outgoing MESSAGE request; otherwise (i.e., if the Authorization or Proxy-Authorization header field of incoming MESSAGE request contains a different realm), the MESSAGE URI-list service MUST copy the value to the respective header field of the outgoing MESSAGE request.

A MESSAGE URI-list service SHOULD create a separate count for the CSeq header field [RFC3261] of the outgoing MESSAGE request.

A MESSAGE URI-list service SHOULD initialize the value of the Max- Forward header field of the outgoing MESSAGE request.

A MESSAGE URI-list service MUST include its own value in the Via header field.

7.3. Composing Bodies in the Outgoing MESSAGE Request

When creating the body of each of the outgoing MESSAGE requests, the MESSAGE URI-list service keeps the relevant bodies of the incoming MESSAGE request and copies them to the outgoing MESSAGE request. The following guidelines constitute exceptions to the general body handling:

A MESSAGE request received at a MESSAGE URI-list service can contain one or more security bodies (e.g., S/MIME, RFC 3851 [RFC3851]) encrypted with the public key of the MESSAGE URI-list service. These bodies are deemed to be read by the URI-list service rather than the recipient of the outgoing MESSAGE request (which will not be able to decrypt them). Therefore, a MESSAGE URI-list service MUST NOT copy any security body (such as an S/MIME as per RFC 3851 [RFC3851] encrypted body) addressed to the MESSAGE URI-list service to the outgoing MESSAGE request. This includes bodies encrypted with the public key of the URI-list service.

The incoming MESSAGE request typically contains a recipient-list body or reference, as indicated in RFC 5363 [RFC5363] with the actual list of recipients. If this URI list includes resources tagged with the 'copyControl' attribute set to a value of "to" or "cc", the URI-list service SHOULD include a URI list in each of the outgoing MESSAGE requests. This list SHOULD be formatted according to the resource list document format specified in RFC 4826 [RFC4826] and the copyControl extension specified in RFC 5364 [RFC5364]. The MESSAGE URI-list service MUST follow the procedures specified in RFC 5364 [RFC5364] with respect to handling of the 'anonymize', 'count', and 'copyControl' attributes.

If the MESSAGE URI-list service includes a URI list in an outgoing MESSAGE request, it MUST include a Content-Disposition header field as per RFC 2183 [RFC2183] with the value set to 'recipient- list-history' and a "handling" parameter as per RFC 3204 [RFC3204] set to "optional".

If a MESSAGE URI-list service includes a URI list in an outgoing MESSAGE request, it SHOULD use S/MIME (RFC 3851) [RFC3851] to encrypt the URI list with the public key of the receiver.

The MESSAGE URI-list service SHOULD copy all the remaining message bodies (e.g., text messages, images, etc.) of the incoming MESSAGE request to the outgoing MESSAGE request.

If there is only one body left, the MESSAGE URI-list service MUST remove the multipart/mixed wrapper in the outgoing MESSAGE request.

The rest of the MESSAGE request corresponding to a given URI in the URI list MUST be created following the rules in Section 19.1.5, "Forming Requests from a URI", of RFC 3261 [RFC3261]. In particular, Section 19.1.5 of RFC 3261 [RFC3261] states:

"An implementation SHOULD treat the presence of any headers or body parts in the URI as a desire to include them in the message, and choose to honor the request on a per-component basis."

SIP allows to append a "method" parameter to a URI. Therefore, it is legitimate that the 'uri' attribute of the <entry> element in the XML resource list contains a "method" parameter. MESSAGE URI-list services MUST generate only MESSAGE requests, regardless of the "method" parameter that the URIs in the list indicate. Effectively, MESSAGE URI-list services MUST ignore the "method" parameter in each of the URIs present in the URI list.

8. Procedures at the UAS

A UAS (in this specification, also known as intended recipient UAS) that receives a MESSAGE request from the MESSAGE URI-list service behaves as specified in RFC 3428 [RFC3428] Section 7.

If the UAS supports this specification and the MESSAGE request contains a body with a Content-Disposition header field as per RFC 2183 [RFC2183] set to 'recipient-list-history', then the UAS will be able to determine the SIP Address-of-Record (AOR) of the other intended recipients of the MESSAGE request. This allows the user to create a reply request (e.g., MESSAGE, INVITE) to the sender and the rest of the recipients included in the URI list.

9. Examples

Figure 1 shows an example of operation. A SIP UAC issuer sends a MESSAGE request. The MESSAGE URI-list service answers with a 202 (Accepted) response and sends a MESSAGE request to each of the intended recipients.

   +--------+        +---------+      +--------+ +--------+ +--------+
   |SIP UAC |        | MESSAGE |      |intended| |intended| |intended|
   | issuer |        | URI-list|      | recip. | | recip. | | recip. |
   |        |        | service |      |   1    | |   2    | |   n    |
   +--------+        +---------+      +--------+ +--------+ +--------+
       |                  |               |          |          |
       | F1 MESSAGE       |               |          |          |
       | ---------------->|               |          |          |
       | F2 202 Accepted  |               |          |          |
       |<---------------- |  F3 MESSAGE   |          |          |
       |                  | ------------->|          |          |
       |                  |  F4 MESSAGE   |          |          |
       |                  | ------------------------>|          |
       |                  |  F5 MESSAGE   |          |          |
       |                  | ----------------------------------->|
       |                  |  F6 200 OK    |          |          |
       |                  |<------------- |          |          |
       |                  |  F7 200 OK    |          |          |
       |                  |<------------------------ |          |
       |                  |  F8 200 OK    |          |          |
       |                  |<----------------------------------- |
       |                  |               |          |          |
       |                  |               |          |          |
       |                  |               |          |          |

Figure 1: Example of operation

The MESSAGE request F1 (shown in Figure 2) contains a multipart/mixed body that is composed of two bodies: a text/plain body containing the instant message payload and an application/resource-lists+xml body containing the list of recipients.

   MESSAGE sip:list-service.example.com SIP/2.0
   Via: SIP/2.0/TCP uac.example.com
       ;branch=z9hG4bKhjhs8ass83
   Max-Forwards: 70
   To: MESSAGE URI-list service <sip:list-service.example.com>
   From: Alice <sip:alice@example.com>;tag=32331
   Call-ID: d432fa84b4c76e66710
   CSeq: 1 MESSAGE
   Require: recipient-list-message
   Content-Type: multipart/mixed;boundary="boundary1"
   Content-Length: 501
   
   --boundary1
   Content-Type: text/plain
   
   Hello World!
   
   --boundary1
   Content-Type: application/resource-lists+xml
   Content-Disposition: recipient-list
   
   <?xml version="1.0" encoding="UTF-8"?>
   <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
             xmlns:cp="urn:ietf:params:xml:ns:copycontrol">
     <list>
       <entry uri="sip:bill@example.com" cp:copyControl="to" />
       <entry uri="sip:randy@example.net" cp:copyControl="to"
                                          cp:anonymize="true"/>
       <entry uri="sip:eddy@example.com" cp:copyControl="to"
                                         cp:anonymize="true"/>
       <entry uri="sip:joe@example.org" cp:copyControl="cc" />
       <entry uri="sip:carol@example.net" cp:copyControl="cc"
                                          cp:anonymize="true"/>
       <entry uri="sip:ted@example.net" cp:copyControl="bcc" />
       <entry uri="sip:andy@example.com" cp:copyControl="bcc" />
     </list>
   </resource-lists>
   --boundary1--

Figure 2: MESSAGE request received at the MESSAGE URI-list server

The MESSAGE requests F3, F4, and F5 are similar in nature. All those MESSAGE requests contain a multipart/mixed body that is composed of two other bodies: a text/plain body containing the instant message payload and an application/resource-lists+xml containing the list of recipients. Unlike the text/plain body, the application/ resource-lists+xml bodies of MESSAGE requests F3, F4, and F5 are not equal to the application/resource-lists+xml body included in the incoming MESSAGE request F1. This is because the URI-list service has anonymized those URIs tagged with the 'anonymize' attribute and has removed those URIs tagged with a "bcc" 'copyControl' attribute; besides, the content disposition of these bodies is different. Figure 3 shows an example of the MESSAGE request F3.

   MESSAGE sip:bill@example.com SIP/2.0
   Via: SIP/2.0/TCP list-service.example.com
       ;branch=z9hG4bKhjhs8as34sc
   Max-Forwards: 70
   To: <sip:bill@example.com>
   From: Alice <sip:alice@example.com>;tag=210342
   Call-ID: 39s02sdsl20d9sj2l
   CSeq: 1 MESSAGE
   Content-Type: multipart/mixed;boundary="boundary1"
   Content-Length: 501
   
   --boundary1
   Content-Type: text/plain
   
   Hello World!
   
   --boundary1
   Content-Type: application/resource-lists+xml
   Content-Disposition: recipient-list-history; handling=optional
   
   <?xml version="1.0" encoding="UTF-8"?>
   <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
             xmlns:cp="urn:ietf:params:xml:ns:copycontrol">
     <list>
       <entry uri="sip:bill@example.com" cp:copyControl="to" />
       <entry uri="sip:anonymous@anonymous.invalid" cp:copyControl="to"
                                                    cp:count="2"/>
       <entry uri="sip:joe@example.org" cp:copyControl="cc" />
       <entry uri="sip:anonymous@anonymous.invalid" cp:copyControl="cc"
                                                    cp:count="1"/>
     </list>
   </resource-lists>
   --boundary1--

Figure 3: MESSAGE request sent by the MESSAGE URI-list server

10. Security Considerations

RFC 5363 [RFC5363] discusses issues related to SIP URI-list services. Implementations of MESSAGE URI-list services MUST follow the security-related rules in RFC 5363 [RFC5363]. These rules include opt-in lists and mandatory authentication and authorization of clients.

If the contents of the instant message needs to be kept private, the User Agent Client SHOULD use S/MIME as per RFC 3851 [RFC3851] to prevent a third party from viewing this information. In this case, the user agent client SHOULD encrypt the instant message body with a content encryption key. Then, for each receiver in the list, the UAC SHOULD encrypt the content encryption key with the public key of the receiver, and attach it to the MESSAGE request.

11. IANA Considerations

This document defines the SIP option tag 'recipient-list-message'

The following row has been added to the "Option Tags" section of the SIP Parameter Registry:

   +------------------------+------------------------------+-----------+
   | Name                   | Description                  | Reference |
   +------------------------+------------------------------+-----------+
   | recipient-list-message | The body contains a list of  | [RFC5365] |
   |                        | URIs that indicates the      |           |
   |                        | recipients of the SIP        |           |
   |                        | MESSAGE request              |           |
   +------------------------+------------------------------+-----------+
   
     Table 1: Registration of the 'recipient-list-message' Option-Tag
                                  in SIP

12. Acknowledgements

Duncan Mills supported the idea of having 1 to n MESSAGEs. Ben Campbell, Paul Kyzivat, Cullen Jennings, Jonathan Rosenberg, Dean Willis, and Keith Drage provided helpful comments.

13. References

13.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
   
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2183]  Troost, R., Dorner, S., and K. Moore, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.
   
   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.
   
   [RFC3204]  Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet,
              F., Watson, M., and M. Zonoun, "MIME media types for ISUP
              and QSIG Objects", RFC 3204, December 2001.
   
   [RFC3261]  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.
   
   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323, November 2002.
   
   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.
   
   [RFC3428]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
              and D. Gurle, "Session Initiation Protocol (SIP) Extension
              for Instant Messaging", RFC 3428, December 2002.
   
   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 2004.
   
   [RFC4826]  Rosenberg, J., "Extensible Markup Language (XML) Formats
              for Representing Resource Lists", RFC 4826, May 2007.
   
   [RFC5363]  Camarillo, G. and A.B. Roach, "Framework and Security
              Considerations for Session Initiation Protocol (SIP) URI-
              List Services", RFC 5363, October 2008.
   
   [RFC5364]  Garcia-Martin, M. and G. Camarillo, "Extensible Markup
              Language (XML) Format Extension for Representing Copy
              Control Attributes in Resource Lists", RFC 5364,
              October 2008.

13.2. Informative References

   [RFC4825]  Rosenberg, J., "The Extensible Markup Language (XML)
              Configuration Access Protocol (XCAP)", RFC 4825, May 2007.
   
   [RFC4975]  Campbell, B., Mahy, R., and C. Jennings, "The Message
              Session Relay Protocol (MSRP)", RFC 4975, September 2007.

Authors' Addresses

   Miguel A. Garcia-Martin
   Ericsson
   Via de los Poblados 13
   Madrid  28033
   Spain
   
   EMail: miguel.a.garcia@ericsson.com
   
   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland
   
   EMail: Gonzalo.Camarillo@ericsson.com

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