Session Initiation Protocol (SIP) INFO Method and Package Framework
draft-ietf-sip-info-events-03

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Abstract

This document defines the new SIP INFO method and a mechanism for defining, negotiating and exchanging Info Packages that use the INFO method. Applications that need to exchange session-related information within a SIP INVITE-created dialog, also known as application level information, use these INFO requests. This draft addresses issues and open items from RFC 2976 and replaces it.

Conventions Used in this Document

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119]. The terminology in this document conforms to the Internet Security Glossary (Shirey, R., “Internet Security Glossary, Version 2,” August 2007.) [RFC4949].

Be mindful of the terms User Agent Server (UAS) and User Agent Client (UAC). This document strictly follows RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261]. The UAC issues a SIP request and the UAS responds. This terminology may be confusing when one combines the INFO case with the INVITE case. For an INVITE, the initiator of the session is the UAC and the target of the session is the UAS. However, it is possible for the target UA of the session, the UAS of the INVITE transaction, to send an INFO to the initiating UA of the session, the UAC of the INVITE transaction. From the perspective of the INFO, the target UA of the session (INVITE UAS) is, in fact, the UAC (sender) of the INFO request. Likewise, from the perspective of the INFO, the initiating UA of the session (INVITE UAC) is the UAS (recipient) of the INFO request. Since this document strictly follows RFC 3261, we refer to the UA that issues the INVITE as the "initiating UA" and the UA that responds to the INVITE as the "target UA" to remove any confusion.

Table of Contents

1.  Introduction
2.  Applicability
3.  Info Package Negotiation
    3.1.  UA Behavior
    3.2.  Package Versions
4.  The INFO Method Request
    4.1.  INFO Requests
    4.2.  INFO Request Body
    4.3.  Responses to the INFO Request Method
    4.4.  Routing Behavior
    4.5.  Behavior of Registrars
    4.6.  OPTIONS Processing
    4.7.  Order of Delivery
5.  Formal INFO Method Definition
    5.1.  INFO Method
    5.2.  INFO Headers
        5.2.1.  Info-Package header
        5.2.2.  Recv-Info header
6.  Legacy Uses of INFO
7.  Info Package Requirements
    7.1.  Applicability
    7.2.  Info Package Name
    7.3.  Info Package Parameters
    7.4.  Info Package Tags
    7.5.  INFO Bodies
    7.6.  UAC generation of INFO requests
    7.7.  UAS processing of INFO requests
    7.8.  Rate of INFO Requests
    7.9.  IANA Registrations
    7.10.  Security Considerations
    7.11.  Examples
8.  Syntax
9.  IANA Considerations
    9.1.  Update to Registration of SIP INFO Method
    9.2.  Registration of the Info-Package Header Field
    9.3.  Registration of the Recv-Info Header Field
    9.4.  Creation of the Info Packages Registry
    9.5.  Registration of the Info-Package Content-Disposition
10.  Examples
    10.1.  Single Info Package
    10.2.  Multipart INFO Example
11.  Modifications to SIP Change Process
12.  Security Considerations
13.  References
    13.1.  Normative References
    13.2.  Informative References
Appendix A.  Info Package Considerations
    A.1.  Appropriateness of Usage
    A.2.  Dialog Fate-Sharing
    A.3.  Messaging Rates and Volume
    A.4.  Is there a better alternative?
    A.5.  Alternatives for Common INFO Use
        A.5.1.  State Updates
        A.5.2.  User Stimulus: Touch Tones and Others
        A.5.3.  Direct Signaling Channel
        A.5.4.  Proxy-Aware Signaling
        A.5.5.  Dialog Probe
        A.5.6.  Malicious Indicator
Appendix B.  Legacy INFO Usages
    B.1.  ISUP
    B.2.  QSIG
    B.3.  MSCML
    B.4.  MSML
    B.5.  Video Fast Update
    B.6.  DTMF
Appendix C.  Acknowledgements
Appendix D.  Change Log
§  Authors' Addresses

1.  Introduction

The SIP protocol (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261] defines session control messages used to setup and tear down a SIP controlled session. In addition, a SIP User Agent (UA) can use the re-INVITE and UPDATE methods during a session to change the characteristics of the session. Most often, this is to change the properties of media flows related to the session or to update the SIP session timer (Donovan, S. and J. Rosenberg, “Session Timers in the Session Initiation Protocol (SIP),” April 2005.) [RFC4028]. The purpose of the INFO message (Donovan, S., “The SIP INFO Method,” October 2000.) [RFC2976] is to carry application level information along the SIP signaling path. Note the INFO method does not change the SIP dialog state. It may, however, change application state for applications using the SIP dialog.

While INFO has been widely adopted for specific application use cases, such as ISUP and DTMF exchange, RFC 2976 (Donovan, S., “The SIP INFO Method,” October 2000.) [RFC2976] neither defined a negotiation mechanism nor a means by which to explicitly indicate the type of application information contained in the INFO message. This led to problems associated with static configuration. In addition, the industry realized there was a potential for interoperability problems due to undefined content syntax and semantics. This draft addresses these deficiencies and provides a framework for explicit negotiation of capabilities and content context using "Info Packages".

The INFO method as defined by RFC 2976 did not provide any context for the information the request carried. While it may sometimes be clear what the content is based on the Content-Type, this is only true where there is only one contextual usage of the content-type. For example, if the Content-Type is "image/jpeg", the MIME-attached content is a JPEG image. However, there is no indication what the purpose of the image is. The image could be a caller-id picture, a contact icon, a photo for sharing, and so on. The sender does not know which JPEG to give the receiver if the receiver supports a JPEG content type, and the receiver does not know which JPEG the client is sending if the receiver supports receiving more than one JPEG content type. Thus we need a well defined and documented statement of what the information sent is for. This situation is identical to the context issue in Internet Mail (Burger, E., Candell, E., Eliot, C., and G. Klyne, “Message Context for Internet Mail,” January 2003.) [RFC3458]. RFC 3458 goes into this and other issues in detail.

Event Packages (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265] perform the role of disambiguating the context of a message for subscription-based events. This document provides a similar framework for INVITE-based application level information exchange. The mechanism defined in this draft has no relationship to the SUBSCRIBE and NOTIFY methods. The mechanism defined here neither creates a separate subscription dialog nor a subscription usage within an existing dialog. Instead, it uses the INVITE method and its responses to indicate and negotiate supported Info Packages, and the INFO method to convey the Info Packages. This mechanism is not appropriate for IANA-registered Subscribe Event (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265] package types. Info Package definitions and registrations indicate support for this mechanism when one registers them with IANA.

Each UA enumerates which Info Packages it can receive. If the far end indicates it can receive a package offered by the near end, the near end can send INFO methods containing the payload for that package. Likewise, if the near end indicates it can receive a package, the far end can send INFO methods containing the payload for that package. The Recv-Info header indicates which packages a UA is willing to receive. The Info-Package header indicates which package a particular INFO method request belongs to. There is a reserved Info Package, "nil", which indicates the UA conforms to this document, but does not wish to receive Info Packages. This enables other UAs that conform with this document to detect legacy UAs, as the legacy UA will not include a Recv-Info header in their SIP dialog establishment or modification requests. Section 3 (Info Package Negotiation) describes the negotiation in detail.

This document does not describe any specific Info Package type extensions. One must extend this protocol by other documents, herein referred to as "Info Packages". Section 7 (Info Package Requirements) describes guidelines for creating these extensions.

The INFO method does not change the state of SIP calls or the parameters of the sessions SIP initiates. It merely sends optional application layer information, generally related to the session.

Applications need to be aware that application level information transported by the INFO method constitutes mid-dialog signaling. These messages traverse the post-session-setup SIP signaling path. This is the path taken by SIP re-INVITEs, BYEs, and other SIP requests within an individual dialog. SIP proxy servers will receive, and potentially act on, mid-dialog signaling information. Application designers need to understand this can be a feature, as when the User Agents are exchanging information that elements in the SIP signaling path need to be aware of. Conversely, this can be a problem, as messages these network elements have no interest in can put a significant burden on those element's ability to process other traffic. Moreover, such network elements may not be able to read end-to-end encrypted INFO bodies.

2.  Applicability

This document replaces the SIP INFO method document (Donovan, S., “The SIP INFO Method,” October 2000.) [RFC2976] to include explicit negotiation of supported Info Packages in the INVITE transaction and indication of the Info Package to use by using a new header field in the INFO request. As described in Section 4.1 (INFO Requests), the mechanism described here is backwards-compatible with legacy, RFC 2976 INFO mechanisms.

3.  Info Package Negotiation

To be abundantly clear, as stated in the Conventions section, the term UAC refers to the UAC (sender) of the INFO method and UAS refers to the recipient of the INFO method. "Initiating UA" refers to the sender of an initial INVITE to establish a session and "target UA" refers to the recipient of that INVITE request.

3.1.  UA Behavior

A UA supporting this document MUST advertise a set of Recv-Info packages in the initial INVITE exchange. This includes the initial INVITE request, as well as provisional 1xx, final 2xx responses, and the ACK. The initiating UA (UAC of the INVITE) may choose not to offer any packages in the initial INVITE and negotiate packages from the target UA's subsequent responses and the ACK, in order to support third-party call control (Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo, “Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP),” April 2004.) [RFC3725].

Info Package negotiation may occur any time the UAs negotiate session parameters. There are two cases to consider for SIP dialog parameter negotiation: the initial INVITE transaction and subsequent renegotiation. By subsequent renegotiation, we mean procedures such as re-INVITE and UPDATE (Rosenberg, J., “The Session Initiation Protocol (SIP) UPDATE Method,” October 2002.) [RFC3311]. In the first case, dialog establishment (the initial INVITE transaction), the UAC MUST NOT send INFO requests for a given Info Package until the UAS lists the given Info Package in a Recv-Info header. If the UAS sends a subsequent message in the dialog establishment exchange that removes a listed Info Package, the UAC MUST NOT send INFO requests for that package. In the second case, dialog renegotiation, the UAC MUST NOT send INFO requests for a newly listed Info Package until the dialog renegotiation exchange successfully completes and the newly listed Info Package is in the UAS' final renegotiation exchange message.

A UAS lists multiple packages by enumerating the package name(s), separated by commas, as values for the Recv-Info header in the session establishment exchange. A UAS can also list multiple packages by including multiple Recv-Info headers. The UAS can also combine multiple Recv-Info headers with one or more packages in each header value. If the UAS has a preference for receiving one package over another, the UAS MUST list the preferred Info Package lexically earlier in the message. That is, by listing it earlier in a list within a given Recv-Info header or listing it in a previous Recv-Info header in a given message. Listing a package multiple times does no harm. As far as a hint to the UAC, the first appearance is what the UAC uses for determining the UAS' preference. Note this order is only a hint to the UAC, as there is no meaningful way of enforcing the use of a preferred package at the UAC.

If a UAS does not wish to receive any Info Packages, the UAS MUST indicate this by including one and only one Recv-Info header with the value 'nil'. This enables the UAC to discern the difference between the UAS understanding Info Packages but not wishing to receive any from a UAS that does not understand Info Packages. A UAC conforming to this document can always send or receive legacy INFO usages without packages.

NOTE: We could allow an empty Recv-Info header to indicate the UAS does not wish to receive Info Packages. Semantically that is what this means, as there is no null package. However, this is sloppy and we may find we need an explicit value here in the event we require a richer negotiation strategy. Since mandating nil at this time is no burden, and it will be a burden in the future if we do not specify it now, we specify it now.

Info Package capability negotiation occurs within the context of a single session negotiation exchange. Moreover, the last capability set received within the exchange is the one the receiver applies against its advertised capability set. For example, if in an INVITE, the initiating UA offers the following.

INVITE ...
...
Recv-Info: P, R
...

The target UA responds with a 200 OK, and the initiating UA then confirms in an ACK, as shown.

ACK
...
Recv-Info: R, T
...

The target UA can now send from package T to the initiating UA. Moreover, in this example, the target UA may not send form package P, as P no longer is in the initiating UA's Info Package set.

The limitation on requiring the negotiation to occur within the context of a session negotiation exchange means that if the initiating UA issues a re-INVITE (after the above ACK) with the following.

INVITE ...
...
Recv-Info: P, R, T
...

The target UA MUST NOT send any package P INFO methods until the target UA sees P in the final ACK from the initiating UA.

In the case of a SIP dialog refresh, if the initiating UA and target UA wishes to keep their Info Package set active, the UAs MUST include the Recv-Info header with the appropriate values. Otherwise, if the UA neglects to include the Recv-Info header, the other UA in the dial will assume the first UA no longer supports INFO as specified by this document.

INFO itself does not necessitate the use of Require or Proxy-Require headers. There is no token defined for "Supported" headers. If necessary, clients may probe for the support of this version of INFO using the OPTIONS request defined in SIP (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261]. One could envision a particular Info Package implementation that relied on either of these headers. See Section 7 (Info Package Requirements) for more on this issue.

The presence of the Recv-Info header in a message is sufficient to indicate support for this version of INFO. The "methods" parameter for Contact (Rosenberg, J., Schulzrinne, H., and P. Kyzivat, “Caller Preferences for the Session Initiation Protocol (SIP),” August 2004.) [RFC3841] is not sufficient to determine if the endpoints support Info Packages, as the INFO method supported might be the obsolete RFC 2976 (Donovan, S., “The SIP INFO Method,” October 2000.) [RFC2976] version.

For Info Packages, this draft does not provide a means of requiring support for a specific Info Package. If the far-end UA does not indicate support for an Info Package that the local server requires, the server MAY terminate the session with a CANCEL or BYE request.

3.2.  Package Versions

The protocol mechanism described herein does not provide for a package versioning mechanism. This is for two reasons. The first is that if an Info Package has a capability for forward and backward compatibility in the Info Package payload, then that compatibility comes from the application level semantics of the information. This means it is the responsibility of the application to handle such compatibility and not the INFO framework. For example, one could use XML versioning techniques in the payload to indicate versions of the Info Package.

The second reason we do not have a package versioning system is if the payload is not sufficient to carry payload versions, then it is highly unlikely payloads would be backwards compatible. That is, what one really is defining is a new Info Package. This is more especially so when one considers User Agents can negotiate package support but cannot negotiate package version support.

4.  The INFO Method Request

4.1.  INFO Requests

The INFO method provides additional, application level information that can further enhance a SIP application. It is important to note there are some types of application information for which using INFO messages are inappropriate. See Appendix A (Info Package Considerations) for a discussion of this.

The UAC MUST include the Info-Package header field when it sends an INFO request carrying an Info Package. The Info-Package header field value in an INFO request MUST contain a single Info Package token. That Info Package token MUST match one of the Info Packages the UAS indicated support for during the negotiation described in Section 3 (Info Package Negotiation).

The UAC MAY send an INFO in a legacy usage context. See Appendix B (Legacy INFO Usages) for examples of legacy usages. In general a legacy usage is where there is no Info-Package header. In this case, if the UAS has never offered a Recv-Info header or never offered a Recv-Info header with a package of a similar function to the legacy INFO usage, the UAC MAY send an INFO without an Info-Package header field and a body appropriate to the said legacy usage.

A UAC MUST NOT use the INFO method outside an INVITE dialog usage. The INFO method has no lifetime or usage of its own, as it is inexorably linked to that of the INVITE. When the INVITE-created dialog terminates, that signals the termination of the negotiated Info Packages. A UAS that receives an INFO message after the INVITE dialog usage terminates MUST respond with a 481 Call Does Not Exist.

The dialog identifiers defined in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261] must match those of the provisional or final responses to the INVITE. As a result, INFO requests cannot fork. The UAC may send INFO requests once the UAS has sent the Recv-Info header field value, indicating what the UAS supports.

The converse is not true during initial session establishment. The initiating UA of the first INVITE MUST be prepared to receive multiple INFO requests, as the first INVITE may fork. Since dialog negotiation has not completed, and we allow early INFO requests, multiple target UAs may respond. This initial dialog establishment phase is the only time the UAS need be prepared to receive multiple INFO requests, as we require post-session-establishment negotiation to fully complete before a UAC can send an INFO request.

The construction of the INFO request is the same as any other request within an existing INVITE-initiated dialog. A UAC MAY send an INFO request on both an early and confirmed dialog.

The INFO request MUST NOT carry a Recv-Info header. The UAC can only negotiate Info Packages using the procedures of Section 3 (Info Package Negotiation).

The signaling path for the INFO method is the signaling path established as a result of the dialog setup. This can be direct signaling between the calling and called user agents or a signaling path involving SIP proxy servers that were involved in the call setup and added themselves to the Record-Route header on the initial INVITE message.

4.2.  INFO Request Body

The purpose of the INFO request is to carry application level information between SIP user agents. The INFO message body SHOULD carry this information, unless the message headers carry the information of interest. Note this is not an invitation to invent SIP headers for the purposes of application level information exchange. Rather, one could envision circumstances where existing SIP headers already convey the information the application has interest in.

If the Info Package defines a payload, and the UAC determines it is appropriate to send that payload to the UAS, the UAC MUST include the payload, with the MIME type specified by the Info Package.

If the Info Package allows the UAC to send a request without a payload, the UAC MAY send the INFO request without a body.

Some SIP extensions, which are orthogonal to INFO proper, may insert body parts unrelated to the INFO payload. User Agents MUST conform to RFC 3261 as updated by body-handling (Camarillo, G., “Message Body Handling in the Session Initiation Protocol (SIP),” October 2008.) [I‑D.ietf‑sip‑body‑handling] to support multipart MIME handling. If there are bodies unrelated to the Info Package, and the Info Package also has a payload, the UAC MUST bundle these elements into a multipart MIME body. In this case, the UAS needs a means to unambiguously identify the body part belonging to the Info Package. To do this, the UAC MUST identify the Info Package payload MIME body part with a Content-Disposition of 'Info-Package'.

If the payload of an Info Package is already a multipart MIME body, the UAC MUST identify the payload with a Content-Disposition of 'Info-Package' in the headers for the appropriate MIME body part.

If there is no payload in the INFO request unrelated to the Info Package and the payload of the Info Package is not a multipart MIME, the UAC MUST identify the message, at the SIP header level, with a Content-Disposition of 'Info-Package'.

If there is no payload for the Info Package, they UAC MAY omit the Content-Disposition header.

NOTE: We could be lazy and even save 33 octets by allowing the UAC to construct a non-multipart MIME payload without a Content-Disposition header. However, mandating the presence makes parsing considerably easier, and it is easier to have it required now than run into a problem later.

NOTE: One could offer that the Info-Package header is redundant, as we could have the Info Package name be a parameter for Content-Disposition. However, there could be corner cases with legacy INFO usage that makes this a poor choice.

4.3.  Responses to the INFO Request Method

If a UAS receives an INFO request it MUST send a final response. A UAS MUST send a 200 OK response for an INFO request with no message body and no Info-Package header if the UAS received the INFO request on an existing dialog. This protocol action supports legacy use of INFO as a keep-alive mechanism.

If the UAS receives an INFO request with an Info-Package the UAS advertised with a Recv-Info in the last dialog state update and the body of the INFO request is an appropriate MIME type for the Info Package, the UAS MUST send a 200 OK response.

If the INFO request contains a body the server does not understand then, in the absence of Info Package associated processing rules for the body, including the absence of an Info-Package header, the server MUST respond with a 415 Unsupported Media Type message.

If the INFO request indicates an Info Package type the server does not understand, then the server MUST respond with a 489 Bad Event. The server then MUST terminate the INVITE dialog, as this represents a protocol failure.

NOTE: Some may think "Bad Event" implies there is a link between INFO and NOTIFY. However, what this does is refine 489 to mean, "Received some package in some context that I do not understand," where today the possible contexts are INFO and NOTIFY. The text is irrelevant and the meaning is clear from the context.

If a server receives an INFO request with a body it understands, but it has no Info-Package header, the UAS MAY use the body as it sees fit. The UAS SHOULD respond to the INFO request with a 200 OK. This enables legacy use of INFO. The UAS MAY reject the request with a 489 if the UAS needs to enforce strict compliance with the current INFO framework described here.

The UAS MUST send a 481 Call Leg/Transaction Does Not Exist message if the INFO request does not match any existing INVITE-initiated dialog.

The UAS MAY send other responses, such as Request Failure (4xx), Server Failure (5xx) and Global Failure (6xx) as appropriate for the request.

4.4.  Routing Behavior

Unless stated otherwise, the protocol rules for the INFO request governing the usage of tags, Route and Record-Route, retransmission and reliability, CSeq incrementing and message formatting follow those in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261] as defined for the BYE request.

The INFO message MUST NOT change the state of the SIP dialog. Of course, outside the INFO machinery specific failure responses as documented in the SIP dialog usages document (Sparks, R., “Multiple Dialog Usages in the Session Initiation Protocol,” November 2007.) [RFC5057], may cause the INVITE dialog to terminate.

4.5.  Behavior of Registrars

Registrars receiving a REGISTER request that includes Recv-Info headers MAY store such information and use it for routing purposes. How the registrar uses this information is beyond the scope of this document.

4.6.  OPTIONS Processing

A UAC, the sender of the OPTIONS request, SHOULD include Recv-Info headers, populated appropriately for the packages the UAC supports. The UAS SHOULD include its set of Recv-Info packages. These strictures are of "should" strength because local policy might restrict the advertisement of full capabilities, the UA may know the best choice of equivalent packages to list from local configuration, and so on.

The UAS and UAC MUST NOT consider the OPTIONS request to be part of a capabilities negotiation. The OPTIONS request is purely a probe. For the UAC or UAS to renegotiate package support, they must use the procedures described in Section 3 (Info Package Negotiation).

4.7.  Order of Delivery

The INFO method does not define mechanisms for ensuring in-order delivery for overlapping INFO requests. That is, the UAC can send another INFO request before receiving a transaction response from the UAS to a prior INFO request. While the UAC will increment the CSeq header upon the transmission of new INFO messages, the UAS cannot use the CSeq to determine the sequence of INFO information. This is due to the fact that there could be gaps in the INFO message CSeq count caused by a user agent sending re-INVITES or other SIP messages.

It is up to the individual Info Package definition to specify what happens when there are overlapping INFO requests. However, since it is legal SIP to have overlapping requests, the application must be able to handle the reception of overlapping requests, even if the Info Package does not allow for it. Since overlapping requests can occur even if the application (Info Package) does not allow it, the Info Package needs to define the appropriate response. This is more especially so given the UAC could send from multiple Info Packages. Some of those packages may allow overlapping INFO requests, while others do not. In this situation, it would be hard to tell if the non-overlapping packages were being violated or not.

5.  Formal INFO Method Definition

5.1.  INFO Method

This document describes one new SIP method: INFO. This document replaces the definition and registrations found in [RFC2976] (Donovan, S., “The SIP INFO Method,” October 2000.).

This table expands on Tables 2 and 3 in [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.).

Table 1: Summary of Header Fields

  Header                    Where    INFO
  ------                    -----    ----
  Accept                      R       o
  Accept-Encoding             R       o
  Accept-Encoding            2xx      o
  Accept-Encoding            415      c
  Accept-Language             R       o
  Accept-Language            2xx      o
  Accept-Language            415      c
  Allow                       R       o
  Allow                      200      -
  Allow                      405      o
  Allow-Events                R       o
  Allow-Events                r       o
  Authentication-Info        2xx      o
  Authorization               R       o
  Call-ID                    gc       m
  Call-Info                   R       o
  Call-Info                   r       o
  Contact                     R       -
  Contact                    1xx      -
  Contact                    2xx      -
  Contact                    3xx      -
  Contact                    485      -
  Content-Disposition         e       o
  Content-Encoding            e       o
  Content-Language            e       o
  Content-Length              e       o
  Content-Type                e       *
  CSeq                       gc       m
  Date                        g       o
  Error-Info               3xx-6xx    o
  Expires                     g       -
  From                       gc       m
  Geolocation                 R       o
  Max-Breadth                 R       -
  Max-Forwards                R       o
  MIME-Version                R       o
  MIME-Version                r       o
  Organization                g       o
  Privacy                     R       o
  Proxy-Authenticate         407      o
  Proxy-Authorization         R       o
  Proxy-Require               R       o
  Reason                      R       o
  Record-Route                R       o
  Record-Route               2xx      o
  Require                     R       o
  Require                     r       o
  Retry-After                 R       -
  Retry-After            404,480,486  o
  Retry-After                503      o
  Retry-After              600,603    o
  Route                       R       o
  Security-Client             R       o
  Security-Server          421,494    o
  Security-Verify             R       o
  Server                      r       o
  Subject                     R       o
  Supported                   R       o
  Supported                  2xx      o
  Timestamp                   g       o
  To                        gc(1)     m
  Unsupported                420      o
  User-Agent                  g       o
  Via                       gc(2)     m
  Warning                     r       o
  WWW-Authenticate           401      o

5.2.  INFO Headers

This table expands on tables 2 and 3 in [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.).

Header field where   ACK BYE CAN INV OPT REG PRA INF MSG UPD SUB NOT
--------------------------------------------------------------------
Info-Package   R      -   -   -   -   -   -   -   m   -   -   -   -
Recv-Info      R      o   -   -   o   o   o   o   -   -   o   -   -
Recv-Info      2xx    o   -   -   o   o   -   o   -   -   o   -   -
Recv-Info      1xx    o   -   -   o   o   -   o   -   -   o   -   -
Recv-Info      r      o   -   -   -   o   -   o   -   -   o   -   -

5.2.1.  Info-Package header

This document adds Info-Package to the definition of the element "message-header" in the SIP message grammar.

For the purposes of matching Info Package types indicated in Recv-Info with those in the Info-Package header field value, one compares the Info-package-name portion of the Info-package-type portion of the Info-Package header octet-by-octet with that of the Recv-Info header value. That is, the Info Package name is case sensitive. Info-package-param is not part of the comparison-checking algorithm.

This document does not define values for Info-Package types. Individual Info Packages define these values. Such documents MUST register such values with IANA. These values are Specification Required (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) [RFC5226].

5.2.2.  Recv-Info header

This document adds Recv-Info to the definition of the element "general-header" in the SIP (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261] message grammar. Section 3 (Info Package Negotiation) describes the Recv-Info header usage.

6.  Legacy Uses of INFO

Several RFC-defined and other standards-defined uses of RFC 2976 INFO (Donovan, S., “The SIP INFO Method,” October 2000.) [RFC2976] exist and are in use, as well as numerous proprietary uses. Appendix B (Legacy INFO Usages) describes some of these usages. By definition, identifying such uses has relied on either static local configuration or implicit context determination based on the body Content-Type or Content-Disposition value or some proprietary mechanism. This draft cannot forbid nor avoid such uses, since local configuration can always override standardized mechanisms.

To maintain backward compatibility with the extant standardized uses of INFO, a server MAY interpret an INFO request with no "Info-Package" header as being of such legacy use.

It should be noted that such legacy use will not "break" the mechanism in this draft. For example, if a UA supports SIP-T (Vemuri, A. and J. Peterson, “Session Initiation Protocol for Telephones (SIP-T): Context and Architectures,” September 2002.) [RFC3372], it does so based on static local configuration or based on acceptance of the application/isup body. If it adds support for this draft's Info Package negotiation mechanism, the local configuration still applies, and the UA will send/receive INFO messages based on SIP-T regardless of the Info Package negotiation. It will also be able to send/receive INFO messages based on the Info Packages it negotiated. If, at some future time, an Info Package is defined for SIP-T, the UA can indicate such in the negotiation, and again local configuration would supersede if need be. The UA would not lose the ability to use SIP-T with legacy devices. Rather, it would gain the ability to use it with devices which support this draft and with which it did not have such local configuration set, and could avoid failures related to unsupported bodies.

It is the hope of this draft's authors that vendors that implement proprietary INFO uses submit their mechanisms as Info Package extension documents, and follow the Info Package negotiation mechanism defined in this draft.

7.  Info Package Requirements

Info Packages SHOULD NOT reiterate any of the behavior described in this document, unless required for clarity or emphasis. However, such packages MUST describe the behavior that extends or modifies the behavior described in this document.

Info Packages MUST NOT weaken any behavior designated with "SHOULD" or "MUST" in this document. However, Info Packages MAY strengthen "SHOULD", "MAY", or "RECOMMENDED" requirements to "MUST" strength if the application requires it.

In addition to the normal sections expected in standards-track RFCs and SIP extension documents, authors of Info Packages need to address each of the issues detailed in the following subsections, whenever applicable.

7.1.  Applicability

This section, which MUST be present, describes why any of the other established user-to-user data transfer protocols are not appropriate for the given Info Package. Common reasons can be a requirement for SIP Proxies or back-to-back User Agents (B2BUAs) to see the application level information. Consideration in this section MUST describe what happens if one or both endpoints encrypt the payload.

7.2.  Info Package Name

This section, which MUST be present, defines the token name that designates the Info Package. The name MUST conform to the token-nodot ABNF production described in Section 8 (Syntax). It MUST include the information that appears in the IANA registration of the token. For information on registering such types, see Section 9 (IANA Considerations).

7.3.  Info Package Parameters

If the "Info-Package" header allows parameters to modify the behavior of the Info Package, this section MUST clearly define the syntax and semantics of such parameters.

7.4.  Info Package Tags

If useful for the Info Package to have SIP option tags, this is the place to define the tag. Note that if the Info Package defines a SIP option tag, the Info Package must conform to the SIP Change Process (Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B. Rosen, “Change Process for the Session Initiation Protocol (SIP),” December 2002.) [RFC3427].

7.5.  INFO Bodies

Each Info Package MUST define what type or types of bodies are expected in INFO requests. Such packages MUST specify or cite detailed specifications for the syntax and semantics associated with such a body.

The UAS MUST enumerate every MIME type associated with the Info Packages advertised in the UAS' Recv-Info header the UAS is willing to receive. If a UAC sends a body that includes something not enumerated by the UAS, this is a protocol error and the UAS MUST respond appropriately.

7.6.  UAC generation of INFO requests

Each Info Package MUST describe the process by which a UA generates and sends an INFO request. This includes detailed information about what events cause the UA to send an INFO request.

If the Info Package does not allow overlapping (outstanding) INFO requests the Info Package MUST disclose this in the section describing UA generation of INFO requests. Note the generic protocol machinery of the INFO method has no way of enforcing such a requirement. Section 7.7 (UAS processing of INFO requests) describes this situation.

7.7.  UAS processing of INFO requests

The Info Package MAY describe the process followed by the UA upon receipt of an INFO request. Since INFO does not change SIP state, and may not even change application state, there may be no useful guidance required in the Info Package specification for UA processing.

If the info Package does not permit overlapping INFO requests, it is important to note the issuance of overlapping INFO requests is an application-layer protocol failure and not an INFO method failure. Therefore, in the event a UAC issues overlapping INFO requests for an Info Package, the UAS MUST NOT return an error response. This section of the Info Package specification MUST describe the application level response to overlapping INFO requests. Examples include a new INFO request back to the offending UAC indicating an application error, ignoring the overlapping request and processing it to the UAS' best effort, or terminating the entire SIP dialog.

7.8.  Rate of INFO Requests

Each Info Package MUST define a requirement of MUST strength which defines an absolute maximum on the rate at which an Info Package of a given type can generate INFO messages by a UA in a dialog.

If possible, a package MUST define a throttle mechanism that allows UAs to further limit the rate of INFO messages.

7.9.  IANA Registrations

The Info Package MUST have an IANA Considerations section that includes definitions for the Info Package Name and, if needed, supported MIME types.

7.10.  Security Considerations

The INFO mechanism transports application level information. One implication of this is INFO messages may require a higher level of protection than the underlying SIP-based session signaling. If the application transports sensitive information, such as credit card numbers, health history, personal identifiers, and so on, the Info Package MUST document security procedures that exceed the default procedures presented in this document. In most circumstances it is not sufficient for a package to attempt to mandate TLS for the signaling channel to secure the data carried by the INFO. This is because there are few protocol mechanisms to enforce this requirement. It may be possible for an Info Package to inform the SIP transport layer stack to be "secure." However, the only way to ensure secure transport at the application level is to have the security be part of the Info Package itself. The most common method of achieving this is to use end-to-end security techniques such as S/MIME (Ramsdell, B., “Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification,” July 2004.) [RFC3851].

7.11.  Examples

We RECOMMEND Info Packages include several demonstrative message flow diagrams paired with several typical, syntactically correct, and complete messages.

Documents describing Info Packages MUST clearly indicate the examples are informative and not normative, with instructions that implementers refer to the main text of the document for exact protocol details.

8.  Syntax

This section describes the syntax extensions required for user-to-user data exchange in SIP. The previous sections describe the semantics. Note the formal syntax definitions described in this document use the ABNF format used in SIP (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261] and contain references to elements defined therein.

The Augmented BNF definitions for the various new and modified syntax elements follow. The notation is as used in SIP (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261]. See SIP for any elements not defined in this section.

INFOm               = %x49.4E.46.4F ; INFO in caps
extension-method    = INFOm / token

Info-Package        =  "Info-Package" HCOLON Info-package-type
Recv-Info           =  "Recv-Info" HCOLON "nil"
                    /  Info-package-type
                       *( COMMA Info-package-type )
Info-package-type   =  Info-package-name *( "." Info-package-param)
Info-package-name   =  token-nodot
Info-package-param  =  token-nodot
token-nodot         =  1*( alphanum / "-"  / "!" / "%" / "*"
                           / "_" / "+" / "`" / "'" / "~" )

NOTE on the Recv-Info production: if the value is "nil", there can be one and only one Recv-Info header in the SIP message.

9.  IANA Considerations

9.1.  Update to Registration of SIP INFO Method

Please update the existing registration in the SIP Methods and Response Codes registry under the SIP Parameters registry that states:

Method:      INFO
Reference:   [RFC2976]

to:

Method:      INFO
Reference:   [RFCXXXX]

9.2.  Registration of the Info-Package Header Field

Please add the following new SIP header field in the Header Fields subregistry under the SIP Parameters registry.

Header Name:   Info-Package
Compact Form:  (none)
Reference:     [RFCXXXX]

9.3.  Registration of the Recv-Info Header Field

Please add the following new SIP header field in the Header Fields subregistry under the SIP Parameters registry.

Header Name:   Recv-Info
Compact Form:  (none)
Reference:     [RFCXXXX]

9.4.  Creation of the Info Packages Registry

Please create a subregistry in the SIP Parameters registry for Info Packages. This subregistry has a modified First Come First Served (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) [RFC5226] policy.

The following data elements populate the Info Package Registry.

• Info Package Name: The Info Package Name is a case-sensitive token. In addition, IANA shall not register multiple Info Package names that have identical case-insensitive values.
• Info Package Payload MIME Types: A list of zero or more registered MIME types from the MIME Type Registry.
• Standards Status: Values are "Standards Track" or empty. See below for a discussion and rules on this field.
• Reference: If there is a published specification describing the Info Package, place a reference to that specification in this column. See below for a discussion on this field.

If there is a published specification, the registration MUST include a reference to such specification. The Standards Status field is an indicator of the level of community review for the Info Package specification. If the specification meets the requirements for Specification Required (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) [RFC5226], the value for the Standards Status field is "Standards Track". Otherwise, the field is empty.

This document uses the Info Package Name "nil" to represent "no Info Package present" and as such IANA shall not honor a request to register the "nil" Info Package.

The initial population of this table shall be:

Name         MIME Type                Standards Status      Reference
nil                                    Standards Track      [RFCXXXX]

9.5.  Registration of the Info-Package Content-Disposition

Please add the following registration to the Content-Disposition registry. The description suitable for the IANA registry is as follows.

The payload of the message carrying this Content-Disposition header field value is the payload of an Info Package.

10.  Examples

10.1.  Single Info Package

In the following example, Alice initiates a call to Bob. Alice can support sending or receiving "foo" Info Packages, and sending "bar" Info Packages.

Alice generates the following: (note: much has been left out for simplicity)

INVITE sip:bob@example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bKnashds10
From: Alice <sip:alice@example.net>;tag=1234567
To: Bob <sip:bob@example.com>
Call-Id: 123456mcmxcix
CSeq: 1 INVITE
Contact: <sip:alice@192.0.2.1>
Recv-Info: foo

Bob does not support anything, so he says so.

SIP/2.0 180 Ringing
Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bKnashds10
From: Alice <sip:alice@example.net>;tag=1234567
To: Bob <sip:bob@example.com>;tag=abcdefg
Call-Id: 123456mcmxcix
CSeq: 1 INVITE
Recv-Info: nil

Bob answers, but still does not support anything.

SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bKnashds10
From: Alice <sip:alice@example.net>;tag=1234567
To: Bob <sip:bob@example.com>;tag=abcdefg
Call-Id: 123456mcmxcix
CSeq: 1 INVITE
Contact: <sip:bob@192.0.2.2>
Recv-Info: nil

Alice could have sent an Info Package as soon as she received the 180, but in this example she would not have been able to do so since Bob didn't say he could receive any Info Packages in his 180 response. Bob, on the other hand, may send an INFO:

INFO sip:alice@192.0.2.1 SIP/2.0
Via: SIP/2.0/UDP 192.0.2.2:5060;branch=z9hG4bKnabcdef
To: Alice <sip:alice@example.net>;tag=1234567
From: Bob <sip:bob@example.com>;tag=abcdefg
Call-Id: 123456mcmxcix
CSeq: 2 INFO
Contact: <sip:bob@192.0.2.2>
Info-Package: foo

10.2.  Multipart INFO Example

This is for where there is a single INFO payload in a multipart/mime.

INFO ....
To: ....
From: ....
Info-Package: foo
Mumble: <cid:abcd9999qq>
Content-Type: multipart/mixed;boundary="theboundary"
...

--theboundary
Content-Type: application/mumble
Content-Id: abcd9999qq
...

<mumble stuff>

--theboundary
Content-Type: application/foo
Content-Disposition: Info-Package

<foo body>

--theboundary--

11.  Modifications to SIP Change Process

[EDITOR'S NOTE: This section may become a separate document in the future.]

This document updates RFC 3427 (Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B. Rosen, “Change Process for the Session Initiation Protocol (SIP),” December 2002.) [RFC3427] to add a process for registering new Info Packages. The process for registering new Info Packages follows the process outlined in Section 4.3 of RFC 3427 for the registration of SIP Event Packages. Namely, the registration of a new SIP Info Package requires the SIPPING chairs to assign an individual to perform expert review of the proposal if the work is not a SIPPING work item in itself.

12.  Security Considerations

By eliminating multiple uses of INFO messages without adequate community review and by eliminating the possibility for rogue SIP User Agents from confusing another User Agent by purposely sending unrelated INFO messages, we expect this document's clarification of the use of INFO to improve the security of the Internet. Whilst rogue UACs can still send unrelated INFO messages, this framework provides mechanisms for which the UAS and other security devices can filter for approved Info Packages.

If the content of the Info Package payload is private, User Agents will need to use end-to-end encryption, such as S/MIME, to prevent access to the content. This is particularly important as transport of INFO is likely not to be end-to-end, but through SIP proxies and back-to-back user agents (B2BUA's), which the user may not trust.

The INFO mechanism transports application level information. One implication of this is INFO messages may require a higher level of protection than the underlying SIP-based session signaling. In particular, if one does not protect the SIP signaling from eavesdropping or authentication and repudiation attacks, for example by using TLS transport, then the INFO request and its contents will be vulnerable, as well. Even with SIP/TLS, any SIP hop along the path from UAC to UAS can view, modify, or intercept INFO requests, as they can with any SIP request. This means some applications may require end-to-end encryption of the INFO payload, beyond, for example, hop-by-hop protection of the SIP signaling itself. Since the application dictates the level of security required, individual Info Packages have to enumerate these requirements. In any event, the INFO Framework described by this document provides the tools for such secure, end-to-end transport of application data.

One interesting property of Info Package use is one can reuse the same digest-challenge mechanism used for INVITE-based authentication for the INFO request. For example one could use a quality-of-protection (qop) value of authentication with integrity (auth-int), to challenge the request and its body, and prevent intermediate devices from modifying the body. However this assumes the device which knows the credentials in order to perform the INVITE challenge is still in the path for the INFO, or that the far-end UAS knows such credentials.

13.  References

13.1. Normative References

13.2. Informative References

Appendix A.  Info Package Considerations

This section covers several issues that one should take into consideration when proposing new Info Packages.

A.1.  Appropriateness of Usage

When designing an Info Package using the method described in this document for application level information exchange, it is important to consider: is INFO and, more importantly, is signaling within a SIP dialog, an appropriate mechanism for the problem set? Is it because it is the most reasonable and appropriate choice, or merely because "it's easy"?

These are difficult issues to consider, especially when presented with real-world deadlines and implementation cost issues. However, choosing to use INFO for inappropriate uses *will* lead to issues in the real world, not the least of which are certain types of middleboxes which will remove the device from the network if it is found to cause damage to other SIP nodes.

Therefore, the following sections provide consideration guidelines and alternatives to INFO use.

A.2.  Dialog Fate-Sharing

INFO, by design, is a method within an INVITE dialog usage. RFC 5057 (Sparks, R., “Multiple Dialog Usages in the Session Initiation Protocol,” November 2007.) [RFC5057] enumerates the problems with using dialogs for multiple usages, and we strongly urge the reader to review RFC 5057. The most relevant issue is a failure of transmission or processing of an INFO request may render the INVITE dialog terminated, depending on the type of failure. Prior to RFC 5057 it was not clear if the INFO usage was a separate usage or not. RFC 5057 clarifies the INFO method is always part of the INVITE usage.

Some uses of INFO can tolerate this fate sharing of the INFO message over the entire dialog. For example, in the SIP-T usage, it may be acceptable for a call to fail, or to tear down the call, if one cannot deliver the associated SS7 information. The same is usually true for DTMF. However, it may not be acceptable for a call to fail if, for example, a DTMF buffer overflows. Then again, for some services, that may be the exact desired behavior.

A.3.  Messaging Rates and Volume

There is no throttling mechanism for INFO. Consider that most call signaling occurs on the order of 7-10 messages per 3 minutes, although with a burst of 5-7 messages in one second during call setup. DTMF tones occur in bursts at a rate of up to 20 messages per second. This is a considerably higher rate than for call signaling. Sending constant GPS location updates, on the other hand, would incur an undue burden on SIP Proxies along the path.

Furthermore, SIP messages tend to be relatively small, on the order of 500 Bytes to 32K Bytes. SIP is a poor mechanism for direct exchange of bulk data beyond these limits, especially if the headers plus body exceed the UDP MTU (Postel, J., “User Datagram Protocol,” August 1980.) [RFC0768]. Appropriate mechanisms for such traffic include MSRP (Campbell, B., Mahy, R., and C. Jennings, “The Message Session Relay Protocol (MSRP),” September 2007.) [RFC4975], COMEDIA (Yon, D. and G. Camarillo, “TCP-Based Media Transport in the Session Description Protocol (SDP),” September 2005.) [RFC4145], or HTTP (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) [RFC2616].

A.4.  Is there a better alternative?

The first alternative for application level interaction is SIP Events, also known as SUBSCRIBE/NOTIFY (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265]. In this model, a user agent requests state information, such as key pad presses from a device to an application server or key map images from an application server to a device. The SUBSCRIBE creates a new dialog that does not share the fate of the related INVITE-initiated dialog. Moreover, using the SUBSCRIBE model enables multiple applications to receive state updates. These applications can be outside the media path and potentially outside the INVITE-initiated dialog's proxy path. In fact, SIUBSCRIBE/NOTIFY is your only option if you need to exchange data outside a communications session.

SUBSCRIBE/NOTIFY messages pass through the SIP signaling infrastructure, such as SIP Proxies and B2BUAs. Application designers need to understand this can be a feature, as when the User Agents are exchanging information that elements in the SIP signaling path need to be aware of. Conversely, this can be a problem, as messages these network elements have no interest in can put a significant burden on those element's ability to process other traffic. Moreover, such network elements may not be able to read end-to-end encrypted SUBSCRIBE or NOTIFY bodies.

Implementers do need to be aware the price of having a protocol that works in all cases, can scale, can easily load balance, and will not mysteriously fail a session in the event of state synchronization failure does come at a cost. Session establishment is a minimum of two messages in addition to the INVITE dialog establishment. If the SUBSCRIBE application is co-resident with the INVITE application, the application will have to manage two SIP dialogs instead of one. Tracking the application level state dominates memory and processing for some applications, and as such the doubling of SIP dialogs is not an issue. However, for other applications, this may be an issue.

The MESSAGE method (Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and D. Gurle, “Session Initiation Protocol (SIP) Extension for Instant Messaging,” December 2002.) [RFC3428] defines one-time instant message exchange, typically for sending MIME contents for rendering to the user.

Another model for application level information exchange is to establish a communication channel in the media plane. One model for this is MRCPv2 (Shanmugham, S. and D. Burnett, “Media Resource Control Protocol Version 2 (MRCPv2),” November 2008.) [I‑D.ietf‑speechsc‑mrcpv2]. Here, the INVITE-initiated dialog establishes a separate reliable, connection-oriented channel, such as a TCP (Postel, J., “Transmission Control Protocol,” September 1981.) [RFC0793] or SCTP (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) [RFC4960] stream. One uses SIP to locate the remote endpoint, but uses a direct connection for the UUI. One then can create whatever protocol one wishes, whether from scratch (as in MRCPv2) or using a substrate such as BEEP (Rose, M., “The Blocks Extensible Exchange Protocol Core,” March 2001.) [RFC3080].

A low latency requirement for the exchange of information is one strong indicator for using a media channel. Exchanging information through the SIP routing network can introduce hundreds of milliseconds of latency. Also, if there will be a lot of information exchanged, and there is no need for the SIP routing network to examine the information, one should use a separate media channel.

Another model is to use a totally externally signaled channel, such as HTTP (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) [RFC2616]. In this model, the user agent knows about a rendezvous point to direct HTTP requests to for the transfer of information. Examples include encoding of a prompt to retrieve in the SIP Request URI in RFC 4240 (Burger, E., Van Dyke, J., and A. Spitzer, “Basic Network Media Services with SIP,” December 2005.) [RFC4240] or the encoding of a SUBMIT target in a VoiceXML (Porter, B., McGlashan, S., Lee, A., Burnett, D., Carter, J., Oshry, M., Bodell, M., Baggia, P., Rehor, K., Burke, D., Candell, E., and R. Auburn, “Voice Extensible Markup Language (VoiceXML) 2.1,” June 2007.) [W3C.REC‑voicexml21‑20070619] script.

MSRP (Campbell, B., Mahy, R., and C. Jennings, “The Message Session Relay Protocol (MSRP),” September 2007.) [RFC4975] defines session-based instant messaging as well as bulk file transfer and other such large-volume uses. It is part of an INVITE-based session, similar to other media. Unlike INFO, MSRP follows a direct media path, rather than through the network elements composing the SIP signaling path.

A common reason people in the past used INFO for application level information exchange is the negotiation is very lightweight compared to SUBSCRIBE/NOTIFY. This is more especially so if it is not certain if there will be application level information exchange. The SUBSCRIBE/NOTIFY machinery requires the user agents to exchange rich capabilities and maintain state for additional SIP dialogs. However, this is a weak argument if there is a high likelihood of application level information exchange. In this case, we recommend the use of a more robust application level information exchange protocol.

A.5.  Alternatives for Common INFO Use

What alternatives to INFO are there for UA-to-UA application session signaling? As noted above, there are three broad classes of session signaling available. The choice depends on the circumstances. Following is a list of situations that have used INFO in the past.

• State updates
• User stimulus
• Direct signaling channel
• Proxy-aware signaling
• Dialog probe

A.5.1.  State Updates

This is the broad class of one User Agent updating another with changes in state. The design goal of the SUBSCRIBE/NOTIFY (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265] event framework is to meet just this need.

A.5.2.  User Stimulus: Touch Tones and Others

This is the class of the user entering stimulus at one User Agent, and the User Agent transporting that stimulus to the other. A key thing to realize is key presses on the telephone keypad is user stimulus. Thus, the appropriate mechanism to use here is KPML (Burger, E. and M. Dolly, “A Session Initiation Protocol (SIP) Event Package for Key Press Stimulus (KPML),” November 2006.) [RFC4730].

A.5.3.  Direct Signaling Channel

State updates and user stimulus tend to have relatively few messages per session. Sometimes, User Agents need to exchange a relatively high number of messages. In addition, User Agents may have a need for a relatively low-latency exchange of messages. In this latter case, the User Agent may not be able to tolerate the latency introduced by intermediate proxies. Likewise, the intermediate proxies may have no interest in processing all of that data.

In this case, establishing a separate, direct control channel, as in MSRP (Campbell, B., Mahy, R., and C. Jennings, “The Message Session Relay Protocol (MSRP),” September 2007.) [RFC4975] or MRCPv2 (Shanmugham, S. and D. Burnett, “Media Resource Control Protocol Version 2 (MRCPv2),” November 2008.) [I‑D.ietf‑speechsc‑mrcpv2] is appropriate.

In addition, not every situation requires a SIP solution. Some signaling is really just one-shot to third-party endpoints. That situation may better be handled using an appropriate protocol, such as HTTP (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) [RFC2616].

A.5.4.  Proxy-Aware Signaling

Sometimes, one does want proxies to be in the signaling path for UA-to-UA application signaling. In this case, the use of a SIP request is appropriate. To date, there are no mechanisms for completely disambiguating INFO requests. For example, one could create a registry of INFO packages. The definition of the package would define the contexts for the various MIME Content-Types, as well as the context of the request itself. However, a package can have multiple content types. Moreover, having the context, or package identifier, at the SIP level precludes bundling multiple contexts responding in the same INFO request. For example, a User Agent might want to bundle two different responses in a multipart/mixed MIME body type.

Because there is no difference in either the protocol machinery or registration process due to these factors, we will not create an INFO framework. If one needs a SIP User Agent-to-SIP User Agent application session signaling transport protocol that touches all Record-Route proxies in a path, one MUST create a new SIP method as described in Section 27.4 of RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261].

A.5.5.  Dialog Probe

Some implementations in the wild use INFO to probe if an INVITE-initiated dialog is alive. While this works, it is NOT RECOMMENDED. In particular, RFC 4028 (Donovan, S. and J. Rosenberg, “Session Timers in the Session Initiation Protocol (SIP),” April 2005.) [RFC4028] describes how to ensure an INVITE-initiated dialog is alive.

A.5.6.  Malicious Indicator

Take the case of Malicious Indicator. This is where a subscriber receives a call, realizes it is a malicious call (threatening, SPIT, etc.). They then press the SPIT button (or press *xx), which tells their service provider to mark the UAC as a bad actor. One might be tempted to think that INFO would be a great option for this service. It follows the return path of the INVITE, and so the INFO will hit the caller's inbound proxy, which it can learn the caller is (statistically) a bad actor. That way the inbound proxy can do stuff like notify law enforcement, add a vote to "this is a SPIT source," or other useful action.

However, consider a few issues. First, since INFO lives exclusively within an established dialog, there is no way to assert this message after the call completes. Second, this mechanism relies on an active service provider topology. If there is no proxy in the chain that will eat the INFO, the caller will see the "this is a bad guy" message, which may have consequences in the real world. Third, there is no a'priori way for the UAS to know whether or not it can issue the INFO. The caller certainly will not advertise, "please tell me if I am bad, particularly I know in advance that I *am* a bad actor."

One approach is for the service provider's proxy to SUBSCRIBE for the SPIT event at the UAS. At this point, life is good, interoperable, and works across networks. This enables events after the dialog is torn down, as presumably the SPIT event will refer not to, "this dialog," which does not exist, but to "that dialog identifier," which exists (and is theoretically unique) forever.

Another approach that saves considerably on the overhead of subscriptions would be for the service provider to insert a HTTP URI in the initial INVITE, noting it is for reporting malicious behavior. When the subscriber presses the SPIT button, an HTTP POST gets executed, delivering the call information to the service provider. The service provider can encode basic call information in the HTTP URI and can instruct the device to send whatever arbitrary data is necessary in the POST. This method has the added benefit of being entirely outside the real-time SIP proxy network.

Appendix B.  Legacy INFO Usages

We do not intend this section to be a comprehensive catalog of INFO usages. However, it should give the reader a flavor for current INFO usages.

B.1.  ISUP

SIP-T uses Content-Type to identify ISUP protocol elements in an INFO message. See RFC3372 (Vemuri, A. and J. Peterson, “Session Initiation Protocol for Telephones (SIP-T): Context and Architectures,” September 2002.) [RFC3372].

B.2.  QSIG

QSIG uses Content-Type to identify QSIG protocol elements in an INFO message. See RFC4497 (Elwell, J., Derks, F., Mourot, P., and O. Rousseau, “Interworking between the Session Initiation Protocol (SIP) and QSIG,” May 2006.) [RFC4497].

B.3.  MSCML

MSCML uses a Require to ensure the UAS understands that INFO messages of the MSCML type are in fact MSCML messages. See RFC5022 (Van Dyke, J., Burger, E., and A. Spitzer, “Media Server Control Markup Language (MSCML) and Protocol,” September 2007.) [RFC5022].

B.4.  MSML

MSML endpoints just know the INFO messages carry MSML and from the Content-Type of the given INFO method request. See the MSML (Saleem, A., “Media Server Markup Language (MSML),” August 2008.) [I‑D.saleem‑msml] draft.

B.5.  Video Fast Update

Microsoft, Polycom, and Radvision used INFO messages as an interim solution for requesting fast video update before the ability to request I-Frames in RTCP was available. See the XML Schema for Media Control (Levin, O., Even, R., and P. Hagendorf, “XML Schema for Media Control,” March 2008.) [RFC5168] for more information.

B.6.  DTMF

[EDITOR'S NOTE: Are there public references? The AS5300 documentation from Cisco describes Cisco's use of INFO to carry DTMF. Anyone else want to belly up to the bar and have us collect your proprietary DTMF INFO payload here?]

Appendix C.  Acknowledgements

We are standing on the shoulders of giants. Jonathan Rosenberg did the original "INFO Considered Harmful" Internet Draft on 26 December 2002, which influenced the work group and this document. Likewise, Dean Willis influenced the text from his Internet Draft, "Packaging and Negotiation of INFO Methods for the Session Initiation Protocol" of 15 January 2003. Four paragraphs come from Jonathan Rosenberg's INFO Litmus draft. My, we have been working on this for a long time!

This and other related drafts have elicited well over 450 messages on the SIP list. People who have argued with its thesis, supported its thesis, added to the examples, or argued with the examples, include the following individuals:

Adam Roach, Bram Verburg, Brian Stucker, Chris Boulton, Cullen Jennings, Dale Worley, Dean Willis, Frank Miller, Gonzalo Camarillo, Gordon Beith, Henry Sinnreich, James Jackson, James Rafferty, Jeroen van Bemmel, Joel Halpern, John Elwell, Johnathan Rosenberg, Juha Heinanen, Keith Drage, Kevin Attard Compagno, Manpreet Singh, Martin Dolly, Mary Barnes, Michael Procter, Paul Kyzivat, Peili Xu, Peter Blatherwick, Raj Jain, Rayees Khan, Robert Sparks, Roland Jesske, Salvatore Loreto, Sam Ganesan, Sanjay Sinha, Spencer Dawkins, Steve Langstaff, Sumit Garg, and Xavier Marjou.

John Elwell and Francois Audet helped with QSIG references. In addition, Francois Audet provided actual text for the revised abstract. Keith Drage gave lots of excellent comments and helped immensely with Figure 1.

The work group version of this document benefited from the close readings and comments from

John Elwell, Paul Kyzivat, Dean Willis, Francois Audet, Dale Worley, Andrew Allen, Adam Roach, Anders Kristensen, Gordon Beith, Ben Campbell, Bob Penfield, Keith Drage, Jeroen van Bemmel, Mary Barnes, and Salvatore Loreto.

Since publication of the first work group version of this document, we have had over 329 messages. New voices in addition to those included above include

Arun Arunachalam, Christian Stredicke, Eric Rescorla, Inaki Baz Castillo, and Roni Evan.

However, any errors and issues we missed are still our own.

Appendix D.  Change Log

[RFC EDITOR NOTE: Please remove this section when publishing]

Changes from -02

• Applicability statement explicitly says we're backwards compatible
• Explicitly state we work like UPDATE (both early and confirmed dialogs)
• Agreed text for IANA Considerations package registry

Changes from -01

• One and only one Info Package per INFO
• Removed Send-Info header, greatly simplifying negotiation
• Multiple body part identification through Content-Disposition: Info-Package
• Note that forking INVITEs may result in multiple INFO's coming back to INVITE originator
• Describe how a UAS can enforce strict adherence to this document
• Remove CANCEL INFO faux pas
• Better explained overlapping INFO issues and resolutions
• Token names are now really case sensitive
• Moved Info Package Considerations to an Appendix
• Introduced stronger, yet more open, IANA registration process
• Took a few more paragraphs from INFO Litmus to cover all bases.
• Added RFC 5168 to legacy usages

Changes from -00

• Corrected ABNF.
• Enabled sending of legacy INFO messages. Receiving legacy INFO messages was already here.
• Negotiation is not Offer/Answer, it is Offer/Offer.
• Created the explicit "nil" Info Package to indicate no info package.
• Fixed CANCEL impacting future transactions.
• Added Registrar behavior.
• Added OPTIONS processing.
• Clarified overlapping INFO method processing.
• Described multiple INFO bodies in a single INFO method.
• Took out Info-Package as a header for responses to the INFO method.
• Expanded on risks of using INFO and filled-in more on the alternatives
• Moved definitions of INFO into the body of the text and cleaned up IANA Considerations section
• Added legacy usages descriptions

Authors' Addresses